交换设备网络互联设备布线系统.PPT

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 Switching Circuit switching Message switching Packet switching Datagram Virtual circuit switching (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 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 (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. 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. 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. MS: Disadvantage It is not suited to real-time or interactive traffic. The delay is relative long and has relativel