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CSE3153 Network Administration Semester-2- 2006 This is an elective unit and aims to develop understanding of the theory and practice in administration and management of modern computer networks. Local area networks, connected by wide area links, form the infrastructure on which many distributed systems are constructed, and a deeper understanding of the installation, operation and management of this infrastructure has become an important area of specialisation within the larger discipline of Information Technology. Standardised specifications of this unit is also available in the Monash University handbook and in the FIT Unit Information pages. Unit Information on Unit web page http:// Unit Book On-line resources on the web Lectures: – 2 hrs per week, Wednesday 6.00pm~8.00pm • Tutorials/Practical Sessions (2 hrs per week): – Wednesday 8:00pm ~10:00pm [3Tutes] – Thursday 9:00pm ~11:00pm • Reading: – Text Book – Other resources on the unit page Unit Details Presented by: Mr. Pravin Shetty 6 point unit 2 hours Lecture 2 hours Tutorial upto 8 hours additional private study Each week for 13 weeks Caulfield campus Unit Objectives The unit is intended to enable you to understand: New Networking Technologies and Administration of: various techniques to transmit data over a transmission medium characteristics of various transmission media various techniques for sharing a communication channel design issues of various flow and error control in data communication identify hardware and software used in developing a Local Area Network (LAN) design issues involved in developing various protocols for Local Area Network analysis, design and implementation of a LAN for a given communication need methods of connecting LAN with other LANs or connecting LAN with Wide Area Network (WAN) architecture of several switching networks Assessment • • • • 40% in two Assignments 50% by examination 10%from Tutorials and Practical Sessions To pass the unit: – both assignments must be attempted – must pass in assignment assessment – must pass the final examination – Final mark according to the following formula final mark = min(A+10, E+10, E*R+A*(1-R)) where A = overall assignment percentage, E = exam percentage, R = exam weighting (50% = 0.5) Assessment-Assignments • Assignment 1 – Due Date: TBA – Weighting15 % • Assignment 2 – Due Date: TBA – Weighting 25 % • Assignment details will be posted on the web • Late submission ONLY with prior permission and VALID reasons Assessment-Examination • 3 hour examination • 50 % of total marks • will test your knowledge in the unit matter Objectives This unit will develop student knowledge of the techniques and systems for network administration. On completion of this unit, the student should have acquired the knowledge needed to identify the tasks or roles required of network administrators, understand current developments and standards for network management, define the principles involved in system and network administration and be able to apply these to practical situations, analyse and classify the requirements for management of a network particularly when it is a critical part of the structure of an organisation, design and implement network management policies, identify and compare different network management techniques and strategies. Students should also have developed practical skills in network administration, including experience of various network management tools, their interface, capabilities and operation, familiarity with typical methods of documenting and modelling networks,be able to effectively and efficiently setup networks and confirm correct operation,be able to monitor networks and diagnose common network faults, be able to construct test strategies and acceptance tests for networks.In addition, students will Experience the need for cooperative management of networks and computer equipment Work effectively in groups to achieve a system implementation Synopsis The unit will provide students with fundamentals and theoretical foundations of network administration. Specific topics include: Introduction to Network Administration Scope, Goals, Philosophy & Standards Challenges and common practice Network Administrators Role Review IT System Components Network Structures Technology (Sockets, Cables, etc) Protocols (TCP/IP, X.25, ATM, etc) Network Operating Systems Network System Management Hosts and Users, System Configuration and Maintenance Administration of Network Services TCP/IP Networks TCP/IP Toolkit Methods of Network Administration Managing devices using SNMP Remote Management using RMON DeskTop Management Network Fault Diagnosis and Recovery Network Performance and Tuning Network Security and Administration Analytical System Administration Network Simulation Network Documentation Future of Network Administration Recommended texts Burgess M Principles of Network & System Administration Second Edition, Wiley 2004 Subramani Network Management: Principles and Pra Addison Wesley ISBN 0-201-35 ISBN: 0-470-86807-4 Support material Stallings W. SNMP, v2, v3 & RMON I and II 3rd Edition Addison Wesley 1998 Stallin Data and Computer Communica 7th Edition, Prentice Hall ISBN 0-13-100 Co ISBN: 0-201-48534-6 Hunt C. TCP/IP Network Administration 3rd Edition, O'Reilly Associates 2002 ISBN: 0-596-00297-1 Mikalsen A., Borgesen P. Local Area Network Cernick P., Degner M., Kruep Cisco IP Routing Hand M&T, IDG Books ISB Burke Network Management: concepts and pra Pearson - PrenticeHall, ISB Lecture 1 Introduction to Communication ─ Data & Information ─ ─ ─ Communication Model Key Communication Tasks Networking Concept Communications Derived from Latin communis - means common communicate - the act of making it known to many (what is known to one) Communication simply means the transmission (transfer) of information Telecommunications communications using electronics, fibre optics and other specialized circuits Data & Information Data - representation of facts, concepts or instructions in a formalized manner suitable for communication and processing by human beings Information is born when data is interpreted Information exchange implies involvement of at least two parties Communication suggest a path and media through which information flows Why Study Data Communications? Information is a primary source for decision making optimum production keeping in line with the technological developments many other things … And we need to exchange information Types of Communication Voice communication human voice (telephone) Video communication pictures, diagrams, images, ... Data communication Numeric and text data, … The distinction between various types of communication are not clear-cut and the domains overlap Computer Communications The Computer-Communications revolution has resulted in – no fundamental difference between data processing and data communication – no fundamental differences among data, voice, video, ..., communications – the boundary between different systems heavily overlap A Communications Model Source Transmitter Carries data Receiver Converts data into transmittable signals Transmission System generates data to be transmitted Converts received signal into data Destination Takes incoming data A Communications Model • Source – generates data to be transmitted • Transmitter – Converts data into transmittable signals • Transmission System – Carries data • Receiver – Converts received signal into data • Destination – Takes incoming data Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control Simplified Communications Model - Diagram Simplified Data Communications Model Networking • Point to point communication not usually practical – Devices are too far apart – Large set of devices would need impractical number of connections • Solution is a communications network – Wide Area Network (WAN) – Local Area Network (LAN) Wide Area Networks • • • • Large geographical area Crossing public rights of way Rely in part on common carrier circuits Alternative technologies – Circuit switching – Packet switching – Frame relay – Asynchronous Transfer Mode (ATM) Circuit Switching • Dedicated communications path established for the duration of the conversation • e.g. telephone network Packet Switching • Data sent out of sequence • Small chunks (packets) of data at a time • Packets passed from node to node between source and destination • Used for terminal to computer and computer to computer communications Frame Relay • Packet switching systems have large overheads to compensate for errors • Modern systems are more reliable • Errors can be caught in end system • Most overhead for error control is stripped out Asynchronous Transfer Mode • • • • • • ATM Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet switching technique Local Area Networks • Smaller scope – Building or small campus • Usually owned by same organization as attached devices • Data rates much higher • Usually broadcast systems • Now some switched systems and ATM are being introduced LAN Configurations • Switched – Switched Ethernet • May be single or multiple switches – ATM LAN – Fibre Channel • Wireless – Mobility – Ease of installation Metropolitan Area Networks • • • • • MAN Middle ground between LAN and WAN Private or public network High speed Large area Networking Configuration Protocols Used for communications between entities in a system Must speak the same language Entities User applications e-mail facilities terminals Systems Computer Terminal Remote sensor What is a protocol? human protocols: • “what’s the time?” • “I have a question” • introductions network protocols: • machines rather than humans • all communication activity in Internet governed by protocols … specific msgs sent … specific actions taken when msgs received, or other events protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection req. Hi TCP connection reply. Got the time? 2:00 Get http://gaia.cs.umass.edu/index.htm time <file> Protocol Its characteristics 1. Monolithic VS Structured 2. Standard VS Non-standard Overview • • • • What is protocol? Protocol Families Function Characteristics – Monolithic VS Structured – Standard VS Non-standard What is protocol? • A protocol is a convention or standard that controls or enables the connection, communication and data transfer between two computing endpoints In its simplest form, a protocol can be defined as the rule governing the syntax, semantics and synchronization of communication. What is protocol? • A protocol is varied in purpose and sophistication – Detection of underlying physical connection (wired or wireless) or the existence of the other endpoint or node – How to start and end message – How to format a message – Termination of the session or connection Protocol Families • Most protocols are layered together into protocol stacks where various tasks listed above are divided among different protocols in the stack Open Standard Proprietary standard • Internet protocol suite • Open Systems Interconnect • • • • • AppleTalk DECnet ISX/SPX SMB System Network Architecture • Distributed Systems Architecture Functions • Segmentation & Reassembly • Encapsulation • Flow Control • Error Control • Addressing • Application Synchronisation • Multiplexing • Transmission Services Characteristics 1. Monolithic VS Structured • Monolithic Protocol Application package includes communication protocols. Modification is difficult with high risk of introducing bugs Characteristics 1. Monolithic VS Structured • Structured Protocol Protocols separated out from the application package using structured design techniques to form a set of layers - A Network architecture or protocol stack Characteristics Standard VS Non-standard • Standard protocol – It conforms to recognised standard – It provides more open network environment • Non-standard – Built for specific communication situation – Proprietary, implemented by a company for use within its networks – Problem of interconnection Protocol Functions • Not all protocols have all functions; this would involve a significant duplication of effort • Many instances of the same type of function being present in protocols at different levels. Protocol Functions • • • • • • • • • Encapsulation and Delineation Segmentation and reassembly Connection control Ordered Delivery Flow Control Error Control Addressing Multiplexing Transmission services Encapsulation • • • Each Protocol Data Unit (PDU) contains not only data but control information Some PDU consists solely of control information Control Information falls into 3 categories: – Address – Error detection code – Protocol control Encapsulation of PDU in TCP/IP Delineation • Trace the outline of the PDU • Indicates the control information and the actual data. Segmentation and Reassembly • packet-switched telecommunication network, segmentation and reassembly (SAR, sometimes just referred to as segmentation) • is the process of breaking a packet into smaller units before transmission and reassembling them into the proper order at the receiving end of the communication • Packets are made smaller to speed them through the network and specifically because of specified packet size restrictions in a given path Segmentation and Reassembly Protocol Architecture • Task of communication broken up into modules • For example file transfer could use three modules – File transfer application – Communication service module – Network access module Two Types of Data Transfer: Connectionless data transfer: Connection Oriented Transfer: – Every PDU is sent to receiver as an independent unit. – ‘Connection’ is opened between sender/receiver – No guarantee of loss, error, misdelivery etc – Preferable and advantageous when sending large data Connection Control – Connection-Oriented Transfer: • Multiple PDU’s sent in a single session • Ability to logically order multiple PDU’s (sequencing) • Consists of three main phases: – Connection Establishment – Data Transfer – Connection Termination • Further phases may exist for error detection/recovery Connection Establishment • • • • Creates a connection with other party Both must be using identical protocols A connection request is sent Connection must be accepted before any exchanges can proceed • Other features may be added to protocol as required Data Transfer • Occurs once connection is established • Data is transferred • Control information transferred, e.g: – Flow control – Error control – Acknowledgements of data sent/received Data Termination • Connection is terminated when request is sent to other party • May also be terminated by a central authority (e.g. security reasons) A typical Connection-Oriented Session • Source: Stallings 7E fig 18.1 p575 Sequencing • • • • An ability of connection-oriented transfer HDLC, IEEE 802.11 make use of this Each PDU is numbered sequentially Incoming/Outgoing numbers are monitored • Three main functions: – Ordered delivery – Flow Control – Error Control Ordered Delivery • A function of sequencing • Every PDU takes a different path, leading to varying arrival times • Each unique PDU can be sequenced according to its sending order Flow Control • Receiving entity can limit amount/rate of incoming data • Helps prevent overflow/overburden • Stop-and wait • Credit system/sliding window techniques • Implemented in several protocols Briefly on TCP/IP • Transmission Control Protocol (TCP) is connection-oriented – Logical created connection is between entities – Employs sequencing devices • Internet Protocol (IP) is connectionless – PDUs delivered to destination as single, stateless packets – Combines with TCP to form basis of internet transfers Why the need for addressing? • Addressing allows for the identification of devices on the network, so that data can be sent to the right device • To further that end, part of the information sent in a PDU (Protocol Data Unit) is the address of the intended recipient of the packet. A problem….. • Will is sitting at his computer, and wishes to send an email to Jane. How can this be achieved? High Level Address • For each device on the network assign a unique logical address to the device. • For TCP/IP this is an IP address: – 130.194.15.119 • For the OSI Model this is a ‘Network Access Service Point’ (NSAP) located within the Network layer High Level Address 2 • The logical Address (whether it is TCP/IP or NSAP) allows data to be sent from one device on the network to another device with a guarantee that the PDU wont go to the wrong device. • Thus Will’s email will get through to Jane’s computer. Another problem • Will has sent his email, but Jane is surfing the web, receiving a file transfer of some music, and talking online through an IM program. • How can Will’s email get to her email client so she will read it? Service Access Points • Within a device that has a single logical Address lies Service Access Points that applications can attach themselves to in order to receive the data they need. An Example of SAPs Picture by Kieran Simpson Ports • Within the TCP/IP Model SAP are referred to as ports. • Some common ports are: – 80 or 8080 for HTTP – 22 for SSH – 23 for Telnet Still another problem • How can Will’s and Jane’s computers be on the network to begin with? Device Identification • Each device on the network has a ‘Network Attachment Point’ through it’s Network Interface Card (NIC) • This is usually a 48bit long number represented as hexadecimal – 00-02-B3-24-5A-51 • Therefore Will and Jane’s NICs will have MAC addresses and be able to get onto the network. Diagrammatic Representation Taken from: “Data and Computer Communications”, William Stallings Page 41 How are these levels achieved? Taken from: “Data and Computer Communications”, William Stallings Page 42 Addressing Scope • There are two types of scope for address. – Local – Global Local Addressing Scope • A local address within the LAN • It is the name/address through which a device is identified within it’s own system. • Typically the MAC address. Global Addressing Scope • The name/address through which a device can be known outside the system. • Typically the logical Address (eg IP address) • Must be unique. An example of Scope • Outside the Monash network, the devices can only be reached through an IP address of: – 130.194.xxx.xxx – Example of global scope • Within the Monash network devices are located through an internal local address scheme => local scope Bit-oriented protocols Bit-oriented protocols interpret a transmission frame or packet as a succession of individual bits, made meaningful by their placement in the frame and by their juxtaposition with other bits. Bit-Oriented protocols SDLC HDLC LAPs LANs THREE TYPES OF STATIONS • Primary station – Controls operation of link – Frames issued are called commands – Maintains separate logical link to each secondary station • Secondary station – Under control of primary station – Frames issued called responses • Combined station – May issue commands and responses Tow link configurations • Unbalanced – One primary and one or more secondary stations – Supports full duplex and half duplex • Balanced – Two combined stations – Supports full duplex and half duplex Transfer Modes • Normal Response Mode (NRM) – Unbalanced configuration – Primary initiates transfer to secondary – Secondary may only transmit data in response to command from primary – Used on multi-drop lines – Host computer as primary – Terminals as secondary Transfer Modes • Asynchronous Balanced Mode (ABM) – Balanced configuration – Either station may initiate transmission without receiving permission – Most widely used – No polling overhead Transfer Modes • Asynchronous Response Mode (ARM) – Unbalanced configuration – Secondary may initiate transmission without permission form primary – Primary responsible for line – rarely used When we say it is a Bit-oriented Protocol • individual bits for control information and are the preferred method for transmitting data • control codes are used to control another device or provide information about the status of the session • for local area networks • data is transmitted as a steady stream of bits • Before date transmission begins , special synchronism characters are transmitted by the sender so the receiver can synchronize itself with the bit stream • transfer data frames regardless of frame contents. • provide full-duplex operation and are more efficient and reliable Each piece of data is encapsulated in an HDLC frame by adding a trailer and a header HDLC address and an HDLC control fields The frames are separated by HDLC flag sequences which are transmitted between each frame and whenever there is no data to be transmitted. at the end of the frame, and contains a cyclic redundancy check 01111110 Frame sent Flag Address Control Address FCS Flag Stuffed and unstuffed bits Frame received Flag 011111010 Control 011111010 FCS Flag 01111110 J • Start of the flag is 01111110that identifies both beginning and end of frame and services for receiver. • May be misread by receiver • The station finds a flag on line determines that the frame is addressed to it and begins reading the transmission. • Watching for the next flag that signifies the end of frame. • Guaranteed a flag lines not appear inadvertently. • Tell the receiver that the current sequence is not a flag Start After 1 Zero and 5 continuous onec 0 Unstuff zero It is part of the data 1 7th bit 0 8th bit 1 Continue counting ones unitlthe next zero IT IS A Flag TOTAL <15 It is an abort stop >=15 It means an idle channel Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that receiver does not mistake the data for flag In summary, a protocol is…. • An agreement about communication between two or more entities • It specifies – Format of messages – Meaning of messages – Rules for exchange – Procedures for handling problems Simplified File Transfer Architecture A Three Layer Model • Network Access Layer • Transport Layer • Application Layer Network Access Layer • Exchange of data between the computer and the network • Sending computer provides address of destination • May invoke levels of service • Dependent on type of network used (LAN, packet switched etc.) Transport Layer • Reliable data exchange • Independent of network being used • Independent of application Application Layer • Support for different user applications • e.g. e-mail, file transfer The OSI Reference Model OSI – Open Systems Interconnect What is the OSI