System information

Technologies Compared
Book Title
21-432
ATM contrasts with TDM in the way it allocates communications channels. In TDM,
communications channels are divided into fixed periods of time called frames. The frames are
divided into a fixed number of time slots of equal duration (see Figure 21-1). Each user is assigned
certain time slots within each frame. As Figure 21-1 indicates, a user can be given more than one
time slot in a frame.
Figure 21-1 User Assignments on Communications Channel Using TDM
The time slots allocated for each user occur at precisely the same time in every frame. Because the
time slots are synchronous, TDM is sometimes referred to as synchronous transfer mode (STM).
Users can access the communications channel only when a time slot that has been allocated to them
is available. For example, User A can send messages over the communications channel only during
the time slot(s) designated for User A. If no traffic is ready to send when the designated time slot
occurs, that time slot is unused. If a user has a burst of traffic that exceeds the capacity of the
designated time slots, additional slots cannot be used, even if they are idle. As a result, a long delay
could result before the burst of traffic is transferred over the TDM network.
In ATM, access to the communications channel is more flexible. Any user needing the
communications channel can use it whenever it is available. In contrast to TDM, ATM imposes no
regular pattern on the way users are given access to the communications channel. ATM is also
described as providing bandwidth on demand.
In other packet-handling technologies, such as High-Level Data Link Control (HDLC), any user can
gain access to the communications channel, but a user who has a long message to send can prevent
other users from gaining access to the channel until the entire message has been passed. However,
with ATM, every message is divided into small, fixed-length cells. Thus, no single user can
monopolize access to the communications channel while other users have messages to send (see
Figure 21-2).
Figure 21-2 User Assignments on ATM Communications Channel
Fitting ATM into the OSI Model
ATM standards define protocols that operate at Layer 2 (the data link layer) of the International
Organization for Standardization (ISO) seven-layer Open Systems Interconnection (OSI) reference
model. Figure 21-3 shows the layered architecture of the OSI model.
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