Signaling system and reason for SS7

Signaling system

A signaling system is the set of signals, codes and procedures used to exchange control information between subscriber and exchange and between exchanges, so that calls can be set up, supervised, charged and released. Signals are of three kinds: supervisory (seize, answer, clear), address (dialled digits) and call-progress (dial tone, ringing, busy tone).

Reasons behind the development of SS7

Older systems (DC loop, single-frequency 2600 Hz, MF R1/R2 and CAS on E1 TS16) sent signals inside or alongside each voice channel. As networks became digital and larger, these methods had serious limits, which SS7 (ITU-T Q.700 series) was designed to remove.

1. Slow call set-up: MF digits are sent one by one with acknowledgements, taking several seconds per link. SS7 sends the whole address in one IAM message at 64 kb/s; set-up falls to well under a second.
2. Wasted trunk capacity: with in-channel signaling the speech trunk is seized for the whole set-up even if the called line is busy. SS7 checks first and seizes the trunk only when needed.
3. Limited signal repertoire: tone systems have only about 15 codes. SS7 messages are flexible and new messages and parameters can be added (ISDN, calling line identification, call forwarding).
4. Fraud: in-band tones could be imitated by users ("blue box" with 2600 Hz). SS7 links are out of reach of subscribers.
5. No database access: modern services (toll-free numbers, number portability, credit-card calling, mobile roaming via HLR/VLR, SMS) need queries to remote databases. SS7 with SCCP/TCAP supports these non-circuit transactions.
6. Signaling during the call: CAS cannot easily exchange information once speech starts; SS7 can at any time (supplementary services).
7. Efficiency: one 64 kb/s SS7 link can serve thousands of trunks, replacing per-trunk signaling equipment.
8. Reliability and management: SS7 has CRC error control, retransmission, redundant STP pairs and automatic re-routing.
9. Digital and ISDN compatibility: a message-based system suits stored-program-control digital exchanges and ISDN/mobile (GSM) networks.
10. International standard: one common system makes interworking between operators and countries simple.

Result

SS7 became the "nervous system" of the PSTN and GSM networks: an out-of-band packet network of SSPs, STPs and SCPs that is fast, secure, flexible and supports intelligent network services.