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London Transport (uk.transport.london) Discussion of all forms of transport in London. |
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Phone roaming in the US and Canada was card numbers, was cards, was E-ZPass, was CharlieCards v.v. Oyster (and Octopus?)
"Roland Perry" wrote in message
... In message , at 15:01:49 on Fri, 16 Mar 2012, MatSav remarked: That has been alleged to have happened on English south coast shores/beaches which are screened from the local transmitter by high cliffs but within range of French base stations. Sometimes, exceptional atmospheric conditions can cause this from elevated inland points as well. I was at the Great Dorset Steam Fair http://www.gdsf.co.uk at Tarrant Hinton, and made some calls to friends who were elsewhere on the very large fair site. When I got the bill, I discovered I had apparently made calls in France *and* the UK within a few seconds of each other. The mobile phone systems utilise a transmission protocol known as TDMA (Time Division Multiple Access). In theory, this should block connections where the propogation delay is more than 28 milliseconds (equating to a distance of about 90km). In my case, the nearest point of the French coast was at least 130km away - so it shouldn't have been possible, but it definitely happened. Coastal base stations (such as the one which you contacted in northern France) can be adjusted to allow twice the normal propagation delay, so that large areas of sea are covered. Thanks Roland - I didn't know that. It makes sense, I suppose, given that there are numerous reports of ill-prepared "sailors" calling for assistance at sea by using their mobile 'phones, rather than using a marine-frequency VHF radio. -- MatSav |
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