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Mobility management is one of the major functions of a GSM or a UMTS network that allows mobile phones to work. The aim of mobility management is to track where the subscribers are, allowing calls, SMS and other mobile phone services to be delivered to them.
A GSM or UMTS network, like all cellular networks, is basically a radio network of individual cells, known as base stations. Each base station covers a small geographical area which is part of a uniquely identified location area. By integrating the coverage of each of these base stations, a cellular network provides a radio coverage over a much wider area. For GSM, a base station is called a base transceiver station (BTS), and for UMTS it is called a Node B. A group of base stations is named a location area, or a routing area.
The location update procedure allows a mobile device to inform the cellular network whenever it moves from one location area to the next. Mobiles are responsible for detecting location area codes (LAC). When a mobile finds that the location area code is different from its last update, it performs another update by sending to the network, a location update request, together with its previous location, and its Temporary Mobile Subscriber Identity (TMSI).
The mobile also stores the current LAC in the SIM card, concatenating it to a list of recently used LACs. This is done to avoid unnecessary IMSI attachment procedures in case the mobile has been forced to switch off (by removing the battery, for example) without having a chance to notify the network with an IMSI detach and then switched on right after it has been turned off. Considering the fact that the mobile is still associated with the Mobile Switching Center/Visitor Location Register (MSC/VLR) of the current location area, there is no need for any kind of IMSI attachment procedures to be done.
There are several reasons why a mobile may provide updated location information to the network. Whenever a mobile is switched on or off, the network may require it to perform an IMSI attach or IMSI detach location update procedure. Also, each mobile is required to regularly report its location at a set time interval using a periodic location update procedure. Whenever a mobile moves from one location area to the next while not on a call, a random location update is required. This is also required of a stationary mobile that reselects coverage from a cell in a different location area, because of signal fade. Thus, a subscriber has reliable access to the network and may be reached with a call, while enjoying the freedom of mobility within the whole coverage area.
When a subscriber is paged in an attempt to deliver a call or SMS and the subscriber does not reply to that page then the subscriber is marked as absent in both the MSC/VLR and the Home Location Register (HLR) (Mobile not reachable flag MNRF is set). The next time the mobile performs a location update, the HLR is updated and the mobile not reachable flag is cleared.
The Temporary Mobile Subscriber Identity (TMSI) is the identity that is most commonly sent between the mobile and the network. Depending on the necessary precision, a TMSI may be set by a VLR, SGSN, or MME. In the least-precise case, TMSI is randomly assigned by the VLR to every mobile in the area, the moment it is switched on, in order to support identity confidentiality. The number is local to a location area, and so it has to be updated each time the mobile moves to a new geographical area.
The VLR, SGSN, and MME must be capable of correlating an allocated TMSI with the IMSI of the MS to which it is allocated. An MS may be allocated three TMSIs; one for services provided through the MSC (the TMSI), one for services provided through the SGSN (the packet-TMSI or P-TMSI), and one for the services provided the MME (the MME-TMSI or M-TMSI; a part of the GUTI).
The network can also change the TMSI of the mobile at any time. And it normally does so, in order to avoid the subscriber from being identified, and tracked by eavesdroppers on the radio interface. This makes it difficult to trace which mobile is which, except briefly, when the mobile is just switched on, or when the data in the mobile becomes invalid for one reason or another. At that point, the global "international mobile subscriber identity" (IMSI) must be sent to the network. The IMSI is sent as rarely as possible, to avoid it being identified and tracked.
A key use of the TMSI is in paging a mobile. "Paging" is the one-to-one communication between the mobile and the base station. The most important use of broadcast information is to set up channels for "paging". Every cellular system has a broadcast mechanism to distribute such information to a plurality of mobiles.
Size of TMSI is 4 octet with full hex digits but can't be all FF because the SIM uses 4 octets with all bits equal to 1 to indicate that no valid TMSI is available. [1]
Roaming is one of the fundamental mobility management procedures of all cellular networks. Roaming is defined [2] as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. This can be done by using a communication terminal or else just by using the subscriber identity in the visited network. Roaming is technically supported by a mobility management, authentication, authorization and billing procedures.
A "location area" is a set of base stations that are grouped together to optimize signaling. Typically, tens or even hundreds of base stations share a single Base Station Controller (BSC) in GSM, or a Radio Network Controller (RNC) in UMTS. The BSC / RNC is the intelligence behind the base stations; it handles allocation of radio channels, receives measurements from the mobile phones, and controls handovers between base stations.
