Telephone number mapping

from Wikipedia, the free encyclopedia

Telephone Number Mapping (also E.164 Number Mapping , ENUM ) is an application of the Domain Name System for the translation of telephone numbers into Internet addresses. ENUM is defined in RFC 6116 . The need for such a solution arose from the availability of VoIP services ( telephone over the Internet) and the need for users to be reachable under the same number both on the Internet and in the traditional telephone network. It is the declared aim of the service developers to make all services that are available on the Internet or that may be made available in the future, accessible under a central Internet address ( domain ). In fact, however, the free telephone routing, in which a VoIP subscriber can reach another VoIP subscriber directly under a landline number (bypassing the network provider and therefore virtually “free of charge”), is the main reason for using ENUM services.

Basic possibilities of ENUM

The basic aim of ENUM is the resolution (conversion) of a classic telephone number into a target name under which a service on the Internet, for example an Internet telephone, is available. For example, the telephone number “123456789” can be resolved or converted into the target name “participant123@dienstdienstleistungen.de” of an Internet telephone using an ENUM lookup . Here, the telephone call is not established by the network provider of participants by choosing a landline number, but by the devices of the participants themselves. The name "teilnehmer123@dienstanbieter.de" can in principle be both a registered Internet phone point as well as a E- Mail address or other services. While an ENUM-enabled phone the entry: In this example, an e-mail server will be resolved e-mail address through ENUM (teilnehmer123@dienstanbieter.de as pop3) " SIP: teilnehmer123@dienstanbieter.de" processed . There are some providers of complete solutions ( DSL , e-mail, telephone) who make such addresses available for both e-mail and VoIP services. They only differ from each other in the protocol used (mail, telephone, other). Since e-mail addresses are usually already known, there is usually no explicit reason to send e-mails via ENUM-capable systems.

With ENUM it is also possible to reach Internet telephones as well as landline telephones and mobile devices via the Internet, provided that several such connections are configured under a uniform telephone number. The subscriber configures the actual availability according to defined priorities. For example, an ENUM-activated phone number (landline, mobile, special phone number, foreign country, etc.) can be configured in such a way that an attempt is first made to reach an Internet phone. If this is not registered with the VoIP provider, a cellular connection is established. If the mobile phone is not logged into the network, the call is forwarded to a landline connection. The caller is usually charged for the service that is configured, regardless of whether they have dialed a landline or mobile number. Whether the caller uses ENUM or instead calls an Internet telephone directly using its Internet name “teilnehmer123@dienstträger.de” is only irrelevant if the destination is an Internet connection anyway.

It should be noted that only a few VoIP telephone numbers have been entered so far. Telephone numbers that are not entered are routed by their own service providers as usual via the telephone network operator's mandatory telephone network gateway (network gateway) in order to reach the subscriber as usual via the fixed network (PSTN). In the case of an ENUM entry, landline and mobile telephones can be reached via ENUM (and thus from the Internet), but must actually still be routed via the landline or cellular network if they do not also access the Internet. Some mobile devices can be configured for Internet telephony, but it is up to the network operator whether he routes the required VoIP protocol (mostly SIP protocol) or blocks it on the provider side.

ENUM and worldwide phone numbers

graphic representation of an ENUM zone based on the DNS hierarchy

Since telephone numbers are created according to country-specific numbering plans, they must be integrated into the global Internet in an international format. Here they follow a numbering plan of the International Telecommunication Union (ITU), which is specified in its recommendation E.164 . Since the ENUM mapping of worldwide telephone numbers is an Internet resource, in contrast to the telephone numbers themselves, which can be viewed as a resource under the administration of the ITU, the classic top-level domain for Internet resources became the Internet Engineering Task Force (IETF), .arpa , is used. E164 was chosen as the subdomain - since it is a matter of mappings from the E.164 numbering plan . Since ENUM mappings are ultimately nothing more than simple DNS entries, it is of course also possible to map your own numbering plans under other domains.

Implementation and query

The conversion of a telephone number into the corresponding ENUM domain (in this case the IETF mapping) happens as follows:

  1. +44 1 2345 6789 - Cover letter with the full E.164 number
  2. 44123456789 - Remove all non-digit characters
  3. 98765432144 - Reverse the order of the digits
  4. 9.8.7.6.5.4.3.2.1.4.4 - Inserting periods between the individual digits
  5. 9.8.7.6.5.4.3.2.1.4.4.e164.arpa - Adding the ENUM domain suffix .e164.arpa

This conversion takes place automatically in most ENUM-compatible end devices , so that only the entry of the telephone number is necessary. Knowledge of the exact domain plan is therefore neither necessary for the caller nor for the participants who configure a customer-specific ENUM server.

