Computer Networks · TEE · practice

Backoff · example 2

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 3 collisions for its current frame. It selects K=5; one slot lasts 51.2 μs. Under binary exponential backoff, find the allowed K range and the selected delay. [3 practice marks]

Answer

Given

n=3; K=5; Tslot=51.2 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(3,10)=3.

Step 2. Find the range

23=8; largest K=8−1=7. Range: 0 to 7.

Step 3. Validate selected K

5 is within 0…7.

Step 4. Calculate delay

5×51.2=256 μs.

Final answer: K may be 0…7; selected delay=256 μs.