Hi I have just completed the opnet CSMA/CD tutorial and my lecturer would like us to run additional simulations and then generate graphs. I know how to do that but what im unsure of is that he wants us to include graphical plots for collision count vs transmission attempts, I'm not sure how to do this part, where do I get the collision count and transmission attempts data from? Sorry for sounding silly but im new to networking so not sure how it all works yet.

Thanks in advance

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Last Post by rubberman

When I taught networking protocols, this is what I did in class - remember that csma-cd means carrier-sense-multiple-access with collision detection. IE, the network interfaces listen for a clear carrier (no data), and then send their next packet onto the wire (this is more broadcast than point-to-point like a switch). They listen for the echo to see if other packets have corrupted it, in which case they wait a random period of time before trying to send it again. The model I used to illustrate collision detection was a bunch of people trying to get through the same door at the same time. If you collide with someone, you back off for a period, and then try to go through again...

As for collision counts, that is the number of times a NIC (network interface controller) tries to send a packet, but detects that it was corrupted by another packet. Transmission attempts would also include the ones that get through without corruption. So, for a specific packet, I would think that transmission attempts would equal collision count + 1, at least under ordinary circumstances.

This is the fundamental algorithm used by the original (non-switched) ethernet. I'm sure you can find good articles about it on Wikipedia. Also, Robert Metcalfe, the inventor of Ethernet at Xerox Parc, is an old friend of mine. When I was selling computers in the Silicon Valley in the early 1980's, I sold Robert's company, 3-Com Systems, their first 100 IBM PC's. :-)

Ok. Here is a good Wikipedia article about the protocol: http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_detection

It basically says the same thing I did above, but with some additional details that may be useful.

Edited by rubberman

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