Infrared LANs typically use the wavelength band between 780 and 950 nanometers (nm), which is somewhere between the visible spectrum of light and microwaves. This is due primarily to the ready availability of inexpensive, reliable system components. The infrared signals from a transceiver-equipped mobile or desktop computer go to a similarly equipped LAN access node, which translates the infrared signals into electrical signals suitable for transmission over the network in standard LAN formats. A line-of-sight connection is needed between transmitters and receivers because infrared will not penetrate walls or windows.
While these limitations may discourage some users from infrared, they may actually be an advantage over other wireless LAN technologies. For example, infrared is not susceptible to interference from radio waves, as are microwave and spread-spectrum transmissions. Infrared may also provide higher security, especially in comparison to microwave, because it can be contained within a building.
Infrared technology, operating at very high frequencies just below visible light in the electromagnetic spectrum, has long been used to operate devices such as TV remote controls. Only in the last few years has infrared become a mainstream feature in mobile computing. Infrared technology is used to implement wireless LANs as well as the wireless interface to connect laptops and other portable machines to the desktop computer equipped with an infrared transceiver.
About 200 vendors now offer infrared products that make it easier to wirelessly exchange information and connect to the corporate network. Interest in infrared technology has taken off since the Infrared Data Association (IrDA) agreed on a de facto 4-Mbps data transfer standard in 1996. Today, just about all new notebooks and handheld computers come equipped with infrared interfaces for direct connection to desktop computers and wired LANs. Because infrared network adapters offer ease of use and relatively low cost, they may end up competing with docking stations and LAN adapter cards on the desktop. Infrared connections are now available for keyboards, joysticks, and monitors. Infrared links can also be set up between cellular phones and desktop computers for the exchange of phone directories, short text messages, and schedules.
The 4-Mbps IrDA standard for commodity products and the recently issued IEEE 802.11 wireless LAN specification—which addresses 2 Mbps infrared transmission, among other issues—open the door to a host of new applications and could spur the entire wireless LAN market.
There are two implementations of infrared: directed and nondirected. Directed infrared uses a tightly focused beam that is capable of transmitting data several miles. This approach is used for connecting users in large offices and for connecting LANs in different buildings. Although the transmissions are virtually immune to electromechanical interference and are extremely difficult to intercept, such systems are not widely used outdoors because their performance can be impaired by atmospheric conditions, which can vary daily. Such effects as absorption, scattering, and shimmer can reduce the amount of light energy that is picked up by the receiver, causing the data to be lost or corrupted.
Nondirected infrared systems use a less focused approach. Instead of a narrow beam to convey the signal, the light energy is spread out and bounced off narrowly defined target areas or larger surfaces such as office walls and ceilings. Nondirected infrared links may be further categorized as either line-of-sight or diffuse. Line-of-sight links require a clear path between transmitter and receiver, and generally offer higher performance.
The line-of-sight limitation may be overcome by incorporating a recovery mechanism in the infrared LAN, which is managed and implemented by a separate device called a multiple access unit (MAU) to which the workstations are connected. When a line-of-sight signal between two stations is temporarily blocked, the MAU's internal optical link control circuitry automatically changes the link's path to get around the obstruction. When the original path is cleared, the MAU restores the link over that path. No data is lost during this recovery process.
Diffuse links rely on light bounced off reflective surfaces. Because it is difficult to block all of the light reflected from large surface areas, diffuse links are generally more robust than line-of-sight links. The disadvantage of diffused infrared is that a great deal of energy is lost and, consequently, the data rates and operating distances are much lower.
Light-emitting diodes (LEDs) or laser diodes (LDs) are used for transmitters. LEDs are less efficient than LDs, typically exhibiting only 10 to 20 percent electro-optical power conversion efficiency, while LDs offer an electro-optical conversion efficiency of 30 to 70 percent. However, LEDs are much less expensive than LDs, which is why most commercial systems use them.
Two types of low-capacitance silicon photodiodes are used for receivers: positive-intrinsic-negative (PIN) and avalanche. The simpler and less expensive PIN photodiode is typically used in receivers that operate in environments with bright illumination, whereas the more complex and more expensive avalanche photodiode is used in receivers that must operate in environments where background illumination is weak. The difference in the two types of photodiodes is their sensitivity.
The PIN photodiode produces an electrical current in proportion to the amount of light energy projected onto it. Although the avalanche photodiode requires more complex receiver circuitry, it operates in much the same way as the PIN diode, except that when light is projected onto it, there is a slight amplification of the light energy. This makes it more appropriate for weakly illuminated environments. The avalanche photodiode also offers a faster response time than the PIN photodiode.
Current applications of infrared technology yield performance that matches or exceeds the data rate of wire-based LANs: 10 Mbps for Ethernet and 16 Mbps for token ring. However, infrared technology has a much higher performance potential—transmission systems operating at 50 Mbps and 100 Mbps have already been demonstrated in controlled environments.
Because of its limited range and inability to penetrate walls, nondirected infrared can be easily secured against eavesdropping. Even signals that go out windows are useless to eavesdroppers because they do not travel far, and may be distorted by impurities in the glass as well as by its placement angle.
Infrared offers more immunity from electromagnetic interference than spread spectrum, which makes it suitable for operation in harsh environments like factory floors. Because of its limited range and inability to penetrate walls, several infrared LANs may operate in different areas of the same building without interfering with each other. Since there is less chance of multipath fading (large fluctuations in received signal amplitude and phase), infrared links are highly robust.
Many indoor environments have incandescent or fluorescent lighting, which induces noise in infrared receivers. This is overcome by using directional infrared transceivers with special filters to reject background light.
