I. Mobile Network Codes (MNC)

The Mobile Network Codes are defined in ITU-T Recommendation E.212 “The international identification plan for public networks and subscriptions”, as part of IMSI (International Mobile Subscriber Identity).

ITU-T Recommendation E.212 initially defined the IMSI code as a string of decimal digits, up to a maximum length of 15 digits, which identifies, at an international level, a unique subscription. The IMSI consists of three fields:

The three fields mean the following:

a) MCC – the mobile country code is three digits in length and identifies a country or a group of providers of mobile public networks and electronic communications services provided at mobile locations using the same MCC for international services;

b) MNC – the mobile network code is two or three digits in length and, in combination with the MCC, identify the home network of a subscription;

c) MSIN – the mobile subscriber identification number is up to 10 digits in length, and identifies individual subscriptions.

The IMSI was used by the mobile public networks employing GSM technology. The advantages of using the IMSI, in particular for ensuring roaming services, determined its employment within most mobile public networks, including 3G networks.

The use of this code enabled the development within other types of networks of certain services initially offered only by mobile networks (e.g. SMS). The technological development and the evolution to NGN opened the perspective for users to be able to benefit from a large range of services, irrespective of the type of network used.

In order to enable the subscribers to fixed public networks to have access to the services initially developed for the mobile public networks based on the IMSI code and in order to ensure the mobile-to-fix interoperability, ITU amended Recommendation E.212. Thus, the new ITU-T Recommendation E.212 (05/2004) provides that this code may also be used for terminal or subscriber identification within fixed or wireline networks that offer mobility services, or to achieve compatibility with networks that have mobility services.

According to these changes, MNC can also identify a fixed public network or a group of public networks that offer mobility services or that use MNC to achieve compatibility with mobile public networks.

The MCC is managed by the ITU, the MNC is regularly managed by national administrations, and the MSIN is managed by each provider which was allotted a MNC. The MCC assigned to Romania is 226. At European level, the MNC is two digits in length. Thus, 100 combinations may be achieved within each MCC.

The MNCs are assigned to the providers of mobile public electronic communications networks, including MVNO. They can also be assigned to the providers of fixed networks that offer mobility services or to achieve compatibility with the mobile public networks. The assignment unit is an MNC code; a provider of electronic communications public networks may benefit from the right to use regularly one MNC.

II. Routing numbers

The called number does not hold sufficient information in order to route calls to ported numbers, and therefore an additional routing information is necessary, i.e. the routing number (RN).

The analysis of the technical boundaries of the fixed public telephone networks and of the mobile public telephone networks showed that it is sufficient for the routing number to consist of numerical features. It was agreed that the routing number should have the 18xyz format, where:

a) the first two digits (18) are the prefix of the routing number;

b) the xyz group (where x, y and z may have values ranging between 0 and 9) is established for each provider, depending on the significance of the routing numbers in the provider’s network. Thus:

1. the providers of publicly available telephone services offered over mobile public networks will each use one routing number to indicate the network to which the ported number is connected;

2. the providers of telephone services over fixed public networks will use either one or several routing numbers, depending on the network structure;

3. the providers of publicly available telephone services  offering portability for several categories of numbering resources may use the same routing number for all categories of numbering resources or may use routing numbers for each category of numbers for which it offers portability. 

c) the combination 18xyz = 18000 will not be allotted, following to be used within administrative processes, in cases where a ported number becomes non-ported (the acceptor provider is the initial donor provider).

The structure of the routing numbers was established under the Technical and commercial terms for number portability implementation, adopted by Decision of the president of the National Regulatory Authority for Communications and Information Technology no.3444/2007. Routing numbers can identify either the destination network (acceptor network), or a switch within the destination network.

The proposed form of the routing numbers (18xyz) offers 999 different possible combinations. Should the routing numbers be insufficient, ANCOM will change their format, and will accordingly establish the allocation and usage rules.

