VoIPowering Your Office with Asterisk – Part 2: Introduction continued

Last week in Part
, 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
[email protected]
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.

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