What is the function of the AJB bits on DS1 using Extended Super Frame (ESF)?

A.
It is used for signaling on-hook and off-hook status.

B.
It carries CRC information for error detection.

C.
It is used for a control and performance information.

D.
It is used for a frame’s synchronization and alignment.

Explanation:
DS1 frame synchronization[edit source | edit]
See also: Synchronization in telecommunications
Frame synchronization is necessary to identify the timeslots within each 24-channel frame.
Synchronization takes place by allocating a framing, or 193rd, bit. This results in 8 kbit/s of framing

data, for each DS1.
Because this 8-kbit/s channel is used by the transmitting equipment as overhead, only 1.536
Mbit/s is actually passed on to the user. Two types of framing schemes are Super Frame (SF) and
Extended Super Frame (ESF). A Super Frame consists of twelve consecutive 193-bit frames,
whereas an Extended Super Frame consists of twenty-four consecutive 193-bit frames of data.
Due to the unique bit sequences exchanged, the framing schemes are not compatible with each
other. These two types of framing (SF, and ESF) use their 8 kbit/s framing channel in different
ways.