PSTN and VoIP networks have co-existed for some time now. Calls can be made from one platform to the other with the help of translation signaling/media gateways. But how is this translation done and what are the main functions of a gateway?
PSTN and VoIP are two very different technologies. PSTN uses SS7/ISUP protocol for signaling and transfers media over traditional TDM channels on E1/T1 trunks. VoIP on the other hand, uses SIP protocol for signaling and RTP protocol for the media. A gateway’s duty is to convert those protocols and formats from one to the other.
A gateway has three main functions:
- Signaling Gateway (SG)
- Media Gateway Controller (MGC)
- Media Gateway (MG)
The picture below depicts the functional structure of a typical gateway.
The Signaling Gateway receives the ISUP messages from the PSTN side (that are transferred over MTP according to the SS7 protocol suite), encapsulates them into IP messages (by replacing the MTP part of the message with IP) and forwards them to the Media Gateway Controller.
The Media Gateway Controller performs the actual translation between ISUP and SIP protocols as well as AAA (Authentication Authorization Accounting) tasks. It is also responsible for communicating and controlling the Media Gateway with the help of the Media Gateway Control Protocol (MGCP), which is a client-server, text-based signaling and control protocol, used for administration of the media gateway.
Finally, the Media Gateway translates the PSTN voice stream to RTP packets to be transferred over the IP network.
This post described the three main gateway functions for the translation between PSTN and VoIP networks. There are different implementations of gateways that can support also other/extra functionalities in order to cover various telephony scenarios.