PDH ( Plesiochronous Digital Hierarchy)

The PDH signals

The term PDH stands for Plesiochronous Digital Hierarchy. The word plesiochronous means 'nearly synchronous'. As we have all seen about the E and DS hierarchy, we will slowly move into PDH and SDH/SONET standards and towards the implementation of it. 

What does a PDH signal carry?

PDH frame will contain nothing other than the E and DS signals we have seen in the previous sections. To be clear, PDH carries either of the above two signals (E and DS). We all know that E and DS signals are of different hierarchies. So which specific hierarchy defines PDH signalling? The answer is all possible hierarchies which are defined comes under PDH signalling.

What is the maximum rate of PDH standard?

We have discussed that standard defines 5 levels of E hierarchy and 4 levels of DS hierarchy. From these we can guess that among the 9 possible levels of PDH, the one with the maximum data rate is the maximum possible rate for PDH standard. So, the answer is 565.148 Mbps (~566Mbps) is the maximum possible data rate of PDH standard.

The disadvantages of PDH

- PDH signals have two different standard signals (E and DS) with each of the standard having its own hierarchy of different levels. Higher the hierarchy, the higher is the multiplexing of lower level signals. This in turn gives the need to demux the signals at different nodes based on to which node each of the multiplexed signals are targeted to. The implementation of muxing and demuxing techniques at every node is harder since two different standards are involved with various levels of hierarchy and each of them have a completely different frame structure.

The rise of SDH/SONET

The above disadvantages resulted in the need of a new technology with higher data rates and easier muxing and demuxing. The one which evolved  as a result is SDH/SONET. SDH stands for Synchronous Digital Hierarchy and SONET stands for Synchronous Optical Networking. The advantages of these standards and their frame structures with their differences will be explained in forthcoming days