计算机网络科大课程ch1-2课件

1、COMPUTER NETWORKSChapter 01Introduction (2)Shoubao YangComputer Science DepartmentTel: 3601540E-mail: syang1/syang2022/10/151/44Computer Networks -1-Part 2COMPUTER NETWORKSChapter 01I1.4 Reference Models1.4.1 OSI Reference Model1983, International Standards Organization (ISO) proposed the “ISO OSI (

2、Open Systems Interconnection) Reference Model”, OSI-rmThe principles of layeringA layer should be created where a different abstraction is neededEach layer should perform a well-defined functionThe function of each layer should be chosen towards defining internationally standardized protocolsThe lay

3、er boundaries should be chosen to minimize the information flow across the interfacesThe number of layers should be large enough that distinct functions and small enough that the architecture does not become unwieldy2022/10/152Computer Networks -1-Part 21.4 Reference Models1.4.1 OSIThe OSI Model2022

4、/10/153Computer Networks -1-Part 2The OSI Model2022/10/113ComputThe Physical LayerEssence: Describes the transmission of raw bits in terms of mechanical and electrical issues:Example: Connect two computers by means of a wire:Setting -3V to -12V on the wire corresponds to a binary 1; +3V to +12V is a

5、 binary 0The wire is not to be longer than 15 metersYou may change the voltage at most 20,000 times per second (Question: whats the transfer rate?)2022/10/154Computer Networks -1-Part 2The Physical LayerEssence: DesThe Data Link LayerData link layer is to transform a raw transmission facility into a

6、 line that appears free of undetected transmission errors to the network layerObservation: We need to at least detect bit transmission errors, send bits in frames that add redundancy to detect something went wrongExamples: Add a parity bit to every 7 transmitted bits: 1 says there were odd number of

7、 1s; 0 says there were an even number of 1s Add a checksum (cyclic redundancy check) that should match the bits before itAlso: Provide the mechanisms so that fast senders dont overwhelm slow receivers (flow control)2022/10/155Computer Networks -1-Part 2The Data Link LayerData link lOSI Data Link Lay

8、erObservation: We also need to specify how a number of computers can share a common channel (e.g. wire), that is medium access control sublayer (MAC):1. Specifies how one out of several competing senders, is eventually allowed exclusive access to the wire2. Common approach 1: listen to each other; r

9、etreat when you hear someone else, and try again later3. Common approach 2: wait your turn by passing a token between all stationsWell-known protocols: Ethernet, token ring, token bus, FDDI2022/10/156Computer Networks -1-Part 2OSI Data Link LayerObservationThe Network LayerEssence: Describes how rou

10、ting (and congestion) is to be done. Mostly needed in subnets. Network layer controls subnet operations.1. How do we find out which computers/routers are in the network?2. How do we calculate the best route from A to B?3. What happens when a computer/router goes down?4. Should multicasting/broadcast

11、ing be supported?5. What happens if a router becomes overloaded and starts dropping packets?6. Can we detect and avoid “hot spots?”2022/10/157Computer Networks -1-Part 2The Network LayerEssence: DescThe Transport LayerObservation: The transport layer is to accept data from above, split it up into sm

12、aller units if need be, pass these to network layer, and ensure that the pieces all arrive correctly at the other end. Generally offers connection-oriented as well as connectionless services, and varying degrees of reliability. This layer provides the actual network interface to applications1. Often

13、 provides network interface through sockets (UNIX, Windows)2. Allows to set up a connection to another application, and subsequently deliver data reliably, and in the order that it was sent3. Often also support for secure connections4. Also support for datagrams: unreliable message passing on a per-

14、message basis2022/10/158Computer Networks -1-Part 2The Transport LayerObservationOSI Session and Presentation LayersSession: The dumbest one (and ill-defined) tells how applications can set up “long-lasting” communications, offers various services, including dialog control, token management, synchro

15、nization.Presentation: Describes everything that is needed to exchange data in a platform-independent way, concerned with the syntax and semantics of the information transmitted.Example: think of byte ordering in different computers, or passing “binary” data through e-mail2022/10/159Computer Network

