PM73487-PI datasheet(45/251 Pages) PMC

45 / 251 page

PM73487 QRT

PMC-Sierra, Inc.

PMC-980618

Issue 3

622 Mbps ATM Traffic Management Device

Released

Datasheet

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.

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):

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.

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, ....

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.

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.

Figure 39. Receive Service Class (SC) Map

Unicast Traffic

Multicast Traffic

Strict Priority SC1

Strict Priority SC2

Timeslot-Based Priority

General Purpose (GP)

Weighted Round-

Robin SCs

Q32

Q34 Q35

Q33

Q36

Q38 Q39

Q37

Q10 Q11

Q40

Q42 Q43

Q41

••

Q12

Q14 Q15

Q13

Q44

Q46 Q47

Q45

Q16

Q18 Q19

Q17

Q48

Q50 Q51

Q49

Q20

Q22 Q23

Q21

Q52

Q54 Q55

Q53

Q24

Q26 Q27

Q25

Q56

Q58 Q59

Q57

Q28

Q30 Q31

Q29

Q60

Q62 Q63

Q61

S125 S126

VC1

VC2

VC4

VC3

Round-Robin among VCs

within an SC

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