Differences from Fax Transmission in the PSTN
In IP networks, individual packets that are part of the same data transmission might follow
different physical paths of varying lengths. They can also experience varying levels
of propagation delay and delay that is caused by being held in packet buffers awaiting the
availability of a subsequent circuit. The packets can also arrive in an order different from
the order in which they entered the network. The destination node of the network uses
the identifiers and addresses in the packet sequencing information to reassemble the
packets into the correct sequence.
Fax transmissions are designed to operate across a 64 kbps pulse code modulation
(PCM) encoded voice circuit, but in packet networks, the 64 kbps stream is often compressed
into a much smaller data rate by passing it through a DSP. The codecs normally
used to compress a voice stream in a DSP are designed to compress and decompress
human speech, not fax or modem tones. For this reason, faxes and modems are rarely
used in a VoIP network without some kind of relay or pass-through mechanism in place.