Specifications

Configuring Modem Signals
11-16
MODEM CONTROL Sequences
Introduction
Modem-controlled communication requires that the access server recognize what
type of device is on a port and detect when this device is ready to communicate
and when the device has ceased to communicate. The following section describes
the general sequences of modem signals involved in establishing, in monitoring,
and in ending communications.
Establishing a Connection
When a connection is initiated at a port, the access server follows the signaling
sequence described in this section.
1. First, the access server examines the DTRWAIT characteristic to determine
whether to assert data terminal-ready signal (DTR) and the request-to-send
signal (RTS) while the port is idle.
If DTRWAIT is DISABLED, the access server asserts DTR and RTS while the
port is logged out.
If DTRWAIT is enabled, the access server delays assertion of DTR and RTS
until either it detects any modem signal or a connection occurs. Then, the
access server asserts DTR and RTS.
When asserting DTR and RTS, if conditions require the data-signal-rate-
selector signal (DSRS), the access server asserts DSRS at the same time.
2. After asserting DTR and RTS, the access server waits 2 seconds and monitors
the data-set-ready signal (DSR), which helps the access server identify the
type of device on the port. The presence of DSR indicates a null modem
device. A delay of DSR indicates a modem.
If DSR is delayed, the access server watches for one of the following
situations:
— A clear-to-send signal (CTS), which indicates a V.25/bis compatible
modem.
— The absence of a signal, which indicates a DIGITAL modem.