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This control unit receives card data, controls the light-emitting diodes at the
card reader, sends and receives SMS for usage rights, unlocks the central locking
system to open the car doors, and so on.
We track identifi ed messages between the system and its actors all the way
into the system. This sounds trivial but requires many design decisions in practice.
We document our considerations in a sequence diagram ( Figure 2.75 ). This diagram
shows us the blocks that are connected (exchange of messages), so that we
can model the structures in an internal block diagram ( Figure 2.76 ).
The message apply card gets into the system over CustomerPort . At the customer
card, we model another (conceptual) CustomerPort as an interaction point.
Together with CardReaderPort, this new CustomerPort stands for the interaction
with customers. In the following, we model the structures to read customer data
from the card.
The card reader emits an energy fi eld. The customer card feeds itself with
electric power from this energy fi eld if it is near enough to the card reader.


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