Nortel DMS-100 Series Maintenance Manual page 121

Message time-out and message checksum or CRC calculation performs
message error detection. In the event of protocol, checksum, or CRC failure
on an outgoing message, the sending node retries the send sequence. On an
incoming message failure, the sending node reroutes the message over an
alternate control side (C-side) link. Hardware redundancies provide a
minimum of one alternative path to and from a node.
The format of DMS–X messages appears in the following figure.
DMS–X message format
Bit
7
Byte
0
1
2
3
4
Drawer
number
5
6
The system transmits the message over a link, with link control messages
before and after the transmission. Messaging occurs between active
programs in the SMS and in the RCC. Many tasks for processes
communicate with each other through messages over the DS30A links.
6 5 4 3
SOM
Destination task ID
Source task ID
Line number
Message data bytes
(variable length)
.
.
.
CRC
EOM
DMS-100 Family SMS Maintenance Manual XPM10 and up
2 1 0
Node
number
SMS signaling 3-5
Message
header
LCM
task ID
Actual
message
EOM = End of message
SOM = Start of message
LCM = Line
concentrating
module
ID = Identification