Data Communications Networking Devices - 4th Ed.pdf

4.10 DIGITAL SERVICE UNITS ______________________________________________________ 511TimingPrecise synchronization is the key to the success of an all-digital network. Timingensures that data bits are generated at precise intervals, interleaved in time and readout at the receiving end at the same interval to prevent the loss or garbling of data.To accomplish the necessary clock synchronization on the AT&T digitalnetwork, a master clock is used to supply a hierarchy of timing in the network.Should a link to the master clock fail, the nodal timing supplies can operateindependently for up to two weeks without excessive slippage during outages. InFigure 4.86, the hierarchy of timing supplies as linked to AT&T's master referenceclock is illustrated. As shown, the subsystem is a treelike network containing noclosed loops.Figure 4.86DDStiming subsystemService unitsWhen DDS was introduced, both a channel service unit CSU) and a data serviceunit DSU) were required to terminate a DDS line.The DSU converts the signal from data terminal equipment into the bipolarformat used with DDS and T1 facilities.The DDS master reference clock is an atomic clock that is accurate to 0.01 partper million PPM). This clock was installed by AT&T at Hillsboro, Missouri,which is the geographic center of the United States and whose location ensures aminimal variance in propagation delay time between DDS nodes connected to themaster reference clock. The master reference clock oscillates at a rate known as thebasic system reference frequency and is the most accurate of three timing sourcesused by digital facilities. The other two sources of timing include channel banksand loop timing where clocking is obtained from a high speed circuit.The DSU interface to the DTE is accomplished by the use of a standard 25 pinEIA RS-232/V.24 female connector on the 2.4 kbps to 19.2 kbps units. The