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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.

Location update procedure

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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.

TMSI

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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

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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.

Types of area

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Location area

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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].

Routing area

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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.

Tracking area

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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.

See also

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References

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Mobility management

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Mobility management in telecommunications refers to the set of protocols, procedures, and entities that enable mobile devices to maintain continuous connectivity and seamless service delivery while moving within or across wireless networks.[1] It encompasses tracking the location of users, managing handovers between cells or networks, and supporting authentication and roaming to ensure uninterrupted access to services like voice calls, messaging, and data.[2] The primary objectives are to minimize service disruptions, optimize network resources, and provide efficient routing of communications to mobile subscribers.[3] This is achieved through collaboration between user equipment, radio access networks, and core network elements, such as the Mobility Management Entity (MME) in LTE systems.[4] Mobility management has evolved from early cellular systems to support advanced features in 4G and 5G networks, addressing increasing demands for high-speed mobility and diverse access technologies.[5]

Fundamentals

Definition and objectives

Mobility management encompasses the set of network protocols and procedures designed to track the location of user equipment (UE) in mobile networks, maintain seamless connectivity as the UE moves, and minimize service disruptions during transitions between cells or networks. In earlier 3GPP generations (2G/3G), it operates across circuit-switched and packet-switched domains; in 4G and 5G, mobility management is handled entirely in the packet-switched domain to ensure that services such as voice calls, data sessions, and messaging remain uninterrupted, regardless of the UE's mobility state. This involves monitoring the UE's position at varying levels of granularity, from broad areas in idle mode to precise cell-level tracking in active mode, thereby enabling efficient delivery of incoming traffic and resource handling.[6][7] The primary objectives of mobility management include efficient resource allocation to support ongoing sessions, reduction of signaling overhead through optimized procedures that limit unnecessary network interactions, and provision of quality of service (QoS) guarantees across cell transitions. It supports both idle and active states of the UE: in idle mode, the focus is on low-overhead location updates and reachability via paging to conserve battery and network resources; in active mode, it prioritizes real-time connectivity maintenance and handover execution to prevent packet loss or delays. These goals collectively aim to balance network performance with user experience, adapting to varying mobility patterns while adhering to bearer-level QoS parameters such as allocation and retention priority (ARP) and guaranteed bit rate (GBR).[6][7] Key components provide an overview of the foundational mechanisms without specifying detailed implementations: location registration allows the UE to periodically inform the network of its position within defined areas like tracking areas, handover initiation coordinates transfers between serving nodes to sustain active connections, and paging locates idle UEs for incoming sessions by broadcasting queries across registered areas. In network architectures, mobility management integrates with core elements such as the Mobile Switching Center (MSC) in 2G/3G systems for circuit-switched handling and location tracking via visitor location registers, and the Mobility Management Entity (MME) in 4G Evolved Packet Systems for packet-switched mobility, authentication, and coordination with gateways for bearer management.[6][7]

Historical evolution

Mobility management originated in the first-generation (1G) analog cellular systems of the 1980s, exemplified by the Advanced Mobile Phone System (AMPS), which primarily supported voice services through basic handoff procedures to maintain connections during movement between cells. These early systems lacked formalized location tracking, relying instead on signal strength measurements by mobile stations and base stations to initiate hard handoffs, often resulting in brief call interruptions without support for idle-mode mobility or global roaming.[8][9] The transition to second-generation (2G) digital networks in the 1990s, led by the Global System for Mobile Communications (GSM) standardized by the European Telecommunications Standards Institute (ETSI), introduced structured location management to enable efficient paging and roaming. GSM defined location areas—groups of cells monitored by mobile stations for periodic location updates—and temporary mobile subscriber identities (TMSI) to enhance privacy by anonymizing permanent identifiers during signaling. These features addressed the limitations of 1G by supporting global interoperability and reducing signaling overhead for voice-centric services across denser deployments.[10][11] Third-generation (3G) systems, such as Universal Mobile Telecommunications System (UMTS) developed under the 3rd Generation Partnership Project (3GPP) starting in the early 2000s, extended mobility management to packet-switched data with the introduction of routing areas for more granular tracking in GPRS/EDGE contexts, alongside combined location and routing area updates to optimize signaling. This evolution, specified in 3GPP Release 99, facilitated seamless transitions between circuit- and packet-switched domains, supporting emerging multimedia applications and higher data rates while managing increased network complexity.[12] Fourth-generation (4G) Long-Term Evolution (LTE), introduced in the 2010s via 3GPP Releases 8–10, refined idle-mode mobility through tracking areas that aggregated cells for reduced update frequency, complemented by X2 interfaces for direct inter-eNodeB handovers and S1 interfaces for mobility involving the core network, enabling faster and more efficient transitions in all-IP environments. The advent of fifth-generation (5G) networks from the 2020s, outlined in 3GPP Release 15 onward, integrates network slicing for tailored mobility profiles across diverse services and support for edge computing to reduce latency, with later enhancements in Release 18 (as of 2024) introducing an AI/ML framework for the NG-RAN, including optimizations for mobility management, and Release 19 studies on AI-based handover enhancements for high-mobility scenarios (ongoing as of November 2025).[13][14] Over these generations, standards bodies like ETSI for GSM and 3GPP for subsequent evolutions have progressively tackled challenges from voice-only circuits to multimedia data streams, escalating bit rates from kilobits per second to gigabits, and accommodating denser urban deployments through scalable signaling and predictive mechanisms.[15][16]

Location management

Registration and update procedures

In mobile networks, registration and update procedures enable user equipment (UE) to inform the network of its presence and location changes, ensuring efficient resource allocation and service continuity. These procedures form the core of location management in mobility protocols, evolving from circuit-switched mechanisms in early generations to integrated packet-switched approaches in modern systems.[17] The primary types of registration include initial attach, periodic location updates, and updates due to changes in service areas. Initial attach occurs when the UE powers on, inserts a SIM, or enters a new public land mobile network (PLMN), allowing the UE to establish a non-access stratum (NAS) security context and register with the access and mobility management function (AMF) in 5G or equivalent entities in prior generations. Periodic location updates are triggered by timer expiry, such as T3512 in 5G (default 54 minutes) or T3212 in GSM/UMTS, to confirm UE reachability and prevent implicit deregistration. Updates for service area changes, often called mobility registration updates, activate when the UE reselects a cell in a new location area (LA) or tracking area (TA), ensuring the network maintains an accurate UE position without unnecessary paging overhead.[17][18] Update procedures vary by network generation and domain. In circuit-switched domains of GSM/UMTS, location area updates (LAU) involve the UE sending a LOCATION UPDATING REQUEST message to the mobile switching center/visitor location register (MSC/VLR), which may trigger authentication and temporary mobile subscriber identity (TMSI) reallocation before responding with a LOCATION UPDATING ACCEPT. For packet-switched domains, routing area updates (RAU) use a ROUTING AREA UPDATE REQUEST to the serving GPRS support node (SGSN), updating the routing area identity (RAI) and potentially reallocating a packet TMSI (P-TMSI). In 4G/5G systems, these consolidate into unified registration updates via radio resource control (RRC) signaling: the UE encapsulates a REGISTRATION REQUEST NAS message in an RRC setup complete to the eNodeB/gNodeB, which forwards it to the mobility management entity (MME) or AMF via S1/N2 interfaces; the network responds with REGISTRATION ACCEPT, including a TA list and restarted timers.[18][17] Triggers for these procedures encompass timer expiry (e.g., T3312 for RAU periodicity), cell reselection crossing area boundaries, and power-on events initiating IMSI attach or GPRS attach. In connected mode, cell reselection may invoke implicit updates to avoid explicit signaling, reducing latency during active sessions.[18][17] Signaling overhead from frequent updates poses challenges, particularly in dense networks, where techniques like enlarging tracking area sizes minimize location update frequency at the cost of broader paging scopes. Optimizations include implicit UE context updates during RRC-connected mobility and timer-based suppression of redundant requests, as specified in 3GPP releases to balance update load and reachability.[19] Error handling ensures robustness, with detach procedures addressing failures. UE-initiated detach sends a DEREGISTRATION REQUEST (or equivalent in older systems), prompting the network to release resources and acknowledge, transitioning the UE to a deregistered state. Network-initiated detach, triggered by issues like unauthorized access or slice unavailability, uses a DEREGISTRATION REQUEST with causes (e.g., #10 for implicit deregistration), followed by reattachment attempts after timers like T3346 expire. Registration rejections, due to congestion (#22) or invalid identity (#9), clear UE context and invoke back-off timers (e.g., T3247 for 30-60 minutes) before retrying initial attach.[17][18]

