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Source: Rec. ITU-T E.164 (11/2010)

CC - Country Code for geographic areaNDC - National Destination CodeSN - Subscriber NumberN - Number of digits in the country code

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An alternative model, often called "Private ENUM", is widely deployed. Private ENUM uses the DNS Protocol, but not the public DNS Database. Instead, the database either uses a private domain suffix/apex reserved for this purpose and known to all participants, or is provided by local DNS servers which do not tie into the public IANA-based tree, or more commonly both privacy tactics are used. The Private ENUM DNS servers typically reside in a private or restricted IP network, and are only accessible to specific clients. Such Private ENUM clients are typically constrained to be ones owned and managed by the carrier,

Unlike Infrastructure ENUM, Private ENUM DNS database entries are not registered and populated by the carrier-of-record for a given E.164 number. Instead, the private database's administrator (the local carrier) directly provisions the entries for all E.164 numbers it cares about, based on various indirect information data sources, and sets the entry URI values relative to their specific "view".

Source: draft-kaplan-enum-sip-routing-01

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Step 1: To resolve TEL URL to SIP URL

The output of the First Well Known Rule for the ENUM Application is the E.164 number minus all non-digit characters except for the +. In order to convert this to a unique key in this Database the string is converted into a domain-name according to this algorithm:

1. Remove all characters with the exception of the digits. For example, the First Well Known Rule produced the Key "+442079460148". This step would simply remove the leading "+", producing "442079460148".

2. Put dots (".") between each digit. Example 4.4.2.0.7.9.4.6.0.1.4.83. Reverse the order of the digits. Example: 8.4.1.0.6.4.9.7.0.2.4.44. Append the string ".e164.arpa" to the end. Example:

8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa. This domain-name is used to request NAPTR records which may contain the end result.

If the query success, the response of ENUM server shall return:$ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa.NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:info@domainB!"

This describes that the domain 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa. is preferably contacted by SIP

Step 2: From the SIP URL, resolve the domain name into IP address

The ENUM/DNS translation mechanism as specified in IETF RFC 3761 [16] can be used by all IMS nodes that require E.164 address to SIP URI resolution.The S-CSCF shall support the ability to translate the E.164 address contained in a Request-URI in the Tel: URI format (as specified in IETF RFC 3966 [15]) to a SIP routable SIP URI using the ENUM/DNS translation mechanism.If this translation succeeds, then the session shall be routed according to the returned SIP URI.

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If this translation fails, then the session may be forwarded to a BGCF for further routing (e.g. to the PSTN).

Based on local configuration, the Breakout Gateway Control Function (BGCF) may be provisioned as the contact point within an operator's network for transit IMS scenarios. Otherwise the BGCF processes requests for routing from an S-CSCF for the case were the S-CSCF has determined that the session cannot be routed using DNS or ENUM/DNS .

The BGCF determines the next hop for routing the SIP message. This determination may be based on information received in the protocol, administrative information, and/or database access. For PSTN terminations, the BGCF determines the network in which PSTN/CS Domain breakout is to occur. If the routing determination is such that the breakout is to occur in the same network in which the BGCF is located, then the BGCF shall select a MGCF that will be responsible for the interworking with the PSTN/CS Domain.

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