Wireless computing is a rapidly emerging technology providing users with network connectivity without being tethered off of a wired network. Wireless local area networks (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 traditional additions, deletions, and changes experienced in wired LAN infrastructures. Physical and environmental necessity is another driving factor in favor of WLANs. Typically, new building architectures are planned with network connectivity 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, emergency relief centers, and tactical military environments.
Ideally, users of wireless networks will want the same services 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.
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 process due to the high demand for available radio spectrum.
Interference and Reliability
Interference in wireless communications 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 waiting for the channel to become idle and then beginning transmission 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 random amplitude and phase fluctuations at the receiver. The reliability of the communications channel is typically measured 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.
In a wired network, the transmission medium can be physically secured, and access to the network is easily controlled. A wireless network is more difficult to secure since the transmission medium is open to anyone within the geographical range of a transmitter. Data privacy is usually accomplished through a radio medium using encryption. While encryption of wireless traffic can be achieved, it is usually at the expense of increased cost and decreased performance.
Typically, devices connected to a wired network are powered by the local 110 or 230 V commercial power provided in a building depending upon the country. Wireless devices, however, are meant to be portable and/or mobile and are typically battery powered. Therefore, devices must be designed to be very energy-efficient, resulting in “sleep” modes and low-power displays, causing users to make cost versus performance and cost versus capability trade-offs.
Research is ongoing to determine whether radio frequency (RF) transmissions from radio and cellular phones are linked to human illness. Networks should be designed to minimize the power transmitted by network devices. For infrared (IR) WLAN systems, optical transmitters must be designed to prevent vision impairment.
Unlike wired terminals, which are static when operating on the network, one of the primary advantages of wireless terminals is freedom of mobility. Therefore, system designs must accommodate handoff between transmission boundaries and route traffic to mobile users.
The capacity of WLANs should ideally approach that of their wired counterparts. However, due to physical limitations and limited available bandwidth, WLANs are currently targeted to operate at data rates between 1–20 Mb/s. To support multiple transmissions simultaneously, spread spectrum techniques are frequently employed.