Duplex Modes

The CSMA/CD media access method used in IEEE 802.3u-1995 is inherently a half-duplex mechanism. That is, at any one time, there can be only one sender of data on the link segment. It is not possible for devices on either end of the link segment to transmit simultaneously.

Since Category 5 UTP contains multiple pairs of wires, it is possible to have devices on both ends of a link segment sending data to each other simultaneously. This is known as full-duplex operation. While the details of full-duplex operation are not currently defined by IEEE 802.3u-1995 (full-duplex mode essentially involves "turning off" the CSMA/CD access method which is the foundation of IEEE 802.3), the autonegotiation mechanism defined in IEEE 802.3u-1995 allows devices to advertise and configure themselves to operate in a full-duplex mode which is essentially vendor-specific. Devices that do not support autonegotiation can sometimes be manually configured to operate in full-duplex mode.

Full-duplex mode is most commonly found in, and indeed only makes sense for, switches rather than hubs. It may be found in either 10 Mbit/s or 100 Mbit/s switch devices. Full-duplex mode may provide a throughput advantage under some circumstances, the degree of the advantage is application-dependent.

The PCI 10/100Base-TX card supports both half and full-duplex operation.

In order to correctly configure the duplex mode of the PCI 10/100Base-TX card, the duplex mode of the associated switch should be configured, and if the switch supports autonegotiation on the ports connected to the cards, this should be enabled as explained in the section in this guide called "Autonegotiation and Autosensing."

Under some circumstances, you may need to manually set the duplex mode of the card. This may be done using the lanadmin -S command as described in the following paragraphs. When this command is issued, the btlan5 driver will reinitiate the speed sensing between the card and hub or switch and set the duplex mode of the card as specified by the command. This may be useful, for example, when a switch is configured for a specific speed and duplex setting and is not doing autonegotiation.

To set the duplex mode of the interface, use the -S option of the lanadmin command as follows:

lanadmin -S mode nmid

where:

a mode of 1 = full-duplex mode
a mode of 0 = half-duplex mode

nmid is the Network Management ID of the card, which you can get from the output of the lanscan command.

For example, if the nmid of the 100Base-Tx interface is 5, the command to set the interface to full-duplex mode would be:

lanadmin -S 1 5

After setting the duplex mode as specified, the lanadmin command will echo the current speed setting of the interface as follows. This output may be ignored (output shown for 100 Mbps operation):

Old Speed = 100000000 New Speed = 100000000

After issuing the lanadmin -S, you must wait at least 8 seconds before attempting to use the specified network interface.

If you want the Duplex Mode setting to be effective in all subsequent reboots, you must create an SD script and include the appropriate command in the /sbin/init.d directory so it gets executed on each reboot.