Numbering Plan – a Technical Approach

“World harmony is numbering harmony – Pythagoras of Samos”

1. Introduction

With the advent of new technologies and new multimedia service offers, there have been increasing questions of complexity concerning telecommunication system planning.

One the most important questions is how to design a numbering plan capable of identifying its subscribers uniquely and satisfactorily in the long term.  

2. Numbering Plans

2.1. Numbering plan requirements

There are three basic requirements that a numbering plan must meet:

• unique identification of telecommunication network or service subscribers;

• accurate information for billing purposes; and

• accurate information for call routing within a telecommunication network.

Unique identification of a subscriber occurs in local, regional, national and international levels. This identification is possible through the use of national and international codes.

This identification allows proper charging for the use of telecommunication networks and services.

To enable communication between subscribers, interconnection between networks of different providers is necessary. One of the techniques used is call/communication routing or appropriate routing through the analysis of dialed subscriber numbers (in other words, through the identification of the network and the subscriber, through a unique numbering plan, nationally and internationally).

A numbering plan must be designed taking into account some important characteristics, such as subscribers’ understanding of the telecommunication services offered and capability of meeting long-term demands for subscriber numbers, new services and new networks.

2.2. Designing a Numbering Plan

2.2.1. Subscriber Numbers

A numbering plan is designed in order to identify the subscriber, that is, his/her “address” in the network. For instance, in an eight-digit national plan, the subscriber number 3245-1234 is divided into two parts: the first part “3245” carries the information on the connected subscriber’s network node, the subscriber’s carrier, the service - either fixed or mobile -, and the routing.

The second part “1234” identifies the subscriber uniquely within the network.

However, the string of digits 32451234 is insufficient to determine the country and the geographical area the number is located in. Therefore, national and international network codes become necessary.

2.2.2. National Codes

ITU-T Recommendation E.164 determines a maximum 15-digit number length for subscriber or service identification, not including the “0” (zero) nor “00” (zero-zero) prefixes.

So, when we add the national code to the subscriber number, we have a national (significant) number. In our example:

National Code (NC) + Subscriber Number

CN + 3245-1234

National codes vary from country to country, from 1 (one) to 4 (four) digits, according to ITU-T Recommendation E.164. If in the numbering plan the national code has 2 (two) digits, we have the following:

78 + 3245-1234, where the used national code is “78”. Then, the number 78 + 32451234 represents the national (significant) number. The rule for “uniqueness” of codes must be respected and complied with within the same country.

As shown in the previous example, the national number has 10 (ten) digits. 

2.2.3. International Codes

If we add a country’s international code to the national subscriber number, we will have the international public telecommunication number. The international code of a country is defined by ITU-T and, once again, Recommendation E.164 determines a combination of digits for the formation of the international public telecommunication number.

For example, in the number “55 + 78 + 3245-1234”, “55” is used as the country code. In Brazil, this code is assigned at world level, uniquely for telecommunication service subscribers.

2.3. Numbering Plan Capability

Numbering plan capability is directly associated to and dependent upon the number of digits used for subscriber identification.

With 1 (one) digit, we have the capability for 9 (nine) subscribers; with 2 (two) digits, 99 (nine-nine) subscribers (102 = 100), excluding the digit “0” (zero).

In the case of a numbering plan in which 8 (eight) digits are used, the capability is of 90 (ninety) million subscribers, excluding the digit “0” (zero).

In the previous example (two-digit national code), if we add a national code or area code to this capacity, the numbering plan capability drastically increases one hundred fold. 

2.4. Uniformity of a Numbering Plan

Whenever the quantity of digits represents a fixed-length number as well as the national code, the numbering plan has uniformity characteristics. In this case, the dialing procedures are simple and easily understood by telecommunication subscribers.

On the other hand, whenever the lengths (or number of digits) are variable, such as the subscriber number or the national codes, there may be subscriber numbers of different formats and number of digits. The dialing procedures are more complicated for the subscribers and require greater network analysis capability - considering the commutation nodes -, therefore increasing the implementation costs.

The advantages of a numbering plan uniformly formatted are unquestionable. ITU-T Recommendation E.164 does not impose restriction to uniformity of numbering plans, enabling each National Administration to choose the plan patterns freely, as long as the maximum length of an number is limited to 15 (fifteen) digits.

2.5. The significance of a Numbering Plan

Differently from the IP address of an e-mail, the significance of a subscriber number is processed from right to left. Thus, if we add the national code “78” to the number “3545-1234”,

(78) 3245-1234 

and add the country code “55”,

(55) 78 2345-1234

we will have the unique identification of this Brazil’s subscriber.

This process, however, is different for e-mail addresses. For instance, calil@ means a name with an internet domain, but the information calil@ does not have meaning until something else is added, such as “anatel”,

calil@anatel

It is clear that the meaning of the code above (e-mail address) is still not sufficient to determine which entity “anatel” is. However, if we add the word “gov”:

calil@anatel.gov

indicating that Anatel is a government entity, and if we add the country abbreviation “br”, we will have the full and unique address in an international level:

calil@anatel.gov.br

Differently from the ITU-T E.164 numbering, the significance of digits in the IP world is processed from left to right. This distinction, as we see, is useful for the development of the ENUM Protocol, which transforms E.164 address in IP address, or vice-versa. (to be continued).

Walter Calil Jabur
Interconnection and Numbering Manager – Anatel
Coordinator of Numbering Aspect Rapporteur Group – PCC.I