交换方法

1.5 1.5 网络平台与交换技术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，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，操作系统等交换设备，网络互联设备，布线系统，操作系统等组成。组成。

网络平台 传输网络平台 传输 交换 交换 网络互联 网络互联 布线系统 布线系统 网络接入 网络接入 网络测试 网络测试 网络服务器 网络服务器

交换方法

SwitchingSwitching

Circuit switching Message switching Packet switching

– Datagram– Virtual circuit switching

(1) Circuit switching(1) Circuit switching I IA 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-Switched Network

Circuit switchingCircuit switching II 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

Switch

(2) Message switching(2) Message switching I IIf 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

Message switchingMessage switching II IIA 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: AdvantagesMS: 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: DisadvantageMS: 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.

(3) Packet switching(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 approachVirtual Circuit

Datagram Approach

Datagram Approach, Multiple Channels

Switched Virtual Circuit

Switched Virtual Circuit

Switched Virtual Circuit

Datagram approachDatagram approachEach 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

Virtual circuit approachVirtual circuit approachA logical connection is established before

any packets are sent.A route between stations is set up prior to

data transfer.

Virtual circuit approachVirtual circuit approachAdvantages:

– Be good for long exchange.– Relieve the stations of unnecessary

communication processing functions.Services:

– Sequencing,Error control,Flow control

Packet SwitchPacket Switch

Three types of delay:– Propagation delay– Transmission time– Node delay

Packet SwitchPacket SwitchPerformance factors:

– Number of stations.– Number and arrangement of nodes.– Total load on system.– Length of typical exchange.– Processing speed of the nodes.– Packet size

Summary ISummary IFor 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.

Summary IISummary 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.

A comparison of circuit switched and packet-switched networks.

(4) Multiplexing(4) MultiplexingFDM ( Frequency Division

Multiplexing )TDM ( Time Division

Multiplexing )