User guide

If you want to implement a network in a developing region you might consider

using ATAs as they are normally cheaper than VoIP phones. ATAs are also

smaller and “customs-friendly” for import. One of the advantages of using ATAs

is that you can attach any type of phone to it, including a pay phone, a fax, or a

cordless phone (DECT)

. One popular ATA solution that supports IAX2 is the

s101i from Digium. The ATA is also known as IAXy

4.6 Codecs

A compressor/de-compressor (codec) algorithm is a set of transformations

used to digitize voice into data or convert digitized voice back to an analogue

signal. A codec takes an analogue signal and converts the signal into a binary

format (0s and 1s).

There are many ways to digitize the audio sound resulting in many different type of codecs. You can

assume that higher compression leads to more distortion (poorer quality). One codec is better than

another if it provides better voice quality using similar amount of bandwidth.

A basic digital circuit in the PSTN (your everyday phone) normally uses a codec known as Pulse Code

Modulation (PCM). PCM is a high quality codec but requires as much as 64 kbps. Two common PCM

compression standards are micro-law and a-law. These standards are also known as G711u and

G711a respectively. While micro-law is commonly used in North America, a-law is commonly used in

Europe. G711 codecs do not require much processing power and are available in most (if not all) VoIP

appliances.

In developing countries, the use of G.711 requires just too much bandwidth for a single phone call and

you need to consider the use of other type of codecs.

Good and free codecs that use low bandwidth are GSM and Speex. G.729 is another good codec but it

requires a licence for commercial usage.

21. The following ATA models have been evaluated as part of this primer: Sipura SPA-3000, GlobelTex IAD

22. The IAXy is a very small form factor ATA box with IAX2 support. It does not support high compression codecs.

23. G.729 is an 8 kbps audio codec (approx. 30 Kbps using SIP per stream). The codec was developed by a consortium of

organisations: France Telecom, Mitsubishi Electric Corporation, Nippon Telegraph and Telephone Corporation (NTT), and

Université de Sherbrooke. The price for the codec is around 10 USD.

Page 17 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008