I was assigned to capture the packets which are used in CSMA/CD protocol over the local network.

I have googled around for more than a day but haven't found any solution or related work that negotiate how to set up the environment.

I have tried to simulate by using 3 computers connected to the same router by wired (Airport Extreme) I called them station A, B, and C.

I install and used the wireshark on these stations to capture the packets.

I let the station A and B ping to C for about 2 minutes after that I stopped and saved the traces from all stations.

I have analyzed the packet by using the error, retransmission, and broadcast filter.

But, I haven't seen any collisions.

Did I do something wrong ? Is there any other suggestion for setting up the environment to simulate the CSMA/CD ?

Thank you.

PS. Sorry for my english.

  • How did you conclude that you should be able to see collisions with wireshark? The Retransmissions Filter: tcp.analysis.retransmission has to do with TCP retransmissions, not wired ethernet retransmissions. FYI, if you use 802.11 there is the 802.11 Retry Filter wlan.fc.retry, which should get you much closer to capturing retransmissions (but that doesn't have to be from CSMA/CD in the wireless world). Be sure you use monitor mode in linux if you try to capture 802.11 Jun 26 '14 at 9:42
  • Thank you for the suggestion, @MikePennington. I have a question about wireshark. From you comment, does it mean that wireshark cannot capture the collision packet ? Jun 26 '14 at 13:53
  • It might using the right network interface. Last time I checked starting price was at about $1.000.
    – user2084
    Jun 26 '14 at 14:00
  • CSMA/CD is not a protocol, by the way. It is a media access control method, a sublayer of layer 2 (OSI model) as far as I know. Edit: OK, now researched it a little bit, and confused. Some say it's a protocol, some just say it is a method of MAC protocol... Which is it? Jun 27 '14 at 12:56
  • @ping_smiley could you finally do it? I am trying to do something similar, But I have a hub. It has an LED for collisions. I see it blink often. However, I cannot get anything on wireshark.
    – Lord Loh.
    Apr 6 '17 at 3:50

Well, your assignment alone leaves room for interpretation.

As Ethernet (at least up to GBit-Ethernet) is defined by it use of CSMA/CD, it might be enough to capture just Ethernet frames.

As YLearn noted you wont see any collisions in a switched network (unless something is terribly broken). Even in an environment using Hubs (good luck finding some) or a shared Media like good old 10Base-T you may not see these packets. These packets are most likely filtered out by your network interface. To capture them you have to used specialized Network interfaces.

What you should see is a counter for collision, e.g. with ifconfig on Linux or show interface on Cisco IOS

  • Great point. So long since I have dealt with CSMA/CD and I overlook important facts like they typically get dropped before you ever see them.
    – YLearn
    Jun 26 '14 at 13:20
  • Thank you !! Now I know what are the mistakes I have made. Jun 26 '14 at 13:42
  • s/most likely/*are*/ unless you've reconfigured the chips to present all/errored frames - which means you'll need special drivers or register level specs for the nic chip(s). And by the fact that a collision has occurred, the frame will be mostly junk.
    – Ricky
    Jun 26 '14 at 19:13

Your problem is that you are trying to capture CSMA/CD behavior in a full duplex environment.

If you want to capture this behavior, you need to connect your devices to a hub, not a switch.

  • And use 10 or 100 mbps links. Gig and beyond are full-duplex by design.
    – Ricky
    Jun 26 '14 at 19:10
  • 2
    @rickybeam fyi gigabit ethernet technically can operate in 1000/half however it does require autonegotiation Jun 27 '14 at 2:26
  • I am trying to do this with a hub It has an LED for collisions. I see it blink often. However, I cannot get anything on wireshark.
    – Lord Loh.
    Apr 6 '17 at 3:51
  • @LordLoh., please see the answer from Jens Link.
    – YLearn
    Apr 6 '17 at 6:30

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