Before the break-up of the Bell System, an accurate synchronization signal was distributed throughout the network from a single location. However with divestiture, increased deployment of digital switches and the introduction of digital data services, service providers started to plan, deploy and maintain their own synchronization networks. Initially, there were separate islands of synchronized equipment even within a central office. Realizing the disadvantages of maintaining separate synchronization networks for switched and digital services, the RBOCs embraced the Building Integrated Timing Supply (BITS) concept. This led to the deployment of robust, reliable and efficiently manageable synchronization networks. In the late eighties, the develoment of SONET introduced additional challenges that were overcome with the use of more Primary Reference Sources and DS1s derived from optical signals for synchronization distribution purposes.
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