 used. A summary is given Short-range wireless data communications 287 Figure 11.2 An example of an ad-hoc network in Table 11.1. FCC channel frequencies are used within USA and the same frequencies are available for use in Canada. ETSI frequencies are adopted within Europe, although frequencies in France are limited to channels 10-13. To prevent interference between neighbouring access points there should be a 30 MHz separation between them although there is a move to use a 25 MHz separation. The 802.11 system is symmetrical with all stations having the same power output capability. This must be the case because the system must be able to operate in an ad hoc mode. Like the different frequency Table 11.1 Channels frequencies and identification Channel ID FCC Channel Frequencies ETSI Channel Frequencies 2412 MHz 2417 MHz 2422 MHz 2422 MHz 2427 MHz 2427 MHz 2432 MHz 2432 MHz 2437 MHz 2437 MHz 2442 MHz 2442 MHz 2447 MHz 2447 MHz 2452 MHz 2452 MHz 2457 MHz 2457 MHz 2462 MHz 2462 MHz 2467 MHz 2472 MHz 288 Newnes Guide to Radio and Communications Technology allocations, different countries have different power limitations. For spread spectrum modes the maximum transmission power is generally specified to be 100 mW (EIRP). However, in the USA this can rise to 1000 mW and in Japan the power is specified slightly differently as 10 mW/MHz. In any country the minimum power level does not fall below 1 mW. Below this level reliable communications would not usually be possible. Within these limits different power levels are set by the system SO that the minimum power level is chosen consistent with reliable communications. In this way interference to nearby cells is minimized, and it also conserves battery power for items like laptop computers. Also the system will reduce the data rate if errors start to be detected. Even SO it is generally accepted that the maximum reasonable distance over which 802.11b will operate is 100 metres for a clear line of sight path. In terms of the actual operation of the system, when a station w