Half-duplex vs. full-duplex media

Half duplex media (e.g. ethernet) need one pipe -- both input and output traffic compete for the same bandwidth

ipfw add 100 pipe 1 ip from any to any
ipfw pipe 1 config bw 10Mbit/s

Full duplex media (e.g. modem, ADSL, etc.) need two pipes, because input and output traffic are decoupled.

ipfw add 100 pipe 1 ip from any to any in
ipfw add 100 pipe 2 ip from any to any out
ipfw pipe 1 config bw 640Kbit/s
ipfw pipe 2 config bw 128Kbit/s