The Plesiochronous Digital Hierarchy (PDH) has been developed in stages from the basic 30-channel PCM (PCM-30) system.
As can be seen in the following Figure, there are three different hierarchical systems available, each supporting different line rates and multiplexing rates. The higher aggregate rates can therefore be achieved by grouping together the lower rates through the use of multiplexers.
The higher bit rate links also require additional bits for framing and control. For example, an 8.4 Mbits signal comprises of 4 × 2.048 Mbits = 8.192 Mbits, with the remaining 256 Kbits being used for framing and control.
The European and North American hierarchy systems are often referred by the letter ‘E’ for European and ‘T’ for North American, with the hierarchy levels being numbered consecutively. These hierarchy levels can be compared in the following Figure −
|Hierarchy Level||Bit Rate (Mbits)||Voice Channels|
These bit rates are often abbreviated to 1.5 meg, 3 meg, 6 meg, 44 meg, 274 meg and 2 meg, 8 meg, 34 meg, 140 meg, and 565 meg respectively.
As the legacy of PDH is so prominent in the telecommunications industry, it became necessary to accommodate these line rates in any new technology to be introduced, therefore many of the PDH line rates are supported by the Synchronous Digital Hierarchy (SDH). The only exception to this is the omission of the 8.4 Mbits level, which no longer has any practical meaning and is not supported by SDH.
In the basic 2 Mbits system, the data is byte interleaved, whereby each 8-bit timeslot is sent one after the other. In the case of the higher hierarchy levels, the data streams are multiplexed together bit-by-bit. A disadvantage of this system is that the bit rate of each tributary signal can vary from the nominal value due to each multiplexer having their own independent clock supplies. These clock deviations are dependent on the line rate and can be compensated for by using justification techniques within the bandwidth remaining after the multiplexing stage. The line rate also dictates the line code used for transmission as can be seen below −
|Bit Rate(Mbits)||Number of 64Kbit Channels||Permitted clock deviation (ppm)||Interface code||Preferred medium/line code|