Collisions

[d0wn_under]

This may sound like a daft question but what causes collisions on an ethernet network?

The reason I'm asking is I've just got a new switch which has collision leds and they are going mad. I knew collisions could happen, but I thought it was a rare thing which happened when IPs were setup wrong (2 matching) or something similar.

If someone could tell me what is causing the problem then I can hopefully fix it and hopefully improve my network speed.

Ta

[fatcat.00]

Heh heh a good question, but the answer is pretty ugly.

Ethernet uses a technique called Collision Detection/ Multiple Access (CDMA, not to be confused with the cell phone standard).

CDMA means this in simple terms:
The various Ethernet adapters on a network are sitting there, listening intently. One of them decides that he wants to send a frame over the line. If he doesn't hear any other NIC transmitting, he goes ahead and sends his frame.

Usually this works fine, but sometimes another NIC decides to send at the same time. This is a collision. When a collision happens, all the NICs go quiet, usually for a random, very brief period of time. Hopefully both NICs that caused the collision the last time will not try transmitting at the same time again, but if they do, then the process simply repeats.

Now, this may seem overly simple but that is pretty much how it really works. Ethernet's biggest advantage over other layer 1 and layer 2 protocols is its simplicity. If you know the difference between RISC and CISC in the microprocessor worlds, you can think of Ethernet as RISC, where Token Ring could be thought of as CISC.

Collisions are rarer in switched environments, but fairly common in non-switched environments. If you are using a hub (not a switch), then you will see collisions all the time. The busier your network, the more you will see. In fact, even though Ethernet commonly is rated at 100Mbps, non-switched networks top out at 50-60Mbps due to collisions. Switched networks can get closer to the full speed.

If your collision light blinks as quickly as a few times a second, you are probably OK. If it is solid, or if the level (if it shows a level) is very high, you might have a problem on your network. A bad hub, switch, NIC or a loop in your network can cause massive collisions which will severely impact your network's performance.

Does that answer your question?
_________________
-- Fatcat

[d0wn_under]

That answers it perfectly. I do have a 10/100 switch but all 3 machines only have 10Mbps cards.

I was getting loads of collisions when I was copy a lot of data off one machine onto the other two at the same time. From what you said it is very reasonable that there should be collisions as there was a lot of data flying about.

As it was a one off sorting my machines out hopefully I won't have to worry about it, and when I can afford it, and have the time, I'll stick some 100Mbps cards in to speed the whole lot up.

Again, ta, good answer.

[kashani]

[d0wn_under]

I am now confused, The new "thing" I have bought is an ADSL modem with 4 network ports built in.

In some shops when I was looking they were calling them routers, others were calling them switches and others were calling them hubs.

From what I managed to pick up a hub is the lowest form, running at the lowest speed of all involved, a switch will run some at 10 and some at 100 if there is need and a router is something that will swap packet from network to another.

Therefore, from my logic I have an ADSL router with built in switch. Is this correct?

There are duplex lights on the box and none of them are lit so I assume that I have half duplex. Two cards are old, the third one was a cheap but new 10Mbps. I'll have a go at turning on full duplex and see what happens if I manage it.

Ta

[kashani]

[fatcat.00]

What kashani is saying is true for consumer-grade devices. You will not see collisions on a switch until its frame buffers fill up, and then you may. Enterprise equipment has very deep buffers, so collisions are quite rare. You only see them if you have hosts that are pounding the hell out of the network, or there is some kind of problem. Very unusual under normal circumstances.

*Most* of the consumer grade devices are switches, but do not have deep buffers, so you tend to see more collisions or dropped frames. If you have a half-duplex device, and if the switch and the NIC try to talk at the same time (which half-duplex doesn't allow) a collision might result.

On the other hand, switches always have a choice when their buffer gets filled with frames. They can either transmit a frame and cause a collision, or they can just drop frames into a bit-bucket (causing TCP retransmits, or lost UDP packets if thats what was encapsulated). I have no idea which one is a more common approach. In either case, lower performance is the result.

kashani is on the money in saying that full-duplex is what you want for the highest throughput. In half-duplex mode, your switch becomes a hub for the NIC that is in half-duplex. In consumer-grade devices, maybe it becomes a hub for all NICs, I don't know. In an enterprise switch, only the half-duplex port is affected.

BTW, the reason people call these devices different things is because they *are* different things. This class of device is called a "multi-layer" device, which means it fulfills the function of at least two layers (using the OSI model). The switch is Layer 1 and 2, the router is Layer 3, and the firewall is Layers 4-7, depending on what protocol you are talking about. This is not important, its just an explanation as to why these things get called different names.

Bah, I am just rambling now...must...sleep.
_________________
-- Fatcat

[d0wn_under]

All the cards seem to be half duplex only so I'm going to save up and get at least a full 100 for my main machine and let the rest suffer for a while.

I think what you say about different names is true, but in most shops I think it is more the staff use the buzz words they've picked up sometimes wrongly. I know that some places do as I had exactly the same product described as a switch, hub and router in different shops each saying that any other description is wrong and that the others don't know what they are saying.

Thanks all

[kashani]

Don't worry it happens at the high end too. Nothing like pointing out a "layer 3 switch" is a router with lots of ports when it comes down to it.

kashani
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Will personally fix your server in exchange for motorcycle related shop tools in good shape.

[paranode]

I think it's actually CSMA/CD for Carrier Sense Multiple Access w/ Collision Detection. The collision detection being a revision to the initial protocol.
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Meh.

[fatcat.00]

Doh! I stand corrected. CSMA/CD is what I meant. I blame a chemical imbalance (to be corrected with more chemicals), a misspent youth, an unhappy childhood, misfirings neurons or cosmic rays...you pick
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-- Fatcat