PM3350-SW datasheet(137/224 Pages) PMC

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ELAN 8X10

PM3350 ELAN 8 X10

DATA SHEET

PMC-970109

ISSUE 3

8 PORT ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND PMC-SIERRA, INC. CUSTOMERS ONLY

135

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if the TypeSpecific subfield of the Flags field is zero, it is changed to a 1, but

nothing more is done with that descriptor; the background task proceeds to the

next transmit queue.

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if the TypeSpecific subfield of the Flags field is non-zero, the descriptor must

have been marked in a previous sweep, and hence must have been present on

the transmit queue for at least one sweep interval. Thus the transmit queue has

been inactive for at least that long; the descriptor (and potentially the frame

data, if no other queues are pointing to the same frame) is removed from the

queue and returned to the free pool.

The mark-and-sweep algorithm thus detects when a frame has been present in a

transmit queue for an excessive amount of time by marking it in one sweep, and

detecting the marked frame in the next sweep. If the frame was transmitted in the

interval between sweeps, it will be deleted from the transmit queue; the next entry in the

transmit queue will be marked in its turn, preparatory to the next sweep. If a transmit

queue is blocked, therefore, frames will be prevented from remaining in it indefinitely,

and their memory resources will be reclaimed.

Transmit Queue Restart

In certain rare cases, it is possible for frames to remain in a transmit queue without any

transmit attempt being made (even if the MAC channel is completely idle). To handle

this situation, the Switch Processor executes a periodic background task (referred to as

the transmit queue restart process) that is invoked at short intervals during idle periods,

when the Switch Processor is not busy with frame switching. This background task

scans all of the MAC channels; for each idle MAC channel, it checks to see if a

corresponding non-empty transmit queue exists. If so, the background task simply

reads the data descriptor at the head of the transmit queue and attempts to start

transmission of that frame over the MAC channel; otherwise, the background task

continues on to the next channel. When all eight channels in the ELAN 8x10 have been

scanned, the background task terminates, and waits until it is invoked again.

The transmit queue restart process is performed at the same time as the mark-and-

sweep algorithm that purges stalled entries from the transmit queues.

Broadcast Rate Limiting

It is desirable to provide some form of limit on the rate of throughput of broadcasts

handled by an ELAN 8x10 system; this limit should be set to allow a relatively small,

'normal' level of broadcasts, but prevent the excessive rates of broadcasts that can

occur during malfunctions such as broadcast storms. It should be noted that the rate at

which broadcasts occur (not the total number of broadcasts being handled by the

system) must be limited; the limit is therefore set in terms of broadcast frames accepted

PM3350-SW Datasheet(PDF) 137 Page - PMC-Sierra, Inc

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