Mathematics of gigabit question

Jim Lux James.P.Lux at
Fri Dec 7 11:15:46 PST 2001

At 11:12 AM 12/7/2001 -0600, you wrote:

>I believe most ethernet devices have the ability to work in either full
>duplex mode or half duplex modes (I think these terms are a little
>weird, especially half duplex.  Seems like it should be just duplex and
>not duplex at all, but that wouldn't sell NICs).  That's 1000 Mbps each
>direction or 125 MB/sec (really 119 MB/sec) one way, 250 MB/sec (238
>MB/sec) in full duplex mode.  So to try to compare apples to apples, for
>a half duplex link you've got (I'll use real MB, since I refuse to
>conform to marketing ploys):
>32/33 PCI = 122 MB/sec vs. half duplex GigE = 119 MB/sec.

duplex terminology comes from radio,etc. links:

full duplex = 2 separate communications channels (1 each way), both 
operating simultaneously
half duplex = 2 separate communications channels (1 each way), but only one 
operating at once
simplex = 1 communication channel (two way), with direction changing.

full duplex requires simultaneous transmit and receive, which the other two 
do not.  simplex requires switching the channel between two devices.

simplex is also used to refer to a unidirectional channel.

Jim Lux
Spacecraft Telecommunications Equipment Section
Jet Propulsion Laboratory
4800 Oak Grove Road, Mail Stop 161-213
Pasadena CA 91109

818/354-2075, fax 818/393-6875

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