SS7 Protocol Stack
The hardware and software functions of the SS7
protocol are divided into functional abstractions called "levels".
These levels map loosely to the Open Systems Interconnect
(OSI) 7-layer model defined by the International Standards Organization
Figure 3. The OSI Reference Model and the SS7
Message Transfer Part
The Message Transfer Part (MTP) is divided into three levels. The
lowest level, MTP Level 1, is equivalent to the OSI Physical
Layer. MTP Level 1 defines the physical, electrical, and functional
characteristics of the digital signaling link. Physical interfaces
defined include E-1 (2048 kb/s; 32 64 kb/s channels),
DS-1 (1544 kb/s; 24 64kb/s channels), V.35 (64 kb/s),
DS-0 (64 kb/s), and DS-0A (56 kb/s).
MTP Level 2 ensures accurate end-to-end transmission of a
message across a signaling link. Level 2 implements flow control,
message sequence validation, and error checking. When an error
occurs on a signaling link, the message (or set of messages) is
retransmitted. MTP Level 2 is equivalent to the OSI Data Link
MTP Level 3 provides message routing between signaling
points in the SS7 network. MTP Level 3 re-routes traffic away from
failed links and signaling points and controls traffic when
congestion occurs. MTP Level 3 is equivalent to the OSI Network
ISDN User Part (ISUP)
The ISDN User Part (ISUP) defines the protocol used to set-up,
manage, and release trunk circuits that carry voice and data between
terminating line exchanges (e.g., between a calling party and a
called party). ISUP is used for both ISDN and non-ISDN calls.
However, calls that originate and terminate at the same switch do
not use ISUP signaling.
Telephone User Part (TUP)
In some parts of the world (e.g., China, Brazil), the Telephone
User Part (TUP) is used to support basic call setup and tear-down.
TUP handles analog circuits only. In many countries, ISUP has
replaced TUP for call management.
Signaling Connection Control Part (SCCP)
SCCP provides connectionless and connection-oriented network
services and global title translation (GTT) capabilities
above MTP Level 3. A global title is an address (e.g., a
dialed 800 number, calling card number, or mobile subscriber
identification number) which is translated by SCCP into a
destination point code and subsystem number. A subsystem
number uniquely identifies an application at the destination
signaling point. SCCP is used as the transport layer for TCAP-based
Transaction Capabilities Applications Part (TCAP)
TCAP supports the exchange of non-circuit related data between
applications across the SS7 network using the SCCP connectionless
service. Queries and responses sent between SSPs and SCPs are
carried in TCAP messages. For example, an SSP sends a TCAP query to
determine the routing number associated with a dialed 800/888 number
and to check the personal identification number (PIN) of a calling
card user. In mobile networks (IS-41 and GSM), TCAP
carries Mobile Application Part (MAP) messages sent between
mobile switches and databases to support user authentication,
equipment identification, and roaming.
Operations, Maintenance and Administration Part (OMAP) and
OMAP and ASE are areas for future definition. Presently, OMAP
services may be used to verify network routing databases and to
diagnose link problems.
Signaling Gateway supports MTP Level 1, 2, and 3, and SCCP
layers of the SS7 Protocol Stack.