Internet Communications Using SIP - RIPHAH INFORMATION ...

DNS and ENUM 71Root“.” and arpaInternationalTier 0e164.arpaCountry Code 1NPA DataTier 1A1.e164.arpaRegistrantRegistrarTier 1BTier 1BTier 1BIndividual zonesTier 2Tier 2 Provider Example:4.3.2.1.5.5.5.4.0.4.1e164.arpaApp ServiceProvider (User)Figure 4.4The ENUM functional architectureENUM and Number PortabilityThe notion of registering blocks of numbers for service providers is complicatedby number portability (NP), which allows end users to keep their telephonenumbers if they change service providers or move from one location toanother within a calling area. The Tel URI can have extensions to inform thePSTN switch that this is a ported number and where it has been ported. To performthis function, a numbering plan database dip has to be performed asdescribed in [17].Implementation IssuesWe have so far considered the Tier 2 address resolution using the ENUMNAPTR approach. Several considerations have to be taken into account whenchoosing an implementation alternative:■■Client complexity—The NAPTR solution requires more complex regularexpression processing and parsing to obtain a result, but a single DNSclient is all that is required.