This page of SDH/SONET tutorial covers PDH concepts and PDH rates with 2Mbps, 8Mbps, 34Mbps and 140 Mbps.

PDH (Plesiochronous Digital Hierarchy) traffics such as DS-1,E1,DS-1C,DS-2 and DS-3 is encapsulated with additional framing bytes/octets to allow PDH traffic over SDH/SONET channel. This is referred by Virtual Tributary (VT) in SONET and Virtual Container (VC) in SDH. With this concept we can have multiple DS-1 to be carried over on a single SDH channel for example, 28 DS-1 carried on single STS-1. It is called Plesiochronous as each multiplexer in the system has its own clock for operation.

There are different series of recommendations finalized by ITU-T in PDH. G -series is developed for transmission systems and multiplexing equipments. O-Series is developed for measuring equipment specifications. M-Series is developed for transmission system maintenance.

Following rates are derived from PDH base rate of 2.048Mbps as mentioned in the table below. These are standards finalized by ITU-T.

Hierarchy type Standard Binary Rate(ppm) Line Code Amplitude Attenuation
1 G.704/732 2048kbit/sec +/- 50 ppm HDB3 2.37 V to 3V 6dB
2 G.742 8448 kbit/sec +/- 30 ppm HDB3 2.37 V 6dB
3 G.751 34368 kbit/sec +/- 20 ppm HDB3 1.00 V 12 dB
4 G.751 139264 kbit/sec +/- 15 ppm CMI 1 V 12 dB


PDH versus SDH
SONET SDH tutorial
SDH Frame structure