ITU Protocol SS7

ITU Protocol SS7
By: Andrey J. Kanevsky

The telecommunication network that has helped to overcome the hurdles of distance for communication is the Signaling System No. 7 (SS7). Some of the ways in which SS7 is used in the communication network are the setting up of phone calls, providing cellular roaming and messaging, supplying congregated voice and data services. The evolution of SS7 and its advantages necessitates an in-depth understanding of the Protocol, Architecture and the Services of SS7.

SS7 (Signaling System #7)

Signaling System #7 (SS7) is just a set of telephony, signaling protocols used in setting up the vast majority of PSTN telephone calls. Though abbreviated as SS7, it is known differently in different places. It is referred to as CCS7 in North America whereas in UK, it is at times known as C7 (CCITT number 7) or number 7 and CCIS7. AT&T had developed the SS7 Protocols in 1975 and was defined as a standard in 1981 by ITU-T in its ITU-T’s Q7XX-series recommendations. SS7 are a replacement for the SS5, SS6 and R2 standards. The out-of-band system of signaling, used by the SS7 is a better option to the in-band signaling system used by SS5 and SS6. The separate signaling channel system used by the SS7 also did away with the security problems faced by the earlier signaling Systems. ITU has standardized an international version of SS7. However, in the US, ANSI governs the standard for SS7.

Network maintenance, messaging, interfacing and providing a universal structure for telephony network are the features of SS7. Establishing a call, exchanging user information, call routing, billing structures and supporting intelligent network (IN) services are the integral part of the SS7. IN technology introduced the concept of separate “service plane” and the most important uses of IN technology include the simplest number translation services and the complex CLASS and prepaid telephone calls.

Uses of SS7
SS7 is an important link between VoIP traffic and PSTN network. It is also used in the cellular networks like GSM and UMTS for circuit switched (voice) and Packet switched (data) applications. The GCM/UMTS CS interfaces in the MSC transported over SS7 are B-> VLR with B as interface “internal”; D-> HLR for location update and attaching to the network; E-> MSC for inter-MSC handover; F-> EIR for identity check of the equipment; H-> SMS-G for SMS over CS. Some to the GMS/UMTS Ps interfaces in the SGSN transported over SS7 include Gr-> HLR to attach to the Ps network and location update; Gd-> SMS-C for SMS over PS; Gs-> MSC for CS+PS signaling over PS; Ge-> charging for CAMEL prepaid and Gf-> EIR for equipment identity check.

SS7: Network
SS7 is a network, which splits the signaling planes from the voice circuits. The network is made up of link types (A, B, C, D, E and F) and three signaling notes – Service Switching Point (SSPs), Signal Transfer Point (STPs) and Service Control Point. There is a unique identification number corresponding to each code. Links between the nodes are full-duplex 56 kbit/s and or 64 kbit/s. They are timeslots (DSOs) within E1 or T1 trunk in Europe whereas in USA the SS7 links are usually carried over a network called non-associated signaling.

SS7 – PROTOCOL
The SS7 network sets up and tears down the call, handles all the routing decisions and supports all telephony services such as 800 numbers, call forwarding, caller ID and local number portability (LNP).

The SS7-Protocol has only 4 levels identified as Message transfer Part (MTP) 1, MTP2, and MTP3 and with level 4 consisting of a number of different user parts, such as TUP, ISUP, TCAP with INAP and MAP and SCCP. Transfer Protocols including network interface, information transfer, message handling and routing to higher levels are controlled by MTP. The end-to-end addressing and routing connection less messages (UDTs) and management services are provided by the MTP3, also known as Network Service Part (NSP). TUP is used to connect calls via link-by-link signaling system. Circuit based protocol to establish, maintain and end connection for calls is taken care of by ISUP. Data base queries and advance network functionalities or links to intelligent networks (INAP), mobile services, MAP is done by TCAP.

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