DS1 signals can carry an indication of the quality level of the source generating the timing information using the SSM transported within the 1544 kb/s Extended Super Frame (ESF) Data Link (DL) of the signal as specified in Recommendation G.704. No such provision is extended to SF formatted DS1 signals.
The format of the data link messages in ESF frame format is "0xxx xxx0 1111 1111", transmitted rightmost bit first. The six bits denoted "xxx xxx" contain the actual message; some of these messages are reserved for synchronization messaging. It takes 32 frames (such as 4 ms) to transmit all 16 bits of a complete DL.