In the case of a station transmitting
at a slower data rate than the preceding station, this could cause PTQ overflow.
The idle shaper performs an optional transmit rate synchronization function that
eliminates the possibility of PTQ overflow by inserting a variable number of small idle
frames in the transmitted data stream. The transmit rate synchronization function is
part of the MAC datapath.
The transmit rate synchronization function supports PHYs with data rate clock
tolerances of up to ?±100 ppm. The idle shaper limits the MAC-supplied idle traffic to
its allocated limits. Idle frames are inserted into the transmit datapath at a fixed rate
equivalent to 0.05% of the line rate. The rate of inserted idle frames is controlled by the
level of the PTQ. As the queue fills, the free space becomes less than the idleThreshold
value. At this threshold, the idle frame rate reduces to 0.025% of the line rate, which increases
the transmit data rate, which eventually reduces the queue depth. Figure 12.17
shows a numerical example of the idle shaper operation.
The idle shaper may optionally be implemented. If the idle shaper is not implemented,
then no idle frames are generated.
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