[nSLUG] Full vs half duplex

Ian Campbell 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.

Shrug. 
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