Operation, Niche applications & Emerging technology | Infrared LANs

Operation

The IrDA-SIR specification takes a standard asynchronous serial character stream from the UART—where a frame is defined as a start bit, 8 data bits, no parity bit, and a stop bit—and encodes the output such that 0 is represented by a pulse and 1 is represented by no pulse. A pulse is further defined as occupying a minimum of 1.6 microseconds to a maximum of 3/16th of a bit period, the length of which is inversely proportional to the bit rate of the data (i.e., the slower the data rate, the longer the pulse). This pulse stream forms the input to the driver for the IR emitter that converts the electrical pulses to IR energy.

IrDA-standard infrared links are half-duplex with the maximum data rate of 115.2 Kbps. There are IrDA high-speed extensions for 1.15 Mbps and 4 Mbps transmission. The hardware consists of an infrared transmit encoder/receiver decoder and the IR transducer, which consists of the output driver and infrared emitter for transmitting and the receiver/detector. The encoder/decoder interfaces to the UART, which is already available in most computers.

IrLAP provides two roles for participating stations, one of which is the Primary (commanding) station and the other is the Secondary (responding) station(s). The primary station has responsibility for the data link. All transmissions over a data link go to, or from, the primary station. IrLAP communication links can be point-to-point or point-to-multipoint. There is always one and only one primary station; all other stations must be secondary stations. Any station that is capable can contend to play the primary station role. The role of primary is determined dynamically when the link connection is established and continues until the connection is closed. The exception is that there is a method provided for a primary and secondary on a point-to-point link to exchange roles without closing the connection.

IrLAP uses most of the standard types of frame defined by the HDLC standards. The frames are classified by function as follows: unnumbered or U frames, supervisory or S frames, and information or I frames.

U frames are used for such functions as establishing and removing connections and discovery of other station device addresses. I frames are used to transfer information from one station to another. S frames are used to assist in the transfer of information and may be used to specifically acknowledge receipt of I frames (I frames can implicitly acknowledge other I frames also) and to convey ready and busy conditions.

IrLAP also describes procedures that support link initialization, device address discovery, connection startup (including link data rate negotiation), information exchange, disconnection, link shutdown, and device address conflict resolution. While each of these procedures is adapted to the IrDA serial infrared environment the link initialization and shutdown, connection startup, disconnection, and information transfer procedures all resemble similar operations in HDLC protocols. However, the discovery and address conflict resolution procedures are unique to IrLAP.

A link operates essentially as follows. A device will want to connect to another device (either by automatic detection via the discovery and sniffing capability of IrLAP, or via direct user request). After obeying the media access rules, the initiator will send connection request information at 9.6 Kbps to the other device, which includes such things as its address and data rate. The responding device will assume the secondary role and, after obeying the media access rules, return information that contains its address and capabilities. The primary and secondary stations will then change the data rate and other link parameters to the common set defined by the capabilities described in the information transfer. The primary station will then send data to the secondary station confirming the link data rate and capabilities. The two devices are now connected and the data is transferred between them under the complete control of the primary station. Rules are defined which ensure that the secondary and primary stations are both able to efficiently transfer data.

Infrared is a communications medium that makes it easy to establish connections between devices. In such a situation, the configuration of devices is not a static configuration, but is highly dynamic. It is determined by the services and protocols offered by the devices within range and the applications the user wishes to use. Three additional elements are necessary for an IrDA IR enabled device: discovery, link control, and multiplexing.

Discovery occurs when two devices first encounter each other. Each service and each protocol on a device will have registered with the link management (i.e., IrLMP). The information registered includes both standard and protocol specific information. An application can query the capabilities of devices within range.

Once an application on one device has determined which service or protocol it wishes to use, it requests the link control to use the protocol. The link management framework allows multiplexing of application or transport protocols on the same link connection at the same time.

Flow control is provided via IrTTP using a credit-based flow control scheme. Not only does IrTTP provide per-transport-connection flow control, but segmentation and reassembly of arbitrary sized packet data units (PDUs) as well.

Niche applications

Infrared systems address a niche market because they impose more restrictive distance limitations that other wireless LAN technologies such as spread spectrum. On the other hand, the infrared systems do offer high levels of throughput. Infrared's primary impact will take the form of benefits for mobile professional users. It enables simple, point-and-shoot connectivity to standard networks, which streamlines users' workflow and allows them to reap more of the productivity gains promised by portable computing. Infrared also confers substantial benefits to network administrators. Infrared is easy to install and configure, requires no maintenance, and imposes no remote-access tracking hassles. It does not disrupt other network operations and it provides data security. And because it makes connectivity so easy, it encourages the use of high-productivity network and groupware applications on portables, thus helping administrators amortize the costs of these packages across a larger base of users.

IrDA-standard infrared ports promise to become pervasive as a low-cost, low-power, dependable two-way data exchange technology on a wide range of products that are increasingly digital and intelligent as the industries of computers, telecommunications, and consumer electronics converge.

Joining the notebooks are other "beaming" products such as mobile printers, mobile digital phones, digital pagers, handheld PCs, PDAs, organizers, modems, PC card adapters, plus a number of flexible printer and computer IrDA adapters. New handheld PCs and organizers from numerous vendors already feature onboard IrDA ports, enabling cordless file exchange and printing functions. Models of cellular phones and pagers from firms such as Nokia Mobile Phones and NEC also have IrDA ports. Sharp and Sony have even announced digital cameras featuring direct IrDA beaming of images to another camera, a computer, or a printer. Future platforms expected to be equipped with IrDA ports include public phones, business phones, USB adapters, watches, and devices for a variety of vertical market applications in the distribution, warehouse, field service, utility, medical, and automotive markets.

Emerging technology

Japanese researchers at NEC have developed infrared wireless network technology that uses the IEEE 1394 high-speed serial bus—also known as Fire Wire—an emerging standard for data transmission in electronics consumer and PC products. The new technology, aimed at the home- and small-office markets, supports multimedia speeds of up to 125 Mbps.

Fire Wire can transmit data from between 100 Mbps and 400 Mbps. Although there are many emerging bus standards, Fire Wire is now appearing in digital camcorders, digital satellite receivers, and digital video recorders. The technology also has the advantages of attracting plug-and-play add-ons and applications. The technology is expected to reach the marketplace in late 1998.

Of note is that NEC researchers have found a way to use ordinary infrared transmission diodes—typically used in remote controllers for consumer products such as TVs—to transfer data at high speeds. Data is converted into an infrared signal that can be transmitted between two transceivers up to 10 meters apart. For distances greater than 10 meters, NEC has developed Fire Wire-based wall sockets to which devices can be physically wired using relatively inexpensive plastic fiber optic cable. By comparison, networks using copper cabling are limited to 4.5 meters, after which they need a repeater to boost the signal.

Because the system used standard parts and diodes, it would be relatively inexpensive to implement in homes and small offices, compared with network systems such as Ethernet. Because infrared is used, there is no concern about electromagnetic interference, which is a growing problem in both home and office environments. The system also meets industry health standards regarding eye protection.

Infrared LANs

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.

Infrared implementations

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.

System components

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.

Operating performance

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.

Media access control

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.

Monitoring and diagnostics

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

Administration

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.

Computer connectivity

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 standards

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.

IrDA protocol suite

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.