Purpose of A1, A2 bytes of STM frame

With this post, let's start discussing about each and every byte in STM frame. Following the discussion of every single byte, we will discuss about ho the frames are multiplexed into higher hierarchial levels.

The bytes A1 and A2 doesn't involves any complex calculations. The value stored in A1 is 0xF6  and A2 is 0x28.

What for?

If there are no calculations involved, what is the purpose of these bytes? Here comes the answer. The purpose of these bytes is just to identify the starting of the frame. Think about the receiver, it is receiving 0s and 1s serially in it. If it is so, there must be something with which the starting of the frame must be identified for differentiating every frame. These bytes serves this purpose. The receiver will search for 'F6' and '28' for identifying the frame.

How?

We will just take a glimpse of the practical scenario. The receiver receives everything serially.

The binary value of F6 is 1111 0110 and that of 28 is 0010 1000. According to the standards framed, SDH bytes are sent in LSB first fashion. (i.e.)  1111 0110 0100 0001 is the pattern which the transmitter transmits for F6 and 28. This pattern is detected by the receiver and the frame is decoded.

The SDH is a synchronous technology so it is not necessary that the receiver should detect every frame. Once a frame is detected, rest of the frames will automatically be synchronized with the clock.
Why F6 and 28 in specific?

There is no specific reason for the bytes to be F6 and 28 in specific. It may be because the people ho framed the standard liked the bytes and there couldn't be any other reason.