Idle frames are always discarded by the
neighbor station receiving them.
Transit Path A MAC transits frames that do not originate or terminate at that MAC.
There are two types of MAC transit queuing designs: single transit queue and dual
transit queue. The single transit queue design places all transit traffic into a primary
transit queue (PTQ). The dual transit queue design places classA transit traffic into a
higher-precedence primary transit queue, and classB and classC transit traffic into a
lower-precedence secondary transit queue (STQ). Neither path supports preemption of
either the transit or ingress frames. Once a frame has begun transmission, its transmission
cannot be interrupted by the transmission of another frame. Single and dual
transit queue MAC can interoperate in the same ring.
Transmit Operation The selection of the frame to be transmitted depends on the transit
path implementation. For single transit queue implementations the intent is to always
Figure 12.17 Idle shaper operation
PTQ
PTQ
Idle threshold
Idle threshold
Resilient Packet Ring (RPR) 351
empty the PTQ before client frame transmissions. This is achieved by assigning a strict
higher priority to the transit traffic (regardless of its class of service) than to any local
added traffic.
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