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Transcript
Introduction to Communication Networks Amit Dvir [email protected] Lecture 1 Introduction and Layering 1 Introduction to Communication Networks 1/2006 Course Information Lectures: – Website: www.cse.bgu.ac.il/Courses/ – Slides, Assignments, Messages Books: – – – 2 Sunday 17:00 – 20:00, 28/303 Computer Networking: A Top-Down Approach Featuring the Internet / Kurose-Ross Computer Networks / Tanenbaum Data Networks / Bertsekas-Gallager Introduction to Communication Networks 1/2006 Course Information Reception hours: – Grading – – 3 Monday 10:00-11:00, 30/302D 80% Final exam 20% Assignments Introduction to Communication Networks 1/2006 Course Objectives 4 Learn the basics of data communication and computer networks To understand the main concepts and principles of communication To present modern communication networks and their applications Introduction to Communication Networks 1/2006 Simplified Communication Model Source Workstation 5 Transmitter Transmission System Receiver Destination Modem Modem Public Telephone Network Introduction to Communication Networks 1/2006 Server Simplified Data Communication Model Analog signal Digital bit stream Analog signal Digital bit stream Text Text Source 1 2 Input Input data information g(t) m 6 Transmission System Transmitter Receiver 3 4 Transmitted signal s(t) Received signal r(t) Introduction to Communication Networks 1/2006 Destination 5 Output data g(t)' 6 Output information m' An Introduction to the mail system BGU Nick Dave Admin Admin (Some of the following slides are taken from lectures by Nick Mckeown, stanford) 7 TLV Introduction to Communication Networks 1/2006 Characteristics of the mail system Each envelope is individually routed. No time guarantee for delivery. No guarantee of delivery in sequence. No guarantee of delivery at all! Things get lost How can we acknowledge delivery? Retransmission How to determine when to retransmit? Timeout? Need local copies of contents of each envelope. How long to keep each copy. What if an acknowledgement is lost? 8 Introduction to Communication Networks 1/2006 An Introduction to the mail system BGU TLV Application Layer Nick Dave Transport Layer Admin Admin Network Layer Link Layer 9 Introduction to Communication Networks 1/2006 An Introduction to the Internet Leland.Stanford.edu Application Layer Athena.MIT.edu Nick Dave Transport Layer O.S. Datagram Data Header Data Header O.S. Network Layer Link Layer 10 Introduction to Communication Networks 1/2006 Characteristics of the Internet Each packet is individually routed. No time guarantee for delivery. No guarantee of delivery in sequence. No guarantee of delivery at all! Things get lost Acknowledgements Retransmission How to determine when to retransmit? Timeout? Need local copies of contents of each packet. How long to keep each copy? What if an acknowledgement is lost? 11 Introduction to Communication Networks 1/2006 Characteristics of the Internet (2) No guarantee of integrity of data. Packets can be fragmented. Packets may be duplicated. 12 All of the above properties and questions are solved by today’s modern networks and communication systems ! Introduction to Communication Networks 1/2006 Data Communication Passing (digital) information throughout the network – – – – Out of the computer from one system to another Usually in a serial manner Using a specific medium (fiber, cables, radio) Option of passing through “intermediate stations” 13 Possible errors before reaching to the destination Introduction to Communication Networks 1/2006 Early communications systems 14 I.e. telephone point-to-point links directly connect together the users wishing to communicate use dedicated communication circuit if distance between users increases beyond the length of the cable, the connection is formed by a number of sections connected end-to-end in series. Introduction to Communication Networks 1/2006 Data Networks - Overview 15 Set of interconnected nodes exchange information Sharing of the transmission circuits = "switching". Many links allow more than one path between every 2 nodes. Network must select an appropriate path for each required connection. Introduction to Communication Networks 1/2006 Data Networks - Overview Overview Hardware:How you can configure a bunch of computers into a network: Local Are Networks (LAN) Metropolitan Area Networks (MAN) Wide Area Networks (WAN) Internetworks Software: This is what actually makes computer networks – not the hardware! Protocols: describe how two communicating parties exchange information. 16 Introduction to Communication Networks 1/2006 Overview – cont. - Overview Data Networks Services: describe what a network offers to parties that want to communicate. Interfaces: describe how a client can make use of network services, i.e. how the services can be accessed. Reference models: Describe how the OSI (Open System Interconnection) and Internet networks are organized. 17 Introduction to Communication Networks 1/2006 ComputerNetworks NetworksHardware Hardware Computer Broadcast network: a single communication channel is shared by all computers: – Point-to-point network: Computers are connected in pairs: – 18 sending a packet implies that all others receive it. sending a packet goes strictly from the sender to the receiver, possibly having to visit intermediate machines (routing). Introduction to Communication Networks 1/2006 Transmission Technology 19 Introduction to Communication Networks 1/2006 Local Area Networks (LAN) 20 A LAN is a computer network (or data communications network) which is confined in a limited geographical area Apart from scale, LANs distinguish themselves from other networks by (generally) using broadcast technology,and having simple topologies: Type (a) (Bus-based): All computers are connected to the same wire. When one of them starts sending, the signal is propagated to all others. If two of them start sending at the same time, packets collide and rubbish is the result. Introduction to Communication Networks 1/2006 Local Area Networks (LAN) Type (b) (Token-based): a token (which is just a small packet) continuously circulates along the ring. A sending computer (1) waits until the token passes and removes it (2) sends its packet along the ring, (3) waits until the packet returns (4) reinserts the token. Why we need the token? 