Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 831)

Introduction
Page 28-5
DTR Dial Backup
Currently, a feature is available to use a dynamic ISDN call as a backup WAN connection for
a primary WAN connection. The primary WAN connection is a permanent virtual circuit (PVC)
with upper layer protocol of Frame Relay or PPP. The DTR Dial Backup feature will allow
another synchronous serial interface to be used for the backup purpose.
The process is analogous to the ISDN backup feature. When the primary circuit fails over a
configurable elapsed time, the initiating side of the backup connection asserts the DTR line of
the designated backup port, causing the attached modem to dial a pre-configured phone
number (i.e. the phone number to call is stored within the modem itself). The modem shall
assert the DSR, CD, and the CTS signals of the serial interface when the call is established and
the modems have fully trained and synchronized. Sensing the call having completed
successfully (via a port-up event), the PPP negotiation commences. Upon a successful PPP
negotiation, the backup virtual circuit, the virtual port and the associated router interface are
activated. The receiving side of the backup connection shall wait for the attached modem to
signal an incoming call (again by looking for all three primary modem control signals: DSR,
CD and CTS to be active and the resulting port-up event). The backup virtual circuit, virtual
port, and router interface are activated after the ensuing PPP negotiation completes
successfully.
After the primary circuit is restored over a configurable elapsed time, the call initiation side
shall turn off the DTR signal causing the modem to terminate the call. It will then also
deactivate the backup circuit at this time. The call receiving side shall receive a indication
from the attached modem on call termination and proceed to deactivate the backup
connection.
This feature only works in conjunction with modems configured for DTR dialing and
synchronous mode operation.
The DTR dial backup feature is only valid on serial ports using Point to Point Protocol (PPP).