Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
Certain multicast SCs may be ineligible for selection due to the aggregate mode and the backpres-
sure from the switch fabric. The QRT can be set to a multicast aggregate mode of either 1 or 4. In
aggregate mode of 1, each of the switch fabric outputs of the QRT are treated as distinct outputs.
Multicast connections must be specifically assigned to an SC in the corresponding column of
multicast SCs (there are 32 multicast SCs, with four columns of eight classes each), since all the
cells of a multicast VC must use the same output. In this mode, only one column (eight) of the
multicast SCs will be eligible for selection (for example, service classes Q32, Q36, Q40, Q44, Q48
,
Q52, Q56, and Q60 correspond to port 0 and service classes Q33, Q37, Q41, Q45, Q49, Q53, Q57, and Q61
correspond to port 1). The other three columns of SCs (total of 24 SCs) will be ineligible. In
aggregate mode of 4, the four outputs are treated as one logical output, and thus all multicast SCs
may be selected for any of the four outputs.
Additional SCs may be ineligible due to backpressure from the switch fabric. There are three
types of backpressure: high, medium and low. High backpressure renders the eight SC1 and SC2
multicast SCs ineligible (Q32 to Q39). Medium backpressure renders the first eight GP SCs ineligi-
ble (Q40 to Q47, two rows of four). Low backpressure renders the last 16 GP SCs ineligible (Q48 to
Q63, four rows of four).
The receive queue controller scheduler provides the following benefits:
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•
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QoS - the strict priority scheme between SC1, SC2, and GP SCs, and the weighted round-
robin algorithms allow satisfaction of QoS guarantees.
CDV minimization - the treatment of the strict priority SCs ensure cells within these SCs
get timely service.
MCR guarantee - the timeslot table ensures all SCs will receive a minimum amount of
servicing (clearly, the aggregate bandwidth given to the SC1 and SC2 VCs affects the
remaining bandwidth to be divided between the GP SCs).
•
Fairness maximization - how SCs (1, 4, 16, or 64) are weighted allows different SCs to
support different bandwidth requirements (for example, high bandwidth SCs are assigned
64 and are serviced 64 times as often as low bandwidth SCs, which are assigned 1).
2.5.6 Receive Sequencing Algorithm
One of the service guarantees ATM offers is the FIFO delivery of cells. Since the QRT can send
multiple cells from a channel simultaneously across the fabric, and not all of those cells will get
through on the first try, the QRT must support an algorithm to make sure the cells can be put back
into order. The algorithm it supports is a classic window algorithm where only N cells are allowed
to be outstanding without acknowledgment. In the QRT, N is either 1 or 2. This limits the data
rate of an individual connection to approximately 155 Mbps. The cells are sequence numbered
and reordered at the transmit side.
This algorithm is implemented by removing the channel from the ring of eligible channels when-
ever two cells are outstanding. The channel is then called run-limited. It also removes the channel
from the ring if the last cell present has been sent to the switch fabric. The channel is then called
cell-limited. In the former case, it will remain off the ring until the fabric transmission results for
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PMC [ PMC-SIERRA, INC ]
