The IMS: IP Multimedia Concepts And Services, Second Edition

16.3. ENUM – the E.164 to Universal Resource Identifier (URI) Dynamic Delegation Discovery System (DDDS) application

ENUM is described as the use of the DNS for storage of international public telecommunication numbers using international format (E.164) numbers [RFC3761]. This is used to discover what services are available for a specific domain.

Imagine that user A has a Public Switched Telephone Network (PSTN) phone and would like to call her friend user B who uses Voice over IP (VoIP). User A cannot enter a SIP address into her PSTN phone and, therefore, enters user B's E.164 number. The call reaches a gateway that performs a DNS ENUM look-up and returns user B's SIP address (see Figure 16.1). IP-to-IP calls are also feasible using E.164 numbers. The steps involved in this procedure are as follows:

1. User A dials the E.164 number, [phone].

2. The Circuit-Switched (CS) domain contacts a gateway.

3. The gateway formats the E.164 number into a Fully Qualified Domain Name (FQDN), 8.7.6.5.4.3.2.1.8.5.3.e164.arpa, and looks up the FQDN in DNS.

4. DNS returns the NAPTR record, sip:[e-mail]

   Figure 16.1. CS-to-IP call example.

   

5. The gateway contacts DNS for the IP address of the SIP server in the domain "example.com", following the procedures in Section 16.12.1.1.

6. DNS returns the IP address of the SIP server.

7. The gateway routes the call to the SIP server.

8. The SIP server contacts user B and ...