1. Introduction to Communication Networks 1/2008 1 Introduction to Communication Networks Amit Dvir azdvir@bgu.ac.il Lecture 1 Introduction and Layering

2. Introduction to Communication Networks 1/2008 2 Course Information Lectures: – Tuesday 14:00 – 17:00, 90/227 Website: www.cse.bgu.ac.il/Courses/ www.cse.bgu.ac.il/Courses/ – Slides, Assignments, Messages Books: – Computer Networking: A Top-Down Approach Featuring the Internet / Kurose-Ross – Computer Networks / Tanenbaum – Data Networks / Bertsekas-Gallager

3. Introduction to Communication Networks 1/2008 3 Course Information Reception hours: – Tuesday 17:00-18:00, 37/306 Grading – 70% Final exam – 30% Assignments

4. Introduction to Communication Networks 1/2008 4 Course Objectives 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

5. Introduction to Communication Networks 1/2008 5 Simplified Communication Model

6. Introduction to Communication Networks 1/2008 6 Simplified Data Communication Model

7. Introduction to Communication Networks 1/2008 7 An Introduction to the mail system NickDave BGU TLV Admin (Some of the following slides are taken from lectures by Nick Mckeown, stanford)

8. Introduction to Communication Networks 1/2008 8  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? Characteristics of the mail system

9. Introduction to Communication Networks 1/2008 9 NickDave BGU TLV Admin Application Layer Transport Layer Network Layer Link Layer An Introduction to the mail system

10. Introduction to Communication Networks 1/2008 10 NickDave Leland.Stanford.edu Athena.MIT.edu Network Layer Link Layer Application Layer Transport Layer O.S. HeaderDataHeaderData Datagram An Introduction to the Internet

11. Introduction to Communication Networks 1/2008 11  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? Characteristics of the Internet

12. Introduction to Communication Networks 1/2008 12  No guarantee of integrity of data.  Packets can be fragmented.  Packets may be duplicated.  All of the above properties and questions are solved by today ’ s modern networks and communication systems ! Characteristics of the Internet (2)

13. Introduction to Communication Networks 1/2008 13 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 ” Possible errors before reaching to the destination

14. Introduction to Communication Networks 1/2008 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. Early communications systems

15. Introduction to Communication Networks 1/2008 15 Data Networks - Overview 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.

16. Introduction to Communication Networks 1/2008 16 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. Data Networks - Overview

17. Introduction to Communication Networks 1/2008 17 Overview – cont. 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. Data Networks - Overview

18. Introduction to Communication Networks 1/2008 18 Computer Networks Hardware Broadcast network: a single communication channel is shared by all computers: – sending a packet implies that all others receive it. Point-to-point network: Computers are connected in pairs: – sending a packet goes strictly from the sender to the receiver, possibly having to visit intermediate machines (routing). Computer Networks Hardware

19. Introduction to Communication Networks 1/2008 19 Transmission Technology

20. Introduction to Communication Networks 1/2008 20 Local Area Networks (LAN) 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.

21. Introduction to Communication Networks 1/2008 21 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?

22. Introduction to Communication Networks 1/2008 22 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 – For example, cover an area the size of a city A typical use of MANs to provide shared access to a wide area network

23. Introduction to Communication Networks 1/2008 23 Wide Area Networks (WAN) 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.

24. Introduction to Communication Networks 1/2008 24 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.

25. Introduction to Communication Networks 1/2008 25 Examples of WAN Topology

26. Introduction to Communication Networks 1/2008 26 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 (bus- based to token-based) within a department.

27. Introduction to Communication Networks 1/2008 27 Internetworks – cont. 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).

28. Introduction to Communication Networks 1/2008 28  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). Networking Issues - Internet

29. Introduction to Communication Networks 1/2008 29 Protocol Layers A way for organizing structure of network The idea: a series of steps

30. Introduction to Communication Networks 1/2008 30 Protocols A protocol is a set of rules and formats that govern the communication between communicating peers set of valid messages meaning of each message Necessary for any function that requires cooperation between peers

31. Introduction to Communication Networks 1/2008 31 A protocol provides a service – For example: the post office protocol for reliable parcel transfer service Peer entities use a protocol to provide a service to a higher-level peer entity – for example, truck drivers use a protocol to present post offices with the abstraction of an unreliable parcel transfer service Protocols

32. Introduction to Communication Networks 1/2008 32 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 – for example, ground transfer is one step in the execution of the example reliable parcel transfer protocol This form of dependency is called layering – Post office handling is layered above parcel ground transfer protocol.

