A call processing service makes its proxy, redirect, and rejection
decisions based on the contents of that database. A CPL speci?¬?cation replaces this
basic database lookup functionality; it takes the registration information, the speci?¬?cs
of a call request, and other external information it wants to reference and chooses the
signaling actions to perform. To put it simply, CPL describes how devices respond to
calls and how a system routes calls.
Services in CPL are normally associated with a particular Internet telephony
address. When a call arrives at a signaling server that is a CPL server, the server
associates the source and destination addresses speci?¬?ed in the request with its
database of CPL services. If one matches, the corresponding CPL service is executed.
At this point, we should emphasize that creation of call processing services is not
bound to any speci?¬?c user type or organization. Services can be created by different
kinds of people as follows:
. An end user can make or modify a service by de?¬?ning a CPL script and
uploading it to a server. This type of user is typically a nonprogrammer.
. A third-party service provider can create or customize services for its clients.
. The administrator of an IP telephony server can create services or describe
policy for the servers they control.
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