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The role of ENUM in NGNs

When PSTN callers wish to contact NGN subscribers, they cannot directly dial their NGN address; instead the network must be able to map between E.164 addresses and NGN addresses. ENUM facilitates this...

Call Scenarios Needing ENUM

Telephone calls which involve VoIP at one or both ends of the call fall into one of three categories:

  • Telephone calls between IP-attached subscribers are routed entirely using the appropriate SIP URI. The Domain Name System (DNS) is used to identify the appropriate SIP server to handle call requests when these involve multiple domains. A DNS service (SRV) record is typically used to identify the SIP server for the destination domain by its canonical name.
  • Telephone calls which originate on a SIP client and terminate on a PSTN line can embed E.164 telephone numbers within the SIP URI, and the SIP server handling these outbound requests can strip the domain part of the address and route to a PSTN gateway using the E.164 destination address.
  • Telephone calls which originate on the PSTN and terminate on a SIP device cannot embed a SIP URI as a called party identity, since they, the PSTN network and its signalling are restricted to dialling and routing E.164 addresses only. Therefore some means of network-based mapping between E.164 addresses and (typically) URIs is required for routing of these calls at the voice application level. The E.164 Numbers (ENUM) initiative from the IETF has been widely adopted to solve this problem in next Generation Networks (NGNs). It makes use of conventional DNS servers, but makes use of a special record type called a Naming Authority Pointer (NAPTR), defined in RFC 2915.

RFC 2916 defines a domain specifically for mapping telephone numbers to URIs in the IP world. Consider the Technology Training Limited main switchboard number

+44 845 672 0175

This is represented within the DNS by the special domain as follows:

To discover the URI corresponding to this E.164 number, a client machine (an actual endpoint or a call server function in the network) makes a request for DNS records for this unique address, and is either provided with one or more NAPTR records indicating the translated address, or directed to a lower-level name server, for example corresponding to the specific country code contained in the E.164 number. This operates in the same way as the hierarchical domains of the conventional DNS.


Many countries are trialling and implementing ENUM systems on a national basis. In the UK, for example, Nominet has been awarded the contract to operate the UK tier 1 ENUM database. Individual operators will run tier 2 ENUM databases for their assigned numbers ranges and subscribers underneath this. A similar model is expected to apply in other countries.

NAPTR Records and Processing

NAPTR records allow string parsing at the DNS server rather than the static lookup of conventional DNS records. In the example shown below, a request to the domain for a UK number (with country code 44) would return the canonical name of the name server with authority for this sub-domain, and to which a subsequent request for the full translation could be made.


A subsequent request to the authoritative name server for the specific record might contain entries similar to the following:


IN NAPTR 10 10 u sip+E2U

IN NAPTR 10 10 u sip+E2U

IN NAPTR 10 10 u sip+E2U tel:+44 1242 650010

All of these records would be returned to the requesting client, which would then choose according to internal policy rules which URI should be used to contact the called party. Several or all of the methods might be used simultaneously.

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