How is overhead and signalling data calculated for T-hierarchy?

While calculating data rate/speed of T-carrier system/T-multiplexer output, we add some overhead and signalling data rate. How are they calculated?

eg-:

1) T1-multiplexer ouptut-:

24 channels of 8-bit PCMed voice signal sampled at 8000 samples/sec are multiplexed into one T1 carrier system.

So,

24*8+1=193 bits in 1 frame.

speed=193*8000

=1.544 Mbps (OK)

2) T2-multiplexer output-:

4 T1 carrier signals are multiplexed. i.e-:

1.544*46.176 (should be this, according to my calculation) BUT the speed is 6.3120 Mbps(0.136 Mbps extra for overhead and signalling data extra).

3) T3-multiplexer ouptut- -:

7 T2 carrier signals are multiplexed into one T3-carrier signal. i,.e

7*6.312=44.184(should be this, according to my calculation) BUT the speed is 44.736 Mbps(0.552 Mbps extra for overhead and signalling).

4) T4-multiplexer ouptut--:

6 T3-signals are multiplexed into one T4 carrier signals.

6*44.736=268.426 Mbps (should be this, according to my calculation)

BUT the speed is 274.176(5.76 Mbps extra)

My question-:

How are they calculated? How are these overhead and signalling data rates calculated?

and

Why are they calculated? While calculating 1.544 Mbps as a T1 multiplexer output,we have already added a synchronization bit. Isn't it?

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