Signaling Protocols - Nortel DMS-100 Series Maintenance Manual

Every fourth bit carries the cyclic redundancy check (CRC), which begins
with the second bit. The system performs a block check field six times in an
extended super frame. The CRC-6 check detects bits that emulate an FSP
bit, and determines when an out-of-frame condition is present.

Signaling protocols

The SMS-RCS subsystem uses the following protocols to communicate and
provide subscriber services:
A- and B-bit messaging
Derived data link (DDL) messaging
Nailed-up cross-connections
A- and B- bit messages
In every sixth and twelfth channel frame, the system steals the least
important bits from each channel. The system steals these bits at the RCS
transmit/receive unit card or at the SMS time switch. The A-bits replace the
least significant bits from the sixth frame. The B-bits replace the least
significant bits from the twelfth frame. The A- and B-bits pass information
that relate to specific channels.
An 8085 microprocessor on the A/B DDL message card sends A- and B-bits
to the time switch. The microprocessor inserts these bits in the PCM data
sent to the DS-1 lines. The 8085 microprocessor also extracts A- and B-bits
from the PCM data that the time switch sends. The SMS signaling processor
(SP) processes the A- and B-bits.
DDL messages
As shown in the previous figure, the framing bits used for channel frames
alternate between frame timing bits (Ft) and frame signaling bits (Fs). Each
superframe contains the same sequence of six Ft bits and six Fs bits.
Ft bits identify the first time slot in each channel frame. Fs bits identify the
A- (sixth) and B- (twelfth) channel frames of a superframe. The A-frame
carries A-bits. The B-frame carries B-bits. The framing pattern of Ft and Fs
bits appears below:
1(Ft) 0(Fs) 0(Ft) 0(Fs) 1(Ft) 1(Fs) 0(Ft) 1(Fs) 1(Ft)
1(Fs) 0(Ft) 0(Fs)
The frame signaling pattern, formed with Fs bits, is as follows:
111 000 111 000
DMS-100 Family SMS Maintenance Manual XPM10 and up
SMS signaling 3-3