通信网络技术课件：chapter 1 Introduction

1、Computer Networks1RoadmapIntroductionPhysical LayerData Link LayerTransport LayerNetwork LayerMedium Access SublayerApplication Layer2IntroductionChapter 13TopicsSome of the overall issues well be dealing with in this course.1.1 Uses of computer networks1.2 Network Hardware1.3 Network Software1.4 Re

2、ference Models1.5 Example Networks1.6 Network Standardization4A Computer Network Communication Link51.1 Uses of Computer NetworksBusiness ApplicationsHome ApplicationsMobile UsersSocial Issues6Business ApplicationsResource Sharing:the goal is to make all programs, equipment, and especially data avai

3、lable to anyone on the network without regard to the physical location of the resource and the user. Equipments: Printers, Scanner, CD-BurnersInformation:Customer records, inventories, accounts receivable, financial statements, tax information 7A network with two clients and one server.Client machin

4、e and Server machine8Client/Server modelThe client/server model involves requests and replies.9A Computer network to do with peopleA second goal of setting up a computer network has to do with people rather than information or even computers. A computer network can provide a powerful communication m

5、edium among employees.EmailCooperate with other to workVideoconferencing10E-CommerceB2B:A third goal for increasingly many companies is doing business electronically with other companies, especially suppliers and customers. B2C:A fourth goal that is starting to become more important is doing busines

6、s with consumers over the Internet. 11Home Network ApplicationsAccess to remote informationPerson-to-person communicationInteractive entertainmentElectronic commerce12Peer to PeerIn peer-to-peer system there are no fixed clients and servers.13E-CommerceSome forms of e-commerce.14Mobile Network Users

7、Combinations of wireless networks and mobile computing.15Social IssuesNewsgroups and BBS;Topics that people actually care aboutEmployee rights vs employer rightsGovernment vs citizenHacker and criminalCopyright protection161.2 Network HardwareLocal Area NetworksMetropolitan Area NetworksWide Area Ne

8、tworksWireless NetworksHome NetworksInternetworks17Network HardwareClassifying networks By transmission technologyBy scale18Transmission technologyTypes of transmission technologyBroadcast NetworksPoint-to-point Networks19Broadcast NetworksBroadcastBroadcast networks have a single communication chan

9、nel that is shared by all the machines on the network. Packets ( short messages) sent by any machine are received by all the others (or a subset ). An address field within the packet specifies the intended recipient. Upon receiving a packet, a machine checks the address field. If the packet is inten

10、ded for the receiving machine, that machine processes the packet; if the packet is intended for some other machine, it is just ignored. 20Broadcast NetworksBroadcastingaddress a packet to all destinations by using a special code in the address field .MulticastingSome broadcast systems also support t

11、ransmission to a subset of the machines 21Point-to-point NetworksPoint-to-point networks consist of many connections between individual pairs of machines. Unicasting Individual connections between pairs of machines. To go from the source to the destination, a packet on this type of network may have

12、to first visit one or more intermediate machines.22Classification by scale.23Local Area NetworksCharacteristics Sizetransmission technology topology 24Local Area Networks (2)Two broadcast networks(a) Bus (IEEE 802.3, Ethernet)(b) Ring (IEEE 802.5, Token Ring)25About the IEEEInstitute of Electrical a

13、nd Electronics Engineers (Eye-triple-E)IEEE 802 LAN/MAN Standards Committee /26Local Area Networks (3)Channel Allocation methodsStatic allocation divide time into discrete intervals and use a round-robin algorithm, allowing each machine to broadcast only when its time slot comes up. Dynamic allocati

14、on Centralizedthere is a single entity which determines who goes next. Decentralized there is no central entity , each machine must decide for itself whether to transmit. 27Metropolitan Area NetworksA metropolitan area network based on cable TV.Cable TV System to MAN28Wide Area NetworksRelation betw

15、een hosts on LANs and the subnet.29Wide Area Networks (2)Communication subnet transmission lines copper wire, optical fiber, or even radio links. switching elements specialized computers called routers 30Wide Area Networks (3)A stream of packets from sender to receiver.31Wide Area Networks (4)Packet

