How Does T1 (DS1) FDL SSM Clock Quality Signaling Work?

DS1/T1 SSM messages indicate the Link or Network clock source Quality Level (QL). In T1 links, they are only carried by the Extended Super Frames' (ESF) Facility Data Link (FDL) channel. The Data Link is a bit-oriented code (BOC)

Synchronization Status Messages (SSM) have the format of 0xxxxxx0 11111111 (right-to-left), where xxxxxx are the six information bits of the message (or status indicator). ANSI standard T1.101 lists the SSM codes to be carried by DS1 timing signals.

The SSM provides the means for Network Elements (NE) downstream from the reference clock, or Building Integrated Timing Supply (BITS), system to be informed the Stratum Level or Quality Level (QL) of the timing signal. With a system of coded messages, transmitted “out-of-band” on the T1 ESF framing (or E1 Sa bits), SSM allows Network Elements to follow an automatic, pre-determined process for selecting the timing source of the highest Stratum Level available; thereby eliminating degradations or outages due to timing distribution failures. 

For SSM to be useful, connected Network Elements must be configured with instructions for actions to be taken in the event of clock quality changes and degradation, indicated by the SSM code. For example, a normal SSM message of Stratum 1 Primary Reference Signal (PRS). or Primary Reference Clock (PRC), indicates that the T1 or E1 signal is traceable back to a Stratum 1 frequency reference source. While the SSM is not a guarantee of signal quality, it can be used to automatically alert connected Network Equipment if the BITS becomes degraded (e.g., due to GNSS reference loss).

Any network element that experiences a condition that degrades its physical layer clock quality, must adjust its SSM QL status to notify the other NEs downstream about the expected quality of its transmit clock.

T1/DS1 Sync Status Message Data Link/FDL (Binary)

For T1 Enhanced Super Frame (ESF), the Facility Data Link (FDL) carries the SSM codes as shown below:

00000100 11111111   Stratum 1 (PRS) Traceable

00001000 11111111   Sync Traceability Unknown (STU)

00001100 11111111   Stratum 2 Traceable

01111100 11111111   Stratum 3E Traceable

00010000 11111111   Stratum 3 Traceable

00101000 11111111   Stratum 4 Traceable

01111000 11111111   Transit Node Clock

00110000 11111111   Do Not Use for Sync (e.g., free-running crystal oscillator)

In this context, "Traceable" means tied to (following)a higher accuracy reference/source, such as a calibrated atomic clock, disciplined by GNSS (a.k.a., GPS) or following a higher accuracy and stable clock source.

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