FastEthernet and Hubs : Auto Select

by Johannes Helmig [Published on 13 April 2001 / Last Updated on 13 April 2001]

FastEthernet cards (like the popular 3COM 3C905-series)
can operate at 10 Mhz or at 100 Mhz, depending on the
type of hub used for the connection.

Some hubs also offer the possibility to connect to
10 Mhz or 100 Mhz network cards : Dual-Speed Hubs

For easy of use, both the FastEthernet card and such Dual-Speed hubs are usually configured
for automatic configuration : They are supposed to detect the type of hub or network card and
then select automatically the proper connection speed.

That works in most installations very well, but (like usual) NOT always:
sometimes the hub and the network card do not work properly together and the automatic
configuration fails: the network does not work (as it can be tested via the TCP/IP PING command).

As a first diagnostics, have a look at the indicators on your hub:

With your PC powered on, hubs will usually indicate whether a port is in use (for example
by a green light). Dual-Speed hubs usually also indicate the detected / configured speed.

try then to define MANUALLY the
configuration on the PC:

In the Properties of the Network /
"Network Neighborhood", select
your network adapter and display
for it the Properties.
Depending on the indicators on
the hub, predefine the "Media Speed"
to 10baseT or 100BaseTx.

Then test again the network
connection (for example using PING )

Another possible reason :
While 10baseT - coax Networking has only 1 wire,
allowing to have only one signal on the network cable,
Twisted-Pair networking uses multiple wires:
4 pairs, each pair twisted .

This allows to operate with multiple signals:
a network signal can be send out, while another one
is received at the same time (if the hub is supporting
this functionality ).
Such simultaneous operation is called : "Full Duplex".

If the network card or hub is only able to handle
ONE signal (sending or receiving), then this is
called "Half Duplex"
Like the "Media speed" , the hub and the network card
will configure themselves automatically for the proper
setting, but it maybe required to configure the
network card under "Duplex Mode" for
"Half Duplex" to get the network to run in some

I can not make a general statement, because in such situations it is usally "trial-and-error" to fix
the problem.

See Also

The Author — Johannes Helmig

Dr.Johannes Helmig is working as Director, Technical Knowledge Management in the Belgium office of Gerber Technology where he is involved in Customer Service and internal training, with special interest in Networking.


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