VOIP Strangeness with Analog Lines - PBX Info :: Your Free PBX, PABX and Telephone Information Resource

rhack · 07-03-2006, 09:05 AM

The system I'm working with is not a Meridian, but shares alot of the same components (IPE's, ALC's, DLC'c etc.). Looking to see if any of you have run into similiar circumstances when deploying voip products.

I am in the process of converting and old Intecom S80 PBX over to a CS2100. The CS2100 uses a voip trunk gateway called and MG3500. The MG3500 connects to the bear callp lan via gigE links. The IPE's are connected behind what is called an MG9000. Connection between the IPE's and MG9000 is TDM, the MG9K also connects to the bearer CallP lan via GigE.

On a couple of occaisions after a cut we have had to revert the analog lines that we have converted back to the old S80 because services would not work on the IPE's.

On one occasion, we had to swing a couple of fax lines back. The fax machines could not establish to fax machines that they previously could.

The other was an analog hunt group that interfaced to proprietary Lanier voice/data modems that interface to a medical transcription service. These modems shift between voice and data during a call. A remote transcriptionist calls in to the service enters some codes via key pad to access a Drs. dictation session. During the call the transcriptionist uses their foot pedal device to forward/rewind/pause etc. For some unkown reason, squeal and noise is generated and the transcriptionist cannot transcribe.

Both instances were temporarily resolved by reverting back to the S80.

I believe the trouble to be on the trunk gateway side. Reason I say this is that with the transcription service when we reverted back to the old phone system there were still issues. Calls were coming in on the voip trunk gateway, then tandeming across PRI tie lines. Issues did not go away until the DID number was ported back to the old trunks.

So, does anything sound familiar?

Thanks!

**Fletch** · 07-03-2006, 02:27 PM

Fax over IP (FoIP) can be troublesome. The FAX protocols are very time sensitive, and when transcoding to T.38 for transmission over VoIP, they become very sensitive to latency and jitter that relate back to timing issues. The same can occur if you are trying to transmit FAX over VoIP in G.711 mode. The slightest bit of echo or packet loss can be devasting to FAX. Hence the fundamental requirement to have T.38 supported. Also another thing to be aware of is any other Fax protocol in use. Just like anything else, transcoding over too many different CODECS will degrade the signal beyond a usable state.

JEHartmann · 07-03-2006, 02:55 PM

The same can occur if you are trying to transmit FAX over VoIP in G.711 mode. The slightest bit of echo or packet loss can be devasting to FAX.

Hi Fletch

You make me nervous whit these remarks, as I am to move an office from Greenland to Denmark.
We are connectet to an Mini 11c in Greenland with VoIP (G729AB) and via sattelite. The connection is actualy bether than via the publiic service, but should I expect that the FAX transfer to Denmark from Greenland will not work???

Regard

JEH

rhack · 07-05-2006, 01:05 PM

It was my understanding that T.38 only comes into play when the whole transmital is over IP. For instance analog gateway at originater is faxing to a fax machine at distant end which is also behind an analog gateway, and the path between them is pure IP. Once you bring a TDM path into place te T.38 protocol is not employed. Both ends have to agree on T.38.

What you said about echo is probably what is causing the problems. We have had problems with echo when terminating to pots lines, or from callers originating calls from POTs lines. Not all POTS lines induced echo.

**Fletch** · 07-06-2006, 07:25 AM

I was troubleshooting a problem where it was a BCM to CS1000 (ITG no SIg Serv) via T.38, then in and out of the CS1000 over a Voice over Frame Relay to a Passport where Fax Relay was in play, to another CS1000, then tandem through that to a Q.Sig link on another Passport link where the Voice over Fram link was G.711 at the PBX, but compressed to to G.729 on the PVC to another BCM where the FAX was. So....

G.711 --> T.38 --> G.711 --> Fax Relay --> G.711 --> G.729 --> G.711

Ouch! Too many hops and CODEC changes! An upgrade to 4.5 and the addition of Sig Servers and peer-to-peer IP changed this to:

G.711 --> T.38 --> G.711

Worked like a champ!

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