Half duplex

Half duplex means that 1 channel is used for communication. Sending data must be synchronized with receiving data and it's possible that both ends of the communication try to send data at the same time causing a collision. The aggregate bandwidth of a 100 Mbps link is 100 Mbps. This is half of what full duplex provides. Ethernet hubs are half duplex, always.

Forcing an interface into half duplex mode

In OpenBSD you can see the media modes with the ifconfig command:

$ ifconfig -m xl0
xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
       lladdr 00:60:08:5a:86:82
       media: Ethernet 100baseTX full-duplex
       status: no carrier
       supported media:
               media none
               media 10baseT
               media 10baseT mediaopt full-duplex
               media 100baseTX
               media 100baseTX mediaopt full-duplex
               media autoselect
       inet 172.16.2.2 netmask 0xfffffe00 broadcast 172.16.3.255
       inet6 fe80::260:8ff:fe5a:8682%xl0 prefixlen 64 scopeid 0x2

To then change it to half duplex mode you would type:

$ ifconfig xl0 media 10baseT

In Linux you'd use mii-tool to do this.