[SLUG] Full or half duplex eth

[Rodney Haywood <rodos@nospam.haywood.org>]

Okay this might be dumb but I am having a run on dumb postings today.

Has anyone experieced differences in eth speed between full or half duplex
modes. The reason I ask some testing someone just did on some Win98 to NT
machines showed that when the cards were set to auto/full duplex they
perfomed slower than being set directly to half duplex. So it got me
wondering on what my Linux boxes were doing (I have a 100Mb hub).

The Ethernet HOWTO has this to say

 3.7.9.  Full Duplex Support

  Will Full Duplex give me 20MBps? Does Linux support it?

  Cameron Spitzer writes the following about full duplex 10Base-T cards:
  ``If you connect it to a full duplex switched hub, and your system is
  fast enough and not doing much else, it can keep the link busy in both
  directions.  There is no such thing as full duplex 10BASE-2 or
  10BASE-5 (thin and thick coax).  Full Duplex works by disabling
  collision detection in the adapter.  That's why you can't do it with
  coax; the LAN won't run that way.  10BASE-T (RJ45 interface) uses
  separate wires for send and receive, so it's possible to run both ways
  at the same time.  The switching hub takes care of the collision
  problem.  The signalling rate is 10 Mbps.''

  So as you can see, you still will only be able to receive or transmit
  at 10Mbps, and hence don't expect a 2x performance increase. As to
  whether it is supported or not, that depends on the card and possibly
  the driver. Some cards may do auto-negotiation, some may need driver
  support, and some may need the user to select an option in a card's
  EEPROM configuration.  Only the serious/heavy user would notice the
  difference between the two modes anyway.

But I was wondering how to tell what my card was being set at, here is whats
being reported in dmesg for one of my machines.

eth0: 3Com 3c905B Cyclone 100baseTx at 0xdc00,  00:c0:4f:68:78:56, IRQ 19
  8K byte-wide RAM 5:3 Rx:Tx split, autoselect/Autonegotiate interface.
  MII transceiver found at address 24, status 782d.
  Enabling bus-master transmits and whole-frame receives.

So does anyone know anything about full/half duplex and how to set/detect it
in Linux?

Rodos

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