fax relay and fax pass through

[ciscoforum]

Have read a lot of document about fax relay and fax pass through, but still not very comfortable with it regarding the difference.

Fox example, both of them can be used on voice over IP. In voip situation, what's the difference? From a IP packet point, both fax content will be encapsulated inside the IP. Anybody can explain in a good way the difference between relay or pass through when fax over IP? fax1-R0-IP-R1-fax2.Thanks

[mcotrone]

There is a ton of good/ok doc on T.38 (FAX Relay) and Fax passthrough.

Basically this is how I view them:

T.38 is a true FAX over IP protocol that operates like this: T.30 FAX call terminates on a gateway that supports T.38. The fax is then sent via IP (t.38) to the far end gateway. This far end GW also running the T.38 protocol then converts the T.38 back to T.30 FAX signaling. Cisco has a proprietary version of this called Cisco FAX Relay.

FAX/Modem Passthrough is when a gateway receives a T.30 FAX tone and forces its codec towards the FAR end VOIP GW to G.711 (Codec Upspeed). The playout buffers are also tuned then the FAX is sent just like a normal G711 Call (T.30 encapsulated inside IP) to the far end gateway. The Gateway then sends the call to the far end FAX.

Nutshell---

T.38/Cisco FAX Relay -- T.30 to T.38(which is FAX over IP) back to T.30

Passthrough is just that --- T.30 to T.30 using G.711 as a normal voice call.

Basically what you have to do up front in a VOIP network is decide which Gateways you will be using in your network and how these gateways will interact with one another. This will usually drive you into a FAX/Modem relay/passthrough method.

For example if you have ATA's, VG248's, and 6608 gateways you have to run FAX/Modem passthrough since the ATA's do not support T.38 faxing.

For example I have recently run into using the new CMM module with MGCP and T.38 code isn't supported on this platform yet.

HTH,

Mike

[hsoep]

Hi,

Sorry to barge in on this thread, but I have also got some issues on t.38.

I have configured an on-ramp gw (827-4v / 12.2(13) ZH4) as a h.323 gw. As off-ramp I have a 3725 (12.3(11)T) as a MGCP gw. As far as IOS sw t.38 should be supported in both gws, but I have a hard time documenting, that I really am utilizing the t.38 protocol across the LAN between the gws when faxing.

Tried the debug voice ccapi inout command. Somewhere in the output I get a cc_api_t38_start line, but that's it. I'm not convinced as Cisco documentation states that other output would be expected....

Any comments on this?

Regards

Henrik S Pedersen

TDC

[mcotrone]

Hendrik,

I haven't worked much with Off-Ramp so I really can't comment either way. You may want to open a TAC case and deliver them the debugs.

Sorry.

Mike

[hsoep]

Hi,

That's ok.... I'll give it one more shot, and then I may open a TAC case on it....

Thanks,

Henrik