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Why would a QR code require Internet, or an app? You could just print it on paper and track the corresponding balance on the server side. To my understanding th

Why would a QR code require Internet, or an app? You could just print it on paper and track the corresponding balance on the server side. To my understanding this is what most transit networks do anyways, to prevent an enterprising user from modifying their balance on the card itself. (The optical scanning argument makes sense, however.)

lucianbr 2y ago
Internet is how you get from the client side to the server side. Maybe not internet but some-kind-of-net, and that has latency, failures etc. Maybe that's not clear: the turnstile needs to connect to a server to check the QR. Need to only have one server, so some turnstiles will be relatively far from it. Latency.
- woodruffw 2y ago
  > Maybe that's not clear: the turnstile needs to connect to a server to check the QR. Need to only have one server, so some turnstiles will be relatively far from it. Latency. Sure, but is this a serious design pressure? I've been on a lot of EU train and trolley networks that have a POS terminal on the train for direct sales, which are already doing networking both for the card transaction and to issue the ticket. (Again to be clear: I'm not saying a QR is better. But I don't think connectivity is a unique problem, since systems that use NFC without tying into payment cards are almost certainly using connectivity to make up for the lack of tamper resistance.)
grishka 2y ago
Even for single-use tickets, the turnstiles on the entire system need to somehow know that you've used your QR code the moment you did it. This requires them all to be connected to some sort of central server. There's a reason why single-use tickets either somehow store the validation mark on the ticket itself (NFC, magnetic stripe, paper that you have to stamp) or get taken away from you (tokens). > To my understanding this is what most transit networks do anyways, to prevent an enterprising user from modifying their balance on the card itself. On those on which this was attempted that I know of, this synchronization is far from instant. I was wrong in my other comment, in St Petersburg metro it only takes two hours for a dumped and restored card to be blocked, but you can apparently do this indefinitely on buses and trams because they aren't (weren't?) networked: https://web.archive.org/web/20170323213524/https://habrahabr.ru/post/324040/ https://web.archive.org/web/20170323213524/https://habrahabr...

lucianbr 2y ago

Internet is how you get from the client side to the server side. Maybe not internet but some-kind-of-net, and that has latency, failures etc. Maybe that's not clear: the turnstile needs to connect to a server to check the QR. Need to only have one server, so some turnstiles will be relatively far from it. Latency.

woodruffw 2y ago
> Maybe that's not clear: the turnstile needs to connect to a server to check the QR. Need to only have one server, so some turnstiles will be relatively far from it. Latency. Sure, but is this a serious design pressure? I've been on a lot of EU train and trolley networks that have a POS terminal on the train for direct sales, which are already doing networking both for the card transaction and to issue the ticket. (Again to be clear: I'm not saying a QR is better. But I don't think connectivity is a unique problem, since systems that use NFC without tying into payment cards are almost certainly using connectivity to make up for the lack of tamper resistance.)

grishka 2y ago

Even for single-use tickets, the turnstiles on the entire system need to somehow know that you've used your QR code the moment you did it. This requires them all to be connected to some sort of central server. There's a reason why single-use tickets either somehow store the validation mark on the ticket itself (NFC, magnetic stripe, paper that you have to stamp) or get taken away from you (tokens). > To my understanding this is what most transit networks do anyways, to prevent an enterprising user from modifying their balance on the card itself. On those on which this was attempted that I know of, this synchronization is far from instant. I was wrong in my other comment, in St Petersburg metro it only takes two hours for a dumped and restored card to be blocked, but you can apparently do this indefinitely on buses and trams because they aren't (weren't?) networked: https://web.archive.org/web/20170323213524/https://habrahabr.ru/post/324040/ https://web.archive.org/web/20170323213524/https://habrahabr...