Paging mechanisms

Paging mechanisms in mobility management enable the network to locate and notify user equipment (UE) in idle or inactive states for incoming calls, data sessions, or system information updates. The process begins when the core network (CN) or radio access network (RAN) initiates paging upon receiving downlink traffic for a UE whose location is known only at a coarse granularity, such as a tracking area. The paging message is broadcast over the paging control channel (PCCH) across multiple cells within the designated paging area, containing the UE's identity (e.g., S-TMSI or 5G-S-TMSI) to alert the specific device. Upon detecting its identity during a monitoring occasion, the UE responds by initiating a random access procedure to re-establish connection, transitioning to RRC_CONNECTED state or resuming from inactive mode. This reactive location finding contrasts with proactive UE-initiated updates, ensuring efficient resource use while minimizing unnecessary signaling.[20] Paging areas form a hierarchical structure to balance signaling overhead, latency, and UE battery life. At the finest level, paging can target individual cells, but to reduce broadcast load in dense networks, larger aggregates like tracking areas (TAs) in RRC_IDLE mode or RAN notification areas (RNAs) in RRC_INACTIVE mode are used. A TA comprises multiple cells, and the network pages all cells within the UE's last registered TA, which serves as the scope for paging operations. In 5G NR, RNAs allow more granular RAN-level management, potentially spanning fewer cells than TAs for faster localization in inactive UEs. This hierarchy starts from cell-level for high-precision needs and scales to TA/RNA levels, optimizing trade-offs where smaller areas reduce paging delay but increase frequent updates, while larger areas conserve battery at the cost of higher broadcast traffic.[20] Optimizations enhance paging efficiency, particularly for power-constrained UEs. Discontinuous reception (DRX) cycles allow UEs to monitor paging occasions (POs) periodically rather than continuously; the UE calculates its paging frame (PF) and PO using formulas based on its identity and DRX value, such as SFN mod T = (T div N) * (UE_ID mod N) for frame determination and i_s = floor(UE_ID / N) mod Ns for subframe indexing, where T is the DRX cycle length, N is the number of PFs per cycle, and Ns is the number of POs per PF. Group paging, introduced in 5G Release 17 via paging early indication (PEI), divides UEs sharing the same PO into subgroups, reducing false wake-ups and achieving power saving gains of up to 16% in low group paging rate scenarios through sub-group notifications before full paging. Location prediction algorithms further refine targeting by forecasting UE positions from mobility history; history-based models using Markov chains or machine learning (e.g., stochastic corrections on past trajectories) predict likely cells, minimizing broadcast scope and search costs in predictive schemes.[20][21][22] In 5G NR, paging differs by UE state. For RRC_IDLE mode, CN-initiated paging occurs across the entire TA to notify the UE of downlink data or system changes, prompting transition to RRC_CONNECTED via random access. In RRC_INACTIVE mode, RAN-initiated paging targets the RNA, enabling quicker resumption of suspended connections without full CN involvement, reducing latency for frequently accessed UEs while maintaining context retention for efficiency. This state-specific approach supports diverse traffic patterns, with inactive paging leveraging anchor points like the last serving gNB for localized broadcasts.[20] Performance metrics evaluate paging effectiveness, focusing on success rate, delay, and signaling load. Paging success rate measures the proportion of initiated pages resulting in UE response, typically tracked via counters like successful 5G paging procedures at the AMF. Delay encompasses time from initiation to UE response, influenced by DRX cycle and area size, often under 100-500 ms in optimized TAs. Signaling load, a key overhead indicator, is commonly modeled as the product of UE density and paging rate divided by area coverage, e.g., paging load = (number of UEs / area size) × paging rate, where higher loads risk occasion overflows and discards, monitored through metrics like discarded paging records ratio (discarded / total received). These ensure scalability, with optimizations like PEI improving load by lowering unnecessary transmissions.[23][24]

