15.7 Location Management

Location management schemes are essentially based on users' mobility and incoming call rate characteristics. It is a two-stage process that enables a network to discover the current attachment point of the mobile user for call delivery. The first stage is location registration or location update. In this stage the mobile terminal periodically notifies the network of it new access point, allowing the network to authenticate the user and revise the user's location profile. The second stage is call delivery. Here, the network is queried for the user location profile and the current position of the mobile host is found. Current techniques for location management involve database architecture design and the transmission of signaling messages between various components of a signaling network. Other issues include security, dynamic database updates, querying delays, terminal paging methods, and paging delays.

There are two standards for location management currently available: Electronic/Telecommunications Industry Associations (EIA/TIA) Interim Standard 41(IS-41) ^[53] and the Global System for Mobile Communications (GSM) mobile application part (MAP). ^[54] The IS-41 scheme is commonly used in North America for Advanced Mobile Phone System (AMPS), IS-54, IS-136, and personal access communications system (PACS) networks, while the GSM MAP is mostly used in Europe for GSM and digital cellular service at 1800 MHz (DCS-1800) networks. Both standards are based on a two-level data hierarchy. Location registration procedures update the location databases (HLR and VLRs) and authenticate the MT when up-to-date location information of an MT is available. Call delivery procedures locate the MT based on the information available at the HLR and VLRs when a call for an MT is initiated. The IS-41 and GSM MAP location management strategies are very similar. While GSM MAP is designed to facilitate personal mobility and to enable user selection of network provider, there are a lot of commonalities between the two standards. ^[55], ^[56] The location management scheme may be categorized in several ways.

15.7.1 Without Location Management

This level 0 method is no location management is realized, ^[57] the system does not track the mobiles. A search for a called user must therefore be done over the complete radio coverage area and within a limited time. This method is usually referred to as the flooding algorithm. ^[58] It is used in paging systems because of the lack of an uplink channel allowing a mobile to inform the network of its whereabouts. It is used also in small private mobile networks because of their small coverage areas and user populations. The main advantage of not locating the mobile terminals is obvious simplicity; in particular, there is no need to implement special databases. Unfortunately, it does not fit large networks dealing with high numbers of users and high incoming call rates.

15.7.2 Manual Registration in Location Management

This level 1 method ^[59] is relatively simple to manage because it requires only the management of an indicator, which stores the current location of the user. The mobile is also relatively simple; its task is just limited to scanning the channels to detect paging messages. This method is currently used in telepoint cordless systems (such as CT2, Cordless Telephone 2). The user has to register when moving to a new island of CT2 beacons. To page a user, the network first transmits messages through the user's registered beacon and, if the mobile does not answer, extends the paging to neighboring beacons. The main drawback of this method is the constraint for a user to register with each move.

15.7.3 Automatic Location Management using Location Area

Presently, this level 2 location method ^[60] most widely implemented in first- and second-generation cellular systems (NMT, GSM, IS-95, etc.) makes use of location areas (LAs) (Figure 15.1). In these wide-area radio networks, location management is done automatically. Location areas allow the system to track the mobiles during their roaming in the network(s): subscriber location is known if the system knows the LA in which the subscriber is located. When the system must establish a communication with the mobile (typically, to route an incoming call), the paging only occurs in the current user LA. Thus, resource consumption is limited to this LA; paging messages are only transmitted in the cells of this particular LA. Implementing LA-based methods requires the use of databases. Generally, a home database and several visitor databases are included in the network architecture.

15.7.4 Memoryless-Based Location Management Methods

All methods are included based on algorithms and network architecture, mainly on the processing capabilities of the system.

15.7.4.1 Database Architecture

LA partitioning, and thus mobility management cost, partly relies on the system architecture (e.g., database locations). Thus, designing an appropriate database organization can reduce signaling traffic. The various database architectures are proposed with this aim. ^[61], ^[62], ^[63], ^[64] An architecture where a unique centralized database is used is well suited to small and medium networks, typically based on a star topology. The second one is a distributed database architecture, which uses several independent databases according to geographical proximity or service providers. It is best suited to large networks, including subnetworks managed by different operators and service providers. The GSM worldwide network, defined as the network made up of all interconnected GSM networks in the world, can be such an example of a large network. The third case is the hybrid database architecture that combines the centralized and distributed database architectures. In this case, a central database (HLR-like) is used to store all user information. Other smaller databases (VLR-like) are distributed all over the network. These VLR databases store portions of HLR user records. A single GSM network is an example of such architecture.

