Cisco Cisco ONS 15454 SONET Multiservice Provisioning Platform (MSPP) Guía De Diseño

OL-4556-01

 

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Cesium Clock 
 
Local Cesium clocks (often referred to as atomic clocks) can also be used to provide stratum 1 
quality synchronization.  The advantage of a Cesium clock is that it never needs recalibration.  
However, the cost for each unit is very high. 
 
Global Positioning System (GPS) 
 
Compared to Cesium clocks, the Navstart Global Positioning Satellites provide a lower cost 
alternative source for network synchronization.  These satellites are accessible throughout North 
America and have internal cesium clocks that can be used as a Stratum 1 source.  A GPS 
receiver costs much less than a Cesium clock. 
 

 
The Cisco ONS 15454 is designed to operate in networks compliant with the Telcordia GR-253-
CORE, and GR-436-CORE. 
 
Timing guidelines for the ONS 15454 evolve around the following conditions: 
 

    Where BITS timing is available, configure the ONS 15454 node to be externally timed 

from the BITS clock. 

 

    Where no BITS timing is available, configure the ONS 15454 node to be lined timed from 

an OC-N signal. 

 

    Where both external and line time references are to be used, configure the ONS 15454 for 

mixed-timing. 

 

 
Timing and synchronization in the ONS 15454 is controlled by the Timing Control Card (TCC), 
which is stratum 3 compliant.  A redundant architecture protects against failure or removal of one 
TCC card.  For timing reliability, the TCC card selects either a recovered clock, a BITS clock, or 
an internal stratum as the system timing reference.  You can provision any of the clock inputs as 
primary or secondary timing sources.  If you identify two timing references,  the secondary 
reference provides protection.  A slow-reference tracking loop allows the TCC to track the 
selected timing reference and synchronize to the recovered clock and provide holdover in the 
event the reference is lost.  In a fail-over scenario, selection of the next timing reference is 
governed by the availability of the next best (clock quality) timing reference as defined by the 
Stratum hierarchy (discussed in the next section).  The timing modes available on the ONS 15454 
include the following: 
 

    External (BITS) timing 

    Line (Optical) timing 

    Mixed (both External and Internal) sources 

    Holdover (automatically provided when all references fail) 

    Free-running (a special case of holdover) 

 

 Timing loops can be created when you select both external and line timing references.  

Use Mixed mode of timing with caution.