A method for defining a context identifier when compressing header fields of data packets, in which method, a context is defined for a compressor and decompressor of a data packet flow, the context controlling the operation of said compressor and decompressor. In accordance with some embodiments, a group of RSM cells in a non-volatile semiconductor memory array is identified for application of a bit set operation. The impedance between the probe and each of the body surface electrodes is measured, and three-dimensional position coordinates of the probe are determined based on the impedance measurements.