same transmitted bandwidth modulation, with four slots per two-bit symbol, gives with a data rate of 2 Mbps, four-level frequency shift- a bit rate of 2 Mbps. keying is employed. Data bits are grouped into symbols Although part of the original IEEE 802.11 specifica- of two bits, SO each symbol can have one of four levels. tion and having what seems to be useful characteristics Nominal deviations of the four levels are +72 kHz and for some applications, products based on the infrared +216 kHz. A 500 kHz Gaussian filter smoothes the physical layer for WLAN have generally not been com- four-level 1 Megasymbols per second at the input to the mercially available. However, point-to-point, very short- FSK modulator. Minimum required receiver sensitivity range infrared links using the IrDA (Infrared Data is -75 dBm. Association) standard are very widespread (reputed to be Although development of Wi-Fi for significantly in- in more than 300 million devices). These links work re- creased data rates has been along the lines of DSSS, liably line-of-site at one meter and are found, for exam- FHSS does have some advantageous features. Many ple, in desktop and notebook computers, handheld PC's, more independent networks can be collocated with vir- printers, cameras and toys. Data rates range from 2400 tually no mutual interference using FHSS than with Bps to 16 Mbps. Bluetooth devices will take over some of DSSS. As we will see later, only three independent the applications but for many cases IrDA imbedding will DSSS networks can be collocated. However, 26 different still have an advantage because of its much higher data hopping sequences (North America and Europe) in any rate capability. of three defined sets can be used in the same area with low probability of collision. Also, the degree of throughput reduction by other 2.4 GHz band users, as FHSS PHY well as interference caused to the other users is lower While overshadowed by the DSSS PHY, acquaintance with FHSS. FHSS implementation may at one time also with the FHSS option