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CPU Limitations

Naturally, there are limits imposed by the available CPU power on these sorts of small format systems. It is generally accepted that Asterisk requires about 30 MHz of CPU power per active voice channel. Thus the 266 MHz CPU on the Net4801, in theory, supports around eight simultaneous calls. This presumes that all calls are G.711 encoded audio; G.711 is the encoding scheme most commonly used by telcos in providing traditional land lines. When combined with IP overhead, G.711 requires around 80 kbps in each direction per call leg. Thus, passing ten calls to our Internet Telephony Service Provider (ITSP) requires a theoretical total of 800 kbps of bandwidth for both inbound and outbound data.

Sometimes it is desirable to limit or reduce the VoIP bandwidth requirements. In my case, my ADSL service is 2.2 Mbps download and 768kbps upload. With this sort of connectivity it's unlikely that I could pass more than five or six G.711 encoded calls at the same time. Thus, if I needed to maximize my call capacity I'd need to use a more advanced voice compression scheme such as G.729a encoding.

Codecs for G.729a compression are not available open source, and thus are not part of the Asterisk or Astlinux base installations. Licenses are commercially available from Digium for $10 USD per stream. Using G.729a on each leg of the call allows it to be compressed down to <10 kbps of data, but real-time compression is a very CPU intensive task. The SC1100 CPU on the Net4801 will only successfully encode two streams of voice using G.729a compression.

There are other voice compression standards, such as GSM, G.723, iLBC and SPEEX. These generally trade compression efficiency against latency, CPU requirements and voice quality. None are as widely accepted by ITSPs as G.711a/u and G.729a.

Again, considering the SOHO/small business nature of the target users for Astlinux, these limitations are probably not significant. It certainly doesn't impact my use of the system in my full time home office, even with four incoming lines. Of course, more demanding installations could run the generic 586 version of Astlinux on a more powerful hardware platform, to avoid being CPU limited.

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