Signalling System 7

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

It is usually abbreviated to SS7 while in North America it is often referred to as CCS7, 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 were 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 signalling, 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 signalling tones from small electronic boxes known as blue boxes. Modern designs of telephone equipment that implement in-band signalling protocols explicitly keep the end-user's audio path—the so-called speech path—separate from the signalling phase to eliminate the possibility that the MF tones used for signalling are introduced by the end-user, which defeats the blue-box phreaking technique.

SS7 moved to a system in which the signalling information was out-of-band, carried in a separate signalling 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 signalling and bearer channels. However it also required a separate channel dedicated solely to signalling, 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 signalling, 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.

In order to move some non-time critical functionality out of the main signalling 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) and data (Packet Switched) 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 signalling over PS

Ge -> Charging for CAMEL prepaid charging

Gf -> EIR for equipment identity check

Physical network

SS7 clearly splits the signalling 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 signalling nodes - Service switching point (SSPs), signal transfer point (STPs), and Service Control Point (SCPs). Each node is identifed 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 signalling links usually also carry bearer channels, called associated signalling. In the US, SS7 links are usually carried over a network that is separated from the bearer channels, called nonassociated signaling. Quasi-associated signalling is similar to nonassociated signaling with a logical separation, but uses a smaller number of dedicated STPs to handle the signalling 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 layer 7. The levels are identified as Message Transfer Part (MTP) 1, MTP 2, and MTP 3 with Level 4 consisting of a number of different user parts, of which TUP, ISUP, TCAP with INAP and MAP , and SCCP 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.

External links

Products

ITU-T recommendations | Signaling System 7 | Telephony

Signalling System 7 | SS7 | SS7 | SS7 | ОКС-7