Nortel BCM 3.7, Manual

Page: 147 / 1004

Defining trunk module types and settings 147
Programming Operations Guide
Determining Clock Sources for DTMs or BRIs
Clock Source allows you to designate the DTM or BRI on the system that obtains the timing
reference for synchronization from the network.
Systems with digital interfaces need to synchronize to the network in order to function.
Synchronization follows a hierarchical path. Each device (switch) obtains the network clock from
the device above it in the synchronization hierarchy. The device then passes the network clock to
the device below it in the synchronization hierarchy. The synchronization levels are referred to as
strata.
Business Communications Manager systems are stratum 4E equipment and are usually used as
termination points in a network.
For each DTM and BRI, choose one of the following settings: Primary , Secondary , or Timing
Master :
• Primary reference —The DTM/BRI obtains the timing reference from the network and the
system synchronizes to it. This is the default value for the first DTM in Business
Communications Manager. Note that there should only be one defined Primary clock source
on a System.
Private network: If this system is in a private network and is intended to provide the master
clock for that private network, the system must have one and only one Primary clock reference
on a DTM or BRI. If this system is intended to act as clock master in a private network, then
all clock sources should be set to Timing Master on this system.
• Secondary reference —The DTM/BRI acts as a standby reference. If there are excessive errors
on the Primary reference link, or the DTM/BRI designated as Primary reference fails, the
Secondary DTM/BRI obtains the timing reference from the network to be used for system
synchronization. This is the default value for the second DTM in a Business Communications
Manager.
Private network: If this system is in a private network and is intended to provide the Master
clock for that private network, then there should be no Secondary reference defined on any
DTM/BRI. Note that there should only be one defined Secondary clock source on a system.
• Timing Master —The DTM/BRI does not obtain timing from the network, but transmits the
internally-generated system timing, which is derived from the Primary/Secondary source, to
equipment connected to it.
Note that while in the absence of a DTM Primary clocking source a BRI module can be used
for the primary timing reference, it is always recommended that, when possible, DTM(s) be
used as primary (and secondary) clock sources and that any remaining DTMs/BRIs be set to
Timing Master.
Warning: Changing the clock source may disconnect calls.
If you change the clock source for your system, you may cause your system DTM
interface(s) to reset, resulting in dropped calls. Choose a suitable time to change the clock
source and use the Page feature to inform users of possible service disruptions.