VoIP – PSTN Gateways and How They Do the Translation

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)

Calling from PSTN to VoIP

As VoIP services are growing, traditional (PSTN) and IP telephony need to co-exist. Users can belong either to one or the other network, and inter-working between the two technologies is necessary. That requires translation between the different protocols used, which is provided by signaling/media gateways. This post makes first a quick introduction to the signaling process of a PSTN call, and then it describes a call scenario where a PSTN subscriber calls a VoIP user.

Traditional telephony (PSTN network) uses a signaling protocol called ISDN User Part (ISUP). When a user initiates a call, an Initial Address Message (IAM) is sent in order to reserve a circuit for the call.