ENVS 1200 Lecture 2: ENVS 1200 Tutorial 2 Notes
13 May 2018
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ENVS 1200 Tutorial 2 Notes – Wiring Bussed
Introduction
• Hub-based Ethernet provides a simple means of wiring a bussed Ethernet together
• The hub does not affect the operation logically.
• Any node may use the bus to send a message to another node any time the bus is not in
use
• There is no specific timing control on the bus.
• When a node has a message to send, it listens to see if the bus is in use.
• If not, it begins to send its packet.
• If the bus is already in use, the node waits until the bus is available.
• This is the CSMA part of CSMA/CD.
• CSMA/CD does not try to prevent the occurrence of collisions.
• As the node sends its frame, it continues to listen to the bus.
• If network traffic is light, the node will usually complete sending the frame and will then
return to listening mode.
• Occasionally, two (or more) nodes may sense that the bus is free and initiate
transmission simultaneously.
• A collision occurs, and the message on the bus becomes scrambled, due to the
interference between the two signals.
• Since each node continues to listen as it transmits, it can recognize that the message is
scrambled
• The signal on the bus is not the same as the message it is transmitting.
• When a node detects a collision, it immediately stops transmitting, waits a short time,
then returns to listening mode, and tries again when the bus is free.
• The amount of time each node waits after a collision is random for each node.
• If both nodes waited the same length of time, collisions would continue to recur
indefinitely.
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