山东大学计算机网络英语课件01 Introduction

1、IntroductionChapter 1CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Uses of Computer NetworksNetwork HardwareNetwork SoftwareReference ModelsExample NetworksNetwork StandardizationMetric UnitsRevised: August 2011Uses of Computer NetworksComputer networks are col

2、lections of autonomous computers, e.g., the InternetThey have many uses:Business Applications Home Applications Mobile Users These uses raise:Social Issues This text covers networks for all of these usesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011requestrespo

3、nseBusiness ApplicationsCompanies use networks and computers for resource sharing with the client-server model:Other popular uses are communication, e.g., email, VoIP, and e-commerce CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Home ApplicationsHomes contain m

4、any networked devices, e.g., computers, TVs, connected to the Internet by cable, DSL, wireless, etc.Home users communicate, e.g., social networks, consume content, e.g., video, and transact, e.g., auctionsSome application use the peer-to-peer model in which there are no fixed clients and servers:CN5

5、E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Mobile UsersTablets, laptops, and smart phones are popular devices; WiFi hotspots and 3G cellular provide wireless connectivity.Mobile users communicate, e.g., voice and texts, consume content, e.g., video and Web, and

6、 use sensors, e.g., GPS. Wireless and mobile are related but different:CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Social IssuesCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Network neutrality no network restrictionsCont

7、ent ownership, e.g., DMCA takedownsAnonymity and censorship Privacy, e.g., Web tracking and profilingTheft, e.g., botnets and phishingNetwork HardwareCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Networks can be classified by their scale:ScaleTypeVicinityPAN (P

8、ersonal Area Network) Building LAN (Local Area Network) CityMAN (Metropolitan Area Network) CountryWAN (Wide Area Network) PlanetThe Internet (network of all networks)Personal Area NetworkCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Connect devices over the ra

9、nge of a personExample of a Bluetooth (wireless) PAN:Local Area NetworksConnect devices in a home or office buildingCalled enterprise network in a companyCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Wireless LAN with 802.11Wired LAN withswitched EthernetMetrop

10、olitan Area NetworksCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Connect devices over a metropolitan areaExample MAN based on cable TV:Connect devices over a countryExample WAN connecting three branch offices:Wide Area Networks (1)CN5E by Tanenbaum & Wetherall

11、, Pearson Education-Prentice Hall and D. Wetherall, 2011Wide Area Networks (2)An ISP (Internet Service Provider) network is also a WAN.Customers buy connectivity from the ISP to use it.CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Wide Area Networks (3)A VPN (V

12、irtual Private Network) is a WAN built from virtual links that run on top of the Internet.CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Network SoftwareCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Protocol layers Design i

13、ssues for the layers Connection-oriented vs. connectionless service Service primitives Relationship of services to protocols Protocol Layers (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Protocol layering is the main structuring method used to divide up netw

14、ork functionality. Each protocol instance talks virtually to its peer Each layer communicates only by using the one below Lower layer services are accessed by an interface At bottom, messages are carried by the mediumProtocol Layers (2)Example: the philosopher-translator-secretary architectureEach p

15、rotocol at different layers serves a different purposeCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Protocol Layers (3)Each lower layer adds its own header (with control inform-ation) to the message to transmit and removes it on receiveLayers may also split and

16、 join messages, etc.CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Design Issues for the LayersCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Each layer solves a particular problem but must include mechanisms to address a se

17、t of recurring design issuesIssueExample mechanisms at different layersReliability despite failuresCodes for error detection/correction (3.2, 3.3)Routing around failures (5.2)Network growth and evolutionAddressing (5.6) and naming (7.1)Protocol layering (1.3)Allocation of resources like bandwidth Mu

18、ltiple access (4.2)Congestion control (5.3, 6.3)Security against various threatsConfidentiality of messages (8.2, 8.6)Authentication of communicating parties (8.7)Connection-Oriented vs. ConnectionlessService provided by a layer may be kinds of either:Connection-oriented, must be set up for ongoing

