Define PURL, Metadata, Checksum

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Define PURL:  Persistent URL, a type of URL that acts as an intermediary for a real URL of a Web resource. When you enter a PURL in a browser, the browser sends the page request to a PURL server which then returns the real URL of the page. PURLs are persistent because once a PURL is established, it never needs to change. The real address of the web page may change but the PURL remains the same.PURLs are managed by the Online Computer Library Center (OCLC).
Example: A PURL is like a PO box with an address that is constant, even if there are changes to the owner's address. When an address changes, the postal service receives forwarding information, so there is no need for the owner to inform others; the postal service simply forwards mail to the owner's new address.

Metadata: Metadata describes other data. It provides information about a certain item's content. For example, an image may include metadata that describes how large the picture is, the color depth, the image resolution, when the image was created, and other data. A text document's metadata may contain information about how long the document is, who the author is, when the document was written, and a short summary of the document.
Web pages often include metadata in the form of meta tags. Description and keywords meta tags are commonly used to describe the Web page's content. Most search engines use this data when adding pages to their search index.




Checksum: A checksum is an error-detection method in the transmitter computes a numerical value according to the number of set or unset bits in a message and sends it along with each message frame. At the receiver end, the same checksum function (formula) is applied to the message frame to retrieve the numerical value. If the received checksum value matches the set value, the transmission is considered to be successful and error-free. A checksum may also be known as a hash sum.














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