33. Introduction to Communication Networks 1/2008 33 Person  delivery of parcel Post office  counter handling Ground transfer  loading on trucks Airport transfer  loading on airplane Airplane  routing from source to destination each layer implements a service via its own internal-layer actions relying on services provided by layer below Peer entities Layer

34. Introduction to Communication Networks 1/2008 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 Advantages of Layering

35. Introduction to Communication Networks 1/2008 35 Open protocols and systems A set of protocols is open if – protocol details are publicly available – changes are managed by an organization whose membership and transactions are open to the public A system that implements open protocols is called an open system International Organization for Standards (ISO) prescribes a standard to connect open systems – open system interconnect (OSI) Has greatly influenced thinking on protocol stacks

36. Introduction to Communication Networks 1/2008 36 ISO OSI reference model Reference model – formally defines what is meant by a layer, a service etc. Service architecture – describes the services provided by each layer and the service access point (the interface with higher and lower layers) Protocol architecture – 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.

37. Introduction to Communication Networks 1/2008 37 The seven Layers Presentation Application Session Transport Network Data Link Physical Presentation Application Session Transport Network Data Link Physical Network Data Link Physical End system Intermediate system

38. Introduction to Communication Networks 1/2008 38 Presentation Application Session Transport Network Data Link Physical Presentation Application Session Transport Network Data Link Physical data DH+data+DT bits data AH PH SH TH Network Data Link Physical dataNH Session and presentation layers are not so important, and are often ignored The seven Layers - protocol stack

39. Introduction to Communication Networks 1/2008 39 Postal network Application: people using the postal system Session and presentation: – 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

40. Introduction to Communication Networks 1/2008 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 ” application transport network link physical Internet protocol stack

41. Introduction to Communication Networks 1/2008 41 application transport network Link physical application transport network Link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame Protocol layering and data

42. Introduction to Communication Networks 1/2008 42 Physical layer Moves bits between physically connected end- systems Standard prescribes – coding scheme to represent a bit – shapes and sizes of connectors – bit-level synchronization Internet – technology to move bits on a wire, wireless link, satellite channel etc.

43. Introduction to Communication Networks 1/2008 43 Datalink layer Reliable communication over a single link. Introduces the notion of a frame – set of bits that belong together Idle markers tell us that a link is not carrying a frame Begin and end markers delimit a frame Internet – a variety of datalink layer protocols – most common is Ethernet – others are FDDI, SONET (fiber-optic), HDLC, wireless (802.11 – WiFi, 802.16 – WiMAX)

44. Introduction to Communication Networks 1/2008 44 Datalink layer (contd.) Datalink layer protocols are the first layer of software Very dependent on underlying physical link properties Usually bundle both physical and datalink 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)

45. Introduction to Communication Networks 1/2008 45 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 end- system 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

46. Introduction to Communication Networks 1/2008 46 Network layer types In datagram networks provides both routing and data forwarding In connection-oriented network separate data plane and control plane data plane only forwards and schedules data (touches every byte) control plane responsible for routing, call- establishment, call-teardown (doesn ’ t touch data bytes)

47. Introduction to Communication Networks 1/2008 47 Internet 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 Network layer (contd.)

48. Introduction to Communication Networks 1/2008 48 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 Network layer (contd.) At end-systems primarily hides details of datalink layer segments and reassemble detects errors

49. Introduction to Communication Networks 1/2008 49 Transport layer Reliable end-to-end communication. creates the abstraction of an error-controlled, flow- controlled 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

50. Introduction to Communication Networks 1/2008 50 Transport layer (contd.) Error control – message will reach destination despite packet loss, corruption and duplication – retransmit lost packets; detect, discard, and retransmit corrupted packets; detect and discard duplicated packets 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-to- end connection – adds an application-specific identifier (port number) so that receiving end-system can hand in incoming packet to the correct application

51. Introduction to Communication Networks 1/2008 51 Session layer Not common Provides full-duplex service, expedited data delivery, and session synchronization Internet – doesn ’ t have a standard session layer

52. Introduction to Communication Networks 1/2008 52 Duplex – if transport layer is simplex, concatenates two transport endpoints together (keep track of whose turn it is) Expedited data delivery – allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint Synchronization – 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) Session layer (cont.)

53. Introduction to Communication Networks 1/2008 53 Presentation layer Usually ad hoc Touches the application data (Unlike other layers which deal with headers) Hides data representation differences between applications – characters (ASCII, unicode, EBCDIC.) Can also encrypt data Internet – no standard presentation layer – only defines network byte order for 2- and 4-byte integers

54. Introduction to Communication Networks 1/2008 54 Application layer The set of applications that use the network Doesn ’ t provide services to any other layer

55. Introduction to Communication Networks 1/2008 55 Discussion Layers break a complex problem into smaller, simpler pieces. Why seven layers?

56. Introduction to Communication Networks 1/2008 56 Next Lecture Physical Layer – Basic Principles – Nyquist Theorem Relation between digital throughput and bandwidth – Shanon Limit Maximum capacity in presence of noise – Basic concepts in encoding and modulation methods