model? • A model for structuring communications software to provide an open communication service that’s independent of manufacturers’ equipment and conventions • A framework for standardization • Split into 7 layers OSI layers • • • • • • • Application Presentation Session Transport Network Data Link Physical Basis of OSI design • Layered Design – Layers can me made independently and simultaneously – Changes to 1 layer should not affect others • Lower levels need to know a great deal of detail. Higher levels should not be concerned with these details • The design is based heavily on having a high level of cohesion, a minimal amount of coupling and using information hiding The Benefits • A high level of cohesion – The operations performed in each layer are all related by their functionality – Reduces complexity – more manageable code • A minimal amount of coupling – The looser the coupling, the easier it is to change 1 module without affecting the others – Keeps interface between modules simple The Benefits • Information hiding – Abstraction allows higher levels to be kept simpler – Also makes the layer reusable in a wider range of situations • All of this leads to interchangeable, reusable and manageable layers Design Principles • Keep it short and simple so the model remains manageable • Not so short as to require grouping of unrelated functions into a single layer • Each layer should follow International standards for the protocol – Interchangeable layers Design Principles • Boundaries between layers should be placed so that the amount of data sent between layers is minimal • Group similar operations into layers • Separate dissimilar operations into different layers • Boundaries should be placed where a different level of abstraction is required Design Principles • Allow changes to a layer without affecting other layers – Adapt to changes in technology.Layers should interface only with the layers directly above or below – eg. Layer 5 only interfaces with layers 4 and 6. Not with 3. – Good cohesion + loose coupling + information hiding = neat, manageable, maintainable & interchangeable design. This is the basis of the OSI model Overview • • • • Standards and Standards organizations Multivendor model Main emphasis - INTERCONNECTION This implies an active relationship between systems • OSI is a set of standards, providing many alternative choices • Divided into 7 different functional layers • Each layer: service provided + protocol A Communications Analogy Business Correspondence Manager Manager Secretary Secretary Mail Clerk Mail Clerk Letter Box Letter Box Post Office Transport Post Office The OSI reference model • The OSI reference model has 7 layers • Each layer is defined so that some easily described operation is performed, and the software or hardware that is responsible can be identified • Each layer thinks it is communicating with an identical layer on another machine and has little concern for what occurs within adjacent layers. The OSI reference model • Each layer must know, of course, what form the adjacent layers use when passing data back and forth, but they don’t need to know what goes on inside. • The direction of passing depends on whether the message is being sent (down through the layers) or received (up). • On sending, each layer will perform some function, add an identifying header to the incoming data, and pass the modified message to the lower layer. • On receiving, the process is reversed. The OSI reference model • In theory, we could have up to six added headers. • The physical layer does not add a header. Added headers OSI Model - Overview OSI Model • Application Layer • • What happens at this layer • What is the justification for having this layer • How does it differ from TCP/IP Model's Application layer Application Layer What happens in this layer? File Transfer - Different file systems have different file naming conventions and ways of representing data. Application Layer handles t Application Layer How does it differ from TCP/IP Model's Application layer Application Layer - TCP/IP Vs OSI Model TCP/IP Application Layer contains the equivalent of OSI Model layers • Application Layer • Presentation Layer • Session Layer Application-to-Application Communication • • • • • • • Set application message in standard form Convert data representation (syntax) Set up session, synchronise data transfer Transfer data in packets Control/organise route Low-level frames transfer, error handling Transfer bits via physical link Seven layers or subsystems Seven layers or subsystems • Each layer accepts services from layer below across an interface • Each layer provides services to layer above • Each layer requires to exchange information with its corresponding (peer) layer in the remote system, to do what is required of it. • The (N)-layer provides services to the (N+1)-layer; • An (N+1)-entity requests the service of an (N)-entity (below it) in order to communicate with its corresponding (peer) (N+1)-entity. Seven layers or subsystems • The (N+1)-protocol provides an apparent horizontal link between corresponding (N+1) layers in 2 systems. It is established via an (N)-connection provided by the (N)layer. Data Encapsulation (Enveloping) • Communication between layers uses protocols. • The data unit increases in size as it is transferred down through the layers. Each layer attaches header and trailer information - an 'envelope'. • These contain control information required by corresponding target (peer) layer • After transfer across the physical medium, the layers in the receiving system; successively strip the protocol control information, and pass the data package to the layer above. Data Encapsulation (Enveloping) standards • What is the problems without standards? Hp Dell Microsoft standards • Advantage – – lower cost Purchaser more flexibility in equipment selection and use • Disadvantage – Freeze the technology – Multiple standards for the same thing Standards Organizations • • • • • IAB - Internet Architecture Board ISO - International Standards Organisation ITU - International Telecommunication Union IEC - International Electrotechnical Commission IEEE - Institute of Electrical and Electronic Engineers • …. different organizations different standards Internet Society • Internet Society (ISOC) - An international organization concerned with the growth and evolution of the worldwide Internet, and the social, political, and technical issues that arise from its use. • ISOC is an organization with individual and organizational members, and is managed by a Board of Trustees elected by the worldwide individual membership Internet Society •Homepage Internet Society • • • • Over 20,000 individual members more than 150 organizational members over 180 countries It can provide the same standards for the world Internet Society • Is made up of – IAB - Internet Architecture Board: responsible for defining the overall architecture of the internet, providing guidance and board direction to the IEEE – IETF- Internet Engineering Task Force : the protocol engineering and development arm of the internet – IESG – Internet Engineering Steering Group: response for technical management of IETF and the Internet standards process RFC • An RFC (Request for Comments ) is a document describing the standards that make the Internet work. • The document series, begun in 1969, which describes the Internet suite of protocols and related experiments • Not all (in fact very few) RFCs describe Internet standards, but all Internet standards are written up as RFCs RFC •RFC Database RFC • Example of RFCs: – RFC 620 - Request for monitor host table updates Mar 1974 – RFC 3869 - IAB Concerns and Recommendations Regarding Internet Research and Evolution August 2004 RFC Publications Developing an ISO standard 1. 2. 3. 4. 5. 6. Proposal Preparatory Consensus Voting Approval publication RFC Publications • An RFC starts life as an Internet Draft. Before it can be published as an RFC, a document must first be published as an Internet Draft (I-D). All RFCs have been I-Ds, but not all I-Ds become RFCs. • In addition, the RFC Editor publishes as independent submissions some RFCs that are outside the IETF process but are relevant to the Internet community. RFCs must first be published as Internet Drafts. RFC Publications RFC Publications • The official specification documents of the Internet Protocol suite that are defined by the Internet Engineering Task Force (IETF) and the Internet Engineering Steering Group (IESG ) are recorded and published as standards track RFCs. • IETF –recommends & publishes RFC • IESG - approves • RFC Submission Process – RFCs from the IETF – Independent Submissions • Final Review Period RFC Publications 6 4 IETF –recommends & publishes RFC IESG - approves RFC Publications Flow chart Standards for physical interface between devices • Voltages, pulses, connectors, switches. • Rules for bits transfer. • Mechanical specifications, e.g. RS232C connector (9-pin or 25-pin); RJ-45; BNC • Electrical specifications, e.g. voltage levels, timing • Functional specifications, e.g. what signal each pin is used for • Procedural specifications, - response and sequencing of signals Service Primitives and Parameters • Interaction between layers – primitives specify the functions to be performed – parameters are used to pass data and control information – four primitives define interaction between adjacent layers • REQUEST, INDICATION, RESPONSE, CONFIRM • may be conformed or not conformed Service Primitives and Parameters • Enable communication between layers in OSI model. • Independent Layers system. • Allow changes of functions or protocols to be made within a layer without affecting other layers. What are The Services Primitives? REQUEST Invoke some service, pass the parameters. INDICATION A primitive issued by a service provider either to: indicate that a procedure has been invoked, provide the associated parameters, or notify the service user of a provider-initiated action. RESPONSE A primitive issued by a service user to acknowledge or complete some procedure. CONFIRM Complete some procedure previously invoked by a request by the service user. Sequences in Services Primitives • Source: http://williamstallings.com Layer 1 = Physical Layer • This is the only layer with DIRECT communication with another corresponding subsystem. • Physical Layer: physical and electrical characteristics of essential hardware Physical Layer • This layer defines: – Physical Media – bounded, unbounded – Hardware devices; Mechanical interfaces, electrical interfaces, • e.g., NICs - Network Interface Cards, repeaters, hubs – Interconnections: Physical topology (Logical topology is Layer 2 MAC) • linear bus, star, ring, etc. – Techniques to transfer bit stream to medium, • i.e., Signalling method, transmission technique (how bits are transmitted) – Interface to Layer 2 = Data Link Layer Introduction Asynchronous protocols: treat each character in a big stream independently. Synchronous protocols: take the whole bit stream and chop it into character of equal size. Asynchronous Protocols • • • • • • Long, long… time ago Not complex and easy to implement Slow Required start/stop bit and space Now mainly used in modem Replaced by high speed synchronous Synchronous Protocols • Character-oriented protocol – – – – Based on one byte (8-bit) Use ASCII for control character Frames are interpreted as a sequence of characters Not efficient because additional DLE character needed – Example: Binary Synchronous Communication (BSC) • Bit-oriented protocol – Going to be presented by Tutor in next question Control characters for BSC Control characters: is used to convey information about the transmission Character STX ASCII Code STX Binary 0000010 SOH SOH 0000001 DLE DLE 0010000 Function Start of text Start of header Data link escape IBM’s Binary Synchronous Communication (BSC) • Character-oriented protocol • Specifies half-duplex transmission with stop-and-wait ARQ • BSC_p divides a transmission into frames 1. Data frame (for transmission of data) 2. Control frame (connect/disconnect and flow/error control) A simple BSC data frame A BSC frame with a header S Y N S Y N S O H Header S T X …Data… E T X B C C B C C • Header Fields: – Include the address of the sending/receiving device – Identifying NO of the frame (0/1 for stop-andwait ARQ) Multiblock frame Block Block S S S Y Y O N N H Header S T X …Data… I B T C B C B C C S T X ITB = Intermediate text block • Reason to divided into blocks …Data… E T X B C C B C C Multiframe transmission S Y N S Y N S O H B C C B C C A S C0 Y K N S Y N S Y N S O H B C C B C C A S C1 Y K N Header S T X …Data… I T B B C C B C C S T X …Data… I T B B C C B C C S T X …Data… E T B B C C B C C …Data… E T X B C C B C C S Y N Header S Y N S T X Control frame A control frame is used by one device to send commands to another device. It contains only control characters but no data Data Transparency • BSC is designed for text message • Now, non-text message like video…graphics.. • Problem? – BSC control character problem • Data transparency: in data communication means we should be able to send any combination of bit as data Byte Stuffing Errors Single-Bit Burst Errors • Interference from heat, magnetism and other forms of electricity. • Errors can alter the meaning of data in binary-encoded data Single Bit error • Only one bit of a given data unit is changed from 1 to 0 or from 0 to 1 • Such as byte, character, data unit or packet • Occurs least likely in serial data transmission • For example, if we send 1mbps of data. It means that each bit last only 1/1000 000 second or 1microsecond. So, for the single Bit error to occur, the sound must have duration of only 1microsec. 0 changed to 1 0 0 0 0 1 0 1 0 Received 0 0 0 0 0 0 1 0 sent Burst error • Two or more bits in the data unit have changed from 1 to 0 or from 0 to 1. • Occurs Most likely in a serial transmission. • For example if we send 1kbps of data. If we have 10 error bits, the noise will have a duration of 1/100seconds. So if we are sending 1mbps, the same noise can affect 10 000bits Burst Length of 4 0 changed to 1 0 1 1 0 1 0 1 0 Received 0 0 0 0 0 0 1 0 sent a. Given the following string of bits was terminated: Transmitted Message : 11011011110111100011111011 Received Message : 11111111110101111111111011 Transmitted Message 1 1 0 11 0 1 1 1 1 0 1 1 1 1 0 0 0 1 1 1 1 1 0 1 1 Received Message 1 1 1 1 1 1 1 1 1 1 0 1 0 1 1 1 1 1 1 1 1 1 0 1 1 16 bits length of burst error Error Control • It happened in data link layer and transport layer. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without ERROR (damage, loss or duplication). It is performed end to end rather than across a single link. • In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error Detection • Despite the best prevention techniques, errors may still happen. • To detect an error, something extra has to be added to the data/signal. This extra is an error detection code. • There are two basic techniques for detecting errors: 1. Parity checking 2. cyclic redundancy checksum (CRC). Error correction Once an error is detected, what is the receiver going to do? • Do nothing • Return an error message to the transmitter • Fix the error with no further help from the transmitter Parity Checks • If performing even parity, add a parity bit such that an even number of 1s are maintained. • If performing odd parity, add a parity bit such that an odd number of 1s are maintained. • For example, if the character 1001010 is to be sent, using even parity, a parity bit = 1 would be added to the character. • If the character 1001011 is to be sent, using even parity, a parity bit = 0 would be added to the character. • Can be defined as two ways VRC and LRC Vertical redundancy check (VRC) In vertical redundancy check, a parity bit is added to every data unit so that the total number of 1s becomes even. Longitudinal redundancy check (LRC) In longitudinal redundancy check, a block of bits is divided into rows and a redundant row of bits is added to the whole block. VRC and LRC Both simple parity and longitudinal parity do not catch all errors. VRC can detect all single-bit errors. But VRC cannot detect errors where the total number of bits changed is even LRC is better at catching errors but requires too many check bits added to a block of data. Example • Given the following frame is to be transmitted: row1 0 0 0 0 0 0 0 row2 0 1 0 1 0 0 0 row3 1 0 0 0 1 1 0 row4 0 1 0 0 0 0 0 row5 0 1 0 1 1 0 1 Example (con.) i) Determine the VRC (row) odd parity and LRC (column) even parity bits for the frame 0 0 1 0 0 0 1 0 1 1 0 0 0 0 0 0 1 0 0 1 0 0 1 0 1 0 0 1 0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 0 1 1 1 Example (con.) ii) Give an example of combination of error bits occurring in the frame that would be undetectable. 0 0 1 0 0 1 0 1 0 1 1 1 0 0 0 0 0 0 0 10 01 0 1 0 0 01 10 0 1 0 0 0 1 0 0 1 0 0 0 0 1 1 1 1 0 0 1 1 Local Area Network (LAN) Fundamentals Reference: Chapter 15 -Stallings Introduction • LANs are usually owned by the organisation that uses a network to interconnect equipment • LANs have much greater capacity than wide area networks (WANs), to carry what is generally a greater internal communications load • A LAN can be used for a variety of applications – A common LAN is one that supports personal computers Introduction Contd. • LANs for the support of personal computers and work stations have become nearly universal in organisations of all sizes • Even the sites that still depend heavily on the mainframe have transferred much of the processing load to networks of personal computers • For personal computer networks, a key requirement is low cost – In particular, the cost of attachment to the network must be significantly less than the cost of the attached device – That is, the data rate of the network may be limited; in general, the higher the data rate, the higher the cost Introduction Contd. – Backend networks are used to interconnect large systems such as mainframes, supercomputers, and mass storage devices • The key requirement here is for bulk data transfer among limited number of devices in a small area • Typically, backend networks are found at sites of large companies of research installations with large data processing budgets • A concept related to that of the backend network is the storage area network (SAN) – The SAN detaches storage tasks from specific servers and creates a shared storage facility across a high-speed network – The collection of networked storage devices can include hard disks, tape libraries, and CD arrays Introduction Contd. – New demands of office environments require high-speed office networks • One reason is that desktop image processors have increased network data flow by an unprecedented amount. – Even with compression techniques, this will generate a tremendous load • These new demands require LANs with high-speed that can support the larger numbers and greater geographic extent of office systems as compared to backend systems Introduction Contd. – Backbone LANs are attractive means of supporting increasing use of distributed processing applications and personal computers of local networking • They employ lower cost, lower-capacity LANs within buildings or departments and interconnect them with a higher-capacity (Backbone) LAN • They are a better alternative than having a single LAN due to: – Better reliability – More scalable – A typical LAN with low cost equipment will not be able to provide overall requirement. Topologies • In the context of a communication network, the term topology refers to the way in which the end points, or stations, attached the network are interconnected • The common topologies for LANs are bus, tree, ring, and star – The bus is a special case of the tree, with only one trunk and no branches Topologies Contd. Topologies Contd. • Bus and Tree topologies: – Both these topologies are characterised by the use of a multipoint medium • For the bus, all stations attach directly to a linear transmission medium, or bus, through appropriate hardware interfacing known as a tap – Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus – A transmission from any station propagates the length of the bus in both directions and can be received by all other stations Topologies Contd. • For the tree topology, the transmission medium is a branching cable with no closed loops – The tree layout begins at a point known as the headend – One or more cables start at the headend, and each of these may have branches – Two problems present themselves in these topologies: • As a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended • A mechanism is needed to regulate transmission Topologies Contd. – To solve these problems, stations transmit data in small blocks, known as frames • Each frame consists of a portion of the data that a station wishes to transmit, plus a frame header that contains control information • Each station on the bus is assigned a unique address, or identifier • The destination address for a frame is included in its header – With the bus or tree, no special action needs to be taken to remove frames from the medium • When a signal reaches the end of the medium, it is absorbed by the terminator Topologies Contd. Topologies Contd. • Ring Topology: – The network consists of a set of repeaters joined by point-topoint links in a closed loop – The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received – The links are unidirectional • Data are transmitted in one direction only (clockwise or counterclockwise) Topologies Contd. – As with the bus and tree, data are transmitted in frames • As a frame circulates past all the other stations, the destination station recognises its address and copies the frame into a local buffer as it goes by • A frame continues to circulate until it returns to the source station, where it is removed – As multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames Topologies Contd. Topologies Contd. • Star Topology: – Each node is directly connected to a common central node • Typically, each station attaches to a central node via two point-topoint links, one for trans mission and one for reception – In general, there are two alternatives for the operation of the central node: • In one approach the central node operates in a broadcast fashion – A transmission of a frame from one station to the node is retransmitted on all of the out going links » This