Partial-Page Rendering
Partial-page rendering makes it unnecessary for the whole ASP.NET page to be refreshed as the result of a postback. Instead, only the region of the page that has to be refreshed is updated. As a result, users do not see the whole page reload with every postback, which makes user interaction with the Web page more seamless.
UpdatePanel Control
UpdatePanel controls work by specifying regions of a page that can be updated without refreshing the whole page. This process is coordinated by the ScriptManager server control and the client PageRequestManager class. When partial-page updates are enabled, controls can asynchronously post to the server. An asynchronous postback behaves like a regular postback in that the resulting server page executes the complete page and control life cycle. However, with an asynchronous postback, page updates are limited to regions of the page that are enclosed in UpdatePanel controls and that are marked to be updated. The server sends HTML markup for only the affected elements to the browser. In the browser, the client PageRequestManager class performs Document Object Model (DOM) manipulation to replace existing HTML with updated markup.
UpdateProgress Control
Provides visual feedback in the browser when the content of UpdatePanel controls is updated. The UpdateProgress control renders a <div> element that is displayed or hidden depending on whether an associated UpdatePanel control has caused an asynchronous postback. For initial page rendering and for synchronous postbacks, the UpdateProgress control is not displayed.
PageRequestManager Class
The PageRequestManager class exposes properties, methods, and events that enable you to customize partial-page updates with client script. The PageRequestManager class exposes a client page event model that you can use in a way similar to how you use the server page event model.
During partial-page updates initiated by asynchronous postbacks, the PageRequestManager class coordinates how the page content is incrementally updated in the browser. The UpdatePanel server control and PageRequestManager client class abstract much of the complexity of partial-page updates. When you use client script and members of the PageRequestManager class, you can customize the partial-page update behavior in the browser.
To use the PageRequestManager class in client script, you must put a ScriptManager server control on the Web page. The EnablePartialRendering property of the ScriptManager control must be set to true (which is the default). When EnablePartialRendering is set to true, the client-script library that contains the PageRequestManager class is available in the page.
ScriptManager Control
The ScriptManager control manages client script for Microsoft ASP.NET AJAX pages. By default, the ScriptManager control registers the script for the Microsoft AJAX Library with the page. This enables client script to use the type system extensions and to support features such as partial-page rendering and Web-service calls.
You must use a ScriptManager control on a page to enable the following features of ASP.NET AJAX:
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Client-script functionality of the Microsoft AJAX Library, and any custom script that you want to send to the browser.
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Partial-page rendering
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JavaScript proxy classes for Web services, which enable you to use client script to access Web services by exposing Web services as strongly typed objects.
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JavaScript classes to access ASP.NET authentication and profile application services.
Timer Control
The Timer control performs postbacks at defined intervals. If you use the Timer control with an UpdatePanel control, you can enable partial-page updates at a defined interval. You can also use the Timer control to post the whole page.
The Timer control is a server control that embeds a JavaScript component into the Web page. The JavaScript component initiates the postback from the browser when the interval that is defined in the Interval property has elapsed. You set the properties for the Timer control in code that runs on the server and those properties are passed to the JavaScript component.
An instance of the ScriptManager class must be included in the Web page when you use the Timer control.
When a postback was initiated by the Timer control, the Timer control raises the Tick event on the server. You can create an event handler for the Tick event to perform actions when the page is posted to the server.
Set the Interval property to specify how often postbacks will occur, and set the Enabled property to turn the Timer on or off. The Interval property is defined in milliseconds and has a default value of 60,000 milliseconds, or 60 seconds.
Reference
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Ramani Sandeep 