交换方法

1. 1.5 网络平台与交换技术网络平台是计算机网络的枢纽，它由传输设备，交换设备，网络互联设备，布线系统，操作系统等组成。网络平台 传输 交换 网络互联 布线系统 网络接入 网络测试 网络服务器

2. 交换方法

3. Switching • Circuit switching • Message switching • Packet switching • Datagram • Virtual circuit switching

4. (1) Circuit switching I • A dedicated communication path between two stations. • That path is a connected sequence of links. • On each physical link, a channel is dedicated to the connection. • Example: Telephone system. • Three phases: • Circuit establishment. • Data transfer • Circuit disconnection

6. Circuit-Switched Network

7. Circuit switching II • Circuit switching can be rather inefficient. • Channel capacity is dedicated. • There is a delay prior to data transfer for call establishment. • Data are transmitted at a fixed data rate with no delay

8. Switch

9. (2) Message switching I • If a station wishes to send a message. • It appends a destination address to the message. • The message is then passed through the network from node to node. • At each node, the entire message is received, stored briefly, and then transmitted to the next node.

10. Message Switching

11. Message switching II • A message-switching node is typically a general-purpose microcomputer, with sufficient storage to buffer message as they come in. • Delay: • The time required to receive all bits of the message + A queuing delay waiting for an opportunity to retransmit to the next node. • Store-and-forward message system.

12. MS: Advantages • Greater line efficiency • Availability of both ends is not required. • When traffic is heavy, messages are still accepted. • Multicast is easy. • Message priorities • Error control and recovery procedures can be built into the network. • Speed and code conversion. • Messages sent to inoperative terminals may be intercepted and either stored or rerouted to other terminals.

13. MS: Disadvantage • It is not suited to real-time or interactive traffic. • The delay is relative long and has relatively high variance. • It cannot be used for voice connections. • It is not suited to interactive terminal-host connections.

14. (3) Packet switching • Each message to be transmitted through the network is firstly divided into a number of smaller message unit known as packet. • Datagram approach • Virtual Circuit

16. Datagram Approach

17. Datagram Approach, Multiple Channels

18. Switched Virtual Circuit

19. Switched Virtual Circuit

20. Switched Virtual Circuit

23. Datagram approach • Each packet is treated independently. • Each packet contains addressing information. • The node make a routing decision for each packet. • Advantages: • The call setup phase is avoided. • More primitive, flexible, and reliable

24. Virtual circuit approach • A logical connection is established before any packets are sent. • A route between stations is set up prior to data transfer.

25. Virtual circuit approach • Advantages: • Be good for long exchange. • Relieve the stations of unnecessary communication processing functions. • Services: • Sequencing,Error control,Flow control

26. Packet Switch • Three types of delay: • Propagation delay • Transmission time • Node delay

27. Packet Switch • Performance factors: • Number of stations. • Number and arrangement of nodes. • Total load on system. • Length of typical exchange. • Processing speed of the nodes. • Packet size

29. Summary I • For interactive traffic, message switching is not appropriate. • For light and/or intermittent loads, circuit switching is the most cost effective ( Public Telephone System ) • For very heavy and sustained loads between two stations, a leased circuit-switched line is the most cost effective.

30. Summary II • For a moderate to heavy load, packet switching is to be preferred. • Datagram packet switching is good for short message and for flexibility. • Virtual circuit packet switching is good for long exchange and relieving stations of processing burden.

31. A comparison of circuit switched and packet-switched networks.

32. (4) Multiplexing • FDM ( Frequency Division Multiplexing ) • TDM ( Time Division Multiplexing )