6.2 Frequency Synchronization in TDM Networks
There are three TDM (time-division multiplexing) technologies currently in use, PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network), which is the North-American equivalent of SDH, and OTN (Optical Transport Network). The SONET requirements have been incorporated in the SDH specifications.
6.2.1 Synchronization Architecture in TDM Networks
Figure 6.1 depicts synchronization network architecture. SDH/SONET carries synchronization on the physical layer, i.e. the clock frequency of the physical signal is synchronized to a central reference. In the case of PDH, the different bit rate signals are not synchronized to each other but the client signal, for example E1, 2 Mbit/s, is carried across the network so that the frequency of the signal is preserved. OTN is similar to PDH, where the client TDM signal preserves frequency when carried over the network.
Usually networks are synchronized centrally by a primary reference clock. The mobile network nodes like base stations and controllers receive synchronization from the transport network. Ring structures can be used for protection purposes. For instance in Figure 6.1 the synchronization chain on the right side ring ends before the bottom node of the ring. However, if a link breaks ...