transmission is received by all the other stations, and only one station at a time may successfully transmit Topologies Contd. – In this case , the central element is referred to as a hub • In the second approach, the central node acts as a frame-switching device – An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station Choice of Topology • The choice of topology depends on a variety of factors, including reliability, expandability, and performance – This choice is part of the overall task of designing a LAN and cannot be made in isolation, independent of the choice of transmission medium, wiring layout, and access control technique • For a bus topology, baseband coaxial cable has achieved widespread use, primarily for Ethernet systems – Comparatively, bus topology using baseband coaxial cable is difficult to work with – Although there is a considerable installed base, a few new installations are being attempted Choice of Topology Contd. • The ring topology is used for very-high-speed links over considerable distances – Hence, the ring has the potential for providing the best throughput of any topology – One disadvantage of the ring is that a single link or repeater failure could disable the entire network • The star topology is generally best for short distances and can support a small number of devices at high data rates Choice of Transmission Medium • The choice of transmission medium in a LAN is determined by a number of factors: – Topology – Capacity – Reliability – Types of data supported • Depends on the application – Environmental scope Choice of Transmission Medium Contd. • Typically, office buildings are wired to meet the anticipated telephone system demand – Voice-grade unshielded twisted pair (UTP) (Category 3) can be used as it is inexpensive and there is no cable installation costs – However, the data rate that can be supported is generally quite limited – Shielded twisted pair and baseband coaxial cabal are more expensive that Category 3 UTP but provide greater capacity Choice of Transmission Medium Contd. – However, in the recent years, the trend has been toward the use of high-performance UTP, especially Category 5 UTP • Category 5 UTP supports high data rates for a small number of devices • Larger installations can be supported by the use of star topology – Optical fibre has a number of attractive features, such as electromagnetic isolation, high capacity, and small size • However, the market penetration of fibre LANs is low as yet, primarily due to high cost of fibre components and lack of skilled personnel to install and maintain fibre systems LAN Protocol Architecture • The architecture of a LAN is best described in terms of layering of protocols that organise the basic functions of a LAN • The standardised protocol architecture for LANs encompasses physical, medium access control (MAC), and logical link control (LLC) layers • The physical layer encompasses topology and transmission medium, which we have discussed so far • The next section provides an overview of the MAC and LLC layers IEEE 802 Reference Model • Protocols defined specifically for LAN and MAN transmission address issues relating to the transmission of blocks of data over the network • In OSI terms, higher layer protocols ( layers 3 or 4 and above) are independent of network architecture and are applicable to LANs, MANs, and WANs • A discussion of LAN protocols is concerned principally with lower layers of the OSI model IEEE 802 Reference Model Contd. IEEE 802 Reference Model Contd. • The figure in the previous slide relates the LAN protocols to the OSI architecture • This architecture was developed by the IEEE 802 committee and has been adopted by all organisations working on the specification of LAN standards – It is generally referred to as IEEE 802 reference model • The lowest layer of the model (physical layer) is responsible for encoding/decoding, preamble generation/ removal, and bit transmission /reception IEEE 802 Reference Model Contd. • The layer above the physical layer is associated with providing service to LAN users; these services include: – On transmission, assemble data into a frame with address and error-detection fields – On reception, disassemble frame, and perform address recognition and error detection – Govern the access to the LAN transmission medium – Provide an interface to higher layers and perform flow and error control IEEE 802 Reference Model Contd. • The set of functions in the last bullet item are grouped into a logical link control (LLC) layer • The functions in the first 3 bullet items are treated as a separate layer, called medium access control (MAC) • Above separation is done for the following reasons: – The logic required to manage access to a shared-access medium is not found in traditional layer 2 data link control – For the same LLC, several MAC options may be provided IEEE 802 Reference Model Contd. Logical Link Control • The LLC layer for LANs is similar in many respects to other link layers in common use • Like all link layers, LLC is concerned with the transmission of a link level PDU between 2 stations, without the need of an intermediate switching node • LLC has 2 characteristics not shared by most other link control protocols: – It must support the multiaccess, shared-medium nature of the link – It is relieved of some details of link access by the MAC layer Logical Link Control Contd. • LLC specifies the mechanisms for addressing stations across the medium and for controlling the exchange of data between two users • The operation and format of this standard is based on HDLC • Three services are provided as alternatives for attached devices using LLC – Unacknowledged connectionless service: Logical Link Control Contd. • A very simple service that does not involve any of the flow- and errorcontrol mechanisms • Thus the delivery of data is not guaranteed – However, in most devices, there will be some higher layer of software that deals with reliability issues (there by avoids duplication) • Used for instances in which the overhead of connection establishment and maintenance is unjustified or even counter-productive – For example, data collection activities that involve periodic sampling data sources, such as sensors and automatic self-test reports from security equipment or network components Logical Link Control Contd. – Connection mode service: • Similar to the service offered by HDLC. • A logical connection is set up between 2 users exchanging data, and flow control and error control are provided • Could be used in very simple devices, such as terminal controllers, that have little software operating above this level • In this mode, the logical link control software must maintain some sort of table for each active connection, to keep track of the status of the connection Logical Link Control Contd. – Acknowledged connectionless service: • This is a cross between the previous two services • If the user needs guaranteed delivery but there are a large number of destinations, this mode is preferred – An example is a process control or automated factory environment where central site may need to communicate with a large number of processors and programmable controllers – Another use of this is the handling of important and time-critical alarm or emergency control signals in a factory Logical Link Control Contd. • The basic LLC protocol is modelled after HDLC and has similar functions and formats • The differences between the 2 protocols are: – LLC makes use of asynchronous balanced mode of operation of HDLC, to support connection mode LLC service • This is referred to as type 2 operation • The other HDLC modes are not employed – LLC supports an unacknowledged connectionless service using the unnumbered information PDU • This is known as type 1 operation Logical Link Control Contd. – LLC supports an acknowledged connectionless service by using two new unnumbered PDUs • This is known as type 3 operation – LLC permits multiplexing of the use of LLC service access points (LASPs) • All three LLC protocols employ the same PDU format, which consists of 4 fields – The DSAP (destination services access point) and SSAP (source service access point) fields each contain a 7-bit address, which specify the destination and source uses of LLC Logical Link Control Contd. – One bit of DSAP indicates whether the DSAP is an individual or group address – One bit of the SSAP indicates whether the PDU is a command or response – The format of LLC control field is identical to that of HDLC, using extended (7-bit) sequence numbers Logical Link Control Contd. Medium Access Control • All LANs and MANs consist of collections of devices that must share the network’s transmission capacity • The function of the MAC protocol is providing some means of controlling access to the transmission medium for an orderly and efficient use of the above capacity • The control can be exercised in a centralised or distributed manner – In the former, a controller is designated that has the authority to grant access to the network Medium Access Control Contd. – In the latter, the stations collectively perform a medium access control function to determine dynamically the order in which stations transmit • How the access control is accomplished is constrained by the topology and is a tradeoff among competing factors, including cost, performance, and complexity • In general, access control techniques are categorised as being either synchronous or asynchronous – With the former, a specific capacity is dedicated to a connection • Such techniques are generally not optimal in LANs and MANs as the needs of the stations are unpredictable Medium Access Control Contd. – In the latter approach, capacity is allocated in an asynchronous (dynamic) manner, more or less in response to immediate demand – The asynchronous approach can be further subdivided into three categories: • Round Robin: – Each station in turn is given the opportunity to transmit – During that opportunity, the station may decline to transmit or may transmit subject to a specified upper bound » The bound is usually expressed as a maximum amount of data transmitted or time for this opportunity Medium Access Control Contd. – When a station has finished, it relinquishes its turn, and the right to transmit passes to the next station in logical sequence » The control of the sequence may be centralised or distributed » Polling is an example of a centralised technique – When many stations have data to transmit over an extended period of time round-robin techniques can be very efficient – If only a few stations have data to transmit over an extended period of time, then there is a considerable overhead in passing the turn from station to station » Under these circumstances, other techniques may be preferable, largely depending on whether the data traffic has stream or burst characteristics Medium Access Control Contd. » Stream traffic is characterised by lengthy and fairly continuous transmissions – examples are voice communications, bulk file transfer » Bursty traffic is characterised by short, sporadic transmissions – interactive traffic • Reservation – Well suited for stream traffic, and reservation can be made in centralised or distributed manner – In general, for these techniques, time on the medium is divided into slots , much as with synchronous TDM – A station wishing to transmit reserves future slots for an extended or even an indefinite period Medium Access Control Contd. • Contention – Usually appropriate for bursty traffic – No control is exercised to determine whose turn it is, all stations contend for time – These techniques are of distributed in nature – Their principal advantage is that they are simple to implement and, under light to moderate load, efficient » For some of these techniques, performance tend to collapse under heavy load • In LANs, round-robin and contention techniques are the most common Medium Access Control Contd. • As with other protocol layers, MAC implements its functions making use of a PDU at its layer – In this case, the PDU is referred to as a MAC frame • The exact format of the MAC frame differs somewhat for the various MAC protocols in use • In general, all of the MAC frames have a format similar to that was shown in the previous figure Medium Access Control Contd. • The fields of this frame are: – MAC control • Contains any protocol control information needed for the functioning of the MAC protocol – For example, a priority level could be indicated here – Destination MAC address • The destination physical attachment point on the LAN for this frame – Source MAC address – LLC • The LLC data from the next higher layer Medium Access Control Contd. – CRC • The cyclic redundancy check field – also known as the frame check sequence (FCS) field • In most data link control protocols, the data link protocol entity is responsible not only for detecting errors using CRC, but for recovering from those errors by retransmitting • In LAN protocol architecture, these two functions are split between MAC and LLC layers – The MAC layer is responsible for detecting errors and discarding any frame that are in error – The LLC layer optionally keeps track of which frames have been successfully received and retransmit unsuccessful ones Local Area Network (LAN) Developments Reference: Chapter 16 -Stallings Introduction • Recent years have seen rapid changes in technology, design, and commercial applications for LANs – A major feature of this evolution is the introduction of a variety of new schemes for high-speed LANs • The most important commercial products available are: – Fast Ethernet and Gigabit Ethernet – Fibre Channel – High-speed wireless LANs Introduction Contd. • Until relatively recently, office LANs provided basic connectivity services- connecting PCs and terminals to mainframes and midrange systems – It provided workgroup connectivity at the departmental level – The traffic pattern was relatively light, with an emphasis on file transfer and electronic mail – The LANs that were used for this type of workload were primarily Ethernet and token ring • In recent years, two significant trends have altered the role of the PC and therefore the requirements of the LAN: Introduction Contd. – The speed and computing power of PCs have continually increased • Today’s more powerful platforms support graphics intensive applications and elaborate graphical user interfaces to the operating system – MIS organisations have recognised the LAN as a viable and essential computing platform, resulting the focus on network computing • The trend began with client/server computing, which has become a dominant architecture in the business environment • These approaches involve frequent transfer of large volumes of data in a transaction-oriented environment Introduction Contd. • The following are examples of requirements that call for higher-speed LANs: – Centralised server farms – Power workgroups • A small number of cooperating users who need to draw massive data files across the network – Examples are software development groups and CAD companies that run simulations regularly Ethernet • Most widely used high-speed LANs are based on Ethernet, which is developed by the IEEE 802.3 standards committee • The access method used by Ethernet is CSMA/CD (carrier sense multiple access with collision detection) • CSMA/CD and its precursors can be termed random access, or contention, techniques – There is no predictable or scheduled time for any station to transmit – They exhibit contention in the sense that stations contend for time on the shared medium Ethernet Contd. • The earliest of these techniques, known as ALOHA (sometimes pure ALOHA), was developed for packet radio networks – However, it is applicable to any shared transmission medium – In ALOHA, a station may transmit a frame at any time – It then listens for an amount of time equal to the maximum possible round-trip propagation delay on network plus a small fixed time increment – If the station hears an acknowledgment during that time, fine; otherwise it resends the frame Ethernet Contd. – If the station fails to receive an acknowledgment after repeated transmissions, it gives up – A receiving station determines the correctness of an incoming frame by examining a FCS field – If the frame is valid and the destination address in the frame address matches the receiver’s address, the station immediately sends an acknowledgment – A frame may be invalid due to noise on the channel or because another station transmitted a frame at about the same time • The latter case is known as a collision Ethernet Contd. • ALOHA is as simple as can be, but the number of collisions rises rapidly with increased load – The maximum utilisation of the channel is only about 18% • To improve efficiency, a modification of ALOHA, known as slotted ALOHA was developed – Time on channel is organised into uniform slots whose size equals the frame transmission time – Some central clock or other technique is needed to synchronise all stations Ethernet Contd. – Transmission is permitted to begin only at a slot boundary • Thus , frames that overlap will do so totally – This increased the maximum utilisation of the system to about 37% • Both ALOHA and slotted ALOHA exhibit poor utilisation – Both fail to take advantage of one of the key properties of both packet radio networks and LANs Ethernet Contd. • That is propagation delay between stations may be very small compared to frame transmission time – A short propagation delay provides the stations with better feedback about the state of the network • This information can be used to improve efficiency • The above observations led to the development of CSMA – A station wishing to transmit first listen to the medium to determine if another transmission is in progress (carrier sense) Ethernet Contd. – If the medium is in use, the station must wait – If the medium is idle, the station may transmit – It may happen that two or more stations attempt to transmit at about the same time • Then, there will be a collision • To account for this, a station waits a reasonable amount of time after transmitting for an acknowledgment • If there is no acknowledgment, the station assumes a collision has occurred and retransmits Ethernet Contd. • The maximum utilisation achievable using CSMA can far exceed that of ALOHA or slotted ALOHA – It depends on the length of the frame and propagation time • The longer the frames or shorter the propagation time, the higher the utilisation • With CSMA, an algorithm is needed to specify what a station should do if the medium is found busy – One algorithm is nonpersistent CSMA • If the medium is idle transmit • If the medium is busy, wait an amount of time drawn from a probability distribution (retransmission delay) and repeat the previous step Ethernet Contd. • A problem with nonpersistent CSMA is that capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are stations waiting to transmit – To avoid the above limitation, 1-persistent protocol can be used • If the medium is idle, transmit • If the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately • If two or more stations are waiting to transmit, a collision is guaranteed – Things get sorted out only after the collision Ethernet Contd. – A compromise that attempts to reduce collisions and idle time is ppersistent • If the medium is idle, transmit with probability p, and delay one time unit with probability (1-p) – The time unit is typically equal to the maximum propagation delay • If the medium is busy, continue to listen until the channel is idle and repeat the previous step • If transmission is delayed one time unit, repeat the first step • CSMA has one glaring inefficiency – When two frames collide, the medium remains unusable for the duration of transmission of both damaged frames Ethernet Contd. – For long frames, compared to propagation time, the amount of wasted capacity can be considerable • This waste can be reduced if a station continues to listen to the medium while transmitting – The above leads to CSMA/CD • If the medium is idle, transmit • If the medium is busy, continue to listen until the channel is idle, then transmit immediately • If a collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease transmission Ethernet Contd. • After transmitting the jamming signal, wait a random amount of time, referred to as the backoff, then attempt transmit again – An important rule followed in most CSMA/CD systems is that frames should be long enough to allow collision detection prior to the end of transmission • If shorter frames are used, then collision detection does not occur – CSMA/CD will exhibit the same performance as the less efficient CSMA protocol Ethernet Contd. Ethernet Contd. • The MAC frame format for 802.3 protocol consists of the following fields: – Preamble • A 7-octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronisation – Start Frame Delimiter (SFD) • The sequence 10101011, which indicates the actual start of the frame and enables the receiver to locate the first bit of the rest of the frame – Destination Address (DA) Ethernet Contd. – Source Address (SA) – Length/Type • Length of LLC data field in octets, or Ethernet Type field, depending on whether the frame conforms to IEEE 802.3 standard or the earlier Ethernet specification – In either case, the maximum frame size, excluding the Preamble and SFD, is 1518 octets – LLC data – Pad • Octets added to ensure that frame is long enough for proper CD operation Ethernet Contd. – Frame Check Sequence (FCS) • A 32-bit CRC, based on all fields except preamble, SFD, and FCS Ethernet Contd. • A traditional Ethernet is half-duplex – A station can either transmit or receive a frame, but it cannot do both simultaneously – If a 100-Mbps Ethernet ran in full-duplex mode, the theoretical transfer rate becomes 200 Mbps • The attached stations must have full-duplex rather than half-duplex adapter cards • The central point in the star wire cannot be a simple multipoint repeater but rather must be a switching hub – In this case each station constitutes a separate collision domain Ethernet Contd. – In fact, there are no collisions and the CSMA/CD algorithm is no longer needed – However, the same 802.3 MAC frame format is used and attached stations can continue to execute the CSMA/CD algorithm, even though no collisions can ever be detected • One of the strengths of the Fast Ethernet approach is that it readily supports a mixture of existing 10-Mbps LANs and newer 100-Mbps LANs – For example, the 100-Mbps technology can be used as a backbone LAN to support a number of 10-Mbps hubs • These hubs are in turn connected to switching hubs that conform to 100BASE-T and that support both 10-Mbps and 100-Mbps links Fast Ethernet • Fast Ethernet refers to a set of specifications developed by the IEEE 802.3 committee to provide a low-cost, Ethernet compatible LAN operating at 100 Mbps – The blanket designation for these standards is 100BASE-T – The committee defined a number of alternatives to be used with different transmission media – All of the 100BASE-T options use IEEE 802.3 MAC protocol and frame format Gigabit Ethernet • In late 1995, the IEEE 802.3 committee formed a High-Speed Study Group to investigate means for conveying packets in Ethernet format at speeds in gigabits per second range – A set of 1000-Mbps standard have now been issued • While defining a new medium and transmission specification, Gigabit Ethernet retains the CSMA/CD protocol and Ethernet format of its 10-Mbps and 100-Mbps predecessors • As more organisations move to 100BASE-T, putting huge traffic loads on backbone networks, demand for Gigabit Ethernet has intensified Gigabit Ethernet Contd. 