Identity and security in mobility

Temporary identifiers

Temporary identifiers, also known as pseudonymous or temporary mobile subscriber identities, are allocated by the network to user equipment (UE) in mobile systems to obscure the permanent subscriber identity, such as the International Mobile Subscriber Identity (IMSI), during communication over the radio interface.[25] These identifiers enhance privacy by minimizing the transmission of the IMSI, which could otherwise be intercepted by unauthorized parties, while also reducing signaling overhead due to their compact format.[25] In 2G/3G systems like GSM and UMTS, the primary temporary identifier is the Temporary Mobile Subscriber Identity (TMSI), a 32-bit value assigned by the Visitor Location Register (VLR) to a visiting subscriber.[25] The TMSI is valid within a specific location area and is used in place of the IMSI for most signaling procedures after initial authentication.[25] In packet-switched domains, such as the General Packet Radio Service (GPRS), the Packet-TMSI (P-TMSI) serves a similar role, functioning as a 32-bit temporary identifier allocated by the Serving GPRS Support Node (SGSN) for data sessions.[26] Evolving to later generations, the Globally Unique Temporary UE Identity (GUTI) in LTE and its counterpart, the 5G-GUTI in 5G, provide network-wide temporary identifiers that are unambiguous across the entire system, comprising components like the Mobility Management Entity (MME) or Access and Mobility Management Function (AMF) identifier and a UE-specific part.[27] These identifiers ensure continuity during mobility events without revealing the permanent identity.[27] Assignment of temporary identifiers occurs primarily during the initial network attach procedure or a Location Area Update (LAU), where the network generates and delivers the identifier to the UE in a ciphered message to prevent interception.[28] Reallocation happens periodically or triggered by events like LAU to a new location area, IMSI attach, or explicit reallocation commands, ensuring the identifier remains fresh and tied to the current ciphering context for added security against tracking.[29] For instance, in GSM, the VLR includes the new TMSI in the Location Update Accept message, encrypted with the ongoing session keys.[28] The key benefits of these temporary identifiers include providing anonymity against passive eavesdroppers on the air interface, as the IMSI is rarely transmitted after initial registration, thereby protecting user location and identity privacy.[30] Additionally, their fixed 32-bit length (for TMSI and P-TMSI) results in shorter over-the-air messages compared to the variable-length IMSI (typically 15 digits), which reduces signaling load and battery consumption on the UE.[25] However, security implications arise from potential TMSI re-use attacks, where an adversary correlates the same identifier across sessions or locations if reallocation is infrequent, enabling tracking despite pseudonymity.[31] Mitigation strategies include periodic refresh through explicit reallocation procedures and ensuring random, unpredictable values for new identifiers, as specified in 3GPP standards to limit exposure windows.[28] In practice, networks are recommended to reassign TMSIs at least on every location update or at configurable intervals to counter such vulnerabilities.[32]

