Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and control. The ITU definition of SS7 allows for national variants such as the American National Standards Institute (ANSI) and Bell Communications Research (Telcordia Technologies) standards used in North America and the European Telecommunications Standards Institute (ETSI) standard used in
The SS7 network and protocol are used for:
- basic call setup, management,
and tear down
- wireless services such as
personal communications services (PCS), wireless roaming, and mobile
subscriber authentication
- local number portability (LNP)
- toll-free (800/888) and toll
(900) wireline services
- enhanced call features such as
call forwarding, calling party name/number display, and three-way calling
- efficient and secure worldwide
telecommunications
Signaling Links
SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bidirectional channels called signaling links. Signaling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-band signaling provides:- faster call setup times
(compared to in-band signaling using multi-frequency (MF) signaling tones)
- more efficient use of voice
circuits
- support for Intelligent Network
(IN) services which require signaling to network elements without voice
trunks (e.g., database systems)
- improved control over fraudulent
network usage
Signaling Points
Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message.There are three kinds of signaling points in the SS7 network (Fig. 1):
- SSP (Service Switching Point)
- STP (Signal Transfer Point)
- SCP (Service Control Point)
Network traffic between signaling points may be routed via a packet switch called an STP. An STP routes each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signaling points. An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message (e.g., the dialed 800 number, calling card number, or mobile subscriber identification number). An STP can also act as a "firewall" to screen SS7 messages exchanged with other networks.
Because the SS7 network is critical to call processing, SCPs and STPs are usually deployed in mated pair configurations in separate physical locations to ensure network-wide service in the event of an isolated failure. Links between signaling points are also provisioned in pairs. Traffic is shared across all links in the linkset. If one of the links fails, the signaling traffic is rerouted over another link in the linkset. The SS7 protocol provides both error correction and retransmission capabilities to allow continued service in the event of signaling point or link failures.
Signaling gateways can be configured as an STP or SEP (Signaling End Point).
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