But since analog fax tones cannot be compressed, fax must be digitized using the full rate G.711. This protocol is sometimes known as “Fax Pass-through.” It would be fine for faxing over digital services like PRI.
However, to convert that digital fax transmission to IP means encapsulating the 64kbps data stream into a series of IP-data packets. This process actually adds header overhead (as shown in the preceding diagram), increasing the total bandwidth to about 88kbps.
That’s 38% more bandwidth than a standard 64kbps voice call. Worse, it’s 175% more than a VoIP call that was compressed down to 32kbps using G.729, and it’s over ten times more bandwidth than 8kbps calls.
And it gets worse, because in addition to the extra bandwidth used, the process of switching between the different encoding protocols can actually cause the fax transmission to fail.
For example, if the SIP Trunking service default is G.729, but then it starts receiving packets encoded as G.711, brief gaps can appear in the fax tones as the system negotiates between the two protocols.
These gaps can cause the VoIP fax machines
to loose synchronization and eventually disconnect. This is especially true with faxes longer than about 4-5 pages.
So the longer the fax, the more likely it is to fail when running over G.711. Pre-deployment VoIP testing should take this into account by sending and receiving multiple 40-page faxes, not just 4 page faxes. 3. Packet Delay, Loss, Jitter explained.
We pointed out before that fax technology has mechanisms like error correction to deal with the types of impairments commonly found on analog telephone networks.
But those mechanisms do not work so well with the types of impairments that occur on IP networks—namely packet delay, jitter, and loss. These terms refer to the tendency of IP packets to be delayed or dropped, by design, as network switches and routers experience congestion. Jitter is the variability of delay.