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The IEEE 802.11 Wireless LAN Standard The Institute of Electrical and Electronic Engineers (IEEE) P802.11 working group has been focusing on the development of wireless LAN standards. The intent of the 802.11 working group is to develop a final draft standard, have it approved by IEEE, and then work toward acceptance of the standard by International Organization for Standardization (ISO). The project authorization request (PAR) submitted to IEEE in May 1991 to initiate the 802.11 working group states the scope of the proposed [wireless LAN] standard is to develop a specification for wireless connectivity for fixed, portable and moving stations within a local area. The PAR further spells out the purpose of the standard is to provide wireless connectivity to automatic machinery and equipment or stations that require rapid deployment, which may be portable, handheld, or which may be mounted on moving vehicles within a local area. In general, the 802.11 standard provides medium access control (MAC) and physical-layer (PHY) functional ability for wireless connectivity of fixed, portable, and moving stations within a local area with raw bit rates exceeding 1M bps. The goal of the standard is for a single MAC to support multiple PHYs using the transmission of electromagnetic waves (e.g., radio signals or infrared light) through the air. The 802.11 standard will support stations moving at pedestrian and vehicular speeds. Diffused Infrared-Based LAN Technique Another wireless LAN technique uses infrared light modulated to carry data between computers and bounce the signal off the ceiling as shown in Exhibit 2-6-4. The cost of these types of products average $200 to $400 per connection. Stations on this network compete for the use of the ceiling using a carrier sense protocol. If no other station is sending data (i.e., beaming light onto the ceiling), another station can send its data, which can be seen by the other stations.
The advantage of infrared connectivity, because of its very high frequency, is that it does not currently require special FCC or government licensing and has very little interference with other emanating devices. A wireless infrared network also provides a greater degree of privacy, because infrared light does not propagate through walls. A disadvantage, however, is that walls restrict wireless infrared connectivity to a single room, which means a radio or wired approach must be used to interconnect rooms. Infrared light is capable of very high bandwidth; however, the diffusing technique severely attenuates the signal and requires slow data transmissions (less than 1M bps) to avoid significant transmission errors. In addition, this technique limits wireless component spacing to around 40 feet, mainly because of geometry. The advantages include relatively easy installation and inexpensive components. Point-to-Point Infrared-Based LAN Technique Point-to-point infrared is another technique used to connect PCS and other peripherals together without wireless. As shown in Exhibit 2-6-5, the point-to-point infrared method replaces the wires in a Token Ring network. The transducers at each station focus the light beam and send it downline to the next station. This highly reduces the attenuation found in the passive infrared systems, offering data rates as high as 16M bps. Currently only one vendor, InfraLAN Technologies, sells a product of this type.
Power Line Techniques Exhibit 2-6-6 illustrates the use of existing power lines within a facility for the transmission of data between computers and peripherals. Some companies refer to this as carrier current technology. This technique avoids the installation of network cabling, and is therefore considered quasi-wireless. With this arrangement, an interface between the computer and the power line acts like a telephone modem and converts the digital data within a computer to an analog signal. As with most wireless techniques, this arrangement uses a carrier sense protocol for gaining access to the medium.
Power lines were designed to carry 60 Hz alternating current (AC), however, these wires have enough bandwidth to easily support 3,000 to 4,000 Hz. This low-cost methodapproximately $150 per connection provides data rates as high as 40K bps (see Exhibit 2-6-7). The disadvantage, however, is that electrical transformers only pass the 60 Hz AC, blocking the higher frequency data signals. Therefore, it is not possible to interconnect users on opposite sides of a power transformer.
POINT-TO-POINT WIRELESS NETWORKS As previously mentioned, wireless LANs are designed to operate within buildings. Their omnidirectional propagation of radio waves and diffused infrared transmission limits operating distances to less than 1,000 feet. On the other hand, as shown in Exhibit 2-6-8, point-to-point wireless networks offer line-of-site ranges of as far as 30 miles. Therefore, point-to-point wireless networks can provide connectivity among local buildings, across a large city, or between a company's headquarters and its branch offices.
With wireless bridges, a physical connection is unnecessary, therefore eliminating many issues related to installing links between sites. In addition, it is not necessary to spend the time and money tearing up sidewalks and parking lots to install cable or optical fiber. This avoids right-of-way restrictions. Point-to-point wireless networks use either radio waves or infrared light to facilitate a carrier for the transmission of data. The radio wave types are less expensive and operate at rates only as high as 4M bps, whereas the infrared ones are fairly expensive but offer rates of 10M bps and higher.
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