From: johnl@iecc.com   
      
   >Nevertheless, the excuse in Chicago for buying Ventra (it's actually   
   >leased) was that the contactless system we already had for 20 years   
   >had to be replaced as it was proprietary and they weren't going to   
   >make cards for it any longer.   
      
   At least NYC is trying to avoid that problem.   
      
   >>As I understand it, that doesn't work for performance reasons.  A   
   >>turnstile can consult a server in the same station, but not the master   
   >>one somewhere else.  TfL says an Oyster transaction has to take no   
   >>more than 350ms, and they grudingly accepted 500ms for bank cards with   
   >>a promise from the banks that the next generation will be faster.   
   >   
   >We're talking about transit fares. All that has to be done in real   
   >time is validate the fare media. The rest of it doesn't have to happen   
   >immediately.   
      
   Transit fares can be pretty complicated.   
      
   Caltrain products for a Clipper include a monthly pass, an 8-ride   
   ticket good for 30 days, and the prepaid credit shared among all the   
   agencies.  So when you tap in, it has to check whether you have a   
   monthly pass OR you have an unexpired 8-ride with at least one ride   
   left OR you have enough prepaid credit.  When you tap out, that's when   
   it knows how many zones you went, so it decides what kind of fare you   
   used and if needed takes a ride off the eight ride or money out of the   
   prepaid credit.  It needs to do that pretty quickly since within a few   
   minutes riders are likely to transfer to a LRV or bus run by a   
   different agency, which will want to know how much prepaid credit is   
   left.  The products and balances are all stored on the card since many   
   of the tap terminals operate offline.   
      
   In London, Oyster fares are zone based, but if you're going from one   
   part of outer London to another, there's a standard fare if you go   
   through the center of the city (via zone 1) and a lower fare if you   
   use train lines that don't go through zone 1.  So in addition to the   
   normal blue terminals to touch in and out, there are pink terminals at   
   some transfer stations where you tap your card to show that you're   
   taking a not-zone-1 route.  Must be fun to do all that in 350 ms.   
      
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