se the IR and FHSS links, and a more detailed review of the same pseudo-random sequence of frequencies, and DSSS. their synthesizers must be in step, that is, they must all be tuned to the same frequency channel at the same time. Synchronization is achieved in 802.11 by sending Infrared PHY the essential parameters-dwell time, frequency se- quence number, and present channel number-in a fre- Infrared communication links have some advantages over quency parameter set field that is part of a beacon radio wave transmissions. They are completely confined transmission (and other management frames) sent peri- within walled enclosures and therefore eavesdropping odically on the channel. A station wishing to join the concerns are greatly relieved, as are problems from ex- network can listen to the beacon and synchronize its hop ternal interference. Also, they are not subject to in- pattern as part of the network association procedure. tentional radiation regulations. The IEEE 802.11 IR The FHSS physical layer uses GFSK (Gaussian fre- physical layer is based on diffused infrared links, and the quency shift keying) modulation, and must restrict receiving sensor detects radiation reflected off ceilings transmitted bandwidth to 1 MHz at 20 dB down (from and walls, making the system independent of line-of-site. peak carrier). This bandwidth holds for both 1 Mbps and The range limit is on the order of 10 meters. Baseband 2 Mbps data rates. For 1 Mbps data rate, nominal fre- pulse position modulation is used, with a nominal pulse quency deviation is +160 kHz. The data entering the width of 250 nsec. The IR wavelength is between modulator is filtered by a Gaussian (constant phase 850 and 950 nM. The 1 Mbps bit rate is achieved by delay) filter with 3 dB bandwidth of 500 kHz. Receiver sending symbols representing 4 bits, each consisting of sensitivity must be better than -80 dBm for a 3% frame a pulse in one of 16 consecutive 250 nsec slots. This error rate. modulation method is called 16-PPM. Optional 4-PPM In order to keep the 