Propagation delay measures the time required for a signal to propagate from one end of the circuit or pair to the other. Delay is the principal reason for distance limitations for structured cabling. Propagation delay skew is the propagation delay difference between the slowest and fastest cable pair. Skew is important because again, Gigabit Ethernet uses all four pairs in a cable. The packet of information may be sent over multiple pairs; thus, if one pair is significantly slower than the others, it may be impossible to recombine the original signal.