16、 / Cellthe message is cut into packets before sendingPacket-switched / store-and-forward the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and then forwarded. Routing algorithm Routing decisions are made locally. 32Wireless Netwo

17、rksCategories of wireless networks:System interconnectionWireless LANsWireless WANs33Wireless Networks (2)(a) Bluetooth configuration( IEEE802.15 )(b) Wireless LAN (Wi-Fi IEEE 802.11, LMDS IEEE 802.16 )34Wireless Networks (3)(a) Individual mobile computers(b) A flying LAN35Home Network CategoriesCom

18、puters (desktop PC, PDA, shared peripheralsEntertainment (TV, DVD, VCR, camera, stereo, MP3)Telecomm (telephone, cell phone, intercom, fax)Appliances (microwave, fridge, clock, furnace, airco)Telemetry (utility meter, burglar alarm, babycam).36InternetInternetworks:Collection of interconnected netwo

19、ernet (lower case i) is generic term.Internet (upper case I) is worldwide connection to all kinds of machines.371.3 Network SoftwareProtocol HierarchiesDesign Issues for the LayersConnection-Oriented and Connectionless ServicesService PrimitivesThe Relationship of Services to Protocols38Proto

20、col HierarchiesThe philosophy of connecting together two entities:“Layering” is the key word.To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. 39Human protocols:“whats the time?”“How are you doing?”“I have a question”

21、- Specific messages sent- Specific actions taken when messages received, or other eventsWhat Is A ProtocolNetwork protocols:Machines rather than humansAll communication activity in network governed by protocols A protocol is an agreement between communicating parties on how communication is to proce

22、ed. Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt 40Human NetworkProtocol ExampleTCP connection requestTCP connection replySNTP requestSNTP response“Got the time?”“Hello”“Hello”“Its 10:30”41Hierarchy ExampleNam

23、eAddressPost OfficePost OfficeNameAddress42Protocol HierarchiesLayers, protocols, and interfaces.43Layers: Network software is organized functionally into levels. A level on one host talks to the same level on another host (its peer).Protocol:The protocol is the convention or standard that a layer u

24、ses to talk to the same layer at the other end. An agreement or standard on the conversation.Interface:Defines the services that one layer offers another (either up or down.)Philosophy Important to keep it simple and clean to reduce complexity.Important that each layer perform specific actions.Termi

25、nology44TerminologyNetwork architecture:A set of layers and protocols. It contains details on what happens in the layer and what the layers says to its peer.Functional interfaces and implementation details are not part of the spec, since thats not visible outside the machine.Protocol stack: A list o

26、f protocols used by a system, one protocol per layer.Header: Control information attached to a message45Protocol Hierarchies (2)The philosopher-translator-secretary architecture.46Protocol Hierarchies (3)Example information flow supporting virtual communication in layer 5.SendtoPeer rather than “Cal

27、lNextLayerDown.47TCP/IP Hierarchies UDPTCPFTPHTTPIPICMPIGMPARPEthernetATMUDPTCPFTPHTTPIPICMPIGMPARPEthernetATMDataApplicationDataFTPHTTPTransportDataUDPTCPFTPHTTPInternetDataUDPTCPIPICMPIGMPARPPreambleDataCRCIPICMPIGMPARPEthernetATMDataEthernetATMDataEthernetATMDataIPICMPIGMPARPEthernetATMDataUDPTCP

28、IPICMPIGMPARPDataUDPTCPFTPHTTPPreambleInternetTransportApplicationDataCRCFTPHTTP48Software or hardware?Although this section is called network software”, it is worth pointing out that the lower layers of a protocol hierarchy are frequently implemented in hardware or firmware. Nevertheless, complex p

29、rotocol algorithms are involved, even if they are embedded (in whole or in part) in hardware. 49Design Issues for the LayersAddressingError ControlFlow ControlMultiplexingRouting50Connection-Oriented and Connectionless ServicesConnection oriented service:Like the telephone system . The system establ

30、ishes a connection, uses it, and closes it. Acts like a tube. Data comes out the other end in the same order as it goes in.Connection SetupData TransferConnection TerminationConnectionless service -Like the postal system . Each message has the entire address on it. Each message may follow a differen

