Signaling Connection Control Part
SCCP provides connectionless and connection-oriented network
services above MTP Level 3.
While MTP Level 3 provides point codes to allow messages to be
addressed to specific signaling points, SCCP provides subsystem
numbers to allow messages to be addressed to specific
applications (called subsystems) at these signaling points.
SCCP is used as the transport layer for TCAP-based
services such as freephone (800/888), calling card, local number
portability, wireless roaming, and personal communications
services (PCS). Global Title Translation
SCCP also provides the means by which an STP can
perform global title translation (GTT), a procedure by which
the destination signaling point and subsystem number (SSN) is
determined from digits (i.e., the global title) present in
the signaling message.
The global title digits may be any sequence of digits (e.g., the
dialed 800/888 number, calling card number, or mobile subscriber
identification number) pertinent to the service requested. Because
an STP provides global title translation, originating signaling
points do not need to know the destination point code or subsystem
number of the associated service. Only the STPs need to maintain a
database of destination point codes and subsystem numbers associated
with specific services and possible destinations.
SCCP Message Format
The Service Indicator of the Service Information Octet (SIO) is
coded 3 (binary 0011) for SCCP. SCCP messages are contained within
the Signaling Information Field (SIF) of an
MSU. The SIF contains the routing label followed by the SCCP message
contents. The SCCP message is comprised of a one-octet message
type field which defines the contents of the remainder of the
message (Fig. 15).
Figure 15. SCCP Message Format
Each SCCP message contains a mandatory fixed part
(mandatory fixed-length parameters), mandatory variable part
(mandatory variable-length parameters), and an optional part
which may contain fixed-length and variable-length fields. Each
optional part parameter is identified by a one-octet parameter code
followed by a length indicator ("octets to follow") field. Optional
parameters may occur in any order. If optional parameters are
included, the end of the optional parameters is indicated by an
octet containing all zeros.