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New Member

no modem conection with voip over frame-relay

I'm trying to connect two modems through voip over frame-relay network using the modem passthrough command but that is no working. The voice calls is operating very well but the modems no connect between them. I'm using G-729 codec with ip header compression and data fragment in 80 bytes.

3 REPLIES
New Member

Re: no modem conection with voip over frame-relay

what is the bandwith on your FR link? as far as i understand you have 64Kbit/s link. because your fragment size is 80 bytes and this is the recomended size for 64Kbit/sec. if so you cannot passthrough modem connection. for passing through modem connection you need "upspeed" your voip flow. routers must change codec to g711 for modem passthrough. as you know g711 requires the bandwith about 80Kbit/s.

New Member

Re: no modem conection with voip over frame-relay

The network bandwith is 128Kbps end the traffic is voice and data. The performance is good. As the network is real time working, I built a prototype at the laboratory and I have changed several parameters like bandwith to 256 Kbps,no fragment, direct codec G.711ulaw without passthrough command, and so forth. the results of that tests be no satisfactory. The IOS software version is c1700-sv3y-mz.121-2.T. What do I can to do?

New Member

Re: no modem conection with voip over frame-relay

There are some know issues with cisco dsps and V.90+ modems, but we have been able to get most modems to work correctly accross our network. If you are using g729 for all other voice calls you will want to use the modem passthrough command to have both gw's turn to modem passthrough. I typically set up each dial peer with the specific commands, since I have found the global commands are not consistently applied. Here is a sample dial peer config....

dial-peer voice 3 voip

description Default voip peer

preference 1

incoming called-number .T

destination-pattern .T

progress_ind setup enable 3

progress_ind connect enable 8

modem passthrough nse codec g711ulaw

voice-class codec 1

session protocol sipv2

session target sip-server

dtmf-relay rtp-nte

fax rate disable

fax protocol pass-through g711ulaw

ip qos dscp cs3 signaling

no vad

Some of these commands are specific to the isdn signaling set up we have at this site. Also, we are using sip hence the sip session target. But the modem and fax passthrough commands are pretty much universal.

Also, some of those commands are not supported until 12.2.13. You might want to think about an ios upgrade.

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