Regardless of the type of transmission media (wire, radio, or fiber), bits are sent and bits are received. Every receive port in the network has to deal with an incoming serial bit stream that includes clocking and payload bits. Clocking and data, also called payload, have their timing and phasing relationships established at the point of creation. Along the way, the serial bit stream may be multiplexed with additional serial bit streams, cross-connected to a different carrier, or switched at the circuit, cell, or packet layer. Yes, this is layer 1 and 2 of the OSI stack. Separating clock information from payload, or mucking around with the time relationship between signal transitions amounts to errors. If, for whatever reason, a network element loses its synchronization reference and wanders outside holding limits, anything and everything using it as a synchronization reference is out of time with the larger network, converting valuable data to invaluable trash.

Clocking and data recovery are a critical function. Considering the clocking concept from the perspective of a receive port on a network element or the receive end of a transmission path gives the ability to look backward toward the source and forward toward other network elements and facilities dependent on the clock for proper operation and delivery of the associated payload.

If there is a single point in the entire end-to-end, top-to-bottom process that is the most critical in moving digitized information through a network, it has to be at...