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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Diffused Infrared-based LAN TechniqueYouve probably been using a diffused infrared device for yearsthe television remote controlwhich allows you to operate your TV from a distance without the use of wires. When you depress a button on the remote, a corresponding code modulates an infrared light signal that is transmitted to the TV. The TV receives the code and performs the applicable function. This is fairly simple, but infrared-based LANs are not much more complex. The main difference is that LANs utilize infrared light at slightly higher power levels and use communications protocols to transport data. When using infrared light in a LAN, the ceiling can be a reflection point (see fig. 2.12). This technique uses carrier sense protocols to share access to the ceiling. Imagine, for example, a room containing four people who can only communicate via flashlights. To send information, the people can encode letters that spell words using a system such as Morse Code. If someone wants to send information, they first look at the ceiling to see if someone is currently transmitting (shining light off the ceiling). If a transmission is taking place, the person wanting to send the information waits until the other person stops sending the message. If no one is transmitting, the source person will point their flashlight to the ceiling and turn the light on and off, according to the code that represents the information being sent. To alert the destination person of an incoming message, the sender will transmit the proper sequence of code words that represent the destination persons name. All people in the room will be constantly looking at the ceiling, waiting for light signal s containing their address. If a person sees their name, they will pay attention to the rest of the transmission. Through this method, each person will be able to send and receive information.
Diffused infrared light LANs work similarly to the previous analogy. The LANs, however, operate much faster. Typical data rates are 14 Mbps versus the 2030 bps the average person can send using flashlights and Morse Code. Prices for these types of wireless adapters range from $200 to $500 each. Due to geometry, diffused infrared light stations are limited in separation distance, typically 3050 feet. The lower the ceiling, the less range between stations. Ceiling heights of ten feet will limit the range to around forty feet. To extend the operating range, you can utilize infrared access points to connect cells together via a wired backbone.
Several companies sell diffused infrared LAN components. The following sections provide an overview of these products. Photonics Cooperative Wireless Network SystemPhotonics is currently the only dual platform wireless networking provider with products for both Apple and PC-compatible computers. Photonics wireless LAN products use diffuse infrared light that bounces off ceilings to provide a coverage area of up to 25 × 25. The Photonics Cooperative system, which operates at 1 Mbps, consists of two product families: adapters that provide peer-to-peer networking capability to computers and access points for linking groups of wireless users. The access points, which support multiple cell configurations by providing roaming from cell to cell, will interface Cooperative users to either AppleTalk or ethernet networks. IBMs Infrared LANIBMs family of infrared LAN communications products includes infrared adapter models that have form factors fitting Micro Channel, ISA, and PCMCIA Type II. IBM offers two types of Access Points. The Access Point Unit gives multiple mobile computing users and wireless cells access to ethernet LANs. It also enables multiple wireless cells to communicate with each other via a wired LAN. The Access Point Unit comes with a built-in ethernet and a PCMCIA Infrared LAN adapter with a tethered transceiver. The Access Point Software enables a 33MHz 486 (or higher) PC with an ethernet or token-ring adapter to act as an infrared LAN access point. This software comes at no charge with the purchase of an IBM Infrared LAN adapter. Spectrixs SpectrixLiteSpectrix produces a diffused infrared LAN called SpectrixLite for portable computers. A SpectixLite Communications Link connects to a PC in a variety of formats: RS-232, TTL, RS-485/RS-422, or PCMCIA Type II. The Comm Link sends and receives diffused infrared light with a wide angle field of view and offers a 4 Mbps data rate. With the signal bouncing off walls and objects, communications are omnidirectional, and the users can move freely within a 50 foot range of the Base Station. The Base Station provides wireless subnet control using Spectrixs patented deterministic reservation/polling wireless protocol called CODIAC (Centralized Operation Deterministic Interface Access Control). CODIAC minimizes consumption of a portables battery power and supports time-bounded services by making stations that want to transmit information reserve time slots to send their data. Antennas are connected to the Base Station by twisted-pair wires, providing a 40,000 square foot service area using a 16-port hub. The system enables users to roam seamlessly between Base Stations while maintaining connectivity. Point-to-Point Infrared TechniquesSome infrared products operate in a point-to-point manner, that is, where the devices maintain direct links with one another. Two highly different products in this category are the point and beam devices that transfer files directly between computers and peripherals, and a point-to-point infrared LAN system that replaces the wire in a token-ring network with infrared light.
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