1、數據通信基礎第2章 (因特網模型)第二章內容提要Chapter 2 因特網模型The Internet Model2.1 分層任務2.2 因特網模型2.3 OSI參考模型20120229Page 2 BitLiuJK20120229BitLiuJKPage 3 數據通信為什么需要參考模型?計算機、網絡和因特網都是軟件和硬件組合在一起的復雜系統。參考模型可以看作是某種“標準參照”，可用來比較上述系統的不同實現方式。本課程和所用教材使用的是因特網五層模型，即物理層、數據鏈路層、網絡層、傳輸層和應用層。這個模型與傳統的OSI參考模型（七層）和TCP/IP模型（四層）是不同的。這是本教材的特色所在，也

2、逐漸被大家接受。OSI參考模型僅應用在特定場合。20120229BitLiuJKPage 4 2.1 分層任務Sender, Receiver, and CarrierHierarchyServices20120229BitLiuJKPage 5 Layered TasksTasks can often be treated as “Layered”.Data communication between two devices can be separated into independent “layers”Each layer has interfaces connecting upper

3、 and lower layers.Each layer defines a family of functions distinct from other layers “sub-task” in seriesBenefit: Each layer can be implemented by different manufacturer.Lets see an example of “Layered task”.20120229BitLiuJKPage 6 Figure 2.1 Sending a letter20120229BitLiuJKPage 7 ServicesEach layer

4、 uses the services of the layer immediately below it.In another word, the upper layer does not communication directly with other upper layer. It sends information to the lower layer to transmit. This is called “Virtual Communication”.20120229BitLiuJKPage 8 2.2 Internet ModelPeer-to-Peer Processes對等過

5、程Functions of Layers分層的功能Summary of Layers分層的小結20120229BitLiuJKPage 9 Figure 2.2 Internet layersThere are FIVE layers in the Internet Model.20120229BitLiuJKPage 10 Peer-to-Peer ProcessPeer-to-Peer Process對等過程 is referred to the processes on two or more machines that communicate at a given (same) lay

6、er.It may pass through many intermediate nodes.Information can be passed by Interface between each pair of adjacent layers.20120229BitLiuJKPage 11 Figure 2.3 Peer-to-peer processesNOTE20120229BitLiuJKPage 12 Figure 2.4 An exchange using the Internet modelEncapsulationDecapsulation20120229BitLiuJKPag

7、e 13 Figure 2.5 Physical layer20120229BitLiuJKPage 14 The physical layer is responsible for transmitting individual bits from one node to the next.Note:Major duties of physical layer include:1. Physical characteristics of media2. Representation of bits3. Data Rate4. Synchronization of bits20120229Bi

8、tLiuJKPage 15 Figure 2.6 Data link layer20120229BitLiuJKPage 16 The data link layer is responsible for transmitting frames from one node to the next.Note:Major duties of data link layer include:Framing, Physical addressing, flow control, error control, access control20120229BitLiuJKPage 17 Figure 2.

9、7 Node-to-node (hop-to-hop) delivery20120229BitLiuJKPage 18 Example 1In Figure 2.8 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only

10、addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection.20120229BitLiuJKPage 19 Figure 2.8 Example 120120229BitLiuJKPage 20 Figure 2.9 Network layer20120229BitLiuJKPage 21 The network layer is respon

11、sible for the delivery of packets from the original source to the final destination.Note:Major duties of network layer include:Logical addressing, Routing20120229BitLiuJKPage 22 Figure 2.10 Source-to-destination delivery20120229BitLiuJKPage 23 Example 2In Figure 2.11 we want to send data from a node

12、 with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdicti

13、on. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic. 20120229BitLiuJKPage 24 Figure 2.11 Example 220120229BitLiuJKPage 25 Figure 2.12 Transport layer20120229BitLiuJKPage 26 The transport layer is responsible

14、 for delivery of a message from one process to another.Note:Major duties of transport layer include:Port addressing, Segmentation and reassembly, connection control, flow control, error control20120229BitLiuJKPage 27 Figure 2.12 Reliable process-to-process delivery of a message20120229BitLiuJKPage 2

15、8 Example 3Figure 2.14 shows an example of transport layer communication. Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data

16、are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet.Segmentation & Reassembling20120229BitLiuJKPage 29 Figure 2.14 Example 320120229BitLiuJKPage 30 Figure 2.15 Application layer20120229BitLiuJ

17、KPage 31 The application layer is responsible for providing services to the user.Note:Major duties of application layer include:Mail services, file transfer, remote log-in, World Wide Web (WWW)20120229BitLiuJKPage 32 Figure 2.16 Summary of duties20120229BitLiuJKPage 33 2.3 OSI 參考模型兩種模型的比較20120229Bit

18、LiuJKPage 34 Figure 2.17 OSI參考模型20120229BitLiuJKPage 35 The OSI ModelOSI = Open System InterconnectionOSI RM = OSI Reference ModelOSI is a standard from the ISO (International Standard Organization) .OSI is not equal to ISO.Open System is a model that allows different systems to communicate with eac

19、h other.OSI is only a model, not a protocol.20120229BitLiuJKPage 36 Layered ArchitectureData communication between two devices can be separated into independent “layers”Each layer has interfaces connecting upper and lower layers.Each layer defines a family of functions distinct from other layersEach

20、 layer can be implemented by different manufacturer, hence Open System.20120229BitLiuJKPage 37 Useful AnalogyTwo philosophers wish to exchange philosophies1st philosopher speaks Spanish and English2nd philosopher speaks Chinese and FrenchEach engages a translator, who agree on a neutral language kno

21、wn to bothe.g. DutchSecretaries FAX w/o interpretingL2 protocol is Dutch, but (w.r.t. L1 and L3) might just as well have been Japanese! 20120229BitLiuJKPage 38 Seven Layers of OSIPhysicalData linkNetworkTransportSessionPresentationApplicationPleaseDoNotTouchStevesPetAlligator20120229BitLiuJKPage 39

22、Communication through OSI ModelSee the figure in next slideTwo devices connected by many Intermediate NodesEnd device uses all the 7 layers, while the intermediate node uses only part of them. Example: Router uses only the lower three layers, i.e., physical, data link and network layers.20120229BitL

23、iuJKPage 40 Figure 3-2OSI Layers20120229BitLiuJKPage 41 Virtual CommunicationAny two layers can communicate with each other in a “virtual” way.For transmission, the lower layer add “Header” to the data (Protocol Data Unit), this is called Packing or Encapsulation.At the receiver, PDU is extracted from the PDU, Header is removed, and the data is sent to upper layer, this is called Unpacking or Decapsulation.20120229BitLiuJKPage 42 Figure 3-3An Exchange Using the