QUESTION: Why can't my PBX just assume it gets good synchronization from the public network's DS1 or E1 signals?
ANSWER: Depending on the application of the signal that your PBX network receives (and how it is tariffed by the public network operator) from the network, it may act as a high-quality synchronization source, or it may be asynchronous, relying on the internal crystal oscillator of a remote PBX in your network to define the DS1 or E1 frequency.
This latter case holds for the "leased lines" that are typically used to connect the elements of a private network, with the public network simply providing the inter-site transport of the signals in a transparent (regenerated, not re-clocked by a network clock) manner. Thus a private network will be restricted to operate asynchronously if no synchronization strategy is defined for it.
On the other hand, when the public network is providing channelized or "value-added" services on individual DS0 channels of the DS1 or E1, (for example, when a PBX is connected to the public network's voice or data switches over several trunks) then the public network signals to the PBX are clocked by the switches, which are synchronous to the network's Primary Reference Source. In this case the PBX can confidently use the received signals from the public network as clock references.
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