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30 Compare the characteristics of wireless communication standards Wireless Radio Channels

- Radio Frequency: RF or Radio Frequency is an important part of wireless communications. It is the RF band that is used for communication. As already mentioned the standards prescribe the RF ranges that can be used with each standard.

- Radio Channels: With respect to the channels, a wide frequency band is available under 802.11a allowing more channels and more data throughput. The standard 802.11a can support up to 8 non-overlapping channels, whereas 802.11b and 802.11g can only support up to three channels as the bandwidth used is much smaller. Non-overlapping channels are the prescribed ones for communication. Overlapping channels cannot be ignored while troubleshooting a wireless network.

- Spread Spectrum Technology: The manner in which data signals travel through a radio frequency is termed as spread spectrum. Data is transmitted using the narrowband transmission. It is a requisite of spread spectrum that data signals change their data pattern or alternate between frequencies. Spread spectrum compromises on bandwidth efficiency for attaining more reliable and secure transmission. There are three types of spread spectrum technologies:

- Frequency-Hopping Spread Spectrum (FHSS) Technology: In FHSS, narrowband signals are used. These signals change frequencies in a predictable pattern. Hopping of data signals between narrow channels is known as Frequency hopping. FHSS is a strong technology when it comes to dealing with interferences. It is best suited for networks covering large areas. Directional antennas cannot be used for reducing the effect of environment. It is not a technology in vogue.

- Direct-Sequence Spread Spectrum (DSSS) Technology: Using this technology, the signal is spread over full transmission frequency spectrum. With every bit of data, a 32 pattern bit called the chip is also sent. This is done for the purposes of security as well as to ensure delivery. It is considered a safe technology as multiple redundant copies are sent. It is immune to interference and noise, but deeply effected by environmental factors.

- Orthogonal Frequency Division Multiplexing: It is a transmission technique, used for transferring large amounts of data. The data is transmitted over frequencies, which are separate and evenly placed. The radio signal is split into separate frequencies by this technology and a simultaneous transmission is made to the receiver. Cross Talk is reduced by the use of this technology as the signal is split and transferred over multiple frequencies.

The relation between 802.11 Standards and FHSS, DSSS, OFDM is illustrated in the table given below:

Table 8: Relation between 802.11 Standards and FHSS, DSSS, OFDM