[nSLUG] Full vs half duplex
ian at slu.ms
Fri Sep 26 18:12:07 ADT 2008
On Fri, Sep 26, 2008 at 05:18:36PM -0300, Jim Haliburton wrote:
> Draker commented
> I am not sure if this statement is completely correct. If using a
> switch, not a hub, then the collision domain is 1. In other words there
> cannot be a collision. One pair transmits, another pair receives. What
> is transmit on one end of the cable is receive on the other. There can
> be no collisions on a switched conection between a card and the switch.
> The transmit and receive paths do not cross.
I haven't been reading this thread up until now, so apologies if this
has already been mentioned...
Collision detection is active in half-duplex mode, no matter what
you're plugged into. If the card/switch sees something on transmit and
receive at the same time when it's in half-duplex mode, that's a
collision, regardless of what's on the other end of the cable.
> On Netware networks with IPX, using full duplex on the server end, can
> get you a 50-80 % reduction in throughput. Setting the card to come up
> in half duplex is an easy setting. In addition, when using IPX, setting
> Link Support Layer Max buffer size to 2048 will gain you some throughput.
It's been a while since I had anything to do with Network but I have a
hard time believing that. Tweaking buffer sizes is one thing, that
part I believe, but I can't think of any reason Netware would perform
more poorly in half-duplex than full. I can't find anything on Google
about it either.
> Now Windows networks are quite different. Because interrupts are not
> always handled elegantly or as quickly in Windows they need to use full
> duplex to fill the buffers on each end while the system is thinking about
> responding to the interrrupts. Netware is much more interupt driven on
> the comm side so it can respond to cards at interrupt time and clear the
> data from the buffer. In FD it is waiting for the buffers to fill rather
> than acting on an interrupt when it is full. So HD is faster. I have
> several clients still using Netware and this behaviour is documented and
> well tested in the Novell world. Have seen this across NW 3.12, 5.0,
> 5.1, 6.0, and 6.5 regardless of the card brand. When set at FD the
> system slows. At HD 10 Mbit gives about 950K bytes / sec and 100Mbit
> gives over 8 MBytes/sec sustained. Have not tested Gig. But it is
> faster again.
See above... I dunno. I can't find anything obvious either in on the
web or in newsgroups about this. The only times I see anyone
mentioning half-duplex is when either someone is having duplex
mismatch problems or they're connected to a hub.
The explanation doesn't really make sense to me either. I mean, Linux
is pretty interrupt driven and nobody has ever tried to sell me on HD
as a speed boost.
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