The domain name resulting from the above implementation is searched for NAPTR resource records via the resolver . The evaluation of these resource records results in one or more Uniform Resource Identifiers under which the desired service of the specified domain or telephone number can be addressed.

administration

The international ENUM top-level domain e164.arpa is currently administered by Réseaux IP Européens (RIPE) on behalf of the ITU . Upon request, the individual country codes can be delegated to local country organizations after consultation with representatives of the ITU. The further design of the national domain space is determined autonomously by the country organization, whereby this is usually done in close cooperation with the national competent authority.

The allocation and activation of these new domain names has been tested in so-called ENUM trials in some countries since the beginning of 2002 . These trials mostly serve to research ENUM and related technologies as well as to prepare for a possible commercial operation.

In Germany, ENUM entries under 9.4.e164.arpa have been available through some DENIC members as part of the experiment since 2002. The transition to active operation was originally planned for December 2005, but was not finally started until January 26, 2006.

After a two-year test, Austria was the first country in the world to sign a contract for the commercial operation of ENUM on August 24, 2004. In this contract, the Austrian regulator, Rundfunk und Telekom Regulierungs GmbH (RTR GmbH), delegates operation until the end of 2007 to enum.at Dienstleistungs GmbH for convergent communication services. Enum.at started production on December 9, 2004. enum.at terminated this contract at the end of 2014. ENUM is now to be continued in Austria by Kernnetz Invent GmbH .

procedure

Technical and organizational requirements for using ENUM

If you call a telephone number with an ENUM lookup terminal or via a provider that supports ENUM lookup, this terminal or the provider queries the destination using the ENUM domain. If there is an ENUM domain and another destination is stored, for example VoIP destination, mobile phone number or landline number, this is preferred for establishing the connection. Several goals can also be stored, which are queried according to priorities. Otherwise the specified telephone number will be called. An email address or a website can also be stored.

In order to register the ENUM domain for a German phone number, several conditions must be met:

  • The number must have an equivalent in the German number plan.
  • The registrant of the ENUM domains must have the right to use the number.
  • In Germany, the number must come from a number range available for ENUM:
    • Local networks
    • Cellphone 015, 016, 017
    • toll-free services 0800
    • personal phone numbers 0700
    • national subscriber numbers (NTR, nomadic use for VoIP ) 032

It is not necessary that your own gateway with this telephone number is connected to the telephone network. In order to still be accessible via the telephone network, the telephone network operator must install gateways that can handle ENUM.

In Austria, the following number ranges are intended for use with ENUM:

  • geographical numbers
  • Numbers for private networks (05 ...)
  • mobile numbers (0664, 0676, 0699, 0650, 0660 ...)
  • location-independent landline numbers (0720)
  • Numbers for convergent services (0780)
  • Numbers in the range 0800

A link to an existing telephone number is made by a registrar. The registrar checks every six months whether the linked phone number is still up to date.

The caller pays additional costs incurred through rerouting via ENUM. Example: You have dialed a landline number, but the ENUM lookup directs you to a mobile phone number or even a value-added service number (0900, previously 0190). But: Since the ENUM lookup is carried out on the user side, it is up to the hardware and software of the user whether such a forwarding is carried out without notice to the user. ENUM only "distributes" the information, what is made of it depends on the user and his hardware and software.

ENUM and conventional telephone routing between VoIP and landline customers

The most common use of ENUM is direct calling between two communication partners using ENUM in order to reach a subscriber directly using a normal telephone number, i.e. bypassing the otherwise usual telephone networks, and thus usually also to save costs.

Schematic representation of how VoIP and telephone subscribers can establish connections; the ENUM resolution is shown by the thick arrow line
The graphic opposite shows the usual connection paths between various technical telephone systems in connection with ENUM
  • The callers are dark gray, the target subscribers called are light gray.
  • Participants with Internet or VoIP telephony are light blue (the frequently used universal SIP protocol is used in the examples), while participants with landline telephony are shown in magenta. Other protocols besides SIP can also be used; usability depends on the technical support on the subscriber side and on the provider side.
  • The data queries are shown with a dashed line:
    Resolution of the telephone number into a SIP destination (thick line)
    Resolution of the SIP name into the IP address of the Internet telephone
    Solid lines are the telephone calls
  • The numbers in the same colors indicate the order in which the call is set up
Caller 1, VoIP customer - dark blue, red and magenta

Caller 1 has an ENUM-enabled device (telephone or SIP adapter) and, after dialing a telephone number, can make inquiries to the SIP server himself (path 1 blue for all connections), which resolves those telephone numbers into a SIP address that were previously configured there by the participants. A telephone connection is only possible if the SIP server of the target subscriber can actually be reached under the resolved SIP address, otherwise calls must be made using the telephone number.

