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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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Sensitivity determines the amount of time the mobile station will spend in cell search mode and when the mobile station will switch to another WavePOINT. Sensitivity parameters set the values of the Cell Search Thresholds, which determine when the mobile station starts or stops looking for another WavePOINT. These thresholds are related to the level of communications quality. There are three Cell Search thresholds:

  Regular Cell Search. The level of communications quality at which the mobile station starts looking for another WavePOINT. The station will only switch over to a new WavePOINT if the level of communications quality with that WavePOINT is higher than the Stop Cell Search threshold.
  Fast Cell Search. The level of communications quality at which the mobile station starts looking for a WavePOINT with any acceptable level of communications quality. In this case, the station will immediately switch over to a WavePOINT that provides better communications quality.
  Stop Cell Search. The level of communications quality at which the mobile station stops looking for a WavePOINT. There are three user-defined Sensitivity presets: Low, Normal and High.
  Low. If sensitivity is Low, a roaming station will stay connected to a WavePOINT as long as possible. It will start searching for another WavePOINT later than with the Normal setting and stop searching earlier. The Low sensitivity settings are best when coverage areas are not adjacent to one another. They will avoid a station looking for WavePOINTs when there is no WavePOINT within range.
  Normal. If sensitivity is Normal, a roaming station will work best in most environments.
  High. If sensitivity is High, a roaming station will try to switch to another WavePOINT as soon as possible. It will start searching for another WavePOINT earlier and stop searching later than with the Normal setting. If Sensitivity is High, for example, a roaming station is likely to spend more time in cell search mode.

In cell search mode, a mobile station has to interpret beacons and network broadcast messages transmitted by different WavePOINTs. If sensitivity is too high, a station is likely to spend more time in cell search mode than needed. This will cause unnecessary use of the processing capacity from the mobile station. A mobile station requires network overhead to switch between two WavePOINTs.

WaveAROUND roaming functionality enables a mobile station to detect an out-of-range situation and reestablish a lost connection. The combined characteristics of applications and the NOS, however, may pose problems for network operations because most of today’s applications are not designed for use in a wireless mobile environment. Future developments of applications should allow for temporarily working offline. When a connection is lost, the application must be able to “synchronize files” as soon as the connection becomes available again.

Radio-based Wireless LAN Performance

Radio-based wireless LANs offer performance similar to ethernet networks. Figure 2.11 compares the performance of WaveLAN versus ethernet. The figure shows the response time of performing a DOS file copy for several different size files between a 80386/25 MHz server and 80386sx/16 MHz workstation via WaveLAN NICs. For file sizes of less than 100 KB, ethernet and WaveLAN performance is nearly the same. For larger files, though, ethernet takes the lead. The actual performance will depend on the application’s file sizes and frequency of network use.

In addition, radio-based wireless bridges were designed to operate within a typical LAN environment. WavePOINT, for example, was designed to operate under the following assumptions:

  Ethernet network utilization = 20%
  Frame size varying from 64–1518 bytes
  WaveLAN throughput of 150 KByte/s
  Traffic to be forwarded = 25%

This allows the bridge to keep up with typical ethernet traffic.


Figure 2.11  WaveLAN versus 10 Mbps ethernet.

Infrared Light-based Wireless LANs

Infrared light is an alternative to using radio waves for wireless LAN interconnectivity. The wavelength of infrared light ranges from about 0.75 to 1,000 microns, which is longer (lower in frequency) than the spectral colors but much shorter (higher in frequency) than radio waves. Under most lighting conditions, therefore, infrared light is invisible to the naked eye. Infrared light LAN products operate around 820 nanometer wavelengths because air offers the least attenuation at that point in the infrared spectrum.


NOTE:  

Sir William Herschel discovered infrared light in 1800 when he separated sunlight into its component colors with a prism. He found that most of the heat in the beam fell in the spectral region where no visible light existed, just beyond the red.


In comparison to radio waves, infrared light offers higher degrees of security and performance. These LANs are more secure because infrared light does not propagate through opaque objects, such as walls, keeping the data signals contained within a room or building. Also, common noise sources such as microwave ovens and radio transmitters will not interfere with the light signal. In terms of performance, infrared light has a great deal of bandwidth, making infrared light possible to operate at very high data rates. Infrared light, however, is not as suitable as radio waves for mobile applications because of its limited coverage.

An infrared light LAN consists mainly of two components—an adapter card or unit and a transducer. The adapter card plugs into the PC or printer via an ISA or PCMCIA slot (or connects to the parallel port). The transducer, similar to the antenna with a radio-based LAN, attaches to a wall or office partition. The adapter card handles the protocols needed to operate in a shared medium environment, and the transducer transmits and receives infrared light signals.

There are two types of infrared light LANs:

  Diffused
  Point-to-point


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