Principles of Synchronous Digital Hierarchy

160 Principles of Synchronous Digital Hierarchy

Clock accuracies have been dened by the strata orders such as strata 1, 2,

3, 4, etc. in the North American standards for SONET applications, whereas

ITU-T has dened the clock accuracies in terms of primary reference clock

(PRC), synchronization supply unit (SSU), synchronization equipment clock

(SEC), etc. in their recommendations of G.811, G.812, and G.813 for SDH

applications. The details of clock accuracies are dealt with in Section 5.5.

This philosophy of plesiochronous network was considered the best at that

time because of the following reasons:

1. The cost of network-wide synchronization, by distributing the tim-

ings from a master clock, was considered too high because t