The start of bit p0, which is the first preamble bit, is defined to occur at the point in time 68 bit periods ,before the instant at which the modulated carrier passes through the nominal channel frequency immediately prior to the deviation corresponding to the first bit of the Access Code trailer for the IUT ,as defined in the Baseband Specification 4.2.The position of the start of bit p0 is calculated using an averaging based on the position of all the zero crossings in the packet: