Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
1.2.4 End-to-End Flow Control
The full queueing and flow control model is shown in Figure 4.
NOTE: Credits or backpressure are used at every transfer point to ensure that cells cannot be lost
due to lack of buffer space.
Figure 4. Queueing and Flow Control
ETT1 core
Linecard
ETT1 port
ETT1 port
Linecard
Egress
queues
Ingress
queues
Ingress queues
Egress queues
ETT1 backpressure
hole requests
LCS credits
cell flow
backpressure/credits
1.2.5 TDM Service
The ETT1 TDM service provides guaranteed bandwidth and zero cell delay variation. These properties,
which are not available from the best-effort service, mean that the TDM service might be used to provide
an ATM CBR service, for example. The ETT1 core provides the TDM service at the same time as the best
effort service, and TDM cells integrate smoothly with the flow of best-effort traffic. In effect, the TDM cells
appear to the Scheduler to be cells of the highest precedence, even greater than level zero best-effort
multicast traffic.
The TDM service operates by enabling a ETT1 port (and linecard) to reserve the crossbar fabric at some
specified cell time in the future. The Scheduler is notified of this reservation and will not schedule any
best-effort cells from the ingress port or to the egress port during that one cell time. Each port can make
separate reservations according to whether it will send and/or receive a cell at each cell time.
Several egress ports may receive the same cell from a given ingress port; therefore, the TDM service is
inherently a multicast service.
In order to provide a guaranteed bandwidth over a long period of time, an ingress port will want to repeat
the reservations on a regular basis. To support this the ETT1 core uses an internal construct called a TDM
22
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
