PM3350-SW datasheet(146/224 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

144

dynamically allocatable buffers you should get the same result if you take a statistical

time average of the data traffic as opposed to an instantaneous frame rate such that

the period of averaging is less than or equal to the time it would take to receive frames

that would utilize 50% of the maximum allocatable number of packet buffers.

Since the PM3351 Elan 1x100 is a 1-port device the limiting factor in constructing a

switch using the chip is the bandwidth that can be supported over the PCI expansion

bus. The PCI bus interface that is used on both the ELAN-1x100 and ELAN-8x10

devices together with the ELAN frame transfer protocol that is used results in a

maximum sustainable throughput of 500 Mbit/sec on the PCI bus.

A switch configuration and traffic mix that requires sustained throughput of less than

500 Mbit/sec on the PCI expansion bus will be non-blocking; a switch configuration

and traffic mix corresponding to a sustained throughput of greater than 500 Mbit/sec will

result in a blocking Ethernet switch configuration. The PM3351 and PM3350 chips, if

used in a blocking configuration, will do one of two things:

if flow control is enabled, then flow control will be enabled until PacketBuffers

are returned to the free queue

if flow control is not enabled, then frames that are being received from the

Ethernet physical layer device will be dropped until there are a sufficient number

of PacketBuffers in the free pool in which to format the frame.

The following derivation shows how the rate of traffic being received on the Ethernet

physical layer corresponds to the transfer rate as seen on the PCI expansion bus.

GIVEN:

Let Mlp represent the percentage of frames that are received on the link media

that require to be switched across the PCI expansion bus. Let Mports

represent the number of ports of the device: for the PM3350 this is 8, for the

PM3351 it is 1.

Since the PM3351 is a 1-port device, each frame received on the 100 Mbit link

media is switched over the PCI expansion bus so Mlp is 1. However, the

PM3350 is an 8-port device so not all unicast traffic may need to be switched

across the PCI bus; hence, Mlp may be less than 1 in that case.

Let Mrfr be the relative frame rate. This is the ratio of the actual frame rate to

the maximum Ethernet frame rate. Restating, this is the relative percentage of

time that the link media is non-idle that does NOT include the standard

interframe gap of 96 bit times.

If you are modelling a network running at maximum Ethernet frame rates, Mrfr

is 1.

PM3350-SW Datasheet(PDF) 146 Page - PMC-Sierra, Inc

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