1. January 2000Communication Networks1 Computer Networks: Architecture & Concepts Abdulaziz Almulhem

2. January 2000Communication Networks2 Communication l People need to communicate with each other. l Why? To exchange information. l What? Voice, Sound, Graphics, Pictures, Text, or Data l One way is to use the postal service.

3. January 2000Communication Networks3 Communication (cont.) l Better, pull wires between each two people. (point-to-point communication) »cost is high and resources are wasted »complexity (mesh, drop and add) l We need to build a shared communication media (links) »Efficient »needs management (data may wait)

4. January 2000Communication Networks4 Communication (cont.) l We build communication networks. l These are arrangements of hardware and software that allow users to exchange information. (computer net) l Thus we have two types of nodes: »terminal nodes »communication nodes

5. January 2000Communication Networks5 Computer Network? l An interconnected collection of autonomous computers and computer resources Terminal node communication node Shared medium

6. January 2000Communication Networks6 Simple Data Communication Model Transceiver Transport System Digital Analog/Digital Digital 001101 Data Network Public Telephone Network

7. January 2000Communication Networks7 Network Services l The objective is to provide services to the users. »Information transportation »signaling »billing l A service is the execution of a sequence of basic actions on network resources. (how to place a phone call?)

8. January 2000Communication Networks8 Network Services (cont.) l Scripts are executed by the communicating entities only. l These scripts could be very complex. l It is more efficient to have a modular construction of scripts: »action it performs »interaction to other modules

9. January 2000Communication Networks9 Network Services (Cont.) l Advantages of such modular: »reusability »upgradability »interoperability l The general organization of services into simpler services is called the network architecture.

10. January 2000Communication Networks10 Communication Protocols l To provide error-free and convenient information transfers, the network communication is regulated by a set of rules and conventions called network protocols. l Protocols define connectors, cables, signals, data formats, error control techniques, and algorithms for message preparation, analysis and transfer.

11. January 2000Communication Networks11 Protocol Data Units (PDU) l Protocol entities exchange PDUs »Each PDU must contain two major parts: –Header: l Identifies how the following parts are to be handled and routed. –Message: l This is the message body itself. l This is where the protocol is determined to be character oriented or bit oriented.

12. January 2000Communication Networks12 OSI Reference Model of ISO l Architecture/structure that defines communication tasks and which would: »Serve as a reference model for international standards »would facilitate efficient internetworking among systems from different technologies, manufacturers, administrations, nationalities, and enterprises.

13. January 2000Communication Networks13 Reference Model

14. January 2000Communication Networks14 Most Important Standards Organizations l ITU-T: International Telecommunication Union (a United Nations specialized agency, was created on March 1, 1993) l ISO: International Organization for Standardization (an international voluntary, nontreaty organization, founded in 1946) l IETF: Internet Engineering Task Force (responsible for publishing RFCs (Requests For Comments)) l IEEE: Institute of Electrical and Electronic Engineers (ATM Forum: This organization is not a standard organization. After ITU defined the ATM concept in Nov 1990, ATM Forum was initiated in October 1991 to accelerate the deployment of ATM products and services. ATM Forum develops implementation agreements and publishes them as “specifications” on its web site: www.atmforum.com)

15. January 2000Communication Networks15 ISO OSI Reference Architecture l The architecture is layered to reduce complexity. »Each layer offers certain services to the layer immediately above it. »Each layer shields the higher layer from the details of implementation of how the services are offered. »Layer "n" on one station carries on a conversation with layer "n" on another network station.

16. January 2000Communication Networks16 7 Application ftp, telnet, email, www, etc. 6 Presentation Data representation 5 Session Negotiation and connection 4 Transport End-to-end delivery 3 Network Addresses and best path (routing) 2 Data Link Access to media (transfer of frames) 1 Physical Binary transmission and cabling Layer Functions

17. January 2000Communication Networks17 Application Application Presentation Presentation SessionSession Transport Transport NetworkNetwork Data LinkData Link PhysicalPhysical Layer Functions bits frames packets segments Host A Host B

18. January 2000Communication Networks18 Data Encapsulation Frames Bits Packets Data Salams 7. application 6. presentation 5. session 4. transport

19. January 2000Communication Networks19 Data Encapsulation Example segment data header network segment data header header data Frame Network Segment Data Frame header header header trailer Data Segment Packet Frame Bits 01111111010101101000100100010110101

20. January 2000Communication Networks20 Summary l Internetworking evolved to support current and future applications l The OSI reference model organizes network functions into seven layers l Data flows from upper-level user applications to lower-level bits transmitted over network media l Peer-to-peer functions use encapsulation and de- encapsulation at layer interfaces l Most network manager tasks configure the lower three layers

21. January 2000Communication Networks21 Interfaces and Services l Each layer in the OSI layered architecture has a function (service) to be provided to the upper layer through some interface (service access point). l The upper layer can request a service from the lower layer through SAP.

