PM7326 datasheet(71/238 Pages) PMC

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PM7326 S/UNI APEX

DATA SHEET

PMC-1981224

ISSUE 6

ATM/PACKET TRAFFIC MANAGER AND SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

The scheduler has 8 polling sequences and 4 different weighting groups. Port

weighting is achieved by configuring the number of polling sequences a port

should participate in. This configuration is done by assigning a 2-bit logarithmic

weight value and a 3-bit sequence number to each port. Software assigns the 2-

bit weight value, and the hardware always maps the 4 ports to the following

sequence numbers: port 0 is assigned 000, port 1 is assigned 010, port 2 is

assigned 101, and port 3 is assigned 111. The scheduler maintains a 3-bit polling

sequence number and increments it after each scheduler polling cycle. During a

scheduler polling cycle each of the 4 ports is evaluated. The port will be selected

for transmission if the following conditions are met:

• the port’s transmit data queue is not empty

• the n LSB’s of the scheduler poll sequence number match the n LSB’s

of the port’s sequence number (n is equal to the port’s weight). For

ports with a weight of zero, this compare is ignored and assumed

successful. For ports with a weight of one, only the LSB is compared.

For ports with a weight of two, only the first two LSBs are compared.

For ports with a weight of three, all three bits are compared.

• the S/UNI APEX internal WAN FIFO for the port is not full

The logarithmic weights are set so that lower speed ports are evaluated less

often relative to higher speed ports. The following formula shows relationship

between the logarithmic weights values and the resulting linear relative weight.

rw = 2

(3-lw)

If port 0 were assigned a weight of 0, port 1 a weight of 1, port 2 a weight of 2,

and port 3 a weight of 3, and all the ports had data to send, and none of the

WAN FIFOs were full, then cells would be transmitted in the following order:

PM7326 Datasheet(PDF) 71 Page - PMC-Sierra, Inc