Authentication during mobility events

In mobile networks, authentication during mobility events verifies the legitimacy of user equipment (UE) while integrating with procedures such as location updates and handovers to maintain secure communication without interrupting service. These events trigger challenge-response mechanisms to confirm the UE's credentials against home network records, preventing unauthorized access as the UE transitions between serving nodes. The process relies on pre-shared secrets stored in the UE's subscriber identity module and the network's authentication center, ensuring both confidentiality and integrity of signaling during movement. In 2G and 3G systems, the Home Location Register/Authentication Center (HLR/AuC) generates authentication vectors upon request from the Visitor Location Register/Serving GPRS Support Node (VLR/SGSN) during location updates or initial mobility registration. For GSM (2G), vectors consist of triplets including a 128-bit random challenge (RAND), a 32-bit signed response (SRES) computed by the mobile station using the A3 algorithm, and a 64-bit cipher key (Kc) derived via the A8 algorithm for subsequent encryption. In UMTS (3G), quintuplets extend this with RAND, an expected response (XRES) up to 128 bits, 128-bit cipher key (CK), 128-bit integrity key (IK), and an authentication token (AUTN) containing sequence number and message authentication code for network verification. These vectors support the Authentication and Key Agreement (AKA) protocol, where the VLR/SGSN challenges the UE with RAND (and AUTN in 3G), the UE responds with its computed RES/SRES, and matching confirms authenticity before key activation for ciphering and integrity protection over radio bearers. During handovers in 2G/3G, authentication is typically avoided for latency reasons by forwarding existing security contexts and keys from the source to target node, but re-authentication occurs if vectors are exhausted or upon explicit triggers like inter-system handovers. Key management derives session keys directly from vectors: Kc enables A5 ciphering algorithms in GSM, while CK and IK support f8 for confidentiality and f9 for integrity in UMTS, protecting mobility-related signaling such as handover commands. Temporary identifiers like Temporary Mobile Subscriber Identity (TMSI) serve as inputs to initiate these procedures, concealing the permanent International Mobile Subscriber Identity (IMSI) during challenges. The Evolved Packet System AKA (EPS-AKA) in 4G LTE evolves this framework, with the Home Subscriber Server (HSS) providing authentication vectors to the Mobility Management Entity (MME) for use in tracking area updates and attach procedures tied to mobility. Vectors include RAND, AUTN, XRES*, CK, and IK, where the UE's Universal Subscriber Identity Module (USIM) verifies AUTN and derives RES* for mutual challenge-response. The master key KASME (256 bits) is then computed from CK, IK, serving network identity, RAND, and sequence number using a key derivation function, serving as the root for subordinate keys like KeNB for radio resource control, NASenc for non-access stratum encryption, and NASint for integrity. In handovers, keys are refreshed via next hop (NH) parameters over X2 or S1 interfaces without full re-authentication, deriving new KeNB from prior KASME and a nonce counter to maintain forward secrecy during intra-LTE mobility. Early 2G/3G systems exhibited mutual authentication gaps, as the UE could not fully verify the network, exposing risks like false base station attacks during handovers or updates. 5G addresses these through enhanced 5G-AKA with explicit mutual authentication and the Subscription Concealed Identifier (SUCI), which encrypts the Subscription Permanent Identifier (SUPI) using public-key methods before transmission in initial messages for location registration or handover signaling, mitigating IMSI exposure. Re-authentication triggers include entry into roaming via combined attach/location update procedures, periodic tracking area updates (e.g., every 30 minutes to hours configurable by network), or key expiry during prolonged mobility, ensuring fresh vectors and keys from the Unified Data Management/Authentication Server (UDM/AUSF) to counter desynchronization or replay attacks.

Handover procedures

Intra-network handovers

Intra-network handovers enable seamless mobility for user equipment (UE) within the same radio access network, ensuring continuity of service without changing the network type or radio access technology (RAT). These handovers are essential for maintaining connection quality as the UE moves between cells served by the same core network, minimizing disruptions in voice, data, or multimedia sessions. Unlike inter-network transitions, intra-network handovers focus on optimizing radio resource allocation and signal quality within a unified administrative domain, such as a single operator's 3G or 4G infrastructure.[33] Key types of intra-network handovers vary by generation and domain. In 2G and 3G circuit-switched networks, hard handovers predominate, where the connection to the source cell is released before establishing the link to the target cell, potentially causing a brief interruption.[34] In 3G code-division multiple access (CDMA) systems, soft handovers allow the UE to maintain simultaneous connections to multiple cells, combining signals for diversity gain before dropping the weaker link, which reduces the risk of call drops.[35] In LTE networks, handovers follow a make-before-break approach using hard handover principles, where the target cell is prepared and data forwarding ensures minimal packet loss, though the radio link is briefly interrupted.[36] The intra-network handover procedure typically involves several coordinated steps to transfer the UE's context efficiently. The process begins with the UE sending measurement reports to the source base station (e.g., eNodeB in LTE), detailing signal quality metrics from neighboring cells based on configured events. The source base station then decides on the handover, prepares the target cell by allocating resources, and transfers the UE's context—including security keys, bearer information, and quality-of-service parameters—via direct interfaces like X2 in LTE or through the core network if direct links are unavailable.[36] Upon receiving the handover command, the UE detaches from the source and attaches to the target cell, completing the handover with path switch updates to the core network for routing continuity. Handovers are triggered by specific conditions to preempt degradation in service quality. Primary triggers include signal strength thresholds, such as LTE's A3 event, where the neighbor cell's reference signal received power (RSRP) exceeds the serving cell's by a predefined offset (hysteresis margin), ensuring proactive mobility.[37] Load balancing serves as a network-initiated trigger, redistributing UEs from congested cells to underutilized ones to optimize resource usage and throughput across the network.[38] In packet-switched domains, intra-network handovers preserve the UE's IP address and session continuity through mechanisms like GTP tunneling and data forwarding from source to target base stations, avoiding reconfiguration of ongoing IP flows.[39] Conversely, circuit-switched handovers in legacy 2G/3G networks reassign dedicated channels without IP considerations, focusing instead on maintaining fixed bearer paths, which can introduce higher latency due to circuit reconfiguration.[40] Performance metrics emphasize low interruption to support real-time applications, with LTE targeting handover latency below 50 ms to minimize user-perceived delays.[41] Handover failure rates, critical for reliability, are often modeled based on signal-to-interference-plus-noise ratio (SINR) margins at the target cell; for instance, the failure probability during handover execution can be approximated using the outage probability under log-normal shadowing as $ P_{\text{fail}} = Q\left( \frac{\gamma - \mu_{\text{SINR}}}{\sigma} \right) $, where $ Q(\cdot) $ is the Gaussian Q-function, $ \gamma $ is the minimum SINR threshold for successful attachment, $ \mu_{\text{SINR}} $ is the mean SINR at handover boundary, and $ \sigma $ is the shadowing standard deviation (typically 8 dB).[42] These intra-handovers generally occur within defined registration areas, avoiding immediate location updates to the core network.

