DS1 signals can carry the quality level value of the timing source via the SSM transported within the 1544 kb/s signal Extended Super Frame (ESF) Data Link (DL), as specified in Recommendation G.704.
The format of the ESF data link messages is 0xxx xxx0 1111 1111, with the rightmost bit transmitted first. The 6 bits denoted by xxx xxx contain the message; some of these messages are reserved for synchronization messaging. It takes 32 frames (4 ms) to transmit all 16 bits of a complete DL message.
SSM over DS1 ESF is supported on the 7705 SAR-18 via the BITS ports and also on T1 ports on the 16-port T1/E1 ASAP Adapter card, the 32-port T1/E1 ASAP Adapter card, and the 7705 SAR-A, 7705 SAR-M, 7705 SAR-H, and 7705 SAR-X chassis.