PM9313 datasheet(28/343 Pages) PMC

28 / 343 page

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

PM9311/2/3/5 ETT1™ CHIP SET

PMC-2000164

ISSUE 3

ENHANCED TT1™ SWITCH FABRIC

Released

Data Sheet

PMC-Sierra, Inc.

A third difference is that the EPP must maintain separate LCS request counters for each of the subports. It

must also maintain separate egress queues. So the number of queues can increase four-fold in order to

preserve the independence of each subport. Section 1.3 “Prioritized Best-Effort Queue Model” on page 30

describes the various queuing models in great detail.

1.2.6.1

Identifying the Source Subport

The EPP manages a single physical stream of cells at 25M cells/second. In subport mode the EPP must

look at each incoming cell and determine which subport has sent the cell. The LCS label field is used to

achieve this. Two bits within the label (referred to in the LCS Specification as MUX bits) are used to denote

the source subport, numbered 0 through 3. The MUX bits must be inserted in the LCS label before the cell

arrives at the EPP. The MUX bits might be inserted at the source linecards themselves. Alternatively, they

might be inserted by a four-to-one multiplexer device placed between the subport linecards and the EPP.

In this latter case, the multiplexer device might not be able to re-calculate the LCS CRC. For maximum

flexibility, the EPP can be configured to calculate the LCS CRC either with or without the MUX bits.

1.2.6.2

Egress Cell Rate

Within the EPP is an Output Scheduler process which is different from, and should not be confused with,

the ETT1 Scheduler device. At every OC-192 cell time, the Output Scheduler looks at the egress queues

and decides which cell should be forwarded to the attached egress linecard(s). In subport mode, the

Output Scheduler will constrain the egress cell rate so as not to overflow any of the 2.5 Gbit/s links. It does

this by operating in a strict round-robin mode, so that at any OC-192 cell time it will only try to send a cell

for one of the subports. The four 2.5 Gbit/s subports are labeled as 0 through 3 ; at some cell time the

Output Scheduler will only try to send a cell from the egress queues associated with subport 0. In the next

time, it will only consider cells destined for subport 1, etc. If, at any cell time, there are no cells to be sent to

the selected subport, then an Idle (empty) cell is sent. For example, if subports 0 and 3 are connected to

linecards but subports 2 and 4 are disconnected, then the Output Scheduler will send a cell to 0, then an

empty cell, then a cell to 3, then an empty cell, and then repeat the sequence. The effective cell rate

transmitted to each subport will not exceed 6.25 M cells per second.

1.2.7

LCS Control Packets

The LCS protocol provides in-band control packets. These packets (cells) are distinct from normal cell

traffic in that they do not pass through the fabric to an egress linecard, but are intended to cause some

effect within the switch.

There are two classes of Control Packets. The first class, referred to as CPU Control Packets, are

exchanged between the linecard and the ETT1 CPU (via the EPP and Dataslices). The intention is that

CPU Control Packets form the basic mechanism through which the linecard CPU and ETT1 CPU can

exchange information. This simple mechanism is subject to cell loss, and so should be supplemented by

some form of reliable transport protocol that would operate within the ETT1 CPU and the linecards.

The second class, referred to as LCS Control Packets, are used to manage the link between the linecard

and the ETT1 port. These LCS Control Packets can be used to start and stop the flow of cells on the link,

PM9313 Datasheet(PDF) 28 Page - PMC-Sierra, Inc

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