10-Gbps Ethernet • The principal driving requirement for 10 Gigabit Ethernet is the increase in Internet and intranet traffic • A number of factors contribute to the explosive growth in both Internet and intranet traffic – An increase in the number of network connections – An increase in the connection speed of each end-station • E.g., 10 Mbps users moving to 100 Mbps, analog 56Kbps user moving to DSL and cable modems 10-Gbps Ethernet Contd. • An increase in the deployment of bandwidth-intensive applications such as high-quality video – An increase in Web hosting and application hosting traffic • Initially network managers will use 10-Gbps Ethernet to provide high-speed, local backbone interconnection between large capacity switches – As the demand for bandwidth increases, 10-Gbps Ethernet will be deployed throughout the entire network and will include server farm, backbone, and campuswide connectivity Token Ring • The IEEE 802.5 token ring standard is an outgrowth of IBM’s commercial token ring LAN product • A ring consists of a number of repeaters, each connected to two others by unidirectional transmission links to form a single closed path – Data are transferred sequentially, bit by bit, around the ring from one repeater to the next • Each repeater regenerates and retransmits each bit Token Ring Contd. • For a ring to operate as a communication network, three functions are required: data insertion, data reception, and data removal – These functions are provided by the repeaters • Each repeater, in addition to serving as an active element on the ring, serves as a devise attachment point • Data are transmitted in packets, each of which contains a destination address field Token Ring Contd. • As a packet circulates past a repeater, the address field is copied – If the attached station recognises the address, the remainder of the packet is copied • Repeaters perform the data insertion and reception functions similar to that of taps, which serve as devise attachment points on a bus or tree • Data removal is, however, is more difficult on a ring – As a ring is a closed loop, a packet will circulate indefinitely unless it is removed Token Ring Contd. • A packet may be removed by the addressed repeater – Alternatively, each packet could be removed by the transmitting repeater after it has made one trip around the loop • This approach is more desirable as it permits automatic acknowledgment and permits multicast addressing • A repeater can be seen to have 2 main functions: – To contribute to the proper functioning of the ring by passing on all data that come its way – To provide an access point for attached stations to send and receive data Token Ring Contd. • Corresponding to the above two purposes, there ate two states: – The listen state – The transmit state • In the listen state, each received bit is retransmitted with a small delay, required to allow the repeater to perform required functions – Scan passing bit stream for pertinent patterns • Chief among these is the address of addresses of attached stations • Another pattern indicates the permission to transmit Token Ring Contd. – Copy each incoming bit and send it to the attached station while continuing to retransmit each bit • This will be done for each bit of each packet addressed to this station – Modify a bit as it passes by • In certain control strategies, bits may be modified to, for example, indicate that the packet has been copied • This would serve as an acknowledgment – When a repeater’s station has data to send and when the repeater has permission to send, the repeater enters the transmit state Token Ring Contd. • The token ring technique is based on the use of a small frame, called a token, that circulates when all stations are idle • A station wishing to transmit must wait until it detects a token passing by • It then seizes the token by changing one bit in the token, which transforms it from a token to a start-of-frame sequence for a data frame • The station then appends and transmits the remainder of the field needed to construct a data frame Token Ring Contd. • When a station seizes a token and begins to transmit a data frame, there is no token on the ring – So the stations wishing to transmit must wait • The frame on the ring will make a round trip and be absorbed by transmitting station • In the default operation, the transmitting station will insert a new token on the ring when – The station has completed transmission of its frame – The leading edge of the transmitted frame has returned Token Ring Contd. Layer 2 = Data Link Layer • Applies to the transfer of data frames between locally, directly connected (linked) devices via the physical layer (wire, fibre, radio). • A set of rules for exchanging messages. Layer 2 = Data Link Layer • Several important tasks: – Delineation of data. Data link framing uses standard fields, each with a specific task. – Addressing. The source and destination addresses are MAC addresses, 6 bytes = 48 bits long. (NOT the IP address, which is 4 bytes long). The MAC is usually burned into the NIC Network Interface Card. Supposedly unique world-wide. Layer 2 = Data Link Layer • Error Control. The FCS Frame Check Sequence, or CRC Cyclic Redundancy Check is a check sum, generated using polynomials with coefficients 0 or 1, that gives a very high probability that errors will be detected. • Flow Control. The flow of data from the sender must not overwhelm the receiver. The receiver must be able to inform the sender when some limit is reached. Synchronisation, sequencing of data frames. Layer 2 = Data Link Layer • Medium Access Control. E.g., 802.3, 802.4, 802.5. • Transparency. The Start-of-frame and End-of-frame bit patterns 01111110 are not confused with bit patterns in the message itself. A 0 bit is inserted whenever five consecutive 1 bits occur in the data. • Code Independence. Any character code, e.g., ASCII or EBCDIC, can be transmitted. In Ethernet, data is sent in octets = groups of 8 bits. Layer 2 = Data Link Layer • Two Sublayers: – Upper is 802.2 LLC Logical Link Control. – Lower is MAC Layer 2 = Data Link Layer Upper layer (cont’d) • Upper is 802.2 LLC Logical Link Control. – Activate, maintain, release link. Make physical link reliable. (The Physical Layer can lose bits and is ignorant of this). – Main services are error detection and control. – Independent of MAC. – The data unit is called a Protocol Data Unit (PDU); three types: • I-PDU = Information PDU; • S-PDU = Supervisory; • U-PDU = Unnumbered. Layer 2 = Data Link Layer Upper layer (cont’d) • Based on the High level Data Link Control (HDLC) protocol. • 2 classes of service: – Type 1: • Type 1a – Unacknowledged Connectionless service • Type 1b – Acknowledged Connectionless service • Use only U-PDUs. – Type 2: • Connection-oriented service: (e.g., telephone call) • Uses all 3 types of PDU • maintain connection using special packets. Layer 2 = Data Link Layer Lower layer (cont’d) • Lower is MAC – Ethernet, CSMA/CD is IEEE 802.3 – Token passing bus is 802.4 – Token passing ring is 802.5 – Wireless is 802.11 Layer 2 = Data Link Layer • Hardware: Bridges, Switches – Switches: Cut-through, Modified cut-through, Store-and-forward. • ARP Address Resolution Protocol (Know IP address, want MAC address). (Broadcast) Who has 130.194.75.44? Tell 130.194.75.89. Layer 3 = Network Layer • Handles data transfer across communications network - routing, relaying, switching logical connections. Has view of entire network (Data Link, Physical layers address subnetworks only). • Provides the means to set up, maintain and clear a connection path. • If direct link between stations - network layer not needed Layer 3 = Network Layer • Responsible for delivery of a packet between source and destination. • Packets are called datagrams. Layer 3 = Network Layer • Tasks: – Internetworking – linking heterogeneous physical networks. – Addressing. Each device on the Internet must be uniquely identified. – Routing – determine optimal path through network. Often there is a choice of pathways. – Packetizing – encapsulates packets from the upperlayer protocols. – Fragmenting. Different physical networks may have different protocols, different frame sizes. Layer 3 = Network Layer Addressing • Unique IP Internetworking protocol address, e.g., 130.194.99.44. • 4 bytes, written as 4 decimal values, 0 <= N <= 255, separated by periods. • 32 bits, hence 232 possible values, 4,294,967,296. Layer 3 = Network Layer Addressing • 5 classes of network: – Class A. First byte is 0 to 127, binary 00000000 to 01111111 • 128 such networks; each can support about 16,777,216 devices. – Class B. First byte is 128 to 191, binary 10000000 to 10111111 • First 2 bytes define the network, 64 x 256 = 16384; each can support about 65,536 devices. Typically allows 256 ‘subnets’. Layer 3 = Network Layer Addressing – Class C. First byte is 192 to 223, binary 11000000 to 11011111 • First 3 bytes define the network, 32 x 256 x 256 = 2,097,152; each can support about 256 devices. – Class D. First byte is 224 to 239, binary 11100000 to 11101111. • Designed for multicasting. – Class E. First byte is 240 to 255, binary 11110000 to 11111111. • Reserved Layer 3 = Network Layer Routing • Hardware: Routers (also routing switches, switching routers). • Routing table at A: – Destination: – Go via: – Hop count: B B 1 A C B 2 B E D B 3 C E E 1 D F F E 2 Layer 3 = Network Layer Routing • Destination address in routing table shows next address along the path – From A to B, C or D – go via B – From A to E or F – go via E A B E C D F Layer 3 = Network Layer Routing • Routing can be: – Fixed/static/non-adaptive – Entered manually by administrator, no automatic updating. – For a small or experimental internet. • Alternative path routing – Alternative routes provided, as well as the first choice. – Can be used if first choice fails. Layer 3 = Network Layer Routing • Dynamic routing, adaptive routing – Route is dynamically generated from moment to moment, using RIP or OSPF protocols optimising use of network resources – Routes change as the traffic loads change. • RIP Routing Information Protocol. – Routers periodically exchange information with their neighbours using RIP broadcast packets. – Workstations can query the nearest router using RIP request packet. – Uses an abstract distance measure called distance-vector. Layer 3 = Network Layer Routing • Open Shortest Path First (OSPF) protocol – Uses link-state(usually cost) is minimized – more complicated than RIP, becoming more popular. Layer 4 = Transport Layer • Objective: provide reliable and efficient data transport, medium independent, from a source machine to a destination machine. Enhance the quality of service provided by the network layer. • Delivers a complete message (within a running process) from source to destination, as compared with individual Network Layer packets. Layer 4 = Transport Layer • Tasks: – Port addressing. There can be several simultaneous network processes. Different ‘port addresses’ are used to distinguish these. – Segmentation and reassembly. As in TCP Transmission Control Protocol. • Sequence numbers for consecutive segments enable reassembly. – Connection control. TCP is connection oriented (machines cooperating). UDP is connectionless (send and hope). – Flow control. End-to-end flow control across the whole network, (rather than across a single link as in the Data Link layer). – Error control. Like flow control, end-to-end control across the whole network. Layer 4 = Transport Layer • Optimise resources, according to requested quality of service. – Quality here means resilience to errors. • Five classes of transport layer protocol, depending on reliability of network layer: – Simple class, no enhancement to network service. Do nothing. – Basic error recovery class – Multiplexing (increase throughput using several network connections) – Error recovery and multiplexing – Error detection and recovery Layer 4 = Transport Layer • Controls data flow rates – Cuts messages into packets, reassembles packets into messages. – Optionally, full end-to-end error checking. • Border between providers of services (layers 1 to 3) and users of services (layers 5 to 7). • Similar to network layer, but: – Network layer can lose packets. Transport layer detects lost/damaged packets. – Transport layer is visible to applications. Network layer is not. Layer 4 = Transport Layer • Some protocols: – TCP Transmission Control Protocol (not all layer 4 OSI, but close) – UDP User Datagram Protocol (‘Unreliable’ Datagram Protocol). – SPX Sequenced Packet Exchange (Novell) – XNS Xerox Network System; Early internet protocol now replaced entirely by TCP/IP Layer 4 = Transport Layer • Two types of transport service: connection-oriented and connectionless. – TCP is connection-oriented. A stream of messages delivered in sequence with any loss or failure signalled to both end systems. – UDP is connectionless. Individual messages might or might not get through. Layer 4 = Transport Layer • TCP Transmission Control Protocol: – Data is organised as a stream of bytes. Usually full duplex, two byte streams in opposite directions. – Reliable delivery using sequence numbers, that count bytes in the data stream. Each packet has the starting sequence number of the data in the packet, and the acknowledgement (sequence) number of the last byte received. – Flow Control using Sliding Window: Remote host is informed of current buffer size (window). Can stop sending, or send multiple packets. Layer 5 = Session Layer • Application programs do not bombard each other with continuous unstructured streams of data. They interact in a structured manner – the basis of the Session layer. • Basic Services: – Session connection: Establish session liaison between 2 applications e.g. flow of data, PC to printer – Data transfer: Manage/monitor session dialogue – Disconnection: Manage orderly release. Layer 5 = Session Layer • Telephone call analogy: – Check phone is working – Obtain number of person being called – If answered, have conversation – a “session” – When finished, sign off appropriately. • Checkpointing is used for recovery/restart Layer 6 = Presentation Layer • A set of data transformation services – – – – – – Transformation of syntax Selection of syntax Provide independence from character representation Provide independence from machine characteristics Compression (improve transmission rates) Encryption (improve security) Layer 6 = Presentation Layer • Not needed on LANs with machines of the same type – Presentation may be a “pass-through” layer. May have direct mapping between Application and Session. • Data conversion - syntax (representation), not semantics (meaning). – Presentation layer negotiates and selects the appropriate transfer syntaxes to be used during a transmission. – Need a common representation of information in order to preserve its meaning – a universally understood language must be defined to allow the transfer language to be negotiated. Layer 6 = Presentation Layer • Data conversion - syntax (representation), not semantics (meaning). – Need a common representation of information in order to preserve its meaning – a universally understood language must be defined to allow the transfer language to be negotiated. Layer 6 = Presentation Layer • What differences are there? • Hardware manufacturers never quite agree about the best way to do things. – Big-Endian vs Little-Endian – can affect ALL data formats – Different word sizes – 16, 32, 36, 60, 64 … bits – Different character codes, • e.g.: ASCII, ANSI, EBCDIC, BCL(Burroughs Common Language) – Different number representations – Binary, BCD, 1’s/2’s complement – Different alignment rules for data within words, longwords e.g. PACKED records in Pascal Layer 7 = Application Layer • Some protocols: – FTAM File Transfer Access and Management:Remote file access and manipulation – MHS Message Handling System X.400:-Electronic mail – Directory (X.500) – ODA Office Document Architecture:- Interchange of generic forms of documents – VT Virtual Terminal – RDA Remote Database Some protocols(cont’d) – Systems Management: • SNMP Simple Network Management Protocol • CMISE Common Management Information Service Element • CMIP Common management Information Protocol Layer 7 = Application Layer • NOT the application programs themselves. • Provides services to Applications/Users – – – – – – File Transfer Protocol (FTP) Record transaction server E-mail Virtual terminal - screen display, keyboard reading Network management Remote systems job control Layer 7 = Application Layer • Application Service Elements ASE are modules which support commonly required services. – Identifies communicators – Authenticates, synchronises communicators – Defines error recovery, flow control protocols, etc. Transmission Media Signals a media dependant. The OSI Model/Internet Model Basic Protocol Functions Contd. » Typically, a unique address is associated with each end system (e.g., workstation or server) and each intermediate system (e.g., router) » Such an address, in general, is a network-level address » In TCP/IP architecture, this is known as an IP address, or simply an internet address » In OSI architecture, this is referred to as a network service access point (NSAP) » The network-level address is used to route a PDU through a network or networks to a system indicated by a network-level address in the PDU » Once data arrive at a destination system, they must be routed to some process or application in a system Basic Protocol Functions Contd. » Typically, a system will support multiple applications and an application may support multiple users » Each application and, perhaps, each concurrent user of an application, is assigned a unique identifier, referred to as a port in the TCP/IP architecture and as a service access point (SAP) in the OSI architecture – Addressing Scope » The internet address or NSAP address referred to previously is a global address » A global address identifies a unique system (global nonambiguity) » It is possible at any global address to identify any other global address, in any system, by means of the global address of the other system (global applicability) Basic Protocol Functions Contd. » Because a global address is unique and globally applicable, it enables an internet to route data from any system attached to any network to any other system attached to any other network » Each network must maintain a unique address for each device interface on the network » Examples are MAC address on an IEE 802 network and an ATM host address » This address enables the network to route data units (e.g., MAC frames, ATM cells) through the network and deliver them to the intended attached system » Such an address is referred to as a network attachment point address Basic Protocol Functions Contd. » The issue of addressing scope is generally only relevant for network-level addresses » A port or SAP above the network level is unique within a given system but need not be globally unique – Connection identifiers » The concept of connection identifiers comes into play when we consider connection-oriented data transfer (e.g., virtual circuit) rather than connectionless data transfer » For connectionless data transfer, a global identifier is used with each data transmission » For connection-oriented transfer, it is sometimes desirable to use only a connection identifier during data transfer phase Basic Protocol Functions Contd. – Addressing mode » Most commonly, an address refers to a single system or port; in this case it is referred to a s an individual or unicast address » It is also possible for an address to refer to more than one entity or port; such an address identifies multiple simultaneous recipients for data » An address for multiple recipients may be broadcast, intended for all entities within a domain, or multicast, intended for a specific subset of entities – Multiplexing • One form of multiplexing is supported by means of multiple connections into a single system Basic Protocol Functions Contd. – For example, there can be multiple data link connections terminating in a single end system » We can say that these data link connections are multiplexed over the single physical interface between the end system and the network • Multiplexing can also be accomplished via port names, which also permit multiple simultaneous connections – For example, there can be multiple TCP connections terminating in a given system, each connection supporting a different pair of ports • Multiplexing is used in another context as well, namely, mapping of connections from one level to another Basic Protocol Functions Contd. – In a network, for each process to process connection established at the higher level, a data link connection could be created at the network access level » This is one-to-one relationship, but need not be so. – Multiplexing can be used in on of two directions » Upward multiplexing, or inward multiplexing, occurs when multiple higher-level connections are multiplexed on, or share, a single lower-level connection » Downward multiplexing, or splitting, means that a single