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Communication networks for digital data transmission can be classified as either circuit switched, message switched, or packet switched. In circuit-switched networks, a direct communication link is established between two users by closing a sequence of switches at appropriate network node points. The message is then transmitted continuously as is done, for example, in the national telephone network. In a message-switched network, the complete message is routed from node to node in the network until it reaches its destination. At each node, the message is placed in a queue to await next-node routing decisions and retransmission to a neighboring mode. Because of this two-step process, message-switched networks are known as store-and- forward networks. The packet-switched network is also a member of the store-and-forward class and differs from the message-switched network in that each message is subdivided into basic units called packets before being offered as input to the network.