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New Member

Fax Disconnects on FXS ports

I have having trouble getting faxes into a fax machine on an FXS port. The call hits the fax machine and start negotiation, but then disconnects. Need help.

Here is the dial peer for the DID:

dial-peer voice 0283 voip

destination-pattern 0283

session target ipv4:

dtmf-relay h245-alphanumeric

codec g711ulaw

New Member

Re: Fax Disconnects on FXS ports

I added these commands to my fax dial peers on the recomendation of the local Cisco SE, and they helped.

fax-relay ecm disable

fax rate disable

no vad

New Member

Re: Fax Disconnects on FXS ports

i think you forgot the most important part: "enabling the fax protocol" on the box itself.

1. in global config:

voice service voip

fax protocol t38 ls-redundancy 0 hs-redundancy 0

2. in the voip dial peer that communicates with the remote gateway, add

fax protocol t38 ls-redundancy 0 hs-redundancy 0

step 2 is not as critical as step 1, but it never hurts to make sure the feature is enabled. As you can see, this configuration assumes you are using the standard T38 fax. The other option is to use cisco proprietary fax protocol. and needless to mention that the remote gateway must have T38 fax enable on it as well, or they won't be able to negociate.

If this doesn't work, you might have to move to sniffing the packets and monitoring signaling, especially h245 signaling. the one that triggers fax is a "RequestMode", followed by an incoming "ACK" and a bunch of "OLCs" exchanges.


Cisco Employee

Re: Fax Disconnects on FXS ports

There are several ways of transporting fax information over VoIP:

1) fax-relay - mentioned above is t.38 fax relay whereby the router actually detects fax tones and tries to negotiate a fax codec. Another alternative is "cisco fax relay" which came out before the t.38 standard

2) fax passthrough - not using a fax codec at all, but passing the fax tones in the voice path. This works best with higher-quality codecs such as g.711, etc. To enable this we need to disable fax-relay. Configure "fax rate disable" under your dial-peer.

Faxes, in general, are sensitive to any jitter and delay, and a feature called ECM will make them even more so. I would try the following:

1) configure "fax rate disable" on your dial peer and try the fax.

2) if it succeeds, great, you're doing fax passthrough with g.711

3) if it fails, make sure that ECM is disabled on both fax machines

4) if it still fails, try with a different fax machine. Some older fax machines have very poor results.

5) if fax passthrough succeeds but you still want to do fax relay (or neither succeeds), I would recommend opening a TAC case as these cases are often tricky to resolve.



New Member

Re: Fax Disconnects on FXS ports


I had encounter the same problem like yours, although my session target was pointing to the call manager. But when I configured the VoIP dial-peer going directly to the ip address of the router where the FxS port is connected, everything works well.

By the way, do you have two separate routers, meaning one which connects to the PSTN then another router within the LAN with an FxS port that connects your fax? If your fax is directly connected to the router that is also connected to the PSTN, you don't need to configure VoIP dial-peer. You just put "connection plar" in the voice-port sub-command. I also noticed that you mention DID, did you put "direct inward-dial in your pots dial-peer?