Transporting Fax Communications - Passthrough Versus Relay

Those that are new to the transport of fax communications over IP (FoIP) are confronted with two different transport methods, passthrough and relay. Conceptually, passthrough is the same as a voice over IP (VoIP) call except that a low compression codec such as G.711 must be used. The reason that a codec such as G.711 must be used is because high compression codecs are optimized for human speech. Consequently, high compression codecs distort the fax tones and will cause FoIP calls to fail. Fax relay, on the other hand, does not use a voice codec to sample and digitize the fax tones like passthrough does. Instead, relay demodulates the fax tones. Faxes communicate using basic HDLC frames that are modulated into audible sounds. Relay strips out the HDLC information carried by the modulated fax tones and then simply transports the fax HDLC information across the IP network using a fax relay protocol such as T.38. On the terminating side the HDLC frames are modulated back into fax tones so that they are understood by the receiving fax machine. So, is one of these transport methods preferable over the other? Well, that depends on a number of factors but I can tell you that from my experience with Cisco customers these days, T.38 fax relay is the unanimous choice. However, passthrough is still used quite a bit as well. Below are some of the key differences between passthrough and relay, which may be helpful if you have to decide upon a FoIP transport solution. - Relay consumes less bandwidth than passthrough. In LAN topologies, this may not be a big deal but over WAN links where bandwidth is often contentious, relay offers significant bandwidth savings. - T.38 fax relay is the de facto standard for fax transport and because it is a standard, interoperability between the devices of different vendors is much easier. For example, if you want to integrate a dedicated IP fax server into your Unified Communications network, then T.38 fax relay is the protocol that you must use. The standard for passthrough is V.152 but I have not seen this implemented yet. Therefore, vendors tend to use different passthrough switchover methods, which can make interoperability difficult. - Because of its multiple levels of configurable redundancy, T.38 can handle much larger amounts of packet loss than passthrough. In networks with packet loss and/or other impairments fax passthrough calls are less reliable. - Passthrough supports Super G3 (SG3) or high-speed V.34 faxing natively. The benefit with SG3 faxing is that the top transmission speed is 33.6 kbps compared with the typical G3 fax call with a maximum speed of 14.4 kbps. Therefore, more faxes can be sent in a much quicker amount of time with SG3 fax communications. The T.38 specification offers a provision for SG3 support but this will not be supported by Cisco's T.38 implementation until sometime next year. - Many customers like passthrough for its simplicity (its just like a voice call) and because it uses the standard voice RTP packet header. This makes it easy to implement a feature such as Secure RTP (SRTP). Cisco's implementation of T.38 does not use RTP so running SRTP is not possible. For most organizations, T.38 fax relay is the best choice for transporting fax communications and it is Cisco's recommended best practice. However, passthrough is still a viable alternative for some and it merits definite consideration if SG3 fax support and/or SRTP support are desired. -David

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