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…
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