System Management
260
Basic System Configuration Guide
3HE 11010 AAAC TQZZA Edition: 01
6.4.9 Synchronization Status Messaging with Quality
Level Selection
Synchronization Status Messaging (SSM) provides a mechanism for downstream
network elements to determine the quality level of the source.
The quality level values are processed by the 7705 SAR system timing module
(SSU) to track the network timing flow and select the highest-quality source. The
selection process is described in Timing Reference Selection Based on Quality
Level. Also see Figure 23. SSM also allows the network elements to autonomously
reconfigure the timing path to select the best possible source for timing and to avoid
timing loops. This function is especially useful in a ring topology where network
timing may be passed in both directions around the ring.
Synchronization status messages containing the quality level values are placed in
prescribed overhead bytes for SONET and SDH signals and in bit-oriented
messages within the data link for DS1 (ESF) and E1 physical ports.
For synchronous Ethernet and DSL interfaces, there is no equivalent fixed location
to convey synchronization status messages; therefore, the quality level values are
transported using Ethernet frames over a message channel. This channel, called the
Ethernet Synchronization Message Channel (ESMC), uses an Ethernet protocol
based on an IEEE Organization Specific Slow Protocol (OSSP). The 4-bit quality
level value is carried within a Type-Length-Value (TLV) byte of an Ethernet OAM
Protocol Data Unit (PDU) that uses the OSSP subtype.
The clock source quality levels identified for the purpose of tracking network timing
flow are listed below. They make up all of the defined network deployment options
given in Recommendations G.803 and G.781 (option I pertains to the SDH model
and Option II pertains to the SONET model).
The received quality level values for the two network options based on the specific
interfaces within these options are provided in the first two columns of Table 30 (for
SONET, SDH, and Synchronous Ethernet interfaces) and Table 31 (for E1 and T1
interfaces). The transmitted quality level values are shown in the last two columns of
Table 30 and Table 31.
• prs — SONET Primary Reference Source Traceable
• stu — SONET Synchronous Traceability Unknown
• st2 — SONET Stratum 2 Traceable
• tnc — SONET Transit Node Clock Traceable
• st3e — SONET Stratum 3E Traceable
• st3 — SONET Stratum 3 Traceable
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