Each location area has an assigned unique identifier, made up of numbers, called a "location area code" (LAC). The LAC is broadcast by each base station at regular intervals. Within a location area, each base station is assigned a distinct "cell identifier" (CI) number, see also Cell Global Identity.
If the location areas are large and moderately populated, there will likely be a high number of mobiles operating simultaneously, resulting in very high paging traffic. This is due to the fact that every paging request has to be broadcast to every base station in the location area. Ultimately, this wastes bandwidth and power on mobile devices by requiring them to listen for broadcast messages too often. Similarly if on the other hand, there are too many small location areas, the mobile device must contact the network very often for changes of location, which will also drain the device’s battery. Therefore, it is important to strike a balance between the size of the location area and the number of mobile device users in the location area[ citation needed ].
The routing area is the packet-switched domain equivalent of the location area. A "routing area" is normally a subdivision of a "location area". Routing areas are used by mobiles which are GPRS-attached. GPRS is optimized for "bursty" data communication services, such as wireless internet/intranet, and multimedia services. It is also known as GSM-IP ("Internet Protocol") because it will connect users directly to Internet Service Providers
The bursty nature of packet traffic means that more paging messages are expected per mobile, and so it is worth knowing the location of the mobile more accurately than it would be with traditional circuit-switched traffic. A change from routing area to routing area (called a "Routing Area Update") is done in an almost identical way to a change from location area to location area. The main differences are that the "Serving GPRS Support Node" (SGSN) is the element involved.
The tracking area is the LTE counterpart of the location area and routing area. A tracking area is a set of cells. Tracking areas can be grouped into lists of tracking areas (TA lists), which can be configured on the User Equipment (UE). Tracking area updates are performed periodically or when the UE moves to a tracking area that is not included in its TA list.
Operators can allocate different TA lists to different UEs. This can avoid signaling peaks in some conditions: for instance, the UEs of passengers of a train may not perform tracking area updates simultaneously.
On the network side, the involved element is the Mobility Management Entity (MME). MME configures TA lists using NAS messages like Attach Accept, TAU Accept or GUTI Reallocation Command.
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile devices such as mobile phones and tablets. GSM is also a trade mark owned by the GSM Association. GSM may also refer to the Full Rate voice codec.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP, UMTS is a component of the International Telecommunication Union IMT-2000 standard set and compares with the CDMA2000 standard set for networks based on the competing cdmaOne technology. UMTS uses wideband code-division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators.
In telecommunication, a public land mobile network (PLMN) is a combination of wireless communication services offered by a specific operator in a specific country. A PLMN typically consists of several cellular technologies like GSM/2G, UMTS/3G, LTE/4G, offered by a single operator within a given country, often referred to as a cellular network.
The international mobile subscriber identity (IMSI) is a number that uniquely identifies every user of a cellular network. It is stored as a 64-bit field and is sent by the mobile device to the network. It is also used for acquiring other details of the mobile in the home location register (HLR) or as locally copied in the visitor location register. To prevent eavesdroppers from identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead.
Roaming is a wireless telecommunication term typically used with mobile devices, such as mobile phones. It refers to a mobile phone being used outside the range of its native network and connecting to another available cell network.
The GPRS core network is the central part of the general packet radio service (GPRS) which allows 2G, 3G and WCDMA mobile networks to transmit Internet Protocol (IP) packets to external networks such as the Internet. The GPRS system is an integrated part of the GSM network switching subsystem.
Network switching subsystem (NSS) is the component of a GSM system that carries out call out and mobility management functions for mobile phones roaming on the network of base stations. It is owned and deployed by mobile phone operators and allows mobile devices to communicate with each other and telephones in the wider public switched telephone network (PSTN). The architecture contains specific features and functions which are needed because the phones are not fixed in one location.
MSISDN is a number uniquely identifying a subscription in a Global System for Mobile communications or a Universal Mobile Telecommunications System mobile network. It is the mapping of the telephone number to the subscriber identity module in a mobile or cellular phone. This abbreviation has several interpretations, the most common one being "Mobile Station International Subscriber Directory Number".
GSM services are a standard collection of applications and features available over the Global System for Mobile Communications (GSM) to mobile phone subscribers all over the world. The GSM standards are defined by the 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators. The common standard makes it possible to use the same phones with different companies' services, or even roam into different countries. GSM is the world's most dominant mobile phone standard.
