The Anatomy of an Open Source Phone System

April 23, 2008 by Garrett Smith

Yesterday I was conducting a product training for three of our new sales hires and we started to talk about open source phone systems. As you can imagine (since you have probably been there yourself) as a newbie, wrapping your head around open source phone system isn’t always the easiest thing to do. Heck, learning about VoIP in general can be difficult and when you throw in the part about open source software you can see how things can derail quite quickly.

In order to help them better understand open source phone systems, I came up with a layered anatomy approach to describing an open source phone system. While this is nothing revolutionary in my eyes, it seemed to help them, so I figured why not throw it out there in hopes that it might help others.

Anatomy of an Open Source Phone System

When I think of an open source phone system I think of a lemon meringue pie (probably because I am a fan of it). Everyone knows that a great lemon meringue has a few “layers” on it and the same is true of an open source phone system.

1. Layer one: Hardware – This is typically a server, tower PC or some sort of appliance. Most look at the hardware as “the” phone system because it is much more tangible than software, but in reality the hardware is like crust. It serves a purpose, but you wouldn’t eat it as a standalone.

2. Layer two: Operating System – In the open source world, this is your favorite (or the recommended) Linux distribution. The operating system is a piece of software that manages computer resources and provides programmers/users with an interface used to access those resources. It is the pie filling (or the meat) or the open source phone system.

3. Layer three: Open Source Software – With the third layer you are adding the actual phone system software (Asterisk, trixbox, freeSWITCH, etc). This is the special layer, like the meringue that turns a lemon pie into a lemon meringue pie and a functional server/tower/appliance into a phone system.

3. Layer four: Connectivity Cards – Most servers, towers or appliances come with Ethernet NIC’s, but some do not. The last layer to this open source phone system is connectivity to the LAN/WAN through an Ethernet NIC and PSTN connectivity through an analog/digital PCI card. Although I called them a layer, connectivity cards are really more like a fork. They are an enabler of the phone systems capabilities just as a fork enables you to each that slice of pie.

I suppose I made some gross generalizations with this one, as there is a lot more to a open source phone system (from a set-up and configuration standpoint), but there is also a lot more to making a pie than just point out the layers. However, I think this is an effective way to gain a better understanding of what a open source phone system is made up of and hopefully it helps a few you out.


  • Is there any difference in the above between a proprietary system and an open source one besides the selection of software that is open source and not proprietary?
    I think the split into layers is helping explain VoIP phone systems as a whole, and is a good abstraction of what a phone system is.

  • Garrett Smith

    @Tsahi You are correct, proprietary systems are made up of the same components, however they are always positioned as the sum of the whole, rather than as individual elements. When speaking with people newer to the VoIP world about Asterisk, this is typically where they fall off as they can’t seem to wrap there head around the whole “software” (intangible) as the phone system.

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