This chapter discusses various elementary concepts of broadband communication networks. The focus is on networks in which information is transported in fixed-length packets called cells. The asynchronous transfer mode (ATM) standardized worldwide, for example, assumes such a transport mechanism. The coverage is brief but should provide enough background information for the more detailed study in Chapter 8.
To understand the motivations behind ATM, let us consider the limitations of the synchronous transfer mode (STM) that lead to the proposal of ATM in the first place. Simply put, STM is time-division multiplexing (TDM) and ATM is fixed-length packet switching.
Recall from Chapter 1 that in TDM, information from several sources is multiplexed onto one physical transmission medium in which the time is slotted. Time slots are grouped into frames. Each time-slot position in a frame is dedicated exclusively to a particular source. Thus, time slot i of frames 1, 2, 3, … can carry information from one and only one source once the connection has been set up. There is no sharing of time slots among different connections.
While this exclusive dedication of bandwidth works well for sources that produce traffic in a continuous fashion (e.g., voice and video), it is wasteful for sources with bursty traffic (e.g., computer data): when the sources are idle, their associated time slots do not carry any information. ...