Station A is sending data to station B over a full duplex error free channel. A sliding window protocol is being used for flow control. The send and receive window sizes are 6 frames each. Each frame is 1200 bytes long and the transmission time for such a frame is 70 μS. Acknowledgment frames sent by B to A are very small and require negligible transmission time. The propagation delay over the link is 300 μS. What is the maximum achievable throughput in this communication?
To compute the throughput we need to check the performance of the system in long term, not just at the starting. Thus we need to find a regular pattern of the transmission that will be repeated for infinitely many times.
Here you didn't mention the details of sliding window protocol, thus I'll assume the worst case scenario. I assume that the receiver don't send the ACK until it gets all of the 6 frames.
The first bit will be reached to the receiver after 300 $\mu$S. After 70 $\mu$S the first packet will be reached. And the other five packets would take 350 $\mu$S more. Then the receiver will send the ACK that will take 300 $\mu$S to reach to the sender. Sender can start transmission only after getting this ACK and a new cycle starts. This pattern is repeated for ever.
Now, you can see only productive time in this 300+420+300 $\mu$S interval is 420 $\mu$S. Hence the throughput is 420/1020 = .41