2.1 Modem Functions
2.1.3 Communication Termination
The Figure shows one of the modem-control functions terminating a connection. Either the DTE device or the DCE device may signal for the connection to be terminated. The signals that are used for this function are DTR from the DTE or the modem recognizing the loss of the CD signal. Therefore, a modem connection can be terminated in two ways:
  • DTE initiated—The access server or computer can drop the DTR signal. The modem must be programmed to terminate the connection on loss of DTR and restore to saved settings in its nonvolatile RAM (NVRAM).
  • DCE initiated—The access server detects CD low and terminates the connection. The modem must be programmed so that the CD reflects the state of the carrier.

When modem control is not configured properly, the following symptoms might occur:

  • The modem does not hang up when you quit your session, meaning the DTR is not dropped or recognized, so the modem is not aware that it should break the connection.
  • You end up in someone else's session, meaning that the CD is not dropped or recognized. This scenario happens when Caller A terminates his/her dialup session, and the modem does not pass the true state of the CD to the DTE. The access server is not aware that Caller A terminated his/her session, so it maintains the line for Caller A. When a new caller, Caller B, comes in by the same line (interface), the access server continues with the session previously initiated by Caller A, instead of starting a new one. Thus, Caller B ends up in Caller A's session without having to authenticate. It is, therefore, very important that the true state of CD is always passed back to the DTE, so the access server terminates sessions when callers hang up.