  • Path to subscriber B: The telephone or the SIP adapter of caller 1 send a request to the ENUM server (path 1 blue, server is brown), which resolves the telephone number into a SIP destination (i.e. converts it).
    The SIP server (orange) can be reached via the SIP destination (path 2 blue), which knows and answers the current IP address of the participant. (The SIP server obtains the current IP address of the subscriber from the user's device each time the DSL router logs in or dials in again.)
    Only with this IP address is a direct connection established between the two subscribers (path 3 blue). If the ENUM server is configured incorrectly, the calling device cannot obtain an IP address from the SIP server and cannot reach the subscriber.
    If the caller knows the SIP destination and can dial it directly, the telephone number cannot be resolved via ENUM (step 1).
    Alternatively, caller 1 can also call the subscriber directly via his SIP address (for example, if ENUM is configured incorrectly, step 1 is not applicable in blue), provided he knows the SIP address of his conversation partner or he can dial the telephone number without ENUM lookup to conduct the conversation over the telephone network (paths 2b / 2c blue, 3 red). In this case, the provider of subscriber B must make an internal SIP request (route 4 red), since the target subscriber can only be reached via the Internet (VoIP) in any case.
  • Path to subscriber A: Provider A's SIP server does not respond to external requests. An ENUM query (path 1 blue) is therefore possible, but pointless.
    Therefore, the caller must use the normal telephone landline number via his VoIP provider (alternative route 2b blue) to establish a connection via the telephone network (route 2c blue), his VoIP provider selects the route to the VoIP or telephone provider of the target subscriber (route 3 red) This provider-controlled internal network can be the telephone network or the Internet, routing is based on the fixed network conditions.
    The VoIP provider of subscriber A sends a request to its own internal SIP server (path 4 red for target subscriber A), which only accesses this internal Inquiries are answered and the call is forwarded to the resolved IP address (path 5 red), which is hidden from the caller's device.
  • Path to subscriber C: Subscriber C has a landline connection. A possible ENUM query (path 1 blue) remains inconclusive. As with subscriber A, the call is routed to the provider's internal network (paths 2b / 2c blue) and from the provider of subscriber C directly to their landline connection (path 3 magenta).
Caller 2, VoIP customer - green, red and magenta

Caller 2 does not have an ENUM-enabled device, but a VoIP provider who makes ENUM requests, i.e. his device always establishes a connection via its own VoIP provider (path 1, green for all connections). This provider now makes an ENUM query (path 2 green)

  • Path to subscriber B: The VoIP provider of caller 2 receives a SIP address from the ENUM server (path 2 green). The caller's VoIP provider now sends a request to the SIP server of destination subscriber B (path 3 green) using the SIP address and receives the subscriber's IP address.
    The VoIP provider establishes a direct connection to the target subscriber for the caller (path 4 green) and switches between caller 2 and target subscriber B.
    If no entry is found in the ENUM request (path 2 green), it is up to the provider which way he chooses. If there is an ENUM entry but it is incorrectly configured, caller 2 cannot generally reach the target subscriber. His provider adheres to the priority settings given to him by the ENUM server. If the target subscriber can only be reached temporarily via the Internet, he can store a second, subordinate route for dialing in to the ENUM server, for example via the telephone network (route 3 red).
    All participants (A, B and C) can be reached via the telephone network (alternative route 2b / c, green, then via the gateway from the Internet to the provider's internal network, route 3 red / magenta).
  • Route to subscriber A: Subscriber A can also be reached via the Internet (route 2b / c green), but only via the gateway of its VoIP provider (route 3 red to A), because its SIP server only answers internal inquiries ( Path 4 red).
  • Path to subscriber C: Subscriber C can only be reached via the telephone network (path 2b / c green and after the gateway from the Internet to the telephone network path 4 magenta)
Caller 3, landline customer

Except via call-by-call, callers calling from the landline network have no influence on the route and tariffs at which they reach a target subscriber.
The call is conducted via the provider's network (path 1 + 2 magenta) to the target participants (path 3 red or magenta) Here, the routing is managed solely by the telephone companies involved, even if an internet telephone (VoIP for participants A and B, red) is called.

Current market acceptance

The number of registered German numbers has stagnated at around 8,000 since December 2006 (as of December 31, 2015). Many German VoIP providers avoid ENUM so that customers can use their landline gateways for a fee. Many VoIP providers even block all VoIP calls from other providers so that the customer can be called and reached via the fixed network for a fee. With flat rates, VoIP providers earn additional money on incoming landline calls. So far, only a few VoIP providers offer the possibility of reaching Internet users via ENUM lookups, but no telephone network operators. In the future, call-by-call providers could try to use ENUM lookups to lower their own costs by making individual connections more cheaply as part of a mixed calculation. In doing so, however, you have to contrast the initial investment in the new technology with the currently still low cost advantage (due to the few ENUM-activated numbers).

