Before VoIP telephony it was PSTN landlines that reigned supreme and, though they may be on the way out, it is not going to happen anytime soon. As such both have to co-exist and interact with each other to facilitate seamless voice communication between two seemingly parallel universes. This is achieved through ENUM protocol.
What is ENUM?
ENUM is telephony protocol (E.164) which translates standard PSTN telephone numbers to internet addresses maintained in the ENUM lookup table in a server with ENUM support. If someone using an IP PBX dials a landline number then ENUM gets into the act by looking up the DNS in order to set up the call. Calls may go through direct to the PSTN number or may be referenced to SIP URI (Uniform Resource Locator) in order to dial a number. The SIP URI is responsible for identifying the communication resource, which may be in email or web page, after which it initiates the sequence of SIP request header, message, user, name, password, port and other parameters. SIP Proxies based on SER, OpenSIPs, Asterisk, Kamailo and Swyx offer full ENUM support.
The ENUM RFC 6116 protocol uses the internet DNS system to translate E.164 numbers to IP addressing schemes through the above mentioned SIP URI and it also makes use of RFC 6117 to register new ENUM services. RFC 6117 is followed by RFC 6118 to define ENUM standards. Currently, ENUM services must be registered with IANA.
How does ENUM work?
ENUM ties together VoIP and PSTN. Servers that have ENUM support will search for the dialed number in the tree and then set up the call using different types of ENUM.
Owner of each ENUM number is free to set rules on how calls must be routed. Owners can put in place one set of rules for a fax and another set for voice calls from phones.
For this system to work the ENUM number must be registered. VoIP service providers usually take care of registering ENUM but it can also be handled through registration service providers who then take it up with ITU and, once registered, the number is available for global use.
You have an E.164 phone number which usually has country code, area code and local code. These numbers are turned into FQDN code and the DNS is queries to show a list of URIs. The digits in the FQDN become definable and distributed zones in terms of the DNS. The DNS also defines authoritative name servers records. It is also possible for country codes, area codes and delegated number blocks to be delegated and the delegated model matches the E.164 number plan. The ENUM global directory or DNS then equates E.164 number to the return list of URI in order to initiate a SIP call. The DNS also returns NAPTR record containing SIP URL to calling party and the calling party proxy UAC queries DNS for end point location. All this goes on in the background even as the user simply dials the PSTN line. This is just how much ENUM simplifies telephony.
Types of ENUM
As stated above ENUM numbers must be registered and defined according to category.
Public Infrastructure ENUM
A National Number Administrator manages this ENUM type and assigns a phone number to a carrier that, in turn, assigns it to an end user and the number is mapped to internal network address for call routing purpose.
Private infrastructure ENUM
Specific groups may operate in a closed structure without involving public domain. In this type of ENUM a domain name is created for each phone number and linked to the URI.
Public user ENUM
End users can manage their own account and directly enter their numbers into the ENUM registry.
What does ENUM have to do with VoIP routing?
Not many people know about ENUM because it works in the background. You may be using a softphone or SIP phone or desk softphone and you dial another VoIP number or landline and the call goes through but ENUM is at work in the background. The URI kicks in and provides a generic naming scheme to define IP service points and it also helps in selection of transport protocol such as TCP/UDP, IP address, port address and address selector. With ENUM you are not tied to a specific service provider. It enables number portability and lets you switch services without worries of losing your number since the ENUM look is global in nature and, in addition, your system can choose method of call transport. Suppose you are using VoIP telephony and would like to call a landline number. Without ENUM and the ENUM lookup that SIP URI refers to, it simply would not be possible to route your VoIP call to that landline. Another advantage is that it also facilitates multiple applications such as video, mobility and presence besides service interoperability.
In case of VoIP to VoIP, ENUM is still functional. A caller dials a number and the query goes to the ENUM lookup over the internet and connections are established to the called party IP PBX. This is how it happens:
The user’s resolver creates ENUM domain name and queries the DNS which, in turn, gives the internet address of the VoIP user and leads to establishment of the call.
ENUM is the solid underpinnings of VoIP telephony that is invisible but indispensable. It leads to unification of global private VoIP dialing plans across VPNs and intranet links as well as PSTN and facilitates common administration as well as access. At the same time, due to flexibility of ENUM types, it is possible to optimize service routing and this facilitation also includes mobile numbers. So, what we have is ENUM that links together VoIP, PSTN and mobile GSM/LTE communications without the end user having to worry about complex steps.
ENUM is gaining in importance as the VoIP market explodes and PSTN shrinks but persists as do GSM networks.