9.1 What Is Plesiochronous Digital Hierarchy?

Analog radio networks were based onfrequency division multiplexing and hence called FDM systems. The digital systems that replaced them were based on time division multiplexing and use pulse code modulation (PCM) to form the primary digital line rate (E1 or T1). To create higher bit rates secondary multiplexers are used. This is not done synchronously but is made to look synchronous by a technique called stuffing. "Plesio" means nearly, hence the term plesiochronous (nearly synchronous) digital hierarchy (PDH).

Higher order multiplexers are used to bit interleave the incoming bit streams into a higher order stream. The multiplexers have to synchronize the incoming primary order streams so that they can be multiplexed into a single higher order bit stream. Each E1 or T1 stream is essentially free running because it is not locked to a central clock signal. The nominal E1 bit rate is 2048 kbit/s 50 ppm. In a 2/8 secondary multiplexer, four 2-Mbit/s streams are multiplexed into an 8-Mbit/s stream as follows. The incoming bit streamsare read into elastic store buffers using a clock that is extracted from the bit stream. The bits are written our each buffer one bit at a time and bit interleaved into the aggregate stream using the main multiplexer clock. To ensure that the fastest incoming stream does not cause the buffer to overflow, the multiplexer clock is run at a rate higher than the fastest incoming stream in other words, 2048 kbit/s + 50 ppm (2048 102 b/s)...