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Subject: Computer Forensics and Cyber Applications UNIT- I Prof. Pansare R.B. Department of Computer Engineering Contents : Basics of Computer Networks: Protocols and Standards, OSI Model, TCP/IP Model, Network topology, LAN standards, Ethernet (802.3) Transmission media: Guided transmission media - Twisted Pair, Coaxial and Fiber-optic cables, Switching techniques: Circuit switching, Packet switching and message switching, Network Hardware Components: Connectors, Repeaters, hubs, NICs, Bridges and Switches • Fundamentals of Mac Protocols: Motivation for a specialized MAC, Fundamentals of MAC protocols, Sensor MAC Case Study (Protocol overview, Periodic listen and sleep operations, Schedule selection and coordination, Adaptive listening, Message passing) IEEE 802.15.4 protocol: Physical, MAC layer, naming and addressing, Assignment of MAC addresses, Distributed assignment of locally unique addresses, content based and geographic addressing STANDARDS AND PROTOCOLS 1. Organizations For Communication Standards Standards are developed by cooperation among standards creation committees, forums, and government regulatory agencies. Standards Creation Committees a) International Standards Organization (ISO) b) International Telecommunications Union (ITU) c) American National Standards Institute (ANSI) d) Institute of Electrical and Electronics Engineers (IEEE) e) Electronic Industries Association (EIA) f) Internet Engineering Task Force (IETF) a) International Standards Organization (ISO) - A multinational body whose membership is drawn mainly from the standards creation committees of various governments throughout the world - Dedicated to worldwide agreement on international standards in a variety field. - Currently includes 82 memberships industrialized nations. - Aims to facilitate the international exchange of goods and services by providing models for compatibility, improved quality, increased quality, increased productivity and decreased prices. b) International Telecommunications Union (ITU) - Also known as International Telecommunications Union-Telecommunication Standards Sector (ITU-T) - An international standards organization related to the United Nations that develops standards for telecommunications. - Two popular standards developed by ITU-T are: i) V series – transmission over phone lines ii) X series – transmission over public digital networks, email and directory services and ISDN. c) American National Standards Institute (ANSI) - A non-profit corporation not affiliated with US government. - ANSI members include professional societies, industry associations, governmental and regulatory bodies, and consumer groups. - Discussing the internetwork planning and engineering, ISDN services, signaling, and architecture and optical hierarchy. d) Institute of Electrical and Electronics Engineers (IEEE) - The largest national professional group involved in developing standards for computing, communication, electrical engineering, and electronics. - Aims to advance theory, creativity and product quality in the fields of electrical engineering, electronics and radio. - It sponsored an important standard for local area networks called Project 802 (eg. 802.3, 802.4 and 802.5 standards.) e) Electronic Industries Association (EIA) - An association of electronics manufacturers in the US. - Provide activities include public awareness education and lobbying efforts in addition to standards development. - Responsible for developing the EIA-232-D and EIA-530 standards. f) Internet Engineering Task Force (IETF) - Concerned with speeding the growth and evolution of Internet communications. - The standards body for the Internet itself - Reviews internet software and hardware. 2. Communication Protocols Definition - Protocol is a set of rules that govern all aspect of data communication between computers on a network. - These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer. - A protocol defines what, how, when it communicated. The key elements of a protocol are syntax, semantics and timing. - Protocols are to computers what language is to humans. Since this article is in English, to understand it you must be able to read English. Similarly, for two devices on a network to successfully communicate, they must both understand the same protocols. Elements of protocol i) Syntax The structure or format of the data. Eg. A simple protocol; Sender address 8 bits Receiver address data 8 bits 64 bits ii) Semantics - Refers to the meaning of each section of bits. - how is a particular pattern to be interpreted, and what action is to be taken based on that interpretation. Eg. Does an address identify the route to be taken or the final of the message? iii) Timing Refers to two characteristics: a. When data to be sent b. How fast it can be sent Eg. If a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and data will be largely lost. Characteristics of protocol a) Direct / indirect - communication between two entities maybe direct or indirect. i) point-to-point link - connection provides a dedicated link between two devices - the entities in these systems may communicate directly that is data and control information pass directly between entities with no intervening active agent. ii) multipoint link - connection more than two devices can share a single link - The entities must be concerned with the issue of access control and making the protocol more complex. b) Monolithic / structured - The task of communication between entities on different systems is too complex to be handled as a unit. Eg. An electronic mail package running on two computers connected by a synchronous HDLC link. To be structured, the package would need to include all of the HDLC logic. If the connection were over a packet-switched network, the packaged would still need the HDLC logic to attach it to the network. c) Symmetric / asymmetric - Symmetric is the most use in protocol and involve communication between peer entities. - Asymmetry may be dictated by the logic of an exchange (eg; client and a server process) the desire to keep one of the entities or systems as simple as possible. d) Standard / nonstandard If K different kinds of information sources have to communicate with L types of information receivers, as many as K x L different protocols are needed without standards and a total of 2 x K x L implementations are required If all systems shared a common protocol, only K+L implementations would be needed. Common protocol used Protocol Acronym Remarks Point To Point PPP Used to manage network communication over a modem Transfer/Transmission Control Protocol TCP / IP Backbone protocol. The most widely used protocol. Internetwork package exchange IPX Standard protocol for Novell NOS NetBIOS extended user interface NetBEUI Microsoft protocol that doesn’t support routing to other network. Running only Windows-based clients. File transfer Protocol FTP used to send and received file from a remote host Simple mail Transfer protocol SMTP Used to send Email over a network Hyper text transfer protocol HTTP Used for Internet to send document that encoded in HTML Apple Talk Apple Talk Protocol suite to network Macintosh computer and a peer-to-peer network protocol OSI Model OSI Layers A way of illustrating how information functions travels through network of its 7 layers. 3. Network Protocols a) Simple Network Management Protocol (SNMP) - Allows simple maintenance and remote monitoring of any device on a network. With SNMP, administrators can address issues such as problems with a network card in a server, a program, or service on the server, or a device such as a hub or a router. When managing a network device using SNMP, an administrator can use the central management system and the management information base. The management system allows the administrator to view performance and operation statistics of the network devices, enabling him to diagnose a network remotely. - b) User Diagram Protocol (UDP) Relay - A connectionless protocol that operates at the transport layer of the TCP/IP and OSI models. - UDP is an unreliable delivery service, it does not require receiving protocols to acknowledge the receipt of the packet. - The advantage of UDP is; it does not concentrate on establishing a connection, it can transmit more information in a smaller amount of time than TCP. c) Virtual LAN(VLAN) - A logical grouping of network devices or users that are not restricted to a physical switch segment. - The devices or users in a VLAN can be grouped by function, department, and application, regardless of their physical segment location. - A VLAN creates a single broadcast domain that is not restricted to a physical segment and is treated like a subnet. d) Routing Information Protocol (RIP) - A protocol supplied with UNIX BSD systems. - Used to transfer routing information between routers that are located in the same domain. - RIP uses hop count as a routing metrics. - Allows the router to determine which path it will use to send, based on a concept known as distancevector routing. e) Open Shortest Path First (OSPF) - A link-state routing protocol based on open standards. A better description, might be “determination of optimum path” because this interior gateway protocol actually uses several criteria to determine the best route to a destination. - These criteria include cost metrics, which factor in such things as route speed, traffic, reliability, and security. f) Quality Of Service (QoS) - Network management traffic - Provide traffic management on network particularly during times of congestion or failure. - QoS also give preferential treatment if a node does not reach the worth levels during the packets transmission. OSI Model Communication Architecture Strategy for connecting host computers and other communicating equipment. Defines necessary elements for data communication between devices. A communication architecture, therefore, defines a standard for the communicating hosts. A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software. Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices. OSI Model Layer Architecture Layer architecture simplifies the network design. It is easy to debug network applications in a layered architecture network. The network management is easier due to the layered architecture. Network layers follow a set of rules, called protocol. The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers. OSI Model Open Systems Interconnection (OSI) Model International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication. Open Systems Interconnection (OSI) reference model is the result of this effort. In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards. OSI Model OSI Reference Model The OSI model is now considered the primary Architectural model for inter-computer communications. The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems . This separation into smaller more manageable functions is known as layering. OSI Model OSI Reference Model: 7 Layers OSI Model OSI: A Layered Network Model The process of breaking up the functions or tasks of networking into layers reduces complexity. Each layer provides a service to the layer above it in the protocol specification. Each layer communicates with the same layer’s software or hardware on other computers. The lower 4 layers (transport, network, data link and physical — Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit. OSI Model Physical Layer Provides physical interface for transmission of information. Defines rules by which bits are passed from one system to another on a physical communication medium. Covers all - mechanical, electrical, functional and procedural aspects for physical communication. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications. OSI Model Data Link Layer Data link layer attempts to provide reliable communication over the physical layer interface. Breaks the outgoing data into frames and reassemble the received frames. Create and detect frame boundaries. Handle errors by implementing an acknowledgement and retransmission scheme. Implement flow control. Supports points-to-point as well as broadcast communication. Supports simplex, half-duplex or full-duplex communication. OSI Model Network Layer Implements routing of frames (packets) through the network. Defines the most optimum path the packet should take from the source to the destination Defines logical addressing so that any endpoint can be identified. Handles congestion in the network. Facilitates interconnection between heterogeneous networks (Internetworking). The network layer also defines how to fragment a packet into smaller packets to accommodate different media. OSI Model Transport Layer Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. Ensures that the data units are delivered error free. Ensures that data units are delivered in sequence. Ensures that there is no loss or duplication of data units. Provides connectionless or connection oriented service. Provides for the connection management. Multiplex multiple connection over a single channel. OSI Model Session Layer Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. This layer requests for a logical connection to be established on an enduser’s request. Any necessary log-on or password validation is also handled by this layer. Session layer is also responsible for terminating the connection. This layer provides services like dialogue discipline which can be full duplex or half duplex. Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint. OSI Model Presentation Layer Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. Also handles (cryptography). data compression and data encryption OSI Model Application Layer 1. Application layer interacts with application programs and is the highest level of OSI model. 2. Application layer contains management functions to support distributed applications. 3. Examples of application layer are applications such as file transfer, electronic mail, remote login etc. OSI Model OSI in Action A message begins at the top application layer and moves down the OSI layers to the bottom physical layer. As the message descends, each successive OSI model layer adds a header to it. A header is layer-specific information that basically explains what functions the layer carried out. Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers. TCP/IP Model OSI & TCP/IP Models TCP/IP Model TCP/IP Model Application Layer Application programs using the network Transport Layer (TCP/UDP) Management of end-to-end message transmission, error detection and error correction Network Layer (IP) Handling of datagrams : routing and congestion Data Link Layer Management of cost effective and reliable data delivery, access to physical networks Physical Layer Physical Media Models and Standards in Communication • Communication – Established standards – Standards are known as protocols • Implementation – A framework is helpful in the design of hardware and software for communication – ISO-OSI Model serves this purpose – ISO-OSI supersedes the TCP/IP model ISO and OSI Defined • ISO – International Standards Organization • OSI – Open Systems Interconnect OSI Model Background • Introduced in 1978 and revised in 1984 • Formulates the communication process into structured layers • There are seven layers in the model, hence the name the 7-Layer model • The model acts as a frame of reference in the design of communications and networking products The Layered Approach to Communication 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical Division of Layers 7. Application 6. Presentation Upper Layers 5. Session 4. Transport Middle Layer 3. Network 2. Data Link Lower Layers 1. Physical The Function of a Layer • Each layer deals with one aspect of networking – Layer 1 deals with the communication media • Each layer communicates with the adjacent layers – In both directions – Ex: Network layer communicates with: • Transport layer • Data Link layer • Each layer formats the data packet – Ex: Adds or deletes addresses Role of Layers Node A 7. Application 6. Presentation Data In To/from Node B 1. Physical Data Out Communication Between Layers 7. Application Data Encapsulation 6. Presentation Data Stripping 5. Session The Role of Layers in Point-to-point Communication Node a 7. Application 1. Physical Node b 7. Application 1.Physical Virtual Communication Between Layers 7. Application 7. Application 3. Network 3. Network Module Objectives • • • • • Application Layer Presentation Layer Session Layer Transport Layer Network Layer 7. Application Layer • Purpose – User application to network service interface • Examples – File request from server – E-mail services – etc. Application Layer Function • General network access • Flow control • Error recovery 6. Presentation Layer • Purpose – Formats data for exchange between points of communication • Ex: Between nodes in a network • Example: – Redirector software • Formats for transmission to the server Presentation Layer Function • • • • • Protocol conversion Data translation Encryption Character set conversion Expansion of graphics command Redirector Example F:/PUR/ORDER C:/CORRES/USDA REDIRECTOR TO SERVER TO LOCAL DISK 5. Session Layer • Purpose – Oversee a communication session • Establish • Maintain • Terminate • Example Session Layer Function • Performs name recognition and related security • Synchronization between sender and receiver • Assignment of time for transmission – Start time – End time etc. 4. Transport Layer • Purpose – Repackage proper and efficient delivery of packages • Error free • In sequence • Without duplication • Example Transport Layer Function • For sending data – Repackage the message to fit into packets • Split long messages • Assemble small messages • On receiving data – Perform the reverse – Send an acknowledgment to the sender • Solve packet problems – During transmission and reception 3. Network Layer • Purpose – Addressing and routing the packets • Example application at the router – If the packet size is large, splits into small packets Network Layer Function • Address messages • Address translation from logical to physical – Ex: nganesa ----------> 102.13.345.25 • Routing of data – Based on priority – Best path at the time of transmission • Congestion control 2. Data Link Layer • Purpose – Manages the flow of data over the physical media • Responsible for error-free transmission over the physical media • Assures error-free data submission to the Network Layer Data Link Layer Function • Point of origin – Packages data for transmission over physical line • Receiving end – Packages data for submission to the network layer • Deals with network transmission protocols – IEEE 802. protocols Data Link Layer Subdivision • Improvement to ISO Model • Logical Link Control (LLC) sub-layer – Manages service access points (logical link) – Error and flow control • Media Access Control (MAC) sub-layer – Applies directly to network card communication – Access