The Single Wire Protocol (SWP) is a specification for a single-wire connection between the SIM card and a near field communication (NFC) chip in a cell phone. It is currently under final review by the European Telecommunications Standards Institute (ETSI).
SWP is an interface between Contactless frontend (CLF) and UICC (SIM card chip). It is a contact based protocol which is used for contactless communication. C6 pin of UICC is connected to CLF for SWP support. It is a bit oriented full duplex protocol i.e. at the same time transmission as well as reception is possible. CLF acts as a master and UICC as a slave. CLF provides the UICC with energy, a transmission clock, data and signal for bus management. The data to be transmitted are represented by the binary states of voltage and current on the single wire.