E1 signals can carry an indication of the quality level of the source generating the timing information using the SSM as specified in Recommendation G.704.
One of the Sa4 to Sa8 bits, (the actual Sa bit is for operator selection), is allocated for Synchronization Status Messages. To prevent ambiguities in pattern recognition, it is necessary to align the first bit (San1) with frame 1 of a G.704 E1 multi-frame.
The numbering of the San (n = 4, 5, 6, 7, 8) bits. A San bit is organized as a 4-bit nibble San1 to San4. San1 is the most significant bit; 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.