control Logical Link Control Media Access Control Application • Network Interface Card driver NETWORK SOFTWARE NETWORK CARD NIC Driver facilitates data transfer 1. Physical Layer • Purpose – Deals with the transmission of 0s and 1s over the physical media • Translation of bits into signals • Example – Pulse duration determination – Transmission synchronization – etc. Physical Layer Function • Encode bits into signals – Carry data from the h higher layers • Define the interface to the card – Electrical – Mechanical – Functional – Example: Pin count on the connector Lower Layers Application Areas • Special significance to network card design • Applies to general LAN hardware design – Exceptions • Routers etc. • 802. standards – Centered around the lower layers – Applies to networks Layer Operations • At each layer, additional information is added to the data packet • An example would be information related to the IP protocol that is added at Layer 3 Formatting of Data Through the Layers Application Header Network Header Data Link Header and Trailer Presentation Header Session Header Transport Header Physical Frame Preamble Packet : General Format Header Trailer Data A general concept of packets serves as a prerequisite to the understanding of the ISO-OSI model. Some Header Information Added at Various Layers • • • • Packet arrival information Receiver’s address Sender’s address Synchronization character Data • Actual data • May contain error correction code – Performed on individual characters of the data – Example: Parity • Size may vary – Depending on the protocol – Example • 802.3 specifies range of data packet length Some Trailer Information Added at Various Layers • Error correction code – Character oriented – VRC (Parity Checking) • Packet oriented error correction codes – LRC – CRC A Note on CRC • Used widely • Sophisticated – Polynomial of deferent degrees are used for error correction – Example: Degrees 16, 32 etc. • CRC-32 is a more stringent error checking procedure than CRC-16 Some of the Major Components of the Data Packet Receiver’s Address Control Data Data Error Correction Protocol Start/synch Information Sender’s Address Standardizing Packet Formatting • Packets must conform to a standard in order for the nodes in a network to be able to communicate with one another • The International Standards Organization (ISO) has provided a reference model • Standards are established for operations at each layer of the ISO/OSI model in the form of protocols IEEE Background • Institution of Electrical and Electronic Engineers (IEEE) – A professional non-profit organization • Project group 802 – Responsible for setting standards relating to the physical link of the network IEEE 802 Focus • OSI Reference – Data Link layer – Physical layer • Areas – Network cards and cables – Network electronic/optical/ wireless communication standard as they apply to the lower two layers mentioned above – WAN connectivity Upper Layer Focus • • • • IETF W3C ISO/IEC The above agencies focus on setting standards on higher level protocol – TCP, IP etc. IEEE 802 Committees And Responsibilities • 802.1 – Internetworking • 802.2 – Logical Link Control (LLC) • 802.3 – CSMA/CD • 802.4 – Token Bus LAN IEEE 802 Committees and Responsibilities (Cont.) • 802.5 – Token Ring LAN • 802.6 – Metropolitan Area Network • 802.7 – Broadband Technical Advisory Group • 802.8 – Fiber-Optic Technical Advisory Group IEEE 802 (Cont.) • 802.9 – Integrated Voice/Data Networks • 802.10 – Network Security • 802.11 – Wireless Networks • 802.12 – Demand Priority Access LANs – Ex: 100BaseVG-AnyLAN OSI Sub-Layer Reference to IEEE 802 Standards Logical Link Control (LLC) 802.2 802.1 for both. Media Access Control (MAC) 802.3 802.4 802.5 802.12 Network Topologies Objectives • Describe the basic and hybrid LAN physical topologies, and their uses, advantages and disadvantages • Describe the backbone structures that form the foundation for most LANs Simple Physical Topologies • Physical topology: physical layout of nodes on a network • Three fundamental shapes: – Bus – Ring – Star • May create hybrid topologies • Topology integral to type of network, cabling infrastructure, and transmission media used Bus • Single cable connects all network nodes without intervening connectivity devices • Devices share responsibility for getting data from one point to another • Terminators stop signals after reaching end of wire – Prevent signal bounce • Inexpensive, not very scalable • Difficult to troubleshoot, not fault-tolerant Bus (continued) Advantages of Bus Topology • Works well for small networks • Relatively inexpensive to implement • Easy to add to it Disadvantages of Bus Topology • Management costs can be high • Potential for congestion with network traffic Ring Simple Physical Topologies • Physical topology – Physical layout of a network • A Bus topology consists of a single cable—called a bus— connecting all nodes on a network without intervening connectivity devices Advantages of Bus Topology • Works well for small networks • Relatively inexpensive to implement • Easy to add to it Disadvantages of Bus Topology • Management costs can be high • Potential for congestion with network traffic Simple Physical Topologies • Ring topology – Each node is connected to the two nearest nodes so the entire network forms a circle – One method for passing data on ring networks is token passing • Active topology – Each workstation transmits data Advantages of Ring Topology • Easier to manage; easier to locate a defective node or cable problem • Well-suited for transmitting signals over long distances on a LAN • Handles high-volume network traffic • Enables reliable communication Disadvantages of Ring Topology • Expensive • Requires more cable and network equipment at the start • Not used as widely as bus topology – Fewer equipment options – Fewer options for expansion to high-speed communication Star Simple Physical Topologies • Star topology – Every node on the network is connected through a central device Star (continued) • Any single cable connects only two devices – Cabling problems affect two nodes at most • Requires more cabling than ring or bus networks – More fault-tolerant • Easily moved, isolated, or interconnected with other networks – Scalable • Supports max of 1024 addressable nodes on logical network Advantages of Star Topology • • • • • Good option for modern networks Low startup costs Easy to manage Offers opportunities for expansion Most popular topology in use; wide variety of equipment available Disadvantages of Star Topology • Hub is a single point of failure • Requires more cable than the bus Hybrid Physical Topologies: Star-Wired Ring Star-Wired Bus Backbone Networks: Serial Backbone • Daisy chain: linked series of devices – Hubs and switches often connected in daisy chain to extend a network • Hubs, gateways, routers, switches, and bridges can form part of backbone • Extent to which hubs can be connected is limited Backbone Networks: Serial Backbone (continued) Distributed Backbone Collapsed Backbone Parallel Backbone Logical Topologies • Logical topology: how data is transmitted between nodes – May not match physical topology • Bus logical topology: signals travel from one network device to all other devices on network – Required by bus, star, star-wired physical topologies • Ring logical topology: signals follow circular path between sender and receiver – Required by ring, star-wired ring topologies Topology • The physical topology of a network refers to the configuration of cables, computers and other peripherals. • The main types of network topologies are: – – – – Linear Bus Star Ring Tree or Hybrid Linear Bus topology • A linear bus topology consists of a main run of cable with a terminator at each end. All servers workstations and peripherals are connected to the linear cable Star topology • A star network is designed with each node (file server, workstation, peripheral) connected directly to a central network hub or server Ring topology • A ring network is one where all workstations and other devices are connected in a continuous loop. There is no central server Tree or hybrid topology • A tree or hybrid topology combines characteristics of linear bus and star and/or ring topologies. • It consists of groups of star-configured workstations connected to a linear bus backbone cable Network Operating Software • Network operating systems coordinate the activities of multiple computers across a network • The two major types of network OS are: – Peer-to-peer – Client/server Peer to peer network OS – In peer to peer network OS, there is no file server or central management source; all computers are considered equal – Peer to peer networks are design primarily for small to medium LANS – AppleShare and Windows for Workgroups are examples of programs that can function as peer to peer Client/Server network OS – Client/server network OS centralise functions and applications in one or more dedicated file servers. – The file server provides access to resources and provides security – Novel Netware and Windows NT Server are examples of client/server network operating systems Ethernet Outline Multiple Access and Ethernet Intro Ethernet Framing CSMA/CD protocol Exponential backoff Shared Access Networks are Different • Shared Access Networks assume multiple nodes on the same physical link – Bus, ring and wireless structures – Transmission sent by one node is received by all others – No intermediate switches • Need methods for moderating