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Troubleshooting Fax Relay

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Other basic connectivity problems not relating to dial peers , including:

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Fax connectivity problems across the PSTN—

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Cisco BTS 10200 Softswitch T.38 Fax Relay Support

It is not always required that you have identical configurations on the outgoing and incoming

VoIP dial-peers. When you have a fax relay problem though, make sure you have a dedicated

incoming VoIP dial-peer on the terminating gateway and that its configuration matches the

configuration of the outgoing VoIP dial-peer on the originating gateway.

Another method you can use to check dial peer matching is to issue the debug voip ccapi inout

command. The debug output from this command will show an ssaSetupPeer message that lists

all the dial-peers matching the called number. A ccCallSetupRequest message follows with the

outbound peer option indicating the outgoing VoIP dial-peer selected. When multiple VoIP dial

peers are configured for the same destination, it is possible that the initial call setup could fail

and another dial peer tried. In this case another ccCallSetupRequest will appear in the debug.

On the terminating voice gateway the first line of the debug voip ccapi inout call trace will be

a cc_api_call_Setup_ind message with a peer_tag option that refers to the incoming VoIP dial

peer on the terminating gateway.

Incorrectly configured dial peers on one or both sides—

After you confirm that the correct dial peer is being matched (in this case dial-peer 100 for the

originating gateway and dial peer 400 for the terminating router), confirm in the configuration

that the dial-peer is configured correctly for fax. Some common errors to check for on both sides

of the call are:

Fax relay is disabled (that is, the fax rate disable command has been issued on the dial

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peer) while a low bandwidth codec has been in use.

The dial peer on one gateway is configured for T.38 fax relay but the other gateway is

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configured for Cisco proprietary fax relay, so the negotiation will fail.

The default dial peer is used inbound on the terminating gateway and these parameters do

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not match with the parameters set for the outbound dial peer on the originating gateway.

Incorrect compand type—

The companding type for the US is µ-law, for EMEA it is a-law. The show voice call command

dispolays which value is currently configured. If, on a BRI or E1 port, the companding type on

the gateway does not match the one on the connected device, calls will sometimes fail and

sometimes connect. When a connection is made the voice becomes heavily distorted, the person

becomes unrecognizable, and a high volume low-pitch noise level is heard.

Cisco IOS Software incompatibilities on gateway pairs.

It is not always required that Cisco IOS Software releases on the gateways match, but it is

recommended to check releases for compatability when fax relay problems occur.

Compressed Real-Time Transport Protocol (cRTP) problems.

There are several known problems associated with cRTP. Fixes are available for these problems

and it makes sense to disable cRTP when fax relay problems occur and to check whether a Cisco

IOS Software upgrade is an appropriate course of action.

Ensure that the VCWare and Cisco IOS Software are compatible.

On Cisco AS5300 series voice gateways, the Cisco IOS software might be incompatible with

the VCWare software.

Cisco BTS 10200 Softswitch T.38 Fax Relay