SBSLITE Telecom Standard Product Data Sheet
Preliminary
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-2010883, Issue 2
254
13 Operation
There are several important aspects regarding the operation of NSE-based switch fabrics; these
are dealt with in turn in the following sections.
13.1
“C1” Synchronization.
Any NSE/SBS fabric can be viewed as a collection of five “columns” of devices:
column 0 consists of the ingress flow from the load devices (e.g., some SBI device)
column 1 consists of the ingress flow through the SBS devices
column 2 consists of the NSE-20G device
column 3 consists of the egress flow through the SBS devices
column 4 consists of the egress flow through the load devices (e.g. some SBI device)
Note that the devices in columns 0 and 4 are SBI bus devices while columns 1 and 3 are SBS or
SBSLITE devices. The dual column references refer to their two separate simplex flows. Path-
aligned STS-12 frames are pipelined through this structure in a regular fashion, under control of a
single clock source and frame pulse. There are latencies between these columns, and these
latencies may vary from path to path. The following design is used to accommodate these
latencies.
A timing pulse for SBI frames (2 KHz, 500
=
μ
s) is generated and fed to each device in the fabric.
Each chip has a
FrameDelay
register (RC1DLY) which contains the count of 77.76 MHz clock
ticks that device should delay from the reference timing pulse before expecting the C1 characters
of the ingress STS-12 frames to have arrived. The base timing pulse is called
t
. The delays from
t
based on the settings of the RC1DLY registers in the successive columns of the devices are called
t0, … t4
. The first signal, t
1
(equal to t
0
), determines the start of an STS-12 frame; this signal is
used to instruct the ingress load devices (column 0) to start emitting an STS-12 frame (with its
special “C1” control character) at that time. ti is determined by the customer, based on device
and wiring delays to be approximately the earliest time that all “C1” characters will have arrived
in the ingress FIFOs of the ti column of devices. ti is selected to provide assurance that all “C1”
characters have arrived at the i
th
column. The i
th
column of devices use the ti signal to
synchronize emission of the STS-12 frames. The ingress FIFOs permit a variable latency in C1
arrival of up to 24 clock cycles.
Note: SBS devices, being a memory switches, add a latency of one complete frame plus a few
clock ticks to the data.