# Q1A · Carrier sensing and collisions

Subject: Computer Networks

Exam: M2

Reader: https://deep-learning-study-room.vercel.app/cn/#m2/mocks/mock-a/q1a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/m2/mocks/mock-a--q1a/

Q1(a)+Q1(b) compulsory; choose any two of Q2/Q3/Q4 · 10 marks attempted

### Question. Why does carrier sensing reduce but not eliminate collisions? [2 practice marks]

#### Answer

Carrier sensing observes the local medium. Because signals have nonzero propagation delay, another station’s new transmission may not yet be detectable. Both stations can then start and their transmissions overlap.

#### Practical example

A starts at time 0. B is far away and starts a moment later, before A’s signal reaches it. B heard silence, but the two transmissions can still collide.