Explain how CSMA/CD is implemented in Ethernet. You may express your answer as an algorithm if you wish.
Try an answer before revealing the guidance below.
Key Concepts
- Carrier sense
- Collision detection
- Jam signal
- Binary exponential backoff
- K × 512 bit times
Answer Approach
- Start with the adapter receiving a datagram and building a frame.
- Describe carrier sensing before transmission.
- Describe what happens during transmission if a collision is detected.
- Describe how the backoff interval is chosen after repeated collisions.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
1. The adapter receives a datagram from the network layer and creates a frame. 2. It senses the channel: if idle it starts transmitting; if busy it waits until the channel is idle, then transmits. 3. If the whole frame is sent without detecting another transmission, it is done. 4. If it detects another transmission while sending, it aborts and sends a 48-bit jam signal so all stations notice the collision. 5. After the m-th collision it enters binary exponential backoff: it picks K at random from {0, 1, …, 2^m − 1} (m capped at 10) and waits K × 512 bit times, then returns to step 2. After too many attempts (16) it gives up.
Backoff = K × 512 bit times, K ∈ {0, …, 2^min(m,10) − 1}Shortcuts: K concepts · A approach · F answer · R reviewed · B bookmark · ← / → previous / next