Number resolution - ENUM and TRIP
The system of telephone numbers is well defined and understood. There are billions of telephones, and billions of telephone numbers, each one no larger than 15 digits and globally unique. Alongside this system many people will be using SIP phones and SIP clients on their PCs and these will have SIP URLs associated with them. Fortunately, SIP can carry phone numbers using the new telephone URL (e.g. tel:208534845).
ENUM
A problem arises when one needs to associate a traditional phone with a resource on the Internet (this would be a requirement of any converged presence-type service). The IETF's ENUM (Telephone Number Resolution) working group is devising a scheme to enter telephone numbers onto the Internet DNS so that any application, including SIP, can discover resources available to a globally unique phone number. A SIP phone or proxy server would do the number domain translation and through classic DNS resolution discover a DNS resource that would give a SIP address at which the dialled number could be reached.
Further resources:
TRIP
Where a telephone number does not have a SIP resource associated with it, the IP network routes a call to a telephone gateway, which connects to the PSTN. In an interconnect environment with many peering relationships between service providers, resources in the IP network need to be able to discover which telephone numbers are associated with which gateways.
The IPTEL working group is developing TRIP (Telephony Routing overIP), a policy driven inter-administrative domain protocol for advertising the reachability of telephony destinations between location servers, and for advertising attributes of the routes to those destinations. TRIP is designed to allow service providers to exchange routing information in order to avoid the over-provisioning or duplication of gateways. It uses established Internet protocols such as BGP, OSPF and SCSP.
For further information see the TRIP charter