PM3350-SW datasheet(135/224 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

133

instead, the Switch Processor firmware will simply cease to transfer frames from the

corresponding remote device across the PCI bus. The effect is that external devices

that attempt to consume too much of the ELAN 8x10 device's local memory will be

forced to stop, and instead accumulate frames in their own local storage. This imposes

fairness between devices.

If the BackPEn bit is set in the PortFlags field of a local port descriptor, and a high

traffic condition causes its MaxBuffers field to become less than or equal to zero, then

backpressure flow control will be started for that port. In this case, the Switch Processor

firmware will enable the MAC channel to begin jamming the medium by continuously

transmitting a bit pattern. The bit pattern is interrupted periodically to prevent

downstream jabber timers from locking out the ELAN 8x10 MAC interface; a standard

interframe gap is inserted at these times, after which the jamming will begin again. If a

collision is encountered during a backpressure jam, the MAC channel logic on the

ELAN 8x10 will perform the normal collision jam process, insert a normal interframe

gap, and then restart the jam (i.e., no backoff will be performed). The backpressure jam

will be maintained until the Switch Processor detects that the MaxBuffers field has risen

above a programmable threshold, at which time the Switch Processor firmware will shut

off the jam and resume normal operation on that channel. Note that if a normal transmit

frame is queued for transmission out a jamming ELAN 8x10 port, then the jamming will

temporarily cease, the frame will be transmitted (after waiting an interframe gap), and

the jamming will restart (again, after a standard interframe gap).

Note that the MaxBuffers variables in the local and expansion port descriptors are

maintained by the Switch Processor firmware in cached data space, and thus cannot be

updated by direct access over the PCI bus interface. Instead, the messaging interface

must be used to modify these variables during system operation, if desired. This

function is TBD.

Broadcast Frame Limit

Broadcast and multicast frames consume a considerable amount of memory resources

of an ELAN 8x10 system for a potentially long time. If, for example, a port is receiving a

high rate of broadcasts, then the broadcast frames (which must be sent out over all the

other ports in the system) may have to be buffered for a long time, and will also require

a large number of data descriptors in order to occupy all of the local port transmit and

expansion port transfer queues in the system. A programmable broadcast frame limit is

therefore provided to permit the system implementer to exercise some control over the

amount of system resources that may be consumed by broadcast and multicast traffic.

The broadcast/multicast frame limit is implemented by the Switch Processor firmware

using a single global MaxBcstFrames variable in the cached data space. This variable

is set up initially to the maximum number of broadcast frames permitted to be buffered

by the ELAN 8x10 device. Whenever a newly received broadcast or multicast frame is

detected by the switching code executed by the Switch Processor, the MaxBcstFrames

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