Solved: fax relay + fax passthrough on mgcp

Alexander.Khlemanov_2 · ‎11-14-2007

Hi All,

Does anybody know if it is possible to use fax relay and fax passthrough at the same time on a MGCP gateway?

Thanks.

Jon Nelson · ‎11-15-2007

Great information from Allan, but the answer is it is one or the other with MGCP. CUCM4.2 and CUCM6 are the only versions of CUCM capable of controlling the gateways running T.38. If you aren't running one of these versions of CUCM, I would stick with fax passthrough and leave the mgcp fax t38 inhibit command in place. I would also check to make sure you have this command in place:

no ccm-manager fax protcol cisco

as well as under voice service voip this command:

fax protocol pass-through g711ulaw

I hope this helps!

Thank you,

Jon

View solution in original post

allan.thomas · ‎11-15-2007

If your are refering to gateway controlled MGCP T.38 fax-relay, then essentially during the capabilities exchange the gateway will switch to T.38. If the gateways do not support T.38 then pass-through will be used for the fax transmission.

This below details were taken from the following link:-

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide_chapter09186a00800b5dce.html#wp1158066

In gateway-controlled mode, the gateways exchange NSEs that provide the following services:

â€¢Instruct the peer gateway to switch to T.38 for the call.

â€¢Either acknowledge the switch and a readiness to accept packets or indicate that a gateway cannot accept T.38 packets.

MGCP-based fax relay in gateway-controlled mode uses the following call flow:

1. An incoming call is initially established as a voice call.

2. The gateways advertise capabilities in an SDP exchange during connection establishment.

3. If both gateways do not support T.38, fax pass-through is used for fax transmission.

If both gateways do support T.38, they attempt to switch to T.38 upon fax tone detection. The existing audio channel is used for T.38 fax relay, and the existing connection port is reused to minimize delay. If failure occurs at some point during the switch to T.38, the call reverts to the original settings that it had as a voice call. If this failure occurs, a fallback to fax pass-through is not supported.

4. Upon completion of the fax image transfer, the connection remains established and reverts to a voice call using the previously designated codec, unless the CA instructs the gateway to do otherwise. If the CA has been configured to control fax relay, the CA instructs the gateway on processing the call with the fx: extension of the local connection option (LCO).

Allan.

Alexander.Khlemanov_2 · ‎11-15-2007

Hi Allan,

Well, I should better explain what kind of information I am looking for :)

We have an MGCP gateway connected to PSTN via Q931 and configured for fax pass-through. When we place a fax call through this gateway from a fax plugged into ATA188 the fax goes through all right. A part of this MGCP configuration is "mgcp fax t38 inhibit" command. As soon as we send "no mgcp fax t38 inhibit" command enabling t38 fax relay in this gateway, all fax transmissions from faxes connected to ATA188 fail. I know that ATA can support only fax pass-through. So I wonder if by enabling fax relay in MGCP gateway we actually disable fax pass-through? In H.323 gateways we can have different fax modes (relay and pass-through) connected under different dial-peers. What about MGCP gateways? Can we have fax relay for some calls and fax pass-through for other calls at the same time?

Alex.

Jon Nelson · ‎11-15-2007