OSI参考模型-英文ppt课件

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 example One 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 tra

2、nslators (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 practic

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

4、nsformed or how it is transmitted by the lower layers Layer 2 is only concerned with transforming the message (i.e. translation) and does not concern itself with how the message is transmitted Layer 1 is only concerned with transmitting the message and does not concern itself with the contents of th

5、e message,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 Reuse Extensibility,Protocols,Used by Network model layers Sets of rules to define how to communicate

6、 at each layer 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,Syntax Data formats Signal levels Semantics Control information Error handling Timing Speed matching Sequencing,Protocol Architectures and Netwo

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

8、nvelope,Addressed Envelope,Layered Software Systems,Network software Operating 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 a

9、n unreliable bit transmission/reception service to the layer above it,The OSI Reference Model Data-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.

10、 This layer needs to be able to recognize a frame in a raw bit stream It needs to be able to identify errors in frames This layer must regulate the speed of data transmission. A fast transmitter could overload a slower receiver If in a broadcast network, then this layer controls access to the shared

11、 channel (Medium Access Control,The OSI Reference Model Network 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 computer This layer adds header information to

12、 each packet to uniquely address hosts in the network Routing Information is kept in tables or built dynamically In 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 Model Transport Layer a

13、ccepts data from the Session Layer 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 sent Data can also be

14、 sent using a datagram service where packets are delivered without assured order of delivery This 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 Model Session Layer This layer allow

15、s different machines to establish sessions between them. E.g. Transferring a file from one machine to another. A session can be resumed after a computer has crashed Presentation Layer This layer provides data coding for integers, floats etc, data compression and secure transmission using cryptograph

16、y Application Layer. End application programs 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,Mes

17、sage Transmission Example,Message Transmission 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-peer Protocols,Network,Process,Transport,Data Link,Netw