A review of Plesiochronous Digital Hierarchy (PDH) and Synchronous   Digital Hierarchy (SDH)
  With the advancement in telecommunications, packet traffic is rapidly becoming the mainstream of data traffic. The use and deployment of Synchronous Digital Hierarchy (SDH) networks for interconnection has gained traction worldwide due to its flexibility and standard for interconnecting multiple vendors, low operating cost and the high quality of service it provides. Plesiochronous Digital Hierarchy (PDH) on the other hand has been used before the introduction of the SDH standard and it also provides a means to transport large quantity of data via digital equipment such as radio wave systems, optic fibre and microwaves. In this paper, we shall discuss both PDH and SDH technologies and identify some of the features of both and the issues in PDH that brought about the introduction of the SDH technology. 
Olabenjo Babatunde, Salim Mbarouk
cs.NI