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New Member

Fax relay and pass through

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?


New Member

Re: Fax relay and pass through


Re: Fax relay and pass through

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.


Re: Fax relay and pass through

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).

New Member

Re: Fax relay and pass through


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.



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