The limited scope makes sure that the CPL server??™s
security will be ensured. Looking from the DSM creation side, the domain is well
restricted and the requirement of complexity hiding is highly relevant. Also,
generators offer obvious advantages here as quality and correctness of services are of
great importance. Nobody wants to miss a call just because the service de?¬?nition had
errors.
A computational model of a CPL speci?¬?cation is a list of condition and action
pairs: if the system condition matches a condition speci?¬?ed in a service, then a
corresponding action or actions are performed. A typical system condition in CPL is,
for example, that a call arrives and the line is busy or it is a certain day of the week.
Example actions redirect the call to the user??™s mobile phone or reject the call.
98 IP TELEPHONY AND CALL PROCESSING
If conditions are not speci?¬?ed in the CPL speci?¬?cation, then the server??™s standard
action is taken.
5.1.1 CPL Architecture
Architecturally, a call processing service is executed in a signaling server. Signaling
servers are devices that relay or control signaling information. In the SIP world
(Rosenberg et al., 2002), examples of signaling servers are proxy servers and redirect
servers. A signaling server also normally maintains a database of locations where a
user can be reached.
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