19、use (and torn down after use), e.g., phone callConnectionless, messages are handled separately, e.g., postal deliveryCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Service Primitives (1)A service is provided to the layer above as primitivesHypothetical example o

20、f service primitives that may provide a reliable byte stream (connection-oriented) service:CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Service Primitives (2)Hypothetical example of how these primitives may be used for a client-server interactionCN5E by Tanenb

21、aum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011ClientServerLISTEN (0)ACCEPT RECEIVESEND (4)DISCONNECT (6)CONNECT (1)SENDRECEIVEDISCONNECT (5)Connect requestAccept responseRequest for dataReplyDisconnectDisconnect(2)(3)Relationship of Services to ProtocolsCN5E by Tanenbaum & W

22、etherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Recap:A layer provides a service to the one aboveverticalA layer talks to its peer using a protocol horizontalReference ModelsCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Reference models describe

23、 the layers in a network architectureOSI reference model TCP/IP reference model Model used for this text Critique of OSI and TCP/IP OSI Reference ModelA principled, international standard, seven layer model to connect different systemsCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall an

24、d D. Wetherall, 2011 Provides functions needed by users Converts different representations Manages task dialogs Provides end-to-end delivery Sends packets over multiple links Sends frames of information Sends bits as signalsTCP/IP Reference ModelA four layer model derived from experimentation; omits

25、 some OSI layers and uses the IP as the network layer.CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011IP is the “narrow waist” of the InternetProtocols are shown in their respective layersModel Used in this BookCN5E by Tanenbaum & Wetherall, Pearson Education-Pre

26、ntice Hall and D. Wetherall, 2011It is based on the TCP/IP model but we call out the physical layer and look beyond Internet protocols.Critique of OSI & TCP/IPCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011OSI:Very influential model with clear conceptsModels, pr

27、otocols and adoption all bogged down by politics and complexityTCP/IP:Very successful protocols that worked well and thrivedWeak model derived after the fact from protocolsExample NetworksCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011The Internet 3G mobile phon

28、e networks Wireless LANs RFID and sensor networks Internet (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Before the Internet was the ARPANET, a decentralized, packet-switched network based on Barans ideas.ARPANET topology in Sept 1972.Nodes are IMPs, or earl

29、y routers, linked to hosts56 kbps linksInternet (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011The early Internet used NSFNET (1985-1995) as its backbone; universities connected to get on the InternetNSFNET topology in 1988T1 links (1.5 Mbps)Internet (3)CN5E

30、by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011The modern Internet is more complex:ISP networks serve as the Internet backboneISPs connect or peer to exchange traffic at IXPsWithin each network routers switch packetsBetween networks, traffic exchange is set by busine

31、ss agreementsCustomers connect at the edge by many meansCable, DSL, Fiber-to-the-Home, 3G/4G wireless, dialupData centers concentrate many servers (“the cloud”)Most traffic is content from data centers (esp. video)The architecture continues to evolveInternet (4)CN5E by Tanenbaum & Wetherall, Pearson

32、 Education-Prentice Hall and D. Wetherall, 2011Architecture of the Internet3G Mobile Phone Networks (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 20113G network is based on spatial cells; each cell provides wireless service to mobiles within it via a base statio

33、n3G Mobile Phone Networks (2)Base stations connect to the core network to find other mobiles and send data to the phone network and InternetCN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 20113G Mobile Phone Networks (3)CN5E by Tanenbaum & Wetherall, Pearson Educatio

34、n-Prentice Hall and D. Wetherall, 2011As mobiles move, base stations hand them off from one cell to the next, and the network tracks their locationHandoverWireless LANs (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011In 802.11, clients communicate via an AP (A

35、ccess Point) that is wired to the rest of the network.Wireless LANs (2)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Signals in the 2.4GHz ISM band vary in strength due to many effects, such as multipath fading due to reflections requires complex transmission s

36、chemes, e.g., OFDMWireless LANs (3)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011Radio broadcasts interfere with each other, and radio ranges may incompletely overlapCSMA (Carrier Sense Multiple Access) designs are usedRFID and Sensor Networks (1)CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D