Figure 4.4 The ENUM functional architecture
ENUM and Number Portability
The notion of registering blocks of numbers for service providers is compli-
cated by number portability (NP), which allows end users to keep their tele-
phone numbers if they change service providers or move from one location to
another within a calling area. The Tel URI can have extensions to inform the
PSTN switch that this is a ported number and where it has been ported. To per-
form this function, a numbering plan database dip has to be performed as
described in [17].
Implementation Issues
We have so far considered the Tier 2 address resolution using the ENUM
NAPTR approach. Several considerations have to be taken into account when
choosing an implementation alternative:
■■ Client complexity—The NAPTR solution requires more complex regular
expression processing and parsing to obtain a result, but a single DNS
client is all that is required.Country Code 1 NPA DataRegistrar Tier 1B Tier 1BTier 1ATier 2Tier 0RootTier 1B Individual zonesTier 2 Provider Example:
4.3.2.1.5.5.5.4.0.4.1e164.arpaInternational“.” and arpa1.e164.arpae164.arpaApp Service
Provider (User)RegistrantDNS and ENUM 71