31、t route to it destination. Ordering not maintained.Data Transfer51Quality of service:Will the message arrive? A reliable connection-oriented service guarantees success. Message sequence - message boundaries and order are maintained. Byte streams - messages are broken up or combined; flow is bytes. C

32、an pair mechanism with upper-layer requirements.Connectionless Service /Datagram Service:Like junk mail. Its not worth the cost to determine if it actually arrived. Needs a high probability of arrival, but 100% not required. Connectionless, no acknowledgment.Acknowledged datagram service:As above, b

33、ut improved reliability via acknowledgment.Request-reply service:Acknowledgment is in the form of a reply.Connection-Oriented/Connectionless Services52Connection-Oriented and Connectionless ServicesSix different types of service.53Service PrimitivesA service is formally specified by a set of primiti

34、ves (operations) available to a user process to access the service. These primitives tell the service to perform some action or report on an action taken by a peer entity. If the protocol stack is located in the operating system, as it often is, the primitives are normally system calls 54Service Pri

35、mitivesFive service primitives for implementing a simple connection-oriented service.55Service Primitives Packets sent in a simple client-server interaction on a connection-oriented network.( but life is not so simple)56Services to Protocols RelationshipA service is a set of primitives (operations)

36、that a layer provides to the layer above it. The service defines what operations the layer is prepared to perform on behalf of its users, but it says nothing at all about how these operations are implemented. A protocol, in contrast, is a set of rules governing the format and meaning of the packets,

37、 or messages that are exchanged by the peer entities within a layer. Entities use protocols to implement their service definitions. 57The relationship between a service and a protocol. In other words, services relate to the interfaces between layers,. In contrast, protocols relate to the packets sen

38、t between peer entities on different machines. 58 1.4 Reference ModelsThe OSI Reference ModelThe TCP/IP Reference ModelA Comparison of OSI and TCP/IP A Critique of the OSI Model and ProtocolsA Critique of the TCP/IP Reference Model59Developed by ISO: International Standards Organization(1983,revised

39、 in 1995)Principles used to develop OSI Layering:1. Need a layer for each different level of abstraction.2. Each layer performs a well defined function.3. Each layer should be standardizable.4. Layer boundaries should minimize data flow across those boundaries.5. The right number of layers - dont pu

40、t too many functions together, but not too many layers either.The OSI Reference Model60Reference ModelsThe OSI reference model.61The OSI Reference ModelPhysical Layer: Purpose - Transmits raw bits across a medium. Electrical - Concerns are voltage, timing, duplexing, connectors, etc.Data Link Layer:

41、 Framing - Breaks apart messages into frames. Reassembles frames into messages. Error handling - solves damaged, lost, and duplicate frames. Flow control - keeps a fast transmitter from flooding a slow receiver. Gaining Access - if many hosts have usage of the medium, how is access arbitrated.62The

42、OSI Reference ModelNetwork Layer: Routing - What path is followed by packets from source to destination. Can be based on a static table, when the connection is created, or when each packet is sent. Congestion - Controls the number packets in the subnet. Accounting - Counts packets/bytes for billing

43、purposes. Heterogeneity - Interfacing so one type of network can talk to another.63The OSI Reference ModelTransport Layer: Reliability - Ensures that packets arrive at their destination. Reassembles out of order messages. Hides network - Allows details of the network to be hidden from higher level l

44、ayers. Service Decisions - What type of service to provide; error-free point to point, datagram, etc. Mapping - Determines which messages belong to which connections. Naming - Send to node xyz must be translated into an internal address and route. Flow control - keeps a fast transmitter from floodin

45、g a slow receiver.64The OSI Reference ModelSession Layer: Sessions - Provides services that span a particular message. For instance, a login session could be logged. Synchronization - Provide way to subdivide a long mechanism for reliability.Presentation Layer: Prettiness - Syntax and semantics of i

46、nformation transmitted. Understands the nature of the data being transmitted. Converts ASCII/EBCDIC, big endian/little endianApplication Layer: it contains a variety of protocols that are commonly needed by users. ftp ,http65 The TCP/IP Reference ModelsU.S. DoD ARPANET (1974,1985,1988)was a research

