802.11 WLAN Deployment
What are RF channels and how does channel allocation differ for 802.11a and 802.11b/g?
A Radio Frequency (RF) channel is the range of frequencies allocated to provide the specified data rate. Each IEEE 802.11b and 802.11g channel offers 11 and 54 Mbps respectively in the 2.4 GHz spectrum while 802.11a offers 54 Mbps in the 5 GHz spectrum.
Each 802.11b or 802.11g channel is defined as having to operate with a center frequency between 2412 and 2484 MHz using a bandwidth of approximately 22 MHz. The channels are spaced at 5 MHz, therefore there are a maximum of 14 overlapping channels available. The number of channels that can be used depends upon the country’s regulatory body.
Each 802.11a channel is defined as having to operate with a center frequency between 5170 and 5320 MHz or 5745 to 5805 MHz. The 5.7 – 5.8 GHz band is dedicated to outdoor applications, typically point to point. In the lower band, there are 12 channels spaced at 10 MHz, each using a bandwidth of 20 MHz. As with 802.11b, the number that can be used depends upon the country’s regulatory body.
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What is the impact of overlapping channels?
Each channel delivers 11 Mbps or 54 Mbps of bandwidth. All clients that associate with an Access Point on a channel share that bandwidth. If more bandwidth is needed, additional Access Points must be added. As wireless is a shared medium like an Ethernet hub, these Access Points must be on non-overlapping channels. Otherwise, if a 2nd or 3rd Access Point on the same channel is used in the same physical area, no additional capacity will be added. Even channels that only partially overlap will cause performance issues. Overlap between the channels will cause interference between radios, making it harder for clients that are far away to connect reliably. Thus in the 2.4 GHz band, only 3 non-overlapping channels are available.
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How does 802.11g differ from 802.11b and what are the advantages of 802.11g?
With a maximum link rate of 54 Mbps and theoretical maximum TCP and UDP data rates of 24.4 Mbps and 30.5 Mbps, respectively, 802.11g should provide a significant increase in throughput over 802.11b. While this is true in ideal conditions, tests have shown that due to the inherent low reliability of the medium used (open air with a variety of interference factors), while 802.11g provides a significant speed and throughput increase in "clean" environments, it provides only a slight advantage as distance and / or interference increase. Another important aspect in overall network performance is the handling of control frames, which establish the link between clients and APs, are used to authenticate clients, to maintain the connection, and to terminate (or disassociate) the connection. While the data rate in an 802.11g system may be higher, all control frames use the same speed as 802.11b
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How does 802.11a differ from 802.11b/g and what are the advantages of 802.11a?
While 802.11b and 802.11g operate in the relatively congested 2.4 GHz band, 802.11a operates in the much less congested 5 GHz-UNII band, and with its 12 non-overlapping channels, allows for deployment of much larger WLANs with a substantial increase in the amount of wireless clients served. Conversely, the range of a single 802.11a station is much smaller than 802.11b or 802.11g, due to the higher free-space path loss experienced in the 5 GHz band. The major disadvantage of 802.11a is in the fact that these devices are substantially more expensive than 802.11b/g.
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What is the future of 802.11n and how does it improve over 802.11a/b/g networks?
The IEEE's study group is currently drafting the 802.11n standard. Expected to be ratified in 2008, 802.11n proposes to up the data rate of wireless LAN networks to speeds of 108 Mbps to 250 Mbps, in the 5 GHz, and potentially, in the 2.4 GHz bands. The new specification proposes the use of multiple (2-4) antennas, and more channels, to overcome multi-path effects and thus to increase the actual throughput.
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