16、s -1-Part 2OSI Session and Presentation LThe Application LayerEssence: Contains the restTraditional: Name services (DNS), security, e-mail(SMTP), News (NNTP), Web (HTTP)Modern: All types of middleware protocols to support distributed systems:New transfer protocols for object systems like Java (RMI),

17、 CORBA (IIOP), DCOM (propriety)Special protocols to handle replication, fault tolerance, caching, data persistence, etc.High-level protocols: Special application-level protocols for e-commerce, banking, EDI, etc.2022/10/1510Computer Networks -1-Part 2The Application LayerEssence: Data Transmission i

18、n the OSI Model2022/10/1511Computer Networks -1-Part 2Data Transmission in the OSI M1.4.2 The TCP/IP Reference ModelTCP/IP is a result of protocol research and development conducted on the experimental packet switched network, ARPANET, funded by the Defense Advanced Research Projects Agency (DARPA),

19、 and is generally referred to as the TCP/IP protocol suite. This protocol suite consists of a large collection of protocols that have been issued as Internet standards by the Internet Activities Board (IAB). There is no official TCP/IP protocol model. However, based on the protocol standards, we can

20、 organize the communication task for TCP/IP into four relatively independent layers, from bottom to top: host-to-network (physical layer+network access layer), internet layer, transport layer (host-to-host), application layer.2022/10/1512Computer Networks -1-Part 21.4.2 The TCP/IP Reference Mod1.4.2

21、 The TCP/IP Reference Model (2)2022/10/1513Computer Networks -1-Part 21.4.2 The TCP/IP Reference Mod2022/10/1514Computer Networks -1-Part 22022/10/1114Computer Networks 1.4.2 The TCP/IP Reference Model (3)The bad thing is that TCP/IP did not make a clear distinction between services, interfaces, and

22、 protocols. That makes it much harder to re-implement certain layers.TCP/IP protocol suite is successful because (1) it was there when needed (OSI implementations were terrible), (2) freely distributed with the UNIX operating system.2022/10/1515Computer Networks -1-Part 21.4.2 The TCP/IP Reference M

23、od1.4.2 The TCP/IP Reference Model (4)The Internet LayerA packet-switching network based on a connectionless internetwork layer, permits hosts to inject packets into any network and have them travel independently to the destination.The internet layer defines an official packet format and protocol ca

24、lled IP (Internet Protocol), delivers IP packets where they are supposed to go.The Transport LayerAllows peer entities on the source and destination hosts to carry on an conversation. TCP (Transmission Control Protocol) is a reliable connection-oriented protocol, while UDP (User Datagram Protocol) i

25、s an unreliable connectionless protocol. 2022/10/1516Computer Networks -1-Part 21.4.2 The TCP/IP Reference Mod1.4.2 The TCP/IP Reference Model (5)The Application LayerContains all the higher-level protocols: Telnet, FTP, SMTP, DNS, HTTP, The Host-Network LayerThe TCP/IP reference model does not real

26、ly say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network. 2022/10/1517Computer Networks -1-Part 21.4.2 The TCP/IP Referenc

27、e Mod1.4.3 Comparing OSI and TCP/IP ModelsConcepts central to the OSI modelServicesInterfacesProtocolsThe TCP/IP model did not originally clearly distinguish between service, interface, and protocol.TCP/IPs protocols came first, and the model was really just a description of the existing protocols.D

28、ifferent numbers of layersAnother difference is in the area of connectionless versus connection-oriented communication. 2022/10/1518Computer Networks -1-Part 21.4.3 Comparing OSI and TCP/IP1.4.4 A Critique of the OSI Model and ProtocolsWhy OSI did not take over the worldBad timingBad technologyBad i

29、mplementationsBad politics2022/10/1519Computer Networks -1-Part 21.4.4 A Critique of the OSI Mo1.4.5 A Critique of the TCP/IP Reference ModelProblemsService, interface, and protocol not distinguishedNot a general modelHost-to-network “layer” not really a layerNo mention of physical and data link lay

