Analysis of a Dual Mode Digital Synchronization System Employing Digital Rate-Locked Loops

When data stations arc connected by wholly digital transmission facilities, it is most efficient to slave the clocks at the data stations to a master clock. To perform this operation, hereafter referred to as master-to-slave operation, an interface unit at the data station extracts timing pulses from the incoming data stream. These timing pulses are passed through a phase-locked loop to eliminate noise and jitter. The output of the phase-locked loop controls the signaling rate of the data station. 1881 1882 T H E BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1972 Unfortunately, a technical problem arises when data stations are synchronized in the above manner. Before digital systems evolve into a well-connected network, data stations are also often connected by wholly analog transmission facilities. When two data stations equipped to operate in the master-to-slave mode are connected by analog facilities, each station will regard the clock at the other station as the master clock, and the two stations will attempt to mutually synchronize each other. This mode of synchronization can be called "slave-to-slave." Conventional phase-locked loops1 which perform well in the masterto-slave mode may not perform well in the slave-to-slave situation, being unusually sensitive to path-length delays and other system parameters. This technical problem can be solved by avoiding the slave-to-slave situation in the following manner: (i) Informing the data stations when analog transmission facilities are used.