higherlevel connection is built on top of multiple lower-level connections, the traffic on the higher connection being divided among the various lower connections Basic Protocol Functions Contd. – Transmission Service • A protocol may provide a variety of additional services to the entities that use it • Three common examples are: – Priority » Certain messages, such as control messages, may need to get through to the destination entity with minimum delay » Thus, priority could be assigned on a message basis, or on a connection basis – Quality of service » Certain classes of data may require a minimum throughput or a maximum delay threshold Basic Protocol Functions Contd. – Security » Security mechanisms, restricting access, may be invoked • All of these services depends on the underlying transmission system and any intervening lower-level entities Principles of Internetworking • Packet-switching and packet-broadcasting networks grew out of a need to allow the computer user to have access to resources beyond that available in a single system – Resources of a single network are often inadequate to meet user’s needs • As the networks that might be interest exhibit so many differences, it is impractical to consider merging them into a single network – Rather, what is needed is the ability to interconnect various networks so that any 2 stations on any of the constituent networks can communicate Principles of Internetworking Contd. • An interconnected set of networks, from a user’s point of view, may appear simply a large network – However, if each of the constituent networks retain its identity and special mechanisms are for communicating across multiple networks, then the entire configuration is often referred to as an internet • Each constituent network in an internet supports communication among the devices attached to the network – These devices are referred to as end systems (ESs) Principles of Internetworking Contd. • In addition, networks are connected by devices referred to in the ISO documents as intermediate nodes (ISs) – ISs provide a communications path and perform the necessary relaying and routing functions so that data can be exchanged between devices attached to different networks in the internet – Two types of ISs of particular interest are bridges and routers • A bridge operates at layer 2 of the OSI 7 layer architecture and acts as a relay of frames between similar networks • A router operates at layer 3 of the OSI architecture and routes packets between potentially different networks Principles of Internetworking Contd. • The overall requirements for an internetworking facility are: – Provide a link between networks • At minimum, a physical and link control connection is needed – Provide for routing and delivery of data between processes on different networks – Provide an accounting service that keeps track of the use of various networks and routers and maintains status information – Provide the services just listed in such a way as not to require modifications to the networking architecture of any of the constituent networks Principles of Internetworking Contd. • This means that the internetworking facility must accommodate a number of differences among networks: – Different addressing schemes » The networks may use different endpoint names and address and directory maintenance schemes » Some form of global network addressing must be provided, as well as a directory service – Different maximum packet size » Packets from one network may have to be broken up into smaller pieces for another; this process is referred to as fragmentation – Different network access mechanisms » The network access mechanism between station and network may be different for stations on different networks Principles of Internetworking Contd. – Different timeouts » Typically, a connection-oriented transport service will await an acknowledgment until a timeout expires, at which it will retransmit its block of data » In general, longer times are required for successful delivery across multiple networks » Internetwork timing procedures must allow successful transmission that avoids unnecessary retransmissions – Error recovery » Network procedures may provide anything from no error recovery up to reliable end-to-end (within the network) service Principles of Internetworking Contd. » The internetwork service should not depend on nor be interfered with by nature of the individual network’s error recovery capability – Status reporting » Different networks report status and performance differently » It must be possible for the internetworking facility to provide such information on internetworking activity to interested and authorised processes – Routing techniques » Internetwork routing may depend on fault detection and congestion control techniques peculiar to each network » The internetworking facility must be able to coordinate these to route data adaptively between stations on different networks Principles of Internetworking Contd. – User access control » Each network will have its own user access control technique » These must be invoked by the internetwork facility as needed » Further, a separate internetwork access control technique may be required – Connection, connectionless » Individual networks may provide connection-oriented or connectionless service » It may be desirable for the internetwork service not to depend on the nature of the connection service of the individual network Principles of Internetworking Contd. • A key characteristic of an internet architecture is whether the mode of operation is connection oriented or connectionless – Connection-oriented operation • It is assumed that each network provides a connection-oriented form of service – That is, it is possible to establish a logical network connection between any two end systems attached to the same network • ISs are used to connect 2 or more networks – Each IS appears as an ES to each of the network to which it is attached Principles of Internetworking Contd. • When ES A wishes to exchange data with ES B, a logical connection is set up between them – This connection consists of the concatenation of a sequence of logical connections across networks • The individual network logical connections are spliced together by ISs – Any traffic arriving at an IS on one logical connection is retransmitted on a second logical connection and vice versa • A connection oriented IS performs the following key functions – Relaying » Data units arriving from one network via the network layer protocol are relayed (retransmitted) on another network Principles of Internetworking Contd. – Routing » When an end-to-end logical connection consisting of a sequence logical connections, is to be set up, each IS in the sequence must make a routing decision that determines the next hop in the sequence • Thus, at layer 3, a relaying operation is performed – It is assumed that all of the end systems share common protocols at layer 4 and above for successful end-to-end communication – Connectionless Operation • Connectionless-mode operation corresponds to the datagram mechanism of a packet-switching network • Each network protocol data unit is treated independently and routed from source ES to destination ES through a series of routers and networks Principles of Internetworking Contd. • For each data unit transmitted by A, A makes a decision as to which router should receive the data unit • The data unit hops across the internet from one router to the next until it reaches the destination network – At each router a routing decision is made (independently for each data unit) concerning the next hop » Thus, different data units may travel different routes between source and destination ES • All Ess and routers share a common network-layer protocol known generally as the internet protocol • An Internet Protocol (IP) was initially developed for the DARPA internet project and published as RFC 791 and has become an Internet Standard Connectionless Internetworking • In this section we refer specifically to the Internet Standard IP, but it applies any connectionless Internet Protocol, such as IPv6 • IP provides connectionless, or datagram, service between end systems • The advantages of this approach are: – Connectionless internet facility is flexible • It can deal with a variety of networks, some of which are themselves connectionless • In essence, IP requires very little from the constituent networks Connectionless Internetworking Contd. – A connectionless internet service can be made highly robust • This is basically the same argument made for a datagram network service versus a virtual circuit service – A connectionless internet service is best for connectionless transport protocols, as it does not impose unnecessary overhead Connectionless Internetworking Contd. Connectionless Internetworking Contd. • The figure in the previous slide depicts a typical example using IP, in which two LANs are interconnected by a frame relay WAN • End System A has a datagram to transmit to end system B – The datagram includes the internet address of B • The IP module in A recognises that the destination B is on another network – So the first step is to send the data to a router, in this case router X Connectionless Internetworking Contd. • To send data to router X, IP passes the datagram down to the next lower layer ( in this case LLC) with instruction to send it to router X • LLC in turn passes this information down to MAC layer, which inserts the MAC-level address of router X into the MAC header • When the packet reaches router X, the router removes MAC and LLC fields and analyse the IP header to determine the ultimate destination of the data – in this case B Connectionless Internetworking Contd. • The router must now make a routing decision; there are 3 possibilities – The destination station B is connected directly to one of the networks to which the router is attached • If so, the router sends the datagram directly to the destination – To reach the destination, one or more additional routers must be traversed • If so, a routing decision must be made: to which router the datagram must be sent? Connectionless Internetworking Contd. • In both above cases, the IP module in the router sends the datagram down to the next lower layer with the destination network address – The router does not know the destination address • Router returns an error message to the source of the datagram • In the above example, the data must pass through router Y before reaching the destination – So the router X constructs a new frame by appending a frame relay header and trailer to the IP data unit – The frame relay header indicates a logical connection to router Y Connectionless Internetworking Contd. • When the frame arrives at router Y, the frame header and the trailer are stripped off – The router determines that this IP data unit is destined for B, which is connected directly to a network to which this router is attached – The router therefore creates a frame with layer-2 destination address of B and sends it out onto LAN 2 • The data finally arrive at B, where the LAN and IP headers can be stripped off Connectionless Internetworking Contd. • At each router, before the data can be forwarded, the router may need to fragment the data unit – This is done to accommodate a smaller maximum packet size limitation on the outgoing network • The data units split into two or more fragments, each of which becomes an independent IP data unit • Each new data unit is wrapped in a lower-layer packet and queued for transmission Connectionless Internetworking Contd. • The process described above continues through as many routers as it takes for the data unit to reach its destination • As with routers, the destination end systems recovers the IP data unit from its network wrapping • If fragmentation has occurred, the IP module in the destination end system buffers the incoming data until the entire original data field can be reassembled Connectionless Internetworking Contd. • The service offered by IP is an unreliable one – That is, IP does not guarantee that all data will be delivered or that the data that are delivered will arrive in the proper order • It is the responsibility of the next higher layer (e.g., TCP) to recover from any errors that occur – This approach provides a great deal of flexibility • As the sequence of delivery is not guaranteed, successive data units can follow different paths through the internet – This allows the protocol to react to both congestion and failure in the internet by changing routes Internet Protocol • In this section, we will look at version 4 of IP, officially defined in RFC 791 • Although it is intended that IPv4 will eventually be replaced by IPv6, it is currently the standard IP used in TCP/IP networks • As with any protocol standard, IP is specified in two parts: – The interface with higher layer (e.g., TCP), specifying the services that IP provides – The actual protocol format and mechanisms Internet Protocol Contd. • The services to be provided across adjacent protocol layers (e.g., IP and TCP) are expressed in terms pf primitives and parameters – A primitive specifies the function to be performed • The actual form of a primitive is implementation dependent • An example is a subroutine call – Parameters are used to pass data and control information • IP provides two service primitives at the interface to the interface to the next higher layer Internet Protocol Contd. – The send primitive is used to request transmission of a data unit – The delivery primitive is used by IP to notify a user of the arrival of data unit • The parameters associated with the two primitives are as follows: – Source address – Destination address – Protocol • Recipient protocol entity (such as TCP) Internet Protocol Contd. – Type of service indicators • Used to specify the treatment of the data unit in its transmission through component networks – Identification • Used in combination with the source and destination addresses and user protocol to identify the data unit uniquely • This parameter is required for reassembly and error reporting – Don’t fragment identifier – Time to live Internet Protocol Contd. – Data length – Option data – Data • The identification, don’t fragment identifier, and time to live parameters are present in the Send primitive but not in the Deliver primitive – These 3 parameters provide instructions to IP that are not of concern to the recipient IP user Internet Protocol Contd. • The options parameter allows for future extensibility and inclusion of parameters that are usually not invoked – The currently defined options are • Security – Allow a security label to be attached to a datagram • Source routing – A sequenced list of router addresses that specifies the route to be followed • Route recording • Stream identification • Timestamping Internet Protocol Contd. Internet Protocol Contd. • The protocol between IP entities is best described with reference to IP datagram format, shown in the previous slide • The fields are: – Version • Indicates version number, to allow evolution of the protocol; the value is 4 – Internet Header Length (IHL) • The length of header in 32-bit words • The minimum value is 5, for minimum header length of 20 octets Internet Protocol Contd. – Type of Service • Specifies reliability, precedence, delay, and throughput parameters • This field is rarely used – Total length • Total datagram length, in octets – Identification • A sequence number that, together with the source address, destination address, and user protocol, is intended to identify a datagram uniquely • Thus this number should be unique for the datagram’s source address, destination address, and user protocol for the time during which the datagram will remain in the internet Internet Protocol Contd. – Flags • Only 2 bits are currently used – The more bit is used for fragmentation and reassembly – The Don’t fragment bit prohibits fragmentation when set – Fragment Offset • Indicates where in the original datagram this fragment belongs, measured in 64-bit units • This implies that fragments other than the last fragment must contain data field that is a multiple of 64 bits in length – Time to Live • Specifies how long, in seconds, a datagram is allowed to remain in the internet Internet Protocol Contd. – Protocol • Indicates the next higher level protocol that is to receive the data field at the destination – Header Checksum • An error-detecting code applied to the header only • Because some header fields may change during transit, this is reverified and recalculated at each router – Source Address – Destination Address – Options Internet Protocol Contd. – Padding • Used to ensure that the datagram header is a multiple of 32 bits in length – Data • Must be an integer multiple of 8 bits in length • The maximum length of that datagram is 65,535 octets • The source and destination address fields in the IP header each contain a 32-bit global internet address, generally consisting of a network identifier and a host identifier • The address is coded to allow a variable allocation of bits to specify network and host, as shown in the next slide Internet Protocol Contd. Internet Protocol Contd. • This encoding provides flexibility in assigning addresses to hosts and allows a mix of network sizes on an internet • The 3 principal network classes are best suited to the following conditions: – Class A • Few networks, each with many hosts – Class B • Medium number of networks, each with a medium number of hosts – Class C • Many networks, each with a few hosts Internet Protocol Contd. • A mixture of classes is appropriate for an internetwork consisting of a few large networks, many small networks, plus some medium-sized networks • IP addresses are usually written in what is called dotted decimal notation, with a decimal number representing each of the octets of 32-bit address – For example, the IP address 11000000 11100100 00010001 00111001 is written as 192.228.17.57 • All class A network addresses begin with a binary 0 Internet Protocol Contd. • Network addresses with a first octet of 0 (00000000) and 127 (01111111) are reserved – So there are 126 potential Class A network numbers, which have a first decimal number in the range 1 to 126 • Class B network addresses begin with a binary 10 – So the range of first decimal numbers in a class B address is 128 to 191 (binary 10000000 to 10111111) – The 2nd octet is also part of the Class B address • So there are 214 = 16,384 Class B addresses Internet Protocol Contd. • For Class addresses, the first decimal number ranges from 192 to 223 (11000000 to 11011111) • The total number of Class C addresses is 221= 2,097,152 Routing in Switched Networks Reference: Chapter 12 -Stallings Introduction • A key design issue in switched networks is that of routing – These networks include circuit switching, packet switching, frame relay, and ATM networks • In general terms, the routing function seeks to design routes through the network for individual pairs of communicating end nodes such that network is used efficiently Routing in Circuit Switching Networks • In a large circuit-switching network, many of the circuit connections will require a path through more than one switch • When a call is placed, the network must devise a route through the network from calling subscriber to called subscriber – This route passes through some number of switches and trunks • There are 2 main requirements for the network’s architecture that bear on the routing strategy: Routing in Circuit Switching Networks Contd. – Efficiency • It is desirable to minimise the amount of equipment (switches and trunks) in the network, subject to the ability to handle that amount of load • The load requirement is usually expressed in terms of a busy-hour traffic load – This is simply the average load expected over the course of the busiest hour of use during the course of a day • From a functional point of view, it is necessary to handle that amount of load • From a cost point of view, we would like to handle that load with minimum Routing in Circuit Switching Networks Contd. – Resilience • Although a network may be sized for the busy hour load, it is possible for the traffic to surge temporarily above that level (during a major storm) • It will also be the case that, from time to time, switches and trunks will fail and be temporarily unavailable • We would like the network to provide a reasonable level of service under such conditions • The key design issue that determines the nature of the tradeoff between efficiency and resilience is the routing strategy Routing in Circuit Switching Networks Contd. • Traditionally, the routing function in public telecommunications networks has been quite simple – In essence, the switches of a network were organised into a tree structure, or hierarchy. – A path is constructed by starting at the calling subscriber, tracing up the tree to the first common node, and then tracing down the tree to the called subscriber – To add some resilience to the network, additional high-usage trunks were added that cut across the tree structure to connect exchanges with high volumes of traffic between them Routing in Circuit Switching Networks Contd. • In general, the above mentioned is a static approach – As the routing scheme is not able to adapt to changing conditions, the network must be designed to meet some typical heavy demands • It is difficult to analyse varying demands, which leads to oversizing and therefore inefficiency • In terms of resilience, the fixed hierarchical structure with supplemental trunks any respond poorly to failures Routing in Circuit Switching Networks Contd. • To cope with the growing demand on public telecommunications networks, virtually all providers have moved away from static hierarchical approach to a dynamic approach • A dynamic routing approach is one in which routing decisions are influenced by current traffic conditions • Typically, the circuit switching nodes have a peer relationship with each other rather than a hierarchical one Routing in Circuit Switching Networks Contd. – All nodes are capable of performing the same function • In such an architecture, routing is both more complex and more flexible – It is more complex because the architecture does not provide a ‘natural’ path or set of paths based on hierarchical structure – It is more flexible because more alternative routes