Nortel BCM 3.7, Manual, Page: 866 / 1004

866 ISDN hardware
N0008589 3.3
Figure 221 T reference point
A T loop can be used in combination with an S loop to provide D-packet service for a point-of-sale
terminal adapter (POSTA) or other D-packet device. D-packet service is a 16 kbps data
transmission service that uses the D-channel of an ISDN line. The T and S loops must be on the
same physical module.
Clock Source for ISDN
Systems with ISDN interfaces need to synchronize clocking with the ISDN network and any ISDN
terminal equipment connected to the network. Systems synchronize clocking to the first
functionally available network connection. If there are excessive errors on the reference network
connection, the next available network connection is used for clock synchronization. The clock
synchronization process generates alarm codes and event messages. Clock synchronization is
supported by the DTM, BRI module, and FEM.
The Business Communications Manager derives timing from the network using T reference points
(loops). Terminal equipment on S reference points (loops) derive timing from the Business
Communications Manager system.
When you configure the network connections to the Business Communications Manager, you
should take into account the system preferences for selecting loops for synchronization:
• lower numbered loops have preference over higher numbered loops
• the loop preference order is: 201, 202, 203, 204 etc.
• the system skips S and analog loops on Mod2 Card 1, when selecting a network connection for
synchronization
Systems with only S loops act as timing masters for the attached terminal equipment (TE), and are
not synchronized to the network. ISDN TE without access to a network connection (BRI lines) has
limited or no functionality.
If your system has both a BRI S/T configured as BRI, and a DTM configured as PRI, it is
recommended that you use PRI as the primary clock source. See
“PRI hardware” on page 864 .
network
connection
T
ISDN
Business Communications Manager