This article was previously published under Q111068
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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 isenabled. 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 LANManager Remote Access" version 1.0 and 1.1 regarding the CALLBACK featureand supplements page 57 of the Windows NT version 3.1 "Remote AccessService" manual regarding the CallBackTime parameter.
LAN Manager RAS 1.0 and 1.1 Server
LAN Manager RAS 1.x servers use the parameter, CALLBACKTIME= to defineCallback Time. Callback Time is dictated by the server regardless of anyparameter that may be defined at the client.
LAN Manager RAS 1.0 and 1.1 Client
RAS 1.x clients do not have an individual Callback Time parameter and donot provide the server with a Callback Time. Thus, they depend on theserver to define Callback Time.
Windows NT version 3.1 and 3.5 RAS Server
Windows NT servers use the CallBackTime: registry parameter to define thedefault Callback Time for any RAS client that does not provide anindividual Callback Time to the server (RAS 1.x clients must use thisdefault).
Windows RAS Client and Windows for Workgroups 3.11 RAS Client
Windows for Workgroups 3.11 RAS clients and Windows NT 3.1 and 3.5 RASclients have a CALLBACKTIME= entry in the MODEM.INF file that determinesthe individual client's Callback Time. The client communicates this to theWindows NT RAS server during initial authentication. The Windows NT RASserver CallBackTime: registry value is ignored when the RAS client suppliesa callback time. Clients connecting to RAS 1.x servers are not able tocommunicate their individual Callback Times and must accept the server'sdefault.
NOTE: On a Windows NT RAS client there are two Callback Time parameters;the CALLBACKTIME= entry in MODEM.INF, which applies to the client functionof RAS and the CallBackTime: registry parameter, which applies to theserver function of RAS. Therefore, when a Windows NT RAS client connects toWindows NT RAS server, it is the client's CALLBACKTIME= entry in MODEM.INFthat determines the Callback Time, not the CallBackTime: registry entry.
How Windows for Workgroups 3.11 and Windows NT Callback Time Works
After the RAS client hangs up the line, it sends an AutoAnswer command tothe modem within approximately one half of the specified Callback Time. Toallow the client modem sufficient time to switch to AutoAnswer mode andreceive the call, the Windows NT RAS server waits for the full CallbackTime before it attempts to call back.
Do Not Set Windows For Workgroups 3.11 CALLBACKTIME= Value Larger Than 170
Windows for Workgroups 3.11 RAS clients with Callback Time values greaterthan 170 seconds do not wait long enough for the server to call back. Theclient times out with the following error message:
Error 638: The request has timed out.
However, this is an unusually long Callback Time and there should not beany need to set the value that high.
The Windows for Workgroups 3.11 RAS client times out because its callbackalgorithm sets the modem to AutoAnswer mode after approximately one halfCallback Time and then only waits for about another 85 seconds for theserver to call back. So, when the Callback Time is set very large, forexample, 200 seconds, the client waits for 100 + 85 seconds before it timesout, but the server won't call until 200 seconds have past.