Dec 30

Faxing – Bearer Capability Not Implemented (65)

Faxes failing the negotiation process. When ringing through, the fax starts to signal, however after a couple of seconds, the signalling is supressed, followed by an error on the Fax Display or results sheet. The Disconnect cause is 65 – Bearer Capability Not Implemented.

Setup Traces in CUCM and display the output in the TranslatorX. TranslatorX displays the key information of a call leg within a couple of mouse clicks. Below we can see the an Internal Fax is attempting to fax out to the PSTN. The fax is connected to an ATA Device.

Due to the nature of faxes, the G729 codec is not compatible with fax signal. The G729 compresses the rtp and hence corrupts the fax transmission. Below we see the negotiated codec is G729. So we’ll have to check how this codec was negotiated and not the G711a/ulaw.

CUCM Fax

Check the codec negotiated.

1. Check the Device Pool of the ATA and Voice Gateway. Collect the Regions, the confirm the Regions between the ATA and the Voice Gateway can indeed negotiate G711a/ulaw.

2. If all looks good, log into the Voice Gateway and check the codecs configured for the dial peers. Voice dial-peers on IOS Voice Gateway default to G729 codec. You must configure a voice-class with a preference to use G711alaw first. This ensures G711alaw is negotiated between the ATA (Fax) and the Voice Gateway. In the below example, only the codecs G711alaw and G711ulaw are permitted on the dial-peers 10 and 11.

CUCM Fax

** With the latest CUCM Deployment models, there is also the option to create/modify Codec Preference list under the Regions Menu. This allows us to the preference codecs between endpoints registered to CUCM and/or the Voice Gateways leading to the PSTN Network.

Sep 20

Faxing T.38 Relay MGCP to SIP

Not so commonly discussed area of the Voice Communication world is the T.38 Fax Relay between multiple signalling protocols. I was tasked to complete a Fax Server install into an existing Cisco UC environment. The Fax Server communicated to the CUCM via the SIP Protocol and the existing UC infrastructure inclusive of VG224 and a 2811 ISR was controlled via the MGCP protocol.

First thing first, gain connectivity between the CUCM and the Fax Server to ensure SIP is correctly working. Make a call to the fax server and you should receive the fax signalling sounds.. done.

Now lets work on the 2811 Voice Gateway. This is an ISDN Gateway controlled via the MGCP Protocol through CUCM. However we don’t need to log into CUCM to allow the T.38 Fax Relay protocol. Enable T.38 Fax Relay Globally under the Voice Services VOIP Menu.

(Config)# voice service voip
(Config-voip)# fax protocol t38 ls-redundancy 2 hs-redundanyc 0 fallback passthru g711alaw

In the above exampled I chose to fallback to Passthru method, as this is the existing method on the Voice Gateway.

Now lets enable the T.38 Fax Relay Protocol under MGCP.

(Config)# mgcp default-package fxr-package
(Config)# no fax t38 inhibit

Now lets restart the MGCP Application. *Depending your MGCP Configuration, this may disrupt existing calls.

(Config)# no mgcp
(Config)# mgcp

Faxing inbound/outbound the UC network should now be working as normal.

The VG224 configuration is exactly the same as the 2811 Voice Gateway. As organisations still have a need to fax internally, the VG224 needs the above configuration mended as well.

Below is the summary of configuration changes made.

(Config)# voice service voip
(Config-voip)# fax protocol t38 ls-redundancy 2 hs-redundanyc 0 fallback passthru g711alaw
(Config)# mgcp default-package fxr-package
(Config)# no fax t38 inhibit
(Config)# no mgcp
(Config)# mgcp