Three types of channels are used:
1) Duplex point-to-point channels
2) Duplex party-line channels
3) Half-duplex channels
Duplex Point-to-Point Channels
For this type of channel, the DCE Request
to Send signals at both terminals are activated continuously. The DCE
transmitters and receivers are both continuously active while the channel is
operational, thereby eliminating DCE response time delays.
Duplex Party-Line Channels
The master terminal DCE Request to Send
signal is operated as described in. The corresponding signal at any remote
terminal is activated only when that terminal responds to a preceding master
terminal message and is deactivated immediately following completion of
transmission of the response message. A DCE response time delay is encountered
at the remote terminal.
Half-Duplex Channels
The DCE Request to Send signals are
activated only when the associated terminal initiates transmission of a message
and are deactivated immediately on completion of each message transmission. DCE
response-time delays are encountered at both master and remote terminals.
Additional DCE Facilities
Remote terminal DCE equipment operating in
party-line communication channels should be equipped with means to monitor the
length of their output transmissions and to deactivate their DCE transmitter if
an excessive length occurs, presumably due to an equipment failure.
This feature, commonly known as
“anti-streaming protection,” minimizes the probability that a failure in one
remote terminal will disable communication with other remote terminals that
share the channel.
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