MODEM CONTROL Sequences

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.

1First, 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.

2After 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.

Note

For dial-out modems, the access server enables data communication before detecting DSR. Otherwise, the access server waits until detecting DSR to enable data communication.

10-18 Configuring Modem Signals