Frequently asked Questions

What is SS7?

In old, plain telephone networks signaling between switches was based on analog transmission schemes. These consisted of pulses and multi-frequency tones transmitted over the voice path. These methods started to limit the capabilities of the switches and became computer controlled during the early 1970s. Digital signaling emerged and was named ‘common channel signaling’. This meant that the signaling did not need to be part of the voice connection itself.  A common, separate data link was established between the switches and a telephone signalling protocol was created to initiate telephone calls faster and more economically.

Where is Common Channel Signalling used?

The principle of common channel signalling is not specific to the type of telephone network. Presently it is in use within the PSTN and mobile networks, but also in ISDN BRI and PRI interfaces and in PBX protocols, such as QSIG.
Between the public network switches the signalling protocol is called Common Channel Signaling System No. 7 (with abbreviations, e.g. CCS No. 7, SS7 or C7). The protocol was standardized by the ITU-T (International Telecommunication Union, formerly CCITT) and is described in the Q-series of the ITU-T recommendations.

SS7 signalling supports the core of fixed (PSTN) and mobile networks (GSM and UMTS) globally and is implemented as the call control protocol of choice by carriers. In addition to managing call control sessions, there are SS7 derivates handling the registration and de-registration of phones in mobile networks and inter-network roaming. These are the foundation of the Intelligent Network.

The Intelligent Network, IN/AIN, facilitates the control and development of network based services, such as Centrex (VPN), Number Translation Services (NTS), pre-paid/calling-card, freephone, tele-voting and number portability (LNP). In addition SS7 is also controls supplementary services, such as messaging in mobile networks.

Why is SS7 important for Asterisk and Callweaver

Asterisk and Callweaver open source switches  are being used more and more also in carrier applications such as VoIP telephony gateways and in IVR servers. This means that the operator of the open source switch takes on the role of a network operator and is therefore able to connect to the PSTN like another Telco instead of using ISDN user access interfaces (i.e. PRI or BRI). This network-to-network interface uses SS7 signalling and allows the use of internal network features, such as number portability, and network provided calling party number (caller id). Carrier wholesale rates are generally available only through the SS7 protocol.

Why not ISDN PRI instead of SS7?

The deployment of Asterisk and Callweaver based IP telephony begins quite often with small VoIP service islands. The starting VoIP service providers hope to first gain experience in order to then move to implementations at larger scale. In such situations, the Asterisk and Callweaver systems are at first attached to the PSTN over ISDN PRI gateways using Q.931. The task of the gateway is to convert the circuit-switched PSTN calls to Asterisk and Callweaver and vice-versa (e.g. to SIP, IAX, H.323 or IVR). However, this method is meant to connect the ISDN CPE and does not deliver the necessary network-to-network capabilities that are present in SS7.