5.4 Wireless LAN Communications

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:

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.

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'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.

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 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.

Beyond 802.11