Chapter List

Part I of this book introduces four widely deployed packet network technologies: X.25, frame relay, asynchronous transfer mode (ATM), and Internet protocol (IP).

Before packet networks, communications technology used circuit-switched telephone networks with dedicated, analog circuits that functioned on a always on once activated basis. A dedicated circuit cannot be used for other purposes even if no communications are taking place at the moment. In regard to telephone conversations, it is estimated that on the average a dedicated circuit carried active traffic only 20 to 25 percent of the time and is idle the other 75 to 80 percent. Moreover, other services such as video data streams cannot be efficiently carried on circuit-switched networks.

Packet networks based on packet switching technologies represent a radical departure. The key idea behind packet switching is that a message or a conversation is broken into independent, small pieces of information called packets that are either equal or variable in size. These packets are sent individually to a destination and are reassembled there. No physical resource is dedicated to a connection, and connections become virtual, thus allowing many users to share the same physical network resource.

The concept of packet switching is attributed to Paul Baran who first outlined its principles in an essay published in 1964 in the journal On Distributed Communications. The term packet switching itself was coined by Donald...