Voice over IP – for home

Voice over IP, or VoIP as it is commonly referred as lets you talk to others using IP as the carrier, as opposed to a switched circut line as the plain old telephony service was.

One day I found out that I wanted to try out this at home. In its simplest form – this is what I need to get started.

• A VoIP PBX. The Private Branch eXchange is in it simplest form a directory that maps phone numbers to IP addresses.
• A phone. This can be either a softphone (running on your computer) or a hardware based phone.
• An external line.

An introduction

Voice over IP has two distinct sessions. One is the control session, this uses a protocol called SIP. Using SIP a phone registers itself, receives notifications about calls, how to set them up and so on. All SIP traffic is routed trough central servers in the phone network.

When a phone want to call anoter phone a SIP message is sent to the SIP server. The SIP server relays that message to the other phone and tries to set up a connection. The voice (data) connection is directly between the two phones using a protocol like RTP.

In the field of VoIP thare seems to be many compatibility issues. So don’t expect everything from different vendors to work 100 % together. Many features, except call setup are hard to get to work between vendors. You get what you pay for.

From a network perspective there are many issues.

• The RTP sessions often uses different port numbers making it hard for firewall admins to know what to open.
• The SIP session tells what IP and port to use. NAT’ing firewalls can have a problem with this, unless it supports SIP and can rewrite the request. (Much like FTP.)

VoIP PBX

There are many on the net, I did not spend much time looking for them. A long time ago I wrote about a small company called NCH. They have many VoIP applications, including Axon, their PBX. 3CX also has one that is free, with some minor limitations. There are also some open-source platforms like the Asterix, but I did not look into them.

Axon is simple to set up and comes with integrated voice mail and support for external add-ons by NCH. It is mostly configured by using a web interface. To get the free version (that does not expire) try uninstall the program.

A phone

To start out it is easiest to go for a Windows based phone that you install on your computer. NCH also happens to have a softphone that works fairly well with Axon. To get the free version, try uninstall Express Talk.

I also found the 3CX softphone for Windows that also seems to work well, it claims to be free. Their PBX on the other hand, is not free.

For Windows Mobile there also are many to choose from. NCH’s Express Talk is also shipped for Windows Mobile.

There are many hardware based phones, but I have not tried any of them. I expect Linksys to have some good and cheap phones, look in your local store to see what you can find.

External line

To receive phones from the public network, or to call out you need some kind of external lines. They come in two flavours.

1. SIP trunk, connecting your PBX to another phone network.
2. Analog line, with converters like voice modems or likewise.

I have not made this work yet. You can read more about this topic from Axons homepage.

Technorati Tags: VoIP