[NTLUG:Discuss] Ethernet Switches

[Dennis Myhand]

Eric Schnoebelen wrote:

> Dennis Myhand writes:
> - No.  It is a switch if it uses memory to store the packets before
> - forwarding them and transistors (or some other suitable electronic
> - devices) to make the connections between ports.  It is an hub if
> - the connection between ports is always there and the packet from
> - one machine on the hub goes to ALL machines on the hub.
>
>         Very close.  The packet doesn't have to be stored before
> forwarding (and the mid-range and higher products don't).
> Basically, in a switch, a port never sees a packet not addressed
> for that port (via the ethernet MAC address.)

Sort of.  Cut-through mode on a Cisco switch reads the first 64 bytes of a
packet before moving it on its way.  Store-and-forward reads the packet into
the memory of the switch before sending it on its way.

>
>
>         A hub is `classic' ethernet, just stuck into a little box,
> so the bus is only three inches long, but the tails are hundreds
> of feet.

I think you meant a classic ethernet "bus."  That was what I was trying to say
at 5:30 a.m. with too little coffee in me.

>
>
> -                                                         Full-
> - or half-duplex merely refers to the speed at which the information
> - is moved.
>
>         Nope.. Duplex refers to the ability to transmit while
> receiving.  Half duplex can only transmit _or_ receive during a
> given timeslice.  Full duplex can transmit and receive
> concurrently.

Because full can transmit and receive simultaneously, it is twice as fast.

>
>
> -            The speed at which the information is moved has nothing
> - to do with whether it is a switch or hub.
>
>         Hmm, I'll disagree there.  I've never seen a full duplex
> hub.  And the nature of an ethernet bus would seem to prohibit
> it. (a standard ethernet bus, as defined by 802.<mumble> is
> carrier-sense collission detection, which means the card listens
> for a collision as it starts to transmit.)

I will retro-qualify my statement by saying that, untill you get into gigabit
networks, switches or hubs "can" work at the same speeds, not counting
full-duplex.  It can also work at gigabit speeds using cddi.  Peace, Dennis

>
>
> --
> Eric Schnoebelen                eric at cirr.com           http://www.cirr.com
>         "Ford, If I were to ask where in the hell we are, would I
>                  regret it?" -- Arther Dent, HHGG
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