TRIP, ENUM and Number Portability

This paper describes the problem of locating terminals using E.164 numbers, and the problem of selecting a suitable gateway for calls from an IP telephony network to the public switched telephone network (PSTN). Generally, these are the problems of mapping the name of a destination into an address, and to find the best route to the destination in a combined IP and PSTN network. Number portability is closely related to these problems. Due to number portability the address of a destination is changed without changing its name. Number portability may also change the optimal route to a destination. Two protocols are being developed by the Internet Engineering Task Force (IETF) to solve these problems. The Telephony Routing over IP (TRIP) protocol solves the gateway location problem by distributing routing information between entities on the IP network. The tElephony NUmbering Mapping (ENUM) provides a solution to the terminal location problem based on DNS. ENUM maps an E.164 number into an URI, which is used to locate the end point. Both protocols aim to add a part to the architecture in order to make a global hybrid PSTN-IP network possible. They also aim to enable number portability in the IP network and between the two types of networks. In this paper, we will introduce the concept of terminal and gateway location. We describe how the current protocols locate terminals and gateways, and what the problems with the current solutions are. The TRIP and ENUM protocols are presented in detail and scenarios based on the protocols are described. Solutions to number portability are presented and some problems are discussed.