6.2.4.6 Number portability

In connection with number portability, a distinction is made between

• operator portability,
• geographical portability,
• services portability.

Operator portability means that a user can change the network operator by "taking along" the number used by him. In these cases, there is a transferring network (operator) and an accepting network (operator).

In the case of geographical portability, a user can take along a telephone number to another geographical location. This may (but need not) be connected to a change in operator (operator portability).

Services portability means that a telephone number can be taken along, although the service is changed. On the international level, there are already countries where a geographical number from the fixed network can be "taken along" to a mobile network (or vice versa). POTS and ISDN are not separate services in this connection, but different forms of the voice telephony service in the fixed network.

As the NVO provides only for portability in the fixed network, the following comments are limited to this sector. There are different types of implementation throughout the world. An essential distinguishing feature in this connection is where the porting information regarding a number is stored, and which networks may access it. Below follows a brief description of the basic variants.

The best solution, in terms of technology, is a process where the transferring (original) network is no longer involved in the switching and accounting processes of calls to the ported number, once the portability option has been implemented. Otherwise, the services quality continues to be influenced by the transferring network. This will cause problems whenever the transferring network no longer meets the technical standards or fails completely and permanently, in an extreme case on account of bankruptcy. The technical implementation of such a process requires that every subscriber network or carrier network always has up-to-date information on porting, and that the calls are forwarded directly to the accepting network. Whenever a transit network is involved, the necessary routing information has to be forwarded to it. When beginning to set up a connection, there is therefore the so-called "all call query", where the current target network for a selected number is retrieved from a database. A technically efficient implementation of this approach requires that an inter-network database exists.

In several European countries, there are network operator associations or companies, where - as a rule - the major network operators, at least, will be represented. These have implemented such central databases - often by commissioning third parties. Changes in the central database, which is controlled and documented by means of workflow applications, lead to updates in the routing-relevant data in the networks of the network operators involved (for example, an update of the routing data on an IN platform). The real-time access in the course of a current connection is therefore not in the central database but in the individual networks.

Unlike this technically sophisticated solution, onward routing is another possibility. Here, after porting, only the transferring network (here also called the anchor network) knows to which network the ported number must be currently assigned. In consequence, all connections are established via the anchor network, which adds special routing information upstream from the dialled number before forwarding a call. This information contains the current target network for the subsequent (transit) networks. After a further porting, the anchor network will keep its function, the first accepting network is no longer involved after porting. This solution was developed in Austria by the AK-TK and implemented as a technical basis in the relevant TKK procedures.

An intermediate step between the two variants described above is a method called "query on release" or "call drop back". In contrast to the "all call query", no check for a possible porting is made in the subscriber or carrier networks every time a connection is established, but only if the wish to be connected, which continues to be forwarded to the original (transferring) network, is rejected by it with a special "porting reference".

A special constellation after porting will exist if the subscriber of a network calls a number that was originally part of another network, but was then ported to the caller's own network. The routing of such a connection via the anchor network should be avoided, even though there may be a basic implementation of onward routing, so as to use the total network resources economically. In case of calls to services numbers, the TKK has imposed the obligation for such cases that the respective accepting network must recognise the situation and must handle the routing within its network. In case of calls to geographical numbers, Telekom Austria was not required to meet this obligation. Other arrangements were made instead, as Telekom Austria claimed technical problems, on account of the resulting high requirements on performance. For the other fixed network operators the aforementioned arrangement for services numbers also applies to geographical numbers.