Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
used by the controller to decide which SC to service. Following this decision, the selected SC’s
VCs are serviced in a round-robin manner. The selected VC then transmits the first cell in its
queue.
Unicast Traffic
Multicast Traffic
VC2
Q0 Q1 Q2 Q3
Q32 Q33 Q34 Q35
Strict Priority SC1
Strict Priority SC2
Q36 Q37 Q38 Q39
S125 S126
Q4 Q5 Q6 Q7
S0 S1
VC1
VC3
Timeslot-Based Priority
•
•
VC4
Q8 Q9 Q10 Q11
Q12 Q13 Q14 Q15
Q16 Q17 Q18 Q19
Q20 Q21 Q22 Q23
Q24 Q25 Q26 Q27
Q28 Q29 Q30 Q31
Q40 Q41 Q42 Q43
Q44 Q45 Q46 Q47
Q48 Q49 Q50 Q51
Q52 Q53 Q54 Q55
Q56 Q57 Q58 Q59
Q60 Q61 Q62 Q63
Round-Robin among VCs
within an SC
General Purpose (GP)
Weighted Round-
Robin SCs
Figure 39. Receive Service Class (SC) Map
The general algorithm for deciding which SC to service is as follows (certain multicast SCs may
be ineligible for selection in particular modes or operating conditions; these will be described
after the numbered list that follows):
1. Strict priority SC1 has primary service priority. If there is an SC1 with a cell, it will be selected. The
SC1 service classes are serviced in a weighted round-robin manner, alternating between unicast and
multicast classes (Q0, Q32, Q1, Q33, Q2, Q34, Q3, Q35, Q0, ...). The SC1 round-robin pointer will remain
pointed at an SC for up to w cell selections, where w is the SC’s weight. If no cells are available in an
SC, the round-robin pointer is advanced. Thus, the most time-critical VCs should be placed in an SC1
service class. The pointer for the SC1 service classes is separate from the pointer to the SC2 and GP ser-
vice classes.
2. Strict priority SC2 has secondary service priority. It is treated in the same fashion as SC1, except it has
its own independent round-robin pointer and the weighted round-robin order is: Q4, Q36, Q5, Q37, Q6,
Q38, Q7, Q39, Q4, ....
3. If no cell exists in the strict priority classes, then the controller accesses the timeslot-based priority table
in a round-robin manner. Each entry of this table contains a GP SC number. If the SC pointed to by the
active entry has cells, that SC is selected. The active entry is incremented to the next timeslot each time
the timeslot table is accessed. The table has 127 entries and wraps around. This servicing mechanism
provides the MCR guarantee on a per-SC basis. The number of times an SC is placed in the timeslot
table can be used to determine its MCR.
4. If no cell exists in the strict priority classes, and no cell exists in the SC pointed to by the active entry of
the timeslot-based priority table, then the GP SCs are serviced in a weighted round-robin manner simi-
lar to the SC1 and SC2 classes (Q8, Q40, Q9, Q41, Q10, Q42, Q11, Q43, Q12, Q44, ..., Q31, Q63, Q8, ...).
Again this has a separate round-robin pointer than that kept for the SC1 and SC2 service classes.
33
PMC [ PMC-SIERRA, INC ]
