There are two kinds of digital transmission series in a digital transmission system, one kind is called "quasi synchronous digital series" referred to as PDH, another kind is called" synchronous digital series " referred to as SDH. 

PDH optical (quasi synchronous digital series) multiplexer is small capacity optical multiplexer. Usually applications in pairs, that is say the point-to-point applications. Capacity is commonly 4×E1,8×E1,16×E1. The capacity of SDH (synchronous digital series) optical multiplexer is larger, general is from 16 E1 to 4032 E1. SDH can replace PDH. But PDH can't replace SDH.

PDH optical multiplexer:

SDH optical multiplexer: