[NTLUG:Discuss] New findings on cable modem issue

[Mark Bickel]

> From discuss-admin at ntlug.org Fri Aug 17 12:13 CDT 2001
> From: Kenneth Loafman <ken at lt.com>
> List-Archive: <http://www.ntlug.org/pipermail/discuss/>
> Date: Fri, 17 Aug 2001 17:11:52 +0000
> 
> > Patrick Parks wrote:

> > Ok, I am willind to  go tet a 10/100 Sitch or hub, I have been
> > wanting to upgrade my hub anyway. I am a little confused, which
> > should I get,a switch or hub? Whats  the difference between the two?

An ethernat hub, by definition, takes every ethernet packet it receives
into every port and retransmits it out every other port. That is why a hub,
while physically being a star topology, logically behaves as a bus, just
like the older thinnet 10-base2 co-axial LAN cable. In this type of LAN
it is not possible for a NIC to send and receive a packet at the same
time (they would collide) - therefore it must run in half-duplex mode.

An ethernet switch on the otherhand learns the source MAC address of every
device that connects to it. Similar to a bridge it creates a lookup table
in memory where MAC addresses are mapped to ports. When a switch receives
an ethernet frame into one of its ports it looks at the destinion MAC
addresses and compares it to the lookup table. It then forwards the frame
on to _only_ the port associated with that frame's destination MAC address.
By operating in this way a switch creates a one-to-one path between to
NICs. Think of a crossover cable. That is why it's possible for 10/100baseT
NICs to run in full-duplex mode when they are connected via a switch,
rather than a hub.

> > I went to Comp USA page, and there is a 5 port switch, and there is
> > also a 5 port hub, and they are around the same price. I did not
> > notice if the switch had a uplink port for my cable modem, doe this
> > matter? Thanks.
> 
> A hub will operate at the lowest common speed, so if you have a 10mbs
> cable modem and a couple of 100mbs computers, the hub will slow things
> down to 10mbs (if the cable modem is active at the time).  A switch
> operates at the best speed possible, so your two 100mbs computers will
> talk back and forth at 100mbs, while the cable modem will still only
> talk at 10mbs.
> 
> ...Ken

A switch does not necesarily have to provide dual-speed or auto-sense
capabilities, although very many of them do. Also note that dual-speed
hubs must buffer input from a 100baseT to a 10baseT NIC. When that buffer
fills the hub must tell the faster NIC to stop transmitting. Throughput
therefore will always be limited by the slower NIC.

Mark.Bickel at ericsson.com