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Follow-up to Can I bypass the PSTN with my current router/gateway setup?

You mentioned on an answer to a previous question which was follows:
If I have an E1 PRI ISDN GSM gateway (VCL-GSM E1 PRI-30), DX 2030 (Digital E1 VoIP router/gateway Quintum and Internet access of 512 upstream and downstream, can I successfully terminate calls without any connection to the PSTN?
With your answer being:
The VCL-GSM E1 PRI-30, this appears only to be a GSM to E1 PRI converter. It therefore has no connection to the IP network to route a call as a VoIP call to the Quintum gateway. You need to have the ability to support VoIP calls on both gateways to bypass the PSTN. A GSM to SIP gateway would be a more appropriate fit here.
Could you also have an ISDN to SIP gateway -- in addition to a "GSM to SIP" gateway?

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One case -- which I had not originally thought of -- could be:

GSM phone ← GSM → GSM to ISDN gateway ← E1 PRI→ Quintum gateway ← SIP→ ITSP (SIP trunking)

If the original goal was simply to be able to call from a GSM phone to the SIP network and have a local E1 connection between the gateways, then this will work without going to the PSTN. A cleaner scenario to accomplish this is to use a single GSM to SIP gateway like that from Quescom. This will also minimize delay.

Another case could be that they were trying to route calls over the Internet to terminate them to a PBX over TDM. This is more of what I had in mind when I made the recommendation of:

GSM phone← GSM→ GSM to SIP gateway← SIP → Quintum gateway← E1 to PBX or other equipment→ PBX or other equipment

The Quintum gateway is an ISDN to SIP gateway. If you are talking about putting another gateway down to build something like this:

GSM phone ← GSM → GSM to ISDN gateway← E1 PRI→ ISDN to SIP gateway← SIP→ Quintum gateway ← E1 to PBX or other equipment→ PBX or other equipment

This would not be advised as it adds even further to the overall latency.

This was first published in June 2006

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