This identifies which logical port (channel) on this physical port is designated as the ACM/SRTS timing master for the line interface, on the 4-port CBR E1/DS1 card. Because SRTS/adaptive clock recovery on this port's other channels derives timing relative to this one master channel, misidentifying the wrong channel here would propagate timing errors to every other CBR channel sharing this physical interface. On a 4-port CBR E1 card, setting pportClockMasterChannel to 2 designates channel 2 as the reference all other channels on this same physical port sync their recovered clock against.