# Backoff · example 3

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/practice/csma/backoff-3

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/practice/csma--backoff-3/

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



$$
k=\min(n,10),\quad K_{\max}=2^k-1
$$





$$
T=KT_{\mathrm{slot}}
$$



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.

#### What this result means

The range is not the actual delay. Use the stated selected K to calculate the delay.