The Packet Data Serving Node, or PDSN, is a component of a CDMA2000 mobile network. It acts as the connection point between the radio access and IP networks. This component is responsible for managing PPP sessions between the mobile provider's core IP network and the mobile station. It is similar in function to the GGSN that is found in GSM and UMTS networks.
An international mobile subscriber identity-catcher, or IMSI-catcher, is a telephone eavesdropping device used for intercepting mobile phone traffic and tracking location data of mobile phone users. Essentially a "fake" mobile tower acting between the target mobile phone and the service provider's real towers, it is considered a man-in-the-middle (MITM) attack. The 3G wireless standard offers some risk mitigation due to mutual authentication required from both the handset and the network. However, sophisticated attacks may be able to downgrade 3G and LTE to non-LTE network services which do not require mutual authentication.
GPRS Tunnelling Protocol (GTP) is a group of IP-based communications protocols used to carry general packet radio service (GPRS) within GSM, UMTS, LTE and 5G NR radio networks. In 3GPP architectures, GTP and Proxy Mobile IPv6 based interfaces are specified on various interface points.
The Mobile Application Part (MAP) is an SS7 protocol that provides an application layer for the various nodes in GSM and UMTS mobile core networks and GPRS core networks to communicate with each other in order to provide services to users. The Mobile Application Part is the application-layer protocol used to access the Home Location Register, Visitor Location Register, Mobile Switching Center, Equipment Identity Register, Authentication Centre, Short message service center and Serving GPRS Support Node (SGSN).
A Network (Layer) Service Access Point Identifier (NSAPI), is an identifier used in GPRS networks.
GSM procedures are sets of steps performed by the GSM network and devices on it in order for the network to function. GSM is a set of standards for cell phone networks established by the European Telecommunications Standards Institute and first used in 1991. Its procedures refers to the steps a GSM network takes to communicate with cell phones and other mobile devices on the network. IMSI attach refers to the procedure used when a mobile device or mobile station joins a GSM network when it turns on and IMSI detach refers to the procedure used to leave or disconnect from a network when the device is turned off.
System Architecture Evolution (SAE) is the core network architecture of mobile communications protocol group 3GPP's LTE wireless communication standard.
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station (MS) and the Base transceiver station (BTS). It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of specifications. Um can also support GPRS packet-oriented communication.
Insert Subscriber Data is a Subscriber Data Handling procedure in LTE services. This procedure is used to manage the subscription data of subscriber in MME and SGSN over S6a/S6d interface. IDR is invoked by Home Subscriber Server for subscription data handling. IDR is MAP subscriber management service utilized in GSM/UMTS networks, standardized by 3GPP, and defined in the MAP specification, TS 29.002. This service is used to provide specific subscriber data in the following environments: by an HLR to update a VLR, by an HLR to update a SGSN, and by an HSS to update a MME via IWF in an EPS. This service is primarily used by the home subscriber management entity to update the serving subscriber management entity when there is either a change in a subscriber parameter, or upon a location updating of the subscriber.
The Mobile Telephone Switching Office (MTSO) is the mobile equivalent of a PSTN Central Office. The MTSO contains the switching equipment or Mobile Switching Center (MSC) for routing mobile phone calls. It also contains the equipment for controlling the cell sites that are connected to the MSC.
The Gateway Mobile Location Centre (GMLC) contains functionality required to support location-based service (LBS). In one public land mobile network (PLMN), there may be more than one GMLC. The GMLC is the first node an external LBS client accesses in a GSM, UMTS or LTE network. The GMLC may request routing information from the home location register (HLR) or home subscriber server HSS). After performing registration authorization, it sends positioning requests to either the visited mobile switching centre (VMSC), SGSN serving GPRS support node (SGSN) or mobile switching centre (MSC) server and receives final location estimates from the corresponding entity.
Cellular network standards | |||||||||
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| 0G radio telephones (1946) | |||||||||
| 1G (1979) |
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| 2G (1991) |
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| 2G transitional (2.5G, 2.75G, 2.9G) |
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| 3G (1998) IMT-2000 (2001) |
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| 3G transitional (3.5G, 3.75G, 3.9G) |
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| 4G (2009) IMT Advanced (2013) |
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| 5G (2018) IMT-2020 (2021) |
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