Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
1.3.7 The LCS Protocol
In this section, we describe the LCS protocol from a functional perspective and explain how it works with
the queueing model that was described in the previous section.
NOTE: The LCS protocol is defined in the “LCS Protocol Specification -- Protocol Version 2”,
available from PMC-Sierra, Inc. This version of LCS supersedes LCS Version 1. Version 2
is first supported in the TT1 Chip Set with the Enhanced Port Processor device (also
referred to as the ETT1 Chip Set) and will be supported in future PMC-Sierra products. A
comparison of the two versions of the LCS protocol is in the same document.
The primary role of the LCS protocol is as a credit-based flow control mechanism that ensures that cells
are transferred between the linecard and the switch core only when sufficient buffer space is available. In
order to do this, the linecard must have knowledge of the queues that exist in the ETT1 core. The LCS
protocol is an asymmetrical protocol; we will first consider the ingress direction. Figure 24 shows the 132
best effort ingress queues in an ETT1 port (this assumes an all-OC-192c switch). The linecard may have a
very different number of queues (typically many more), and thus must map its own internal queues to the
physical queues present in the ETT1 core.
Figure 24. ETT1 Port Ingress Queues
Payload
LCS Header
ETT1 Port
Linecard
Cell requests and payloads
Credit Information
Mapping
128 unicast
4 multicast
128 unicast
4 multicast
linecard
ingress
queues
For example, the linecard might provide an ATM interface which can manage many thousands of virtual
circuit flows, and might have a separate internal queue for each flow. The linecard must map its own
ingress queues to the 132 ingress queues present in the ETT1 port. The 132 queues shown in the linecard
do not have to be physically implemented provided that the linecard can perform the requisite mapping
from linecard queues to ETT1 queues on-the-fly. The ETT1 ingress queues are Virtual Output Queues; the
mapping function simply has to map each linecard queue to the desired egress port within the ETT1 core.
The mapping function must also select the appropriate priority if more than a single priority is required.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
51
PMC [ PMC-SIERRA, INC ]
