802.11-oveview - ABSTRACT The draft IEEE 802.11 Wireless...

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IEEE Communications Magazine • September 1997 116 IEEE 802.11 Wireless Local Area Networks 0163-6804/97/$10.00 © 1997 IEEE ireless computing is a rapidly emerging technology providing users with network connectivity without being tethered off of a wired network. Wireless local area net- works (WLANs), like their wired counterparts, are being developed to provide high bandwidth to users in a limited geographical area. WLANs are being studied as an alternative to the high installation and maintenance costs incurred by tra- ditional additions, deletions, and changes experienced in wired LAN infrastructures. Physical and environmental neces- sity is another driving factor in favor of WLANs. Typically, new building architectures are planned with network connec- tivity factored into the building requirements. However, users inhabiting existing buildings may find it infeasible to retrofit existing structures for wired network access. Examples of structures that are very difficult to wire include concrete buildings, trading floors, manufacturing facilities, warehouses, and historical buildings. Lastly, the operational environment may not accommodate a wired network, or the network may be temporary and operational for a very short time, making the installation of a wired network impractical. Examples where this is true include ad hoc networking needs such as conference registration centers, campus classrooms, emergen- cy relief centers, and tactical military environments. Ideally, users of wireless networks will want the same ser- vices and capabilities that they have commonly come to expect with wired networks. However, to meet these objectives, the wireless community faces certain challenges and constraints that are not imposed on their wired counterparts. Frequency Allocation — Operation of a wireless network requires that all users operate on a common frequency band. Frequency bands for particular uses must typically be approved and licensed in each country, which is a time-consuming pro- cess due to the high demand for available radio spectrum. Interference and Reliability — Interference in wireless com- munications can be caused by simultaneous transmissions (i.e., collisions) by two or more sources sharing the same frequency band. Collisions are typically the result of multiple stations wait- ing for the channel to become idle and then beginning transmis- sion at the same time. Collisions are also caused by the “hidden terminal” problem, where a station, believing the channel is idle, begins transmission without successfully detecting the presence of a transmission already in progress. Interference is also caused by multipath fading, which is characterized by ran- dom amplitude and phase fluctuations at the receiver. The reliability of the communications channel is typically mea- sured by the average bit error rate (BER). For packetized voice, packet loss rates on the order of 10 –2 are generally acceptable; for uncoded data, a BER of 10 –5 is regarded as acceptable. Automatic repeat request (ARQ) and forward error correction (FEC) are used to increase reliability.
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