ITU-T Signaling System Number 7 (SS7)

SS7 (or C7) is an ITU-T (formerly CCITT) standard that defines how equipment
in the PSTN digitally exchange data regarding call setup and routing. Other ITU-T
signaling systems are still in use throughout the world, particularly:
■ ITU-T 4, Channel-Associated Signaling (CAS) with a 2VF (voice frequency)
code in the voice band and a 2040/2400 Hz supervisory tone
■ ITU-T 5 CAS with 2VF and a 2400/2600 Hz supervisory tone, plus interregister
codes with Multi-Frequency (MF) tones
■ ITU-T [5] R2 is a revision of ITU-T 5 but uses different frequencies
What sets SS7 apart above all is the fact that it is Common Channel Signaling
(CCS), not CAS like its predecessors.Throughout the telecommunications industry
the SS7 can be used for call session setup, management and tear down, call forwarding,
caller identification information, toll free, LNP, and other service as implemented
by carriers. Information passed through SS7 networks are communicated
completely out of band meaning that signaling and media do not travel down the
same path.The SS7 was loosely designed around the OSI 7-layer model. Figure 4.6
illustrates their basic similarities.
Message Transfer Parts 1, 2, and 3 (MTP)
MTP level 1 is much the same as the Physical layer (1) of the OSI. Here the electrical
and physical characteristics of the digital signaling are addressed.The physical
interfaces defined here are those such as our previously discussed DS0 and T1. MTP
level 2 aligns with the Data Link layer of the OSI. MTP level 2 takes care of making
sure transmissions are accurate from end to end, just like the Data Link layer issues
such as flow control and error checking are handled in the MTP level 2 area. MTP
level 3 aligns itself with the Network layer of the OSI. MTP level 3 reroutes calls
away from failed links and controls signaling when congestion is present.
Telephone User Part (TUP)
This is an analog system component. Prior to digital signaling the TUP was used to
set up and tear down calls.Today most countries are using the ISDN User Part
(ISUP) to handle this requirement.
ISDN User Part (ISUP)
Most countries are using ISUP to handle basic call components. ISUP works by
defining the protocols used to manage calls between calling and called parties.
Automatic Number Identification (ANI), or—when it’s passed on to a subscriber,
known as Calling Party Identification Presentation (CLIP)—caller ID is
passed to the PSTN (or back again) through ISDN trunks and displays the calling
party’s telephone number at the called party’s telephone set during the ring cycle.
ANI is used for all Custom Local Area Signaling Services (CLASS) such as custom
ringing, selective call forwarding, call blocking, and so on.