All shapers other than the downstream shaper are applied only to add traffic. All
unreserved add and transit traffic flows through the downstream shaper. The function
of the downstream shaper is to constrain the MAC to sustain the downstream allocated
subclassA0 rate.
Optionally, rate synchronization idle frames are added by the MAC and shaped by
the idle shaper.
Idle Frames RPR stations and networks can be implemented with either synchronous
(e.g., SONET/SDH) or asynchronously-timed PHYs (e.g., Gigabit Ethernet). In a synchronous
network, the transmit clock for each station is locked to a common timing
source that may be recovered from the received data stream or provided through an external
timing interface, and the transmit data rate from each station is exactly identical
to the received data rate. However, in an asynchronous network, the transmit data rate
at each station is determined by a local clock source, and the transmit data rate from
Figure 12.16 MAC data path??”Dual transit queue
Client
MAC
control
sublayer
MAC
medium
access
control
Checker addMac shl
shl
PTQ
Idle
addMac shM Fairness
control
Send A Send B Send C
To other
attachment
Rate
monitors
shA0
shB shF
shD
Stage
addA
addB
addC
350 Chapter 12
each station varies slightly from the network data rate.
Pages:
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795