Solution

The correct answer is Option 3) 30,000 bits

Key Ideas

• Bandwidth–delay product (BDP) is the amount of data that can be “in flight” on the link:
  BDP = Bandwidth × Round-Trip Time (RTT)
• Bandwidth is given as 1 Mbps = 1,000,000 bits/second.
• “1 bit takes 30 ms to make a round trip” means the RTT = 30 ms = 0.03 s.

Step-by-step Calculation

1. Convert units: 30 ms → 0.03 s.
2. Apply the formula:
   BDP = 1,000,000 (bits/s) × 0.03 (s) = 30,000 bits

Why this matters

• BDP tells you the pipeline capacity: to keep a 1 Mbps link fully busy, up to 30,000 bits can be on the wire before the first acknowledgement returns.
• In stop-and-wait, if the frame size is much smaller than the BDP, the link stays idle while waiting for ACKs, reducing utilization.

Why other options are incorrect

• 15,000 bits / 20,000 bits / 60,000 bits: These come from using the wrong time (half-RTT, some other value) or wrong bandwidth unit. The given RTT is 30 ms round trip, and the bandwidth is 1,000,000 bits/s, so only 30,000 bits fits the formula exactly.