Shutting down a CDPD session in an orderly fashion after an abnormal termination at the F-ES side of the session, eliminates the possibility of the DART 200 having a half open TCP/IP connection, which usually requires a power cycle to clear. This can be a serious problem if the DART is in a remote location. The default for this feature is not enabled (*K0).
This facility works in transmit only mode, receive only mode, or in both modes. It uses a keep-alive character (S-Register 85) and a keep-alive time-out (S-Register 86) to implement the function. The timer restarts when data is sent or received. The default value for the keep-alive character is binary 0 with an allowable range of 0 to 255, and the time-out period default is 120 minutes with an allowable range of 1 to 255.
Timer expiry in transmit mode
If the timer expires in transmit (or transmit/receive) mode, pending data or the keep-alive character is forwarded. This ensures that the receiving session does not time out. For a half open connection (the other end terminated abnormally) this transmission eventually causes the connection to close, because the other end cannot acknowledge receipt of the data packet or the keep-alive character.
Timer expiry in receive mode
In receive (or transmit/receive) mode, incoming keep-alive packets are discarded if the incoming keep-alive character matches the value in S-Register 85. If the timer expires in this mode, meaning no data or keep-alive character was received within the timeout period, an escape is performed and an ATH command is issued to close the connection. This leaves the DART 200 ready to receive another incoming connection request.
Timer expiry for keep-alives
For one way keep-alives (transmit only at one end and receive only at the other) set the transmit time-out value as less than the receive time-out value to ensure proper operation. For keep-alives in both directions (transmit/receive) set the time-out values at each end as equal.
The most common use of this feature is with an unattended MAS, where the modem is set up with the receive keep-alive option specified (*K2) and S-Register 86 set in the 3 to 5 minute range. This permits the modem to break the session, if the central site crashes, and be ready for another call without manual intervention when the central site restarts.
