Wireless LAN communications are most often located within a single office building, and utilize a wireless transceiver (also called access point) in a fixed location. Wireless LANs eliminate the need to interconnect workstations with cable connections since transceivers broadcast data to and receive data from the computers. This is possible because of a new high-bandwidth frequency allocation. In a typical configuration, the transceiver unit connects to servers and other equipment with standard Ethernet cable.
Wireless LANs and other wireless technologies are specified under IEEE 802.11 standard. The standard covers both infrared and spread spectrum radio technologies. IEEE 802.11 also allows for privacy by encrypting transmitted signals, allowing for recovery of lost messages, and overcoming fading and interference that occurs when signals travel over several paths from transmitter to receiver. By using different frequencies, multiple users can exist in the same radio space. To promote interoperability, a new IEEE standard, P802.11, includes the Wired Equivalent Privacy algorithm as its encryption method.
CSMA/CA and Wireless
Similar to the 802.3 Ethernet standard, the MAC Layer specification for IEEE 802.11 uses a slightly different method of directing data at the MAC Layer. The protocol for the wireless LAN standards avoids collisions instead of detecting collisions as done in IEEE 802.3. The protocol scheme is called Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA).
The advantages for wireless LANs include better flexibility and more mobility since wiring isn't required, plus the fact that an FCC license is not required. Some possible drawbacks include security concerns and the lack of complete standardization.
Techniques for utilizing wireless data transmission include the following:
- Point-to-Point Infrared
- Broadcast Infrared
- Spread Spectrum Radio
- Narrowband Radio
- Infrared Transmission Systems
An infrared transmission system employs light as its media. Infrared light signals fall into the electromagnetic spectrum's TeraHertz (THz) range. Infrared signals are created by a device called a Light Emitting Diode (LED). Infrared is supported at the Physical Layer of the OSI model.
On the receiving end, light signals are read by components called photodiodes that interpret binary code-embedded light signals. A common, everyday tool containing photodiodes is the infrared remote control—a stereo or television set's usual sidekick.
- Point-to-Point Infrared Systems
Infrared signals are electromagnetic waves or photons. In order to work properly, infrared signals require a line-of-sight or point-to-point broadcast, or an omni-directional broadcast in which light is redirected off objects to reach its desired location.
Point-to-point broadcast features superior data rates, while omni-directional broadcast allows for greater flexibility in positioning the transmitter and receiver.
Point-to-point infrared transmissions systems offer the option to tightly focus the infrared beam and direct it at a specific target. Certain laser-transmitting, infrared systems are capable of sending signals more than two or three thousand meters.
Point-to-point infrared transmission systems have many advantages. No licenses are required to broadcast an infrared transmission, eavesdropping is extremely difficult, and signals rarely suffer from attenuation.
Computer networks use infrared technology through carefully placed receivers and transmitters that work much the same way as a television remote control.
Point-to-point transmissions utilize low-range light frequencies ranging from 100 GHz to 1000THz.
Cost depends on whether the system is a short- or long-range transmission system. Long-range systems' exact component alignment needs generally require expensive equipment, powerful lasers, and expert installation. Short-range systems can be adapted for use with ordinary computer networks for a reasonable cost.
Point-to-point systems have a data rate ranging from 100Kbps to 16Mbps. The degree of attenuation depends largely on the quality and type of light, disturbances present in the atmosphere, and line-of-sight clarity.
- Broadcast Infrared Transmission Systems
Broadcast infrared systems operate similarly to point-to-point systems. The main difference is the distribution of the infrared signal.
While an infrared THz's signals have excellent throughput, they also present some drawbacks. Infrared signals cannot pass through solid objects like walls or buildings. And a strong, external light source can diminish or dilute an infrared signal's potency.
Broadcast transmission systems beam light signals in a less concentrated fashion, spreading them throughout a wider area. Broadcast systems often have several receivers, some of which are mobile and frequently moved.
- Spread Spectrum Radio
While infrared requires a clear line-of-sight or can operate over short distances within a single room, spread spectrum is far more practical. Spread spectrum technology can penetrate through barriers, is more secure and greatly reduces the interference that infrared suffers from. Spread spectrum technology is currently being used to make wireless phones more secure from unwanted listeners.
However, a less concentrated signal also means less throughput. Broadcast systems are generally limited to one Mbps or less, and operate between 100GHz and 1000THz—the lowest of all light frequencies. Broadcast infrared transmission systems are usually too limited for use with a network.
A broadcast infrared system's costs depend primarily on light signal quality. Compared to laser-quality infrared systems, broadcast equipment is relatively inexpensive and easier to install. In addition, the only necessities for a broadcast system are a clear path for the infrared signal to travel and a signal strong enough to cover the required distance. Another factor to consider is whether or not any powerful light sources may interfere with an infrared transmission.
Broadcast infrared systems have data rates equal to approximately 1 Mbps, but the node capacity varies greatly corresponding to the amount of data involved. Attenuation for broadcast infrared systems depends largely on the strength of light emitted by the system, outside light sources, and atmospheric conditions. Broadcast infrared systems are not commonly obstructed, but eavesdropping—stemming from infrared light's spread-out nature—is relatively easy.
