Xircom CardBus PCMCIA Network Adapter

[mark at buttery.org (Mark J. Dulcey)]

On Thu, 30 Mar 2000, Jerry Feldman wrote:

> Speaking of switches...
> A while back, we were trying to get a good difference between a switch 
> and a hub. What are the advantages between a hub, and active hub, and 
> a switch. In general, can a hub support full duplex if all the nodes have 
> full duplex NICS? 

A hub connects all the computers on a single Ethernet. Packets sent to
any destination are seen by all the computers connected to the hub. The
Ethernet interface in your computer will normally reject packets that
aren't addressed to it or to the broadcast address, unless it's in
promiscuous mode.

A switch puts each computer on a private Ethernet. Packets are forwarded
by the hub to the Ethernet of the destination. Broadcast packets are
forwarded to all the connected Ethernets.

Passive hubs were used back in the days of ARCNET; they just did signal
combining and impedance matching of the incoming lines, and contained no
powered circuits (transistors, ICs, etc.). All Ethernet hubs are active. I
believe that all USB hubs are also active.

Full duplex isn't supported by hubs. To enable full duplex, you need a
private Ethernet connection between two, and only two, computers - either
a switch or a simple crossover cable. The latter case means that you
could enable full duplex between your computer and your cable or DSL modem
- except, of course, that the network interface in the modem probably
doesn't support it.

Full duplex is really a hack of Ethernet that depends on the fact that
there are only two nodes involved. A hub can't possibly implement it,
because data on the out path from one node has to be replicated on the in
path of all the other nodes.

The basic advantage of a switch is that it can make the network faster,
especially if there is more than one server on the network. A switch can
handle full Ethernet bandwidth between any two computers on the switch -
and simultaneously handle full bandwidth between another two computers,
and so on until all the ports on the switch are busy. (Some early switches
couldn't switch packets fast enough to reach 100% utilization of all the
ports. Most current switches can.) In addition, a switch makes full duplex
operation possible, increasing the potential speed of the network by
another 100%.

In practice, the gain from a switch isn't nearly as large as all that. On
a typical network, a small number of computers act as servers and send and
receive all the network traffic, so the potential speed is (100% x number
of servers). In addition, there is usually more traffic in one direction
than in the other, so you don't gain the full 100% improvement from
full-duplex operation. But it's still enough of a win to make sense for a
lot of networks, especially since switches aren't nearly as expensive as
they used to be.

The disadvantage of a switch is that it costs more than a hub. It's also
likely to be larger, consume more power, and have noisier fans.

A potential advantage and disadvantage is that a switch sends packets only
to the computers that they are intended for. This is a data security win
(a packet analyzer on your computer can't possibly spy on packets intended
for others), but does make network testing more difficult.


-- 
Mark J. Dulcey               mark at buttery.org
Visit my house's home page:  http://www.buttery.org/
Visit my home page:          http://www.buttery.org/markpoly/

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