FHSS systems, the 802.11 system will come off worse because its hopping rate is typically 160 times slower than the Bluetooth radio. Chapter Nine Channel 1 Channel 6 Channel 11 2.412 GHz 2.437 GHz 2.462 GHz 22 MHz 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 24 interfering FHSS channels per DSSS channel 79 X 1 MHz Bluetooth channels Figure 9-2: Bluetooth and 802.11 DSSS Spectrum Overlap This means that, in hopping over the 79 channels, the Bluetooth radio is likely to land on the same frequency as the 802.11 radio several times for each transmitted 802.11 packet. The 802.11 MAC will be issuing continued requests to repeat lost packets and network throughput will be degraded. Fortunately few 802.11 systems use the optional FHSS PHY layer specification. The situation is a little more complex for a DSSS 802.11 system (Figure 9-2), since the direct sequence detection is inherently more robust against narrow band interference, and as the probability of a collision between a FHSS packet and a DSSS packet depends on the WLAN data packet length. In this situation the Bluetooth link is likely to be more susceptible to interference since the DSSS interference will affect 24 of the 79 hopping channels, SO that some 30% of the WPAN packets could be lost. This will seriously degrade throughput, particularly for synchronous links such as voice transmission to a Bluetooth headset. The IEEE 802.15 Task Group TG2 has developed recommended practices to reduce the interference between 802.11 and 802.15.1 radios, using two types of coexistence mechanism - collaborative and non-collaborative. Collaborative mechanisms are possible when information to minimise interference can be exchanged between the WLAN and WPAN, while non-collaborative mechanism do not require exchange of information between the two networks, but are inherently less effective. The types of Wireless LAN Troubleshooting 802.11 beacon period Bluetooth traffic WLAN traffic Period timing WLAN interval WPAN interval TWLAN TWPAN Figure 9-3: WLAN and WPAN Transmit Perio