QUESTION:
I heard Japan was using 64kbps(CC) clock as one of their timing sources for their network transport system, my question is does any ITU or Bellcore requirement apply for the Japan application?
ANSWER:
Composite Clock ("CC") is a timing distribution format that transports both frequency (time-base) and phase. It is nominally a 64 kbps signal comprising an "all-ones" signal encoded using Alternate Mark Inversion (AMI). The notion of AMI is that the pulses corresponding to the binary "1" are made to alternate in polarity. A simple clock-recovery circuit will generate a 64 kHz reference signal (typically digital logic) from a CC signal. A "marker" can be introduced by violating the AMI rule, also called a bipolar violation (BPV). The CC standard calls for a BPV every 8 bits (i.e. every octet) and this does not impact the 64 kHz clock recovery mechanism. By monitoring the BPVs, the receiver can extract an 8 kHz signal that can specify octet boundaries. Since all CC feeds are required to be aligned, the effect is that all receivers thus have both frequency and phase.
The Japanese version of CC differs, albeit very slightly, from that used in North America, the differences being related to the interpretation of what the BPV signifies. Furthermore, in the Japanese a sub-pattern corresponding to 400 Hz is superimposed onto the 8 kHz because the ISDN scheme in Japan requires this phasing capability.
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