Fax Pass-through Mode vs Fax Mode

Answered Question
Dec 15th, 2009

Hi,

I am planning to use FAX machine under Callmanager 6.1.

The documentation below says that " Use "Fax mode" when the gateways in the network do not support "Fax pass-through mode"."

http://www.cisco.com/en/US/docs/voice_ip_comm/cata/186_188/3_0/english/administration/guide/sccp/sccpaapd.html

Is "Fax mode" just an alternative solution when you cannot use "Fax pass-through" mode?

Does "Fax mode" have any benefit over "Fax pass-through mode"?

I would like to pick more reliable solution.

Thanks in advance.

I have this problem too.
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Correct Answer by Jonathan Schulenberg about 7 years 1 month ago

"Fax mode" refers to fax relay (Cisco proprietary or T.38-based). Fax pass-through attempts to use a standard G.711 with VAD disabled audio call for the T.30 signals to pass through. Pass-through is extremely sensitive to packet loss and consumes lots of bandwidth by doubling the packets sent to minimize the chances of a lost packet. This sensitivity means a minor interruption or jitter will typically fail the entire fax. Relay consumes less bandwidth and can handle a packet-based network better.

The ATA186/188 do not support relay of any kind. They suck to put it bluntly. I would recommend using the VG202 or VG204 instead if you can.

Here's a lot more on the topic:

Cisco IOS Fax, Modem, and Text Support over IP Configuration Guide, Release 12.4T

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Correct Answer
Jonathan Schulenberg Tue, 12/15/2009 - 05:34

"Fax mode" refers to fax relay (Cisco proprietary or T.38-based). Fax pass-through attempts to use a standard G.711 with VAD disabled audio call for the T.30 signals to pass through. Pass-through is extremely sensitive to packet loss and consumes lots of bandwidth by doubling the packets sent to minimize the chances of a lost packet. This sensitivity means a minor interruption or jitter will typically fail the entire fax. Relay consumes less bandwidth and can handle a packet-based network better.

The ATA186/188 do not support relay of any kind. They suck to put it bluntly. I would recommend using the VG202 or VG204 instead if you can.

Here's a lot more on the topic:

Cisco IOS Fax, Modem, and Text Support over IP Configuration Guide, Release 12.4T

taroyamada9999 Tue, 12/15/2009 - 17:17

Thank you very much.

Your answer helped me understand the difference between them!!

You mentioned that the ATA186/188 do not support relay of any kind and "Fax mode" refers to fax relay.

What is the "Configuring the Cisco ATA for Fax Mode" section in the ATA documentation?

http://www.cisco.com/en/US/docs/voice_ip_comm/cata/186_188/3_0/english/administration/guide/sccp/sccpaapd.html

Jonathan Schulenberg Wed, 12/16/2009 - 06:39

Those are the instructions for properly configuring the ATA186 to do fax pass-through. For example, it disables VAD and other settings.

taroyamada9999 Wed, 12/16/2009 - 21:18

Thanks for your reply.

But under "Configuring the Cisco ATA for Fax Pass-through mode" section, it enables VAD.

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