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Callback Time is the time interval that a Remote Access Service (RAS) server waits before it calls back a client when the Callback feature is enabled. This article describes how Callback Time works and when to use the "CALLBACKTIME=" and "CallBackTime:" parameters in various RAS versions.
This information supplements the "Administrator's Guide for Microsoft LAN Manager Remote Access" version 1.0 and 1.1 regarding the CALLBACK feature and supplements page 57 of the Windows NT version 3.1 "Remote Access Service" manual regarding the CallBackTime parameter.
LAN Manager RAS 1.0 and 1.1 ServerLAN Manager RAS 1.x servers use the parameter, CALLBACKTIME= to define Callback Time. Callback Time is dictated by the server regardless of any parameter that may be defined at the client.
LAN Manager RAS 1.0 and 1.1 ClientRAS 1.x clients do not have an individual Callback Time parameter and do not provide the server with a Callback Time. Thus, they depend on the server to define Callback Time.
Windows NT version 3.1 and 3.5 RAS ServerWindows NT servers use the CallBackTime: registry parameter to define the default Callback Time for any RAS client that does not provide an individual Callback Time to the server (RAS 1.x clients must use this default).
Windows RAS Client and Windows for Workgroups 3.11 RAS ClientWindows for Workgroups 3.11 RAS clients and Windows NT 3.1 and 3.5 RAS clients have a CALLBACKTIME= entry in the MODEM.INF file that determines the individual client's Callback Time. The client communicates this to the Windows NT RAS server during initial authentication. The Windows NT RAS server CallBackTime: registry value is ignored when the RAS client supplies a callback time. Clients connecting to RAS 1.x servers are not able to communicate their individual Callback Times and must accept the server's default.
NOTE: On a Windows NT RAS client there are two Callback Time parameters; the CALLBACKTIME= entry in MODEM.INF, which applies to the client function of RAS and the CallBackTime: registry parameter, which applies to the server function of RAS. Therefore, when a Windows NT RAS client connects to Windows NT RAS server, it is the client's CALLBACKTIME= entry in MODEM.INF that determines the Callback Time, not the CallBackTime: registry entry.
How Windows for Workgroups 3.11 and Windows NT Callback Time WorksAfter the RAS client hangs up the line, it sends an AutoAnswer command to the modem within approximately one half of the specified Callback Time. To allow the client modem sufficient time to switch to AutoAnswer mode and receive the call, the Windows NT RAS server waits for the full Callback Time before it attempts to call back.
Do Not Set Windows For Workgroups 3.11 CALLBACKTIME= Value Larger Than 170Windows for Workgroups 3.11 RAS clients with Callback Time values greater than 170 seconds do not wait long enough for the server to call back. The client times out with the following error message:
However, this is an unusually long Callback Time and there should not be any need to set the value that high.
Error 638: The request has timed out.
The Windows for Workgroups 3.11 RAS client times out because its callback algorithm sets the modem to AutoAnswer mode after approximately one half Callback Time and then only waits for about another 85 seconds for the server to call back. So, when the Callback Time is set very large, for example, 200 seconds, the client waits for 100 + 85 seconds before it times out, but the server won't call until 200 seconds have past.
Article ID: 111068 - Last Review: January 9, 2015 - Revision: 3.2