Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
1.5.3 TDM Frame Timing
A single TDM table is shown in Figure 29. In addition to the 1,024 entry portion, there is an additional
“guard band” of G entries preceding the table. These G entries are unusable and are explained below. An
additional 16 time slots (plus uncertainty/jitter in TDM sync timing) are unusable following the last valid
entry in any of the TDM tables due to pipeline latency through the Scheduler and EPP. These 16 entries
cannot be used for TDM traffic.
Figure 29. Single TDM Table
TDM_Period
TDM_Sync
TDM_Sync
Guard Band
Main TDM Frame
Unusable
16 entries
N Entries
G Entries
+ uncertainty
0 ≤ N ≤ 1024
(128 cells)
The time between TDM Sync signals is referred to as the TDM period. This period is set to the sum of G
plus the length of table N plus the final latency allowance of 16 + uncertainty. In the EPP chapter, G is
referred to as the “TDM offset value”.
1.5.4 TDM Synchronization
There are several different levels of synchronization required for the TDM service to work properly. First, all
EPPs must point to the same entry in their TDM tables in order for the Send and Receive reservations to
match up in the Scheduler. Second, each linecard must be synchronized (approximately) with its ETT1 port
so that it can send TDM cells at the right time and third, so that the linecard will switch TDM tables
synchronously with the ETT1 port.
The first level of synchronization, between ETT1 ports, is handled automatically by the ETT1 Chip Set. All
of the ETT1 devices operate cell synchronously.
The second and third levels of synchronization, between the linecard and ETT1, are handled by the TDM
Sync mechanism. The Scheduler is configured in one of three ways, described below, to periodically send
out a TDM_Sync to all ports. All of the ETT1 ports receive the TDM_Sync signal from the Scheduler at the
same cell time. Upon receiving the TDM_Sync from the Scheduler, every port then (1) flushes any cells
currently in the TDM input queue, (2) transmits a TDM Control Packet to the linecard and (3) starts a
counter which increments every cell time. When this counter reaches the value of the TDM offset
(guardband), the EPP begins comparing the tag of the cell at the head of the TDM FIFO with the entry in
TDM table location 0, and normal TDM processing commences.
The TDM_Sync and TDM Control Packet both include a Frame Select bit to tell the ports and linecards
which TDM table to use. This allows all ports and linecards to switch TDM tables synchronously.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
59
PMC [ PMC-SIERRA, INC ]