Digital Radio Frequency modulation has been around since the mid 1970's. Digital modulation features greater noise reduction than analog. Reduced noise levels improve transmissions signals, especially over long distances.
Spread Spectrum Radio often uses two of the Federal Communications Commission's unlicensed ISM (Industrial, Scientific and Medical) radio frequency bands, 902MHz to 928 MHz and 2.4GHz to 2.4835 GHz, spreading its transmissions across a range of frequencies instead of using just one frequency.
Spread Spectrum's energy levels are too weak to interfere with any conventional radio signals and its signaling distance is generally under 1,000 feet at 2Mbps.
- Direct Sequencing Spread Spectrum (DSSS)
The distinguishing characteristic of spread spectrum is that the original signal is spread out to a wide bandwidth 200 times the original signal's bandwidth. The two most often used spread spectrum techniques are direct sequence and frequency hopping.
- Frequency Hopping Spread Spectrum (FHSS)
Data bits are spread into an 11 bit sequence, known as a chipping code, before being transmitted. This splits the radio frequency into a wider bandwidth that is required to send the raw data. It is then re-assembled by the receiver. It is more secure than infrared because the data is coming through in split up, unsequenced chunks that are theoretically uninterruptible by a receiver other than the one for which the message is intended.
FHSS uses a narrowband carrier that changes frequency in patterns that are known to both the transmitter and receiver, so it appears to be a single channel. This provides security above and beyond that of infrared technologies as each transmission is delivered in such a way that makes it very challenging for an intercepting receiver to translate an airborne transmission.
FHSS has 22 hop patterns to hop across the 2.4 GHz ISM band that covers 79 channels. Each channel occupies 1 MHz of bandwidth and must hop at the minimum rate specified. In the United States this means a minimum hop rate of 2.5 hops per second.
| Frequency Hopping Spread Spectrum (FHSS) Ranges Around the World | |||
|---|---|---|---|
| Geography | Lower Limit (GHz) | Upper Limit (GHz) | Range (GHz) |
| North America | 2.402 | 2.480 | 2.400 - 2.4835 |
| Europe | 2.402 | 2.480 | 2.400 - 2.4835 |
| France | 2.448 | 2.482 | 2.4465 - 2.4835 |
| Spain | 2.447 | 2.473 | 2.445 - 2.475 |
| Japan | 2.473 | 2.495 | 2.471 - 2.497 |
- Narrowband Radio
Very similar to radio station broadcasting, narrowband radio tunes into a "tight" frequency on both the transmitter and receiver.
The two spread spectrum technologies used in wireless communication do not interoperate with each other. Both makes claims for superiority. DSSS supporters say that it has greater range and a faster data rate (11 Mbps vs. 2 Mbps). Conversely, FHSS supporters claim that it is more scalable and can adapt to new technologies. The FCC is considering proposals to make these two spread spectrum technologies interoperate.
Narrowband radio can spread over a wide area and go through walls, so focusing is not required. On the other hand, you may experience fuzzy reception called ghosting. Also, certain frequencies are FCC regulated.
Wireless LANs, though they are relatively cheap to maintain and have no cabling requirement, have not gained a large market share yet due to low data rates of under 10 Mbps for most of wireless LAN products. This may change in the near future.
In 1997 the FCC opened up additional spectrums from unlicensed wireless LANs from 5.15 to 5.35 GHz and 5.725 to 5.825 GHz. These frequencies are high enough to allow data rates as high as 20Mbps and are free. Radio LAN introduced the first wireless LAN to operate in the 5.8GHz band, and it achieves data rates of 10Mbps.
While IEEE has approved interoperability standards for 1Mbps to 10Mbps wireless LAN devices in the 802.11 wireless LAN specification, enhancements at higher data rates are being developed. To ensure interoperability, Aironet Wireless, Digital Ocean, and Lucent Technologies are developing common access points.
Current limitations may restrict usage, however. While interoperability is improving, the different systems are not yet totally in synch. Another weakness lies with the fact that the speed does not yet match that of the fastest Ethernets.
Beyond 802.11
- Wireless Personal Area Networks
The IEEE 802.15 standard defines Wireless Personal Area Networks (WPAN)s and was formed as a working group during the summer of 1999. This is an area that promises to see many new developments, as portable and mobile devices become more widespread. These standards will focus on wireless networking of PCs, laptops, Personal Digital Assistants (PDAs), cell phones, pagers and other consumer electronics. The goal is to allow these various devices to communicate and interoperate with each other on a global scale. - Broadband Wireless Access
IEEE 802.16, a wireless working group formed in July 1999, defines a standard for Broadband Wireless Access (BWA). The focus here will be on fixed broadband wireless access systems that operate near 30 GHz but will be applicable from 10GHz to 66 GHz. This project will also apply to data, video and voice services with licensed bands designated for public network services—applying to systems that operate between 2GHz and 11 GHz.