Inter-network handovers

Inter-network handovers, also known as vertical handovers, enable seamless mobility between disparate network technologies or operators, such as transitions from 3G to 4G or across different radio access technologies (RATs). These handovers are critical in heterogeneous environments where devices must switch between cellular generations to maintain connectivity, often involving inter-RAT (IRAT) measurements to assess signal quality and reselection algorithms to decide the target network. Unlike intra-network handovers, vertical handovers require coordination across core network elements to handle protocol differences and ensure service continuity. The procedure for vertical handovers typically begins with preparation phases, including dual connectivity setups where the device maintains simultaneous links to source and target networks for smoother transitions. In LTE contexts, this involves anchor point relocation, such as migrating the serving gateway (S-GW) to align with the new RAT, which minimizes data path disruptions during the handover execution. Measurements from the source network trigger the process, followed by signaling exchanges via interfaces like the S1 in LTE to reconfigure bearers and update location information in the mobility management entity (MME). Success criteria include low handover failure rates, often below 1% in optimized deployments, as defined in 3GPP specifications. Key challenges in inter-network handovers include quality of service (QoS) mapping between differing RAT capabilities, where parameters like latency and throughput must be renegotiated to avoid service degradation—for instance, a voice call from 3G to 4G may experience brief interruptions due to codec mismatches. Latency spikes can arise from extended signaling delays across network boundaries, potentially exceeding 100 ms in suboptimal conditions, while spectrum differences between technologies (e.g., licensed vs. unlicensed bands) complicate interference management and power control. Authentication is briefly invoked during these events to verify the device's identity across networks, ensuring secure key derivation without halting the handover. In 5G networks, intra-RAT inter-network handovers (e.g., between gNBs of different operators) leverage the Xn interface for direct communication between gNBs, enabling faster context transfers and reduced core network involvement compared to earlier generations. Inter-RAT handovers involve core network coordination. Conditional handover mechanisms further enhance reliability by pre-configuring target cells and allowing the device to execute the switch based on real-time radio conditions, mitigating ping-pong effects and improving success rates to over 99% in lab tests. In Release 18 (as of 2024), further enhancements include L1/L2 triggered mobility for near-zero handover interruption times in 5G Advanced networks.[43] These advancements are outlined in 3GPP Release 15 and beyond, with IRAT handover procedures specifying blind and compressed mode operations for measurement accuracy during transitions.

