DMS-100 Automatic Dial Back


Supplementary Information

This section provides information on product descriptive information related to table DIALBACK.

Dial Back

The special dial-back login sequence is performed only if the correct hardware and firmware are available and the dial-back flag associated with the modem is set.

The first login is a special dial-back login which requires a dial-back ID and password.  The second login is the normal login currently used to gain access to the system.  The passwords associated with the dial-back IDs are assigned and maintained by the operating company.  Passwords are required to have a minimum length.

After the dial-back ID and password are obtained, the modem is disconnected and the remote user is expected to hang up.  A brief explanatory message is displayed before the disconnect.  A random number of garbage characters can appear on the user's terminal due to the disconnect.  This is expected and is no cause of alarm.  No attempt is made to inform the user whether the ID and password which were entered were correct.  A person trying to break into the system would not immediately know whether he was successful.  If the attempt is successful (a correct ID and password pair are entered), a dial-back is performed.

After a short delay to allow the originator to hang up the line and set up the modem, the system then begins to dial-out on a second modem using the Directory Number (DN) associated with the dial-back ID that the user first entered.  There is a one-to-one mapping between dial-back IDs and DNs (that is, the DN is a function of the dial-back ID).  The manner in which this number is determined provides an indirect level of security since the called number (the one the system calls) is not calling number (the number of the remote site) unless the proper dial-back ID is entered.  The amount of time elapsed between the modem disconnect and the completion of the return call varies between 80 and 240 seconds for the Rixon modem (slightly less for the CTS212AH modem).  The time is dependent on the baud rate of the port, the load on the switch, the length of the DN dialed, the number of dial-backs attempted and the type of modem used for the dial-back.  The operating company is responsible for assigning and maintaining the DNs associated with the dial-back IDs.

Once the call is connected, the following message is displayed and the user is automatically prompted to login:

DIALBACK COMPLETE

It is important that the user does not hit the <BREAK> key in order to obtain the logon prompt as this reinitiates the dial-back sequence and causes the automatic login to abort.




Commands

If feature package NTX293AA (Enhanced Security Package II) is present in the switch, the following command interpreter (CI) commands are available: LOGINCONTROL, DIALBACKPW and SHOWDBPW, and office parameter DIALBACKPW_ENCRYPTED in table OFCOPT.

The CI command LOGINCONTROL permits the operating company to turn dial-back on and off for a specific port, as well as change three dial-out-related values (number of rings for each dial-back attempt, number of dial-back attempts and the type of dial line).

The CI command DIALBACKPW allows the operating company to change dial-back passwords.  This must be a privileged command in order to prevent security violations.  The security of this feature depends on the operating company assigning appropriate command classes for this command.

The CI command SHOWDBPW can be used to show dial-back passwords.  It is only available if the office parameter DIALBACKPW_ENCRYPTED is not set.

The dial-back, by the system, is made on a line different from the one which was used for the incoming call.  At least two modems must be connected to the switch.  The CI command LOGINCONTROL is used to specify whether a modem is used as an answer modem or a dial-out modem when the dial-back function is active.

Tables

Field MODEM specifies which type of modem (if any) is connected to the port.  This field can be changed to any acceptable value by using the table editor.

Table DIALBACK, stores dial-back-related data.  It contains fields for the dial-back ID, the DN and one for any miscellaneous data which the operating company wishes associated with a specific DN or dial-back ID.

Modems

The Companion CTS212AH Smart Modem and the Rixon R212A Intelligent Modem can be used with feature BC1043 (Automatic Dial Back).  Both of the above mentioned modems satisfy the following requirements:

Note 1:  The operation of the Rixon is sensitive to cable length.  If the cable is too long, the MODEM UNSTABLE log report can occur frequently.

Note 2:  Outgoing dial-back modems as set with the CI command LOGINCONTROL with device DIALBACK DIAL require that the Digital Coupling Device (DCD) (Input/Output Controller [IOC] connector pin-31) and Call-Through Simulator (CTS) (pin-34) leads are tied high with a strap in the IOC connector.  Otherwise, the DMS cannot send the modem initialization string and the SECU122 log reports (DIALBACK FAILED.  NO MODEM AVAILABLE) are generated.

Cable CAOX15 must be used for the dial-back modems to operate correctly.  This cable has pin-31 and pin-37 strapped at the IOC end to allow the DMS to send initialization strings to the modem.

Since outgoing calls are placed on one of these other modems, the requirement that the line connected to the first modem be dropped is unnecessary.  In most cases, the line is dropped when the system disconnects its modem.  The only time this does not occur is when the original call was placed through a cross-bar or step-by-step switch.  In these two cases, only the originator can drop the call.

An abnormal modem disconnect can also occur if the modem detects noise on the line or the physical connection between the user's modem and the DMS is broken.  Telephone lines to which the modems are connected generally do not have Call Waiting feature since it produces an audible tone that the modem regards as noise.  This is not mandatory requirement, but only a suggested one since modems disconnected in this way can be hung.  It is then necessary to force busy (BSY) and Return-the-Port-to-Service (RTS) twice before the modem is again available.  It can also be necessary to re-enable the consoles (that is, the dial-in and dial-out modems) using the command LOGINCONTROL.

Disconnects that occur during login prompting (for dial-back password or CI password) can cause the login process to hang the port indefinately.  All dial-back ports must have a login time-out and an idle time-out set using the command LOGINCONTROL.

Note:  Read and write access to this table must be restricted by the operating company.  Datafill dial-back IDs and DNs of the modems on which feature Automatic Dial Back is enabled in table CUSTPROF.