[K12OSN] 100Mbit switch vs. 100Mbit hub
Les Mikesell
les at futuresource.com
Mon Mar 19 22:12:21 UTC 2007
Rob Owens wrote:
> I've been on this list long enough that I've heard "use a switch, not a
> hub" about a hundred times. But I have a question about the reasoning:
>
> The server has the most traffic going to/from it. So if you were using
> a switch, the server port of the switch would be saturated first--as
> soon as 100Mbit/sec is going to/from the server. If
> you instead used a hub, the only difference I see is that *all* of the
> ports would saturate at the same time--as soon as 100Mbit/sec is going
> to/from the server.
>
> Since there is no traffic going from one terminal
> to another, it seems like a 100Mbit/sec hub would become a bottleneck at
> the same time that a 100Mbit/sec switch would. What am I missing?
What people are generally recommending is a switch with a gig uplink and
100M to the clients, but even with 100M on all ports there is a big
difference. With TCP traffic, the receiver has to acknowledge the data
at least within the transmit window size or the sender will wait. So
for about every 3 packets or less that the server sends, an ack must
come back from a client. When the link is saturated by the server, on a
hub the clients are going to have collisions and delays sending the
acks. On a full duplex switch, the directions are independent so
nothing has to wait for anything else.
--
Les Mikesell
les at futuresource.com
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