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Wireless Radio Channels

Radio frequency (RF) channels are an important part of wireless communication. A channel is the band of RF used for the wireless communication. Each IEEE wireless standard specifies the channels that can be used. The 802.11a standard specifies radio frequency ranges between 5.15 and 5.875GHz. In contrast, 802.11b and 802.11g standards operate in the 2.4 to 2.497GHz range. IEEE wireless standards are discussed later in this chapter.

As far as channels are concerned, 802.11a has a wider frequency band, allowing more channels and therefore more data throughput. As a result of the wider band, 802.11a supports up to eight nonoverlapping channels. 802.11b/g standards use the smaller band and support only up to three nonoverlapping channels.

It is recommended that nonoverlapping channels be used for communication. In the U.S., 802.11b/g use 11 channels for data communication, as mentioned; three of these—channels 1, 6, and 11—are nonoverlapping. Most manufacturers set their default channel to one of the nonoverlapping channels to avoid transmission conflicts. With wireless devices you can select which channel your WLAN operates on to avoid interference from other wireless devices that operate in the 2.4GHz frequency range.

When troubleshooting a wireless network, be aware that overlapping channels can disrupt the wireless communications. For example, in many environments, APs are inadvertently placed close together—perhaps two access points in separate offices located next door to each other or between floors. Signal disruption results if channel overlap exists between the access points. The solution is to try to move the access point to avoid the overlap problem, or to change channels to one of the other nonoverlapping channels. For example, you could switch from channel 6 to channel 11.

Typically you would change the channel of a wireless device only if it overlapped with another device. If a channel must be changed, it must be changed to another, nonoverlapping channel. Table 7.2 shows the channel ranges for 802.11b/g wireless standards. Table 7.3 shows the channel ranges for 802.11a. 802.11n has the option of using both channels used by 802.11a and b/g.

Table 7.2. RF Channels for 802.11b/g

Channel

Frequency Band

1

2412MHz

2

2417MHz

3

2422MHz

4

2427MHz

5

2432MHz

6

2437MHz

7

2442MHz

8

2447MHz

9

2452MHz

10

2457MHz

11

2462MHz

Table 7.3. RF Channels for 802.11a

Channel

Frequency

36

5180MHz

40

5200MHz

44

5220MHz

48

5240MHz

52

5260MHz

56

5280MHz

60

5300MHz

64

5320MHz

Tables 7.2 and 7.3 outline the available wireless channels. When deploying a wireless network, it is recommended that you use channel 1, grow to use channel 6, and add channel 11 when necessary, because these three channels do not overlap.

Data Rate Versus Throughput

When talking about wireless transmissions, it is important to distinguish between throughput and data rate. From time to time these terms are used interchangeably, but technically speaking, they are different. As shown later in this chapter, each wireless standard has an associated speed. For instance, 802.11g lists a speed of up to 54Mbps. This represents the speed at which devices using this standard can send and receive data. However, in network data transmissions, many factors prevent the actual speeds from reaching this end-to-end theoretical maximum. For instance, data packets include overhead such as routing information, checksums, and error recovery data. Although this might all be necessary, it can impact overall speed.

The number of clients on the network can also impact the data rate; the more clients, the more collisions. Depending on the network layout, collisions can have a significant impact on end-to-end transmission speeds. Wireless network signals degrade as they pass through obstructions such as walls or doors; the signal speed deteriorates with each obstruction.

All these factors leave us with the actual throughput of wireless data transmissions. Throughput represents the actual speed to expect from wireless transmissions. In practical application, wireless transmissions are approximately one-half or less of the data rate. This means that you could hope for about 20 to 25Mbps for 802.11g. Depending on the wireless setup, the transmission rate could be much less.

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