Solution

First we find the Transmission Delay

= Packet size / Bandwidth

= 64 bytes / 1.536 Mbps

= (64 × 8 bits) / (1.536 × 10⁶ bits per sec)

= 333.33 μsec

Then we find the Propagation Delay

For 1 km, propagation delay = 6 μsec

For 3000 km, propagation delay = 3000 x 6 μsec = 18000 μsec

Value of a is calculated by  :

= T_p / T_t

= 18000 μsec / 333.33 μsec

= 54

Bits required in sequence number field

= ⌈log₂(1+2 x a)⌉

= ⌈log₂(1 + 2 x 54)⌉

= ⌈log₂(109)⌉

= ⌈6.76⌉

= 7 bits

Hence, option 2) is correct.