Faxing - protocol selection

Unanswered Question
Jul 5th, 2006
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I am trying to understand the order of fax protocol selection. For example, lets take a PSTN gateway that is MGCP enabled. Fax passthrough, T38 relay and Cisco fax relay are enabled. Which protocol will be used to negotiate with the far end device first(ATA, vg248 etc) ? Is there a particular order in which negotiation takes place ?

On the mgcp gateway the follwing commands are present.

mgcp modem passthrough codec g711ulaw

mgcp modem passthrough voip mode nse

ccm-manager fax protocol cisco (default)

no mgcp t38 fax inhibit (default)

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Overall Rating: 5 (2 ratings)
Daniele Giordano Sat, 07/08/2006 - 01:18
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In my opinion the best fax protocol is T38 (standard) followed by CISCO Fax relay (similar to T38) and the last option is fax pass through.

ATA 188 doesn't support T38. You can find more info about ATA MGCP here:


Info about VG248 here:


On your gateway you can configure T38 as first option. If T38 negotiation fail the second option will be CISCO or pass through:

fax protocol {t38 [nse [force]] [ls-redundancy value [hs-redundancy value]] [fallback {cisco |

none | pass-through {g711ulaw | g711alaw}}]}


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