tor recreates the same hopping pattern. This allows the receiver to make the same channel-to-channel hops as the transmitter, SO that the data signal can be decoded. Because the probability of two networks selecting the same channel at the same time is very low, many more FHSS networks can overlap physically without interference than is the case for DSSS networks. Frequency hopping spread spectrum in the 2.4 GHz ISM band is specified alongside DSSS as an option in the IEEE 802.11b standard and is also used in Bluetooth networks. All 79 available channels are normally used in Bluetooth, although an alternative hopping sequence that uses 23 channels (2.454 to 2.476 GHz), is available for use in France where special regulatory conditions apply. Frequency hops occur after each data packet, which will be a multiple of 1-, 3- or 5-times the time slot duration of 625 microseconds (320 to 1600 hops/second). The frequency hopping pattern is determined by the unique 48-bit device ID of the master device in each Bluetooth piconet, and synchronisation to the hopping pattern is part of the process of device discovery when a new device joins the piconet. The use of FHSS in Bluetooth is described further in Chapter 10. Time Hopping Spread Spectrum In a time hopping spread spectrum system, time is divided into frames, with each frame divided into a number of transmission slots. Within each Chapter Four User 1 bit stream = 1101... User 2 bit stream = 0011 Bit time Tb User 1 pulses TH code = 2767 User 2 pulses TH code = 4483 Frame time Tf Transmission slot number Figure 4-12: Time Hopping Spread Spectrum frame, data is transmitted only during one time slot and the specific time slot used during each frame is determined using a PN code. Figure 4-12 shows a THSS system with two clients using different hopping codes. Impulse radio is an ultra-wide band transmission technique that is a candidate for the IEEE 802.15.4a (ZigBee) physical layer specification. This is a time hopping spread spectrum technique where a very short pulse is tr