At 2:29 AM +0100 7/10/01, Iggy Drougge wrote:
>H. Spencer Young skrev:
>
>>Can someone give me a quick primer on the advantages of switches vs. hubs on
> >a small appletalk network with about 20 machines using file sharing?
>
>Ethernet networks are very chatty by nature, without any particular
>moderation, unlike Tokenring networks, where a token is passed around in a
>ring-like fashion, alloting a slice of network time/bandwidth to each node.
>Any Ethernet node will just "talk" when it feels like it, and if several nodes
>happen to talk at once, they will have to step back and try to negotiate which
>will talk first. This is called a collision, and disrupts network traffic. It
>usually isn't as bad as it sounds, though.
>On a two-node network, there are virtually no collisions, since any traffic
>will be between the two nodes. In this case, a switch will help in no way.
>When there are thirty nodes on the network, all speaking out loud, though,
>there will be a lot of collisions and thus the effective throughput drops.
>What a switch does is to analyse all incoming packets, look at their
>destination, and then passing them on only two that particular node.
>Effectively, the switch establishes a private Ethernet between the two nodes
>involved, since their traffic isn't broadcast to all hosts on the network.
>This minimises the amount of collisions, effectively splitting the network
>into lots and lots of smaller nets.
There seems to be a misunderstanding about how Ethernet works.
Ethernet is described as CSMA/CD or Carrier Sense Media Access /
Collision Detection. When two nodes attempt to talk at roughly the
same time they both attempt to assert the media. The specifics of
this depend somewhat on the media (10BaseT, 10Base2, 10Base5,
10BaseF). If another device has already asserted the media the
second one will back off and wait. This is the CSMA part. In a
normal network there is only a very short period of time in which two
devices can both assert the media and both think they have it. In
this condition they both then try to send data which causes a
collision and both then quit and wait a random amount of time to
retry. This is the CD part. Because of this collisions SHOULD be
rare. In a well configured network they are.
This short period of time is dependent on the time it takes for a
signal to go from one end of the system to the other. This is both
the propagation time through the cable and through any hubs. If the
network has troubles such as too many hubs, over long cables or
excessive noise collisions can happen more than they should.
>The larger your networks grows, the more simultaneous traffic, and the more
>collisions. Sooner or later, those collisions will have a serious impact on
>your throughput, and you may feel as though you're on a two-megabit network
>instead of a ten-megabit one. This is the idal application for a switch.
Switches break collision domains, i.e. collision related problems on
one leg of a switch do not cause collision problems on any other.
Switches can mask network problems. This is good and bad. It's good
because often the problem remains small and may not be noticeable.
It's bad because it makes it harder to locate problems which may
later come back to bite you.
--
Clark Martin
Macintosh Consultant
Another designated driver on the Information Super Highway
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