[nSLUG] Full vs half duplex

Jim Haliburton jim at on-site.ns.ca
Fri Sep 26 17:18:36 ADT 2008

Draker commented

"Full duplex is faster.  Half duplex results in collisions when both
sides attempt to communicate with each other at the same time.
Doesn't matter if you cannot sustain 100 Mbit data stream -- I'm
talking about latency, not bandwidth."

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.

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.

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.

Just my personal experience.

Jim H

James A. Haliburton
On-Site Computer Services of Halifax
Suite 100, 25 Walton Drive
Halifax, Nova Scotia
Canada     B3N 1X6
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e-mail      : jim at on-site.ns.ca

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