BOOK_CONTENT
From Next Generation SONET/SDH

3.2.3   Timing Aspects

In modern networks, the bit rate has exceeded by far the gigabit per second rate and,
therefore, the performance of timing circuitry must meet stringent specifications.
Timing affects the bit error rate (BER) and signal performance, and circuits are
characterized by an accuracy that is defined in parts per million (ppm) pulses, by jitter,
and by wander. If a path consists of several nodes or stations, each having its
own timing circuitry, then the timing effects may be compounded over the path. Because
of the importance of this issue, standards agencies have issued recommendations
pertaining to the accuracy of timing characteristics and their measuring methods.

ITU-T has issued recommendations on performance, timing, and measurements
(G.823, G.828, O.150, O.151). Per GR-253-CORE specification, jitter is defined as
the “short-term variation” of a signal’s significant instants from their ideal position
in time. “Short-term variation” implies some frequency oscillation greater than or
equal to a frequency demarcation. In the North American hierarchy (DS1–DS3) the
demarcation between jitter and wander is 10 Hz. Jitter network element (NE) criteria
(see GR-499-CORE for details) are specified as:

  • Jitter Transfer (per interface category)—defined as the jitter transfer characteristics
    (limits) of an NE.
  • Jitter Tolerance (per interface category)—defined as the point-to-point amplitude
    of sinusoidal jitter applied on the OC-N (SONET/SDH) input signal
    that causes a 1-dB power penalty.
  • Jitter Generation (per interface category)—defines the limits of jitter generated
    by an NE without jitter or wander at its inputs. In communications systems,
    payload mapping, bit stuffing, and pointer adjustments are sources of
    jitter.

 

© 2004

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