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84 Chapter 4

Not all the multiplexing systems were synchronous. In the
United States, anything above DS-1 bandwidth was
asynchronously multiplexed, timed, and mapped.

Synchronous Communications

What does synchronous mean anyway? Why is it so important to the
telecommunications industry? The easiest way to describe the need
for synchronization is that the “bits” from one telephone call are
always in the same location inside a digital transmission frame such
as a DS-1. In the United States, telephone calls using digital trans-
mission systems create a DS-0. The DS-0 signals are multiplexed 24
per DS-1 channel. DS-1 lines are synchronously timed and mapped;
therefore, it is easy to remove or insert a call. Finding the location
creates an easy add-drop multiplexing arrangement.

Plesiochronous

Plesiochronous means “almost synchronous.”Variations occur on the
timing of the line, so bits are stuffed into the frames as padding. The
digital bits (1s and 0s) vary slightly in their specific location within
the frame, creating jitter. This occurs on a frame-to-frame basis, cre-
ating ill timing and requiring some other actions to make everything
bear some semblance of timing. An example seen in the preceding
chapter with the multiplexing of a DS-3 stream occurred when
Four DS-1 lines were bit-interleaved and multiplexed together
to create a DS-2.
Seven DS-2 lines were bit-interleaved and multiplexed to create
a DS-3.
We needed to isolate a particular call from a DS-3 stream, and
the entire DS-3 stream had to be demultiplexed to the DS-1
level, where we could then extract the individual DS-0 signals.