• Telephone Over Internet

VOIP, Voice Over Internet Protocol, is about making phone calls over the Internet. In addition, video can be added to the voice in the call, and also text chat, instant messaging, file sharing, and so on.

This page is about getting a start with VOIP.

(some sections are not yet completed. See also: Telefoneren over Internet (in Dutch))

What is VOIP, terminology

See also VOIP in Wikipedia (also in other languages).

Traditional circuit switched telephony

The traditional telephone system (POTS: Plain Old Telephone Service, or PSTN: Public Switched Telephone Network) uses handsets (the actual telephone) connected with wire (subscriber line, up to, say, 5 km in distance) to a central office.

Central offices are connected with cables containing many wires (circuits). When a call is placed between two telephones, one of such circuits will be used, dedicated, for just that call, for the duration of the call. When the call has ended (hangup), the circuits can be reused for another call.

The central offices contain switches to connect circuits and subscriber lines. When a call is placed, there will be an electrical circuit all the way between the two telephones involved. Therefore, the traditional telephone system is said to be circuit-oriented.

There are only so many circuits available, and at times subscribers may have to compete to get a circuit. It is logical that the cost of a call is proportional to the duration of the call and the lenght of the circuit needed.

With modulation techniques, one physical circuit can carry many calls in separate frequency bands and multiplex time slots. For this purpose, the analog voice signals are transferred as digital signals in the telephone system. But even then, every call will still occupy a fixed portion of the physical circuit for the entire duration of the call.

Packet based Internet

The Internet is based on the Internet Protocol (IP). Originally, it uses the same telephone circuit, but instead of using it for a single communication, it can carry many unrelated computer communications by chopping up each communication into packets of bytes. Also, the routing of packets is done outside the telephone system. As a result, once the Internet network has been established by allocating telephone circuits and computers at each network node, the cost of transferring each packet of bytes between whatever end nodes is basically nothing. Therefore, email is said to be free.

Internet telephony

Combining the principle of transferring the analog voice signal in digital form and the Internet Protocol (IP) results in transferring the analog voice signal in IP packets. And because transferring IP packets are free (you still have to pay for access to the Internet and for your own computer), telephone over Internet is free.

The jargon term is Voice Over Internet Protocol, or VOIP.

Internet, and therefore email, VOIP, etc, is packet oriented in contrast to circuit oriented traditional telephone service. The advantage is lower cost, but the sound quality may also be lower as packets may get time-delayed by traveling through routers,

Codecs

There are several methods to convert back and forth between the analog voice signal and a digital format. Each converter-deconverter combination, or codec, has different properties, such as the number of bits needed per second, and the sound quality. The most basic codec is G.711 which uses PCM, and exists in two variants: mu-law (logarithmic) used in North America and part of the Far East, and a-law (linear) used elsewhere.

The most advanced is G.729A, which is uses compression. The GSM codec is used in mobile telephony. If there is a problem with a voip call, just try the G.711 codec first.

Why VOIP

Just like so much communication goes over the Internet, also voice can be transmitted over the Internet.

Just as email is free, VOIP is free.

Of course, you first need to pay for access to the Internet. In The Netherlands, 78 percent of households have Internet access, and 54 percent even have broadband access. (Eurostat, 2006)

The issue is not whether consumers should use VOIP, but in what form. The telephone companies already are routing many voice calls over the Internet. In Holland there are companies offering rates to many far-away countries that are equal or even lower than regular domestic calls. This means that VOIP is done for you by companies. Also, because of competition, the regular rates of the telephone companies are going down too.

There is a concept that the value of traditional telephone service is falling rapidly to practically nothing. Telephone companies are scrambling to convert to providers of internet service and even digital television service, to compensate for the loss of telephone revenues.

So why should consumers care about voip?

By moving VOIP to your own PC, you will save a little money, but also you will improve on comfort. You will not need to hassle with companies doing voip for you, with special phone numbers before the phone number you want to call, but you can just click on a name in the phone book on your PC.

But there is more.

You can use a VOIP number from home, the office, a hotel, or any other place on Earth where you have Internet access. That means that you can call and be called on the same number no matter what your actual location is.

Why Skype is a bad choice

Standards and applications

Linux

Windows

Mac

WLAN SIP Phones

Siemens Gigaset SL75

Nokia E60 with GSM and WLAN

(I have no relation to Siemens or Nokia. These are just examples, there are more types.)

VOIP Accounts

PC to PC calls

Voip-Out to and from normal phones

Your own VOIP server

Setup and testing

Most VOIP servers provide for a test number, but you need to register to that server first. A test number that you can use from any server is: sip:904@voiptalk.org

The test number will sound a welcome message, and offer an echo test. If everything works, you will hear your own voice and you can experience the latency.

If your installation works with the test number, you could in principle call any other sip number, and start enjoying the advantages of VOIP.