RE: Network Setup Opinion Needed
From: Otto Haliburton (ottohaliburton_at_comcast.net)
Date: 09/30/03
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- In reply to: Benjamin J. Weiss: "RE: Network Setup Opinion Needed"
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To: <redhat-list@redhat.com> Date: Tue, 30 Sep 2003 15:14:51 -0500
Ben you didn't read my latter email, which I conceded that I was talking
something different than what was being presented. You have probably gotten
that far now so.
> -----Original Message-----
> From: redhat-list-admin@redhat.com [mailto:redhat-list-admin@redhat.com]
> On Behalf Of Benjamin J. Weiss
> Sent: Tuesday, September 30, 2003 2:44 PM
> To: redhat-list@redhat.com
> Subject: RE: Network Setup Opinion Needed
>
> > > This is not quite always the case. Ethernet's CSMA/CD (Carrier Sense
> > > Multiple Access with Collision Detection) was invented during a time
> when
> > > a hub or bus were the primary method of connection. Collision was
> indeed
> > > a problem then, and keeping the LAN small was a way to ensure network
> > > performance.
> > >
> > > However, these days, switches are much cheaper and are easily within
> the
> > > reach of most organizations.
> > >
> > > If your users are hooked up to a switch instead of a hub, you can
> ignore
> > > the "collisions problem", as it no longer exists. At that point, the
> > > limiting factors are the speed/RAM of the gateway and the speed/RAM of
> the
> > > switch.
> > >
> > > A good, short explanation can be found at
> > > http://www.duxcw.com/faq/network/hubsw.htm
> > >
> > >
> > > Ben
> > >
> > Actually CSMA/CD is the problem on a large single area network. I read
> the
> > article and see the point. Here is the problem. When a node transmits
> it
> > first listen for no traffic then it tries to transmit, if a collision
> occurs
> > then it goes into an algorithm to make a attempt again after it selects
> it's
> > new time slot, well the larger the number of nodes the greater the
> > probability that they will select the same time slot and cause a
> collision
> > again. Etc. etc .... Therefore large networks always bottle neck under
> > Ethernet and that is why no company will place a large number of
> computers
> > in the same area no matter what the transport medium is. There is
> always a
> > optimum number that should be in an area before it is broken down.
> That's
> > the theory.
>
> Otto,
>
> I'm sorry, but you have absolutely no idea as to how ethernet works in a
> *switched* environment. You are describing a LAN that is on a hub. In
> that case, you are correct with all of your above comment.
>
> On the other hand, in a situation where all of the hosts are connected to
> a *switch*, instead of a hub, then each "segment" consists of exactly two
> devices: the host and the switch port. In that case, the chance for
> collision is greatly reduced. The reason for this is simple:
>
> If host-a transmits a packet while connected to a hub, then all other
> hosts connected to that hub will see the packet, whether or not it is
> intended for them.
>
> If host-a transmits a packet while connected to a switch, then something
> entirely different happens. The switch looks at the packet and decides
> where it is to go. If it is a multicast or broadcast packet, then most or
> all of the other hosts on the switch will see it (I won't go into those
> rules, it's beyond the scope of this particular discussion.)
>
> If, on the other hand, it is a *unicast* packet, and the destination host
> is on the switch, then the switch will only transmit it on the port to
> which that destination host is connected. If the host is not connected to
> the switch, then it will send it on it's "uplink" port, depending upon
> configuration.
>
> Now, what this means is that any host that is connected to a switch will
> only see broadcast traffic, multicast traffic to which it is subscribed,
> or unicast traffic that is addressed to it. This sharply decreases the
> number of packets that the host's ethernet card will see as inbound, which
> will thereby reduce the number of collisions during transmission and
> subsequently increase the perceived bandwidth.
>
> At that point the bottleneck becomes the switch's backplane capabilities,
> not collisions.
>
> Switched ethernet is vastly superior to ethernet on a hub, and has become
> very cheap. It is now easily in reach for most organizations, including
> the home office.
>
> Ben
>
>
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- Previous message: Simpson, Doug: "cd burning software"
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