request queues ASP.NET

ASP.NET uses two request queues:
•The global queue, which is managed by the process that runs ASP.NET (Aspnet_wp). The global queue is configured in the Machine.config file by the property.
•The application queue, or virtual directory queue, which is managed by the HttpRuntime class. The HttpRuntime class provides run-time services for ASP.NET applications. There is one queue for each virtual directory. The application queue is configured in Machine.config by the property.
When either queue exceeds its default limit, the request is rejected with a 503 error (Service Unavailable).