VoIPowering Your Office with Asterisk - Part 2: Introduction continued

by Carla Schroder

Step one in setting up your own office PBX is dimensioning the system and figuring out how much it will cost.

Last week in Part 1, we learned what Asterisk is (a free, open-source IP PBX software package) and some of the many things it can do. Today we'll look at what hardware you need to make it work, and how much it costs.


  • Software: free
  • Hardware: it depends
  • Bandwidth: it depends

That really nails it down, doesn't it. To get a bit more precise, let's put together a sample system for a 10-person office currently equipped with analog phone lines. I'm not deliberately recommending any particular pieces of hardware, just making random picks to illustrate the examples.

Resources needed:

  • a computer (Asterisk server)
  • a broadband Internet connection
  • an interface card to connect to the PSTN
  • adapters for your analog phones, or
  • new IP phones
  • a commercial VoIP service

Pricing out the basics
Your Asterisk software must run all alone on a PC; the machine can not be shared.(While Asterisk versions are available that run on Linux, the BSD Unixes, and Mac OS X, please note that driver support for the various interface cards is the strongest in Linux.)

For this scenario an ordinary middle-range PC works fine—something with at least a 1.5 GHz CPU, 512 Mb of RAM, an Ethernet card, and at least a 20-Gb hard drive. These days $400 buys a machine with twice these specs brand-new.

VoIP calls consume between 20 and 90 kbps each way. A typical business DSL service costs around $80/month for 1.5Mbps/896kbps (down/upstream). If your ten users all jump on the phone at the same time, they could theoretically saturate your uplink: 10 x 90kbps = 900. But that's unlikely, so this type of DSL service should work fine.

To connect to your main phone line (analog trunk line), you'll need an adapter with an FXO port (FXO gateway) on the Asterisk server—something like the Handy Tone 488. These cost around $80. The Handy Tone comes with a raft of excellent features; it's more than just a dumb interface.

You may keep your existing fleet of analog phones by using ATAs (Analog Telephone Adapters). These are also called FXS-to-Ethernet gateways, because they connect your analog phones to your computer network. An example of these is the Linksys SPA-1001, which costs about $60.

NB: Beware VoIP products that are linked to certain commercial services! For example, some Linksys devices work only with Vonage. Don't chain yourself to a single service provider! Besides, Vonage does not work with Asterisk anyway.

You may choose to purchase new IP phones instead of ATAs. The prices on these vary a lot, from around $70 for bare-bones phones to several hundred dollars for "PBX" phones. The sweet spot for value and quality is between $100 and $200; you have a lot of good choices in this range.

Finally, you need a commercial VoIP service provider, or, someone who provides "PSTN service termination." This is necessary so you can call any phone number, and not be limited to other VoIP users. Coverage and prices vary a lot, so shop around. And be sure to look for a provider that supports customer-owned equipment, aka "BYOD." Broadvoice charges BYOD customers $5.95/month.

Adding it up
So our ten-person office will spend $1,100–$2,500 on hardware, and have monthly expenses of maybe $86 for broadband and commercial VoIP services.

Bigger systems
If you are fortunate to have a nice T1/T3 line, you'll get better service quality and more capacity. T1/T3 can be divided into separate voice and data channels, so routing and QoS are easy to manage. Your service provider should be your first stop: Find out what sort of voice/data services they offer, and what kind of deals they are willing to make to keep you happy, such as free interface hardware and bundle discounts.

Linux and the BSD Unixes have powerful routing engines and traffic shaping built-in, so you don't need separate routers. Of course, the more users you plan to support, the more powerful your Asterisk server hardware needs to be and the more storage you'll need. A computer with an Athlon 64 3000 CPU, one gigabyte of RAM, and a three-disk SATA RAID5 array with a hardware controller will run around $1,200, and ought to handle 50 or more medium-talkative users.

You'll need an interface card that supports both voice and data over your T1/T3, like the Digium Wildcard TE110P. This supports up to 50 users. The TE110P can be uplinked to another TE110P card, so you have an easy upgrade path as your user base grows. Digium is the sponsor of Asterisk, and provides an extensive line of both analog and digital telephony hardware.

FXO gateways (also known as PSTN interfaces) come in several sizes, from the single-port Handy Tone 488 to the four-port Audiocodes MP-104-FXO, for about $950. You need one port per analog trunk line.

Deciding what type of telephones you want to use, how robust your Asterisk server needs to be, how many Asterisk servers you need, and how much bandwidth you need depends on so many different factors it's hard to give simple answers. Please visit the Asterisk dimensioning page for a lot of great real-world examples..

Looking ahead
Next week we'll cover how to build an Asterisk testbed in an hour. Those old classic Pentiums may not be powerful enough for production use, but they make dandy testing labs.

Telephonyware.com is a great site to see the different brands and types of telephony hardware
Asterisk: the Open Source PBX
The Asterisk Documentation Project
The Asterisk dimensioning page has a lot of good real-world examples.
voip-info.org is a treasure-trove of information.

This article was originally published on Monday Mar 27th 2006