Monday, April 05, 2010

use asterisk to implement heavy duty fax server

it is possible to implement a fax server that can send out over 10,000 faxes per day using below components.

asterisk - pbx
hylafax - provides fax server software
iaxmodem - software modem using the dsp library

the fax path would be like this:
hylafax->iaxmodem->asterisk->sip peer->pstn

there are some tips to improve the successful rate.
1. use g711 codec
- fax transmission just does not favour any lossy codec which inevitably varies the fax tones

2. turn off ECM
- ECM involves data retransmission which could aggravate problems due to network latency and jitter

3. limit transmission rate below 14.4 kpbs can sometimes improve successful rate

4. upgrade your server hardware
- software-based fax transmission is cpu-bound job

5. Add the two lines below to modem configuration files (in /var/spool/hylafax/etc/) to alleviate HDLC problems.
Class1FrameOverhead: 2
Class1SwitchingCmd: ""

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