AT&T MERLIN LEGEND Communications System PBX System Planning Manual page 77

Trunks
Clock Synchronization
3-22 Trunk Options
Clock synchronization is an arrangement in which digital facilities operate from a
common clock. Whenever digital signals are transmitted over a communications
link, the receiving end is synchronized with the transmitting end to receive the
digital signals.
The system can synchronize itself to the far-end connection by extracting the
timing signal from the DS1's incoming digital stream. The 100D module passes
the signal to the time division multiplex (TDM) bus for use by the system. The
factory setting is that the first 100D module in the control unit extracts the signal
and provides primary synchronization.
if the system has more than one 100D module, you can assign the other
module(s) to provide backup synchronization in the event of maintenance
failure. You can also change the priority in which the modules provide
synchronization by reassigning primary synchronization from Module 1 to either
the second or third 100D module.
The source of clock synchronization is factory set to the external endpoint of the
DS1 facility connected to Module 1 (loop clock reference source). For example,
if the far end of the DS1 facility is another communications system (PBX), your
system synchronizes itself to that PBX's clock. The factory setting can be
changed to 'local clock reference source,' which means that the clock in your
system is free-running (not synchronized to the far end of the DS1 facility). You
must choose the clock synchronization source for all 100D modules in the
system.
When the 100D module is used only to provide tie trunks to other communi-
cations systems, the two systems must be synchronized and one of the two
systems will be the clock source for both.
If this system is the source, program 'local clock reference,' and then the far-
end system will use your local clock for synchronization.
Be careful not to assign backup synchronization by other modules in your
system in such a way that the system at the far end of the tie trunk derives its
clock signal source from an improper source. For example, if the module
providing secondary synchronization is programmed for Imp clock, it extracts
the timing signal from its far-end connection. In a maintenance failure, that far-
end clock will be used by your system and passed through the tie trunk to the
system connected to Module 1. This is called a timing loop. (A timing loop exists
if two systems derive their timing from each other, even if it passes through a
third system).