VoIP – PSTN Gateways and How They Do the Translation

TelcoNotes

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.

VoIP PSTN 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…

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