are available • A form of routing used in circuit-switching networks is known as alternate routing – In this routing scheme, the possible routes to be used between two end offices are predefined Routing in Circuit Switching Networks Contd. – It is the responsibility of the originating switch to select the appropriate route for each call – Each switch is given a set of preplanned routes for each destination, in order of preference • If a direct trunk connection exists between two switches, this is usually the preferred choice • If this trunk is unavailable, then the 2nd choice is to be tried, and so on – The routing sequences reflect an analysis based on historical traffic patterns and designed to optimise the use of network resources Routing in Circuit Switching Networks Contd. Routing in Packet Switching Networks • The primary function of a packet-switching network is to accept packets from a source station and deliver them to a destination station • To accomplish this, a path or route through the network must be determined – Generally more than one route is possible • Thus, a routing function must be performed • The requirements for this function include – Correctness, simplicity, fairness, optimality, robustness, stability, and efficiency Routing in Packet Switching Networks Contd. » Robustness has to do with the ability of the network to deliver packets via some route in the face of localised failures and overloads » The designer who seeks robustness must cope with competing requirements for stability » A tradeoff also exists between fairness and optimality; » Some performance criteria may give higher priority to the exchange of packets between nearby stations compared to an exchange between distant stations » Routing techniques involves some processing overhead at each node and often a transmission overhead as well, both of which impair network efficiency Routing in Packet Switching Networks Contd. • Various design elements that contribute to a routing strategy are: – Performance Criteria • The selection of a route is generally based on some performance criterion – The simplest criterion is to choose the minimum-hop route through the network – A generalisation of the minimum-hop criterion is least-cost routing • In either the minimum-hop or least-cost approach, the algorithm for determining the optimum route for any pair of stations is relatively straightforward, and the processing time would be about the same for either computation Routing in Packet Switching Networks Contd. Routing in Packet Switching Networks Contd. – Decision Time and Place • Two key characteristics of a routing decision are the time and place that the decision is made • Decision time is determined by whether the routing decision is made on a packet or virtual circuit basis – When the internal operation of the network is datagram, a routing decision is made individually for each packet – For internal virtual circuit operation, a routing decision is made at the time the virtual circuit is established » In the simplest case, all subsequent packets using that virtual circuit follow the same route Routing in Packet Switching Networks Contd. – The term decision place refers to which node or nodes in the network are responsible for the routing decision » Most common is distributed routing, in which each node has the responsibility of selecting an output link for routing packets as they arrive » For centralised routing, the decision is made by some designated node, such as a network control centre » A third alternative, used in some networks, is source routing • The decision time and decison place are independent design variables Routing in Packet Switching Networks Contd. – Network Information Source and Update Timing • Most routing strategies require that decisions be based on knowledge of the topology of the network, traffic load, and link cost • However, some strategies use no such information and yet manage to get packets through • With distributed routing, in which the routing decision is made by each node, the individual node may make use of only local information from adjacent nodes, such as the amount of congestion experienced at that node • In the case of centralised routing, the central node typically makes use of information obtained from all nodes Routing in Packet Switching Networks Contd. • The concept of information update timing is a function of both the information source and the routing strategy • If no information is used, there is no information to update • If only local information is used, the update is essentially continuous – That is, an individual node always knows its local condition • For all other information source categories, update timing depends on the routing strategy – For a fixed strategy, the information is never updated – For an adaptive strategy, information is updated from time to time to enable the routing decision to adapt to changing conditions Routing in Packet Switching Networks Contd. • A large number of routing strategies have evolved for dealing with the routing requirements of packet-switching networks • Many of these strategies are also applied to internetwork routing • Four key routing strategies are: – Fixed Routing • A single permanent route is configured for each source-destination pair of nodes in the network Routing in Packet Switching Networks Contd. • The routes are fixed, or at least only change when there is a change in the topology of the network – Thus, the link costs used in designing routes cannot be based on any dynamic variable such as traffic – They could, however, be based on expected traffic or capacity • Fixed routing can be implemented using a central routing matrix, to be stored perhaps at a network control centre – In a routing matrix, it is not necessary to store the complete route for each possible pair of nodes » Rather, it is sufficient to know, for each par of nodes, the identity of the first node on the route Routing in Packet Switching Networks Contd. Routing in Packet Switching Networks Contd. • From the above routing matrix, routing tables can be developed and stored at each node – Each node needs only t store a single column of the routing directory » A node’s directory shows the next node to take for each destination • With fixed routing, there is no difference between routing for datagrams and virtual circuits – All packets from a given source to a given destination follow the same route • The advantage of fixed routing is its simplicity, and it should work well in a reliable network with stable load • Its disadvantage is its lack of flexibility – does not react to network congestion or failures Routing in Packet Switching Networks Contd. – Flooding • A packet is sent by a source node to every one of its neighbours • At each node, an incoming packet is retransmitted on all outgoing links except for the link on which it arrived • Eventually, a number of copies of the packets will arrive at the destination • The packet must have a some unique identifier so that the destination node knows to discard all but the first copy • This technique requires no network information whatsoever • Unless something is done to stop the incessant retransmission of packets, the number of packets in circulation grows without bound Routing in Packet Switching Networks Contd. • One way to prevent the above situation is for each node to remember the identity of those packets it has already transmitted – When a duplicate copies arrive they are discarded • A simpler technique is to include a hop count field with each packet – The count originally be set to some maximum value, such as the diameter of the network – Each time a node passes on a packet, it decrements the count by one » When the count reaches zero, the packet is discarded • The flooding technique has 3 remarkable properties: Routing in Packet Switching Networks Contd. – All possible routes between source and destination are tried » If at least one path between source and destination exists, a packet will always get through – Because all routes are tried, at least one copy of the packet arrive at the destination will have used a minimum-hop route – All nodes are directly or indirectly connected to the source node are visited • Because the flooding technique is highly robust, it could be used to send emergency messages • Flooding can also be useful for the dissemination of important information to all nodes • The principal disadvantage of flooding is the high traffic load that it generates, which is directly proportional to the connectivity of the network Routing in Packet Switching Networks Contd. Routing in Packet Switching Networks Contd. – Random Routing • A node selects only one outgoing path for retransmission of an incoming packet • The outgoing link is chosen at random, excluding the link on which the packet arrived • If all links are likely to be chosen, then a node may simply utilise outgoing links in a round-robin fashion • Random routing has the simplicity and robustness of flooding, with far less traffic load Routing in Packet Switching Networks Contd. • A refinement of the above technique is to assign a probability to each outgoing link and to select the link based on that probability – The probability could be based on data rate, in which case we have Pi Ri Ri Where Pi = probability of selecting link I Ri = data rate on link I • Like flooding, random routing requires the use of no network information • Because the route taken is random, the actual route will typically not be the least cost route nor the minimum-hop route Routing in Packet Switching Networks Contd. – Adaptive Routing • The routing decisions that are made change as conditions on the network change – The conditions that influence routing decisions are: » Failure » Congestion • In virtually all packet-switching networks, some sort of adaptive routing techniques is used • For adaptive routing to be possible, information about the state of the network must be exchanged among the nodes Routing in Packet Switching Networks Contd. • The drawbacks associated with adaptive routing, compared to fixed routing are: – The routing decisions are more complex » Therefore the processing burden on the network nodes increases – In most cases, adaptive strategies depend on status information that is collected at on place but used at another » There is a tradeoff between the quality of the information exchanged and the amount of the overhead » The more information that is exchanged, and the more frequently it is exchanged, the better will be the routing decisions » On the other hand, this information is itself a load on the constituent networks, causing a performance degradation Routing in Packet Switching Networks Contd. – An adaptive strategy may react too quickly, causing congestionproducing oscillations, or too slowly, being irrelevant • Despite the above drawbacks, adaptive routing strategies are by far the most prevalent, for 2 reasons: – An adaptive routing strategy can improve performance, as the network user seen by – An adaptive routing strategy can aid in congestion control » As it tends to balance loads, adaptive routing can delay onset of severe congestion • By and large, adaptive routing is an extraordinarily complex task to perform properly Routing in Packet Switching Networks Contd. • A convenient way to classify adaptive routing strategies is on the basis of information source: – Local » A node routes each packet to the outgoing link with the shortest queue length » This would have the effect of balancing the load on outgoing links; however, some outgoing links may not be headed in the correct general direction – Adjacent nodes – All nodes » Both strategies, adjacent and all nodes, are commonly used » They take the advantage of information that each node has about delays and outages » Such adaptive strategies can be either distributed or centralised Routing in Packet Switching Networks Contd. • First Generation Routing – The original routing algorithm, designed in a969, was a distributed adaptive algorithm using delay as the performance criterion – For this algorithm, each node maintains two vectors: d i1 d i2 Di . . d iN i si1 s i2 S i . . siN – Where Di = delay vector for node I dij = current estimate of minimum delay from node I to node j N = Number of nodes in the network Si = successor node vector for node I sij = the next node in the current minimum-delay route from I to j Routing in Packet Switching Networks Contd. – Periodically (every 128ms), each node exchanges its delay vector with all of its neighbours – On the basis of all incoming delay vectors , a node k updates both of its vectors as follows: d kj skj = i – min d ij lki i A using i that minimises the preceding expression Where A = set of neighbour nodes for k lki = current estimate of delay from k to i Routing in Packet Switching Networks Contd. – The estimated link delay is simply the queue length for that link • Thus, in building a new routing table, the node will tend to favour outgoing links with shorter queues • This tends to balance the load on outgoing links – However, as the queue lengths vary rapidly with time, the distributed perception of the shortest route could change while a packet is en route – This could lead to a thrashing situation in which a packet continues to seek out areas of low congestion rather than aiming at the destination – The major shortcomings of the above algorithm were: • It did not consider line speed, merely queue lengths – Thus higher-capacity links were not given the favoured status they deserved Routing in Packet Switching Networks Contd. – Queue length is , in any case, an artificial measure of delay, because some variable amount of processing time elapses between the arrival of a packet at a node ad its placement in an outbound queue – The algorithm was not very accurate » In particular, it responded slowly to congestion and delay increases • Second Generation Routing – The new algorithm was also a distributed one, using delay as the performance criterion, but the difference was significant • Rather than using queue length as a surrogate for delay, the delay was measured directly Routing in Packet Switching Networks Contd. – At a node, each incoming packet was timestamped with an arrival time; a departure time was recorded when the packet was transmitted » If a positive acknowledgment is returned, the delay for the packet was recorded as the departure time minus the arrival time plus transmission time and propagation delay » The node must therefore know the link data rate and propagation time » If a negative acknowledgement comes back, the departure time is updated and the node tries again, until a measure of successful transmission delay is obtained – Every 10 seconds, the node computes the average delay on each outgoing link » If there are any significant changes in delay, the information is sent to all other nodes using flooding » Each node maintains an estimate of delay on every network link; when information arrives, it recomputes its routing table Routing in Packet Switching Networks Contd. – Experience with this second strategy indicated that it was more responsive and stable than the previous one – However, as the load on the network grew, a shortcoming in the new strategy began to appear, and it was revised in 1987 – The problem was the assumption that the measured packet delay on a link is a good predictor of the link delay encountered after all nodes reroute their traffic based on this reported delay • Thus, it is an effective routing mechanism only if there is some correlation between the reported values and those actually experienced after re-routing • This correlation tends to be rather high under light and moderate traffic loads, but there is little correlation under heavy loads Routing in Packet Switching Networks Contd. • Therefore, immediately after all nodes have made routing updates, the routing tables are obsolete • The ARPANET designers concluded that the essence of the problem was that every node was trying to obtain the best route for all destinations, and these efforts conflicted • It was concluded that under heavy loads, the goal of routing should be to give the average route a good path instead of attempting to give all routes the best path • The designers decided that it was unnecessary to change the overall routing algorithm – Rather, it was sufficient to change the function that calculates link costs Routing in Packet Switching Networks Contd. • The calculation begins with measuring the average delay over the last 10 seconds • The value is then transformed with the following steps: – Using a simple single server queueing model, the measured delay is transformed into an estimate of link utilisation » From queueing theory, utilisation can be expressed as a function of a delay as follows: 2(Ts T ) Ts 2T Where ρ = link utilisation T = measured delay Ts= service time » The service time was set at network-wide average packet size (600bits) divided by the data rate of the link Routing in Packet Switching Networks Contd. – The results was then smoothed by averaging it with the previous estimate of utilisation U(n+1) = 0.5 * ρ(n+1) + 0.5* U(n) Where U(n) = average utilisation calculated at sampling time n ρ(n) = link utilisation measured at sampling time » Averaging increases the period of routing oscillations, thus reducing routing overhead – The link cost is then set as a function of average utilisation that is designated to provide a reasonable estimate of cost while avoiding oscillations Wide Area Networks (WANs) Reference: Chapter 10 -Stallings Introduction • Traditional approaches to wide area network design are circuit-switching and packet switching • Since the invention of the telephone, circuit switching has been the dominant technology for voice communications, and has remained so well into the digital era • Around 1970, research began on a new form of architecture for long-distance digital data communications known as packet switching – Although the technology of packet switching has evolved substantially, it is remarkable that: Introduction Contd. • The basic technology of packet switching is fundamentally the same today as it was in early stages: • Packet-switching remains one of the few effective technologies for long-distance data communications – Many advantages of packet-switching, such as flexibility, resource sharing, robustness, and responsiveness, come with a cost • The packet-switching network is a distributed collection of packetswitching nodes • Ideally, all packet-switching nodes would always know the state of the entire network Introduction Contd. • Unfortunately, there is a time delay between a change in status in one portion of the network and the knowledge of that change elsewhere • Further, there is overhead involved in communicating status information • As a result, a packet-switching network can never perform “perfectly”, and elaborate algorithms are used to cope with the time delay and overhead penalties of network operation Switched Communications Networks • For transmission of data beyond a local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes • The switching nodes are not concerned with the content of the data – Rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination – The end devices that wish to communicate may be referred to as stations – The switching nodes whose purpose is to provide communication are referred to as nodes Switched Communications Networks Contd. • Each station attaches to a node, and the collection of nodes is referred to as a communication network • The types of networks that are discussed in this lecture are referred to as switched communication networks – Data entering the network from a station are routed to the destination by being switched from node to node • In switched communication networks, some nodes connect only to other nodes – Their sole task is the internal (to the network) switching of data Switched Communications Networks Contd. – Other nodes have one or more stations attached as well • In addition to their switching functions, such nodes accept data from and deliver data to the attached stations – Node-node links are usually multiplexed, using either frequency division multiplexing (FDM) or time division multiplexing (TDM) – Usually, the network is not fully connected; that is , there is not a direct link between every possible pair of nodes • However, it is always desirable to have more than one possible path through the network for each pair of stations Switched Communications Networks Contd. Circuit Switching Networks • Communication via circuit switching implies that there is a dedicated communication path between two stations – That path is a connected sequence of links between network nodes – On each physical link, a logical channel is dedicated to a connection – Communication via circuit switching involves 3 phases: • Circuit establishment – Before any signals can be transmitted, an end-to-end (station-tostation) circuit must be established Circuit Switching Networks Contd. • Data transfer – Information can be transferred from the source to destination, once a connection is established – The data may be analog or digital, depending on the nature of the network – Generally the connection is full duplex • Circuit disconnect – After some period of data transfer, the connection is terminated, usually by the action of one of the two stations – Signals must be propagated through the path to deallocate resources Circuit Switching Networks Contd. • In circuit switching, the switches must have intelligence to make resource allocations and to devise a route through the network • Circuit switching can be rather inefficient – Channel capacity is dedicated for the duration of a connection, even if no data are being transferred – For a voice connection, utilisation may be rather high, but still does not approach 100% – For a terminal-to-computer connection, the capacity may be idle during most of the time of the connection Circuit Switching Networks Contd. – In terms of performance, there is a delay prior to signal transfer for call establishment • However, once the circuit is established, the network is effectively transparent to the users – Information is transmitted at a fixed data rate with no delay other than the propagation delay through the transmission link – The delay at each node is negligible • Circuit switching was developed