VG224 Modem Speed

[Donald St. Croix]

What is the max modem speed of vg224?


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[George Thomas]

I believe the highest would be T38 version 3 so 33.6kbps.

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[Gordon Ross]

I've struggled to get much over 24Kb/s.

GTG

[Jonathan Schulenberg]

Modem speed is separate from fax. If using Cisco-proprietary modem relay, the highest you can achieve is V.34 which runs at 33.6kbps best case. Technically V.34 caps out at 28.8kbps and V.34bis is required for 33.6kbps but none of Cisco's documentation makes this distinction as IOS supports V.34bis. Even if the modems support V.90 the training will be forced down to V.34. If using modem passthrough, you may be able to achieve a higher speed; however, there is no guarantee and the call will be extremely vulnerable to delay/loss.

For faxing, George is correct that you can achieve SuperG3 (i.e. V.34) faxing with T.38v3 though this requires support on both the VG224 and the PSTN-faxing gateway. Prior to T.38v3 an alternate option was to enable fax relay for G3 (i.e. V.17) and use modem passthrough for SuperG3 faxes. You had to disable the sg3-to-g3 command for this to happen though as IOS forced the faster machines down to G3 by default.

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[Donald St. Croix]

We are unable to get above 14.4.  The VG is using MGCP.  The fax rate command shows 14.4 as max speed but there is also a voice option.  It states it is based on dial peer codec, We have g.711 but it does not list what is capable.  I have looked through the SRND and can not find anything like a matrix that list type of fxs and max speed both from modem and Fax machine.  I have opened a TAC case and hope to get a clear answer.

So far this is the best documenation I can find. 

https://supportforums.cisco.com/docs/DOC-25210

[Jonathan Schulenberg · Accepted solution]

14.4kbps is the maximum speed supported under V.17 which is all T.38 version 0 and Cisco-proprietary fax relay support. MGCP does not support T.38 version 3 (only SIP/H.323 do). Your only option for faster speeds with MGCP would be to enable modem passthrough and disable SG3-to-G3 stepdown. The former is a dropdown in CUCM on the Gateway configuration page, the second is a command on the router 'no mgcp fax-relay sg3-to-g3'

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[Donald St. Croix]

I had the VG224 MGCP config page set to Modem Passthrough enabled, I added the cli command "no mgcp fax-relay sg3-to-g3" to the VG224.  Customer is on the West Coast, I will have them test when they get in, Thanks for the suggestions and time.

Call flow PSTN -> T1 CAS -> SIP -> CUCM -> VG224 -.Modem

Any changes needed on the Voice Gateway.

I added these based on the previous listed url.

voice service voip

fax protocol pass-through g711ulaw