Envoy Configuration and Management Manual

Introduction to Envoy

Envoy Configuration and Management Manual—429861-001

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Half-Duplex and Full-Duplex Lines

An Envoy, point-to-point or multipoint data link can use either half-duplex or full-

duplex line facilities. For half-duplex facilities, the transmission lines can physically

pass data in only one direction at any given time (Figure 2-5

). The direction of the data

flow can be switched only by a line turnaround: one station disables its read unit and

enables its write unit while the other station does the opposite. Communications pass

over a half-duplex line like traffic passes over a one-lane bridge. A station can either

send or receive a message over the line; a station cannot send and receive at the same

time.

Full-duplex facilities, on the other hand, can pass data in both directions simultaneously,

without physical line turnaround (Figure 2-6 on page 2-11).

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.

Figure 2-5. Half-Duplex Lines

Send

Receive

Time

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