The Facility Data Link (FDL), in a T1 (DS1) link, carries the Synchronization Status Message (SSM) that tells other network elements (NE) the quality level (QL) of its clock reference signal.
T1/DS1 Synchronization Status Messages (SSM) indicate the clock source quality level (QL). They are only carried by the Extended Super Frames (ESF) in their Facility Data Link (FDL) channel. The data link is a bit-oriented code (BOC), or Bit-patterned Message (BPM), defined by T1.403. SSMs have the format of 0xxxxxx0 11111111, where xxxxxx are the six information bits of the message. ANSI standard T1.101 lists the SSM codes carried by DS1 timing signals.
Sync Status Messaging (SSM) provides a means for Network Elements downstream from the BITS system to be alerted the Stratum Level of the timing signal used for transmission. With a system of coded messages transmitted “out-of-band” on a T1 ESF or E1 signal, SSM allows Network Elements to follow an automatic, pre-determined process for selecting the timing source of the highest Stratum Level available; thereby eliminating outages due to timing failures. For T1-ESF, the Facility Data Link carriers the SSM codes as shown below.
T1 Sync Status Message - DS1 Data Link/FDL (Binary)
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
For SSM to be useful, connected Network Elements must be configured with instructions for actions to be taken in event of degradation of the SSM code. For example, a normal SSM message of Stratum 1 (PRS) indicates the T1 or E1 signal is traceable back to a Stratum 1 source. While the SSM is not a guarantee of signal quality, it can be used to automatically alert connected Network Equipment if the BITS knowingly becomes degraded.