The Single Wire Protocol (SWP, ETSI TS 102 613) is intended as direct interface
between a mobile phone's SIM card (UICC) and the mobile phone's contactless
front-end (CLF). The SWP's final technical specification has just been released.
The first devices implementing this communication protocol, mainly in its draft
versions, are already in production. As a consequence there will be a demand for
a test suite implementing a reference design and test methods for both the SWP
master and the SWP slave.

With communication protocols it is usually important to debug communication
problems between multiple devices. One way to trace and decode the transferred
data packets are packet sniffers. These systems contain hardware components and
software implementations to wiretap and analyze the physical interface of the
connection, to capture the data and to decode the packets into human readable
information.

The SWP uses a single wire for full-duplex communication between one master and
one slave device. While master-to-slave data transfers take place in the voltage
domain, slave-to-master data transfers take place in the current domain. In a
first step, this paper discusses approaches to intercept the communication on
the SWP's data wire without influencing the actual communication.

The information tapped from the SWP's data wire is still difficult to be read by
hand. Thus, in a second step, a method for retrieving the state of the single
wire interface is developed. Moreover, this paper gives an overview on how to
decode the data link layer communication from the intercepted data streams.