30、ersMinor protocols deeply entrenched, hard to replaceThe hybrid reference model to be used in this book.2022/10/1520Computer Networks -1-Part 21.4.5 A Critique of the TCP/IP1.5 Example NetworksNovell NetwareThe ARPAnet2022/10/1521Computer Networks -1-Part 21.5 Example NetworksNovell Net1.5.1 The Int

31、ernet(a) Structure of the telephone system.(b) Barans proposed distributed switching system.The ARPANET2022/10/1522Computer Networks -1-Part 21.5.1 The Internet(a) StructurThe ARPANET (2)The original ARPANET design.2022/10/1523Computer Networks -1-Part 2The ARPANET (2)The original ARThe ARPANET (3)G

32、rowth of the ARPANET (a) December 1969. (b) July 1970.(c) March 1971. (d) April 1972. (e) September 1972. 2022/10/1524Computer Networks -1-Part 2The ARPANET (3)Growth of the ANSFNETThe NSFNET backbone in 1988.2022/10/1525Computer Networks -1-Part 2NSFNETThe NSFNET backbone in 1Internet UsageTraditio

33、nal applications (1970 1990) E-mailNewsRemote loginFile transfer2022/10/1526Computer Networks -1-Part 2Internet UsageTraditional applArchitecture of the InternetFig. 1.29 Overview of the Internet.2022/10/1527Computer Networks -1-Part 2Architecture of the InternetFiATM Virtual CircuitsFig. 1.30 A vir

34、tual circuit.Fig. 1.31 An ATM cell.2022/10/1528Computer Networks -1-Part 2ATM Virtual CircuitsFig. 1.30 The ATM Reference ModelThe ATM reference model.2022/10/1529Computer Networks -1-Part 2The ATM Reference ModelThe ATMThe ATM Reference Model (2)The ATM layers and sublayers and their functions.2022

35、/10/1530Computer Networks -1-Part 2The ATM Reference Model (2)The1.5.3 EthernetFig. 1.34 Architecture of the original Ethernet.2022/10/1531Computer Networks -1-Part 21.5.3 EthernetFig. 1.34 Arch1.5.4 Wireless LANs: 802.11(a) Wireless networking with a base station.(b) Ad hoc networking.2022/10/1532C

36、omputer Networks -1-Part 21.5.4 Wireless LANs: 802.11(a)Wireless LANs (2)The range of a single radio may not cover the entire system.2022/10/1533Computer Networks -1-Part 2Wireless LANs (2)The range of Wireless LANs (3)A multicell 802.11 network.2022/10/1534Computer Networks -1-Part 2Wireless LANs (

37、3)A multicell 81.6 Network StandardizationWhos Who in the Telecommunications WorldWhos Who in the International Standards WorldWhos Who in the Internet Standards World2022/10/1535Computer Networks -1-Part 21.6 Network StandardizationWhoITUMain sectorsRadio communicationsTelecommunications Standardiz

38、ationDevelopmentClasses of MembersNational governmentsSector membersAssociate membersRegulatory agencies2022/10/1536Computer Networks -1-Part 2ITUMain sectors2022/10/1136ComIEEE 802 StandardsThe 802 working groups. The important ones are marked with *. The ones marked with are hibernating. The one m

39、arked with gave up.2022/10/1537Computer Networks -1-Part 2IEEE 802 StandardsThe 802 work1.7 Metric UnitsFig. 1-39. The principal metric prefixes.2022/10/1538Computer Networks -1-Part 21.7 Metric UnitsFig. 1-39. TheCarrier Data Communication ServicesObservation: As long as carriers rule the world (i.

40、e.they have the cables), and competition increases, carriers will increase and improve their data communication services for the public:SMDS: Switched Multimegabit Data Service primarily intended to connect a number of LANs through long-haul networks (owned by the carrier)2022/10/1539Computer Networ

41、ks -1-Part 2Carrier Data Communication SerX.25: The OSI network protocol (also covering datalink and physical layer), intended to offer a data network on top of an (existing) cable infrastructure. Pretty old.Frame relay: Extremely simple facility that allows a customer to hire a single high-bandwidth link. There is hardly any support for error detection, routing,