Nortel DMS-100 Series Maintenance Manual page 120

3-4 SMS signaling
These 12 bits transfer the signaling pattern from two superframes. The
system takes the Fs bits from the next four superframes. This action occurs
at the transmit receive unit card in the RCS or at the time switch card in the
SMS. Special supervisory bits are inserted in the place of the robbed
signaling bits. The 24 consecutive supervisory bits form a special data link
called a derived data link (DDL).
The DDL is a 2.2 kilobit/second (Kb/s) data path of supervisory bits always
present on the RCS A-link. The DDL is also present on the C-link when
shelf group CD is in Mode II. The system uses the C-field part of the DDL.
DMS–X protocol
The DMS–X protocol is a half-duplex protocol used on DS30A links
between the remote cluster controller RCC and the SMS. The DMS–X
protocol includes a cyclic redundancy check (CRC) code for error detection.
The DMS–X protocol is an inter-terminal process that contains handshaking
messages. With handshaking messages, two terminals inform each other of
the current terminal condition that applies to messaging through message
transfer. The following figure illustrates a general form of handshaking
protocol that creates DMS–X protocol.
DMS–X handshaking protocol
297-8231-550 Standard 09.01 August 1998
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Message header:
DMS–X
Message data
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Checksum or cyclic
redundancy check
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Link control messages
(request to send, send)
Message
Link control messages
(acknowledgment)