Chapter 2. Preparing a System for Asterisk

Very early on, I knew that someday in some "perfect"
future out there over the horizon, it would be
commonplace for computers to handle all of the
necessary processing functionality internally,
making the necessary external hardware to connect up
to telecom interfaces VERY inexpensive
and in some cases trivial.

Jim Dixon, "The History of Zapata Telephony and
How It Relates to the Asterisk PBX"

By this point, you must be anxious to get your Asterisk system up and running. If you are building a hobby system, you can probably jump right to the next chapter and begin the installation. For a mission-critical deployment, however, some thought must be given to the environment in which the Asterisk system will run. Make no mistake: Asterisk, being a very flexible piece of software, will happily and successfully install on nearly any Linux platform you can conceive of, and several non-Linux platforms as well.^[*] However, to arm you with an understanding of the type of operating environment Asterisk will really thrive in, this chapter will discuss issues you need to be aware of in order to deliver a reliable, well-designed system.

^[*] People have successfully compiled and run Asterisk on WRAP boards, Linksys WRT54G routers, Soekris systems, Pentium 100s, PDAs, Apple Macs, Sun SPARCs, laptops, and more. Of course, whether you would want to put such a system into production is another matter entirely. (Actually, the AstLinux distribution, by Kristian Kielhofner, runs very well indeed on the Soekris 4801 board. Once you've grasped the basics of Asterisk, this is something worth looking into further. Check out http://www.astlinux.org.)

In terms of its resource requirements, Asterisk's needs are similar to those of an embedded, real-time application. This is due in large part to its need to have priority access to the processor and system buses. It is therefore imperative that any functions on the system not directly related to the call-processing tasks of Asterisk be run at a low priority, if at all. On smaller systems and hobby systems, this might not be as much of an issue. However, on high-capacity systems, performance shortcomings will manifest as audio quality problems for users, often experienced as echo, static, and the like. The symptoms will resemble those experienced on a cell phone when going out of range, although the underlying causes will be different. As loads increase, the system will have increasing difficulty maintaining connections. For a PBX, such a situation is nothing short of disastrous, so careful attention to performance requirements is a critical consideration during the platform selection process.

Table 2-1 lists some very basic guidelines that you'll want to keep in mind when planning your system. The next section takes a close look at the various design and implementation issues that will affect its performance.

Table 2-1. System requirement guidelines

With large Asterisk installations, it is common to deploy functionality across several servers. One or more central units will be dedicated to call processing; these will be complemented by one or more ancillary servers handling peripherals (such as a database, voicemail, conferencing, management, a web interface, a firewall, and so on). As is true in most Linux environments, Asterisk is well suited to growing with your needs: a small system that used to be able to handle all your call-processing and peripheral tasks can be distributed between several servers when increased demands exceed its abilities. Flexibility is a key reason why Asterisk is extremely cost-effective for rapidly growing businessesthere is no effective maximum or minimum size to consider when budgeting the initial purchase. While some scalability is possible with most telephone systems, we have yet to hear of one that can scale as inexpensively as Asterisk. Having said that, distributed Asterisk systems are not simple to designthis is not a task for someone new to Asterisk.^[*]

^[*] If you are sure that you need to set up a distributed Asterisk system, you will want to study the DUNDi protocol. You should probably get the interest of the Asterisk-Users mailing list as well, but be sure to wear your flame-retardant suit; for some reason, this subject can spur a heated (but generally very educational) debate.