15.7.4.2 Optimizing Fixed Network Architecture

In 2G cellular networks and 3G systems, the intelligent network (IN) manages signaling. ^[65] Appropriately organizing mobility functions and entities can help reduce the signaling burden at the network side. The main advantage of these propositions is that they allow one to reduce the network mobility costs independent of the radio interface and LA organization.

15.7.4.3 Combining Location Areas and Paging Areas

In current systems, an LA is defined as both an area in which to locate a user and an area in which to page him. LA size optimization is therefore achieved by taking into account two antagonistic procedures, locating and paging. Based on this observation, several proposals have defined location management procedures, which make use of LAs and paging areas (PAs) of different sizes. ^[66] One method often considered consists of splitting an LA into several PAs. An MS registers only once, i.e., when it enters the LA. It does not register when moving between the different PAs of the same LA. For an incoming call, paging messages will be broadcast in the PAs according to a sequence determined by different strategies. For example, the first PA of the sequence can be the one where the MS was last detected by the network. The drawback of this method is the possible delay increase due to large LAs.

15.7.4.4 Multilayer LAs

In present location management methods, LU traffic is mainly concentrated in the cells of the LA border. Based on this observation and to overcome this problem, Okasaka et al. have introduced the multiplayer concept. ^[67] In this method, each MS is assigned to a given group, and each group is assigned one or several layers of LAs. This location updating method, although it may help reduce channel congestion, does not help reduce the overall signaling load generated by LUs.

15.7.5 Memory-Based Location Management Methods

The design of memory-based location management methods has been motivated by the fact that systems do a lot of repetitive actions, which can be avoided if predicted. This is particularly the case for LUs. Indeed, present cellular systems achieve every day, at the same peak hours, almost the same LU processing. Systems act as memoryless processes.

15.7.5.1 Dynamic LA and PA Size

The size of LAs is optimized according to mean parameter values, which, in practical situations, vary over a wide range during the day and from one user to another. Based on this observation, it is proposed to manage user location by defining multilevel LAs in a hierarchical cellular structure. ^[68] At each level, the LA size is different, and a cell belongs to different LAs of different sizes. According to past and present MS mobility behavior, the scheme dynamically changes the hierarchical level of the LA to which the MS registers. LU savings can thus be obtained.

An opposite approach considers that instead of defining LA sizes a priori, these can be adjusted dynamically for every user according to the incoming call rate (a) and LU rate (u_k), for instance. In Xie and coworkers, ^[69] a mobility cost function denoted C(k, a, u_k) is minimized so that k is permanently adjusted. Each user is therefore related to a unique LA for which size k is adjusted according to the particular mobility and incoming call rate characteristics. Adapting the LA size to each user's parameter values may be difficult to manage in practical situations. This led to the definition of a method where the LA sizes are dynamically adjusted for the whole population, not per user. ^[70]

15.7.5.2 Individual User Patterns

Observing that users show repetitive mobility patterns, the alternative strategy (AS) is defined. ^[71], ^[72] Its main goal is to reduce the traffic related to mobility management — and thus reduce the LUs — by taking advantage of users' highly predictable patterns. In AS, the system handles a profile recording the most probable mobility patterns of each user. The profile of the user can be provided and updated manually by the subscriber himself or determined automatically by monitoring the subscriber's movements over a period of time. For an individual user, each period of time corresponds to a set of location areas, k. When the user receives a call, the system pages him sequentially over the LA ai s until getting an acknowledgment from the mobile. When the subscriber moves away from the recorded zone {a₁,...,a_k}, the terminal processes a voluntary registration by pointing out its new LA to the network. The main savings allowed by this method are due to the nontriggered LUs when the user keeps moving inside his profile LAs. So, the more predictable the user's mobility, the lower the mobility management cost. The main advantage of this method relies on the reduction of LUs when a mobile goes back and forth between two LAs.

