Signaling System #7 (SS7) is a telecommunications protocol suite defined by the ITU-T which is used by the telephone companies for interoffice signalling SS7 uses out of band or common-channel signalling (CCS) techniques, which uses a separated packet-switched network for the signalling purpose. SS7 is known as C7 outside North America.
The primary function of SS7 / C7 is to provide call control, remote network management, and maintenance capabilities for the inter- office telephone network. SS7 performs these functions by exchanging control messages between SS7 / C7 telephone exchanges (signalling points or SPs) and SS7 / C7 signalling transfer points (STPs). Basically, the SS7 / C7 control network tells the switching office which paths to establish over the circuit-switched network. The STPs route SS7 control packets across the signalling network. A switching office may or may not be an STP.
The SS7 network and protocol are used for providing intelligent network services such as:
- 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
- 911, 411 services
- enhanced call features such as call forwarding, caller ID display, and three-way calling
- efficient and secure worldwide telecommunications
The current SS7 / C7 network, one of the largest data network in the world, connects together local telcos, cellular, and long-distance networks nationwide and worldwide.
Protocol Structure
SS7/C7 protocol suite covers all 7 layers of the OSI model as showing in the following diagram:
SS7 / C7: Signaling System # 7 for Telephony Signalling
| ASP | Application Service Part | ASP provides the functions of Layers 4 through 6 of the OSI model.td |
| BICC | Bearer Independent Call Control protocol | BICC is a call control protocol based on ISUP used between serving nodes to support the ISDN services independent of the bearer technology and signalling message transport technology used.td |
| BISUP | B-ISDN User Part | BISUP is an ATM protocol intended to support services such as high-definition television (HDTV), multilingual TV, voice and image storage and retrieval, video conferencing, high-speed LANs and multimedia. |
| DUP | Data User Part td | DUP defines the necessary call control, and facility registration and cancellation related elements for international common channel signalling by use of SS7 for circuit-switched data transmission services. |
| ISUP | ISDN User Parttd | ISUP supports basic telephone call connect/disconnect between end offices. ISUP was derived from TUP, but supports ISDN and intelligent networking functions. ISUP also links the cellular and PCS network to the PSTN. |
| MAP | Mobile Application Parttd | MAP is used to share cellular subscriber information among different networks. |
| MTP | Message Transfer Parttd | MTP acrosses physical, data link and network layers. It defines what interface to be used, provides the network with sequenced delivery of all SS7 message packets; and provides routing, message discrimination and message distribution functions. |
| SCCP | Signalling Connection Control Parttd | SCCP provides end-to-end routing. SCCP is required for routing TCAP messages to their proper database. |
| TCAP | Transaction Capabilities Application Parttd | TCAP facilitates connection to an external database |
| TUP | Telephone User Part | TUP is an analog protocol that performs basic telephone call connect and disconnect. |
Related Terms: ASP, BICC, BISUP, DUP, ISUP, MTP, SCCP, TCAP, TUP, MAP
Sponsor Source: SS7/C7 is a suite of ITU-T (http://www.itu.int/ITU-T/) standards defined in the Q.700 documents.
Reference Links:
http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/: SS7 Fundamentals
http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent=T-REC-Q.700 : Introduction to Signaling System No. 7