Codecs used to compress and encode voice transmissions are not effective when the stream is a fax transmission.  As the stream is digitised data the codec can cause adverse affects.

  • Cisco Fax Relay (pre-standards), detects a fax call and downgrades the stream to an appropriate size for transmission for the IP network.  DSPs are required for conversion from analog to digital.
  • T.38 fax relay - standards based transmission over IP.  DSPs convertor the stream into an appropriate format, e.g. 9600bps group 3.  T.38 is non-propriatory, devices such as servers can support sending/recieving if they meet the T.38 standard.
  • T.39 store and forward - supports the above, but with the ability to digitally scan/email.  Devices are configured as on/off ramps depending on the direction of the transmission.
  • Fax Pass-through - treated as a normal call with G.711 codec, 64Kb/sec, without VAD.  Fax transmission is suceptable to packet loss in the IP network, which could break transmission.