Internet Information Services (IIS) limits the maximum number of application pool at any given time can wait in the queue request. If this limit is reached, all new requests will be rejected, and the user will receive an error message "503 - Service Unavailable." The default, IIS queue length to 1,000 requests. The default value may be too low, because if the queue length of 1,000, Communicator Web Access (2007 R2 release) will only be able to handle approximately 650 concurrent users, then will begin to deny new user.
To help ensure the availability of Communicator Web Access service, it is recommended to set the IIS queue length to the expected value of 1.5 times the maximum number of users. For example, suppose you expect at a given time a maximum of 2,000 users log on to Communicator Web Access. In this case, the queue length should be set to 3000 (i.e., 1.5 times 2,000 users).
Change queue length in IIS 7.0
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Click "Start", point to "Administrative Tools" and then click "Internet Information Services (IIS) Manager."
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In IIS Manager, expand the name of the IIS server, and then click "application pool."
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In the "Application Pool" pane, right-click the Communicator Web Access application pool, and then click "Advanced Settings."
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In the "Advanced Settings" dialog "General", click the "Queue Length", then type up to 1.5 times the number of concurrent users of Communicator Web Access deployment of value.
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Click "OK."
Change the queue length setting in IIS 6.0
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Click "Start," click "All Programs", point to "Administrative Tools" and then click "Internet Information Services (IIS) Manager."
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In IIS Manager, expand the "Sites" node, right-click the Communicator Web Access application pool, and then click "Properties."
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In the "Properties" dialog box, click the "Performance" tab.
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Under "request queue limit", click the "core limit request queue (number of requests)", then type up to 1.5 times the number of concurrent users of Communicator Web Access deployment of value.
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Click "OK."