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.

Frequency hopping | Spread-Spectrum LANs

Frequency hopping entails the transmitter jumping from one frequency to the next at a specific hopping rate in accordance with a pseudo-random code sequence. The order of frequencies selected by the transmitter is taken from a predetermined set as dictated by the code sequence. For example, the transmitter may have a hopping pattern of going from channel 6 to channel 1 to channel 5 to channel 9 to channel 3 and so on. The receiver tracks these changes. Since only the intended receiver is aware of the transmitter's hopping pattern, then only that receiver can make sense of the data being transmitted.

Other frequency hopping transmitters will be using different hopping patterns that usually will be on noninterfering frequencies. Should different transmitters coincidentally attempt to use the same frequency and the data of one or both become garbled at that point, retransmission of the affected data packets is required. Those data packets will be sent again on the next hopping frequency of each transmitter. Most LAN protocols have an integral error detection capability. When the protocol's error checking mechanism recognizes incoming packets that are bad or determines that there are missing packets, the receiving station requests a retransmission of only those packets. When the new packets arrive to rendezvous with those held in queue, the protocol's sequencing capability puts them in the correct order.

Some wireless LAN vendors use algorithms for dynamic data rate switching in conjunction with frequency hopping spread spectrum. The algorithms dynamically determine without user intervention whether the wireless signal is uncorrupted and whether the number of retransmissions is sufficiently low to allow data to be transmitted at a higher speed. The transmitting device can then select the maximum reliable data rate on the fly.

As noted, the FCC has authorized the use of spread-spectrum transmitters without requiring individual user licenses. However, only direct sequence or frequency hopping techniques may be used—no other spreading techniques are permitted, per FCC Rule 15.247. Users may define their own channel width up to a maximum of 500 kHz in the 900-MHz band or 1 MHz in the 2.4-GHz band. Frequency hopped systems must not spend more than 0.4 seconds on any one channel each 20 seconds, or 30 seconds in the 2.4-GHz band. Furthermore, they must hop through at least 50 channels in the 900-MHz band, or 75 channels in the 2.4-GHz band. These rules reduce the chance of repeated packet collisions in areas with multiple transmitters.

Direct sequence spread spectrum offers better performance, but frequency-hopping spread spectrum is more resistant to interference and is preferable in environments where many other devices generate electromechanical noise. Direct sequence is more expensive than frequency hopping and uses more power.

What hardware the user chooses will largely be dictated by applications. Users concerned with good performance in environments where interference is not a problem, will generally opt for the direct sequence solution. Users who need a small, inexpensive portable wireless adapter for their notebook or PDA, generally go for frequency-hopping spread spectrum.

Spread-spectrum technology is also used in bridges and routers for LAN extension. Although line-of-sight connections are required, such devices allow users to extend the reach of wireless LANs (Ethernet and token ring) by as much as 25 miles. For example, a bridge can be used to connect to a central server to access e-mail, groupware, and client/server applications without any modifications. Bridges and routers can also be used to link wireless and wireline LANs together into a seamless enterprise network.

Direct sequence | Spread-Spectrum LANs

In direct sequence spreading, the radio energy is spread across a larger portion of the band than is actually necessary for the data. This is done by breaking each data bit into multiple sub-bits called "chips" to create a higher modulation rate. The higher modulation rate is achieved by multiplying the digital signal with a chip sequence. If the chip sequence is ten, for example, and it is applied to a signal carrying data at 300 Kbps, then the resulting bandwidth will be ten times wider. The amount of spreading is dependent upon the ratio of chips to each bit of information. Because data modulation widens the radio carrier to increasingly larger bandwidths as the data rate increases, this chip rate of 10 times the data rate spreads the radio carrier to 10 times wider than it would otherwise be for data alone. Figure 8.2 compares the bandwidth required for 300 Kbps of data and the 10:1 increase in bandwidth due to spreading.

Figure 1: Spreading 300 Kbps of data across a wider band causes it to resemble random noise during transmission, making it difficult to intercept. (a) Original signal. (b) Spread signal.