22. January 2000Communication Networks22 Interface and services (cont.) l When layer n+1 requests a service from layer n, Layer n will encapsulate the PDU of layer n+1 into a new PDU to be handed to layer n-1. l Services provided by each layer: »Connection-oriented (telephone) »Connectionless (post)

23. January 2000Communication Networks23 Service Primitives l Layer services are specified by a set of operations available to the layer above it. l These operations either: »Ask for the service »report the status of the service

24. January 2000Communication Networks24 Service Primitives (cont.) Layer n+1 Layer n Request Indication Response Confirm

25. January 2000Communication Networks25 Services and Protocols l Services are set of operations provided to upper layers or user. l Protocols are sets of rules governing how the same peers should be communicating. l Protocols are transparent from user and may change without changing services.

26. January 2000Communication Networks26 Protocols and Services Session Transport Network Session Transport Network Services Protocols

27. January 2000Communication Networks27 Application, Presentation, and Session Layers

28. January 2000Communication Networks28 Application Layer Computer Applications l Word Processing l Presentation Graphics l Spreadsheet l Database l Design/Manufacturing l Project Planning l Others Network Applications l Electronic mail l File Transfer l Remote Access l Client/Server Process l Information Location l Network Management l Others

29. January 2000Communication Networks29 Application Layer (cont.) Network Applications (For enterprise communication) l Electronic mail l File Transfer l Remote Access l Client/Server Process l Information Location l Network Management l Others Internetwork Applications (Extend beyond the enterprise) l Electronic Data Interchange l World Wide Web l E-mail Gateways l Special-Interest Bulletin Boards l Financial Transaction Services l Internet Navigation Utilities l Conferencing (Video, Voice, Data)

30. January 2000Communication Networks30 Presentation Layer l Text l Data »ASCII »EBCDIC »Encrypted l Sound l Video »MIDI (Musical Instrument Digital Interface) »MPEG (Motion Picture Experts Group) »QuickTime

31. January 2000Communication Networks31 Presentation Layer l Graphics l Visual Images »PICT(format to transfer QuickDraw graphics between Macintosh or PowerPC programs) »TIFF (Tagged Image File Format) »JPEG (Joint Photographic Experts Group) »GIF l Provides code formatting and conversion for applications

32. January 2000Communication Networks32 Session Layer l Coordinates applications as they interact on different hosts (dialogue control and synchronization) Service Request Service Reply

33. January 2000Communication Networks33 Session Layer (contd.) l Network File System (NFS) –Allows transparent access to remote network resources l Structured Query Language (SQL) l Remote-Procedure Call (RPC) –RPC procedures are built on clients and executed on servers l X Window System –Allows intelligent terminals to communicate with remote UNIX machines l AppleTalk Session Protocol (ASP) –Establishes and maintains sessions between an AppleTalk client and server l DNA Session Control Protocol (SCP)

34. January 2000Communication Networks34 Transport & Network Layers

35. January 2000Communication Networks35 Transport Layer Overview l Segments upper-layer PDUs l Establishes an end-to-end connection l Sends segments from one end host to another l Ensures end-to-end data reliability

36. January 2000Communication Networks36 Segment Upper-Layer PDUs l Transport segments share traffic stream Application Data Application Data port port Electronic mail File transfer Terminal session Application Presentation Session Transport

37. January 2000Communication Networks37 Establishes Connection sender receiver synchronize Negotiate connection synchronize Acknowledge Connection established Data transfer (send segments )

38. January 2000Communication Networks38 Establishes Connection sender receiver transmit not ready ready Resume Transmission Buffer full process segments Buffer OK

39. January 2000Communication Networks39 Reliability with Windowing l In the most basic form of reliable connection-oriented transfer, data segments must be delivered to the recipient in the same sequence that they were transmitted. l Windowing is a method to control the amount of information transferred end-to- end. Some protocols measure information in terms of number of packets

40. January 2000Communication Networks40 Reliability with Windowing senderreceiversender receiver Send 1 Send 2 Send 1 Send 2 Send 3 Window size 1 Window size 3 Receive 1 Receive 2 Receive 3 Receive 1 Receive 2 ACK 2 ACK 3 ACK 4 Send 4

41. January 2000Communication Networks41 PAR Technique l Reliable delivery guarantees that a stream of data sent from one machine will be delivered through a functioning data link to another machine without duplication or data loss. Positive acknowledgement with retransmission is one technique that guarantees reliable delivery of data streams. l The sender keeps the record of each segment it sends and waits for an acknowledgement. l The sender also starts a timer when it sends a segment, and it retransmits a segment it the timer expires before an acknowledgement arrives.

42. January 2000Communication Networks42 PAR Technique (contd.) send 1 send 2 send 3 Ack 4 send 4 send 5 send 6 Ack 5 send 5 Ack 7 sender receiver 1 2 3 4 5 6 X

43. January 2000Communication Networks43 Transport to Network Layer End-to-end segments Routed packets

44. January 2000Communication Networks44 Network Layer l Controlling the operation of the network. l How to route data from source to destination? »Static routing tables »Dynamic routing tables l Datagram is similar to a postal letter.

45. January 2000Communication Networks45 Summary l Presentation layer formats and converts network application data to represent text, graphics, images, video, and audio. l Session-layer functions coordinate communication interactions between applications. l Reliable transport-layer functions include »Multiplexing »Flow control »Error recovery »Reliability through windowing

46. January 2000Communication Networks46 Physical and Data Link Layers

47. January 2000Communication Networks47 Physical and Data-Link Standards l The data link layer provides data transport across a physical link. To do so, the data link layer handles physical addressing, network topology, line discipline, error notification, orderly delivery of frames, and optional flow control. l The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link between end systems. l These requirements and characteristics are codified into standards.

48. January 2000Communication Networks48 LAN Data-Link Sublayers Logical Link Control Media Access Control MAC Frame 802.2 LLC Packet or datagram Network Physical Data Link LLC MAC

49. January 2000Communication Networks49 LAN Data-Link Sublayers l LLC refers upward to higher-layer software functions. l MAC refers downward to lower-layer hardware functions. l LAN protocols occupy the bottom two layers of OSI reference model: the physical layer and data link layer.

50. January 2000Communication Networks50 LAN Data-Link Sublayers l The IEEE 802 committee subdivided the data link layer into two sublayers: »The logical link control (LLC) sublayer »The media access control (MAC) sublayer l The LLC sublayer provides for environments that need connectionless or connection- oriented services and the data link layer. l The MAC sublayer provides access to the LAN medium in an orderly manner.

51. January 2000Communication Networks51 LLC Sublayer Functions l Enable upper layers to gain independence over LAN media access. l Allow service access points (SAPs) from interface sublayers to upper-layer functions. l Provide optional connection, flow control, and sequencing services.

52. January 2000Communication Networks52 Client-Server Model

53. January 2000Communication Networks53 Client Server Model l Client-Server paradigm is the primary pattern of interactions among cooperating applications. l This model constitutes the foundation on which distributed algorithms are built.

54. January 2000Communication Networks54 What is the Client-Server Paradigm? l The paradigm divides communicating applications into 2 broad categories, depending on whether the application waits for communication or initiates it. »An application that initiates a communication is called a client. »End users usually invoke a client software when they use a network service.

55. January 2000Communication Networks55 Client Server Model (cont.) l Server: Any program that offers a service reachable over the network »If a machine’s primary purpose is to support a particular server program, the term server is usually applied to both, the machine and the server program l Client: An executing program becomes a client when it sends a request to a server and waits for a response

56. January 2000Communication Networks56 Client Server Model (cont.) l A server is any program that waits for incoming communication requests from a client. »Each time a client application needs to contact a server, it sends a request and awaits a response. »The server receives a client’s request, performs the necessary computation, and returns the result to the client. »When the response arrives at the client, the client continues processing.

57. January 2000Communication Networks57 Client Server Model (cont.) Request Reply Machine Running Client Application Machine Running Server Application Client Program Server Program

58. January 2000Communication Networks58 Client Server Model (cont.) l A Misconception: »Technically, a server is a program and not a piece of hardware. »However, computer users frequently (mis)apply the term to the computer responsible for running a particular server program. –For example, Web Server, is usually a computer running the http server program.

59. January 2000Communication Networks59 Summary l Internetworking evolved to support current and future applications. l The OSI reference model organizes network functions into seven layers. l Data flows from upper-level user applications to lower-level bits transmitted over network media. l Peer-to-peer functions use encapsulation and de- encapsulation at layer interfaces. l Client-Server paradigm constitutes the foundation on which distributed algorithms are built.