15.7.6 Location Management in 3G-and-Beyond Systems

The next generation in mobility management will enable different mobile networks to interoperate to ensure terminal and personal mobility and global portability of network services. However, in order to ensure global mobility, the deployment and integration of both wire and wireless components are necessary. The focuses are given on issues related to mobility management in a future mobile communications system, in a scenario where different access networks are integrated into an IP core network by exploiting the principles of Mobile IP. Mobile IP, ^[73] the current standard for IP-based mobility management, needs to be enhanced to meet the needs of future fourth-generation (4G) cellular environments. In particular, the absence of a location management hierarchy leads to concerns about signaling scalability and handoff latency, especially for a future infrastructure that must provide global mobility support to potentially billions of mobile nodes and accommodate the stringent performance bounds associated with real-time multimedia traffic. In this chapter, the discussion is confined to Mobile IP to describe the aspects of location management in 3G and beyond.

The 4G cellular network will be used to develop a framework for truly ubiquitous IP-based access by mobile users, with special emphasis on the ability to use a wide variety of wireless and wired access technologies to access the common information infrastructure. While the 3G initiatives are almost exclusively directed at defining wide area packet-based cellular technologies, the 4G vision embraces additional local area access technologies, such as IEEE 802.11-based wireless local area networks (WLANs) and Bluetooth-based wireless personal area networks (WPANs). The development of mobile terminals with multiple physical or software-defined interfaces is expected to allow users to seamlessly switch between different access technologies, often with overlapping areas of coverage and dramatically different cell sizes.

Consider one example ^[74] of this multitechnology vision at work in a corporate campus located in an urban environment. While conventional wide area cellular coverage is available in all outdoor locations, the corporation offers 802.11-based access also in public indoor locations such as the cafeteria and parking lots, as well as Bluetooth-based access to the Internet in every individual office. As mobile users drive to work, their ongoing Voice over IP (VoIP) calls are seamlessly switched, first from the wide area cellular to the WLAN infrastructure, and subsequently from the 802.11 access point (AP) to the Bluetooth AP located in their individual cubicles or offices. Because a domain can comprise multiple access technologies, mobility management protocols should be capable of handling vertical handoffs (i.e., handoffs between heterogeneous technologies).

Due to the different types of architecture envisaged in the multiaccess system, three levels of location management procedures can be envisaged ^[75]

Internet (interdomain) network location management: Identifies the point of access to the Internet network
Intrasegment location management: Executed by segment-specific procedures when the terminal moves within the same access network
Intersegment location management: Executed by system-specific entities when the terminal moves from one access network to another

In Mobile IP (Figure 15.4), ^[76] each mobile node is assigned a pair of addresses. The first address is used for identification, known as the home IP address, which is defined in the address space of the home subnetwork. The second address is used to determine the current position of the node and is known as the care-of address (CoA), which is defined in the address space of the visited/foreign subnetwork. The continuous tracking of the subscriber's CoA allows the Internet to provide subscribers with roaming services. The location of the subscriber is stored in a database, known as a binding table, in the home agent (HA) and in the corresponding node (CN). By using the binding table, it is possible to route the IP packets toward the Internet point of access to which the subscriber is connected.

click to expand
Figure 15.4: Mobile IP architecture.

The terminal can be seen from the Internet perspective as a mobile terminal (MT). Once the MT selects an access segment, the access point to the Internet network is automatically defined. The MT is therefore identified by a home address of the home subnetwork and by a CoA of the access segment. In the target system, location management in the Internet network is based on the main features of Mobile IP. Nevertheless, a major difference can be identified between the use of Mobile IP in fixed and mobile networks. In the fixed Internet network, IP packets are routed directly to the mobile node, whereas in the integrated system considered in this chapter, packets will be routed up to the appropriate edge router. Once a packet leaves the edge router and reaches the access network, the routing toward the final destination will be performed according to the mechanisms adopted by each access segment (intrasegment mobility). When the MMT decides to change access segments, its CoA will be changed. Therefore, the new CoA has to be stored in the corresponding binding tables. Because these binding tables can be seen as a type of location management database, this binding update also can be seen as a form of location update on the Internet.

Intersegment location management is used to store information on the access segments at a particular time. The information is then used to perform system registration, location update, and handover procedures. Using certain parameters, including the condition of the radio coverage and QoS perceived by the user, the MT continuously executes procedures with the objective of selecting the most suitable access segment. Any modifications to these parameters could therefore lead to a change of access segment. This implies also a change in the point of access to the Internet network. Therefore, in order to route these packets correctly it is necessary to have information on the active access segment, particularly information concerning the edge router that is connected to the node of the access segment. Thus, from the Internet point of view, no additional procedure or database is required because the information is implicitly contained in the CoA assigned to the MT.

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