access (MAC protocols) – Fairness – Performance – How can this be done? Multiple Access Methods • Fixed assignment – Partition channel so each node gets a slice of the bandwidth – Essentially circuit switching – thus inefficient – Examples: TDMA, FDMA, CDMA (all used in wireless/cellular environments) • Contention-based – Nodes contends equally for bandwidth and recover from collisions – Examples: Aloha, Ethernet • Token-based or reservation-based – Take turns using the channel – Examples: Token ring A Quick Word about Token Ring • Developed by IBM in early 80’s as a new LAN architecture – Consists of nodes connected into a ring (typically via concentrators) – Special message called a token is passed around the ring • When nodes gets the token it can transmit for a limited time • Every node gets an equal opportunity to send – IEEE 802.5 standard for Token Ring • Designed for predictability, fairness and reliability – Originally designed to run at either 4Mbps and 16Mbps • Still used and sold but beaten out by Ethernet Our Focus is Ethernet • History – – – – Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox, DEC, and Intel in 1978 LAN standards define MAC and physical layer connectivity • IEEE 802.3 (CSMA/CD - Ethernet) standard – originally 2Mbps • IEEE 802.3u standard for 100Mbps Ethernet • IEEE 802.3z standard for 1,000Mbps Ethernet • CSMA/CD: Ethernet’s Media Access Control (MAC) policy – CS = carrier sense • Send only if medium is idle – MA = multiple access – CD = collision detection • Stop sending immediately if collision is detected Ethernet Standard Defines Physical Layer • 802.3 standard defines both MAC and physical layer details Metcalfe’s original Ethernet Sketch Ethernet Technologies: 10Base2 • • 10: 10Mbps; 2: under 185 (~200) meters cable length Thin coaxial cable in a bus topology • Repeaters used to connect multiple segments – Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! 10BaseT and 100BaseT • 10/100 Mbps rate • T stands for Twisted Pair • Hub(s) connected by twisted pair facilitate “star topology” – Distance of any node to hub must be < 100M Physical Layer Configurations for 802.3 • Physical layer configurations are specified in three parts • Data rate (10, 100, 1,000) – 10, 100, 1,000Mbps • Signaling method (base, broad) – Baseband • Digital signaling – Broadband • Analog signaling • Cabling (2, 5, T, F, S, L) – – – – 5 - Thick coax (original Ethernet cabling) F – Optical fiber S – Short wave laser over multimode fiber L – Long wave laser over single mode fiber Ethernet Overview • Most popular packet-switched LAN technology • Bandwidths: 10Mbps, 100Mbps, 1Gbps • Max bus length: 2500m – 500m segments with 4 repeaters • Bus and Star topologies are used to connect hosts – Hosts attach to network via Ethernet transceiver or hub or switch • Detects line state and sends/receives signals – Hubs are used to facilitate shared connections – All hosts on an Ethernet are competing for access to the medium • Switches break this model • Problem: Distributed algorithm that provides fair access Ethernet Overview (contd.) • Ethernet by definition is a broadcast protocol – Any signal can be received by all hosts – Switching enables individual hosts to communicate • Network layer packets are transmitted over 64 48 48 16 32 an Ethernet by encapsulating Src Dest Preamble Type Body CRC addr addr • Frame Format Switched Ethernet • Switches forward and filter frames based on LAN addresses – It’s not a bus or a router (although simple forwarding tables are maintained) • Very scalable – Options for many interfaces – Full duplex operation (send/receive frames simultaneously) • Connect two or more “segments” by copying data frames between them – Switches only copy data when needed • key difference from repeaters • Higher link bandwidth – Collisions are completely avoided • Much greater aggregate bandwidth – Separate segments can send at once Ethernet Frames • Preamble is a sequence of 7 bytes, each set to “10101010” – Used to synchronize receiver before actual data is sent • Addresses – unique, 48-bit unicast address assigned to each adapter • example: 8:0:e4:b1:2 • Each manufacturer gets their own address range – broadcast: all 1s – multicast: first bit is 1 • Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to • Body can contain up to 1500 bytes of data A Quick Word about Aloha Networks • Developed in late 60’s by Norm Abramson at Univ. of Hawaii (!!) for use with packet radio systems – Any station can send data at any time – Receiver sends an ACK for data – Timeout for ACK signals that there was a collision • What happens if timeout is poorly timed? – If there is a collision, sender will resend data after a random backoff • Utilization (fraction of transmitted frames avoiding collision for N nodes) was pretty bad – Max utilization = 18% • Slotted Aloha (dividing transmit time into windows) helped – Max utilization increased to 36% Ethernet’s MAC Algorithm • In Aloha, decisions to transmit are made without paying attention to what other nodes might be doing • Ethernet uses CSMA/CD – listens to line before/during sending • If line is idle (no carrier sensed) – send packet immediately – upper bound message size of 1500 bytes – must wait 9.6us between back-to-back frames • If line is busy (carrier sensed) – wait until idle and transmit packet immediately • called 1-persistent sending • If collision detected – Stop sending and jam signal – Try again later State Diagram for CSMA/CD Packet? No Sense Carrier Send Detect Collision Yes Discard Packet attempts < 16 attempts == 16 Jam channel b=CalcBackoff(); wait(b); attempts++; Collisions Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences) • Both found line to be idle • Both had been waiting to for a busy line to become idle A starts at time 0 A A B B Message almost there at time T when B starts – collision! How can we be sure A knows about the collision? Collision Detection • How can A know that a collision has taken place? – – – – There must be a mechanism to insure retransmission on collision A’s message reaches B at time T B’s message reaches A at time 2T So, A must still be transmitting at 2T • IEEE 802.3 specifies max value of 2T to be 51.2us – This relates to maximum distance of 2500m between hosts – At 10Mbps it takes 0.1us to transmit one bit so 512 bits (64B) take 51.2us to send – So, Ethernet frames must be at least 64B long • 14B header, 46B data, 4B CRC • Padding is used if data is less than 46B • Send jamming signal after collision is detected to insure all hosts see collision – 48 bit signal Collision Detection contd. A B A B A B time = 0 time = T time = 2T Exponential Backoff • If a collision is detected, delay and try again • Delay time is selected using binary exponential backoff – 1st time: choose K from {0,1} then delay = K * 51.2us – 2nd time: choose K from {0,1,2,3} then delay = K * 51.2us – nth time: delay = K x 51.2us, for K=0..2n – 1 • Note max value for k = 1023 – give up after several tries (usually 16) • Report transmit error to host • If delay were not random, then there is a chance that sources would retransmit in lock step • Why not just choose from small set for K – This works fine for a small number of hosts – Large number of nodes would result in more collisions MAC Algorithm from the Receiver Side • Senders handle all access control • Receivers simply read frames with acceptable address – Address to host – Address to broadcast – Address to multicast to which host belongs – All frames if host is in promiscuous mode Fast and Gigabit Ethernet • Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet – Uses different physical layer encoding (4B5B) – Many NIC’s are 10/100 capable • Can be used at either speed • Gigabit Ethernet (1,000Mbps) – – – – – – Compatible with lower speeds Uses standard framing and CSMA/CD algorithm Distances are severely limited Typically used for backbones and inter-router connectivity Becoming cost competitive How much of this bandwidth is realizable? Experiences with Ethernet • Ethernets work best under light loads – Utilization over 30% is considered heavy • Network capacity is wasted by collisions • Most networks are limited to about 200 hosts – Specification allows for up to 1024 • Most networks are much shorter – 5 to 10 microsecond RTT • Transport level flow control helps reduce load (number of back to back packets) • Ethernet is inexpensive, fast and easy to administer! Ethernet Problems • Ethernet’s peak utilization is pretty low (like Aloha) • Peak throughput worst with – More hosts • More collisions needed to identify single sender – Smaller packet sizes • More frequent arbitration – Longer links • Collisions take longer to observe, more wasted bandwidth – Efficiency is improved by avoiding these conditions Why did Ethernet Win? • • • • • • There are LOTS of LAN protocols Price Performance Availability Ease of use Scalability Transmission Media Overview • Guided - wire • Unguided - wireless • Characteristics and quality determined by medium and signal • For guided, the medium is more important • For unguided, the bandwidth produced by the antenna is more important • Key concerns are data rate and distance Design Factors • Bandwidth – Higher bandwidth gives higher data rate • Transmission impairments – Attenuation • Interference • Number of receivers – In guided media – More receivers (multi-point) introduce more attenuation Electromagnetic Spectrum Guided Transmission Media • Twisted Pair • Coaxial cable • Optical