Card Clash on the London Underground

07.03.2014 by Mike Small

Recently there have been posters in London Underground stations warning users of Oyster Cards – the Transport for London (TfL) NFC enabled electronic travel wallet – that there is a risk of “card clash”.  These posters warn that they need to keep other contactless NFC payment cards separate from their Oyster Card when they “touch in” on a bus to avoid the risk that the wrong card would be charged.  TfL will be rolling out the ability to use NFC enabled payment cards on the Tube (London Underground), Overground and DLR later in 2014, and this could lead to further problems.  The charges on the London Underground are based on the journey made and the system depends upon the same card “touched in” on a reader at the origin of the journey being “touched out” at the destination.  If a different card is used at each end of the journey both cards are charged the maximum fare.

NFC technology is an important enabling technology for the Internet of Things (IoT) and the vision for the IoT makes bold promises of benefits for individuals and businesses.  These benefits include making life easier for the individual while allowing businesses to be more efficient.  Being charged twice for the same journey doesn’t seem to square with these claims – so what is happening here?

Near Field Communications (NFC) is a set of standards for devices including smartphones and contactless cards that allow a radio frequency connection to be established quickly and simply by bringing two devices close together (within 10cm to 20cm).  NFC standards cover communications protocols and data exchange formats, and are based on existing radio-frequency identification (RFID) standards.

An important aspect of these protocols is singulation. When different NFC devices are in the RF field of a reader, it needs a way to discriminate between them in order to establish single interactions with one or each of them. This is achieved through the singulation protocol, which is usually run at the time the reader starts a new communication session.  During this initial phase each device identifies itself to the reader, communicating an identifier that will be then used by the reader to contact them individually.

At the NFC device protocol level the ability to distinguish between cards is taken care of, so it looks like the problem lies at the application or system level.  The whole system relies on the same card being used on entry and on exit. The technical protection provided by the NFC protocols cannot protect the system if the application does not take account of the possibility for more than one card being detected at either end. In view of the number of passengers entering and leaving the Tube at peak times it is understandable that throughput may need to take priority over flexibility, however getting to grips with details like this will be essential to realize the potential benefits of the Internet of Things.


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© 2014 Mike Small, KuppingerCole