What's SS7 ?
Signaling System #7 (SS7) is a set of telephony signalling protocols which are used to set up the vast majority of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls.
It is usually abbreviated to SS7 though in North America it is often referred to as CCSS7, 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), 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 multi-frequency tones into the telephone lines (known as bearer channels in the parlance of the telecom industry). This led to security problems with blue boxes. Modern designs of telephone equipment that implement out-of-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 end users may introduce tones that would be mistaken for those used for signaling.
SS6 and 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.

Functionality
Signaling refers to the exchange of information between call components required to provide and maintain service.
Users of the public switched telephone network (PSTN), exchange signaling with network elements constantly. Examples of signaling between a telephone user and the telephone network include: dialing digits, providing dial tone, accessing a voice mailbox, sending a call-waiting tone, dialing *66 (to retry a busy number), etc.
SS7 is a means by which elements of the telephone network exchange information. Information is conveyed in the form of messages. SS7 messages can convey information such as:
* I’m forwarding to you a call placed from 212-555-1234 to 718-555-5678. Look for it on trunk 067.
* Someone just dialed 800-555-1212. Where do I route the call?
* The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone.
* The route to 'X' is congested. Please don’t send any messages to 'X' unless they are of priority 2 or higher.
* I’m taking trunk 143 out of service for maintenance.
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 PSTN. 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 signaling) 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 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.

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, D, 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 the SS7 network.
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.
In an effort to increase the signaling capacity between nodes in a signaling network; newer technologies have been developed to support this. They are High Speed Links and SIGTRAN signaling links.
High Speed Links (HSL) are documented in Telcordia specification GR-2878. High-Speed Links utilize the entire bandwidth of a T1/E1 for the transport of SS7 signaling messages. The physical and data links layers of HSL uses Asynchronous Transfer Mode (ATM). One or more HSL's can be used for a linkset (1 or more physical signaling links that connect two endpoints together - more than one link per linkset is needed to give the required throughput between the endpoints, sort of like multiple lanes on a highway).
SIGTRAN provides signaling using SCTP associations over the Internet Protocol. The protocols for SIGTRAN are M2PA, M2UA, and M3UA.

MAP Signaling
In mobile cellular telephony networks like GSM and UMTS the SS7 application MAP is used. Voice connections are Circuit Switched (CS) and data connections are Packet Switched (PS) applications.
Some of the GSM/UMTS CS interfaces in the Mobile Switching Center (MSC) transported over SS7 include the following:
B -> VLR (uses MAP/B). Most MSCs are associated with a Visitor Location Register (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 Short Message Service (SMS) over CS
There are also several GSM/UMTS PS interfaces in the Serving GPRS Support Node (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 Customised Applications for Mobile networks Enhanced Logic (CAMEL) prepaid charging
Gf -> EIR for equipment identity check