to handle voice traffic but is now also used for data traffic Circuit Switching Networks Contd. – The best-known example of a circuit-switching network is the public telephone network • This is actually a collection of national networks interconnected to form the international service • Although originally designed and implemented to service analog telephone subscribers, it handles substantial data traffic via modem and is gradually being converted to a digital network – Another well-known application of circuit switching is the private branch exchange (PBS), used to connect telephones within a building or office – Circuit-switching is also used in private networks Circuit Switching Networks Contd. • A public telecommunications network can be described using four generic architectural components: – Subscribers • The devices that attach to the network • It is still the case that most subscriber devices to public communications networks are telephones – But the percentage of data traffic increases year by year – Subscriber line • The link between the subscriber and the network, also referred to as the subscriber loop or local loop • Almost all local loop connections use twisted-pair wire • The length of a local loop is typically in a range from a few kilometres to a few tens of kilometres Circuit Switching Networks Contd. – Exchanges • The switching centres in the network – A switching centre that directly supports subscribers is known as an end office » Typically, an end office will support many thousands of subscribers in a localised area – In addition, intermediate switching nodes are used – Trunks • The branches between exchanges • Trunks carry multiple voice frequency circuits using either FDM or synchronous TDM • Earlier these were referred to as carrier systems Circuit Switching Networks Contd. Circuit Switching Networks Contd. Circuit-Switching Concepts • A network built around a single circuit-switching node consists of a collection of stations attached to a central switching unit – The central switch establishes a dedicated path between any two devices that wish to communicate • The heart of a modern system is digital switch – The function of the digital switch is to provide a transparent signal path between any pair of attached devices – The path is transparent in that it appears to the attached pair of devices that there is a direct connection between them Circuit-Switching Concepts Contd. Circuit-Switching Concepts Contd. – The network interface element represents the functions and hardware needed to connect digital devices, such as data processing devices and digital telephones, to the network – Analog telephones can also be attached if the network interface contains the logic for converting to digital signals – Trunks to other digital switches carry TDM signals and provide the links for constructing multiple-node networks – The control unit performs 3 general tasks: • First, it establishes connections – This is generally done on demand, that is, at request of an attached device Circuit-Switching Concepts Contd. – To establish the connection, the control unit must handle and acknowledge the request, determine if the intended destination is free, and construct a a path through the switch • Second, The control unit must maintain the connection – Because the digital switch uses time division principles, this may require ongoing manipulation of the switching elements – However, the bits of communication are transferred transparently • Third, the control unit must tear down he connection, either in response to a request from one of the parties or for its own reasons • An important characteristic of a circuit-switching device is whether it is blocking or nonblocking Circuit-Switching Concepts Contd. – Blocking occurs when the network is unable to connect two stations because all possible paths between them are already in use • A blocking network is one in which such blocking is possible – A nonblocking network permits all stations to be connected (in pairs) at once and grant all possible connection requests as long as the called party is free – When a network is supporting only voice traffic, a blocking configuration is generally acceptable, because it is expected that most phone calls are of short duration and that therefore only a fraction of the telephones will be engaged at any time Circuit-Switching Concepts Contd. – However, when data processing devices are involved, these assumptions may be invalid • For example, for a data entry application, a terminal may be continuously connected to a computer for hours at a time • Hence, for a data applications, there is a requirement for a nonblocking or nearly nonblocking configuration • One of the switching techniques internal to a single cirswitching node is space division switching – It was originally developed for the analog environment and has been carried over into the digital realm – As the name implies, a space division switch is one which the signal paths are physically separate from one another Circuit-Switching Concepts Contd. – Each connection requires the establishment of a physical path through the switch that is dedicated solely to transfer of signals between the two end points – The basic building block of the switch is a metallic cross-point or semiconductor gate that can be enabled and disabled by a control unit – The crossbar switch has a number of limitations: • The number of crosspoints grows with the square of the number of attached stations – This is costly for a large switch Circuit-Switching Concepts Contd. Circuit-Switching Concepts Contd. • The loss of a crosspoint prevents connection between the two devices whose lines intersect at that crosspoint • The crosspoints are inefficiently utilised; – even when all of the attached devices are active, only a small fraction of the crosspoints are engaged – To overcome these limitations, multiple-stage switches are employed • This type of arrangement has two advantages over a single-stage crossbar matrix – The number of crosspoints is reduced; in the example, the total number of crosspoints for 10 stations is reduced from 100 to 48 Circuit-Switching Concepts Contd. Circuit-Switching Concepts Contd. – There is more than one path through the network to connect two endpoints, increasing reliability • However, a multistage network requires a more complex control scheme • Another consideration with a multistage space division switch is that it may be blocking – A single-stage crossbar matrix is nonblocking; that is a path is always available to connect an input to an output • With the advent of digitised voice and synchronous time division multiplexing techniques, both voice and data can be transmitted via digital signals Circuit-Switching Concepts Contd. – This has led to a fundamental change in the design and technology of switching systems – Instead of relatively dumb space division systems, modern digital systems rely on intelligent control of space – and time division systems – Virtually all modern circuit switches use digital time division techniques for establishing and maintaining circuits – Time division switching involves the partitioning of a lowerspeed bit stream into pieces that share a higher-speed stream with other bit streams Packet-Switching Principals • When circuit switching networks began to be used increasingly for data connections, two shortcomings became apparent: – In typical user/host data connection, much of the time the line is idle • Thus, with the data connections, a circuit-switching approach is inefficient – In a circuit-switching network, the connection provides for transmission at a constant data rate • Thus, each of the two devices that are connected must transmit and receive at the same data rate as the other – This limits the utility of the network in interconnecting a variety of host computers and workstations Packet-Switching Principals • In packet switching, data are transmitted in short packets – A typical upper bound on packet length is 1000 octets • If a source has a longer message to send, the message is broken up into a series of packets • Each packet contains a portion (or all for a short message) of the user’s data plus some control information • The control information, at a minimum, includes the information that the network requires to be able to route the packet through the network and deliver it to the intended destination Packet-Switching Principals Contd. Packet-Switching Principals Contd. • At each node en route, a packet is received, stored briefly, and passed on to the next node • The packet-switching approach has a number of advantages over circuit-switching: – Line efficiency is greater, because a single node-to-node link can be dynamically shared by many packets over time • The packets are queued up and transmitted as rapidly as possible over the link – By contrast, with circuit switching, time on a node-to-node link is preallocated using synchronous time division multiplexing Packet-Switching Principals Contd. – A packet-switching network can perform data-rate conversion • Two stations of different data rates can exchange packets because each connects to its node at its proper data rate – When traffic becomes heavy on a circuit-switching network, some calls are blocked • On a packet-switching network, packets are still accepted, but delivery delay increases – Priorities can be used • If a node has a number of packets queued for transmission, it can transmit the higher-priority packets first Packet-Switching Principals Contd. • A network uses two approaches to handle a stream of packets as it attempts to route them through the network and deliver them to the intended destination – Datagram • Each packet is treated independently, with no reference to packets that have gone before • Each node chooses the next node on a packet’s path, taking into account information received from neighbouring nodes on traffic, line failures, and so on Packet-Switching Principals Contd. • So the packets, each with the same destination address, do not all follow the same route, and they may arrive out of sequence at the exit point – It is up to the exit node or the destination to restore the packets to original order – Further, it is up to the exit node or destination to detect the loss of a packet and decide how to recover it – Virtual circuits • A preplanned route is established before any packets are sent • Once the route is established, all the packets between a pair of communicating parties follow this same route through the network Packet-Switching Principals Contd. • Because the route is fixed for the duration of the logical connection, it is somewhat similar to a circuit in a circuit-switching network and is referred to as a virtual circuit – This does not mean that there is a dedicated path, as in circuit switching » A packet is still buffered at each node, and queued for out put over a line, while other packets on other virtual circuits may share the use of the line • Each packet contains a virtual circuit identifier as well as data – Each node on the preestablished route knows where to direct such packets; no routing decisions are required • At any time, each station can have more than one virtual circuit to any other station and can have virtual circuits to more than one station Packet-Switching Principals Contd. Packet-Switching Principals Contd. Comparison of Circuit Switching and Packet Switching • When a comparison of performance between the two types is done, we are concerned with 3 types of delay: – Propagation delay • The time it takes a signal to propagate from one node to the next • This time is generally negligible – Transmission time • The time it takes for a transmitter to send out a block of data • For example, it takes 1s to transmit a 10,000-bit block of data onto a 10-kbps line Comparison of Circuit Switching and Packet Switching – Node delay • The time it takes for a node to perform necessary processing as it switches data • In circuit switching, once a connection is established, a constant data rate is provided to the connected stations • In the case of packet switching, a variable delay is introduced and packets arrive in a choppy manner • For packet switching, analog data must be converted to digital before transmission Comparison of Circuit Switching and Packet Switching Asynchronous Transfer Mode (ATM) Reference: Chapter 11 -Stallings Introduction • ATM is the transmission technology that is the foundation of broadband ISDN (Integrated Services Digital Network) • ATM is also finding widespread application beyond its use as part of ISDN • ATM is, in essence, a packet switching technology, but is far more streamlined and efficient than traditional packet switching – It is designed to support very high data rates Protocol Architecture • Asynchronous Transfer Mode (ATM), also known as cell relay, takes advantage of the reliability and fidelity of modern digital facilities to provide faster packet switching than X.25 • Like packet switching and frame relay, ATM involves the transfer of data in discrete chunks • Also like packet switching and frame relay, ATM allows multiple logical connections to be multiplexed over a single physical interface • In ATM, the information on each logical connection is organised into fixed-size packets, called cells Protocol Architecture Contd. • ATM is a streamlined protocol with minimal error- and flow control capabilities – This reduces the overhead of processing ATM cells and reduces the number of overhead bits required with each cell • Thus ATM is able to operate at high data rates • Use of fixed-size cells simplifies the processing required at each ATM node – This also supports the use of ATM at high data rates Protocol Architecture Contd. • The standards issued for ATM by ITU-T govern the basic architecture for interface between user and network • The physical layer involves the specification of a transmission medium and a signal encoding scheme • The data rates specified at the physical layer range from 25.6Mbps to 622.08Mbps • Two layers of the protocol architecture relate to ATM functions Protocol Architecture Contd. – ATM layer • Common to all layers that provides packet transfer capabilities • Defines the transmission of data in fixed-size cells and defines the use of logical connections – ATM adaptation layer (AAL) • This layer is service dependent • Use of ATM creates the need for an adaptation layer to support information transfer protocols not based on ATM – The AAL maps higher-layer information into ATM cells to be transported over an ATM network – It also collects information from ATM cells for delivery to higher layers Protocol Architecture Contd. Protocol Architecture Contd. • The ATM protocol reference model involves 3 separate planes: – User plane • Provides for user information transfer, along with associated controls (e.g., flow control error control) – Control plane • Performs call control and connection control functions – Management plane • Includes plane management, which performs management functions related to a system as a whole and coordination between all planes • Also includes layer management, which performs management functions relating to resources and parameters residing in its protocol entities ATM Logical Connections • Logical connections in ATM are referred to as virtual channel connections (VCCs) • A VCC is the basic unit of switching in an ATM network • A VCC is set up between two end users through the network and a variable-rate, full-duplex flow of fixed-size cells is exchanged over the connection • VCCs are also used for user-network exchange (control signalling) and network-network exchange ( network management and routing ATM Logical Connections Contd. • For ATM, a second sublayer of processing has been introduced that deals with the concept of virtual path – A virtual path connection (VPC) is a bundle of VCCs that have the same endpoints • Thus, all of the cells flowing over all of the VCCs in a single VPC are switched together • The virtual path concept was developed in response to a trend in high-speed networking – In that the control cost of the network is becoming an increasingly higher proportion of the overall network cost ATM Logical Connections Contd. – The virtual path technique helps contain the control cost by grouping connections sharing common paths through the network into a single unit • Network management actions can then be applied to a small number of groups of connections instead of a large number of individual connections – The advantages of using virtual paths are: • Simplified network architecture – Network transport functions can be separated into those related to an individual logical connection ( virtual channel) and those related to a group of logical connections (virtual paths) • Increased network performance and reliability – the network deals with fewer, aggregated entities ATM Logical Connections Contd. ATM Logical Connections Contd. • Reduced processing and short connection setup time – Much of the work is done when virtual path is setup » By reserving capacity on a virtual path connection in anticipation of later call arrivals, new virtual channel connections can be established by executing simple control functions at the endpoints of the virtual path connection » No call processing is required at transit nodes » Thus addition of new virtual channels to an existing virtual path involves minimal processing • Enhanced network services – The virtual path is used internal to the network but is also visible to the end user » Thus, the user may define closed user groups or closed networks of virtual channel bundles ATM Logical Connections Contd. ATM Logical Connections Contd. • The process of setting up a virtual path connection is decoupled from the process of setting up an individual virtual channel connection: – The virtual path control mechanisms include calculating routes, allocating capacity, and storing connection state information – To set up a virtual channel, there must first be a virtual path connection to the required destination node • Further, connection must have sufficient available capacity to support the virtual channel with the appropriate quality of service • virtual channel is setup by storing the required state information VCC Uses • The endpoints of a VCC may be end users, network entities, or an end user and a network entity – In all cases, cell sequence integrity is preserved within a VCC • Examples of uses of a VCC are: – Between end users • Can be used to carry end-to-end user data • Can also be used to carry control signalling between end users • A VPC between end users provides them with an overall capacity VCC Uses Contd. – Between an end user and a network entity • Used for user-to-network control signalling • A user-to-network VPC can be used to aggregate traffic form an end user to a network exchange or network server – Between two network entities • Used for network traffic management and routing functions • A network-to-network VPC can be used to define a common route for the exchange of network management information VCC Characteristics • ITU-T recommendation I.150 lists the following as characteristics of VCCs – Quality of Service • A user of a VCC is provided with a quality of service specified by parameters such as cell loss ratio and cell delay variation – Switched and semipermanent virtual channel connections • A switched VCC is an on-demand connection, which requires call control signalling for setup and tearing down • A semipermanent VCC is one that is of long duration and is setup by configuring or network management action VCC Characteristics Contd. – Cell sequence integrity • The sequence of transmitted cells within a VCC is preserved – Traffic parameter negotiation and usage monitoring • Traffic parameters can be negotiated between a user and network for each VCC • The input of cells to the VCC is monitored by the network to ensure that negotiated parameters are not violated • The types of traffic parameters that can be negotiated include average rate, peak rate, burstiness, and peak duration – The network may need a number of strategies to deal with congestion and manage existing and requested VCCs VCC Characteristics Contd. » At the crudest level, the network may simply deny new requests for VCCs to prevent » Additionally, cells may be discarded if negotiated parameters are violated or if congestion becomes severe » In extreme situations, existing connections might be terminated VPC Characteristics • I.150 also lists characteristics of VPCs – The first four characteristics listed are identical to those for VCCs • That is, those listed in the last 2 slides apply for VPCs as well – There are a number of reasons for this duplication: • Provides some flexibility in how the network service manages the requirements placed upon them • The network must be concerned with the overall requirements for a VPC, and within a VPC may negotiate the establishment of virtual channels with given characteristics VPC Characteristics Contd. • Once a VPC is setup, it is possible for the end users to negotiate the creation of new VCCs – In addition, a fifth characteristic is listed for VPCs: • Virtual channel identifier restriction within a VPC – One or more virtual channel identifiers, or numbers, may not be available to the user of the VPC but may be reserved for network use » Examples include VCCs used for network management Control Signaling • In ATM, a mechanism is needed for the establishment and release of VPCs and VCCs • The exchange of information involved in this process is referred to as control signaling and take place on separate connections from those that are being managed • For VCCs, I.150 specifies 4 methods for providing an establishment/release facility – Semipermanent VCC • May be used for user-to-user exchange • No control signalling is required Control Signaling Contd. – If there is no preestablished call control signalling channel, then one must be setup • For this purpose, a control signalling exchange must take place between the user an network on some channel • Hence, a permanent channel is required, probably of low data rate – This can be used to setup a VCCs that can be used for call control – Such a channel is called a meta-signaling channel, as the channel is used to setup signaling channels – The meta-signaling channel can be used to set upa VCC between the user and network