fiber Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) Coaxial cable 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Twisted Pair Twisted Pair - Applications • Most common medium • Telephone network – Between house and local exchange (subscriber loop) • Within buildings – To private branch exchange (PBX) • For local area networks (LAN) – 10Mbps or 100Mbps Twisted Pair - Pros and Cons • • • • Cheap Easy to work with Low data rate Short range Twisted Pair - Transmission Characteristics • Analog – Amplifiers every 5km to 6km • Digital – Use either analog or digital signals – repeater every 2km or 3km • • • • Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise Near End Crosstalk • Coupling of signal from one pair to another • Coupling takes place when transmit signal entering the link couples back to receiving pair • i.e. near transmitted signal is picked up by near receiving pair Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) – Ordinary telephone wire – Cheapest – Easiest to install – Suffers from external EM interference • Shielded Twisted Pair (STP) – Metal braid or sheathing that reduces interference – More expensive – Harder to handle (thick, heavy) UTP Categories • Cat 3 – up to 16MHz – Voice grade found in most offices – Twist length of 7.5 cm to 10 cm • Cat 4 – up to 20 MHz • Cat 5 – up to 100MHz – Commonly pre-installed in new office buildings – Twist length 0.6 cm to 0.85 cm • Cat 5E (Enhanced) –see tables • Cat 6 • Cat 7 Comparison of Shielded and Unshielded Twisted Pair Attenuation (dB per 100 m) Frequency (MHz) Category 3 UTP Category 5 UTP 1 2.6 2.0 4 5.6 16 13.1 150-ohm STP Near-end Crosstalk (dB) Category 3 UTP Category 5 UTP 150-ohm STP 1.1 41 62 58 4.1 2.2 32 53 58 8.2 4.4 23 44 50.4 25 — 10.4 6.2 — 41 47.5 100 — 22.0 12.3 — 32 38.5 300 — 21.4 — — — 31.3 Twisted Pair Categories and Classes Category 3 Class C Category 5 Class D Bandwidth 16 MHz 100 MHz Cable Type UTP Link Cost (Cat 5 =1) 0.7 Category 5E Category 6 Class E Category 7 Class F 100 MHz 200 MHz 600 MHz UTP/FTP UTP/FTP UTP/FTP SSTP 1 1.2 1.5 2.2 Coaxial Cable Coaxial Cable Applications • Most versatile medium • Television distribution – Ariel to TV – Cable TV • Long distance telephone transmission – Can carry 10,000 voice calls simultaneously – Being replaced by fiber optic • Short distance computer systems links • Local area networks Coaxial Cable - Transmission Characteristics • Analog – Amplifiers every few km – Closer if higher frequency – Up to 500MHz • Digital – Repeater every 1km – Closer for higher data rates Optical Fiber Optical Fiber - Benefits • Greater capacity – Data rates of hundreds of Gbps • • • • Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing – 10s of km at least Optical Fiber - Applications • • • • • Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz – Portions of infrared and visible spectrum • Light Emitting Diode (LED) – Cheaper – Wider operating temp range – Last longer • Injection Laser Diode (ILD) – More efficient – Greater data rate • Wavelength Division Multiplexing Optical Fiber Transmission Modes Frequency Utilization for Fiber Applications Wavelength (in vacuum) range (nm) Frequency range (THz) 820 to 900 366 to 333 1280 to 1350 234 to 222 1528 to 1561 1561 to 1620 Band label Fiber type Application Multimode LAN S Single mode Various 196 to 192 C Single mode WDM 185 to 192 L Single mode WDM Attenuation in Guided Media Wireless Transmission Frequencies • 2GHz to 40GHz – – – – Microwave Highly directional Point to point Satellite • 30MHz to 1GHz – Omnidirectional – Broadcast radio • 3 x 1011 to 2 x 1014 – Infrared – Local Antennas • Electrical conductor (or system of..) used to radiate electromagnetic energy or collect electromagnetic energy • Transmission – – – – Radio frequency energy from transmitter Converted to electromagnetic energy By antenna Radiated into surrounding environment • Reception – Electromagnetic energy impinging on antenna – Converted to radio frequency electrical energy – Fed to receiver • Same antenna often used for both Radiation Pattern • Power radiated in all directions • Not same performance in all directions • Isotropic antenna is (theoretical) point in space – Radiates in all directions equally – Gives spherical radiation pattern Parabolic Reflective Antenna • Used for terrestrial and satellite microwave • Parabola is locus of point equidistant from a line and a point not on that line – Fixed point is focus – Line is directrix • Revolve parabola about axis to get paraboloid – Cross section parallel to axis gives parabola – Cross section perpendicular to axis gives circle • Source placed at focus will produce waves reflected from parabola in parallel to axis – Creates (theoretical) parallel beam of light/sound/radio • On reception, signal is concentrated at focus, where detector is placed Parabolic Reflective Antenna Antenna Gain • Measure of directionality of antenna • Power output in particular direction compared with that produced by isotropic antenna • Measured in decibels (dB) • Results in loss in power in another direction • Effective area relates to size and shape – Related to gain Terrestrial Microwave • • • • • Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates Satellite Microwave • Satellite is relay station • Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency • Requires geo-stationary orbit – Height of 35,784km • Television • Long distance telephone • Private business networks Satellite Point to Point Link Satellite Broadcast Link Broadcast Radio • • • • • Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath interference – Reflections Infrared • • • • Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port Wireless Propagation • Signal travels along three routes – Ground wave • Follows contour of earth • Up to 2MHz • AM radio – Sky wave • Amateur radio, BBC world service, Voice of America • Signal reflected from ionosphere layer of upper atmosphere • (Actually refracted) – Line of sight • Above 30Mhz • May be further than optical line of sight due to refraction • More later… Ground Wave Propagation Sky Wave Propagation Line of Sight Propagation Refraction • Velocity of electromagnetic wave is a function of density of material – ~3 x 108 m/s in vacuum, less in anything else • As wave moves from one medium to another, its speed changes – Causes bending of direction of wave at boundary – Towards more dense medium • Index of refraction (refractive index) is – Sin(angle of incidence)/sin(angle of refraction) – Varies with wavelength • May cause sudden change of direction at transition between media • May cause gradual bending if medium density is varying – Density of atmosphere decreases with height – Results in bending towards earth of radio waves Optical and Radio Horizons Line of Sight Transmission • Free space loss – Signal disperses with distance – Greater for lower frequencies (longer wavelengths) • Atmospheric Absorption – – – – Water vapour and oxygen absorb radio signals Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz Rain and fog scatter radio waves – – – – Better to get line of sight if possible Signal can be reflected causing multiple copies to be received May be no direct signal at all May reinforce or cancel direct signal • Multipath • Refraction – May result in partial or total loss of signal at receiver Free Space Loss Multipath Interference Overview • Characteristics and quality determined by: – Medium – Signal • Medium – Guided - wire – Unguided - wireless • For Guided Medium – The medium is more important • For Unguided – The bandwidth produced by the antenna is more important • Key concerns are data rate and distance Design Factors • Bandwidth – • Transmission impairments – • Attenuation Interference – • Higher bandwidth gives higher data rate Issue especially in case of unguided medium Number of receivers – – Unicast (one sender, one receiver) Multicast (multiple receivers can introduce more errors) Guided Transmission Media • Twisted Pair • Coaxial Cable • Optical Fiber Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) Coaxial cable 0 to 1 MHz 0.7 dB/km @ 1 kHz 50 µs/km 2 km 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Twisted Pair Twisted Pair Architecture Two Insulated copper wires Issues: (1) Interference due to unwanted electrical coupling of two copper (2) Interference due to unwanted electrical coupling between the neighboring twisted pairs Twisted Pair Applications • Most commonly used medium • Telephone network – Between house and local exchange (subscriber loop) • Within buildings – To private branch exchange (PBX) • For local area networks (LAN) – 10Mbps or 100Mbps Twisted Pair - Pros and Cons • Advantages – Less expensive – Easy to work with • Disadvantages – Low data rate – Short range Twisted Pair (TP) Characteristics • Analog transmission – Amplifiers every 5km to 6km • Digital transmission – Use either analog or digital signals – repeater every 2km or 3km • TP is Limited – Distance – Bandwidth – Data rate • Susceptible to interference and noise – Easy coupling of electromagnetic fields Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) – Ordinary telephone wire – Less expensive – Weak immunity against noise and interference – Suffers from external EM interference • Shielded Twisted Pair (STP) – An extra metallic sheath on each pair – Relatively more expensive – Provide better performance than UTP • Increased Data rate • Increased Bandwidth UTP Categories • Cat 3 – up to 16MHz – Voice grade found in most offices – Twist length of 7.5 cm to 10 cm • Cat 4 – up to 