ASYNCHRONOUS TRANSFER MODE
EVOLVING TOWARD ATM
Frame relay (Section 12.5) began a march toward an optimized
digital format for
multimedia transmission (i.e., voice, data, video, and facsimile). There were new con-
cepts in frame relay. Take, for example, the trend toward simplicity where the header
was notably shortened. The header was pure overhead, so it was cut back as practically
possible. The header also signified processing. By reducing the processing, delivery time
of a data frame is speeded up.
In the effort to speed up delivery, frames were unacknowledged (at least at the frame
relay level); there was no operational error correction scheme. It was unnecessary because
it was assumed that the underlying transport system had excellent error performance (BER
better than 1
There was error detection for each frame, and a frame found in
error was thrown away. Now that was something that we would never do for those of us
steeped in old-time data communication. It is assumed that the higher OSI layers would
request repeats of the few frames missing (i.e., thrown away). These higher OSI layers
(i.e., layer 3 and above) were the customer's responsibility, not the frame relay provider.
Frame relay also moved into the flow control arena with the BECN and FECN bits
and the CLLM. The method of handling flow control has a lot to do with its effectiveness
in preventing buffer overflow. It also uses a discard eligibility (DE) bit, which set a type
of priority to a frame. If the DE bit was set, the frame would be among the first to be
discarded in a time of congestion.
DQDB (distributed queue dual bus) was another antecedent of ATM. It was developed
by the IEEE as a simple and unique network access scheme. Even more important, its
data transport format is based on the slot, which is called a cell in ATM. This slot has a
format very similar to the ATM cell, which we will discuss at length in this chapter. It
even has the same number of octets, 53; 48 of these carry the payload. This is identical
to ATM. This "cell" idea even may be found in Bellcore's SMDS (switched multimegabit
data service). In each case the slot or cell was 53 octets long. DQDB introduced a
comparatively new concept of the HCS or header check sequence for detecting errors
in the header. In neither case was there a capability of detecting errors in the payload.
SMDS/DQDB left it to layer 3. Slots or cells carry pieces of messages. The first "piece"
Compromise might be a better term.
In North America, PSTN BER can be expected to be better than 5
Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman
2005 by Roger L. Freeman