In packet switching there are no dedicated circuits. Each circuit in a packet-switching network carries many different transmissions at the same time. The only rule is that every data unit sent through a packet-switching network must have enough information in the header that the nodes in the network can determine how to route the data unit. This tends to add overhead to the data unit, but the trade-off is well invested.

Another advantage of packet switching is the ability to route data units over any route, rather than a fixed route. For example, if I have a lot of data to send, the data will have to be divided into many different data units. These data units do not have to follow the same route in a packet-switching network.

The trick is being able to place the data units in the proper order when they are received. If data units are routed over different paths, it is highly likely that the first data unit may be received after subsequent data units, which means the order of transmission is now mixed up. The protocols used in packet-switching networks have the ability to reassemble the data units into their proper order.

Packet switching is favored over circuit switching for many different reasons. It is more reliable than circuit switching because if a particular circuit in the network should fail, the routers in the network simploy route data units over different circuits, taking a different route altogether. In a circuit-switched network, this is not possible. If a circuit fails in the middle of a transmission, the entire connection must be released and a new one established, which means the conversation must start over again (think of being disconnected from a telephone call; the whole process of connecting must be repeated).

Packet switching is not new. The industry recognized the need for a more efficient way of transmitting data over long-haul networks and deployed the first X.25 networks in the 1960s. These packet-switching networks were used by many corporations for years, and many still use them today. Many corporations are looking toward the Internet and a packet-switching network using TCP/IP as their WAN solution.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 149-155.