PM7326 Datasheet, PDF (90/238 Pages) PMC-Sierra, Inc – ATM/PACKET Traffic Manager and Switch

DATA SHEET
PMC-1981224
ISSUE 6
PM7326 S/UNI APEX
ATM/PACKET TRAFFIC MANAGER AND SWITCH
• strict priority, round robin or modified strict priority between classes, evaluated
after the transmission of each cell.
• strict priority between classes, evaluated after the transmission of an entire
packet, and available only to those VCs configured for FCQ.
• strict priority between classes, evaluated after the transmission of a partial
packet of a programmable length, and available only to those VCs configured
for FCQ.
10.9.3.1 Cell and Packet Scheduling
In order to ensure that the lower priority classes are not starved when the high
priority classes are under heavy utilization, a minimum bandwidth reservation
scheme is employed. The user can program the minimum bandwidth
requirements of classes one, two, and three and thus avoid starvation. Setting
the minimum bandwidth requirements to zero (ClassXCellLmt = 0) on all classes
will result in the class scheduler acting as a strict priority scheduler. Setting the
minimum bandwidth requirements to three (ClassXCellLmt = 3) on all classes will
result in the class scheduler acting as a round robin scheduler.
The mechanism utilized to ensure that a class does not starve is as follows. The
class scheduler keeps track of the number of missed transmit opportunities the
lower priority classes within a port have had. When a cell is transmitted on a
particular class the ClassXCellCnt counters are incremented for all other classes
which have missed an opportunity to transmit a cell. Once the ClassXCellCnt for
a class reaches a maximum value (as defined by ClassXCellLmt), the class is in
starvation. On the next cell transmit opportunity for that port, the starving class
will be allowed to transmit one cell. If multiple classes were indicating starvation
then the highest priority class would transmit first, then the next class until all
starving classes have been serviced.
A starving class is only allowed to transmit one cell at a time. This ensures that
the higher priority classes do not experience a large amount of CDV caused by
the lower priority classes. When a class has an opportunity to transmit (due to
starvation avoidance or otherwise), its ClassXCellCnt is reset and the above
procedure is repeated.
A per-Port parameter, ClassPacket, is provided to support continuous packet
transmission. In this packet mode, a VC that is configured for FCQ will retain
permission to transmit cells for the length of the entire packet, regardless of the
starvation states of the other classes, including class 0. This feature enables
traffic to be emitted from the S/UNI APEX packet contiguously and thus
minimizing the buffering requirements for an external SAR device. Strict priority
must be set whenever packet class scheduling is selected.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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