Editing Global title

{{Short description|Address used for routing SS7 messages}}
{{SS7stack}}
{{More citations needed|date=December 2016}}

A '''global title''' ('''GT''') is an [[Network Layer|address]] used in the [[Signalling Connection Control Part|SCCP]] protocol for routing signaling messages on [[telecommunications]] networks. In theory, a global title is a unique address which refers to only one destination, though in practice destinations can change over time.

==Overview==
The global title is similar in purpose on the [[PSTN]] to the [[host name]] on the [[Internet]]. In design, however, global titles are quite different. The structure is usually hierarchical, the value can be of variable length, and is not necessarily a wholly numeric value—though it often is for issues of [[backward compatibility]] and association with regular [[telephone number]]s.

==Structure of the global-title value==
The structure of a global title for [[ITU-T]] applications is officially defined in [http://www.itu.int/rec/T-REC-Q.713/en/ ITU-T Recommendation Q.713], and further extended in the supporting [[numbering plan]] standards. Other national variants of [[Signalling Connection Control Part]] ([[Signalling Connection Control Part|SCCP]]), such as the [[American National Standards Institute]] variant specified in ANSI T1.112/2000, define their own format for the Global Title. The value of a global title is a sequence of attributes which modify the address value. To summarize:

===Global title format===
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.

===Numbering plan indicator===
The [[Telephone numbering plan#Numbering plan indicator|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.

===Type of number===
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 [[List of E.164 country codes|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]]).

===Translation type===
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 America]]n mobile networks, different translation types are used for analysis of the [[International mobile subscriber identity|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 [[Local number portability|number portability]] resolution.

==Global title translation==
'''Global title translation''' ('''GTT''') is the [[Signaling System 7|SS7]] equivalent to [[Internet Protocol|IP]] routing. Translation examines the destination address (e.g. the number being called) and decides how to identify it over the telephone network. This process can include  ''global title analysis'', which is the act of looking up the number and finding a result address, and ''global title modification''.

It is possible for the result of global title translation to be '''route on [[Subsystem number|SSN]]'''. This means that, instead of the Global Title routing, lower level [[Message Transfer Part|MTP]] routing will be used for this message from this point on. Equivalently, in a system using [[SS7 over IP]] (for example, [[SIGTRAN]]), the result from Global Title Translation may be to route to an IP server, though the exact details depend greatly on which variant of SS7 over IP is being used.<ref>{{IETF RFC|3868}}</ref>

===Global title analysis===
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 [[Message Transfer Part|MTP]] network, but could also be an IP system if we are using SS7 over IP.<ref>{{IETF RFC|3868}}</ref>

===Routing structure===
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.

==Global title routing in mobile networks==
{{See also|Mobile Application Part}}

In mobile networks, there are database queries such as "how can I tell if this subscriber is really who he says he is" (MAP_Send_Authentication_Info) which have to be routed back to the database which holds the subscriber's information (the [[Network switching subsystem#Home location register (HLR)|home location register]] [HLR], or in this case, the [[Network switching subsystem#Authentication center (AuC)|authentication center]] [AuC]).

When the subscriber first arrives the subscriber's HLR is unknown. For this reason, the queries have to be routed on the subscriber's identity (IMSI) which is used to generate the '''called party address''' in the message. How this is done depends on the location. There are three types of GT in use in mobile networks known as E.164 (MSISDN), E.212(IMSI) and E.214(MGT):<ref>{{cite tech report |url=http://www.3gpp.org/ftp/Specs/html-info/29002.htm |title=3GPP TS 29.002: Mobile Application Part (MAP) Specification |at=SCCP Addressing |publisher=[[3GPP]] |id=29.002 |access-date=2025-04-22 }}</ref>

* E.164(MSISDN) = CC+NDC+SN - (Country Code + National Destination Code + Subscriber Number) e.g. 91-98-71405178
* E.212(IMSI) = MCC+MNC+MSIN - (Mobile Country Code + Mobile Network Code + Mobile Subscription Identification Number) e.g. 404-68-6600620186 (MTNL delhi)
* E.214(MGT) = combination of E.212 and E.164(CC+NDC+MSIN) (Exact combination is defined in the operators IR21 document)

===Mobile global title routing (except North America)===
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 [[International mobile subscriber identity|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 [[Mobile number portability|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.

===IMSI routing (North America)===
In World Area 1 (corresponding to North America) [[American National Standards Institute|ANSI]] [[Signalling Connection Control Part|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 [[Signal transfer point|signalling transfer points]] where it has never been needed before.

==References==
{{Reflist}}

[[Category:ITU-T recommendations]]
[[Category:Signaling System 7]]