20 MHz • Cat 5 – up to 100MHz – Commonly pre-installed in new office buildings – Twist length 0.6 cm to 0.85 cm • Cat 5E (Enhanced) –see tables • Cat 6 • Cat 7 Coaxial Cable Coaxial Cable Architecture Coaxial Cable Applications • Television (TV) signals distribution – Ariel to TV – Cable TV • Long distance telephone transmission – Can carry 10,000 voice calls simultaneously – Being replaced by fiber optic • Short distance computer systems links – Local area networks (LAN) – Metropolitan area network (MAN) Coaxial Cable Characteristics • Analog – Amplifiers every few km – Closer if higher frequency – Up to 500MHz • Digital – Repeater every 1km – Closer for higher data rates • Problem – Inter-modulation noise – Thermal noise Optical Fiber Optical Fiber Architecture Optical Fiber Benefits • Greater capacity – Data rates of hundreds of Gbps • Smaller size & weight – Made up of extremely thin fibers • Lower attenuation – Electromagnetic isolation • Greater repeater spacing – 10s of km at least Optical Fiber - Transmission Characteristics • Operational range – 1014 to 1015 Hz • Light source – Light Emitting Diode (LED) • Cheaper • Wider operating temperature range • Last longer – Injection Laser Diode (ILD) • Operates on laser principle • More efficient • Greater data rate • Wavelength Division Multiplexing (WDM) Transmission Media Transmission medium and physical layer Classes of transmission media GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. Topics discussed in this section: Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable Twisted-pair cable UTP and STP cables Table 7.1 Categories of unshielded twisted-pair cables UTP connector Coaxial cable Categories of coaxial cables Fiber optics: Bending of light ray Optical fiber Propagation modes Modes Fiber types Fiber construction Fiber-optic cable connectors UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Topics discussed in this section: Radio Waves Microwaves Infrared Electromagnetic spectrum for wireless communication Propagation methods Bands Wireless transmission waves Omnidirectional antenna Unidirectional antennas Wireless Channels • Are subject to a lot more errors than guided media channels. • Interference is one cause for errors, can be circumvented with high SNR. • The higher the SNR the less capacity is available for transmission due to the broadcast nature of the channel. • Channel also subject to fading and no coverage holes. Circuit Switching and Packet Switching Overview • Networks are used to interconnect many devices. • We have checked with Local Area Networks. • Now, wide area networks – Since the invention of the telephone, circuit switching has been the dominant technology for voice communications. – Since 1970, packet switching has evolved substantially for digital data communications. It was designed to provide a more efficient facility than circuit switching for bursty data traffic. • Two types of packet switching: – Datagram (such as today’s Internet) – Virtual circuit (such as Frame Relay, ATM) Switched Communications Networks • Long distance transmission between stations (called “end devices”) is typically done over a network of switching nodes. • Switching nodes do not concern with content of data. Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination (the end device). • A collection of nodes and connections forms a communications network. • In a switched communications network, data entering the network from a station are routed to the destination by being switched from node to node. Simple Switching Network Switching Nodes • Nodes may connect to other nodes, or to some stations. • Network is usually partially connected – However, some redundant connections are desirable for reliability • Two different switching technologies – Circuit switching – Packet switching Circuit Switching • Circuit switching: – There is a dedicated communication path between two stations (endto-end) – The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. • Communication via circuit switching has three phases: – Circuit establishment (link by link) • Routing & resource allocation (FDM or TDM) – Data transfer – Circuit disconnect • Deallocate the dedicated resources • The switches must know how to find the route to the destination and how to allocate bandwidth (channel) to establish a connection. Circuit Switching Properties • Inefficiency – Channel capacity is dedicated for the whole duration of a connection – If no data, capacity is wasted • Delay – Long initial delay: circuit establishment takes time – Low data delay: after the circuit establishment, information is transmitted at a fixed data rate with no delay other than the propagation delay. The delay at each node is negligible. • Developed for voice traffic (public telephone network) but can also applied to data traffic. – For voice connections, the resulting circuit will enjoy a high percentage of utilization because most of the time one party or the other is talking. – But how about data connections? Public Circuit Switched Network Subscribers: the devices that attach to the network. Subscriber loop: the link between the subscriber and the network. Exchanges: the switching centers in the network. End office: the switching center that directly supports subscribers. Trunks: the branches between exchanges. They carry multiple voice-frequency circuits using either FDM or synchronous TDM. Packet Switching Principles • Problem of circuit switching – designed for voice service – Resources dedicated to a particular call – For data transmission, much of the time the connection is idle (say, web browsing) – Data rate is fixed • Both ends must operate at the same rate during the entire period of connection • Packet switching is designed to address these problems. Basic Operation • Data are transmitted in short packets – Typically at the order of 1000 bytes – Longer messages are split into series of packets – Each packet contains a portion of user data plus some control info • Control info contains at least – Routing (addressing) info, so as to be routed to the intended destination – Recall the content of an IP header! • store and forward – On each switching node, packets are received, stored briefly (buffered) and passed on to the next node. Use of Packets Advantages of Packet Switching • Line efficiency – Single node-to-node link can be dynamically shared by many packets over time – Packets are queued up and transmitted as fast as possible • Data rate conversion – Each station connects to the local node at its own speed • In circuit-switching, a connection could be blocked if there lacks free resources. On a packet-switching network, even with heavy traffic, packets are still accepted, by delivery delay increases. • Priorities can be used – On each node, packets with higher priority can be forwarded first. They will experience less delay than lower-priority packets. Packet Switching Technique • A station breaks long message into packets • Packets are sent out to the network sequentially, one at a time • How will the network handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination? – Two approaches • Datagram approach • Virtual circuit approach 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. • • • • Packets can take any possible route. Packets may arrive at the receiver out of order. Packets may go missing. It is up to the receiver to re-order packets and recover from missing packets. • Example: Internet Datagram Virtual Circuit • In virtual circuit, a preplanned route is established before any packets are sent, then all packets follow the same route. • Each packet contains a virtual circuit identifier instead of destination address, and each node on the preestablished route knows where to forward such packets. – The node need not make a routing decision for each packet. • Example: X.25, Frame Relay, ATM Virtual Circuit A route between stations is set up prior to data transfer. All the data packets then follow the same route. But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded. Virtual Circuits v Datagram • Virtual circuits – Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes). – Packets are forwarded more quickly • Based on the virtual circuit identifier • No routing decisions to make – Less reliable • If a node fails, all virtual circuits that pass through that node fail. • Datagram – No call setup phase • Good for bursty data, such as Web applications – More flexible • If a node fails, packets may find an alternate route • Routing can be used to avoid congested parts of the network Comparison of communication switching techniques Switching Networks • Long distance transmission is typically done over a network of switched nodes • Nodes not concerned with content of data • End devices are stations – Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data routed by being switched from node to node Nodes • Nodes may connect to other nodes only, or to stations and other nodes • Node to node links usually multiplexed • Network is usually partially connected – Some redundant connections are desirable for reliability • Two different switching technologies – Circuit switching – Packet switching Circuit Switching • Dedicated communication path between two stations • Three phases – Establish – Transfer – Disconnect • Must have switching capacity and channel capacity to establish connection • Must have intelligence to work out routing Circuit Switching - Applications • Inefficient – Channel capacity dedicated for duration of