The rationale behind this technique is that a spread-spectrum signal with a unique spread code cannot create the exact spectral characteristics as another spread-coded signal. Using the same code as the transmitter, the receiver can correlate and collapse the spread signal back down to its original form, while other receivers using different codes cannot.

This feature of spread spectrum makes it possible to build and operate multiple networks in the same location. By assigning each one its own unique spreading code, all transmissions can use the same frequency band, yet remain independent of each other. The transmissions of one network appear to the other as random noise and are filtered out because the spreading codes do not match.

This spreading technique would appear to result in a weaker signal-to-noise ratio, since the spreading process lowers the signal power at any one frequency. Normally, a low signal-to-noise ratio would result in damaged data packets that would require retransmission. However, the processing gain of the despreading correlator recovers the loss in power when the signal is collapsed back down to the original data bandwidth. This process is not the same as the signal enhancement techniques used by certain devices on wireline networks, since the signal is not strengthened beyond what would have been received had the signal not been spread. (In wireline data transmission, DSUs, for example, perform data regeneration by reshaping the transmit signal before it is sent out over the digital facility to ensure optimal network performance.)

The FCC has set rules for direct sequence transmitters. Each signal must have ten or more chips. This rule limits the practical raw data throughput of transmitters to 2 Mbps in the 902-MHz band and 8 Mbps in the 2.4-GHz band. Unfortunately, the number of chips is directly related to a signal's immunity to interference. In an area with a lot of radio interference, users will have to give up throughput to successfully limit interference.

Spread-Spectrum LANs

Spread spectrum is assigned the industrial, scientific, and medical (ISM) bands of the electromagnetic spectrum. The ISM bands include the frequency ranges of 902 MHz to 928 MHz and 2.4 GHz to 2.484 GHz, which do not require an FCC site license.

Spread spectrum is based on a digital coding technique in which the signal is taken apart or "spread" so that it sounds more like noise, making interception difficult. The coding operation increases the number of bits transmitted and expands the bandwidth used. With the signal's power spread over a larger set of frequencies, the result is a more robust signal that also is less susceptible to impairment from electrical noise and other sources of interference.

Using the same spreading code as the transmitter, the receiver correlates and collapses the spread signal back down to its original form. Spread spectrum is used for wireless Ethernet LANs and is the basis for other advanced wireless transmission techniques such as code division multiple access (CDMA), which is being used to support a variety of services, including emerging personal communications services (PCS).

Spread spectrum is a highly robust wireless data transmission technology that offers substantial performance advantages over conventional narrowband radio systems. As noted, the digital coding technique used in spread spectrum takes the signal apart and spreads it over the available bandwidth, making it appear as random noise. Noise has a relatively flat, uniform spectrum with no coherent peaks and can generally be removed by filtering. The spread signal has a much lower power density, but the same total power.

This low power density, spread over the expanded transmitter bandwidth, provides resistance to a variety of conditions that can plague conventional narrowband radio systems, including:

• Interference: a condition in which a transmission is being disrupted by external sources, such as the noise emitted by various electromechanical devices, or internal sources such as crosstalk.
• Jamming: a condition in which a stronger signal overwhelms a weaker signal, causing a disruption to data communications.
• Multipath: a condition in which the original signal is distorted after being reflected off solid objects.
• Interception: a condition in which unauthorized users capture signals in an attempt to determine its content.

Spread spectrum also is able to achieve a higher transmission rate than narrowband radio systems (nonspread spectrum). Narrowband radio systems transmit and receive on a specific frequency that is just wide enough to pass the information, whether voice or data. By assigning users different channel frequencies, confining the signals to specified bandwidth limits, and restricting the power that can be used to modulate the signals, undesirable crosstalk—interference between different users—can be avoided. These rules are necessary because any increase in the modulation rate widens the radio signal bandwidth, which increases the chance for crosstalk.

For example, current narrowband radio systems generally cannot transmit data beyond 9.6 Kbps without violating the FCC-established narrowband channel spacing of 25 kHz between radio carriers. Figure 1 shows radio systems operating within their own assigned 25-kHz radio channels (top) and a radio system impaired by crosstalk from an adjacent radio system that is exceeding its channel bandwidth to achieve a higher transmission rate (bottom). Some narrowband radio systems operate this way, but they require FCC approval.

