DS1 signals can carry the quality level value of the timing source information using the SSM that is transported within the 1544 kb/s signal Extended Super Frame (ESF) Data Link (DL), as described in ITU-T Recommendation G.704. No such provision is extended to SF formatted DS1 signals.
The format of the ESF DL 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.