Part XV: SS7/C7 Protocols: Signaling System # 7 for Telephony

Description Signalling System #7 (SS7) is a telecommunications protocol suite defined by the ITUT 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 CCS7 or C7 outside North America and SS7 and C7 are often combined together as SS7/C7 for worldwide audience.

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 telephone exchanges (signalling points or SPs) and SS7 signalling transfer points (STPs). Basically, the SS7 control network tells the switching office which paths to establish over the circuitswitched network. The STPs route SS7 control packets across the signalling network. A switching office may or may not be an STP.

The SS7/C7 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 telecoms, cellular, and long-distance networks nationwide and worldwide.

Protocol Structure The SS7 / C7 protocol suite covers all 7 layers of the OSI model as shown in the following diagram:

Figure 2.54. SS7/C7 Protocol Suite Architecture

ASP	Application Service Part	ASP provides the functions of Layers 4 through 6 of the OSI model.
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.
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	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 Part	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 Part	MAP is used to share cellular subscriber information among different networks.
MTP	Message Transfer Part	MTP crosses 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 Part	SCCP provides end-to-end routing. SCCP is required for routing TCAP messages to their proper database.
TCAP	Transaction Capabilities Application Part	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 Protocols ASP, BICC, BISUP, DUP, ISUP, MTP, SCCP, TCAP, TUP, MAP

Sponsor Source SS7 / C7 protocols are defined by ITU-T in Q.700 documents series.

Reference 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