The sharing of frequencies at the international level by Member States is accomplished by means of: the Table of Frequency Allocations, assignment and allotment plans, and coordination procedures as prescribed in the Radio Regulations.

The part of the spectrum that may be employed for communication purposes is divided into small portions, varying in width according to requirements. These portions, called frequency bands, are allocated to the various radiocommunication services on either an exclusive or a shared basis. Such allocations may be made world-wide or be limited to a region of the world.

The Table, and the Radio Regulations of which it forms part, can be amended only by a World Radiocommunication Conference to which all Member States of the Union are invited. Amendments are introduced following negotiations between the national delegations aimed at reconciling the demand for frequencies with the capacity of the spectrum. If a country or group of countries request the use of a band by a service different from that on which the rest of the world has agreed, such use may, if there is consensus to this effect, be included in the Table in the form of a footnote or authorized by the application of a procedure introduced in the Regulations whereby the countries concerned secure, prior to the planned use of the frequencies in this band, the agreement of other countries which may be affected.

A page of the Table of Frequency Allocations is reproduced (see box) to illustrate the complexity of regulation. It shows that in the band 138 - 144 MHz, each of the three regions of the world has decided to use frequencies for different services and that, in addition, exceptions have been accepted as indicated in the additional allocations in the footnote.

In Region 2, for instance, the frequencies between 138 and 143.6 MHz are shared between the fixed, mobile, radiolocation and space research services in the following order of priority: first, the fixed, mobile and radiolocation services are on a primary basis and space research is on a secondary basis (i.e., without right of protection against signals from stations in the primary services category but with right of protection against signals from other stations in the secondary services category). The same frequencies are allocated differently in Regions 1 and 3. Since a similar scenario applies to all frequencies between 9 kHz and 400 GHz, it is an extremely difficult and complex matter to negotiate amendments to the Table and manage their application.

Moreover, all amendments proposed must first be carefully scrutinized, since any decisions taken affect operational systems which would have to cease emitting if they no longer met the Regulations. Such factors are of course taken into consideration during the negotiations held at ITU conferences, and transitional periods - often based on equipment life - are established by common agreement.

Depending on the requirements expressed by Member States, the Plenipotentiary Conference � the supreme organ of the Union � establishes a schedule of conferences for roughly the next four years (i.e. until the next Plenipotentiary Conference).

Besides the Table of Frequency Allocations, conferences adopt assignment or, if appropriate, allotment plans for certain services in specified frequency bands in which transmission and reception are not necessarily confined to the territory of a given country. On the basis of the requirements expressed by each country, assignment plans allocate frequencies for each station in a given service; allotment plans grant to each country frequencies intended for use by a given service, frequencies which the country then assigns to the stations within its jurisdiction according to their needs.

When such plans are established, delegations study and adopt technical criteria, applicable planning parameters and criteria for sharing with other services if appropriate. They also have to assess all the risks of harmful interference and establish implementation procedures before assigning or allotting frequencies. Such plans must in every case comply with the current Table of Frequency Allocations.

When plans are not established, procedures are developed for international coordination to ensure that a country planning to use a given frequency obtains the agreement of all countries which may be affected by its use.

The Radiocommunication Bureau must be notified of any use of frequencies that might cause interference outside the territory of the country in which the relevant station is located or if the country wishes to have international recognition for its usage of frequencies.

The Master International Frequency Register, in which all frequencies notified to the Bureau are registered, now contains 2 117 367 assignments, each one of which must be able to be used without causing interference to any other. There were 200 000 registered frequencies in 1952.

The notification of frequency assignments, the examination of notices and the recording of assignments in the Register constitute an ongoing process which the Bureau carries out throughout the year; once every two weeks, the Bureau publishes on a CD ROM and sends to all Member States all of the information concerning the notices received and the results of their processing, leading either to an entry in the Register or the return of the notice to the notifying administration. The Bureau receives some 1 200 new notices every week!

A primary responsibility of the Radiocommunication Bureau concerns the examination of notices, from administrations, of new frequency assignments as well as changes to frequency assignments, and their entry in the Master Register following the application of any coordination procedures between Members of the Union, in accordance with the relevant provisions of the Radio Regulations as established, or subsequently updated, by a competent Radiocommunication Conference.

Procedures governing notification and registration of frequency assignments in the Master International Frequency Register may be broadly subdivided into the acts of co-ordination, notification, examination, and registration. The intent of these procedures is in effect to ensure a coordinated approach by administrations to the use of spectrum, thus reducing the probability of harmful interference.

To mitigate the problem of harmful interference, each administration wishing to put into service a new station that is likely to cause interference outside the territory of the country in which it is located, is under obligation to send a notice of its intention to the Bureau, giving the technical characteristics of the station concerned. The Bureau examines the notice for its conformity with the Table of Frequency Allocations and the other relevant provisions of the Radio Regulations and, in some specific cases, assesses the extent to which the use of the frequency, under the notified conditions, could cause interference to stations of any other administrations whose assignments are recorded in the Master International Frequency Register. If all Bureau�s findings are favourable, the particulars of the assignment are entered in the Master Register, accompanied by all relevant remarks, that establish the legal status of and confer international recognition to the assignment, as is indeed the case with other assignments (existing or future) similarly recorded. If some Bureau�s findings are unfavourable, the notice is returned to the notifying administration with a view to modifying the characteristics of the station in such a way as to obviate the probable harmful interference. The administration may alternatively decide to use the frequency on a non-interference basis, in which case the frequency assignment is recorded in the Master Register with the indication that the administration will not cause interference to conforming assignments.