Understanding the network.pdf - Back to Home

SS7 Components
The SS7 signaling system is constructed outside the PSTN transport network
through dedicated circuits that interconnect telecommunications switches of IXCs,
ILECs, and CLECs. The use of an out-of-network system provides some significant
advantages. Among the more significant is the capability to enhance the signaling
system without impacting the performance of the transport network. Faster call
processing is one result, because the signaling is out-of-band and is not impacted by
transport network congestion or contention. The out-of-band signaling system also
removes direct customer access, which prevents fraud and hacking attacks. It also
provides facilities needed for implementing the Advanced Intelligent Network (AIN),
which is a uniform maintenance, support, and service system for managing the
PSTN. The major purpose of the AIN is to provide the capability for IXCs and LECs to
be able to implement enhancements and new services to the PSTN in an efficient
manner that ensures continuity. The AIN also provides the "intelligence" for
enhanced subscriber services such as Centrex, Network-wide Automatic Call
Distribution, Enhanced 800 service, Network-wide ISDN switched digital service
support, and universal subscriber IDs.
There are two types of messages sent by SS7: circuit-control and database-access
messages. Circuit-control messages are messages relating to circuit operation and
management. Database-access messages query call completion databases to
retrieve network resource availability information to complete call setups. There are
five components to the SS7 network:
• PSTN/ISUP trunks—These are PSTN CO trunk lines (generally DS4/OC-3
links) that will transport ISDN and non-ISDN voice and data calls. These
lines are connected to the service switching points (SSPs), which use them to
complete calls that originate on their subscriber local loop lines (DS0).
• Service Control Point (SCP)—SCP is a database of call completion
information, network resource and configuration information, and AIN
information. SCP databases are typically installed as a pair to provide
redundancy and load-sharing capabilities.
• Signaling Transfer Point (STP)—STP is a switching device that routes
signaling information requests from CO switches to SCP databases. STPs do
not typically generate their own signaling messages, except for SS7 network
management messages. STPs, like SCPs, are commonly installed in pairs.
• Service Switching Point (SSP)—The SSP is any CO-class switch (LEC CO,
LEC-Tandem, or IXC-POP) that is connected to the SS7 network by way of a
CCSL. The SSP relays signaling and database queries to the STPs, which
generate the actual data requests to the SCPs.
• Common Channel Signaling Links (CCSL)—These are the 56Kbps links used
to carry SS7 signaling messages.