connection – If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone) Public Circuit Switched Network Circuit Establishment Circuit Switching Principles revisited • Circuit switching designed for voice – Resources dedicated to a particular call – Much of the time a data connection is idle – Data rate is fixed • Both ends must operate at the same rate Packet Switching: Basic Operation • Data transmitted in small packets – Longer messages split into series of packets – Each packet contains a portion of user data plus some control info • Control info – Routing (addressing) info • Packets are received, stored briefly (buffered) and past on to the next node – Store and forward Packet-Switched Network Use of Packets Advantages • Line efficiency – Single node to node link can be shared by many packets over time – Packets queued and transmitted as fast as possible • Data rate conversion – Each station connects to the local node at its own speed – Nodes buffer data if required to equalize rates • Packets are accepted even when network is busy – Delivery may slow down • Priorities can be used Switching Technique • Station breaks long message into packets • Packets sent one at a time to the network • Packets handled in two ways – Datagram – Virtual circuit Datagram • • • • • Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing packets Datagram Diagram Network Hardware and Physical Media • Network hardware includes: – – – – Computers Peripherals Interface cards and Other equipment needed to perform data processing and communications within the network File servers • A very fast computer with a large amount of RAM and storage space along with a fast network interface card • The network operating system software resides on this computer Workstations • All computers connected to the file server on a network are called workstations Network interface cards • The network interface card (NIC) provides the physical connection between the network and the computer workstation. • Most NICs are internal with the card fitting into an expansion slot in the computer. • Three common network interface connections are Ethernet cards, Local Talk connectors and Token Ring cards Ethernet cards • The most common Network Interface Cards are Ethernet cards • They contain connections for either coaxial or twisted pair cables, or both Co-axial cable Twisted pair cable Concentrators / Hubs • A concentrator is a device that provides a central connection point for cables from workstations, servers and peripherals • Hubs are multi-slot concentrators Switches • hubs provide an easy way to scale up and shorten the distance that the packets must travel to get from one node to another • they do not break up the actual network into discrete segments. That is where switches come in. Switches (continued) • A vital difference between a hub and a switch is – all the nodes connected to a hub share the bandwidth among themselves. – while a device connected to a switch port has the full bandwidth all to itself. • Think of a switch as a ‘clever’ hub Repeaters • A signal loses strength as it passes along a cable, so it is often necessary to boost the signal with a device called a repeater • A repeater might be a separate device, or might be part of a concentrator Bridges • A bridge is a device that allows you to segment a large network into two smaller, more efficient networks An example of a network with a bridge Router Hub Bridge Hub Internet Segment Node Routers • A router translates information from one network to another • The router directs traffic to prevent “head-on” collisions • If you have a LAN that you want to connect to the Internet, you will need a router to serve as the translator between information on your LAN and the Internet Routers (continued) Physical Media • Physical media provide the connections between network devices that make networking possible • There are four main types of physical media in widespread use today: – Coaxial Cable – Twisted Pair – Fiber Optic Cable LAN Technologies Ethernet Physical Media :10 Base5 10 Base2 10 BaseT 10 BaseFL - Thick Co-axial Cable with Bus Topology - Thin Co-axial Cable with Bus Topology - UTP Cat 3/5 with Tree Topology - Multimode/Singlemode Fiber with Tree Topology Maximum Segment Length 10 Base5 - 500 m with at most 4 repeaters (Use Bridge to extend the network) 10 Base2 - 185 m with at most 4 repeaters (Use Bridge to extend the network) 10 BaseT - 100 m with at most 4 hubs (Use Switch to extend the network) Thick Coaxial Cable • • • • • • • • • Used in the first Ethernet networks Type RG-11 / 10Base5 Usually orange/black Thickness of a small garden hose Very expensive and heavy cable Two strands along the axis Conductor down the center Insulator surrounds conductor Shielded mesh serves as outside Thin Coaxial Cable • • • • • • • Alternative to Thick Ethernet Cable Type RG-58 / 10Base2 / “Cheapnet” Usually black Thickness of a pencil More flexible than thick Ethernet Reduced the cost of the cabling Flexible Coaxial cable connectors • The most common type of connector used with coaxial cables is the BNC connector Twisted Pair Cable • Phone Systems • Twisted Pair Cable consists of two copper wires, usually twisted around each other to cancel out any noise in the circuit • Two main type of Twisted Pair Cabling – Shielded Twisted Pair (STP) – Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) • STP is the original media used for token ring networks • STP can be used for high-speed networks, such as FDDI or ATM, where shielding is important. RJ-45 Unshielded Twisted Pair (UTP) • UTP has four pairs of wires inside the jacket • Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs UTP (Continued) • Most commonly used twisted pair cable • Uses common telephone wire • UTP was standardized by the IEEE 802.3 committee in October of 1990 • UTP for LANs is now classified as: – Category 3 - used for LANs up to 10 Mbps – Category 4 - used for LANs up to 16 Mbps – Category 5 - used for LANs up to 100 Mbps Fiber Optic Cable • Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials • It transmits light rather than electronic signals • It is the standard for connecting networks between buildings, due to its immunity to the effects of moisture and light Fiber Optic (continued) • Fiber optic cable has the ability to transmit signals over much longer distances than coaxial or twisted pair • It can also carry information at vastly greater speeds • Fiber optic cable is more difficult to install than other cabling Wireless LANS – Wireless networks use high frequency radio signals to communicate between the workstations and the fileserver or hubs. – Disadvantages of wireless networks are: • • • • they are expensive (relatively), provide poor security, are susceptible to interference and are slower than cabled networks MAC Layer in WSNs Outline • • • • • • Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary Introduction to MAC • The role of medium access control (MAC) – Controls when and how each node can transmit in the wireless channel • Why do we need MAC? – Wireless channel is a shared medium – Radios transmitting in the same frequency band interfere with each other – collisions – Other shared medium examples: Ethernet Where Is the MAC? • Network model from Internet Application layer Transport layer End-to-end reliability, congestion control Network layer Routing Link/MAC layer Physical layer Per-hop reliability, flow control, multiple access Packet transmission and reception • A sublayer of the Link layer – Directly controls the radio – The MAC on each node only cares about its neighborhood Media access in wireless • In wired link, – Carrier Sense Multiple Access with Collision Detection – send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) • In wireless – Signal strength decreases in proportional to at least square of the distance – Collision detection only at receiver – Half-duplex mode – Furthermore, CS is not possible after propagation range What’s New in Sensor Networks? • A special wireless ad hoc network – – – – – – Large number of nodes Limited computation ability and RAM Battery powered Topology and density change Nodes for a common task In-network data processing Primary Concerns of MAC Attributes • Collision avoidance – Basic task of a MAC protocol – Determine when and how to access the medium • Energy efficiency – One of the most important attributes for sensor networks, since most nodes are battery powered – Affect the overall node lifetime Primary Concerns of MAC Attributes • Scalability and adaptivity – Network size, node density and topology change • Deployed ad-hoc and operate in uncertain environments • Nodes die • Nodes join later • Nodes move – Good MAC accommodates changes gracefully Other Concerns of MAC Attributes • Channel utilization – How well is the channel used? – Also called bandwidth utilization or channel capacity • Latency – Delay from sender to receiver – Its importance depends on application – single hop or multi-hop Other Concerns of MAC Attributes • Throughput – The amount of data transferred from sender to receiver in unit time – Affected by efficiency of collision avoidance, channel utilization, latency, control overhead… Energy Efficiency in MAC Design • Energy is primary concern in sensor networks • What causes energy waste? – Collisions • Retransmission – Long idle time Dominant factor • Idle listening consumes 50—100% of the power for receiving Energy • What causes energy waste? – Overhearing unnecessary traffic • Can be a