Routing numbers are stored in the centralised database and in the providers’ operational databases. The complete routing information consists of the routing number and the dialled number (DN). Transmission of the routing information to the point of interconnection between networks is performed using the method of the address concatenated within the SS7 signalling message.

III. Common channel signalling system (SS7)

Electronic communications networks ensure the mechanism required for establishing the links between the terminals connected to the networks. The activities of creation, maintenance and monitoring of the links require information transfer between the terminals and the commuting systems they are connected to, between the commuting systems within a network and respectively of those of distinct networks, in case of interconnection. This type of information is called signalling information. Signalling information is transmitted by means of the signalling systems.

In analogue networks, the signalling information is usually transferred by signalling systems with associated channel (e.g.: the signalling system R2). This means that each voice channel is associated with a signalling channel. The development of digital networks allowed the introduction of new services and triggered considerable higher quantity and diversification levels of the signalling information to be transmitted. In order to answer the new requirements of the digital networks, ITU-T standardised an efficient signalling system, called SS7 (the ITU-T Recommendations of the series Q.7XX).

SS7 belongs to the category of common channel signalling systems. The signalling messages are transmitted through signalling-dedicated circuits (other than the voice circuits, for example). The signalling network may be regarded as a parallel network beside the network transporting the useful signal. Generally, each commuting system is associated with a signalling network node. The signalling network nodes are called signalling points – SP. These SP’s may be identified by means of the signalling point codes - SPC.

The signalling information is conveyed as packages called signalling units. There are three types of signalling units:

• Message signal units (MSU) - useful information
• Link status signal units (LSSU) - useful information
• Fill-in signal units (FISU) - passive information

LSSU indicate the state of the signalling channel, FISU are transmitted in the absence of useful signals. The actual signalling information transmitted between two network nodes are encoded in the MSU.

The MSU standard structure is:

F Flag – Frame edge

CK Check bits – controls errors

SIF Signalling information field – consists of the following fields:

• Information – contains the message delivered from the application
• Head – indicates the type of the message
• Message label – contains three indicators:

-    CIC – circuit identity code – indicates the link in the message transport network;

- OPC – originating point code – indicates the signalling point where the message entered the signalling network (i.e. the respective signalling point code);

-  DPC – destination point code – indicates the signalling point where the message is expected to arrive (the respective signalling point code).

SIO Service information octet – contains two sub-fields:

• Service indicator (e.g. message from the application, signalling network internal test message, signalling network management message etc.);
• Network Indicator – NI. NI consists of 2 bits, with the following significance:

-   00 – international network

- 01 – not used, but reserved for international use

-  10 – national network;

- 11 – reserved for national use. In Romania, NI = 11 characterises the providers’ internal networks.

LI Length indicator – indicates the length of the SIF and SIO fields.

FIB Forward indicator bit – a bit indicating forward transmission

FSN Forward sequence number – number of the transmission signalling unit

BIB Backward indicator bit – a bit indicating reception transmission.

BSN Backward sequence number – number of the reception signalling unit.

Allocation of signalling codes refers to the consistent defining of the addressing fields OPC, respectively DPC in the SIF, identifying the corresponding signalling network nodes.

1. Signalling network

On global level, SS7 signalling is hierarchically organised, featuring three independent levels represented by:

• Each provider’s internal signalling network (NI = 11);
• The national signalling network (NI = 10);
• The international signalling network (NI = 00)

NI represents the network indicator and is transmitted by means of the SIO within the MSU.

2. Signalling points

The nodes of each of the three signalling networks are identified by means of the signalling point codes. Thus, there are three categories of signalling point codes:

• Codes of the signalling points of the providers’ internal networks;
• National signalling point codes (NSPC);
• International signalling point codes (ISPC).

These codes have a 14 bit-format, grouped (for easier administration) in three distinct fields. The codes are allotted using the decimal representation of the three binary fields (Z-XXX-Y type):