Is a DSP needed when using a Cisco ATA186 ?

Unanswered Question
Apr 9th, 2009

Would I still need a DSP installed on my router if it will act as a H323 gateway with a Cisco ATA186 connected to it instead of FXS cards ? Also, this ATA186 will have a FAX machine off it and as I understand, it will use the G711 codec. My H323 codec is G729, will I need a DSP on the router for this also ? Thanks.

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Paolo Bevilacqua Thu, 04/09/2009 - 10:59

You will not need DSP for the ATA itself, but if you connect to PSTN via voice ports, you will need DSP anyway.

bspear123 Thu, 04/09/2009 - 11:09

No, the ATA will connect to the IP network at a remote site and contact a Gatekeeper at a central location. So , eventho I will be switching between two different CODECS (711 & 729) for voice and FAX, the remote sites will not need a DSP to do this ?

Paolo Bevilacqua Thu, 04/09/2009 - 11:21

You will then need a DSP on the closest router for the transcoding function.

Then you will find that fax may not work at all with G.729, and you will need fax relay.

Are you alone in doing this? The people that designed the network initially should know about all these issues.

bspear123 Thu, 04/09/2009 - 11:27

That confirms my suspicion that every remote router will need a DSP for transcoding the FAX to 711. I have read that the Cisco ATA186 does NOT support fax relay and that I will need to configure this as a "fax pass-through". My H323 voice traffic will use 729 codec and the fax pass-through will negotiate 711 codec. Can anyone confirm this for me before I attempt to roll this out ? Thanks !!

Paolo Bevilacqua Thu, 04/09/2009 - 11:33

You do not need a DSP for pass-through. In fact, from my understanding you do not need transcoding at all.

bspear123 Thu, 04/09/2009 - 11:43

Ok. Would I need to configure every remote site with a fax for the fax pass-through, or only the one Gatekeeper site ?

Paolo Bevilacqua Thu, 04/09/2009 - 12:04

Just configure the ATA for G.711, and let the GK be ok with that.

Note, G.711 on a WAN does not imply that fax will work 100%.

CHRIS CHARLEBOIS Thu, 04/09/2009 - 12:13

First, you only need transcoding if the two end devices cannot negoitiate the same codec or if you want to use a specific codec (say, over a low-bandwidth WAN link) that the endpoints do not support. As long as you configure the H323 gateway to support 711 (even if it prefers 729), you do not need to xcode.

The difference between Fax Relay and Fax Pass-through, as I understand it, is that Fax Relay interprets the fax signals, passes through the IP network digitally, and regenerates it on the far end, whereas Fax Pass-through simply encodes the analog signal as it would if it were voice. When you configure Fax Pass-through, all you are really doing is disabling any fax recoginition on the line so that the router does not use some other transport protocol that is designed for fax. So you only need to configure it on the site that will first receive the fax call from the analog port.


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