Note for Computer Networks_Circuit Switching & Packet Switching

Packet Switching:

- In a packet switched network data is transmitted in blocks(packets), typically less than 1KB in length.

- Each packet contains a chunk of data and sufficient information for the packet to navigate to its destination and the original data reconstituted.

- A packet arriving at a switch is queued until it is possible to send it on to the next switch by the appropriate route.

1. Datagrams Switching

　　1. Each datagramis a packet which is treated independently as it traverses the network, each switch in the network must decide where to send that datagram next.

　　2. It is possible therefore that two datagrams from the same message may take entirely different routesto reach a destination, with the possibility that the order of packets may be changed or than packets may become lost (or terminally delayed) en route.

　　3. Datagram protocols (such as IP) must therefore include sufficient information so that order can be re-instated, missing packets detected and that lost packets are eventually terminated.

　　4. Users compete for bandwidth

　　　

2. Virtual Circuit Switching(Connection-oriented packet switching)

　　1. Connection-oriented

　　2. This kind of packet switching work as circuit switching, the lower layer shield higher layer from the action of cut the data into packge.

　　3. It works in a similar way to the telephone system
　　　　1.Establish a connection (“Are you there?”)
　　　　2.Once the connection is established, use it for data transfer (push data in one end of the “wire” and take it out at the other)
　　　　3.Release the connection (disconnect)

　　4. In a virtual circuita preplannedrouteis set for all packets constituting a message.

　　5. This is achieved by a control packet preceding all the data: each switch makes a decision as the where to send the control, but all subsequent packets follow the control.

　　6. This does not constitute a dedicated route, the packets will still be queued at each switch and other virtual circuits may use the same equipment.

　　7. The main advantage of the virtual circuit is that each of the switches makes only one route choice per message.

　　8. There is an additional delay in setting up the virtual circuit and the switches must remember the next step of the route for every virtual circuit

　　9. Users are guaranteed bandwidth