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The one thing you don't want with a portable PC is a cable to tether you down, yet most of the time you must plug into one thing or another. Even a simple and routine chore like downloading files from your notebook machine into your desktop PC gets tangled in cable trouble. Not only do you have to plug both ends in, reaching behind your desktop machine only a little more elegantly than fishing into a catch basin for a fallen quarter-and, more likely than not, unplugging something else that you'll inevitably need later only to discover the dangling cord-but you've got to tote that writhing cable along with you wherever you go. There has to be a better way.

There is. You can link your PC to other systems and components with a light beam. On the rear panel of many notebook PCs, you'll find a clear LED or a dark red window through which your system can send and receive invisible infrared light beams. Although originally introduced to allow you to link portable PCs to desktop machines, the same technology can tie in peripherals like modems and printers, all without the hassle of plugging and unplugging cables.

On June 28, 1993, a group of about 120 representatives from 50 computer-related companies got together to take the first step in cutting the cord. Creating what has come to be known as the Infrared Developers Association or IrDA, they aimed at more than making your PC more convenient to carry. They also saw a new versatility and, hardly incidentally, a way to trim their own costs.

The idea behind the get together was to create a standard for using infrared light to link your PC to peripherals and other systems. The technology had already been long established, not only in television remote controls but also in a number of notebook PCs already in the market. Rather than build a new technology, the goal of the group was to find common ground, a standard so that the products of all manufacturers could communicate with the computer equivalent of sign language.

Hardly a year later on June 30, 1994, the group approved its first standard. The original specification, now known as IrDA Version 1.0, essentially gave the standard RS-232C port an optical counterpart, one with the same data structure and, alas, speed limit. In August 1995, IrDA took the next step and approved high speed extensions that pushed the wireless data rate to four megabits per second.

More than a gimmicky cordless keyboard, IrDA holds an advantage that makes computer manufacturers-particularly those developing low cost machines-eye it with interest. It can cut several dollars from the cost of a complex system by eliminating some expensive hardware, a connector or two, and a cable. Compared to the other wireless technology, radio, infrared requires less space because it needs only a tiny LED instead of a larger and more costly antenna. Moreover, infrared transmissions are not regulated by the FCC as are radio transmissions. Nor do they cause interference to radios, televisions, pacemakers, and airliners. The range of infrared is more limited than radio and restricted to line-of-sight over a narrow angle. However, these weaknesses can become strengths for those who are security conscious.

The original design formulated by IrDA was for a replacement for serial cables. The link was envisioned as a half-duplex system. Although communications go in both directions, only one end of the conversation sends out data at any given time.

To make the technology easy and inexpensive to implement with existing components, it was based on the standard RS-232C port and its constituent components, such as UARTs. The original IrDA standard called for asynchronous communication using the same data frame as RS-232C and the most popular UART data rates from 2400 to 115,200 bits per second.

To keep power needs low and prevent interference among multiple installations in a single room, IrDA kept the range of the system low. The expected separation between devices using IrDA signals to communicate was about one meter (three feet). Some links are reliable to two meters.

Similarly, the IrDA system concentrates the infrared beam used to carry data because diffusing the beam would require more power for a given range and be prone to causing greater interference among competing units. The laser diodes used in the IrDA system consequently focus their beams into a cone with a spread of about 30 degrees.

After the initial serial port replacement design was in place, IrDA worked to make its interface suitable for replacing parallel ports as well. That goal lead to the creation of the IrDA high speed standards for transmissions at data rates of 0.576, 1.152 and 4.0 megabits per second. The two higher speeds use a packet based synchronous system that requires a special hardware based communication controller. This controller monitors and controls the flow of information between the host computer's bus and communications buffers.

A watershed of differences separate low speed and high speed IrDA systems. Although IrDA designed the high speed standard to be backwardly compatible with old equipment, making the higher speeds work requires special hardware. In other words, although high speed IrDA devices can successfully communicate with lower speed units, such communications are constrained to the speeds of the lower speed units. Low speed units cannot operate at high speed without upgrading their hardware.

IrDA defines not only the hardware but also the data format used by its system. The group has published six standards to cover these aspects of IrDA communications. The hardware itself forms the physical layer. In addition, IrDA defines a link access protocol termed IrLAP and a link management protocol called IrLMP that describe the data formats used to negotiate and maintain communications. All IrDA ports must follow these standards. In addition, IrDA has defined an optional transport protocol and optional Plug-and-Play extensions to allow the smooth integration of the system into modern PCs. The group's IrCOMM standard describes a standard way for infrared ports to emulate conventional PC serial and parallel ports.