Signalling System No. 7

SS7 protocol suite
SS7 protocols by OSI layer
Application	INAP, MAP, IS-41...; TCAP, CAP, ISUP, ...
Network	MTP Level 3 + SCCP
Data link	MTP Level 2
Physical	MTP Level 1
	v; t; e;

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Signaling System #7 (SS7) is a set of telephony signaling protocols which are used to set up the vast majority of the world's PSTN telephone calls.

It is usually abbreviated to SS7 though in North America it is often referred to as CCS7, an acronym for "Common Channel Signaling System 7". In some European countries, specifically the United Kingdom, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7. (ITU-T was formerly known as CCITT.)

History

The SS7 protocols have been developed by AT&T since 1975 and defined as standard by ITU-T during 1981 in ITU-T's Q.7XX-series recommendations. SS7 was designed to replace Signalling System #5 (SS5) and Signalling System #6 (SS6) and R2, all of which are ITU standards defined by ITU-T prior to SS7 and were once in widespread international use. SS7 has substantially replaced SS6, SS5, and R2, with the exception that R2 variants are still used in numerous nations. SS5 and earlier used in-band signaling, where the call-setup information was sent by playing special tones into the telephone lines (known as bearer channels in the parlance of the telecom industry). This led to a number of security problems when users discovered on certain telephone switching equipment that they could play these tones into the telephone handset and control the network even without the "special keys" on an operators handset. So-called phreaks experimented with fooling the telephone exchanges by sending their own user-generated signaling tones from small electronic boxes known as blue boxes. Modern designs of telephone equipment that implement in-band signaling protocols explicitly keep the end-user's audio path—the so-called speech path—separate from the signaling phase to eliminate the possibility that the MF tones used for signaling are introduced by the end-user, which defeats the blue-box phreaking technique.

SS7 moved to a system in which the signaling information was out-of-band, carried in a separate signaling channel. This avoided the security problems earlier systems had, as the end user had no connection to these channels. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) or Common Channel Signaling (CCS) due to their hard separation of signaling and bearer channels. However it also required a separate channel dedicated solely to signaling, but due to the rapid rise in the number of available channels at the same time this was a moot point.

Uses of SS7

SS7 provides a universal structure for telephony network signaling, messaging, interfacing, and network maintenance. It deals with establishment of a call, exchanging user information, call routing, different billing structures, and supports Intelligent network (IN) services.

The most fundamental use of SS7 is to deliver a telephone call across the large public switched telephone network. To do this the call must make several hops (from my phone company, to a long distance company, to your local company, and so forth). At each hop along the way the telephone switches need to know from where the call is coming in (which phone line or which channel of a trunk) and to where it needs to go. This takes a lot of coordination. ISUP (or ISDN user part signalling) is a type of SS7 communication which deals with getting all these various links of the end to end call lined up. The ISUP messages get passed along from hop to hop, and at each point the the call's circuit gets extended a little further until it is built end to end.

In order to move some non-time critical functionality out of the main signaling path, and for future flexibility, the concept of a separate "service plane" was introduced by the IN technology. The initial, and still the most important use of IN technology has been for number translation services, e.g. when translating toll free numbers to regular PSTN numbers. But much more complex services have since been built on IN, such as CLASS and prepaid telephone calls.

SS7 is also important in linking VoIP traffic to the PSTN network.

SS7 is also used in the mobile cellular telephony networks like GSM and UMTS for voice (Circuit Switched [CS Below]) and data (Packet Switched [PS Below]) applications.

Here are some of the GSM/UMTS CS interfaces in the MSC transported over SS7:

B -> VLR (uses MAP/B). Most MSCs are associated with a VLR, making the B interface "internal".

D -> HLR (uses MAP/D) for attaching to the CS network and location update

E -> MSC (uses MAP/E) for inter-MSC handover

F -> EIR (uses MAP/F) for equipment identity check

H -> SMS-G (uses MAP/H) for SMS over CS

There are also several GSM/UMTS PS interfaces in the SGSN transported over SS7:

Gr -> HLR for attaching to the PS network and location update

Gd -> SMS-C for SMS over PS

Gs -> MSC for combined CS+PS signaling over PS

Ge -> Charging for CAMEL prepaid charging

Gf -> EIR for equipment identity check

Physical network

SS7 clearly splits the signaling planes and voice circuits. An SS7 network has to be made up of SS7-capable equipment from end to end in order to provide its full functionality. The network is made up of several link types (A, B, C, E, and F) and three signaling nodes - Service switching point (SSPs), signal transfer point (STPs), and Service Control Point (SCPs). Each node is identified on the network by a number, a point code. Extended services are provided by a database interface at the SCP level using X.25.

The links between nodes are full-duplex 56 kbit/s and/or 64 kbit/s. In Europe they are usually timeslots (DS0s) within an E1 or T1 trunk. In contrast to the US, trunks with signaling links usually also carry bearer channels, called associated signaling. In the US, SS7 links are usually carried over a network that is separated from the bearer channels, called nonassociated signaling. Quasi-associated signaling is similar to nonassociated signaling with a logical separation, but uses a smaller number of dedicated STPs to handle the signaling path.

SS7 protocol

The SS7 protocol stack borrows partially from the OSI Model of a packetized digital protocol stack. The SS7 protocol has only four levels, matching the OSI layers 1 (physical), 2 (data link), and 3 (network), with level 4 corresponding to OSI layers 4 through 7. The levels are identified as Message Transfer Part (MTP) 1, MTP 2, and MTP 3 with Level 4 consisting of the Signaling Connection and Control Part (SCCP) and a number of different user parts, of which Telephone User Part (TUP), ISDN User Part (ISUP), Transaction Capabilities Application Part (TCAP) with Intelligent Network Application Part (INAP), and Mobile Application Part (MAP) are examples.

The MTP covers the transport protocols including network interface, information transfer, message handling and routing to the higher levels. SCCP is a sub-part of other L4 protocols, together with MTP 3 it can be called the Network Service Part (NSP), it provides end-to-end addressing and routing, connectionless messages (UDTs), and management services for the other L4 user parts. TUP is a link-by-link signaling system used to connect calls. ISUP is the key user part, providing a circuit-based protocol to establish, maintain, and end the connections for calls. TCAP is used to create database queries and invoke advanced network functionality, or links to intelligent networks (INAP), mobile services (MAP), etc.

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