This is an interesting subject and one the automotive companies would like to see cracked so they can sell us MP3/Wi-Fi downloading solutions etc.
802.11 as it stands today is far from the ideal mobile platform, we all know that. I was involved in some trials of Mobitex (developed by Ericsson) over the RAM network. We achieved 70MPH speeds with the RAM radio towers able to hand over the session with ease and with no session drops. I heard there was an 802.11b-based solution devised whereby you'd install a custom client which would obtain a pair of IPv4 addresses, one would stay with you till you timed out or left the network, the second one would be released and a fresh one subsequently be renewed along the way as per normal. By this method, the data stream apparently stayed up. Anyone remember who did this? Cheers Nigel Nigel Ballard [EMAIL PROTECTED] www.joejava.com > We are engaged in a project that needs to provide internet access in a > highway 150 kms with seamless mobility. You should understand that this is going to be a challenging project. At vehicular velocities, the amount of time spent in the coverage area, let alone the coverage overlap area, isn't very large. For example, 60 mph = 88 feet/second. If the coverage overlap area is 10 feet, this means that the vehicle is only present in that area for 114 ms. In that time it is necessary to complete the 802.11 scan, authentication and association as well IP layer network attachment detection, in order for connectivity to not be interrupted. Bill Arbaugh's team at the University of Maryland has been doing measurements and as I understand it, scan times alone can run 40-400 ms. With WPA, the 4-way handshake is always run, and depending on the client and AP, this can take as little as 10 ms or more than 40 ms. In a highway situation, key caching won't help much because vehicles will typically not retrace their steps within a time period in which the cached keys would remain valid. Therefore, it probably makes sense to do either pre-authentication or pre-emptive key generation. With pre-authentication, it is probably necessary for the AP to advertise the neighbor graph in the Beacon in order to allow pre-authentication to begin prior to entering the coverage overlap area. Otherwise pre-authentication would be unlikely to complete in time; measurements show authentication times from 250 ms (fast resume) to 700+ ms (full EAP TLS authentication). For references to papers on pre-authentication, pre-emptive key distribution and other aspects of 802.11 handoff, see: http://www.drizzle.com/~aboba/IEEE/ -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
