Contact: phone: <tel:+1-214-555-1212; service=pager
>;mobility=”mobile
“;duplex=send-only ;media=
;text; ;q=0.1; ;priority=”urgent
“ ;description=”For emergency only”Contact: <mailto:[email protected]>This contact list for Henry shows the most preferred contact to be voicemail,
followed by an ISDN phone number to call, pager, and e-mail in that order.
The callers having only a telephone, PBX, or PSTN, must only dial a single
phone number that will be used in the tel URI on the Internet side.
■■ DNS and ENUM resolution will route the call to the SIP server.
■■ The SIP server can map the address of record (AOR) to all the Contact
addresses of the called party.
■■ The called party’s contact addresses are kept confidential.
■■ The called party has full control if and where to receive the call.
■■ If the PBX or the PSTN telephone switch has no support for Internet
telephony, the call will be routed over the PSTN, usually at higher cost.The ENUM Functional Architecture
The ENUM application for DNS presented in the previous sections is quite
straightforward for DNS developers and administrators but raises significant
issues regarding the registration of users and their telephone phone numbers.
The mapping of telephone numbers to IP addresses falls at the intersection of
two very different networks and standards bodies, as shown in Table 4.3.
Besides the technical standards, there are huge global industries dependent
on the addressing on both the PSTN and the Internet, and the registration of
users is big business indeed. For these reasons, the schema for registration of
telephone users in the DNS databases has required good collaboration
between the IETF and IUT-T and, as of the writing of this book, a reasonable
agreement has been reached.
Table 4.3 Networks and Standards Bodies
NETWORK STANDARDS ADDRESS ADMINISTRATION
Internet IETF ICANN
Fixed & mobile phones ITU-T, 3GPP/3GPP2 ITU-TDNS and ENUM 69