Computer Networks · TEE · practice

Backoff · example 3

CSMA, collision detection and avoidance · optional practice

Optional extension · no matching calculation in the two supplied papers

Optional worked extension. This calculation format was not found in the supplied December 2025 final or February 2026 re-exam. The numbers and practice marks are authored. Study the core concept and actual-paper questions first; this is extra explanation, not a TEE prediction.

Practice question. A station has experienced 5 collisions for its current frame. It selects K=17; one slot lasts 5.12 μs. Under binary exponential backoff, find the allowed K range and the selected delay. [3 practice marks]

Answer

Given

n=5; K=17; Tslot=5.12 μs.

To find

The random range and the delay for the selected K.

Formula and why

k=\min(n,10),\quad K_{\max}=2^k-1LaTeX: k=\min(n,10),\quad K_{\max}=2^k-1
T=KT_{\mathrm{slot}}LaTeX: T=KT_{\mathrm{slot}}

Count starts at zero for K. The selected K must fit the allowed range.

Step-by-step solution

Step 1. Determine exponent

k=min(5,10)=5.

Step 2. Find the range

25=32; largest K=32−1=31. Range: 0 to 31.

Step 3. Validate selected K

17 is within 0…31.

Step 4. Calculate delay

17×5.12=87.04 μs.

Final answer: K may be 0…31; selected delay=87.04 μs.