 802.11 DSSS Channels Chapter Four Channel 5 Channel 10 2.432 GHz 2.457 GHz Channel 4 Channel 9 2.427 GHz 2.452 GHz Channel 3 Channel 8 2.422 GHz 2.447 GHz Channel 2 Channel 7 2.417 GHz 2.442 GHz Channel 1 Channel 6 Channel 11 2.412 GHz 2.437 GHz 2.462 GHz 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 GHz 11 channels in 5 MHz steps Figure 4-10: DSSS Channels in the 2.4 GHz ISM Band (US) wide channels into an 83 MHz wide band results in considerable overlap between the channels (as shown in Figure 4-10), resulting in the potential for interference between signals in adjacent channels. The 3 non-overlapping channels allow 3 DSSS networks to operate in the same physical area without interference. The use of DHSS in 802.11 wireless networks is described further in Chapter 6. Frequency Hopping Spread Spectrum in the 2.4 GHz ISM Band In frequency hopping spread spectrum transmission (FHSS) the data is modulated directly onto a single carrier frequency, but that carrier frequency hops across a number of channels within the RF band using a pseudo-random hopping pattern. In the 2.4 GHz ISM band for example, a maximum channel width of 1 MHz is specified for FHSS systems, and 79 such channels are available. A transmitter switches between these channels many times a second, moving on to the next channel in its sequence after a predetermined time, known as the "dwell time". The IEEE 802.11b standard specifies that the hop must be to a new channel a minimum of 6 MHz from the previous channel, and that hops must occur at least 2.5 time per second (Figure 4-11). The spectrum regulators specify the allowable limits for transmission parameters such as Radio Communication Basics Over 2.5 hops per second 6 MHz minimum hop 1 MHz channel width 78 hopping patterns 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 79 X 1 MHz Bluetooth channels Figure 4-11: FHSS Channels Within the 2.4Ghz ISM Band the maximum dwell time and individual standards, like IEEE 802.11, have to work within these boundary conditions. In the receiver, a PN genera