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Full Discussion: duplex
Special Forums IP Networking duplex Post 20651 by Perderabo on Wednesday 1st of May 2002 09:44:53 AM
Old 05-01-2002
With half duplex ethernet in a 10baseT environment, collisions can occur between the network card and the hub port when they both try to talk at they same time. If either one detects a collision, I would guess that they both will, but it might depend on the length on the cable. In any event both must be ready to detect collisions. And when one detects a collisions, it transmits a jamming signal that the other would see anyway. NB, with a modern 10baseT hub, the only collisions possible are between a host's incoming and outgoing packets. One host's traffic will not collide with another host's traffic. This is very different from the old 10base5 ethernet.

Full duplex ethernet eliminates collisions and this is a major performance boost. (This is what the host sees and describes the connection between the hub and the host. I don't know what in happening inside the hub.) Full duplex 10baseT did exist for awhile, but mostly you see full duplex 100baseT. 1000baseT is out, but I have never worked with it. Using 100baseT as my example, in addition to the no collisions concept, some folks claim that you also get a 100 megabits in both directions. Others claim that the 100 megabits is shared bandwidth. I don't know who is correct.

But there is a nasty landmine here and I have stepped on it dozens of times. The standards guys allow something called autonegoiation. This is intended for, say, a visitors office. You are supposed to be able to have a network connection that allows your visitor to just plug in a laptop and everything automatically works.

The trouble is that this autonegoiation doesn't work right and you end up with one side in half-duplex and the other in full-duplex. This is a disaster because the HD side is detecting collisions and sending jamming signals which the FD side ignores. The performance impact is incredible. So we have to turn off autonegoiation on all of our boxes and lock them in to 100 FD mode.
 

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LEARN ABOUT REDHAT

txp

TXP(4)                                                     BSD Kernel Interfaces Manual                                                     TXP(4)

NAME

     txp -- 3Com 3XP Typhoon/Sidewinder (3CR990) Ethernet interface

SYNOPSIS

     To compile this driver into the kernel, place the following line in your kernel configuration file:

           device txp

     Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):

           if_txp_load="YES"

DESCRIPTION

     The txp interface provides access to the 10Mb/s and 100Mb/s Ethernet networks via the 3Com Typhoon/Sidewinder chipset.

     Basic Ethernet functions are provided as well as support for vlan(4) tag removal and insertion assistance, receive ip(4), tcp(4), and udp(4)
     checksum offloading, and transmit ip(4) checksum offloading.  There is currently no support for transmit tcp(4) or udp(4) checksum offload-
     ing, tcp(4) segmentation, nor ipsec(4) acceleration.

     Each of the host's network addresses is specified at boot time with an SIOCSIFADDR ioctl(2).  The txp interface employs the address resolu-
     tion protocol described in arp(4) to dynamically map between Internet and Ethernet addresses on the local network.

     When a txp interface is brought up, by default, it will attempt to auto-negotiate the link speed and duplex mode.  The speeds, in order of
     attempt, are: 100Mb/s Full Duplex, 100Mb/s Half Duplex, 10 Mb/s Full Duplex, and 10 Mb/s Half Duplex.

     The txp supports several media types, which are selected via the ifconfig(8) command.  The supported media types are:

     media autoselect
             Attempt to autoselect the media type (default)

     media 100baseTX mediaopt full-duplex
             Use 100baseTX, full duplex

     media 100baseTX [mediaopt half-duplex]
             Use 100baseTX, half duplex

     media 10baseT mediaopt full-duplex
             Use 10baseT, full duplex

     media 10baseT [mediaopt half-duplex]
             Use 10baseT, half duplex

HARDWARE

     The txp driver supports the following cards:

           o   3Com 3CR990-TX-95
           o   3Com 3CR990-TX-97
           o   3Com 3cR990B-TXM
           o   3Com 3CR990SVR95
           o   3Com 3CR990SVR97
           o   3Com 3cR990B-SRV

SEE ALSO

     altq(4), arp(4), inet(4), intro(4), ip(4), miibus(4), tcp(4), udp(4), vlan(4), ifconfig(8)

HISTORY

     The txp driver first appeared in OpenBSD 2.9.

BSD                                                               March 12, 2009                                                               BSD
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