Page 78 - Optical Switching And Networking Handbook

04_200023_CH03/Batesx 1/17/01 9:42 AM Page 63

SONET 63

Why Bother Synchronizing?

In the past, transmission systems have been primarily asynchro-
nous. Each terminal device in the network was timed independently.
In a digital synchronous transmission system, clocking is all-
important. Clocking uses a series of pulses to keep the bit rate con-
stant and to help recover the 1s and 0s from the data stream.
Because these past clocks were timed independently, large varia-
tions occurred in the clock rate, making it extremely difficult (if not
impossible) to extract and recover the data. A DS-1 stream operates
at 1.544 Mbps 150 pulses per second (pps), whereas a DS-3 stream
operates at 44.736 Mbps 1789 pps. This difference means that one
DS-1 stream may be transmitting at up to 300 pps more or less than
another (assuming that one DS-1 signal is at -150 pps and the other
is at 150 pps). The differences can make it difficult to determine
the actual data across a common receiver.
Earlier we discussed the asynchronous method of multiplexing a
DS-3 stream. We saw that bit stuffing was used. Therefore, the
method of synchronously multiplexing in the SONET architecture
provides for better efficiency and problem resolution. Using SONET,
the average frequency of all the clocks will be the same. Every clock
can be traced to a common reference, which is highly reliable and
stable. Bit stuffing can be eliminated in the preparation of the STS-
1 signal, and therefore, the lower speed signals are more readily
accessible. The benefits outweigh the possible overhead associated
with the SONET multiplexing scheme. In SONET, the hierarchy of
clocking follows the master-slave clocking architecture. Higher-level
(stratum 1) clocks will feed the timing across the network to lower-
level devices. Any jitter, phase shifts, or drifting by the clocks can be
accommodated through the use of pointers in the SONET overhead.
The internal clock in a SONET multiplexer also may draw its timing
from a building-integrated timing system (BITS) used by switching
systems and other devices. This terminal will then serve as the mas-
ter for other SONET nodes downstream, providing timing on its out-
going signal. The receiving SONET equipment will act in a slave
mode (loop timing) with their internal clocks timed by the incoming
signal. The standard specifies that SONET equipment must be able