this is quite basic smartcard terminology.
a smart card reader is the master, it drives the clock, reset lines and issues commands to
rhe card.
the card is a slave to the clock and reset lines. on a protocol level, it only responds to
commands by the reader.
see iso 7816-3 for details on this. asynchronous t=0 is the predominant protocol used in
sim cards.
a sniffer just passively listens to the communication between card and reader.
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Sent from a mobile device, excuse my short response