Figure 1: How crosstalk impairs the proper operation of a radio system. (a) Proper operation. (b) Channel interference.

Current spread-spectrum LANs provide transmission rates of up to 6 Mbps. The higher speed of spread spectrum over conventional narrowband radio is primarily attributable to the use of very wide channels, which can be up to 500 kHz in the 900-MHz band and 1 MHz in the 2.4-GHz band, versus the 25 kHz channels typically used for narrowband radio.

The main advantage of spread-spectrum radio waves is that the signals can be manipulated to propagate fairly well through the air, despite electromagnetic interference, to virtually eliminate crosstalk. In spread-spectrum modulation, a signal's power is spread over a larger band of frequencies. This results in a more robust signal that is less susceptible to impairment from electrical noise and interference from similar radio-based systems, since they too are spreading their signals, but with different spreading algorithms.

There are several spreading techniques. Direct sequence and frequency hopping are the most common spreading techniques used in the LAN environment.

Overview | Wireless LANs

Most computers in the corporate environment are tied together over wired LANs so that users can access and share data, applications, and services. However, a growing number of applications require mobility as well as network access. One way of achieving both objectives is for a notebook computer to plug into a docking station, which is wired to the LAN. Another way is for the notebook's PCMCIA card to establish a wireless connection to the nearest access point, which is wired to the LAN. Of course, desktop computers can be interconnected with each other over a wireless LAN, and connect to a wired LAN only when necessary through an access point. A network interface card (NIC) equipped with a transceiver links individual network nodes. External antennas allow for omnidirectional transmission instead of requiring a clear line of sight.

Coverage can be extended to other floors, between buildings, or across a metropolitan area using wireless bridge/routers. Since it is not necessary to install new cabling, wireless LANs offer a convenient alternative for adding or moving users. Both Ethernet and token ring LANs are supported over wireless links and the devices can be managed using standard SNMP-based management packages or vendor-specific configuration tools.

Notebook and desktop computers are not the only devices that require wireless connections. Mobile terminals—PDAs, specialized handheld terminals, and barcode scanners—connected to wireless LANs are being increasingly used to enhance business operations. Mobile data applications are raising the productivity of essential personnel and eliminating unnecessary paperwork, cutting operations costs in the process. These devices are also used to increase revenues by bringing products, services and transaction points closer to users via wireless connections.

While the use of wireless networks answers the need for mobility and solves many network administration problems, they do have their share of drawbacks. For example, wireless LANs usually transmit at slower speeds than wired LANs, and the frequencies used for data transmission are subject to interference which can impair performance. The fact that signals are radiated in the air may present security concerns. The products of many vendors are not interoperable with each other; wireless LANs are often too small to make interoperability a strong issue. And although prices are dropping, wireless LANs are still more expensive than wired LANs.

Despite these limitations, however, wireless LANs are here to stay and will continue to improve and grow. With the IEEE 802.11 standard for wireless LAN communication released in 1997, a number of basic media and configuration issues, transmission procedures, throughput requirements, and range characteristics are addressed which can help reduce the risk of product incompatibility and early obsolescence. Over the long term, the 802.11 standard is expected to help make wireless LANs price-competitive with wired networks.

One source of multivendor product incompatibility is that different wireless technologies are used to implement wireless LANs. The three popular technologies currently in use are spread spectrum, infrared, and microwave.

Spread-spectrum modulation is a more complex form of AM/FM. It uses low-power, 900-MHz radio waves. The maximum attainable speeds are 1 Mbps or 2 Mbps, which is far too slow for current 10 Mbps and 16 Mbps LANs. Infrared uses short-wavelength light for transmission and it works well at higher speeds, but it offers the least amount of coverage and requires a line-of-sight connection between devices. These problems can be easily overcome, but at the greater cost. Microwave transmission at 18 GHz is a very effective communications medium, but it requires an FCC license. This is not an obstacle, if the vendor acts on the customer's behalf to obtain the license. Although offering greater range, microwave is more expensive than either spread spectrum or infrared.