[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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