Traditionally, faxes and modems made use of the timing nature of TDM based, with the whole network being tied to a strict clock that ensured synchronization throughout the network. This strict clocking and the dynamic range of a TDM circuit allowed modems and faxes to use very complex audio techniques to achieve high rates of throughput. In packet based networks, the clocking synchronization is relinquished in favor of more flexible call routing options. This imposes some restrictions in the transfer of fax and modem traffic across packet based networks. Mechanisms such as fax and modem relay are widely deployed in order to achieve reliable high-speed throughput whilst still retaining the flexible nature of packet networks.
Given the bursty nature of IP networks, the transmission of the various facsimile and modem standards embedded inside of digital voice codecs across them can be problematic. Hence the various additional standards for the use of faxes and modems in IP networks to deal with the variations in delay and jitter.
There are three techniques for sending faxes over IP voice networks consisting of fax pass through, real-time fax relay and fax store and forward. There are two techniques for utilizing modems in an IP voice network consisting of modem pass through and modem relay.