for call control signaling • This user-to-network signaling virtual channel can then be used to set up VCCs to carry user data Control Signaling Contd. – The meta-signaling channel can also be used to set up a userto-user signaling virtual channel • Such a channel must be set up with in a preestablished VPC • It can then be used to allow the 2 end users, without network intervention, to establish and release user-to-user VCCs to carry user data • For VPCs, three signaling methods are defined in I.150: – A VPC can be established on a semipermanent basis by prior agreement • No control signaling is required in this case Control Signaling Contd. – VPC establishment/release may be customer controlled • A customer uses a signaling VCC to request the VPC from the network – VPC establishment/release may be network controlled • Network establishes a VPC for its own convenience • The path may be network-to-network, user-to-network, or user-touser ATM Cells • ATM makes use of fixed-size cells, consisting of a 5octet header and a 48-octet information field • There are several advantages to the use of small, fixedsize cells: – Reduce the queueing delay for a high-priority cell, because it waits less if it arrives slightly behind a lower-priority cell – It appears that fixed-size cells can be switched more efficiently • This is important for very high data rates of ATM ATM Cells Contd. ATM Cells Contd. – It is easier to implement the switching mechanism in hardware with fixed-size cells • In the cell header format, generic flow control (GFC) field does no appear in the cell header internal to the network – It only appears at the user-network interface – Hence, it can be used for control of cell flow only at the local- usernetwork interface – The field could be used to assist the customer in controlling the flow of traffic for different qualities of service – In any case, GFC mechanism is used to alleviate short-term overload conditions in the network ATM Cells Contd. • The virtual path identifier (VPI) constitutes a routing field for the network – It is 8 bits at the user-network interface and 12 bits at the networknetwork interface • The latter allows support for an expanded number of VPCs internal to the network, to include supporting subscribers and those required for network management • The virtual channel identifier (VCI) is used for routing to and from the end user • The payload type (PT) field indicates the type of information in the information field ATM Cells Contd. – A value of 0 in the first bit indicates user information • In this case, the 2nd bit indicates whether congestion has been experienced • The 3rd bit, known as the service data unit )SDU) type bit, is a onebit field that can be used to discriminate two types of ATM SDUs associated with a connection, – The term SDU refers to the 48-octet payload of the cell – A value of 1 in the first bit of the payload type field indicates that this cell carries network management or maintenance information • This indication allows the insertion of network-management cells onto a user’s VCC without impacting the user data – Thus, the PT field can provide inband control information ATM Cells Contd. • The cell loss priority (CLP) bit is used to provide guidance to the network in the event of congestion – A value 0 indicates a cell of relatively higher priority, which should not be discarded unless no other alternative is available – A value of 1 indicates that this cell is subject to discard within the network • The user might employ this field so that extra cells (beyond the negotiated rate) may be inserted into the network, with a CLP of 1, and delivered to the destination if the network is not congested • The header error control field is used for both error control and synchronisation ATM Service Categories • An ATM network is designed to be able to transfer many different types of traffic simultaneously – These include real-time flows such as voice, video, and bursty TCP flows • Each such traffic is handled as a stream of 53-octet cells travelling through a virtual channel – However, the way in which each data flow is handled within the network depends on the characteristics of the traffic flow and the requirements of the application ATM Service Categories Contd. • The following service categories have been defined by ATM Forum: – Real-Time Services • The most important distinction among applications concerns the amount of delay and variability of delay (jitter) that the applications can tolerate • Real-time applications typically involve a flow of information to a user that is intended to reproduce that flow at a source – A user expects a flow of audio or video information to be presented in a continuous, smooth fashion – Applications that involve interaction between people have tight constraints on delay ATM Service Categories Contd. » Typically, any delay above a few hundred milliseconds become noticeable and annoying • Constant Bit Rate (CBR) – Used by applications that require a fixed data rate that is continuously available during the connection lifetime and a relatively tight upper bound on transfer delay – Commonly used for uncompressed audio and video information – CBR applications include: » » » » Videoconferencing Interactive audio (e.g., telephony) Audio/Video distribution (e.g., television) Audio/Video retrieval (e.g., video on demand) ATM Service Categories Contd. • Real_Time Variable Bit Rate (rt-VBR) – Intended for time sensitive applications » That is, those requiring tightly constrained delay and delay variation – The main difference with CBR traffic is that rt-VBR applications transmit at a rate that varies with time » Equivalently, an rt-VBR source can be characterised as somewhat bursty – The rt-VBR service allows the network more flexibility than CBR » The network is able to statistically multiplex a number of connections over the same dedicated capacity and still provide the required service to each connection ATM Service Categories Contd. – Non-Real-Time Services • Intended for applications that have bursty traffic characteristics and do not have tight constraints on delay and delay variation – The network has greater flexibility in handling such flows and can make greater use of statistical multiplexing to increase network efficiency • Non-Real-Time Variable Bit Rate (nrt-VBR) – For some non-real-time applications, it is possible to characterise the expected traffic flow so that the network can provide substantially improved quality of service in the areas of delay and loss » Such applications can use the nrt-VBR service ATM Service Categories Contd. – With this service, the end system specifies a peak cell rate, a sustainable or average cell rate, and a measure of how bursty or clumped the cells may be » With this information, the network can allocate resources to provide relatively low delay and minimum cell loss • Unspecified Bit Rate (UBR) – At any given time, a certain amount of capacity of an ATM network is consumed in carrying CBR and the two types of VBR traffic – Additional capacity is available for one or both of the following reasons: » Not all of the total resources have been committed to CBR and VBR traffic » The bursty nature of VBR traffic means that at some times less than the committed capacity is being used ATM Service Categories Contd. » All of this unused capacity could be made available for the use of UBR service – This service is suitable for applications that can tolerate variable delays and some cell losses, which is typically true of TCP-based traffic – With UBR, cells are forwarded on a FIFO basis using the capacity not consumed by other services » No initial commitment is made to a UBR source and no feedback concerning congestion is provided » This is referred to as a best-effort service • Available Bit Rate (ABR) – To improve the service provided to bursty sources that would otherwise use UBR, the ABR service has been defined ATM Service Categories Contd. – An application using ABR specifies a peak cell rate (PCR) that it will use and a minimum cell rate (MCR) that it requires – The network allocates resources so that all ABR applications receive at least their MCR capacity » Any unused capacity is then shared in a fair and controlled fashion among all ABR sources – The ABR mechanism uses explicit feedback to sources to assure that capacity is fairly allocated – Any capacity not used for ABR sources remains available for UBR traffic ATM Service Categories Contd. • Guaranteed Frame rate (GFR) – The most recent addition to ATM service categories – Designed specifically to support IP backbone subnetworks – GFR provides better service than UBR for frame-based traffic, including IP and Ethernet – The major goal of GFR is to optimise the handling of frame-based traffic that passes from a LAN through a router onto an ATM backbone network – Such ATM networks are increasingly being used in large enterprise, carrier, and Internet service provider networks to consolidate and extend IP services over the wide area ATM Service Categories Contd. – ABR is also an ATM service meant to provide a greater measure of guaranteed packet performance over ATM backbones » However, ABR is relatively difficult to implement between routers over an ATM network – With the increased emphasis on using ATM to support IP-based traffic, especially traffic that originates on Ethernet LANs, GFR may offer the most attractive alternative for providing ATM service – One of the techniques use by GFR to provide improved performance compared to UBR is to require the network elements be aware of frame or packet boundaries » Thus, when congestion requires the discard of cell, network elements must discard all the cells that comprise a single frame ATM Service Categories Contd. ATM Adaptation Layer • The use of ATM creates the need for an adaptation layer to support information transfer protocols not based on ATM – Two examples are PCM voice and the IP • PCM voice is an application that produces a stream of bits from a voice signal • To employ this application over ATM, it is necessary to assemble PCM bits into cells for transmission and to read them out on reception in such a way to produce a smooth constant flow of bits • When IP-based networks interconnect with ATM networks, a convenient way of integrating the two is to map IP packets into ATM cells ATM Adaptation Layer Contd. – This will usually mean segmenting 1 IP packet into a number of cells on transmission and reassembling the frame from cells on reception » By allowing the use of IP over ATM, all the existing IP infrastructure can be used over an ATM network • ITU-T I.362 lists the following general examples of services produced by AAL: – Handling transmission errors – Segmentation and reassembly, to enable larger blocks of data to be carried in the information field of ATM ATM Adaptation Layer Contd. – Handling of lost and misinserted cell conditions – Flow control and timing control • In essence, the AAL layer provides the mechanisms for mapping a wide variety of applications onto the ATM layer – It provides protocols that are built on top of the traffic management capabilities of the ATM layer – Accordingly, the design of the AAL protocols must relate to the service categories discussed earlier ATM Adaptation Layer Contd. • The types of applications that AAL and ATM together can support include: – Circuit emulation • Refers to the support of synchronous TDM transmission structures over an ATM network – VBR voice and video • Real-time applications that are transmitted in compressed format • One effect of the compression is that a variable bit rate can support the application, which requires a continuous bit-stream delivery to the destination ATM Adaptation Layer Contd. – General data services • These include messaging and transaction services that do not require real-time support – IP over ATM • Transmission of IP packets in ATM cells – Multiprotocol encapsulation over ATM (MPOA) • Supports a variety of protocols other than IP (e.g., IPX, Apple Talk) over ATM – LAN emulation (LANE) • Supports LAN-to-LAN traffic across ATM networks, with emulation of LAN broadcast capability ATM Adaptation Layer Contd. • AAL layer is organised in two logical sublayers: – Convergence sublayer (CS) • Provides the functions needed to support specific applications uising AAL • Each AAL user attaches to AAL at a service access point (SAP), which is simply the address of the application • This sublayer is service dependant – Segmentation and reassembly sublayer (SAR) • Responsible for packaging information received from CS into cells for transmission and unpacking the information at the other end ATM Adaptation Layer Contd. – Thus, SAR must pack any SAR headers and trailers plus CS information into 48-octet blocks • General protocol architecture for ATM and AAL typically encapsulate a higher-layer block of data into a single protocol data unit (PDU) – This PDU consists of the higher-layer data and possibly a header and trailer containing protocol information at the CS level – This CS PDU is then passed down to the SAR layer and segmented into a number of blocks • Each of these blocks is encapsulated into a single 48-octet SAR PDU ATM Adaptation Layer Contd. OSI Model Layer 1 – Physical. • What media can be used to transmit messages in a network? • What are their characteristics? • What criteria are important when selecting the right medium and connection structure? Conducting Media - Bounded • Coaxial Cable – Thick – Thin • Twisted Pair – Shielded – Unshielded – Category 1 – 6 (mostly 5) • Optical Fibre Radiating (Electromagnetic) Media - Unbounded • • • • Broadcast Radio Microwave Infrared Laser (Specialised infrared linking building LANs) Medium Selection Criteria • • • • • Speed – Aggregate data rate – capacity: Kbps, Mbps, Gbps = 1000, 106, 109 bits per second; ‘b’ = ‘bit’, ‘B’ = Byte – Response time (less than c = speed of light = 300,000 Km/Sec) Distance – How far can the signal be propagated? Security – Radiation, tapping, interception Reliability – Interference, noise Cost – Materials and Equipment – Installation and Labour – Operation Medium Selection Criteria(cont’d) • • • • • Availability Expansibility, Adaptability – Additional buildings, extensions Environmental scope: – Office, manufacturing, city,… – Harsh, clean,… Maintenance – Manageable infrastructure Accessibility – Easy access for maintenance • Safety – Conform to safety standards and legal requirements Medium Selection Criteria(cont’d)Conducting Medium: Twisted Pair TP • Shielded (STP) and Unshielded (UTP) – Structure: Two copper wires, twisted in helix (at a constant rate). – One at zero V, the second carries the signal. – Usually, bundle several pairs together, each with a different rate of twist – Twisting averages out the interference equally wires in the pair. Conducting Medium: Twisted Pair TP • Installation: relatively simple, flexible, easy to configure. • Electrical characteristics: – Considerable radiation (hence easily eavesdropped). – Susceptible to interference (electric motors, fluorescent lights). – STP (shielded) is resistant to interference, but more expensive. – Crosstalk between adjacent cables can occur; this is reduced by using different twist rates. Conducting Medium: Twisted Pair TP • Performance: – Commonly, 10 Mbps is used (older Category 3, VG = Voice grade). – 100 Mbps baseband per pair for quality cable (Category 5). – 380 MHz => 150 Mbps for glued/plasticised manufacture. – STP usually 16Mbps in token ring. – Good over short distances. Maximum length 100 metre. For long distance, need repeaters, amplifiers. • Costs: relatively low. Higher technology UTP is more expensive. • Reliability: Good. Conducting Medium: Coaxial Cable Conducting Medium: Coaxial Cable • Structure: – Central copper conductor; concentric dielectric (PVC, teflon); concentric solid/mesh screen; insulation. – Two forms, thin and thick coax. • Installation: – Usually fairly simple. – Thick coax 1 cm diameter is difficult to bend (“frozen yellow garden hose”) Conducting Medium: Coaxial Cable • Electrical characteristics: – High frequency signals – Reduced radiation – Almost immune to interference. – Little crosstalk between adjacent cables. • Performance: – High transmission rates 10 Mbps – Ordinary cable TV coax is similar, but not for use in LAN. – Equipment and expertise, amplifiers and taps readily available. – For long distance, need repeaters, amplifiers. – Thin coax length 185 metre maximum. – Thick coax length 500 metre maximum. Conducting Medium: Coaxial Cable • Baseband: – Copper mesh, 50 ohm. 10 Mbps easily. – Unmodulated. Bit = discrete signal level. – Passive, easy to tap. But security problem. – Unmodulated, radiates more, making eavesdropping easier. • Broadband: – Aluminium screen, 75 ohm. 300 MHz, 150 Mbps. – Analogue Modulation. • Reliability: Good • Cost: Moderate Conducting Medium: Fibre Optic = Optical Fibre • • • • • • • • • Structure: Total internal reflection, along a filament of glass or plastic. – Higher bandwidth and transmission rate than Copper. – 2 - 125 M (= 0.002 to 0.125 mm) Electrically isolated (needed between buildings) Not affected by external electromagnetic fields (magnets or static electricity). Less attenuation, so longer transmission distances Higher data capacity, Gbps Physically, smaller and lighter than Twisted Pair, Coax. High reliability Excellent security Most expensive Conducting Medium: Fibre Optic = Optical Fibre • Infrared and (not quite) visible light. • Three preferred wavelengths; 850, 1300, 1550 nm = 3.5, 2.5, 2.0 × 1014 Hz • Longer wavelengths better for longer distances, higher data rates Three types of optical fibre • Multimode step-index – Thickest 125 M – Can use LED source – Multipath propagation – 20 Mbps for 1 km – Wavelength 850 nm Three types of optical fibre • Multimode graded-index – Commonest 125 M – Multipath propagation – Varying refractive index – slow in centre, faster at outside. – So outside ‘catches up’ to inside. – Can use LED source – 50 times higher data rate than step-index. 100, 155 Mbps Three types of optical fibre • Single mode – Very fine central filament 2 - 8 M – Long distances – Needs laser source – Wavelength 1550 nm (more infrared) – Up to 2 Gbps Radiating Media (Unbounded) • Speed = c = 299,792.458 km/sec = 299,792,458 m/sec ≈ 3 108 m/sec • c = f λ where f = frequency and λ = wavelength. The Electromagnetic Radiation Spectrum Radiating Media • Different frequencies have different behaviours. • Some properties: – Reflection: An object is visible because it reflects light. – Refraction: Change of direction at interface of two media. – Diffraction: Can bend around corners. – Penetration: Can pass through walls Radiating Media • Penetration: – In general, the lower the frequency, the better the penetration – e.g. Radio, low frequency microwave. – (But: Light can penetrate glass – special case; – Xrays, Cosmic rays penetrate most solids [inter-atomic distances]) Radiating Media • General Positives: – Can achieve high data rates – Cost effective – Easy to implement • General Negatives: – Susceptible to interference – May need line-of-sight, see below – Low security Radiating Media • Typical Uses: – Wireless network 802.11 (microwave) – Local LAN, e.g., within a room (infrared) – Connecting LANs between buildings (infrared) – Long distance (20 Km) high data rate connections (microwave) Radio Waves • Mostly, omnidirectional. Antenna radiates in all directions. – Therefore, good for multicast transmissions – radio, TV, paging. – But, inverse square law: Power proportional to 1 / distance2. • Can travel long distances. • Lower frequencies can penetrate walls. – Advantage: Can receive signals inside a building. – Disadvantage: Insecure, cannot isolate transmissions within a building. Microwave • • • • • Frequency 1 – 300 GHz High data rates For some frequencies, need special licence Easy installation Used in both unidirectional and omnidirectional applications. • Less directional than laser and infrared, and hence easier to eavesdrop, intercept, interfere. Microwave • Less sensitive to external interference, e.g. rain, fog • Long distance MAN, WAN connections. • Concentrated line-of-sight directional transmission, parabolic dish antenna and receiver. • Typical frequencies 7 to 38 GHz licensed. • Data Rates 34, 155 Mbps • Wireless networks (802.11) use unlicensed frequencies 2.4 – 2.4835 GHz, 5.725 – 5.85 GHz, omnidirectional. • 2.4 GHz is microwave ISM Industrial, Scientific, Medical band, and microwave oven frequency (water molecule resonates). Security in Wireless Networks; Spread Spectrum Transmission • Two common methods; both use pseudo-random sequences. • DSSS Direct Sequence Spread Spectrum – Multiply the radio frequency carrier with pseudo-random noise. – This spreads the signal over a wider band • FHSS Frequency Hopping Spread Spectrum – Jump from narrow band to narrow band across a wide range of frequencies – Spend Less than 10 milliseconds at each frequency – Pre-arranged sequence, known only to sender and receiver. Infrared • Common use: TV, VCR, etc., remote controller (directional) • Frequency 300 GHz to 400 THz • Very directional. Difficult to intercept/interfere with. • Reflects off surfaces. • Mostly, used inside buildings (sun interferes outside) • Can be used for internal networks. Use ceiling access point, or rely on reflection. Infrared • Adjacent rooms could have separate LANs. These can be connected by wired medium. • Easy installation • Inexpensive – cost effective • Capable of high bandwidth, typically, 100 Kbps to 16 Mbps • Externally, susceptible to external interference, e.g. rain, fog, sun. • But see infrared laser, below. Infrared Laser • http://www.canon.com/bctv/canobeam/p df/dt100.pdf Infrared Laser • Easy installation. • Essentially for links between buildings, not between individual workstations. • Frequency: borderline visible red / infrared 3.82 x 10 14 , wavelength 785 nm. • Infrared frequency allows high transmission rates, 25 Mbps – 1.25 Gbps. • No radio interference caused or received. • Parallel beam - bi-directional. • Limited to line-of-sight. Infrared Laser • Susceptible to external interference, e.g. rain, fog, birds, building cranes, … • Sensitive to atmospheric attenuation, building movement (hence auto-tracking). • Class 1M laser, safe for human eye and skin at output. • Security good, e.g., 4m diameter footprint at 500m.