Knowledgebase

1. The purpose

Asterisk can be used to transcode one voice codec to another. This document will show how to make transcoding between g711u and g729. Main scope is to send call in ulaw from Voipswitch to Asterisk box and get back the call in g729.

2. Prerequisites

• Linux Slackware
• Installed Asterisk with all modules (Zaptel, libpri, asterisk), Intel IPP libraries, compiled G.729 & G.723 Codecs
  (how to do this you can read here http://docs.voipswitch.com:8080/display/VDoc/Transcoding+on+Asterisk

On kernel 2.6.27.7-smp please use Intel l_ipp_ia32_p_6.0.2.076.tar.gz libs instead of l_ipp_itanium_p_6.0.2.076.tar.gz

3. Configuration files

The main configuration files are:
sip.conf - /etc/asterisk/sip.conf
extensions.conf - /etc/asterisk/extensions.conf

It's worth to check if the codecs are compiled correctly under Asterisk before further configuration. To do this please check if you have codec_g723.so* & codec_g729.so* files in /usr/lib/asterisk/modules/.

a) configuration of the sip.conf file

(general)
port=5060
bindaddr=209.160.20.243
srvlookup=yes
maxexpiry=6400
minexpiry=60
qualifyfreq=10
relaxdtmf=yes
nat=yes
dtmfmode=rfc2833
trustrpid=yes ; remote party id
sendrpid=yes ; remote party id
threewaycalling=yes
transfer=yes
insecure=very
disallow=all
allow=g729
allow=ulaw

(101)
;fullname=101
type=friend
context=testujemy
username=101
secret=101
host=dynamic
nat=yes
disallow=all
allow=ulaw

(102)
;fullname=102
type=friend
context=testujemy
username=102
secret=102
host=dynamic
nat=yes
disallow=all
allow=g729

b) configuration of the extensions.conf file

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