47、 network sponsored by the DoD network be able to survive loss of subnet hardware, with existing conversations not being broken off .a flexible architecture was needed since applications with divergent requirements were envisioned, ranging from transferring files to real-time speech transmission 66Th

48、e Internet Layer a packet-switching network based on a connectionless internetwork layer 67 The TCP/IP Reference ModelsThe TCP/IP reference model68The Internet LayerHost to Network Layer:This lowest level is not defined in this model. Various mechanisms abound.Internet Layer:Connector - Provides pac

49、ket switched connectionless service.Routing - The IP (Internet Protocol) does delivery and congestion control.Used in the Arpanet and in the Internet. Common mechanism that is gaining on/surpassing the OSI Model. 69 The TCP/IP Reference ModelsTransport Layer:Allows peer entities to communicate. TCP

50、- Transmission Control Protocol provides a reliable connection oriented protocol that delivers a byte stream from one node to another. Guarantees delivery and provides flow control. UDP - User Datagram Protocol provides an unreliable connection-less protocol for applications that provide their own.A

51、pplication Layer:Terminal - TelnetFile transfer - FTPThe Web - HTTP70 The TCP/IP Reference ModelsProtocols and networks in the TCP/IP model initially.71Comparing OSI and TCP/IP ModelsConcepts central to the OSI modelServicesInterfacesProtocols72Comparing OSI and TCP/IP ModelsOSI has good definition

52、of service, interface, and protocol as discussed before. Fits well with object oriented programming concepts. Protocols are better hidden.With TCP, the protocols came first; model was just a description of the protocols. But then the model isnt good for any other protocols.73A Critique of the OSI Mo

53、del and ProtocolsWhy OSI did not take over the worldBad timingBad technologyBad implementationsBad politics74Bad TimingThe apocalypse of the two elephants.75A Critique of the TCP/IP Reference ModelProblems:Service, interface, and protocol not distinguishedNot a general modelHost-to-network “layer” n

54、ot really a layerNo mention of physical and data link layersMinor protocols deeply entrenched, hard to replace76Hybrid ModelThe hybrid reference model to be used in this book.771.5 Example NetworksThe InternetConnection-Oriented Networks: X.25, Frame Relay, and ATMEthernetWireless LANs: 802.1178The

55、historyThe story begins in the late 1950s. At the height of the Cold War, the DoD wanted a command-and-control network that could survive a nuclear war. this lead to the ARPANET:1968 Originally intended as reliable network, with multiple routing. Used TCP/IP precursor, which got built into early UNI

56、X.(ARPA, the Advanced Research Projects Agency )79The ARPANET(a) Structure of the telephone system.(b) Barans proposed distributed switching system.80The ARPANET (2)The original ARPANET design.IMPs (Interface Message Processors) 81The ARPANET (3)Growth of the ARPANET (a) December 1969. (b) July 1970

57、.(c) March 1971. (d) April 1972. (e) September 1972. NAP (Network Access Point).82NSFNETThe NSFNET backbone in 1988.83NSFNETNSFNET:Late 1970s - Many other folks wanted to get on the net, but Arpanet was essentially limited to military contractors. NSF set up another network to handle this need. Star

58、ted at 448 Kbps and by 80s upgraded to 1.5 Mbps.1990 Formed ANS (Advanced Networks and Services) - MERIT, MCI, IBM took over from the government running at 45 Mbps.1995 ANSNET sold to AOL, who now runs it.84Internet UsageGrowing exponentially.All nodes run TCP/IP. Means that all nodes have an IP add

59、ress by which they can be contacted.Traditional applications (1970 1990) E-mailNewsRemote loginFile transfer85Internet Usage1990s : the WWW (World Wide Web) invented by CERN physicist Tim Berners-Lee Together with the Mosaic browserWritten by Marc Andreessen at the National Center for Supercomputer

60、Applications in Urbana, Illinois ISPs (Internet Service Providers)Offer individual users at home the ability to call up one of their machines and connect to the Internet86Architecture of the InternetOverview of the Internet.POP (Point of Presence), NAP (Network Access Point).87Connectionless vs. Con