# Backoff · example 2

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/practice/csma/backoff-2

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/practice/csma--backoff-2/

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



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





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



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

## Step-by-step solution

#### Step 1. Determine exponent

k=min(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.

#### What this result means

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