Fax relay and pass through

Unanswered Question
Apr 3rd, 2007


This link says that Fax-relay is the preferred fax method.

1. Is there a particular reason Cisco recommends this instead of using Pass through,besides using less bandwidth?

2. If I have big enough bandwidth , say OC3 for about 20 sites where the GW will connect to PBX and fax is on the PBX. GW are all Cisco's 28xx. Do you think pass through will be good idea?


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
johnnylingo Fri, 04/20/2007 - 09:31

Yes - Error Correction (ECM). If errors are received, the fax machines will try to re-transmit the frames and complete the page.

I'm deploying Fax Relay in a campus environment that has plenty of bandwidth and there shouldn't be any packet loss. But I'm still planning to use T.38 with H.323 and MGCP gateways since it provides ECM in case of an error, and also it will be easier to identify fax calls.

johnnylingo Fri, 04/20/2007 - 10:19

Forgot to mention - one downside of Fax Relay is you are limited to 14.4 kbps transmission speed. Passthrough can go up to 64 kbps with G.711, although realistically you'll see speeds 9.6 - 33.6 kbps depending on the speed of the fax machine(s).

jassem_aldulaimi Sun, 11/21/2010 - 22:25


I'm trying to implment fax over IP, but I'm not sure whether I fully understood what's needed, can you please help? This is what I want to achive:

FAX-->Linksys PAP2T--------->Brekeke SIP Server----------->AS5300-----------Fax (PSTN)

PSTN (fax)--------------->AS5300----------->Brekeke----------->PAP2T-------->Fax

I tried to use T38 Fax relay, the fax answer but nothing coming out in eitherway, can someone please direct me in the right path and put some examples of what's the correct configurations/ dial-peer on the AS5300, I belive I'm doing something not right at the AS5300.  Please note my voice call have no issue what soever "SIP-to-PSTN, SIP-to-SIP, PSTN-to-SIP". Please post your answer to both the forum and my email.



[email protected]


This Discussion