The single wire protocol consists of a single-wire transmission line for connecting an NFC interface
(CLF, contactless front end) as master and a secure element as slave. ETSI SCP in TS 102 613 standardised and adopted this NFC interface (Mohrs, 2008). The single wire protocol is mainly intended as secure element for (U)SIM cards in mobile phones as there is only one contact of the standard eight SIM contacts available for this function. The remaining seven contacts are already allocated to other functions.
The data to be transmitted are represented by the binary states of voltage (S1) and current (S2) on the single wire. The data transmission from NFC interface to secure element is carried out by modulating signal S1 through modulation of voltage U CL between the states Logic-1 and Logic-0. In the reversed direction, the data are transmitted by modulating signal S2 through modulating current I CL between states Logic-1 and Logic-0. The process of modulating current I CL can also be described as ‘wire-bound load modulation’. In that case, signal S1 has to be in state Logic-1 (Praca, 2006).
An HDLC protocol is used for controlling data transmission between the NFC interface and the secure element. The HDLC protocol (high-level data link control) is ISO standardised and is one of the oldest communication protocols. It implements efficient error detection and correction, sign synchronization and ‘flow control’.The protocol for the contactless data transmission between the NFC interface and another contactless device is completely processed by the NFC interface. Only application data are forwarded to the secure element via the single wire protocol. Both the protocol for contactless data transmission (ISO/IEC 18092 and ISO/IEC 14443) and a host control interface (HCI) for data transmission via SWP interface have to be implemented on the NFC interface. Thus the SWP interface is open to all future, new contactless transmission standards or to the expansion of existing standards and specifications of NFC interfaces.
A special case constitutes the wiring of (U)SIM cards as secure elements with an NFC interface in mobile phones. As shown in Figure 11.36, the voltage (VDD) of the (U)SIM card is not directly supplied by the phone, but via the NFC interface. This is necessary for contactless data transmission with secure elements, even if the battery is flat. If the NFC interface is close to an RFID reader, the reader field supplies NFC interface and secure element with power, similarly to the process used for passive transponders. This way NFC interface and secure element can, at least, be used in card emulation mode. Typical ticketing and payment applications using card emulation mode thus have a high operational security as the contactless functionality does not rely on the battery’s charging state any longer (Praca, 2006; Mohrs, 2008).