Infrared supports both contention-based and deterministic media access control techniques, making it suitable for Ethernet as well as token ring and, eventually, FDDI LANs.
To implement Ethernet's contention protocol—carrier sense multiple access (CSMA)—each computer's infrared transceiver is typically aimed at the ceiling. Light bounces of the reflector in all directions to let each user receive data from other users. CSMA ensures that only one station can transmit data at a time. Only the station(s) to which packets are addressed can actually receive them.
Deterministic media access control relies on token passing to ensure that all stations get a chance to transmit data in their turn. This technique is used in FDDI as well as token ring LANs. In both types of LAN, each station uses a pair of highly directive (line-of-sight) infrared transceivers. The outgoing transducer is pointed at the incoming transducer of a station down line, thus forming a closed ring with the wireless-infrared links among the computers. With this configuration, much higher data rates can be achieved because of the gain associated with the directive infrared signals. This approach improves overall throughput, since fewer bit errors will occur, which minimizes the need for retransmissions.
Advanced diagnostic tools make the infrared connection easy to manage. The vendor's management application verifies the complete communication path by sending test packets from the application, through the portable PC, to the LAN access node, out onto the network, and back. In addition, the LAN access node usually has LEDs that provide visual feedback on infrared link integrity and network traffic.
When the connection is established, the management application provides performance statistics, such as:
§ Total number of packets transmitted
§ Total packets received
§ Packets received with CRC errors
§ Total received errors
§ Total transmit errors
§ Total transmit collisions
In addition to providing advantages to mobile professional users, infrared also confers substantial benefits to network administrators. The most obvious of these is easing the considerable strains that have built up in network administration in the attempt to accommodate the demands of mobile users. Infrared is easy to install and configure, requires no maintenance, and imposes no remote-access tracking hassles. Because it is standardized and backward compatible, it preserves existing investments in infrared components. For example, when there are a mix of infrared components that support different transmission rates—115 Kbps, 1.15 Mbps, and 4 Mbps—the LAN access node automatically negotiates the highest common speed.
Infrared does not disrupt other network operations and it guarantees data security. And because it makes connectivity so easy, it encourages the use of high-productivity network and groupware applications on portables, thus helping companies amortize the costs of these packages across a larger user base.
To transfer information between portable devices and a desktop computer requires that both machines be equipped with an infrared transceiver and software. The software includes the infrared communication driver which interacts with the device's operating system. The software also provides connection status information.
Establishing an infrared connection between a portable device and a desktop computer, for example, is as simple as placing the portable device within 1 to 3 feet of the desktop computer. The connection is made automatically within seconds. Data is transferred at rates of 115 Kbps to 4 Mbps, depending on the type of transceiver used at both ends. To disconnect, the user just moves the portable device out of the range of the desktop computer. To reconnect, the user just places the portable device back in range of the desktop computer. If the beam is unintentionally broken, the user has a few seconds to remove any obstructions and/or realign the infrared beams before being disconnected. In either case, the user receives audible and/or visual alerts anytime the infrared beam is broken.
Infrared products for computer connectivity conform to the standards developed by the Infrared Data Association (IrDA), an industry consortium. The IrDA Serial Infrared Data Link Standard (IrDA-SIR) was developed with the following advantages in mind:
§ Low-cost implementation. No special or proprietary hardware is required. The standard was developed to make use of components costing only a few dollars per device. With integrated chips that include IrDA functionality, the use of common opto-electronic components adds less than a dollar to the cost of components.
§ Low-power requirements. IrDA-SIR is designed to be power efficient so that it will not be a drain on the batteries of portable devices like notebook computers, PDAs, mobile phones, and other handheld devices. The use of directed IR, rather than diffuse IR, results in very low power consumption when transmitting.
§ Directed, point-to-point connectivity. The use of a directed IR beam avoids unintentional "spilling" of the transmitted data to nearby devices. However, the angular spread of the IR beam does not require the user to align the handheld device perfectly at the target device to achieve an IR link.
§ High noise immunity. IrDA-SIR is specified to achieve bit error rates of better than 1 in 10[9] at ranges of up to 1 meter, while still providing a high level of noise immunity within a typical office environment illuminated with fluorescent light, as well as in environments with full sunlight.
§ Optimized for data transfers. IrDA-SIR is a half-duplex system with the maximum UART-based (Universal Asynchronous Receiver/Transmitter) data rate of 115.2 Kbps. Because the design can be driven by a standard UART, its data rate can be easily programmed from software to a lower data rate to match with slower devices. Of note is that version 2.0 of the IrDA-SIR specification also defines non-UART environments.
The IR LED peak wavelength is specified to range from 0.85 µm to 0.90 µm. The IrDA-SIR physical hardware is very simple. It consists of an encoder/decoder (which performs the IR transmit encoder and IR receiver decoder) and the IR transducer (which consists of the output driver and IR emitter for transmitting and the receiver/detector). The encoder/decoder interfaces to the UART, which most computers already have.
The complete IrDA protocol suite contains five interdependent layers as follows:
§ Infrared Physical Layer (IrPL) specifies infrared transmitter and receiver optical link, modulation and demodulation schemes, and frame formats.
§ Infrared Link Access Protocol (IrLAP) is responsible for link initiation, device address discovery, address conflict resolution, and connection startup. Also ensures reliable data delivery and provides disconnection services.
§ Infrared Link Management Protocol (IrLMP) allows several software applications to operate independently and concurrently, sharing a single IrLAP session between a portable PC and network access device via multiplexing.
§ Infrared Tiny Transport Protocol (IrTTP) is responsible for data flow control, packet segmentation, and re-assembly.
§ Infrared LAN (IrLAN) is a protocol defining how a network connection is established over an IrDA link.
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