PDH was created in the early 1960s. It derives its names in the Greek term "plesio," meaning near, and "chronos," meaning time. The name refers back to the fact that networks using PDH run in a state of just about, although not quite, perfect synchronization. PDH was the very first standardized multiplexing hierarchy based on time-division multiplexing. It works by channeling numerous individual channels into higher-level channels.

Work Theory Of PDH Multiplexer
The PDH product is in line with the theory when you've two identical clocks, each the same brand, style and everything, there is no guarantee that they'll run at the exact same speed. Most likely one of these is going to be slightly out of synchronization using the other. The transmitting multiplexer combines the incoming data streams, compensates for just about any slower incoming information, reconstructs the initial data and sends it back out at the correct rates. This system allows for that slight variation in speed and corrects it during transfer to keep the machine constantly running without pausing and waiting for certain slower data to arrive before sending it on. PDH simply fills within the missing bits to match a smooth change in data.

PDH made little provision for management of the network, and the need to fully de-multiplex a high level carrier to extract a lower level signal resulted in enhancing the capacity of PDH networks beyond a certain point wasn't economically viable. The main economic factor was the cost of the equipment required at each cross-connect point within the network where either individual channels or low-level multiplexed data streams should be extracted or added. Additionally, it added additional latency and increased the possibility of errors occurring, thereby reducing network reliability.

Available Kinds of PDH multiplexer
Traditionally, each channel in PDH was a digitized voice, but video information and knowledge can also be sent of these channels. The fundamental channel is 64 Kbits per second, which is the bandwidth that is required to deliver a voice call that's been converted from analog to digital.

N*E1 PDH Fiber Optic Multiplexers make use of the PDH fiber transmission technologies. The 2M (E1) interfaces can interact with the exchanger, light loop tool and multi-diplexer directly to form the micromini or the special network. Complete alarm function for N*E1 PDH Fiber Optic Multiplexers, it's stable, easy to maintenance and install, small in size. It can support one digital service telephone.

PDH Multiplexer can multiplex 4/8/16E1, Ethernet Media Converter (2*10/100Mbps) and V.35 signals in one fiber channel to deliver. It's ideal for low capacity, point-to-point application of remote transmission. The PDH Multiplexer can be applied to create economical and flexible multi-service transmission networks, employed for relay between switch offices, data transmission of LAN, 2M access of lease service for key clients, voice cutover for residental areas/intelligent buildings, and connection of base stations and other various digital transmission networks. Fiber Optic Multiplexer is reliable, stable, simple to install and keep, which may be monitored from Fi-view-MST management software, that is popular in voice and knowledge application field.

Source from PDH Optical Multiplexer Wiki