21 Introduction to Communication Networks 1/2006 Metropolitan Area Networks (MAN) A network which is bigger than a LAN but smaller then WAN. A MAN typically covers an area of between 5 and 50 km diameter – 22 For example, cover an area the size of a city A typical use of MANs to provide shared access to a wide area network Introduction to Communication Networks 1/2006 Wide Area Networks (WAN) 23 A WAN spans a large geographic area, such as a state, province or country WANs often connect multiple smaller networks, such as LANs or MANs. A network device called a router connects LANs to a WAN. Introduction to Communication Networks 1/2006 Wide Area Networks (WAN) – cont. Routers generally adhere to a store-and-forward principle: incoming packets are first buffered (stored), the router takes a decision on where the packet has to go, and forwards the packet across the selected output line. Note: In contrast to LANs and MANs, the organization of a WAN in terms which hosts are interconnected is Important. 24 Introduction to Communication Networks 1/2006 Examples of WAN Topology 25 Introduction to Communication Networks 1/2006 Internetworks The assumption so far is that a network is homogeneous: there is hardly any variation in hardware and software. In practice, large networks can only be constructed by interconnecting different kinds of networks internet(work). Examples: Connecting a collection of different kinds of LANs (busbased to token-based) within a department. 26 Introduction to Communication Networks 1/2006 Internetworks – cont. 27 Connecting LANs to each other through a WAN (think of enterprise networks for multinationals). The WAN acts as a subnet. Connecting WANs to each other (the Internet). Introduction to Communication Networks 1/2006 Networking Issues - Internet Addressing - identify the end user IP addresses 132.66.48.37, Refer to a host interface = network number + host number Routing- How to get from source to destination packet switching. move packets (chunks) of data among routers from source to destination independently. Information Units - How is information sent Self-descriptive data: packet = data + metadata (header). 28 Introduction to Communication Networks 1/2006 Protocol Layers 29 A way for organizing structure of network The idea: a series of steps Introduction to Communication Networks 1/2006 Protocols A protocol is a set of rules and formats that govern the communication between communicating peers 30 set of valid messages meaning of each message Necessary for any function that requires cooperation between peers Introduction to Communication Networks 1/2006 Protocols A protocol provides a service – Peer entities use a protocol to provide a service to a higher-level peer entity – 31 For example: the post office protocol for reliable parcel transfer service for example, truck drivers use a protocol to present post offices with the abstraction of an unreliable parcel transfer service Introduction to Communication Networks 1/2006 Protocol Layers A network that provides many services needs many protocols Some services are independent, But others depend on each other A Protocol may use another protocol as a step in its execution – This form of dependency is called layering – 32 for example, ground transfer is one step in the execution of the example reliable parcel transfer protocol Post office handling is layered above parcel ground transfer protocol. Introduction to Communication Networks 1/2006 Layer Person delivery of parcel Post office counter handling Peer entities Ground transfer loading on trucks Airport transfer loading on airplane Airplane routing from source to destination 33 each layer implements a service via its own internal-layer actions relying on services provided by layer below Introduction to Communication Networks 1/2006 Advantages of Layering 34 Explicit structure allows identification & relationship of complex system’s pieces – layered reference model for discussion Modularization eases maintenance & updating of system – change of implementation of layer’s service transparent to rest of system Introduction to Communication Networks 1/2006 Open protocols and systems A set of protocols is open if – – A system that implements open protocols is called an open system International Organization for Standards (ISO) prescribes a standard to connect open systems – 35 protocol details are publicly available changes are managed by an organization whose membership and transactions are open to the public open system interconnect (OSI) Has greatly influenced thinking on protocol stacks Introduction to Communication Networks 1/2006 ISO OSI reference model Reference model – Service architecture – – 36 describes the services provided by each layer and the service access point (the interface with higher and lower layers) Protocol architecture – formally defines what is meant by a layer, a service etc. set of protocols that implement the service architecture compliant service architectures may still use non-compliant protocol architectures Grouping together all the protocols of the different layers is called: Protocol Stack. Introduction to Communication Networks 1/2006 The seven Layers Application Application Presentation Presentation Session Session Transport Transport Network Network Network Data Link Data Link Data Link Physical Physical Physical End system 37 Intermediate system Introduction to Communication Networks 1/2006 End system The seven Layers - protocol stack data Application Presentation Session Transport Network Data Link Physical AH PH data SH TH Network Data Link Physical data data data NH data DH+data+DT bits Application Presentation Session Transport Network Data Link Physical Session and presentation layers are not so important, and are often ignored 38 Introduction to Communication Networks 1/2006 Postal network Application: people using the postal system Session and presentation: – – 39 chief clerk sends some priority mail, and some by regular mail ; translator translates letters going abroad. Mail clerk sends a message, retransmits if not acked Postal system computes a route and forwards the letters Datalink layer: letters carried by planes, trains, automobiles Physical layer: the letter itself Introduction to Communication Networks 1/2006 Internet protocol stack 40 application: supporting network applications ftp, smtp, http transport: host-host data transfer tcp, udp network: routing of datagrams from source to destination ip, routing protocols link: data transfer between neighboring network elements Ethernet, wireless LAN physical: bits “on the wire” Introduction to Communication Networks 1/2006 application transport network link physical Protocol layering and data source M Ht M Hn Ht M Hl Hn Ht M 41 application transport network Link physical destination application Ht transport Hn Ht network Link Hl Hn Ht physical Introduction to Communication Networks 1/2006 M message M segment M M datagram frame Physical layer Moves bits between physically connected endsystems Standard prescribes – – – Internet – 42 coding scheme to represent a bit shapes and sizes of connectors bit-level synchronization technology to move bits on a wire, wireless link, satellite channel etc. Introduction to Communication Networks 1/2006 Datalink layer Reliable communication over a single link. Introduces the notion of a frame – Idle markers tell us that a link is not carrying a frame Begin and end markers delimit a frame Internet – – – 43 set of bits that belong together a variety of datalink layer protocols most common is Ethernet others are FDDI, SONET (fiber-optic), HDLC, wireless (802.11 – WiFi, 802.16 – WiMAX) Introduction to Communication Networks 1/2006 Datalink layer (contd.) 44 Ethernet (broadcast link) end-system must receive only bits meant for it need datalink-layer address also need to decide who gets to speak next these functions are provided by Medium Access sublayer (MAC) Datalink layer protocols are the first layer of software Very dependent on underlying physical link properties Usually bundle both physical and datalink Introduction to Communication Networks 1/2006 Network layer Carries data from source to destination. Logically concatenates a set of links to form the abstraction of an end-to-end link Allows an end-system to communicate with any other endsystem by computing a route between them Hides specific behavior of datalink layer Provides unique network-wide addresses Found both in end-systems and in intermediate systems 45 Introduction to Communication Networks 1/2006 Network layer types In datagram networks In connection-oriented network 46 provides both routing and data forwarding separate data plane and control plane data plane only forwards and schedules data (touches every byte) control plane responsible for routing, callestablishment, call-teardown (doesn’t touch data bytes) Introduction to Communication Networks 1/2006 Network layer (contd.) Internet 47 network layer is provided by Internet Protocol found in all end-systems and intermediate systems provides abstraction of end-to-end link segmentation and reassembly packet-forwarding, routing, scheduling unique IP addresses can be layered over anything, but only best-effort service Introduction to Communication Networks 1/2006 Network layer (contd.) 48 At end-systems primarily hides details of datalink layer segments and reassemble detects errors At intermediate systems – participates in routing protocol to create routing tables – responsible for forwarding packets – schedules the transmission order of packets – chooses which packets to drop Introduction to Communication Networks 1/2006 Transport layer Reliable end-to-end communication. creates the abstraction of an error-controlled, flowcontrolled and multiplexed end-to-end link (Network layer provides a ‘raw’ end-to-end service) Some transport layers provide fewer services e.g. simple error detection, no flow control, and no retransmission – Internet TCP provides error control, flow control, multiplexing UDP provides only multiplexing 49 Introduction to Communication Networks 1/2006 Transport layer (contd.) Error control – – Flow control – match transmission rate to rate currently sustainable on the path to destination, and at the destination itself Multiplexes multiple applications to the same end-toend connection – 50 message will reach destination despite packet loss, corruption and duplication retransmit lost packets; detect, discard, and retransmit corrupted packets; detect and discard duplicated packets adds an application-specific identifier (port number) so that receiving end-system can hand in incoming packet to the correct application Introduction to Communication Networks 1/2006 Session layer Not common Provides full-duplex service, expedited data delivery, and session synchronization Internet – 51 doesn’t have a standard session layer Introduction to Communication Networks 1/2006 Session layer (cont.) Duplex – Expedited data delivery – allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint Synchronization – 52 if transport layer is simplex, concatenates two transport endpoints together (keep track of whose turn it is) allows users to place marks in data stream and to roll back to a pre-specified mark (for example, resume option when downloading a big file) Introduction to Communication Networks 1/2006 Presentation layer Usually ad hoc Touches the application data (Unlike other layers which deal with headers) Hides data representation differences between applications – Can also encrypt data Internet – – 53 characters (ASCII, unicode, EBCDIC.) no standard presentation layer only defines network byte order for 2- and 4-byte integers Introduction to Communication Networks 1/2006 Application layer 54 The set of applications that use the network Doesn’t provide services to any other layer Introduction to Communication Networks 1/2006 Discussion 55 Layers break a complex problem into smaller, simpler pieces. Why seven layers? Introduction to Communication Networks 1/2006 Next Lecture Physical Layer – – Basic Principles Nyquist Theorem – Shanon Limit – 56 Relation between digital throughput and bandwidth Maximum capacity in presence of noise Basic concepts in encoding and modulation methods Introduction to Communication Networks 1/2006