For dual transit queue implementations the intent is to always empty the PTQ before
client frame transmissions, but to allow the STQ to fill somewhat while adding client
frame transmissions.
A dual transit queue MAC implementation is more complicated, but it has the advantages
of supporting the more efficient subclassA1, and safely delivering higher amounts
of ring traffic since the low priority transit traffic can not preempt the local high priority
(classA) traffic.
Frame Formats
There are four types of RPR frames:
?– Data frames
?– Control frames
?– Fairness frames
?– Idle frames
The minimum data frame size is 24 bytes, and the maximum is 1616 bytes if jumbo
frames are not supported and 9216 bytes if jumbo frames are supported. The minimum
control frame size is 24 bytes and the maximum is 1616 bytes, no jumbo control frames
are defined.
Figure 12.18 illustrates the data frame options and shows the HEC and FCS error
coverage. The extended frame option is used for bridging.
Figure 12.19 illustrates the control frame formats and the control type values.
DA and SA are 48-bit fields that specifies the destination and source stations for the
frame.
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