Why T.38 Fax Relay is the De Facto Standard for FoIP?

As touched on in some of my previous blog postings, T.38 fax relay is the de facto standard for fax communications today. This was not always the case however. When I first started working with FoIP regularly about 8 years ago, the numerous Cisco customers that I worked with almost always implemented Cisco fax relay (Cisco's pre-standard fax relay implementation) or passthrough for transporting their fax communications over IP. Only rarely did I see T.38 being implemented at all. A lot of this probably had to do with Cisco fax relay being the default fax transport option on Cisco products, and fax passthrough's reputation for simplicity and reliability. However, starting about 4-5 years ago this trend started changing drastically. It was not long before it seemed most customers were deploying T.38 and the usage of Cisco fax relay and passthrough was declining sharply. So, why did this transition away from Cisco fax relay and passthrough to T38 fax relay occur? Well, I see three main reasons why T.38 has pretty much taken over as the FoIP transport method today: 1) T.38 is standards-based. Cisco fax relay is proprietary and only implemented on Cisco voice products. While the ITU-T V.152 specification offers a standards-based form of fax passthrough, it has yet to be implemented by Cisco and the other FoIP companies that I work with. Therefore, fax passthrough is implemented differently by vendors and even Cisco itself has two different fax passthrough implementations. However, T.38 has been widely adopted by numerous vendors. So, for Cisco customers who may want to integrate fax communications with third party products, such as voice gateways and fax servers, T.38 fax relay is a necessity. 2) T.38 fax relay offers a robust redundancy mechanism. With packet loss being the most harmful impairment for fax communications in IP networks, T.38 fax relay offers multiple redundancy levels that can be separately configured for the low-speed T.30 signaling messages and the high-speed page transmissions. Cisco fax relay does not offer redundancy at all and while some passthrough implementations offer redundancy in the form of RFC 2198, T.38's redundancy is superior. 3) T.38 consumes the least amount of bandwidth. When dealing with fax communications over WAN links where bandwidth is usually contentious, T.38 fax relay is more efficient than Cisco fax relay and passthrough. Over the frame relay WAN protocol, Cisco fax relay uses 48 kbps per call and passthrough with its G.711 codec is over 80 kbps. Meanwhile, T.38 fax relay is only 25 kbps per call. Even with redundancy enabled, T.38 fax relay still only consumes about 41 kbps per call. Deploying T.38 fax relay over the WAN can free up precious bandwidth for other types of traffic and this is especially important for low-speed WAN links. For the reasons listed above, T.38 fax relay is recommended as a design best practice by Cisco. While Cisco fax relay and passthrough are still in use by customers today, any new FoIP deployment should use T.38 fax relay for transporting fax communications.

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