Roaming support

Domestic roaming processes

Domestic roaming processes enable mobile subscribers to access services from partner networks within their home country when the home network's coverage is unavailable, ensuring continuity without service interruption. These processes rely on established agreements and standardized procedures between operators sharing the same national spectrum and regulatory framework. Unlike international roaming, domestic processes emphasize seamless integration to mimic home network experience, often included in standard plans without extra cost to users, supported by regulatory mandates.[44] Bilateral contracts form the foundation of domestic roaming, outlining terms for service access, quality, and settlement between operators. These agreements typically cover number portability to allow subscribers to retain their numbers across networks and ensure service continuity, such as voice, data, and SMS, without reconfiguration. For instance, operators negotiate interconnection points and traffic routing to support portability databases, enabling calls and messages to route correctly regardless of the serving network. Such contracts are essential for smaller operators or MVNOs to leverage larger networks' infrastructure for nationwide coverage.[45][46][47] In 2G/3G networks, key procedures involve location management through Visitor Location Register (VLR) and Home Location Register (HLR) interactions. When a user equipment (UE) enters a partner network's area, it initiates a location update request to the serving Mobile Switching Center (MSC), which forwards it to the VLR. The VLR then queries the HLR in the home network for subscriber authentication and profile data, including IMSI and service subscriptions. Upon verification, the HLR updates the subscriber's location and sends necessary parameters back to the VLR, enabling automatic redirection of incoming calls and messages via global title translation. This process ensures minimal latency, with the old VLR (if applicable) forwarding any pending data to the new VLR for seamless handover. Analogous processes apply in 4G/5G networks using elements like MME/HSS (LTE) or AMF/UDM (5G); see modern enhancements for details.[48] Charging and billing in domestic roaming integrate with centralized hubs or direct bilateral systems for efficient settlement. Operators use roaming hubs—interconnected platforms like those supporting Billing and Charging Evolution (BCE)—to exchange usage data in real-time or near-real-time formats, facilitating debit from subscriber accounts without batch processing delays common in international scenarios. For prepaid users, real-time charging applies policy rules to deduct balances instantly during sessions, while postpaid billing aggregates usage across networks via standardized records. This setup minimizes disputes through automated validation of inter-operator tariffs.[49][50] User equipment behavior during domestic roaming follows Public Land Mobile Network (PLMN) selection and reselection rules defined in 3GPP standards. In automatic mode, the UE prioritizes its home PLMN (HPLMN) or equivalent upon power-on or signal loss, scanning for available networks and selecting the strongest signal from the operator's preferred list or any viable domestic PLMN if the HPLMN is unavailable. Reselection occurs periodically or on signal degradation, evaluating criteria like signal quality and access technology, while avoiding forbidden PLMNs unless in limited service. Manual mode allows user override to choose a specific domestic partner, bypassing automatic priorities for that session. These rules promote attachment to the best available coverage within the country.[51][52] Regulations enforce domestic roaming to achieve seamless national coverage, preventing coverage gaps. In the United States, the Federal Communications Commission (FCC) mandates automatic voice and data roaming agreements on reasonable, nondiscriminatory terms, eliminating home market exclusions and requiring providers to negotiate commercially viable deals for broadband access outside licensed areas. Similarly, in the European Union, early directives like the UMTS Decision (1999) and Access Directive (2002) permitted or required national roaming to support 3G rollout in underserved areas, promoting competition and uniform service quality across member states, though ex ante regulation has diminished with market maturity. These frameworks ensure operators collaborate for ubiquitous connectivity.[44][53][54][55]

International roaming agreements

International roaming agreements enable mobile subscribers to access services across national borders through bilateral or multilateral contracts between operators, standardized by organizations like the GSMA to ensure interoperability and service continuity. The GSMA's IR.21 (Roaming Database, Structure and Updating) serves as a core framework, providing a standardized mechanism for operators to exchange essential roaming data, including network configurations, TADIG codes, contact information, and technical details for various network types such as terrestrial, non-terrestrial, and M2M dedicated integrated systems. This reduces implementation time and supports charging and settlement processes, facilitating reliable global connectivity for users traveling internationally. In the EU, the Roaming Regulation was extended until 2032, ensuring continued "Roam Like at Home" with no extra charges for intra-EU roaming, subject to fair use policies, as confirmed in the 2025 review.[56][57] To verify interoperability before activating agreements, the GSMA's International Roaming Expert Group (IREG) conducts testing via a structured three-stage methodology outlined in IR.23. Stage 1 involves interface self-certification for protocols like MAP, CAMEL, Diameter, and GTP; Stage 2 covers exchange of numbering data, operational procedures, and fault reporting; and Stage 3 tests end-to-end functionality and performance. This process ensures that networks from different operators can seamlessly interwork, minimizing service disruptions in roaming scenarios.[58] Signaling procedures between the home network (HPLMN) and visited network (VPLMN) rely on established protocols to manage location updates, service provisioning, and billing. Traditional circuit-switched roaming uses SS7 signaling with the Mobile Application Part (MAP) for mobile-specific operations, such as subscriber authentication and profile retrieval from the home location register (HLR). In packet-switched and IMS-based environments, SIP signaling supports VoLTE and data services, while Diameter handles modern authentication and policy control in 4G/5G contexts. Authentication is adapted for visited networks by querying the home network's keys via MAP or Diameter messages.[59] Key challenges in these agreements include regulatory variances across jurisdictions, which impose differing requirements on data privacy, spectrum usage, and interconnection fees, complicating negotiations and compliance. Time zone differences further hinder accurate billing, as call detail records must align with local times for fair settlement, often leading to disputes. Fraud prevention is paramount, with schemes like roaming recharge—where fraudsters exploit rate arbitrage by recharging in low-cost areas for high-value international use—causing significant revenue loss; operators mitigate this through real-time monitoring and GSMA-recommended blacklisting of suspicious numbers.[60][61] The evolution of international roaming has progressed from 2G's voice-focused services, reliant on SS7/MAP for basic connectivity, to 5G's emphasis on high-speed data roaming with enhanced security via the Security Edge Protection Proxy (SEPP) to address legacy vulnerabilities. eSIM support, standardized by GSMA's SGP.22 and integrated into 5G ecosystems, allows remote provisioning of roaming profiles, eliminating physical SIM swaps and enabling seamless transitions across networks.[62] Performance metrics highlight areas for improvement: field studies indicate roaming success rates where download speeds degrade compared to home networks in approximately 39% of cases, upload speeds in 59%, and latency in 62%, often due to suboptimal agreements. International handover latency typically incurs additional delays of 60 ms or more from cross-border signaling, impacting real-time applications.[63][64]

