[nSLUG] Full vs half duplex
jim at on-site.ns.ca
Fri Sep 26 17:18:36 ADT 2008
"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
Just my personal experience.
James A. Haliburton
On-Site Computer Services of Halifax
Suite 100, 25 Walton Drive
Halifax, Nova Scotia
Canada B3N 1X6
Office/ Cell : (902)499-5250
Home/After Hours : (902)477-8342
e-mail : jim at on-site.ns.ca
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