Significance

• PDH functions like two synchronized clocks in different time zones. Although both watches are in sync with each other, there is an allowance in the time zones for a period of difference between the watches. This allowance also provides coverage for a slight difference in speed between the two watches. In PDH, this "almost synchronization" provides an increased bandwidth or a wider highway for information to travel on as well as an increase in efficiency of data transfer.
  
  

History

• PDH was developed as the first SMH or "standardized multiplexing hierarchy," which was based upon TDM "time-division multiplexing." The increase in transfer rate of PDH allowed information to be transmitted over new media solutions such as microwave and fiber optics. This system became highly popular in the early 80's as system networking became more diversified and the need for higher bit streams and synchronized information transfer became more in demand.
  
  

Features

• With PDH, the usage of time generates faster signals. The process of primary multiplexing is conducted in stages. First PDH combines E1 or T1 links into E2 or T2 links (multiple transfer cables) or a combination of E2 and T2 links together. Then the combination of transfer links irrigates data encryption into a synchronized transfer (plesiochronization) into the multiplexed system and transfers the data. This increases the transfer rate of the data and decreases delay times in data transfer across lines.
  
  

Function

• PDH units in general provide a host of functions. One function of PDH is providing a PBX (private branch exchange) using a CAS system "channel associate signaling." A T1 channel signaling can also be utilized in transfer of multiplexed data. A second function of PDH is providing a multiplexer interface service (T2,E2). Today, a prominent function of PDH is providing a digital cross connection between older units and T3 channels.
  
  

Benefits

• Some modern systems still function with PDH such as the ATM. The conversion between digital network system data to an ATM still requires a multiplexing form of communication to transfer data along its secured network platform. With the utilization of PDH, ATMs are capable of providing real-time data transfers between stand-alone units and a bank's secured network system.
  
  

Considerations

• PDH data transmission format is different from the modern SDM "synchronous data-transfer mode" data transmission format. The complex hierarchy of SDM is not easily converted along a PDH data transfer system. The transfer of existing conversion solutions utilize high cost and often complex hardware schemes in the conversion and transfer of data between the separate channels. This situation provides an increase in complexity of the technology and makes the utilization of PDH more obsolete as complexity increases.