Packet switching refers to protocols in which messages are broken up into small packets before they are sent. Each packet is transmitted individually across the net, and may even follow different routes to the destination. Thus, each packet has a header information about the source, destination, packet numbering, etc. At the destination the packets are reassembled into the original message. Most modern Wide Area Networks (WANs) protocols, such as TCP/IP, X.25 and Frame Relay, are based on packet switching technologies.

Packet switching's main difference from Circuit Switching is that that the communication lines are not dedicated to passing messages from the source to the destination. In Packet Switching, different messages (and even different packets) can pass through different routes, and when there is a "dead time" in the communication between the source and the destination, the lines can be used by other routers.

Circuit Switching is ideal when data must be transmitted quickly, must arrive in sequencing order and at a constant arrival rate. Thus, when transmitting real time data, such as audio and video, Circuit Switching networks will be used. Packet Switching is more efficient and robust for data that is bursty in its nature, and can withstand delays in transmission, such as e-mail messages, and Web pages.

Two basic approaches are common to Packet Switching:

Virtual Circuit Packet Switching Networks

An initial setup phase is used to set up a route between the intermediate nodes for all the packets passed during the session between the two end nodes. In each intermediate node, an entry is registered in a table to indicate the route for the connection that has been set up. Thus, packets passed through this route, can have short headers, containing only a virtual circuit identifier (VCI), and not their destination. Each intermediate node passes the packets according to the information that was stored in it, in the setup phase.

In this way, packets arrive at the destination in the correct sequence, and it is guaranteed that essentially there will not be errors. This approach is slower than Circuit Switching, since different virtual circuits may compete over the same resources, and an initial setup phase is needed to initiate the circuit. As in Circuit Switching, if an intermediate node fails, all virtual circuits that pass through it are lost.

The most common forms of Virtual Circuit networks are X.25 and Frame Relay, which are commonly used for public data networks (PDN).
 

Datagram Packet Switching Networks

This approach uses a different, more dynamic scheme, to determine the route through the network links. Each packet is treated as an independent entity, and its header contains full information about the destination of the packet. The intermediate nodes examine the header of the packet, and decide to which node to send the packet so that it will reach its destination. In the decision two factors are taken into account: Thus, in this method, the packets don't follow a pre-established route, and the intermediate nodes (the routers) don't have pre-defined knowledge of the routes that the packets should be passed through. Packets can follow different routes to the destination, and delivery is not guaranteed (although packets usually do follow the same route, and are reliably sent). Due to the nature of this method, the packets can reach the destination in a different order than they were sent, thus they must be sorted at the destination to form the original message. This approach is time consuming since every router has to decide where to send each packet.

The main implementation of Datagram Switching network is the Internet which uses the IP network protocol.
 

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References:

  1. Data Communications, Computer Networks and OSI / F. Halsall.
  2. Computer Networks and Internets / D.E. Comer.

    3. Resources on the Web:

  3. Publications of Sangoma Technologies Inc.
  4. www.pcwebopedia.com

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