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

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The Cisco ONS 15454 is designed to operate in a network that complies with recommendations 
stated in GR-253-CORE and GR-436-CORE. 
 

1.  SONET timing parameters must be set for each ONS 15454.  Each ONS 15454 

independently accepts its timing reference from one of three sources: 

 

2.  The BITS (building integrated timing supply) pins on the ONS 15454 backplane. 

 

3.  An OC-N card installed in the ONS 15454.  The card is connected to a node that receives 

timing through a BITS source. 

 

4.  The internal ST3 clock on the TCC+/TCC2 card. 

 
You can set ONS 15454 timing to one of three modes: external, line, or mixed.  If timing is coming 
from the BITS pins, set the ONS 15454 timing to external.  If the timing comes from an OC-N 
card, set the timing to line.  Node timing for a typical ONS 15454 network would be as follows: 
 

    One node will be set to external timing.  The external node will derive its timing from a 

BITS source wired to the BITS backplane pins.  The BITS source will derive its timing from 
a Primary Reference Source (PRS) such as a Stratum 1 clock or GPS signal. 

 

    The other ONS 15454 nodes will be set to line.  The line nodes will derive timing from the 

externally timed ONS 15454 node through the OC-N trunk (span) cards. 

 

Mixed timing allows you to select both external and line timing sources.  However, Cisco 

does not recommend its use because it can create timing loops. Use this mode with caution. 
 
You can set three timing references for each ONS 15454.  The first two references are typically 
two BITS-level sources, or two line-level sources optically connected to a node with a BITS 
source.  The third reference is the internal clock provided on every ONS 15454 TCC+/TCC2 card.  
This clock is a Stratum 3 (ST3).  If an ONS 15454 becomes isolated, timing is maintained at the 
ST3 level. 
 

 
Synchronization status messaging (SSM) is a SONET protocol that communicates information 
about the quality of the timing source.  SSM messages are carried on the S1 byte of the SONET 
Line layer.  They enable SONET devices to automatically select the highest quality timing 
reference and to avoid timing loops. 
 
SSM messages are either Generation 1 or Generation 2.  Generation 1 is the first and most 
widely deployed SSM message set.  Generation 2 is a newer version.  If you enable SSM for the 
ONS 15454, consult your timing reference documentation to determine which message set to 
use.  Table 7-18 and Table 7-19 show the Generation 1 and Generation 2 message sets.