QUESTION:
Why is an 8kHz signal always used as a frame synchronization signal in PDH and SDH Systems? Why not 4kHz or 2kHz?
ANSWER:
It was determined many years ago, after much experimentation, that for digitizing voice signals a sampling rate of 8 kHz was appropriate and that each sample could be encoded in one octet ( 8 bits) following either the mu-law or A-law companding characteristic. A DS0 (64 kbps) thus can be viewed as one octet every 125 microseconds (8 kHz rate). Both DS1 (1.544 Mbps) and E1 (2.048 Mbps) framing structures thus use 8 kHz for the frame rate so that the octet for each constituent DS0 can be easily identified. Note that both DS1 and E1 also use superframes (wherein frames can be numbered). For example, the extended superframe (ESF) used for DS1 has a modulo-24 superframe structure, corresponding to a super-frame-rate of (1/3) kHz.
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