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New Member

Fax passthrough & call admission control

We have a customer with a CCM-cluster (4.1.3) at central site and 5 branches connected over WAN (256 kb). Over WAN we use g729, within the sites g711. At each site we have two or more fax, which are connected on fxs-ports on the local SRST-Router or on ATAs. The customer wants to fax over the WAN-links, so i have configured fax passthrough on the gateways (H323, MGCP) and on the ATAs, which is working well.

The problem is, when a fax-call is established over WAN, i see in RTMT that CCM takes away only 24kb of the available bandwidth and not 96kb, which is the required BW for fax passthrough. So the CAC is not right anymore.

Any ideas?

New Member

Re: Fax passthrough & call admission control


As long as you configured G.729 as the max audio codec to be used between sites, and fax pass-through means that the G.3 fax will be transported as a voice call ( not necesarry g.711 i think) sounds logical to me.

As you know g.729 s 8kb/s with headers and etc it takes about 24-25 kbit/s.

Any way can you check what codec you use in the call legs that are transporting the fax call - this will tell us more:)

New Member

Re: Fax passthrough & call admission control

Fax passthrough upspeeds the call from the selected voice codec (g729) to g711 as soon as the gateway detects that the call ist a fax. And that is exactly what our gateways and the ATAs are doing. The 96 kb BW are specified in the Cisco-documentation for fax-passthrough:


Pass-Through Method of Transport

Fax pass-through takes place when incoming T.30 fax data is not demodulated or compressed for its transit through the packet network. The two endpoints (fax machines or modems) communicate directly to each other over a transparent IP connection. The gateway does not distinguish fax calls from voice calls.

On detection of a fax tone on an established VoIP call, the gateways switch into fax pass-through mode by suspending the voice codec and loading the pass-through parameters for the duration of the fax session. This process, called upspeeding, changes the bandwidth needed for the call to the equivalent of G.711.

With pass-through, the fax traffic is carried between the two gateways in RTP packets using an uncompressed format resembling the G.711 codec. This method of transporting fax traffic takes a constant 64-kbps (payload) stream plus its IP overhead end-to-end for the duration of the call. IP overhead is 16 kbps for normal voice traffic, but when switching to pass-through, the packetization period is reduced from 20 ms to 10 ms, which means that half as much data can be put into each frame. The result is that you need twice as many frames and twice as much IP overhead. For pass-through, the total bandwidth is 64 plus 32 kbps, for a total of 96 kbps. For normal voice traffic, total bandwidth is 64 plus 16 kbps, for a total of 80 kbps.

New Member

Re: Fax passthrough & call admission control


thanks for the detailed answer.

I wasn't sure anyway i just guesed.

New Member

Re: Fax passthrough & call admission control

As i tested now, there is no difference between MGCP-GWs and H323-GWs. In both cases the subtracted BW in CCM-RTMT is only 24kbps.

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