Send Receive Fax-T38

Howto send&receive faxes with T.38 with almost 100% success rate

In this article i will show you how to enable fax -ing with T.38 support. For those who don't know what T.38 is, it's a protocol especially for FAX over VoIP. You can send & receive faxes with almost 100% success rate, even if you're using a low-quality codec (g726,g729).

The main technology is simple: Because standard FAX is mainly designed for POTS media, it's very sensitive to quality of the line. From the view of VoIP it works only with g711u/a (PCM) and with very good network conditions (no packet loss, very low round-trip). The T.38 makes a tunnel between two (T.38 capable) devices and code-decode the fax at the ends of that tunnel. The data-sending protocol of T.38 is (in most cases) UDP based and it can be sent via very poor network environments.

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Because of it's mechanism, you will need a device (ATA) with T.38 support and your provider must support T.38 too.

My device is a Linksys SPA2102 (settings are the same for SPA3000). Of course you can use other ATA, which supports T38.

• 1. First, you'll need the source tarball of Asterisk (same version as installed). (My version: 1.4.17)

 After you untar it, edit chan_sip.c file located in channels/ directory of the source tree.
 
 Search for this line:  
    static int global_t38_capability = T38FAX_VERSION_0 | T38FAX_RATE_2400 |
                          T38FAX_RATE_4800 | T38FAX_RATE_7200 | T38FAX_RATE_9600;

 and replace with this line (If your provider & ATA supports T.38 version 1, change T38FAX_VERSION_0 to T38FAX_VERSION_1):
    static int global_t38_capability = T38FAX_VERSION_0 | T38FAX_RATE_2400 |
                          T38FAX_RATE_4800 | T38FAX_RATE_7200 | T38FAX_RATE_9600 | T38FAX_RATE_12000 | T38FAX_RATE_14400;

 Save the file and change directory to the root of the source tree:
    
     cd ..

 Build the source:
     make

 Copy the newly created chan_sip.so over the old one:
     cp channels/chan_sip.so /usr/lib/asterisk/modules/

• 2. Turn on T.38 in MOR gui at both the provider and the device!

• 3. Make sure T.38 is enabled in /etc/asterisk/sip.conf!

  T.38 can be enabled by this setting:
  t38pt_udptl = yes

• 4. Edit /etc/asterisk/udptl.conf like this:

 ;
 ; UDPTL Configuration (UDPTL is one of the transports for T.38)
 ;
 [general]
 ;
 ; UDPTL start and UDPTL end configure start and end addresses
 ;
 udptlstart=4000
 udptlend=4999
 ;
 ; Whether to enable or disable UDP checksums on UDPTL traffic
 ;
 udptlchecksums=no
 ;
 ; The error correction type to be sent
 ;
 ;T38FaxUdpEC = t38UDPFEC
 T38FaxUdpEC = t38UDPRedundancy
 ;
 ; The maximum length of a UDPTL packet
 ;
 T38FaxMaxDatagram = 200
 ;
 ; The number of error correction entries in a UDPTL packet 
 ;
 udptlfecentries = 3
 ;
 ; The span over which parity is calculated for FEC in a UDPTL packet
 ;
 udptlfecspan = 3

• 5. Settings on SPA2102

 Make sure you have "FAX Enable T.38" enabled in your device! You should also set other settings to make it work:

 FAX Passtru method: ReINVITE
 Call Waiting: Disable
 Jitter level: Very High
 Silence Supp: Disable
 Echo Cancel: Disable

 By the way, it should work with factory settings out-of-box (T.38 is enabled by default).

I hope you will get working for now. If not, try to tune settings in udptl.conf!

--Szab100 14:22, 12 March 2008 (CET)