PDH Over SONET/SDH: A Comprehensive Guide
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This section of the SDH/SONET tutorial explains PDH concepts and the various PDH rates, including 2Mbps, 8Mbps, 34Mbps, and 140 Mbps.
PDH (Plesiochronous Digital Hierarchy) traffic, such as DS-1, E1, DS-1C, DS-2, and DS-3, is encapsulated with extra framing bytes/octets. This encapsulation allows the PDH traffic to be carried over an SDH/SONET channel.
This encapsulation is achieved using Virtual Tributaries (VT) in SONET and Virtual Containers (VC) in SDH. This means we can carry multiple DS-1 signals over a single SDH channel. For instance, 28 DS-1 signals can be carried on a single STS-1.
The term “Plesiochronous” is used because each multiplexer in the system uses its own clock for operation. ITU-T has finalized different series of recommendations for PDH. The G-series focuses on transmission systems and multiplexing equipment. The O-Series details measuring equipment specifications, and the M-Series covers transmission system maintenance.
The following rates are derived from the PDH base rate of 2.048Mbps, as described in the table below. These standards are 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 |