Market providers who offer ENUM services need differentiated business models for their economic success. For example, some VoIP providers advertise on their websites with low basic fees and free ENUM-mediated calls, but earn money from the other landline, cell phone and international calls. Other providers offer a free VoIP connection and lower tariffs, but market the customer's landline number separately. The latter can be used by customers who make incoming calls via the number of a DSL provider and outgoing calls via a separate connection. Customers who use ENUM services often do so to reduce costs, so that no cost-intensive marketing and advertising campaigns are to be expected in the future either.

Market participants who are interested in ENUM services need a differentiated awareness of the possibilities and advantages that are available with ENUM. The main reason for your own advantages is that as many other telephone customers as possible, or at least your own conversation partners, also participate in this system. Before ENUM services achieve a breakthrough in the mass market, it is in the interest of interested customers to “make advance payments” to potential future users so that they can use the services themselves in the future. For this purpose, DENIC currently offers the registration of ENUM-activated landline numbers, at least in Germany, free of charge and most registrars are currently still passing this cost advantage on to end customers, so that there are no additional costs other than the time required to set up the service. If the service is chargeable, tariffs are to be expected that are likely to correspond roughly to the prices of domains , i.e. a few euros per year or cents per month.

Alternative services to ENUM

In the area of ​​Internet telephony between two VoIP participants, direct dialing via a VoIP adapter or a so-called softphone (VoIP-capable telephone software) is a viable alternative. The prerequisite, however, is that the caller knows the Internet name of the destination connection and that the VoIP server of the called customer (usually a SIP server) answers external inquiries. Many SIP servers answer external inquiries, but the associated phone numbers are often not ENUM-enabled. If a provider does not answer SIP inquiries, the ENUM configuration of the IP telephone and therefore also no direct connection is possible, since the IP address of the VoIP subscriber usually changes constantly.

The problem of a constantly changing IP address with DSL connections can be avoided by using DynDNS services. The router reports its current IP to a DNS server. With the Fritz! Box Fon, which is very common in Germany, it is possible in this way to call directly with a SIP URI call using the pattern SIP: <username> @ <DynDNS-Name>. The user name must be the login name of a VoIP account entered in the Fritz! Box. In addition, this VoIP account must connect via the Internet. This SIP-URI offered itself as an entry in ENUM. Since the spread of complete connections, large providers such as United Internet (1 & 1, GMX, ...) have switched to establishing VoIP connections via a second virtual Internet connection (2nd PVC) for quality assurance. Such accounts can no longer be reached from the outside simply via SIP-URI, since they cannot be reached via the public Internet. For this reason, the ENUM support that was previously available was no longer provided in the more recent FBF firmware. By registering an independent VoIP account, which establishes the connection via the 1st PVC (i.e. the Internet), you can still be reached via SIP-URI. Since the Fritz! Boxes no longer carry out ENUM queries, a direct call is only possible by dialing the SIP URI of the call participant. To do this, you have to store the SIP URI as the destination in the phone book, since telephones do not allow you to choose letters and @.

If a telephone customer only wants the individual configuration of his various terminals, i.e. the routing of telephone calls under a uniform telephone number to different terminals, a second telephone line and a telephone system can be installed, as was customary up to now. There are also telephone providers who enable the configuration of call forwarding on the provider side, which does not require a second telephone line and special devices on the customer side. In this case, there are costs for the forwarding, even with the existing flat-rate telephone tariff. For these configurations, however, it is also irrelevant which telephone service the caller uses.

Risks of ENUM

The direct connection of end devices, as implemented through the use of SIP URIs, makes it possible to disguise the origin of a call. Similar to e-mails, the sender's metadata (their “phone number” or SIP URI) can be falsified and the data can be routed via botnets (as a proxy). This opens the door to spam and telephone terrorism; The call does not cost the caller anything; if foreign servers, proxies or botnets are used, the caller can hardly be identified. This is an essential difference to classic telephony; a "flying restart" is not possible.

While the use of SIP URIs in private circles (e.g. within the family) is relatively unproblematic, since the caller has to know the user name and DynDNS name, this information is made public by ENUM and can therefore be used by everyone. It is particularly dangerous if the telephone number that is registered with ENUM can be found in the public telephone book.

Due to the low distribution of ENUM, these risks have not yet materialized.

Web links

Further information

Registration offices (registries)

VoIP provider list

Other free ENUM zones (registries)

Individual evidence

  1. ↑ https://www.rtr.at/de/tk/ENUM/ENUM_Vertrag_2015.pdf
  2. ↑ https://www.rtr.at/de/tk/ENUM
  3. ↑ DENIC ENUM annual report 2015