PDH

PDH, or Plesiochronous Digital Hierarchy, is a family of early digital transmission systems developed to carry multiple telephone channels over long-distance networks. The term “plesiochronous” reflects the use of clocks in different network elements that are almost, but not perfectly, synchronized. To accommodate small differences in timing, PDH networks employ bit stuffing and framing techniques to align data streams during multiplexing.

In PDH, data are carried as time-division multiplexed channels built from 64-kbit/s basic units (DS0). The European (E-carrier) and North American (T-carrier) families define different hierarchies. The common European levels are E1 at 2.048 Mbps, containing 32 64-kbit/s channels with a framing channel; higher levels are E2 at 8.448 Mbps, E3 at 34.368 Mbps, and E4 at 139.264 Mbps, each formed by multiplexing four of the previous level. North American levels are T1 at 1.544 Mbps (24 DS0 channels plus a framing bit) and T3 at 44.736 Mbps (composed from multiple T1 streams). The PDH hierarchy thus provides a scalable, serial transport system for telephony traffic, but at each level the signal is limited to the capacity of the multiplexed lower-rate streams.

A key limitation of PDH is the difficulty of performing flexible add/drop and the relatively cumbersome management of synchronization across networks, which complicates reconfiguration and monitoring. These constraints contributed to the replacement of PDH by synchronous technologies such as Synchronous Digital Hierarchy (SDH) and SONET, which offer easier multiplexing, monitoring, and cross-network interoperability. Today, PDH remains in use primarily in legacy networks or where older infrastructure is still active, but it is largely superseded by SDH/SONET in modern backbone networks.