PM3350-SW datasheet(113/224 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

111

Switching Via The Expansion Bus

If a frame received on a physical port of any ELAN 8x10 device is addressed to a

destination host connected to a MAC port of an external (remote) ELAN 8x10 device,

then the frame will be passed to the external device via the PCI expansion bus

interface.

Frame switching via the PCI expansion bus is carried out in two stages: the source

ELAN 8x10 receiving the frame appends the frame to the frame transfer queue that it

maintains for the destination device (and signals the destination device that another

frame is available); the destination device is then expected to copy the data from the

source ELAN 8x10 to its own memory space, and notify the source ELAN 8x10 when

the transfer is complete (i.e., acknowledge the transfer). The destination device is then

completely responsible for subsequently transmitting the data on the correct Ethernet

link.

Note that broadcast or multicast transfer to multiple remote devices is done by having

each destination device separately copy the same frame across the PCI bus to their

own memory spaces, and separately acknowledge the copy to the source of the frame.

This will be followed by multiple transmissions of the frame over the local ports within

the destination devices, without any further use of the PCI bus. A bandwidth

multiplication effect is thus obtained when the destination devices (such as other ELAN

8x10 chips) transmit the frame over multiple ports, as a single copy of the frame over

the PCI bus can result in multiple transmissions within the destination devices.

Request and Acknowledge Counters and Protocol

The ELAN 8x10 (and other devices that expect to communicate with the ELAN 8x10)

implements a set of hardware counters that assist in the signaling across the PCI bus.

A total of eight request counters and eight acknowledge counters are implemented, one

corresponding to each device in the system. These counters are mapped into the upper

64 kB of the 16 MB address space assigned to each ELAN 8x10 device, and are

incremented as required to notify target devices that frames are available to be

transferred, or that frames have been transferred and the memory resources used by

them may be freed.

A relatively simple protocol is followed when transferring frames between ELAN 8x10

devices across the PCI bus. The following figure shows a high-level view of the

handshake employed:

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