计算机网络英文课件.ppt

1、5: DataLink Layer,5-1,Chapter 5: The Data Link Layer,5: DataLink Layer,5-2,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,Data,10100110100101 Raw bits 110101110101,Data,Review and introduction,5: DataLink Layer,5-3,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,10100110100101 Raw bits 110101110101,Review and

2、introduction,5: DataLink Layer,5-4,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,H2,T2,Review and introduction,5: DataLink Layer,5-5,H3,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,Review and introduction,5: DataLink Layer,5-6,H4,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,Review and introduction,5: DataLink

3、 Layer,5-7,Data,H5,Data,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,Review and introduction,5: DataLink Layer,5-8,5,4,3,2,1,5,4,3,2,1,host A,AP2,AP1,host B,Get the data！,Review and introduction,5: DataLink Layer,5-9,Review and introduction,The task of the data link layer is to convert the raw bit stre

4、am offered by the physical layer into a stream of frames for use by the network layer. Unfortunately, communication circuits make errors occasionally. Furthermore, they have only a finite data rate, and there is a nonzero propagation delay between the time a bit is sent and the time it is received.

5、The protocols used for communications must take all these factors into consideration.,5: DataLink Layer,5-10,Chapter 5: The Data Link Layer,Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing

6、 reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies,5: DataLink Layer,5-11,Link Layer,5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet,5.6 Hubs and switche

7、s 5.7 PPP 5.8 Link Virtualization: ATM and MPLS,5: DataLink Layer,5-12,Link Layer: Introduction,5: DataLink Layer,5-13,Link Layer: Introduction,network link physical,M,frame,DATA LINK PROTOCOL,NIC,5: DataLink Layer,5-14,Link Layer: Introduction,Some terminology: hosts and routers are nodes communica

8、tion channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame, encapsulates datagram,data-link layer has responsibility of transferring datagram from one node to adjacent node over a link,5: DataLink Layer,5-15,Link Layer Servi

9、ces,Framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify source, dest different from IP address! Reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom u

10、sed on low bit error link (fiber, some twisted pair) wireless links: high error rates,5: DataLink Layer,5-16,Link Layer Services (more),Flow Control: pacing between adjacent sending and receiving nodes Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors:

11、signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time,5: DataLink Layer,5-17,Adaptors Communicati

12、ng,link layer implemented in “adaptor” (aka NIC, Network Interface Card) Ethernet card, PCMCI card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc.,receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving

13、 node adapter is semi-autonomous link latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub,5: DataLink Layer,5-106,Hubs,Hubs are essentially physical-layer repeaters: bits coming from one link go out all other links

14、 at the same rate no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality,5: DataLink Layer,5-107,Gbit Ethernet,uses standard Ethernet frame format allows for point-to-point links and shared broadcast channels in shared mode, CSMA/CD is used; short dist

15、ances between nodes required for efficiency uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now !,5: DataLink Layer,5-108,Link Layer,5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addr

16、essing 5.5 Ethernet,5.6 Interconnections: Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM,5: DataLink Layer,5-109,(a) Which device is in which layer. (b) Frames, packets, and headers.,These devices operate in different layers. The layer matters because different devices use different pieces o

17、f information to decide how to switch.,Repeaters, Hubs, Bridges, Switches, Routers and Gateways,5: DataLink Layer,5-110,Repeaters,A signal appearing on one of them is amplified and put out on the other. Repeaters do not understand frames, packets, or headers. They understand volts. Classic Ethernet,

18、 for example, was designed to allow 4 repeaters, in order to extend the maximum cable length from 500 meters to 2500 meters.,5: DataLink Layer,5-111,Hubs,A hub has a number of input lines that it joins electrically. Frames arriving on any of the lines are sent out on all the others. If two frames ar

19、rive at the same time, they will collide. In other words, the entire hub forms a single collision domain.,HUB,5: DataLink Layer,5-112,Hubs,Hubs differ from repeaters in that they do not (usually) amplify the incoming signals, but the differences are slight. Like repeaters, hubs do not examine the 80

20、2 addresses or use them in any way.,5: DataLink Layer,5-113,Bridges,A bridge connects two or more LANs. When a frame arrives, software in the bridge extracts the destination address from the frame header and looks it up in a table to see where to send the frame.,5: DataLink Layer,5-114,Switches,Swit

