CSMA/CD

Computer Network & Network Security System — Data Link Layer, NEC licence examination syllabus (Nepal Engineering Council).

CSMA/CD: the algorithm behind classic wired Ethernet

Listen. Talk. Keep listening. If someone else talks too — stop, and try again after a random wait that grows each time.

CSMA/CD solves one problem: several devices share a single cable, any of them may want to talk at any moment, and there is no central authority deciding whose turn it is. The solution is entirely local — every device follows the same rules and no one coordinates.

The sequence, and why each step exists

🎧 The Full Sequence

Carrier senseListen to the channel first, and transmit only if it is idle. Cheap, and it avoids most collisions outright.

Collision detectionKeep listening WHILE transmitting. If what is on the wire differs from what you are sending, someone else started too.

Jam signalOn detecting a collision, transmit a short jam sequence so every other device notices it too, rather than some of them continuing.

Binary exponential backoffWait a random time before retrying, and DOUBLE the range of that random wait after each successive collision.

Carrier sense alone is not enough, and the reason is propagation delay. Two devices at opposite ends of a cable can both listen, both hear silence, and both start transmitting — because neither one's signal has reached the other yet. Collisions are not caused by carelessness; they are caused by the finite speed of the signal.

Why the backoff doubles

After a collision, every device involved wants to retry. If they all waited the same time they would collide again immediately, so the wait is random. But a fixed random range has a subtler failure: as more devices join the contention, the same range gets crowded and collisions keep repeating.

Doubling the range after each collision makes the algorithm adapt to load without measuring it. A first collision might mean bad luck between two devices; a fifth collision means the network is busy, and spreading retries over a much wider window is the correct response.

Binary exponential backoff: After collision number n, wait a random number of slot times chosen from 0 up to (2ⁿ − 1). collision 1 → choose from {0, 1} 2 choices collision 2 → choose from {0, 1, 2, 3} 4 choices collision 3 → choose from {0 … 7} 8 choices collision 4 → choose from {0 … 15} 16 choices The range doubles each time, so the chance of two devices picking the same slot falls sharply.

💡 Two details examiners like. The range grows as a power of two, so the number of choices doubles rather than the maximum wait increasing by a fixed amount. And the growth does not continue forever — after a limit the range stops expanding, and after a further number of attempts the frame is abandoned rather than retried indefinitely, because a network that never gives up on a hopeless frame degrades for everyone.

Why CSMA/CD is largely historical

Everything above assumes a shared medium. Modern wired networks are switched: each device has a dedicated full-duplex link, so it can send and receive simultaneously and there is no one to collide with. CSMA/CD is still specified and still examined, but on a modern switched network it never activates.

Wireless is the opposite case — collisions are possible but cannot be detected, because a radio cannot easily listen while transmitting on the same frequency. That is why wireless uses collision avoidance instead, covered in the wireless LAN chapter.

Commonly confused

CSMA/CD and CSMA/CA. Detection versus avoidance. Wired can detect; wireless cannot, so it avoids.

Carrier sense and collision detection. The first happens before transmitting, the second during. Neither replaces the other.

Backoff and retransmission delay. The backoff is random and grows; it is not a fixed timeout.

Rapid revision

Listen before sending, keep listening while sending, jam, then back off.

Collisions come from propagation delay, not from failing to listen.

Backoff range doubles after each collision — 2ⁿ slots after n collisions — and is capped, with the frame eventually abandoned.

Switched full-duplex Ethernet never collides, so the algorithm is dormant on modern wired networks.

Syllabus points

Carrier sense, collision detection
Binary exponential backoff

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