Short note: Digital transmission hierarchies

A digital transmission hierarchy is a standard set of bit rates in which lower-rate digital signals are multiplexed step by step into higher-rate signals, starting from the 64 kbps PCM voice channel (8000 samples/s × 8 bits). Two generations exist: PDH and SDH/SONET.

Plesiochronous Digital Hierarchy (PDH)

Tributaries at each level have their own clocks, which are almost but not exactly equal (plesiochronous). Higher-order multiplexers use bit interleaving with pulse stuffing to absorb rate differences, so each level's rate is slightly more than the sum of its inputs.

Bit rate examples: $T1 = (24 \times 8 + 1) \times 8000 = 1.544$ Mbps; $E1 = 32 \times 8 \times 8000 = 2.048$ Mbps.

Drawbacks of PDH: to drop one 2 Mbps stream from a 140 Mbps line, the whole signal must be demultiplexed level by level ("multiplexer mountain"); the three regional standards do not interwork; little overhead for management and protection.

Synchronous Digital Hierarchy (SDH) / SONET

All equipment is locked to a common master clock, and byte-interleaved frames with pointers locate each tributary directly.

Benefits: a tributary (e.g. one E1 out of the 63 in an STM-1) can be added or dropped directly with an add-drop multiplexer; one world standard; rich overhead for OAM; ring protection; carries PDH signals and data. It is the backbone of present-day optical networks, usually over WDM.