AS Computer Science 9618: How CSMA/CD Handles Ethernet Collisions

Oakwood Community Centre's older classroom LAN connects six computers to a hub. The hub simply repeats every signal it receives out to all of its other ports, so all six computers share the same transmission medium, and only one signal can be present on that medium at any instant.

(a) State what the abbreviation CSMA/CD stands for. [1]

(b) Describe the steps a computer on this hub-based LAN follows, using CSMA/CD, when it wants to transmit data, including what happens if two computers happen to start transmitting data at exactly the same time. [4]

(c) After a collision has been detected, explain why each of the two computers involved waits for a random amount of time before attempting to retransmit its data, rather than retransmitting immediately. [2]

Part (a): What CSMA/CD stands for

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection.

[1 mark] for the correct full expansion of the abbreviation.

Part (b): The CSMA/CD process

Before transmitting, a computer senses (listens to) the shared medium to check whether it is currently carrying another computer’s signal. This is the “carrier sense” part of CSMA/CD.
If the medium is sensed as free, the computer begins transmitting its data, and continues to monitor the medium throughout its own transmission.
If two computers both happen to sense the medium as free at the same moment and start transmitting together, their two signals overlap on the shared medium. This is a collision.
Both computers detect the collision, because the signal actually present on the medium no longer matches what each of them is individually sending. Both immediately stop transmitting, and each sends a short jam signal, so that every other computer on the segment also becomes aware that a collision has just happened.

[4 marks]: [1] for sensing the medium before transmitting, [1] for transmitting once the medium is free while continuing to monitor it, [1] for describing that a collision occurs when two computers transmit together and is detected by both, [1] for both computers stopping and sending a jam signal.

Part (c): Why a random wait before retransmitting

If both computers simply retransmitted straight away after the collision, they would very likely sense the medium as free again at exactly the same instant, since nothing about their situation has changed, and collide a second time, and then a third time, and so on.

Instead, each computer independently picks a random amount of time to wait before attempting to retransmit. Because the two computers choose their waiting times independently, it becomes far less likely that they will both finish waiting and attempt to retransmit at exactly the same moment again. Whichever computer’s random wait happens to be shorter is then far more likely to sense the medium as free, start transmitting first, and successfully claim the shared medium before the other tries again.

[2 marks]: [1] for explaining that immediate retransmission would likely cause a repeat collision, [1] for explaining that an independently chosen random wait time makes both computers retransmitting at the same moment again far less likely.

Final answers

(a) CSMA/CD = Carrier Sense Multiple Access with Collision Detection.
(b) Sense the medium; transmit if free while continuing to monitor; if two computers transmit together their signals collide, which both detect and respond to by stopping and sending a jam signal.
(c) A random wait makes it unlikely both computers will retransmit at exactly the same moment again, reducing the chance of an immediate repeat collision.