Broadband Access Networks WLAN coverage is usually quite
Broadband Access Networks WLAN coverage is usually quite small. Depending on the power of the transmitter and receiver, reliable coverage is usually restricted to a radius of twenty metres although with line of sight and sophisticated antennae much greater distances can be covered. However, its purpose is primarily to provide coverage over a reasonably small area. Consequently, WLAN is usually deployed in association with some other access network technology, such as ADSL or cable modem. 8.5.1 IEEE 802.11 Wireless LAN Standards WLANs use the family of protocols defined by the Institute of Electrical and Electronic Engineers (IEEE) who have also been instrumental in standardising other popular protocols such as Ethernet. WLAN standards are collectively known as the 802.11 standards [IEEE2004], [GAST2002]. 802.11 is a family of standards. All 802.11 networks use a common MAC layer but vary in the physical layer details. There have been a number of different physical layer standards released since the original 802.11 standard in 1997, but the most commonly used are the 802.11b and the 802.11g standards operating in the 2.4 GHz ISM band. The ISM band is an area of minimally regulated bandwidth in which anyone may operate radio equipment subject to a minimal set of restraints, primarily on power levels. Consequently, although 802.11 equipment does not need special licensing to install and run, it is potentially subject to interference from other equipment. These include short range Bluetooth communications devices, some cordless telephones and microwave ovens. In designing the IEEE 802.11 protocol, care was taken to make it resistant to interference from other sources. However, this resistance is implemented as a graceful degradation from high bit rates where there is no interference to lower bit rates as interference increases. Consequently, WLANs can be subject to seemingly random changes in bit and error rates when other equipment that broadcasts in the ISM band is used nearby. 802.11b and 802.11g divide the ISM band into 14 overlapping channels with centre frequencies 5 MHz apart. Channels are numbered consecutively from 1 to 14. Within the same coverage area, users may access channels separated by 25 MHz. So in one coverage area users might access channels 1, 6 and 11, while in a neighbouring area they might use 2, 7 and 12. Separation of these channels is important. If user equipment or access points do not maintain this separation, then the result can be additional interference and consequent increases in errors and lower bit rates. 8.5.2 Wireless LAN Architectures The 802.11 standard specifies two kinds of network architectures: infrastructure and ad hoc networks. Infrastructure networks make use of an Access Point (AP) to control communication between users and to provide a communication path to the Internet. In ad hoc networks (or peer-to-peer), users equipment communicates directly without mediation by an AP. For 802.11 to be used as an access network, communication must be via an AP. However, the AP itself needs to be connected to the Internet. This is often via another broadband access network such as an ADSL or a Cable Modem network. Figure 8.4 shows an infrastructure network connecting three APs, each with two wireless nodes. In this kind of network, communication between wireless nodes is usually via the AP rather than directly between them.
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