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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)

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How to Analyze VoIP SIP Calls in Wireshark

Analyzing SIP packets is one of the most common ways to troubleshoot VoIP issues in the network or systems. Wireshark is a very helpful tool when it comes to analyzing VoIP SIP calls. Therefore, it is good to be able to monitor the VoIP traffic and capture the SIP packets. Wireshark provides the possibility to detect the VoIP calls in a trace and analyze them accordingly.

In order to detect the VoIP calls in a Wireshark trace, you need to select from the top menu: Telephony –> VoIP Calls. Then a new window appears which lists all the calls that were found in the trace, and information regarding the start/end time, From/To headers, etc. Select a call from the list, and press “Flow“. Continue reading