Re: Kinda On Topic - network question

["Paul Gordon" <paul_gordon@xxxxxxx>]

Just add another 2p worth, it can also be described by saying that each
port
on a switch is a seperate "collision domain" - to explain:

Ethernet uses a collision detection mechanism called CSMA/CD "Carrier
Sense
Multiple Access with Collision Detection" - what this basically means
is
that any device on the ethernet that wants to transmit a packet must first
listen, and can only transmit if no other device on the wire is currently
transmitting. The upshot is that on a hub, where all the ports are in the
same collision domain, only 1 device on the entire hub can transmit at any
instant in time. Conversely, with a switch, each port on the switch is a
seperate collision domain, so 1 device on EACH PORT can be transmitting
packets simultaneously. It's therefore easy to see that by replacing an
8-port hub with an 8-port switch, it is theoretically possible to increase
LAN bandwidth by a factor of 8. (Of course it doesn't actually happen like
that, as that is a massively simplified explanation) In practice, a device
on any one port obviously wants to transmit to another device on the LAN,
and that other device may be either on the same port on the switch, or on
another port on the switch. To facilitate this, the switch keeps an
internal
table of the MAC addresses of every device which is connected to it (either
directly or indirectly), and which port that device is connected to. Whan a
packet is received by the switch that is destined for a device on another
port, the switch looks up the MAC address of the destination node and
creates a virtual circuit between the port the packet came in on, and the
port it has to go out on. Thus you can see that in an 8-port switch there
could potentially be a maximum of 4 VC's active at any one moment. The
creation/removal of these VC's is totally dynamic. Things that will
determine a switch's performance (and therefore it's price) will be things
like the backplane speed, the memory capacity for the MAC table, and the
switching speed of the VC creation. - It's not uncommon to see switches
that
have a limit of 1024 MAC addresses which can be stored in its table.

Basically, buy a switch! - they're hardly any dearer than hubs now
anyway...

Paul G.




>From: "the-croft" <alan@xxxxxxx>
>Reply-To: ukha_d@xxxxxxx
>To: <ukha_d@xxxxxxx>
>Subject: Re: [ukha_d] Kinda On Topic - network question
>Date: Fri, 14 Sep 2001 07:02:58 +0100
>
>A hub simply sends all network traffic to every port.
>
>A switch looks at the destination mac address of each packet and only
sends
>it to the relevent port(s), this means it needs to be quite a bit more
>intelligent (and expensive) than a hub.
>
>If you have a lot of machines sending a lot of data at once (to
different
>destinations) a switch can make quite a difference in network
performance.
>
>If most of the traffic is travelling to/from one machine it probably
won't
>make any difference.
>
>Alan Shields
>alan@xxxxxxx
>
>----- Original Message -----
>From: "Jon Bhargava" <jonathan.bhargava@xxxxxxx>
>To: <ukha_d@xxxxxxx>
>Sent: Thursday, September 13, 2001 10:26 AM
>Subject: Re: [ukha_d] Kinda On Topic - network question
>
>
> > Hi
> >
> > I need to upgrade my old 8-port network hub to something faster
> > I have just seen a 16-port network 'switch' on the Scan site.
> > Please can somebody excuse my ignorance & tell me the
difference between
>a
> > hub & a switch?
> >
> > Many thanks
> >
> > Jon Bhargava
> >
> >
> >
> >
> > For more information: http://www.automatedhome.co.uk
> > Post message: ukha_d@xxxxxxx
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> >
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>
>
>
>For more information: http://www.automatedhome.co.uk
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