ABSTRACT The draft IEEE 802 11 Wireless Local Area Network WLAN specification is approaching completion In this article the IEEE 802 11 protocol is explained with particular emphasis on the medium access control sublayer Performance results are provided for packetized data and a combination of packetized data and voice over the WLAN Our performance investigation reveals that an IEEE 802 11 network may be able to carry traffic with time bounded requirements using the point coordination function However our findings suggest that packetized voice traffic must be handled in conjunction with an echo canceler IEEE 802 11 Wireless Local Area Networks Brian P Crow The MITRE Corporation Indra Widjaja Fujitsu Network Communications Jeong Geun Kim University of Arizona Prescott T Sakai Cypress Semiconductor W ireless 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 Frequency Allocation Operation of a wireless network requires that all users operate on a common frequency band The views and opinions expressed in this article are those of the authors and do not reflect MITRE s or Fujitsu Network Communications current position 116 0163 6804 97 10 00 1997 IEEE 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 Security 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 over 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 Power Consumption Typically devices connected to a wired network are powered by the local 110 V commercial power provided in a building 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 effi IEEE Communications Magazine September 1997 Of particular interest in the specification is the support for two fundamentally different MAC schemes to transport asynchronous and timebounded services The first scheme STA STA distributed coordination function Human Safety Research is ongoIndependent BSS DCF is similar to traditional legacy ing to determine whether radio frepacket networks supporting bestquency RF transmissions from radio effort delivery of the data The DCF and cellular phones are linked to Figure 1 Sketch of an ad hoc network is designed for asynchronous data human illness Networks should be transport where all users with data designed to minimize the power to transmit have an equally fair transmitted by network devices For chance of accessing the network The point coordination funcinfrared IR WLAN systems optical transmitters must be tion PCF is the second MAC scheme The PCF is based on designed to prevent vision impairment polling that is controlled by an access point AP The PCF is primarily designed for the transmission of delay sensitive trafMobility Unlike wired terminals which are static when fic While the DCF has been studied by several researchers operating on the network one of the primary advantages of 4 7 the combined performance of the DCF and PCF operwireless terminals is freedom of mobility Therefore system ating in a common repetition interval is much less understood designs must accommodate handoff between transmission In this article the performance of an ad hoc network DCFboundaries and route traffic to mobile users only and an infrastructure network DCF and PCF are investigated by means of simulation We also investigate the Throughput The capacity of WLANs should ideally effect of channel errors on the performances of PCF and approach that of their wired counterparts However
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