The OpenPhone Project—Internet Telephony for Everyone!
The OpenPhone Project (http://www.openphone.org/) has a simple goal—to phone-enable every computer on the planet. If a computer can browse the Web and play audio from Internet radio stations, it should be able to place and receive phone calls, too. The basic technology is available today. The OpenPhone Project aims at fostering the development of the software that can make this a reality.
Internet telephony, or voice-over IP (VoIP) technology, has come a long way in the last few years, and the path is cluttered with a confusing array of ever-changing standards. I will aim to provide an introduction to the technology and standards on Internet telephony with the hope that more people will participate in the OpenPhone Project. Internet Telephony is one of the most exciting and fastest-growing areas in today's telecommunications world—and it's perfect for Linux!
Phone-enable every computer—what does that mean? It means every computer should to be able to act as a phone—hopefully, a very smart and programmable phone. There are several practical ways to accomplish this feat. The simple way is to mimic some of the capabilities of a phone with a computer's normal resources. For example, use the sound card and a microphone/speaker to communicate, and use either a screen-pop dialog or play a sound file to indicate ringing. Another much more sophisticated technique is to use a telephony interface board that allows you to plug a normal phone into your computer. Given today's inexpensive and powerful phones (especially cordless phones) and the availability of low-cost telephony interface cards, this is the approach favored by the OpenPhone Project.
Telephony interfaces come in a wide and confusing array of types and capabilities. They seem to fall into two basic categories: high-density multiline digital interface cards (T-1 or better) and low-density analog cards. Since most folks don't have a T-1 circuit in their home, the OpenPhone Project focuses on the low-density analog cards. These cards are no more expensive than a decent video card, and provide a whole host of critical features that make Internet telephony work. The simplest thing they do is let you plug a normal, inexpensive analog phone into your computer and provide full control over the ringing and audio. However, they also provide the hardware-based audio compression so critical to voice quality. This hardware technology will be discussed in more detail below.
The OpenPhone Project is not Linux-specific, although it certainly has a strong Linux leaning. Like fax machines, the usefulness of Internet phones depends upon how many other devices are out there that they can interoperate with. If a Linux-based OpenPhone can only call other Linux computers, it's of limited value. However, if an OpenPhone can call any other computer regardless of operating system, or any other phone anywhere in the world—that's powerful! This is the goal of the OpenPhone Project.
At the time of this writing, the OpenPhone Project is using the telephony interface boards made by Quicknet Technologies, Inc. The Internet PhoneJACK is available in PCI (peripheral connection interface) and ISA (industry standard architecture) bus versions and provides an RJ-11 interface into which any normal analog phone can be plugged. It also has headset/microphone and handset jacks. The Internet LineJACK has the RJ-11 POTS (plain old telephone system) port and an additional PSTN (public switch telephone network) port for use as a gateway to the normal phone system. The Internet LineJACK is presently available with only an ISA interface. These low-cost interface boards have Linux and Win32 drivers and provide the ability to use a single standard telephone with your computer. More information is available at Quicknet's web site (http://www.quicknet.net/) or in my article in last September's issue of Linux Journal (“Voice-Over IP for Linux”).
Talks are underway with several other hardware vendors to participate in the OpenPhone project. We encourage such vendors to make drivers available for as many operating systems as possible, and join us in making OpenPhone work across all platforms.
Telephony can be thought of as having two major parts: the audio channel used to communicate and the signaling channel(s) used to control the audio channel. In the traditional public switched telephone network (PSTN), the signaling happens on a separate private network owned and operated by the telephone companies. This separate signaling network uses a protocol called Signaling System 7 (SS7); it is used to control the setup and ending (teardown) of calls, using the switched circuits in the system.
The audio-channel portion of the PSTN is mostly composed of two parts: the local loop, and the central office (CO) equipment that links all the local loops together. The local loop is the pair of copper wires that comes into your house or business—the analog line. The CO equipment is made up of high-speed digital links; it is beyond the scope of this article. The local loop uses analog signals to carry your voice to the CO, where it is digitized and sent to the CO on the other end of the call. The other CO takes the digital signal, converts it back into an analog signal and sends it down the analog line to the called party.
Internet telephony works the same way, except that the digitization process happens at your computer, and the high-speed digital link between end stations is the Internet. Your telephony interface (or sound card) converts the analog signal to digital and sends it in IP packets to the destination. The destination computer converts it back to analog sound signals and plays it out your phone. Simple, right? Ah, but like most things that involve computers, the details are the tricky part.
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