\section{Introduction to T.38} \label{sec-t38intro}
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The ITU T.38 recommendation defines how to transfer fax in realtime between fax gateways and/or IP fax machines in an IP network. Transport of the fax signals is done either by TCP or UDP, and reliability with UDP is achieved through error control mechanisms in T.38, which can be either parity FEC or packet redundancy. The recommendation assumes that a network connection has already been established by the two (or more) peers. This is similar to traditional fax, where a phone line is allocated before the actual fax signalling starts.  The issues that are left to be handled by other mechanisms are addressing, identification, authentication, and creation of the fax connection. The fax machines must also have agreed on whether to use UDP or TCP for transport, and in case of UDP, the error control scheme to use.






