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Press the Asterisk now

Posted on 22 Nov 2005 at 16:24

Simon Brock and Ian Wrigley look into an open-source VoIP package

ISDN provides more information than an analog line. Typically, it delivers not only the number of the person who initiated the call, but also the number they're calling. This lets the phone company allocate a collection of numbers to a set of ISDN lines, allowing multiple calls to come in via the same number, but also direct dialling to specific numbers. The problem is there are slightly different versions of ISDN in use within the UK, and there are ISDN cards with different capabilities.

Whether you have analog or digital phone lines, implementing a telephone exchange on a general-purpose computer is problematic. If you connect an analog phone, something has to digitise the voice signal for processing, then turn it back into analog to deliver it. And even with different digital systems, re-encoding will be required between phone line and computer.

Setting up Asterisk

In our London offices, we now have an Asterisk system set up and working that supports VoIP, ISDN and analog phones. It's connected to our old telephone exchange and allows us to do many of the things we need to do. So how did we achieve it? Well, the first things we needed to make it all work were a PC and some form of telephony interface card.

After a short period of hunting around, we discovered that the favourite cards supported by Asterisk are manufactured by Digium. To be more precise, we bought a Digium TDM-400 card, which comes with two daughterboards - one to do FXO and one to do FXS. It's a PCI card, so everything appeared quite straightforward; the board wasn't supplied with any documentation, but we downloaded some from the website and worked out what to do. The site says that the minimum specification is a 700MHz Pentium III PC, and since we had one of those going spare, we kicked off our experiment.

Installing the card was simple except for one thing. Remember above when we said that the big difference between an FXO and an FXS is that an FXS has to supply the voltage to ring a bell? Well, a PCI slot doesn't supply enough power to make this happen, so the TDM card needs to be plugged into the PC's power supply too, using one of those spare disk drive power plugs that hangs around inside most machines. Unfortunately, the PC we were using didn't have the usual collection of such power plugs, so we had to steal one from the CD-ROM drive. This might present a problem if, for example, you want to use this card in a 1U-high server, which generally won't have long cable runs trailing round the inside of the box.

Our machine already had Linux installed (the excellent Centos system mentioned in Enterprise Linux for free), so we were now ready to go. Having looked on the Asterisk site, we found an installation document that recommended downloading the source and compiling it. We subsequently found that there are some precompiled packages out there, but compiling wasn't a problem. After compiling and installing, we followed the configuration instructions. Configuring the TDM-400 can be confusing because you have to configure an FXO card to use FXS signalling. Having finally got that right, we plugged in a phone and an exchange line, dialled a number and... got the world's worst phone call.

The problem was that the PC we'd chosen was just too slow to support the phone card - the card is really a collection of 'soft modems' that relies on the PC's CPU doing all the hard work, and unfortunately our 700MHz machine wasn't up to it - and, you've guessed it, the only other machine we had to hand was a 1U-high rack-mount server with no trailing power cables. Eventually, though, we found a machine powerful enough, so we were in business.