cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
1383
Views
0
Helpful
4
Replies

Collisions, The truth

wrwiii122
Level 1
Level 1

You can search all day long and not find a direct answer about collisions and NIC settings. Why not always just use full duplex? Doesn't it just talk and receive at the same time on doifferent wires. What is the threshold of collions? Before the switches collisions were normal. Why are there still collision and when is it a problem?

4 Replies 4

thomas.chen
Level 6
Level 6

In half duplex operation the transmit and receive pairs of the NIC are looped (short circuited) in order to detect collision. So when a NIC sends out a frame it loops the sent frame on to its receive pair as well.This concept can be explained with a hub scenario wherein you have say four PC's (PC1, PC2, PC3, PC4) connected to a hub. Consider a situation where PC2 and PC4 are transmitting at the same time. The hub repeats the frame on all its ports except the source port. The signal sent by PC2 is sent to PC4 on its receive pair, and similarly the signal sent by PC4 is sent to PC2 on its receive pair. Let us now see how PC2 detects the collison. Since the receive pair is already looped with the transmit pair, the incoming signal from PC4 plus the looped signal lets PC2 know that there is a collison. Thats the reason in Half duplex operation the NIC cant send and receive at the same time

Right! That makes sense. So why use half duplex anymore and why are there still collisions when using 3550/2950's in full duplex? What are they caused from? That is a great explination above.

You should have collissions if the switchport and the connected device are both set to full duplex. If you have them something is seriously wrong.

Hans

Excuse me typo, ofcourse I meant to state that: "You shouldn't have collissions."

Hans

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: