Notes on Building Asterix and SIP Express VOIP PBX

Last updated: April 5, 2005

Here are some notes on building multi-site VOIP PBX services using Asterisk and SIP Express Router.

General approach:
1. Use Asterisk for PBX functionality at each site
2. Use IAX for inter-site traffic to minimize NAT-related issues
3. Use SIP Express as a front end to SIP clients at single sites

Platform technology:
Both Asterisk and SIP Express Router run on Linux and don’t require very powerful hardware. SIP Express in particular can handle hundreds of calls on a small generic Intel-compatible server. Asterisk provides more extensive functionality, including voicemail, transcoding, and conferencing, and requires somewhat more server resource. For a small office scenario, any current Intel-compatible server should be adequate. In the recent GeekGazette article Kerry Garrison implements Asterisk on a Pentium II/450MHz/386MB RAM/12GB HDD/48x CD-ROM/Intel 10/100 system combined with a generic Intel Winmodem card for line access.

The Asterisk@Home project packages a pre-built CD image for Asterisk running on Linux

SIP Express Router installation is simple, and it can easily be downloaded and run nearly out of the box, especially if call accounting is not required.

Line interface technology:
There are two kinds of analog phone interfaces of interest here:
FXS port – (subscriber) the end that looks like a telephone company and provides power and ringing current
FXO port – (office) the end that looks like a handset

To connect an analog handset to a computer, the computer needs to provide an FXS port to provide power and ringing current to the handset.

To connect a computer to the analog PSTN line, the computer needs to provide an FXO port capable of receiving ringing signals and generating dial tone or pulses.

Digium sells a range of line interface hardware including multiport analog cards and T1/E1 digital cards. As mentioned earlier, the least expensive option is to use generic Winmodem cards. Digium used to sell a rebadged Winmodem bundled with support as the X100P, but it has been discontinued. Units are still available on Ebay as of 4/4/2005. These low-end cards can be obtained for under $15. Other generic Winmodems may also work, but the results with different models and even units from different production runs may vary.

For general development use, and for an entry level office PBX, the Asterisk Developers Kit provides a Digium Wildcard TDM400P with one FXS and one FXO module, leaving 2 additional module slots open for any combination of FXS or FXO modules. The card occupies a single PCI slot.


Asterisk Download Page (
SIP Express Router Download Page (

Build Your Own PBX (Slashdot)
Build Your Own PBX (
Using BroadVoice with Asterisk How-To
Start Your Own Open Source-based Telecom (Slashdot)
Interview with Asterisk Creator Mark Spencer (Slashdot)
Asterisk and Linux to build Secure VoIP Connection
Asterisk Open Source PBX 1.0 Release (Slashdot)
Clever CallerID Tricks with Asterisk (Slashdot)
List of VOIP service providers at
Asterisk Wiki at
SIP Express Wiki at
One user’s experience building Asterisk-based PBX (

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