Global Title
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A Global Title (GT) is an address used in the SCCP protocol for routing signalling messages. The name Global Title derives from the idea that in most instances this is a globally unique address which can refer to only one destination system. In this, the Global Title is rather similar to a public IP address. In many other aspects, the Global Title is quite different: the structure is often strictly hierarchical; the Global Title is of variable length; the Global Title is not just numeric, but includes several attributes which vary its interpretation. Apart from their other attributes, global titles are often similar to telephone numbers although this is not always true.
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Structure of the Global Title
The global title structure is defined in the SCCP standard (Q.713) plus the supporting numbering plan standards. What follows is a brief, from memory description of key aspects.
Global Title Format
The Global Title can have different formats, this parameter says which one is in use . A number of the following parameters are optional; the format parameter decides which of these are present.
Numbering Plan Indicator
The Numbering Plan Indicator (NPI) describes which numbering plan will be used for the global title. Values for this are given in the Wikipedia Numbering Plan Indicator article. Selecting the numbering plan normally decides the routing hierarchy used for routing the global title.
Type of Number
The Type of Number (TON) parameter, which is used in E.164 numbers, for example, has values such as International, National or Subscriber. International numbers have a clearly defined meaning everywhere in the world and include the country code of the system they address. National and subscriber can be seen as equivalent to site local and link local addresses in IPv6; the same identical could legitimately be used elsewhere in a different network to refer to a different target system.
Translation Type
The translation type (TT) parameter is used in a network to select different sets of Global Title analysis (see below). Normally in European networks, this parameter is set to 0 (the default) value. In American Mobile networks, different translation types are used for analysis of the IMSI and for inter system messages. This parameter can be valuable in complex routing problems, where the same number has to be routed differently depending on circumstances. For example, before and after number portability related translations.
Global Title Translation
Global Title Translation is SS7's equivalent to IP's routing. It is the process of looking at the "destination address" (in SS7 this is the called party) and deciding what to do with that. There are two identifiable processes, Global Title Analysis, the act of looking up the number and finding a result then Global Title Modification is an option.
It is possible for the result of Global Title Translation to be Route on SSN. This means that, instead of the Global Title routing, lower level MTP routing will be used for this message from this point on. Equivalently, in a system using SS7 over IP, the result from Global Title Translation may be a to route to an IP server, though the exact details depend greatly on which variant of SS7 over IP is being used.
Global Title Analysis
Global Title Analysis together with Global Title 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 are 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.
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 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 networks own internal structure
In America, the limitations of the North American Number 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 Modification
In Global Title Translation it is quite normal that at some point the Global Title will have to be changed. This happens, for example, as GSM mobility management messages enter and leave networks in America. In America, typically most routing of mobility management messages for all mobile networks is done using the E.212 (IMSI) number. In international networks, E.214 is always used.
At the boundary incoming toward America (this can mean the Signalling Transfer Point at the edge of the American operator's network), numbers routed from European networks are converted from E.214 numbers into E.212 numbers. In the outgoing direction, from America toward the rest of the world, are converted from E.212 numbers into E.214 numbers.
Global Title Routing in Mobile Networks
In mobile networks, there are database queries such as "how can I tell if this subscriber is really who she says she is" (MAP_Send_Authentication_Info) which have to be routed back to the database which holds the subscriber's information (the HLR, or in this case, the AUC).
Unfortunately, at the time the subscriber first arrives, we don't know which HLR is the subscriber's HLR. For this reason, the queries have to be routed on the subscriber's identity (IMSI) is used to generate the called party address in the message. How this is done depends whether we are in world area 1 (North America) or somewhere else.
Hybrid Number Routing (Except North America)
Everywhere in the world, except North America, the subscriber's IMSI is converted to an 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.
IMSI Routing (North America)
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 in 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.
- this section could do with some detailed expansion by a North American experienced in configuration of SCCP signalling in World Area 1.
- translation type in use !!!
- number format (TT only?) ???
Routing of mobility messages on the ANSI / ITU Boundary
Where a signalling message travels from North America to the rest of the world or from the rest of the world to North America, there must be a conversion done from E.212 based global title to E.214 based global title. This conversion is reasonably simple, well defined and fully reversible. The conversion is not totally simple since each individual network must be listed.
Please note, that recommendation E.214 has been interpreted as suggesting that the analysis of the Mobile Country Code (MCC) and Mobile Network Code (MNC) should be done separately. The relationship between the MNC and the Network Code (NC), however, varies from country to country as does the length of the MNC (two or three digits). This means that the analysis of the MNC is dependent on the analysis of the MCC, or alternatively that the analysis must be done for all five or six digits at once (which is how it is done in practise across at least five separate switch vendors).
Examples
Outbound from America:
- NPI=E.212: 24801 xxxxxxxxxx (248 = Bulgaria MCC + 01 = MobilTel MNC) ->
- NPI=E.214: 359888 xxxxxxxxx (359 = Bulgaria country code)
Please note the truncation of the number by one digit since E.214 numbers, as with E.164 numbers have a maximum length of 15 digits.
Inbound toward America:
- NPI=E.214: 14054 xxxxxxxxx (1 = U.S. country code) ->
- NPI=E.212: 310150 xxxxxxxxx (310 = U.S. MCC + 150 = Cingular MNC)
External links
CISCO guide to configuring GT analysis (http://www.cisco.com/en/US/products/sw/wirelssw/ps2153/products_user_guide_chapter09186a00801cd8f6.html) SS7 architecture discussion (http://netlec.com/telecommunication/ss7architecture.html)