dominant factor of energy waste when – Heavy traffic load – High node density – Control packet overhead • Reduce effective goodput – Computation complexity – With Motes, radio and CPU are two major energy consumers Hidden terminal problem • Hidden terminals – – – – A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C A B C Exposed terminal problem • Exposed terminals – B sends to A, C wants to send to D – C has to wait, CS signals a medium in use – but A is outside the radio range of C, thus waiting is not necessary – C is “exposed” to B A B C D Energy Efficiency in Contention Protocols Energy Efficiency in Contention Protocols • Contention-based protocols need to work hard in all directions for energy savings – Reduce idle listening – support low duty cycle – Better collision avoidance – Reduce control overhead – Avoid unnecessary overhearing Sensor Mac: Case Studies Case Study 1: S-MAC • By Ye, Heidemann and Estrin • Tradeoffs Latency Fairness • Major components in S-MAC – Periodic listen and sleep – Collision avoidance – Overhearing avoidance – Message passing Energy Coordinated Sleeping • Problem: Idle listening consumes significant energy • Solution: Periodic listen and sleep listen sleep listen sleep • Turn off radio when sleeping • Reduce duty cycle to ~ 10% (120ms on/1.2s off) Latency Energy Coordinated Sleeping • Schedules can differ Node 1 Node 2 listen sleep listen listen sleep sleep listen sleep • Prefer neighboring nodes have same schedule — easy broadcast & low control overhead Schedule 1 Schedule 2 Border nodes: two schedules or broadcast twice Coordinated Sleeping • Schedule Synchronization – New node tries to follow an existing schedule – Remember neighbors’ schedules — to know when to send to them – Each node broadcasts its schedule every few periods of sleeping and listening – Re-sync when receiving a schedule update • Periodic neighbor discovery – Keep awake in a full sync interval over long periods Coordinated Sleeping • Adaptive listening – Reduce multi-hop latency due to periodic sleep – Wake up for a short period of time at end of each transmission 2 1 3 4 RTS CTS listen CTS listen t1 Reduce latency by at least half listen t2 Periodic Listen and Sleep • Choosing schedules – The node randomly choose time to go to sleep. – The node receives and follows its neighbor’s schedule by setting its schedule to be the same. – If the nodes receives a different schedule after it selects its own schedule, it adopts its own schedule. Periodic Listen and Sleep • Maintaining synchronization – Update schedule by sending a SYNC packet periodically. • SYNC packet contains address of the sender and the time of its next sleep. – The new node follows the same procedure to choose schedule. • The initial listen period should be long enough. S-MAC: Coordinated Sleeping Neighbor Discovery • chance of failing to discover an existing neighbor – corrupted SYNC packet, collisions, interference – sensor – border of two schedules; discovers only the first schedule, if schedules do not overlap • Periodically, listen for the complete SP – frequency? • - if a sensor has no neighbors • S-MAC experimental values: – SP = 10 seconds – Neighbor discovery period = 2 minutes, if at least 1 nbr S-MAC: Coordinated Sleeping Maintaining Synchronization • Listen is split into 2 parts – for SYNC and RTS/CTS • Once RTS/CTS is established, data sent in sleep interval Listen Receiver for SYNC for RTS for CTS Sleep S-MAC: Coordinated Sleeping Example Scenarios Listen Receiver for SYNC for RTS for CTS Sleep Tx SYNC Sender 1 CS Tx RTS Got CTS Send data CS Sender 2 Tx SYNC CS Sender 3 Tx RTS CS Got CTS Send data S-MAC: Collision Avoidance • S-MAC is based on contention • Similar to IEEE 802.11 ad hoc mode (DCF) – Physical and virtual carrier sense – Randomized backoff time – RTS/CTS for hidden terminal problem – RTS/CTS/DATA/ACK sequence S-MAC: Collision Avoidance • Collision avoidance – The same procedure as 802.11. – To adopt RTS/CTS exchange and physical/virtual carrier sense. – Randomized carrier sense time. Overhearing Avoidance • Problem: Receive packets destined to others • Solution: Sleep when neighbors talk • Who should sleep? – All immediate neighbors of sender and receiver • How long to sleep? – The duration field in each packet informs other nodes the sleep interval Overhearing Avoidance • To avoid overhearing by letting interfering nodes go to sleep after they hear a RTS or CTS packet. • Who must go to sleep? E C A B D F – All immediate neighbors of both the sender and receiver should sleep. • Sleep until NAV becomes zero. Message Passing • Problem: Sensor net in-network processing requires entire message • Solution: Don’t interleave different messages – Long message is fragmented & sent in burst – RTS/CTS reserve medium for entire message – Fragment-level error recovery — ACK — extend Tx time and re-transmit immediately • Other nodes sleep for whole message time Fairness Energy Msg-level latency Message Passing • Only one RTS and CTS packet are used to send the fragmented long packet. – To avoid control overhead. • To do overhearing avoidance.. – Each RTS/CTS/DATA/ACK packet has its duration field. – The duration field include expected transmission time of all fragment. – Sleep until NAV becomes zero. S-MAC: An Energy-Efficient MAC Protocol • S- MAC protocol designed specifically for sensor networks to reduce energy consumption while achieving good scalability and collision avoidance by utilizing a combined scheduling and contention scheme • The major sources of energy waste are: – – – – collision overhearing control packet overhead idle listening • S-MAC reduce the waste of energy from all the sources mentioned in exchange of some reduction in both per-hop fairness and latency IEEE 802.15.4 and Zigbee Overview Topics • • • • • 802.15.4 ZigBee Competing Technologies Products Some Motorola Projects IEEE 802.15.4 Applications Space • Home Networking • Automotive Networks • Industrial Networks • Interactive Toys • Remote Metering Some needs in the sensor networks Thousands of sensors in a small space Wireless but sensors are frequently stand alone Low Power and sensors are frequently isolated Moderate Range. Some of the challenges facing the standards committee 802.15.4 General Characteristics Data rates of 250 kb/s, 40 kb/s and 20 kb/s. Star or Peer-to-Peer operation. Support for low latency devices. Fully handshaked protocol for transfer reliability. Low power consumption. Frequency Bands of Operation 16 channels in the 2.4GHz ISM* band 10 channels in the 915MHz ISM band 1 channel in the European 868MHz band. * ISM: Industrial, Scientific, Medical 802.15.4 / ZigBee Architecture Applications ZigBee IEEE 802.15.4 MAC IEEE 802.15.4 868/915 MHz PHY IEEE 802.15.4 2400 MHz PHY • Packet generation • Packet reception • Data transparency • Power Management IEEE 802.15.4 PHY Overview Operating Frequency Bands 868MHz / 915MHz PHY 2.4 GHz PHY 2.4 GHz Channel 0 Channels 1-10 868.3 MHz 902 MHz Channels 11-26 2 MHz 928 MHz 5 MHz 2.4835 GHz IEEE 802.15.4 PHY Overview Packet Structure PHY Packet Fields • • • • Preamble (32 bits) – synchronization Start of Packet Delimiter (8 bits) PHY Header (8 bits) – PSDU length PSDU (0 to 1016 bits) – Data field Preamble Start of Packet Delimiter 6 Octets PHY Header PHY Service Data Unit (PSDU) 0-127 Octets 802.15.4 Architecture Applications ZigBee IEEE 802.15.4 MAC IEEE 802.15.4 868/915 MHz PHY IEEE 802.15.4 2400 MHz PHY • Channel acquisition • Contention mgt • NIC address • Error Correction IEEE 802.15.4 MAC Overview Design Drivers Extremely low cost Ease of implementation Reliable data transfer Short range operation • Very low power consumption Simple but flexible protocol IEEE 802.15.4 MAC Overview Typical Network Topologies IEEE 802.15.4 MAC Overview Device Classes • Full function device (FFD) – Any topology – Network coordinator capable – Talks to any other device • Reduced function device (RFD) – Limited to star topology – Cannot become a network coordinator – Talks only to a network coordinator – Very simple implementation IEEE 802.15.4 MAC Overview Star Topology PAN Coordinator Master/slave Full function device Reduced function device Communications flow IEEE 802.15.4 MAC Overview Peer-Peer Topology Point to point Full function device Cluster tree Communications flow IEEE 802.15.4 MAC Overview Combined Topology Clustered stars - for example, cluster nodes exist between rooms of a hotel and each room has a star network for control. Full function device Reduced function device Communications flow IEEE 802.15.4 MAC Overview General Frame Structure PHY Layer MAC Layer Payload Synch. Header (SHR) MAC Header (MHR) PHY Header (PHR) 4 Types of MAC Frames: • Data Frame • Beacon Frame • Acknowledgment Frame • MAC Command Frame MAC Service Data Unit (MSDU) MAC Protocol Data Unit (MPDU) PHY Service Data Unit (PSDU) MAC Footer (MFR) IEEE 802.15.4 MAC Overview Traffic Types • Periodic data – Application defined rate (e.g. sensors) • Intermittent data – Application/external stimulus defined rate (e.g. light switch) • Repetitive low latency data – Allocation of time slots 802.15.4 Architecture Applications • Network Routing • Address translation • Packet Segmentation • Profiles ZigBee IEEE 802.15.4 MAC IEEE 802.15.4 868/915 MHz PHY IEEE 802.15.4 2400 MHz PHY ZigBee Stack Architecture Typical ZigBee-Enabled Device Design Typical design consist of RF IC and 8-bit microprocessor with peripherals connected to an application Wireless Technology Comparison Chart 34KB /14KB 356 mA