Page 42 - Asterisk™: The Future of Telephony

Performance Issues
Among other considerations, when selecting the hardware for an Asterisk installation
you must bear in mind this critical question: how powerful must the system be? This
is not an easy question to answer, because the manner in which the system is to be used
will play a big role in the resources it will consume. There is no such thing as an Asterisk
performance-engineering matrix, so you will need to understand how Asterisk uses the
system in order to make intelligent decisions about what kinds of resources will be
required. You will need to consider several factors, including:
The maximum number of concurrent connections the system will be expected to support
Each connection will increase the workload on the system.
The percentage of traffic that will require processor-intensive DSP of compressed codecs
(such as G.729 and GSM)
The Digital Signal Processing (DSP) work that Asterisk performs in software can
have a staggering impact on the number of concurrent calls it will support. A system
that might happily handle 50 concurrent G.711 calls could be brought to its knees
by a request to conference together 10 G.729 compressed channels. We’ll talk more
about G.729, GSM, G.711, and many other codecs in Chapter 8.
Whether conferencing will be provided, and what level of conferencing activity is expected
Will the system be used heavily? Conferencing requires the system to transcode
and mix each individual incoming audio stream into multiple outgoing streams.
Mixing multiple audio streams in near-real-time can place a significant load on the
CPU.
Echo cancellation
Echo cancellation may be required on any call where a Public Switched Telephone
Network (PSTN) interface is involved. Since echo cancellation is a mathematical
function, the more of it the system has to perform, the higher the load on the CPU
†
will be. Do not fear. Echo cancellation is another topic for Chapter 8.
Dialplan scripting logic
Whenever Asterisk has to pass call control to an external program, there is a per-
formance penalty. As much logic as possible should be built into the dialplan. If
external scripts are used, they should be designed with performance and efficiency
as critical considerations.
As for the exact performance impact of these factors, it’s difficult to know for sure. The
effect of each is known in general terms, but an accurate performance calculator has
not yet been successfully defined. This is partly because the effect of each component
of the system is dependent on numerous variables, such as CPU power, motherboard
chipset and overall quality, total traffic load on the system, Linux kernel optimizations,
network traffic, number and type of PSTN interfaces, and PSTN traffic—not to mention

† Roughly 30 MHz of CPU power per channel.

14 | Chapter 2: Preparing a System for Asterisk