Global title

A global title can be in a variety of formats, most of which are each defined in separate standards. The format parameter indicates which of the available formats are in use. Each format can include any of the subsequent parameters.

The numbering plan indicator (NPI) describes which numbering plan will be used for the global title. The chosen numbering plan will aid the routing system in determining the correct network system to direct the message.

The type of number (TON) or Nature of Address Indicator (NAI) parameter, which is of relevance to E.164 (regular telephone) numbers for example, indicates the scope of the address value, such as whether it is an international number (i.e. including the country code), a "national" or domestic number (i.e. without country code), and other formats such as "local" format (e.g. in the U.S., without an area code).

The translation type (TT) parameter is used in a network to indicate the preferred method of global title analysis (see below). Normally in European networks, this parameter is set to 0 (the default) value. In North American mobile networks, different translation types are used for analysis of the IMSI and for messages between telephone systems. This parameter is valuable in complex routing problems, where the same number has to be routed differently depending on the circumstances, such as those introduced by number portability resolution.

The situation in this case is somewhat complicated by the additional parameters possible in the global title. Each set of parameter values (TT=0 NP=E.164, TON=INT) can be treated separately from each other one (TT=0 NP=E.214, TON=INT). This means that, instead of one single table, we potentially need a separate table for each possible set of values.

The variable length of the global title makes certain optimisations that can be used in IP routing not so easy to use here. The number analysis of a Global Title is most often done in a tree structure. This allows reasonably efficient analysis to any depth which is chosen.

In the end, global title analysis gives some result. The exact possibilities vary from system to system, is sometimes called an "action" or is integrated into the analysis table.

The destination would typically be given as a signalling point code in an MTP network, but could also be an IP system if we are using SS7 over IP.[2]

The most commonly used numbering plans for global title routing are E.164 and E.214 (although E.212 is also common in America). These simply look like telephone numbers. That is to say, in the most common, international, variant there is a country code at the start of the number and a Network Code immediately following the country code. Beyond that is the subscriber number or mobile subscriber identity number, though even that may be divided into sections. This structure allows for the use of hierarchical routing:

• International SCCP gateways know which systems handle each of the other countries
• The international SCCP gateway belonging to each country knows which SCCP gateways handle each network
• The SCCP gateway of each network knows the network's own internal structure

In America, the limitations of the North American Numbering Plan mean that the destination country is not immediately obvious from the called party address. However, the fact that there is unified administration means that this can be overcome by having complete analysis at every point where it is needed.

Everywhere in the world, except North America, the subscriber's IMSI is converted to a Mobile Global Title (MGT) E.214 number. See the entry about the IMSI for more details. The E.214 number has a structure which is similar to the E.164 number, and, except in a mobile network it can be routed identically. This means that the same routing tables can be used for both and means considerably reduced administrative overhead in maintaining the tables.

Once a signalling message with an E.214 number enters a mobile network in its own country, the routing is dependent on the operator of that mobile network. In networks without number portability, it is normal that the MSIN has a structure and that, by analysing the first few digits we can further route the message to the right element.

In World Area 1 (corresponding to North America) ANSI SCCP is in use. In this case, due to North American standards, the routing of mobility related messages must be done with the E.212 number directly. This has the advantage that it is easier to identify to which country messages should be routed based on the mobile country code. The design of the North American Number Plan means that there is not a separate country code for each country in North America. Working with E.214 numbers would not be an insurmountable challenge, as can be seen from the fact that routing of phone calls using E.164 numbers is normal, but it would mean adding full E.164 routing tables to signalling transfer points where it has never been needed before.