ENUM is perhaps the most talked about protocol of the RCTS VoIP network and the pillar on which the network exists.
But what is ENUM anyway?

ENUMor E.164 Number Mapping is the protocol that allows the association of phone numbers in IP resources, such as URL(Uniform Resource Locators). It plays a key role in the RCTS VoIP network because it is through it that voice calls are allowed to be made through the RCTS network and not through the PSTN (operators).

ENUM, maps phone numbers to services or equipment connected to the network.

Similar to DNS(Domain Name System), ENUM functions as a name resolution system. The protocol converts numbering into domain names where each digit of the number is represented by a string of characters. The response of a NAPTR(Name Authority Pointer) record from this conversion should tell us the location of the contact, i.e. the domain where the number is registered.

In order to understand the ENUM process, we can manually perform the protocol process. We used the FCCN numbering as an example, however, any numbering registered in e164.arpa, or also known as "Golden Tree" will work as well.

FCCN Contact: (+351) 218 440 100

First, we manually convert this numbering to a specific format, that is, we invert the number and add the domain e164.arpa. Next, we perform an information analysis on the NAPTR type record with that same domain, as shown in the image below.

Picture1 FCCN Computation Unit

The answer to this request should be the location of the numbering, in this case, the domain to which this numbering belongs is voip.fccn.pt which in turn will have a record A associated with the domain.

Picture2 FCCN Computation Unit

It is to this same IP that the SIP signal will be sent and thus the call is successfully carried out. It is important to note that the various SRVs with the desired priorities must be configured in the domain's NAPTR record.

Picture3 FCCN Computation Unit

The reason why it is important that in the NAPTR registry of the domain the SRVs with the different transport types are configured is because the ENUM will use one of these services as a resource to make the call. The record of each SRV should answer us with the domain of the "owner"ofthe numbering itself. For example:

Picture4 FCCN Computation Unit

In short, ENUM uses the NAPTR, SRV and A records to identify the location of the registered numbering in the ENUM tree (e164.arpa). Converting this numbering into a domain allows us to register the numbering in a manner identical to DNS. The call is made to the A record of the final domain. This allows calls between institutions not to go out to the PSTN, using only the RCTS network to make the calls without paying any euro.

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