Computer Networks

1. Computer Networks A.S. Tanenbaum 고려대 전산학과 1998.9 조 충 호

2. Chapter 1 Introduction • Computer network - An interconnected group of independent computers and data terminals which communicate with one another. • Why do we need a network? • Cost of processing • # of computer • volume/frequency of information exchange • Method of information exchange • transport of physical media • magnetic tape • floppy disk • potical disk

3. Computer Networks vs Distributed Systems • CN • Interconnected collection of autonomous computers to exchange information • Not master and slave • DS • the existence of multiple autonomous computers is transparent to the users • a special case of CN

4. Uses of Computer Networks • Networks for Companies • resource sharing • high reliability - multiple CPU • saving money - client/server model • Networks for peoples • Access to remote information • Person-to-person communication • Interactive entertainment • Social Issues • child pornography • security

5. Applications of computer networks • Resource sharing - processor, program, storage, printer, plotter etc. • Access to remote data bases - library books, weather information, reservation services. • Electronic mail - majority of the first class mail is generated by computers. • Electronic news • Teleconferencing • Electronic fund transfer • Computer aided training/education

6. 1.2 Network Hardware • Broadcast Network • allow the possibility of addressing a packet to all destination by using a special code in the address field. - Bus , radio network - ground radio, satellite, optical star • Multicast Network : a subset of machines • Point-to-point networks • connections b/w individual pairs of machines • Classification of interconnection processors by scale

7. Computer Computer dedicated or dial-up link Point-to-Point networks • It is expensive to provide direct links between all pair of nodes.(link, transceiver cost) • # of full-duplex links of the fully-connected one hop network with N nodes N(N-1)/2. • eg. N=10, # of links = 45 • N = 100, # of links 5000.

8. computer computer switch computer computer computer computer computer Switched Network • Switched network • communications subnetwork • Links and switching facilities are to be shared among the computers.

9. Switching techniques • Circuit (line) switching - eg. telephone system • A path from a source to a destination is established before a message is transmitted. • All links along the path between the source and the destination are used during the entire transmission. • When the traffic is bursty (the use of links is sporadic) it tends to poorly use the network resources (links and switches). • Packet switching • A message is segmented into blocks of data, called packet. • Packets are transmitted without prior reservation of link capacity. When a packet is received at a switch, it is inspected to determine the appropriate output link. If the output link is available, it is transmitted. Otherwise, it is stored and then forwarded to the next switch on its way to the destination. (Store-&-Forward network) • Packet header - contains address and sequence #. • Message is reassembled at the destination. • Dynamic sharing of network resources is possible. But because of statistical fluctuation of demand for network resources, buffer overflow at the switch (finite buffer) and delay variation (queneing delay) occur.

10. computer computer switch computer computer computer computer computer Store-&-forward network • computer, host : application program • packet switch, node, IMP (interface message processor) - provides access to subnet acts as a store and forward node • topology design • capacity assignment • routing, congestion, flow control

11. LAN(Local Area Network) • classification • by size • by transmission techniques • traditional LAN : 10 to 100 Mbps • new LAN : up to hundreds of Megabits/sec • topology • broadcast LAN • ring (IEEE 802.5, IBM token ring) and bus (IEEE 802.3 Ethernet) • statistic and dynamic channel allocation • statistic : time slot or round robin • dynamic : distributed or centralized • Point-to-point LAN

12. MAN (Metropolitan Area Network) • a bigger version of LAN and similar technology • support data and voice • have a one or two cable, not contain the switching element • DQDB(Distributed Queue Dual Bus) :IEEE802.6

13. WAN (Wide Area Network) • The hosts are connected by a communication subnet • The subnet consists of transmission line and switching elements (packet switching nodes) • Relation b/w hosts and the subnets

14. WAN - topology for PtoP subnet tree star ring complete mesh Intersecting ring Irregular ring

15. Wireless Network • wireless networks and mobile computing • combination of wired and wireless networking • CDPD(Cellular Digital Packet Data)

16. Internet • A collection of interconnected networks • 국내 인터넷

17. 1.3 Network Software • Protocol Hierarchies • protocol - a set of rules and conventions by which physically separated entities interact. • network architecture - a set of layers and protocols • layering - achieves a functional modularity • eg. computer system

18. Layers, Protocols and interface

19. Translator example

20. Information flow in layers

21. Example example (A,B) peer processes -- interaction between peer processes is governed by a protocol. (iA, IB) (tA, tB) set up a connection maintain a dialog terminate a connection (A,B) --layer 3 protocol -- -- independent of lower layer protocol.

