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.
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Add Fax capabilities in Asterisk >=13

AsteriskRecomplieWithSpanDspBeginning with Asterisk 13 Digium will no longer make available the res_fax_digium module (FFA or commercial versione): if you want fax capabilities you should use the res_fax_spandsp module.

Yes, I know, it is possible to use IAX modem, but sometimes I prefer use T.38 protocolo to receive fax, and only res_fax_spandsp can support this protocol. I’ll show hot to install res_fax_spandsp in a working IncrediblePbx 13 inst.
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How to configure a Nortel/Avaya 4550T-PWR switch – Part 2/2

NewtworkSchemaIn this part I’ll show the configuration steps to obtain the situation in this network schema. This is a really simple and real situation: a simple network using two separated VLAN one for Data and the other for Voip.

Att.: In this example for to keep everything simple, clear and practical we’ll use Asterisk PBX and Snom IP Phone, but the same considerations apply to other brands.

Att.: Generally it’s important that we separate the voip traffic from the data traffic so we’ll utilize two different VLANs: one VLAN will carry the data traffic destined to the desktop and server and the other will carry only the Voip/voice traffic. Continue reading