OSI参考模型英文ppt课件.ppt

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1、Open Systems Interconnection Reference Model (1984), or simply the OSI model.,通信网络ISO-OSI七层模型,Analogy - 2 Philosophers exampleOne English speaking philosopher wishes to send a message to a colleague in France (Peers in layer 3)Since they have no common language, they both hire a translator. The tran

2、slators (Peers in layer 2) decide which language they will translate to.The translators send their message to their secretary (Peers in Layer 1) who may decide to send the message by fax, or email or telephone (Layer 1 protocol)Note that logically each layer communicates with its peer. In practice,

3、each layer communicates with the layers below them,Analogy - Things to rememberMessage is passed from layer 3 down through layers 2 and 1 before transmissionAt the opposite side, the message travels up through layer 1 and 2 before arriving at layer 3Layer 3 does not care how the message is transform

4、ed or how it is transmitted by the lower layersLayer 2 is only concerned with transforming the message (i.e. translation) and does not concern itself with how the message is transmittedLayer 1 is only concerned with transmitting the message and does not concern itself with the contents of the messag

5、e,Layering,Divide a task into pieces and then solve each piece independently (or nearly so).Establishing a well defined interface between layers makes porting easier. Major Advantages:Code ReuseExtensibility,Protocols,Used by Network model layersSets of rules to define how to communicate at each lay

6、er and how to interface with adjacent layers,receiver,sender,Layer N,Layer N-1,Layer N+1,Layer N,Layer N-1,Layer N+1,Key Elements of a Protocol,SyntaxData formatsSignal levelsSemanticsControl informationError handlingTimingSpeed matchingSequencing,Protocol Architectures and Networks,Layering Example

7、: Federal Express,Letter in envelope, address on outsideFedX guy adds addressing information, barcode.Local office drives to airport and delivers to hub.Sent via airplane to nearest city.Delivered to right officeDelivered to right person,Letter,FedX Layers,Letter,Addressed Envelope,Addressed Envelop

8、e,Layered Software Systems,Network softwareOperating systems Windowing systems,Unix is a Layered System,Applications,Libraries,System Calls,Kernel,The OSI Reference Model,Physical Layer is the lowest layer and is concerned with wiring and electrical standards. It provides an unreliable bit transmiss

9、ion/reception service to the layer above it.,The OSI Reference ModelData-link Layer is concerned with using the physical layer to transmit information from one computer to another computer connected to the same network.Data to be sent is broken into frames and transmitted.This layer needs to be able

10、 to recognize a frame in a raw bit streamIt needs to be able to identify errors in framesThis layer must regulate the speed of data transmission. A fast transmitter could overload a slower receiverIf in a broadcast network, then this layer controls access to the shared channel (Medium Access Control

11、),The OSI Reference ModelNetwork Layer provides a frame delivery service across a network or across different networks.The key problem that this layer solves is how to route packets from a source computer to a destination computerThis layer adds header information to each packet to uniquely address

12、hosts in the networkRouting Information is kept in tables or built dynamicallyIn broadcast networks, routing is simple since every host receives every packet, thus the network layer in this case is very thin or non-existent.,The OSI Reference ModelTransport Layer accepts data from the Session Layer

13、and breaks it up into smaller units (called fragmentation), if necessary, before passing it to the Network Layer.Data can be sent using an error free logical connection between two computers that delivers packets the same order that they were sentData can also be sent using a datagram service where

14、packets are delivered without assured order of deliveryThis layer can support many transport layer connections entering and leaving it. The transport layer header in each packet will contain this information.,The OSI Reference ModelSession LayerThis layer allows different machines to establish sessi

15、ons between them. E.g. Transferring a file from one machine to another. A session can be resumed after a computer has crashedPresentation LayerThis layer provides data coding for integers, floats etc, data compression and secure transmission using cryptographyApplication Layer.End application progra

16、ms OR services used by end application programs. Typical applications included in this layer would be e-mail, remote login, directory services,Data,Data,Data,Data,Data,Data,Data,Bits,AH,PH,SH,TH,NH,NH,Application Layer Protocol,Actual Path taken by data,Message Transmission Example,Message Transmiss

17、ion Using Layers,sender,receiver,A receiving layer wraps incoming message with an envelope Adds layer related addressing information,A receiving layer removes the layer related envelope and forwards the message up,Simplified Network Model,Process,Transport,Data Link,Interface Protocols,Peer-to-peerP

18、rotocols,Network,Process,Transport,Data Link,Network,802 Layers,Source: IEEE Std 802-2001, IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture, IEEE Std 802 -2001,Comparison of Network Models,Comparing the Models,Source: Computer Networks, Andrew Tannenbaum, Prentice-Hall, 1981, Section 1.4,