Collisions

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Ethernet
IPv4
IPv6
Networking
If I?m experiencing a high number of collisions on a 802.3 network what could they be and how can I solve them.
ASKED: February 23, 2005  2:45 PM
UPDATED: February 24, 2005  10:56 PM

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Sorry to seem a little hard-nosed, but you really need to give us (collectively) more information.

When you say 802.3, that means Ethernet (CSMA/CD) at 10 megabit on a coaxial network – that being the original 802.3. If you mean something else, then you need to specify exactly what you do mean.

If that is really the case though, how do you know? What indication is telling you about the collisions? If it’s a coax network, have you tried segmenting it by inserting terminators to isolate the problem – strictly as a temporary troubleshooting measure? I could go further, but I’ll wait until you confirm the precise nature of your layout.

Since coax is getting rare these days, then let’s lay out the other possibilities.

If it’s switched ethernet (of any speed), do you have a managed or unmanaged switch? A managed switch will allow you to identify the precise port from which the collisions are originating. As for how-to, that depends on the brand and model of the switch. Not even going to try without that info.

Lacking that (a managed switch), if you have status lights, which are indicating the problem, then unplug one station at a time until the status lights stop indicating collisions. Then go investigate what’s on the other end of that wire.

Also – Please feed back the answers to the group as a whole. One of the key reasons that we’re part of this group is so that we can learn from the experiences of others. Questions that offer little in the way of information are likely to get ignored.

Bob

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  • Jaysea
    A good read, check out the link: http://searchnetworking.techtarget.com/infoCenter/tip/0,294276,sid7_gci1043332_tax300805_iid2651,00.html
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  • Bobkberg
    Just to fill everyone in, a private exchange between youngs and me confirmed that this IS a coaxial network. On with the troubleshooting...at this point, I'm going to go waaaay back in history, since I know nothing else about the network other than that it's coaxial. 1) Are there any repeaters (layer 1) devices on the network? 2) Is there thick-net (10Base5 typically yellow or orange, usually used above drop ceilings) cabling on the network? 3) If there is thick-net, are there any "vampire-taps" onto the network (Usually above a suspended/dropped ceiling). The vampire tap is usually part of an external transceiver, and drop an AUI cable (Originally known as a DIX cable (DEC, Intel, Xerox) which uses a 15-pin D-Shell connector which closely resembles a PC game port. 4) Is there any thin-net (10Base2) cabling on the network? In the case of thin-net cabling, the connection to the workstations is done in 2 ways: Either with an external transciever which attaches between coaxial segments by the use of BNC connectors, or with a BNC "Tee" connector which connects directly to a matching BNC connector on the back of the workstation (or other device). If you have repeaters, then the number one rule is that the total length of the ethernet segment (including repeaters) must NOT EXCEED 500 meters (1625 feet) for thick net, or 200 meters (650 feet) for thin-net. For a mixed network, the number will be somewhere in-between. Now - let's classify collisions There are two general types of collisions - normal ones, where both parties back off since they started sending at the same time, reflective collisions (technically not a collision) and "late collisions" where the timing of the signals could not prevent a collision because the total length of the ethernet exceeded the timing specifications. This latter (late collisions) type would be the first thing I'd look for, because people who don't know any better simply decide to extend the network much as most folks would use an extension cord. However this WILL BREAK the network. Another problem - very similar is when someone attaches a LONG TEE section to the network - again like the late collision problem, but all TEE connections off the main ethernet bus (and yes, it's a bus) must be 6" or less. Anything longer than that introduces reflections, and makes all the traffic look like gibberish from the point of view of the ethernet hardware. Think of it as something akin to "ghosts" on your TV set, since those are also caused by signal reflections. The first type (ordinary collisions) is considered part of the automatic operation of the network, unless one or more stations starts "jabbering" (and yes, that's a technical term) meaning that it transmits garbage, or traffic, but without bothering to check whether the line is clear or not. One more background item - the ethernet bus is a 50 Ohm coaxial cable - terminated by 50 Ohm resistors on each physical end of the cable. There must those two terminators and ONLY those two terminators on the entire network. In any event, troubleshooting proceeds as follows: a) Find a point approximately at the middle of the network, and disconnect it - and then terminate each of the open ends. This temporarily creates two networks, which, of course, cannot communicate. If there are still collisions on either side, then find the middle of the side with collisions (Not exactly the middle - what we're doing here is a binary search or "divide and conquer") and repeat the process, until it's narrowed down. b) If all collisions cease when you split the network, then it's likely that the network is extended beyond the distance permitted in the specification. At this point, your simplest option is to place a bridge (layer 2 device) in the middle of the network, and that will be that. c) If one side continues to see collisions, also look for any "TEE" cables that have been placed on the network in a misguided attempt to extend the reach. If this doesn't cover what you need, then write back. Bob
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  • Drmikec
    Good job, Bob. I had to recall my ancient history, also. Only one other possibility: connectors. The BNC connectors on ThinNet are usually crimped on, but there are screw-on connectors available. In my experience, screw-on means screw-up, and both can be pulled loose so there is an intermittent connection on the center conductor, or the ground sheath is crossing the dielectric and touching the center connector. Check your connectors. HTH. Mike
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  • Bobkberg
    Thanks drmikec. That reminds me of a "funny" story from years back. I worked at 3Com at the time, and they had donated a bunch of servers to some local schools. In following up, one school told me that our servers were defective, since they couldn't connect more than 20 workstations at a time. To summarize: I went up to the school with a technician from the workstation vendor, and they were "right" - the server would crash when more than 20 were connected - the workstation vendor had built the entire computer lab network with screw-on BNC connectors!
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  • Astronomer
    Bob's technique is almost guarenteed to find the problem but I wanted to add one more datum. We used thin net exclusively for several years at one of the Intel campuses and it worked pretty well. Near the end we discovered some collision problems in several segments. It turned out to be the quality of the terminators. They had become intermittent. When a terminator gets flakey the collisions can go thru the roof since the signal isn't absorbed at the end of the cable. If you continue to have problems you might try a different brand of terminator on the ends. Also the limit for thin net is 185 meters, in case you are near that length. I would recommend converting to switches and twisted pair for any segments you can replace. This will minimize the scope of collisions.
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  • Tepertyu
    If you network is segmented, start up you network with one segment, check for collisions, if all OK, connect up the second segment, check for collisions, and so on. Anyway, how do you know that collisions causing the problem and not something elese, like heavy broadcast? If you are absolutely SURE that collisions, I guess you alredy have a network monitoring tool, then you easily can locate the source of the problem.
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