Envoy ACP/XF Configuration and Management Manual

Introduction to EnvoyACP/XF
EnvoyACP/XF Configuration and Management Manual—429232-003
2-7
Half-Duplex and Full-Duplex Lines
Full-duplex facilities, on the other hand, can pass data in both directions
simultaneously, without physical line turnaround (Figure 2-7).
Communications pass over a full-duplex line like two-way traffic passes over a divided
highway. A full-duplex line enables a station to send messages and receive messages
at the same time. Although full-duplex line facilities are more expensive than half-
duplex facilities, the additional cost usually is offset by the decreased response time
and increased throughput over the line. Full-duplex lines are sometimes referred to
simply as duplex.
Besides being either half-duplex or full-duplex, communications are affected by logical
states that enable either two-way alternate or two-way simultaneous transmission.
Two-way alternate transmission can occur over either half-duplex or full-duplex lines;
the protocol never attempts to transmit simultaneously in both directions. Two-way
simultaneous transmission can occur only over full-duplex lines; the protocol transmits
Figure 2-6. Half-Duplex Lines
Figure 2-7. Full-Duplex Lines
VST008.vsd
Send
Time
Receive
VST009.vsd
Send
Receive