Slow transfer with 21140 card

Robert G. Brown
Thu Jul 23 18:00:09 1998

On Thu, 23 Jul 1998, Chance Reschke wrote:

> Hubs can not be full-duplex.  The resource you site should have
> described the CSMACD protocol used by ethernet.  It should have also
> defined hubs as connecting circuits within a single collision domain.
> Given the media access protocol and the definition of a collision
> domain, it is impossible for a hub to support full-duplex operation:
> A host must wait until no one else is transmitting before attempting
> to transmit itself.  No full duplex.

Yes, I (finally) found exactly that line on the website:

"Full duplex operation is quite simple compared to normal Ethernet, and
devices at each end of a full duplex link can send and receive data
simultaneously over the link. One advantage of this approach is that
the full duplex link can theoretically provide twice the bandwidth of
normal (half duplex) Ethernet. The full duplex mode of operation
requires that each end of the link only connects to a single device,
such as a workstation or a switched hub port."

...which, as you observe, is effectively excluded by the definition of
a collision domain in the case of a hub.  As I should have known...

Obviously switched ports cannot ALWAYS be full duplex, as you can hang
a hub off of one.  N-way negotiation allows the switch to decide what
duplex it can support given the number of hosts on the node and the
characteristics of the MII device on the other end.  Many switches
(including ours:-) certainly support FD if one hangs a single host on
the port.  The question still remains -- are all switches (these days)
made intrinisically full duplex capable?  Also, are all NIC's (these
days) intrinsically FD capable?

At the moment, it sounds like the answer may be no, as one person
suggested that their switch (with a mix of 10B and 100B ports) was
half-duplex only.  I also wouldn't be surprised if there existed older
cards that only can handle half duplex.  However, I think that in
almost all cases, one should avoid both of these and make sure that
your 100BT switch supports Nway autonegotiation and that your ethernet
card support MII/Nway on the other end.  I do think that all the
mainstream 100BT cards (tulip, 3com, intel) are FD-capable
autonegotiating cards.  Don't know about ALL the clone tulips though...


Robert G. Brown	             
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525