Introduction to Plesiochronous Digital Hierarchy (PDH)

PDH Microwave Hardware IDU and ODU

The signals are actually used in the practical communication systems to enable the media to carry a large number of channels.

The 64-kbps digital signal is never used as it is for the communication link,for the simple reason that it is too costly to use two pairs of copper wires over long distances for only one channel. Thus, a large number of 64-kbps signals are multiplexed into one physical channel, and sent across the distances, for much better utilization of physical resources such as copper cables or any other media.

The time division multiplexing began with the multiplexing of 24 channels. Besides time division multiplexing, a “pulse code modulation” (PCM) technique was used to convert the analog signals to digital signals. Sincethis was the first modern digital communication system and used PCM techniques, the system was popularly called a PCM. Thus, the PCM was the first digital communication system with 24 voice channels multiplexed together, and it employed PCM/TDM technology.

This PCM subsequently formed the first level of hierarchical system of multiplexing. The system was called “plesiochronous digital hierarchy” (PDH). Plesio is a Greek word that means “nearly the same” and knonous is another Greek word meaning “time.” Thus, the meaning of plesiochronous is “nearly the same time.” The nomenclature plesiochronous is related to the means of achieving the synchronization; however, since the multiplexing hierarchy was developed to work with this system, it was called PDH.

Also read PCM (E1/T1) Communication Systems

The first system was introduced by Bell Labs in 1961 and was called a “T1” or “DS1” (digital signal 1) consisting of 24 multiplexed channels. Subsequently, the PCM standards defined by International Telecommunication Union (ITU) specified a system of 32 channels (30 speech channels and 2 signaling channels) called an “E1.” Further developments facilitated multiplexing many more voice channels on one physical channel. The multiplexing hierarchy developed is the PDH, as brought out in the previous paragraph. There are four levels in the PDH: I, II, III, and IV. Level I is popularly known as PCM; however, in PDH, its proper terminology is T1 or E1, as stated earlier. Likewise, level II is called T2 or E2; level III is called T3 or E3; level IV is called T4 or E4. While T1, T2, etc. standards are used in North America and Japan, the rest of the world uses the E1, E2, etc., per ITU–Telecommunication Standardization Sector (ITU-T) standards. (ITU, which was earlier known as Committee Consultative International for Telegraphy and Telephony, or CCITT, is an international body responsible for developing telecommunication standards.)