How does it work ?

Internet telephony

First, let's see about the computing requirements for telephony. The basics are applicable to any platform (Windows, Macintosh, Unix, OS/2) and they are:
486-DX/33 or better
8 megabytes of RAM (minimum)
16-bit sound card
14.4 KB/sec v32bis modem (minimum)
SVGA display
Speakers and a Microphone
Internet Connection

Audio conferencing programs work by digitalizing your speech asyou talk and sending the digital data over the Internet. However there's aproblem: a typical modem connection has limited bandwidth - 14.4KBPSmodems can send and receive a maximum of 1,800 bytes of non-compressibledata each second. Telephone quality speech needs 8,000 bytes per secondof bandwidth. There are two solutions to the problem: either get morebandwidth, or compress the sound information before transmitting it.Although both solutions are used, most programs compress the audio.

There are a variety of methods for encoding and compressing sound data,and as you might expect, the standards aren't necessarily very standard,yet. The quality of the audio you'll send and receive depends on theapplication you're using, the speed of your computer and the compressionmethod used. In general, audio is usually understandable, although it is lessclear than a phone call. Still, talking around theworld for the cost of an Internet connection is amazing !

The Standard compression methods and transport protocols are:
GSM: for more information: www.cs.tu-berlin.de/~jutta/toast.html
CVSD
RTP: Real Time Protocol: for more information:www.fokus.gmd.de:80/step/employees/hgs/rtp/faq.html

Full and half duplex

In a full duplex conversation, you can speak and hear the other person at the same time. In half duplex, only one person can speak at the same time,and if you speak while the other person speaks, then you will not be able tohear each other. In these applications, you usually press a button to speak.Most applications are starting to support both full duplex and half duplex modes. Your sound card will either be full duplex or half duplex, which will control which mode you can operate in. Most new Macs and many Unixstations come equipped with full duplex sound cards, but most PC's do not.When you load the application, if it does not let you choose full-duplexmode, then you probably have a half duplex card.

Radio broadcasting

In radio broadcasting the server plays live and on-demand audio across the Internet with no wait for download.
The technology of compression is based on the MPEG format compression - (the international standard for digital audio and video on a multimedia PC without requiring expensive hardware add-ons).
This technology is World Wide Web Integrated - It means it works as a helper application for webbrowsers such as Netscape's Navigator and Microsoft's Internet Explorer,providing an easy to use, easy to view experience.
The quality of the sound can sometimes be quite high - with CD quality, 44KHz, audio. However it dependsof the bandwidth of the Internet at the moment. Freer the bandwidth, better the sound quality.
It uses scaleable Datarates - From 8.5 Kilobits to 2.0 Megabits, includingISDN and popular 14.4 and 28.8 modems.
More info: www.real.co.uk