Network synchronization is a comprehensive expression that addresses in a wide sense any distribution of time and frequency over a network of clocks [4.1]–[4.3]. To give some examples, in telecommunications its goal may be either:
(1) to align the absolute time scales of network nodes, thus aiming for instance at aligning local clocks to the Universal Time Coordinated (UTC) (examples: synchronization of equipment real-time clocks, Internet NTP);
(2) to align the timing signals (or, more precisely, their significant instants) generated by local clocks, independently from a constant phase offset among them, thus aiming at minimizing phase fluctuations around such average phase offset (examples: the synchronization of SDH NEs in order to avoid pointer action, the synchronization of synchronous digital multiplexers or digital switching exchanges in order to avoid slips at input elastic stores);
(3) to equalize the frequencies of local clocks, without controlling their phase relationship (example: the distribution of a frequency-standard signal to FLL-based slave clocks).
In a network synchronized as in case (1), local timing signals are synchronous (cf. the definition provided in Section 2.1.2) and their total phases are aligned. Therefore, this network synchronization requires estimation and compensation of transmission delays on synchronization signals directed to each node.
In a network synchronized as in case (2), local timing signals ...