network protocols handbook.pdf

304Protocols GuideSS7 / C7 Protocols: Signaling System # 7 forTelephonyProtocol DescriptionSignalling System #7 (SS7) is a telecommunications protocol suite definedby 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 networkmanagement, and maintenance capabilities for the inter- office telephonenetwork. SS7 performs these functions by exchanging control messagesbetween SS7 telephone exchanges (signalling points or SPs) and SS7 signallingtransfer points (STPs). Basically, the SS7 control network tells theswitching office which paths to establish over the circuit-switched network.The STPs route SS7 control packets across the signalling network. A switchingoffice may or may not be an STP.The SS7 / C7 network and protocol are used for providing intelligent networkservices 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, andthree-way calling• efficient and secure worldwide telecommunicationsThe current SS7 / C7 network, one of the largest data network in the world,connects together local telcos, cellular, and long-distance networks nationwideand worldwide.