CSMA/CD — Collision Detection in Ethernet Networks for GCSE

Why is a collision-avoidance protocol necessary?

In an early Ethernet network connected by a hub, all devices shared the same electrical cable. When one device transmitted, every other device received the signal — including all the bits in that frame. If two devices transmitted at the same moment, their signals overlapped and became unreadable. This overlap is a collision.

Without a protocol, devices would simply retransmit immediately, causing another collision, and the network would never successfully deliver a frame. CSMA/CD is the solution.

What does each part of "CSMA/CD" mean?

How does the CSMA/CD process work step by step?

1. Listen before transmitting — the device checks the cable for a carrier signal. If the medium is busy, it waits until it becomes idle.
2. Transmit — once the medium is idle, the device begins sending its frame.
3. Monitor while transmitting — the device continues to listen as it sends. If the voltage on the cable differs from what it is sending, a collision has occurred.
4. Detect collision and jam — on detecting a collision, the device immediately sends a jam signal (a 32-bit burst) to ensure all devices on the network are aware that a collision has occurred.
5. Stop and wait — all transmitting devices stop.
6. Randomised backoff — each device waits for a random period before attempting to retransmit. The random delay is chosen using exponential backoff: after the kth collision, the wait is a random number of time slots between 0 and 2^k − 1. This reduces the probability of another immediate collision.
7. Retry — after the backoff period, the device returns to step 1.

What is a collision domain?

A collision domain is the set of network devices that can cause a collision with each other — i.e., devices that share the same physical medium. In a hub-based network, every device belongs to the same collision domain: any two can collide.

A switch breaks the network into separate collision domains — one per port. Because a switch stores and forwards frames individually (rather than broadcasting electrically to all ports), two conversations can happen simultaneously without collision. This is why switches replaced hubs in almost all networks.

Is CSMA/CD still used today?

Modern switched Ethernet uses full-duplex connections: each device has a dedicated connection to the switch, with separate transmit and receive wires. This means a device can transmit and receive simultaneously, and collisions are physically impossible. CSMA/CD therefore has no work to do on a modern switched network and is disabled.

CSMA/CD survives in:

• Legacy half-duplex Ethernet — older installations using hubs.
• Some industrial Ethernet — older factory networks.
• Academic study — CSMA/CD is in GCSE and A-level specifications because it illustrates fundamental networking principles (contention, arbitration, backoff).

A related protocol, CSMA/CA (Collision Avoidance), is used in Wi-Fi networks, where collision detection is not practical (a device cannot listen while transmitting on a wireless medium). CSMA/CA devices announce their intention to transmit and wait for explicit clearance, rather than detecting collisions after they occur.

Frequently asked questions

Why can't devices always just check whether the medium is clear before transmitting?

They do — that is the "Carrier Sense" part. But two devices can both check simultaneously, both find the medium idle, and both start transmitting at the same instant. The tiny propagation delay (the time the signal takes to travel down the cable) means a device cannot know another has started transmitting until the signal arrives. For long cables, this propagation window is larger, which is why CSMA/CD imposes a minimum frame size (64 bytes): the frame must be long enough that a collision is detected before the device finishes transmitting.

What is the difference between CSMA/CD and CSMA/CA?

CSMA/CD (used in wired Ethernet) detects collisions after they happen and uses backoff to recover. CSMA/CA (used in Wi-Fi, 802.11) tries to avoid collisions before they happen by using random delays before transmission and, in some modes, explicit request-to-send / clear-to-send (RTS/CTS) handshakes. CA is needed on wireless networks because a device physically cannot transmit and receive simultaneously on the same frequency, making collision detection impossible.

Why do GCSE specifications still teach CSMA/CD if it is no longer used in modern networks?

CSMA/CD illustrates several important networking concepts: the nature of shared media, contention, non-deterministic access, and probabilistic backoff. These ideas generalise to wireless networking (CSMA/CA), satellite communication, and any system where multiple agents compete for a shared resource. Understanding CSMA/CD also makes the reason for switching and full-duplex Ethernet clear — you can explain what problem switches solved.

How does exponential backoff prevent the network from being permanently paralysed by collisions?

If all devices backed off for the same fixed time, they would retransmit simultaneously and collide again. Random backoff means the probability of two devices choosing the same slot decreases as the slot range grows. Exponential growth of the range ensures that even under sustained heavy load, the chance of repeated collisions falls rapidly. The 16-collision limit prevents devices from waiting indefinitely — after that, the frame is dropped and the error is reported to higher layers.

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Want to trace through the CSMA/CD process with your own worked examples? Professor Turing at aitutors.me can walk you through every collision and backoff step.