> If the access points are spaced closely enough, and if one can hand you > off to another quickly enough, and if they are distant enough from the > roadway, then you can probably drive at highway speeds without being > disconnected. But those are an awful lot of ifs.
Here's one way to think about this. Assume a coverage overlap of C, and a car velocity of V. If pre-authentication + reassociation can take place in time T = C/V then you can have seamless handoff without disconnection. In other words Vmax = C/T, where Vmax is the maximum velocity at which seamless handoff can be achieved. Let's assume for a moment, that we have open authentication, so that security doesn't enter into the calculations. From Bill Arbaugh's recent measurements, the reassociation time alone may range between 40 and 400 ms. With a coverage overlap C = 5 feet, and T = 40 ms, then Vmax = 125 ft/second = 85 mph. However, with T = 400 ms, then Vmax = 12.5 ft/second = 8.5 mph. So a lot depends on the reassociation times. Note that where Vmax > D/T where D = total coverage Diameter, then you will be out of the coverage area entirely by the time reassociation is completed. So at velocities greater than Vmax 802.11 can no longer be used. Typically this velocity is characteristic of plane flight (0.1 - 0.3 Mach) rather than vehicular travel, though. -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
