Re: [SLUG] Full or half duplex eth

[DaZZa <dazza@nospam.zip.com.au>]

On Fri, 3 Dec 1999, Rodney Haywood wrote:

> Has anyone experieced differences in eth speed between full or half duplex
> modes. 

I have - but it's _really_ application dependant.

> 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).

Full duplex is only possible when connected to a switch rather than a hub.
The CD part of the CSMA/CD technology used in ethernet is removed by the
switch.

As for 10 Mb/s being faster than 100 Mb/s - auyto negotiation can often
have all sorts of _weird_ problems - the main reason being that there is
NO standard as to how cards/switches negotiate data rates and duplex - so
what one manufacturer does MAY work with another manufacturer's switch
gear, but don't bet on it.

Maybe you got caught in one of these.

> 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.

You should look at the hub/switch on the other end and see what it says.

I'd read this as 100 Mb/s half duplex {indicated by the 100BaseTX - it'd
read 100BaseFX if it was full duplex}. But I could be wrong - the only way
to find out would be to check the switch itself and see what it says.

Of course, if you're only connected to a hub, as per above, then half
duplex is your only possibility.

DaZZa

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