E1 signals can carry the quality level value of the timing source information using the SSM, as described in ITU-T Recommendation G.704.
One of the Sa4 to Sa8 bits is allocated for SSMs; choosing the Sa bit that carries the SSM is user-configurable. To prevent ambiguities in pattern recognition, it is necessary to align the first bit (San1) with frame 1 of a G.704 E1 multiframe.
The San bits are numbered (n = 4, 5, 6, 7, 8). A San bit is organized as a 4-bit nibble San1 to San4. San1 is the most significant bit, and San4 is the least significant bit.
The message set in San1 to San4 is a copy of the set defined in SDH bits 5 to 8 of byte S1.