23. IDU Layer N+1 ICI (N+1)- PDU SAP: Service Access Point IDU : Interface Data Unit PDU: Protocol Data Unit SDU : Service Data Unit ICI : Interface Control Information PCI : Protocol Control Information interface Layer N (N)-SDU ICI header to/from other layer N entities (N)-PCI (N)-PDU Interface and Services SAP Relation b/w layers and interfaces

24. Connection oriented & connectionless • Connection-oriented operation - logical connection (virtual circuit) is setup between peer entities prior to data exchange • ordered delivery of data units, flow/error control is provided. • connectionless operation - each data unit is independent of previous and subsequent data units • small overhead, best effort delivery of data units. • efficiency, reliability (service quality) trade-off

25. Service Primitives 1/2 • A Service id formally specified by a set of primitives(operations) available to user or other entity to access the service • 4 classes • Primitives can have parameters • CONNECT.request (type of service desired , max message size,etc) • Services can be either confirmed or unconfirmed

26. Service User (N+1 layer entity) Service User (N+1 layer entity) Service provider (N-1 layer entity) Request Indication Confirmed service Response Confirm Request Unconfirmed service Indication Service Primitives 2/2 • Each entity communicates with entities in the layers above and below it across an interface. The interface is realized as one or mode service access point(SAP). The (N-1) entity provides services to an (N) entity via one of the service primitives.

27. Example of CO service 1. CONNECT.request - You dial ANNE’s phone number 2. CONNECT.indication- Her phone ring 3. CONNECT.response - She picks up the phone 4. CONNECT.confirm - You hear the ringing stop 5. DATA.request - She hear your invitation 6. DATA.indication - She hears your invitation 7. DATA.request - She say OK 8. DATA.indication - You hear her acceptance 9.DISCONNECT.request - you hang up the phone 10. DISCONNECT.indication -she hear it and hang up

28. Relationship of Service to protocols • Service • a set of primitives that a layer provides to the layer above it • relates to an interface b/w 2 layers (service provider - lower layer , service user - upper layer) • Protocol • a set of rules governing the format and meaning of the frame, packets, or messages that are exchanged by the peer entities within layer • Entities use protocols in order to implement their service definition • Two aspects of communication: • Peer to peer - protocol - conceptual understanding. • Layer N and layer N-1 entities - interface - practical implementation. • Note: • Peer to peer protocol is achieved using the services provided by their lower layer entities. • Communication functions are partitioned into a vertical set of layers. • Ideally, upper layer entities are independent of the details of the lower layer entities => easy to modify protocol at one layer independent of other layers.

29. Reference model • Open System Interconnection(OSI) Reference Model Principles used: • Create a layer when a different level of abstraction is needed. • Each layer performs well defined functions. • The functions of each layer should be chosen to facilitate protocol design. • The amount of information flow across interface between adjacent layers should be minimized.

30. OSI reference model 1/5

31. OSI reference model 2/5 • Physical layer • Provides a link for transmitting a sequence of bits between any pair of noded joined by a physical communication channel. • Maps incoming bits from data link layer into signals appropriate to the channel - modulation/demodulation. • defines standards for: • mechanical - dimensions of connector • electrical - voltage level • functional - data, control, timing • procedural - sequence of events • eg. RS-232C, physical layer of X.21

32. OSI reference model 3/5 • Data link layer • Converts the unreliable bit pipe at layer 1 into a higher level, virtual communication link for sending packets asynchronously but error free. • Error control. • Flow control - coordinates transmitter and receiver. • eg. HDLC, LAP-B, LAP-D • For a multi-access/broadcast network, an additional function is needed to coordinate the access to the common channel among the competing users • medium access control (MAC). • MAC involves >- 2 nodes.

33. OSI reference model 4/5 • Network layer • Provides a ‘transparent pipe‘ for transport layer. • Routing(switching) • Congestion/flow control. • It involves >- 2 nodes. • eg. X.25 layer 3 • Transport layer - (end-to-end protocol) • Provides a reliable delivery of data units (error free, in sequence, no loss or duplication), • Segmentation/reassembly. • End to end error recovery. • Depending on the type of services it receives from layer 3, various transport layer protocols are defined.

34. OSI reference model 5/5 • Session layer • Controls the dialogue between applications. • Allows two applications to establish and use a connection (session). • Authentication, billing. • Presentation layer • Resolves differences in data representation. - code conversion • Provides data transformation - text compression, encryption. • Application layer : eg. file transfer protocol (FTP), e-mail

35. Data Transmission in the OSI model

36. TCP/IP reference model • ARPANET by DoD(Department of Defence) • using leased telephoneline • adding satellite and radio network => trouble interworking • TCP/IP reference model • OSITCP/IP Application Application Presentation Not present Session Transport Transport Network Internet Data Link Host-to-Network Physical

37. Internet Transport Layer • IP(Internet Protocol) • Packet-switching network based on connectionless internetwork layer • packetrouting routing to avoid congestion • TCP(Transmission Control Protocol) • reliable connection oriented protocol • UDP(User Datagram Protocol) • unreliable, connectionless protocol

38. TCP/IP Model TELNET FTP SMTP DNS Application TCP UDP Transport IP Network Data Link ARPANET SATNET Packet radio LAN Physical

