Excessive Collisions?

Posted on 2000-05-06
Last Modified: 2013-12-07
Whenever I copy files between computers on my home network I seem to be receiving a possibly excessive number of collisions.

For example, I just copied files between our Windows 98 computer and a Linux box, and in one minute there were approximately 3125 collisions.  This is 52 collisions/sec.  Is this excessive?

If so, what can I do about it?

Thank-you for your time.
Question by:deadhead
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Expert Comment

ID: 2784515
I was having problems with database access at work.  I racked my brain and couldn't figure out what was going on.  Finally one day I was watching one of the hubs and noticed the collisions light going nutsy.  I asked around and it was suggested that I get a switch and put between each hub.  I got a switch (I got an 8-port but you could probably get along well with a sub $100 5-port model) and plugged my computer, the server, color copier/printer, ISDN router, and all of the hubs into it.  I haven't had that problem since.  You can get a router from
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Expert Comment

ID: 2784668
That does sound like excessive collisions. With only a few nodes on a local LAN you shouldn't have collision problems unless there's a lot of other traffic occuring at the same time. It's possible that you might have a hardware problem. How are the nodes connected, via a crossover cable or hub? What ethernet cards are you using? Have you tried changing cables or NIC's?

Author Comment

ID: 2784701
There are 4 nodes connected by a hub.  The hub is an Asante FriendlyNet 8-Port.

The network cards are:
- 1 x AMD PCNet32 (Linux)
- 1 x AMD PCNet32 (Win95)
- 1 x AcerLAN ALN-201 PCI (Win98)
- 1 x Intel EtherExpress? (Win95)

The previous number I gave you for collisions was between the AMD PCNet (Linux) and the AcerLAN.  However, I just tried between the two AMD PCNet's and I received around 10964 collision in one minute (183 collisions/sec).

I have tried changing cables, but this does not help.

I have not tried physically changing the network cards within the computers.

All network cards are working in Half-Duplex mode... when I change it to full Duplex the network becomes unusable.

All my network cables are new Cat-5.

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Expert Comment

ID: 2784768
Humm ... first of all is it excessive for ethernet? What is your activity?

Collisions occur on ethernet as the load increases @ about 40 - 50 % you are running flat out. That is to say there are so many collisions that you spend more time dealing with them than you do actually passing data. This is the draw back of std ethernet, it's a digital broadcast medium but the collision detection is unfortunatley analog (CSMACD With BEB) - Carrier Sence Multiple Access Collision Detect with Binary Exponential Back Off.

That basically means sence the wire (to see if any comms if not send frame if a collison is detected then backoff wait a random time and repeat if, another collision then back off of the square of the of the original random number ad infinitum.

The real bummer about the system is that when you do a straight file copy from a to b the acks sent back by the receiver collide with the incomming data. I'm FTP'ing data now between a 98 box and a Linux box guess what the activity is about 5% rate of collisions exceeding 20% Naff init :-) You could tweak things by increasing or decreasing the size of the sliding window used by TCP/IP protocol by gradual amounts untill you find the best performance if you feel up to it.  

Right the simple answer is there is probably nothing wrong @ all. So what's the solution .... Yes buy a switch as Slink9 suggested. This will make things either fast or missile speed depending on the switch type (fast = store and forward Missile speed = Adaptive cut through) These devices get around collisions by setting up dedicated virtual comms channels between machines bit like a phone call. This is why Slink9 is a happy bunny.

Setting your network cards to full duplex was a cool idea as in theory you double the band width available as you can talk both ways at once, however if your hub is not a full duplex hub then all hell would indeed break loose as the hub would have a fit and drop packets all over the place.  

Hope this helps


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Accepted Solution

jlevie earned 50 total points
ID: 2785216

I'll have to take issue with you statements about the collision rates. With only two active nodes on a network doing an FTP transfer you've got serious problems if you are seeing 5% or more in collision rates. For good hardware in that environment the rates ought to be on the order of 0.3-1% for an FTP.

On my local LAN an ftp transfer of 110Mb yielded:

win95 -> Solaris 0.7%
win95 -> Linux 0.6%
Linux -> Solaris 0.4%
Linux -> Linux 0.9%

The typical transfer rate for all combinations was about 895Kb/sec, which pretty good for a 10Mbps HDX network on a sustained basis. And doing an FTP is about the most stress you can apply to a network short of specialized load tests.

Transfering the same file with NFS yielded:

Linux -> Solaris 0.7%
Linux -> Linux .09%

The much lower numbers for NFS aren't much of a surprise given the way the NFS protocol works.

The OS/HDW in the above are:

Solaris 8 beta on an Ultra 10
Win95 OSR2 on a Thinkpad 1472 w/IBM EtherJet
Linux 6.2 on a 500Mhz AMD w/3c905b
Linux 6.1 on a 500Mhz AMD w/FA310Tx

The hub is a run-of the mill Addtron 10Mbps only. None of the TCP/IP stacks have been tuned in any way.

Expert Comment

ID: 2785609
No the whole point is I don't have any problems and the chances are neither does DH.

I'm just watching hub led's if you really want to find out what is going on get a demo copy of Newtork Associates Sniffer Master Pro. That will actually diagnose any problems you may be having for you. Intrestingly enough it found no problems with my set up



Author Comment

ID: 2785979
I had a tough time deciding which comment to accept as an answer.  Taken as a whole, they all answered my question.

Right now I am using Redhat Linux to tell me how many collisions there were before the transfer, and how many there are after; however, I did not consider changing that into a percent.

When calculating the percent, I am receiving approximately 16.0% collisions when transfering 60 mb from one computer to another.

When I looked at the hub, it looked more like 90% were collisions, because the collision like was on solid, and the activity light was flashing.

Thanks all.
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Expert Comment

ID: 2786024
16% seems a bit high for that size of file, but it still (as sime788 pointed out) really isn't going to kill transfer rates. How were you doing the file transfer (FTP or Netbui over TCP)?

Author Comment

ID: 2786671
Netbui over TCP
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Expert Comment

ID: 2788090
That's probably ok for Netbui.

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