Calling from PSTN to VoIP

2 Sep

TelcoNotes

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. The destination switch receives the IAM, initiates ringing to the called party and sends back to the caller an Address Complete Message (ACM). When the called party picks up the phone, an Answer Message (ANM) is sent back to the caller. Finally, when a…

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