Chapter 5. Timing, Clocking,and Synchronization in the T-carrier System
Time is the extension of motion.
Faster networks depend on accurate timing. As the number of bits per second increases, the time in which to look for any particular bit decreases. Getting both sides to agree on timing becomes more difficult at higher speeds. Synchronous networking is largely about distribution of accurate timing relationships.
A Timing Taxonomy
Synchronous communications do not
depend on start and stop flags to mark the beginning and end of
meaningful data. Instead, the network constantly transmits data and
uses a separate clock signal to determine when
to examine the incoming stream to extract a bit. Distributing clock
information to network nodes is one of the major challenges for
synchronous network designers. Three major types of timing are used
on networks: asynchronous, synchronous, and plesiochronous. All three
terms derive from the Greek word
time. The three differ in how they distribute timing information
through the network.
systems do not share or exchange timing
information. Each network element is timed from its own free-running
clock. Analog modems are asynchronous because timing is derived from
start and stop bits in the data stream. Free-running clocks are
adequate for dial-up communications because the time slots are much
longer than on higher-speed digital networks.
systems distribute timing information from an extremely ...