21、ches are similar to bridges in that both route on frame addresses. In fact, many people uses the terms interchangeably. The main difference is that a switch is most often used to connect individual computers.,5: DataLink Layer,5-115,(a) A hub. (b) A bridge. (c) A switch.,Repeaters, Hubs, Bridges, Sw

22、itches, Routers and Gateways,5: DataLink Layer,5-116,Routers,When a packet comes into a router, the frame header and trailer are stripped off and the packet located in the frames payload field is passed to the routing software. This software uses the packet header to choose an output line. For an IP

23、 packet, the packet header will contain a 32-bit (IPv4) or 128-bit (IPv6) address, but not a 48-bit 802 address. The routing software does not see the frame addresses and does not even know whether the packet came in on a LAN or a point-to-point line.,5: DataLink Layer,5-117,gateways,Transport gatew

24、ays: To connect two computers that use different connection-oriented transport protocols. e.g., suppose a computer using the connection-oriented TCP/IP protocol needs to talk to a computer using the connection-oriented ATM transport protocol. The transport gateway can copy the packets from one conne

25、ction to the other, reformatting them as need be. Application gateways: To understand the format and contents of the data and translate messages from one format to another. e.g., an e-mail gateway could translate Internet messages into SMS （Short Messaging Service）messages for mobile phones.,5: Data

26、Link Layer,5-118,Interconnecting with hubs,Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain Cant interconnect 10BaseT full duplex Switching: A-to-A and B-to-B simultaneously, no collisions,switch,A,A

27、,B,B,C,C,5: DataLink Layer,5-127,More on Switches,cut-through switching: frame forwarded from input to output port without first collecting entire frame slight reduction in latency combinations of shared/dedicated, 10/100/1000 Mbps interfaces,5: DataLink Layer,5-128,Institutional network,hub,hub,hub

28、,switch,to external network,router,IP subnet,mail server,web server,5: DataLink Layer,5-129,Switches vs. Routers,both store-and-forward devices routers: network layer devices (examine network layer headers) switches are link layer devices routers maintain routing tables, implement routing algorithms

29、 switches maintain switch tables, implement filtering, learning algorithms,5: DataLink Layer,5-130,Summary comparison,5: DataLink Layer,5-131,Link Layer,5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet,5.6 Hubs and s

30、witches 5.7 PPP 5.8 Link Virtualization: ATM,5: DataLink Layer,5-132,The Data Link Layer in the Internet,Local networks - LANs Wide-area network - built mainly on point-to-point leased lines.,Point-to-point situations: LAN of organization attaches to Internet. All comms of organization goes through

31、router, which has point-to-point leased line connection with outside router. Home user. Point-to-point connection with ISP. Like having temporary leased line (dial-up) or permanent (broadband, on 24/7). Point-to-point between backbone routers on Internet.,5: DataLink Layer,5-133,The Data Link Layer

32、in the Internet,A home personal computer acting as an internet host.,5: DataLink Layer,5-134,Point to Point Data Link Control,one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-poin

33、t DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!,5: DataLink Layer,5-135,The Data Link Layer in the Internet,Internet protocol stack: TCP - Transport Layer IP - Network Layer PPP - Data Link layer for p

34、oint-to-point lines,5: DataLink Layer,5-136,PPP Point to Point Protocol,The Internet needs a point-to-point protocol for a variety of purposes, including router-to-router traffic and home user-to-ISP traffic. This protocol is PPP (Point-to-Point Protocol). a framing method a link control protocol, L

35、CP(Link Control Protocol) a way to negotiate network-layer options, NCP (Network Control Protocol),5: DataLink Layer,5-137,PPP Point to Point Protocol,The PPP full frame format for unnumbered mode operation.,The PPP frame format closely resemble the HDLC frame format. The major difference between PP

36、P and HDLC is that PPP is character oriented rather than bit oriented. So all frames are an integral number of bytes.,5: DataLink Layer,5-138,PPP Data Frame,Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protoco

37、l: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc),5: DataLink Layer,5-139,PPP Data Frame,info: upper layer data being carried check: cyclic redundancy check for error detection,5: DataLink Layer,5-140,Byte Stuffing,“data transparency” requirement: data field must be allowed to include flag pattern Q: is received data or flag? Sender: adds (“stuffs”) extra byte after each data byte Receiver: two 01111110 bytes in a row: discard first byte, continue data reception single 01111110: flag byte,5: DataLink Layer,5-141,Byte Stuffing,flag byte pattern in data to