PM3350-SW datasheet(104/224 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

102

Structure

Size

Quantity Required

Operating Environment

40 kB

Single block of memory starting at address 0x000000

Packet Buffers

80 bytes

Buffering per port multiplied by number of ports and

divided by packet buffer size

Data Descriptors

20 bytes

Number of packet buffers plus (7*broadcast limit) plus

(number of devices * broadcast limit)

Hash Buckets /

Forwarding Tags

64 bytes

Number of MAC addresses to be supported in the

system

System Boot Image Copy

32 kB

Temporary use; held reserved for simplicity

Of the different structures, the computation of how many data descriptors are required

constitutes the most difficult, principally because of the differing requirements for

unicast and multicast frames. Unicast frames require only a single data descriptor per

frame, because, by definition, they can be placed on only one queue at a time.

Broadcast/multicast frames, however, are queued to multiple destinations and thus

require more than one data descriptor per frame. An upper limit on the number of data

descriptors may be computed by observing that a given frame can be queued to at

most (P -1) + (D - 1) queues, where P is the number of ports on a particular device, and

D is the number of devices in the system. (The computation can be easily derived by

noting that a broadcast frame cannot be retransmitted out the port it came in on, and

also cannot be transferred over the PCI bus to the device that originally received it)

A good upper bound on the number of data descriptors required in the system may

therefore be obtained by assuming minimum-sized frames (i.e., every packet buffer

contains a complete Ethernet frame), and then taking into account the configurable

system broadcast limit set up at initialization time to determine the additional number of

data descriptors required to handle broadcast frames. The maximum number of data

descriptors required is then given by:

N + (P - 1) * B + (D - 1) * B + E

where N = number of packet buffers in free packet buffer pool, P = number of ports in

ELAN 8x10 (i.e., 7), B = broadcast limit in frames, D = number of devices in system

including this ELAN 8x10, and E is the number of additional data descriptors that are

required to support the messaging system. (A reasonable value for E is about 8 per

port.)

As an example, consider a minimum system with two ELAN 8x10 devices, using 64 kB

of buffering per port and supporting up to 2048 MAC addresses in the system. Assume

that the broadcast limit is set to 1400 broadcast frames outstanding at a time, and

about 128 data descriptors are reserved for holding system messages. The parameters

for such a system would be:

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