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The one thing you don't want with a portable computer is a cable to tether you down; yet most of the time you have to plug into one thing or another. Even a simple and routine chore like downloading files from your notebook machine into your desktop computer gets tangled in cable trouble. Not only do you have to plug in both ends, reaching behind your desktop machine is only a little more elegantly done 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 computer to other systems and components with a light beam. On the rear panel of many notebook computers, 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 computers to desktop machines, the same technology can tie in peripherals such as 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 (IrDA), this group aimed at more than making your computer more convenient to carry. It also saw a new versatility and, hardly incidentally, a way to trim its own costs.
The idea behind the get together was to create a standard for using infrared light to link your computer 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 computers already on 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 4Mbps.
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 the 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. 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. The original IrDA standard used asynchronous communication using the same data frame legacy serial ports as well as its 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, about one meter (three feet). 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 led to the creation of the IrDA high-speed standards for transmissions at data rates of 0.576, 1.152, and 4.0Mbps. 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.
Consequently, a watershed of differences separate low-speed and high-speed IrDA systems. Although IrDA designed the high-speed standard to be backward 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 speeds without their hardware being upgraded.
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 for the smooth integration of the system into modern computers. The group's IrCOMM standard describes a standard way for infrared ports to emulate conventional computer serial and parallel ports.
Infrared light is invisible electromagnetic radiation that has a wavelength longer than that of visible light. Whereas you can see light that ranges in wavelength from 400 angstroms (deep violet) to 700 angstroms (dark red), infrared stretches from 700 angstroms to 1000 or more. IrDA specifies that the infrared signal used by computers for communication has a wavelength between 850 and 900 angstroms.
The IrDA specification allows for all the usual speed increments used by conventional serial ports, from 2400bps to 115,200bps. All these speeds use the default modulation scheme, Return-to-Zero Inverted (RZI). High-speed IrDA version 1.1 adds three additional speeds, 576Kbps, 1.152Mbps, and 4.0Mbps, based on a pulse-position modulation scheme.
Regardless of the speed range implemented by a system or used for communications, IrDA devices first establish communications at the mandatory 9600bps speed using the Link Access Protocol. Once the two devices establish a common speed for communicating, they switch to it and use it for the balance of their transmissions.
The infrared cell of an IrDA transmitter sends out its data in pulses, each lasting only a fraction of the basic clock period or bit-cell. The relatively wide spacing between pulses makes each pulse easier for the optical receiver to distinguish.
At speeds up to and including 115,200 bits per second, each infrared pulse must be at least 1.41 microseconds long. Each IrDA data pulse nominally lasts just 3/16th of the length of a bit-cell, although pulse widths a bit more than 10 percent greater remain acceptable. For example, each bit cell of a 9600bps signal would occupy 104.2 microseconds (that is, one second divided by 9600). A typical IrDA pulse at that data rate would last 3/16th that period, or 19.53 microseconds.
At higher speeds, the minimum pulse length is reduced to 295.2 nanoseconds at 576Kbps and to only 115 nanoseconds at 4.0Mbps. At these higher speeds, the nominal pulse width is one-quarter of the character cell. For example, at 4.0Mbps, each pulse is only 125 nanoseconds long. Again, pulses about 10 percent longer remain permissible. Table 11.4 summarizes the speeds and pulse lengths.
At the 4.0Mbps data rate, the IrDA system shifts to pulse position modulation. Because the IrDA system involves four discrete pulse positions, it is abbreviated 4PPM.
IrDA requires data to be transmitted only in eight-bit format. In terms of conventional serial-port parameters, a data frame for IrDA comprises a start bit, eight data bits, no parity bits, and a stop bit, for a total of 10 bits per character. Note, however, that zero insertion may increase the length of a transmitted byte of data. Any inserted zeroes are removed automatically by the receiver and do not enter the data stream. No matter the form of modulation used by the IrDA system, all byte values are transmitted with the least significant bit first.
Note that with RZI modulation, long sequences of logical ones will suppress pulses for the entire duration of the sequence. To prevent such a lengthy gap from appearing in the signal and causing a loss of sync, moderate speed IrDA systems add extra pulses to the signal with bit-stuffing (as discussed in Chapter 8).
The IrDA system doesn't deal with data at the bit or byte level but instead arranges the data transmitted through it in the form of packets, which the IrDA specification also terms frames. A single frame can stretch from 5 to 2050 bytes (and sometimes more) in length. As with other packetized systems, an IrDA frame includes address information, data, and error correction, the last of which is applied at the frame level. The format of the frame is rigidly defined by the IrDA Link Access Protocol standard, discussed later.
Whenever a receiver detects a string of seven or more consecutive logical ones—that is, an absence of optical pulses—it immediately terminates the frame in progress and disregards the data it received (which is classed as invalid because of the lack of error-correction data). The receiver then awaits the next valid frame, signified by a start-of-frame flag, address field, and control field. Any frame that ends in this summary manner is termed an aborted frame.
A transmitter may intentionally abort a frame or a frame may be aborted because of an interruption in the infrared signal. Anything that blocks the light path will stop infrared pulses from reaching the receiver and, if long enough, abort the frame being transmitted.
High-speed systems automatically mute lower-speed systems that are operating in the same environment to prevent interference. To stop the lower-speed link from transmitting, the high-speed system sends out a special Serial Infrared Interaction Pulse (SIP) at intervals no longer than half a second. The SIP is a pulse 1.6 microseconds long, followed by 7.1 microseconds of darkness, parameters exactly equal to a packet start pulse. When the low-speed system sees what it thinks is a start pulse, it automatically starts looking for data at the lower rates, suppressing its own transmission for half a second. Before it has a chance to start sending its own data (if any), another SIP quiets the low-speed system for the next half second.
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