How to use the Sliding Window Protocol Efficiency Calculator
- 1. Describe the linkEnter bandwidth, frame size and the propagation delay, or distance and signal speed.
- 2. Pick the protocolChoose Stop-and-Wait, Go-Back-N or Selective Repeat and enter the window size.
- 3. Read the resultSee Tt, Tp, a, efficiency, throughput, the window needed for full use and sequence bits.
Efficiency formulas
Let Tt = frame size ÷ bandwidth (transmission time) and Tp = distance ÷ signal speed (propagation time). The key ratio is a = Tp ÷ Tt. Ignoring ack and processing time, one frame's cycle takes Tt + 2Tp.
- Stop-and-Wait: efficiency = 1 ÷ (1 + 2a).
- Go-Back-N and Selective Repeat with window N, error-free: efficiency = N ÷ (1 + 2a), up to 100%.
- A window of 1 + 2a frames keeps the link fully busy.
- Throughput = efficiency × bandwidth.
With m sequence bits, Go-Back-N allows a window of at most 2m − 1 frames. Selective Repeat allows at most 2m−1, so new frames are never confused with old ones.
Worked example
A 1 Mbps link sends 1,000-bit frames with a 10 ms one-way delay. Tt = 1000 ÷ 106 = 1 ms, so a = 10.
- Stop-and-Wait: 1 ÷ 21 = 4.76%, or about 47.6 kbps of useful throughput.
- Go-Back-N with N = 7: 7 ÷ 21 = 33.3%.
- Full use needs a window of 1 + 2a = 21 frames. Go-Back-N then needs 5 sequence bits (25 − 1 = 31 ≥ 21). Selective Repeat needs 6 bits (25 = 32 ≥ 21).
These are the tool's default values.
With frame errors
If each frame is lost with probability P, Stop-and-Wait drops to (1 − P) ÷ (1 + 2a). Selective Repeat resends only the lost frame. Go-Back-N resends the whole window, so it loses more at high error rates. The tool uses Stallings' formulas for these cases. Some books write Stop-and-Wait efficiency as Tt ÷ (Tt + 2Tp), which is the same value. For error detection, see the CRC calculator and Hamming code calculator.
Frequently asked questions
What is a in sliding window problems?
a is propagation time divided by transmission time. A large a means the link sits idle while waiting for acks, so a bigger window is needed.
Why is the Selective Repeat window only half the sequence space?
If it were larger, a resent old frame could look like a new frame with the same number after wrap-around.
Does the tool include ack transmission time?
No. Like most textbook problems, it treats acks as tiny. Add the ack time to the delay yourself if the question needs it.
What propagation speed should I use?
Use the value in the question. If none is given, 2 × 10^8 m/s (200,000 km/s) is the usual figure for copper and fibre.