| half duplex
Twenty years ago, modem communication
was either asychronous full duplex (one character at a time sent in both
directions at once) or synchronous half duplex (a block of characters at a
time, but sent in only one direction). These days, modems are all
synchronous half duplex. Why? Because it's more efficient: at 28.8 kbps, 2
modems can send and receive 25 packets of data every second. That looks
like duplex transfer to a casual observer, but if you're counting in
milliseconds, the duplex is only half full. |