Fax & Voip – Part 3/3 – Baud Rate & Fax Relay

oldfaxmachineDecrease Baud Rate and turning off ECM
Packet loss, jitter, and latency all add up to corrupt a fax transmission in VoIP env
and unfortunately also in Asterisk PBX.

When a fax machine determines that some data is lost, it will shut down the entire fax transmission, even though you would still have a workable document.

By slowing down the transmission speed up to 9600 bps, you’ll increase the probability that all the faxes will be transmitted or received correctly.

To slowing down the BAUD rate to 9600 bps you need to check the parameteres of the fax device. Continue reading

Fax & Voip – Part 2/3 – From Vad to Clock……

fax-machineCodec Translation
I have already mentioned before. In the path caller->called codecs can change several times, and every transformation can introduce distortion and delay.

To minimize fax issues all the legs involved must be using the same codec to avoid codec translation. It is really important the kind of codec: you must use in all the path alaw or ulaw ! Continue reading

Fax & Voip – Part 1/3 – All about codec in Fax transmission

faxmachinemurderCertainly there are huge benefits in adopting VoIP technology (i.e. Asterisk PBX), and certainly there will be many people who will be wise to use Voip PBX trunk instead of traditional phone lines.

But using Voip PBX you may start to notice that faxing just doesn’t seem to work as well as you remember it working on the old analog phone line.

Definitely: faxes work so well in the VoIP PBX/line as the old analogue line/Analog Pbx ?
The answer is: usually no !

Faxes in Voip env do not work as well as you in “old” phone line. FAX was designed for analog networks, and can not travel easly over a digital VoIP network.

The reasons are many: starting from the codec used by VoIP that are designed to compress voice and save bandwidth, and that corrupts the signals used by modems/fax, until the fact that the signal is transformed several times in the path caller-called, and Pbx Voip add more transformation point making more difficult transmit/receive fax.

Remember: it is very important understand that each transformation step introduces some distortion to the signal, imperceptible to the voice but not for the fax: and for faxes it is very important the signal quality !

In this series I’ll try to suggest some tips to increase the fax quality in Voip env.
Continue reading