Referring to the digital network hierarchy of T-carrier and E-carrier systems. Originally, the PDH involved highly only link layer synchronization, where the device on one end sent at its own not very precise rate, and the device at the other end sent return data at the same rate. Multiplexing, as when combining 28 T1 circuits into a T3, provided stuff bits to compensate for the variations in clock rates among T1 lines. When digital cross connect systems (DACS) were introduced, they required all attached devices to clock off them. Quickly there was a need for a precise master clock, off which all digital network elements slaved in a hierarchical fashion to sync up, i.e., all operate at the same precise bit rate. In contemporary T/E/J-carrier networks, the switches and muxes sync up with a Global Positioning System (GPS) master clock, which is distributed by radio rather than land line. See also E-carrier, GPS, J-carrier, stuff bits, synchronization, and T-carrier.