39. OSI vs TCP/IP • OSI model central concepts • Services : what the layer does, not how entities above it access it or the how the layer work • Interfaces : the process above it how to access it • Protocols : the peer protocol used in a layer are the layer’s own business • TCP/IP not clearly distinguish b/w service, interface, protocol • The protocols in the OSI model are better hidden than in the TCP/IP model • The OSI model was devised before the protocols were invented, TCP/IP is reversed • OSI- NL (connectionless, connection oriented), TL(connection oriented only) TCP/IP - IP(connectionless) TCP(connectionless, connection oriented)

40. A Critique of the OSI model and TCP/IP • OSI • Bad timing • Bad technology • Bad Implementations • Bad Politics • TCP/IP • not clearly distinguish the concepts of service • for data traffic

41. 1.5 Example Networks • Novell NetWare - Xerox Network System • The ARPANET - DoD • NSFNET - backbone • Internet • Internet II • 정의 • 연구 및 교육기관 중심의 차세대 인터넷 프로젝트 • 등장배경 • 인터넷의 성장 및 저속의 트래픽 • 멀티미디어 데이터량의 폭발적 증가 • 분산된 자원의 통합 필요성 • IP규약의 확장성 제한 및 구현시 구조적 병목현상 대두

42. Internet II • GigaPOP에 의한 동적 QoS 보장 • 새로운 기술 및 기존 기술의 복합체 • 다음 세대의 상업적 인터넷 서비스 제공을 위한 서비스 및 기술의 새로운 집합체 개념 • internet 2 개발 프로젝트 구성원이나 서비스 제공자간의 서비스 전송 및 연동의 접속점 • ATM 또는 SONET 서비스를 통한 연결 • 매우 넓은 대역폭을 가짐(T-3, OC-3, OC-12) • 차세대 인터넷 Testbed • 미국 : vBNS • 캐나다 : Canet II • 유럽 : TEN-34 • 아시아 : APAN

43. Example of Data Communication Services • SMDS-Switched Multimegabit Data Service • Connected Multiple LAN using DQDB (45Mbps) • SMDS packet format

44. X.25 X.25 Internal network operation not specified by x.25 Packet Assembler/ Disassemblera(PAD) X.29 Terminal(Asych) X.25 Network • X.25 standard governs interface between user computer & packet-switched Network. • Dates from 1976, with updates every four years. • Very common standard for public data networks and value-added networks throughout the world. • Provides connection-oriented(virtual circuit) service. • Accommodates speeds up to 56Kbps.

45. Flag Flag Address Address Flag Flag Control Control Data Field Data Field CRC CRC 0 SEQ No. P/F SEQ No. 1 op P/F Supervisory frame 0 SEQ No. (RR,RNR,REJ,SREJ) op P/F op 1 1 LAPB - HDLC frame in X.25 (n*8) (8 or 16) (16 or 32) (8) (n*8) Information frame Unnumbered frame LAP-D 0 C/R SAPI 1 TEI TEI: Terminal Endpoint Id. SAPI: SAP Id Frame-Relay DLCI ( DL Connection Id.) identifies the connection and Multiplexing (16 or 32) Flag Address DLCI Flag Data Field CRC

46. Frame Relay was defined for packet transmission n over telephone company fiber-optic links It provides a high speed alternative to X.25 FR differs from X.25 n A variation of LAPD X.25: 3 layer Packet multiplexing FR: 2 layer frame multiplexing Separate 2 plane : control plane : signaling user plane : data transfer FR is based on 2 assumption n Communications have a very low BER Communications are b/w 2 intelligent device(Error recovery) Frame Relay

47. Service provided by Frame Relay • Connection Oriented Service • Preservation of order • No duplication packets • Some differences • A few lost packets • No flow control • Up to 2 Mbps • FR is one form of “fast packet switching” • Minimal processing at intermediate nodes • No Network Layer processing required

48. Broadband ISDN and ATM • Broadband Integrated Services Digital Network • 2 Mbit/s 이상의 대역폭 전달 • 특징 • ITU-T, ATM Forum 에서 표준화 • WAN, MAN용 기반 구조 • B-ISDN은 SONET/SDH망을 통해 전송되는 ATM • ATM : 유연, 통합서비스, 고속 스위칭 • SONET/SDH : 고속, 관리가 용이한 점대점 전달

49. B-ISDN으로 진화 • 기존 통신망 • POTS • X.25 • TV (CATV) network • LAN (IEEE 802 series) • ISDN • 주요 문제점 • Service Dependence of Information Types, Equipments • Inflexibility on Audio, Video, Speech coding • Inefficiency on resources of multiple networks

50. B-ISDN으로 진화 (계속) • N-ISDN의 확장 • N-ISDN은 기존 음성 중심 통신망 요구사항을 지원 • B-ISDN은 N-ISDN 기능을 지원 • 기존의 기술 및 신규 기술의 접목 • SS7, N-ISDN, X.25 • 광선로 기술 • ATM 고속 스위칭 • SONET/SDH • 광대역화 • 서비스 영역 • 신호 종류 • 대역폭