Derived Data Link - Nortel DMS-100 Series Maintenance Manual

In Mode III, the RCS operates with one DS-1 link for each shelf group.
The RCs uses a multiplexer to multiplex channels from the two shelves
on a shelf group on one DS-1 link.

Derived data link

Messaging between the RCS and the SMS uses the derived data link (DDL).
To create the DDL, data messages are placed on the framing bit of the PCM
stream on DS-1 links A and C. Refer to "DDL message bits and field
names" table in the next module for an explanation of the DDL message
fields. The DDL consists of:
the C-field (slot assignment)
the M-field (maintenance messages)
the A-field (alarm messages)
the S-field (protection switch messages)
All fields are present on the A-link. When shelf group AB operates in Mode
II, use of the C-field can occur. When shelf group CD operates in Mode II,
the DDL is present on the C link. The C-field is the only field present on the
C-link DDL. The data link unit processes DDL messages.
The A- and B-bit on each DS-1 channel transmits signaling and supervision
information. Signaling and supervision information allows the RCS to
perform call processing on link B, link C and link D. A failure does not
affect call processing. A failure results in the loss of link A and the
associated DDL messaging between the SMS and RCS. To take advantage
of this ability of the RCS, define all the links to the RCS as message links.
This definition is necessary because of the configuration of the I/O system
software. The defined links are not message links.
To keep track of simulated message links when the RCS remains in-service
(INSV) with loss of the DDL link is difficult. The following are additional
problems that occur when the RCS remains INSV with loss of the DDL link:
You must disable all PM maintenance on the RCS. Maintenance
messages to the RCS use the DDL link. The state of the RCS is not
known from the time of the failure until the A-link returns to service.
You must turn off different audits that use the DDL link. Node audit and
line diagnostics are examples of audits that must be closed.
The RCS has one message link (link A) because of the previous problems.
Loss of service on this link places the RCS out-of-service (OOS). Call
processing is still possible on link B, link C, and link D.
DMS-100 Family SMS Maintenance Manual XPM10 and up
Maintenance overview 1-57