CCITT SIGNALING SYSTEM NO. 7
The standard bit rate on a digital bearer is 64 kbps. The minimum signaling bit rate
for telephone call control applications is 4.8 kbps. For other applications such as network
management, bit rates lower than 4.8 kbps may also be used.
The following is applicable for a digital signaling data link derived from a 2.048-Mbps
digital path (i.e., E1). At the input/output interface, the digital multiplex equipment or
digital switch block will comply with CCITT Recs. G.703 for electrical characteristics and
G.704 for the functional characteristics--in particular, the frame structure. The signaling
bit rate is 64 kbps. The standard time slot for signaling is time slot 16. When time slot 16
is not available, any time slot available for 64-kbps user transmission rate may be used.
No bit inversion is performed.
For a signaling data link derived from an 8.448-Mbps (E2) digital link, the following
applies: At the multiplex input/output interface, there should be compliance with CCITT
Recs. G.703 for electrical characteristics and G.704 for functional characteristics--in
particular, the frame structure. The signaling bit rate is 64 kbps. The standard time slots
for use of a signaling data link are time slots 6770 in descending order of priority.
When these time slots are not available, any channel time slot available for 64-kbps user
transmission rate may be used. No bit inversion is performed (Ref. 5).
For North American applications of SS No. 7, Telcordia Notes on SS No. 7 and CCS
Network Evolution (Ref. 6) states that data rates from 4.8 to 64 kbps may be used.
THE SIGNALING LINK LAYER (LAYER 2)
This section deals with the transfer of signaling messages over one signaling link directly
connecting two signaling points. Signaling messages delivered by upper hierarchical layers
are transferred over the signaling link in variable-length signal units. The signal units
include transfer control information for proper operation of the signaling link in addition
to the signaling information. The signaling link (layer 2) functions include:
Signaling unit delimitation
Signal unit alignment
Signal link error monitoring
All of these functions are coordinated by the link state control as shown in Figure 14.4.
Signal Unit Delimitation and Alignment
The beginning and end of a signal unit are indicated by a unique 8-bit pattern called the
flag. Measures are taken to ensure that the pattern cannot be imitated elsewhere in the unit.
Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (i.e.,
more than six consecutive ls) is received, or when a certain maximum length of signal
unit is exceeded. Loss of alignment will cause a change in the mode of operation of the
signal unit error rate monitor.