PM3350 Datasheet, PDF (146/224 Pages) PMC-Sierra, Inc – 8-Port 10 Mbit/s Switch

ELAN 8X10
DATA SHEET
PMC-970109
ISSUE 3
PM3350 ELAN 8 X10
8 PORT ETHERNET SWITCH
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:
1. if flow control is enabled, then flow control will be enabled until PacketBuffers
are returned to the free queue
2. 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:
1. 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.
2. 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.
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