Registration areas

Circuit-switched areas

In circuit-switched mobility management, a Location Area (LA) constitutes a geographical grouping of cells served by a single Mobile Switching Center (MSC) within a Public Land Mobile Network (PLMN). This area is uniquely identified by the Location Area Identity (LAI), which combines the Mobile Country Code (MCC), Mobile Network Code (MNC), and a 16-bit Location Area Code (LAC). The LAC, encoded as a fixed-length 2-octet value ranging from 0000 to FFFE in hexadecimal (with 0000 and FFFF reserved), distinguishes the LA from others in the same PLMN.[65] The LA forms the foundational unit for key procedures in the Circuit-Switched (CS) domain of GSM and UMTS networks, specifically Location Area Updates (LAU) and paging. During LAU, a mobile station (MS) registers its presence upon entering a new LA, enabling the Visitor Location Register (VLR) associated with the MSC to maintain accurate location records without necessitating updates for movements within the same LA. This mechanism supports efficient call routing and service delivery for voice-centric operations. For incoming calls or other CS services, the network initiates paging by broadcasting messages across all cells in the MS's registered LA, allowing the device to respond and establish a connection while minimizing unnecessary signaling outside the known area.[48][18] LA sizing involves a critical trade-off between signaling overheads: smaller LAs (e.g., encompassing 10-100 cells) increase the frequency of LAU procedures as MSs cross boundaries more often, but reduce paging load by limiting broadcast scope; conversely, larger LAs decrease updates at the cost of higher paging traffic. Operators configure LA sizes based on factors like subscriber density and terrain, typically balancing these costs to optimize network efficiency in urban or rural deployments.[66] In terms of evolution, LAs in 2G GSM systems are statically defined during network planning, with fixed boundaries tied to MSC coverage. Later UMTS releases introduce greater flexibility through dynamic paging areas, where the network can adjust the effective paging scope within or across LAs based on real-time conditions, enhancing adaptability for CS mobility without altering core LA structures.[67]

Packet-switched areas

In packet-switched networks such as those introduced by the General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE), the Routing Area (RA) serves as a fundamental unit for managing data mobility. An RA is defined as a geographical area comprising one or more cells within which a mobile station (MS) can move freely without needing to update its serving GPRS Support Node (SGSN), thereby maintaining an active packet-switched (PS) session. Each RA is a subset of a Location Area (LA) from the circuit-switched (CS) domain and is identified by a Routing Area Identity (RAI), which includes the Mobile Country Code (MCC), Mobile Network Code (MNC), Location Area Code (LAC), and Routing Area Code (RAC). The RAC, a one-octet fixed-length code, uniquely identifies the RA within its encompassing LA.[68] The primary procedure for handling mobility within PS domains is the Routing Area Update (RAU), which occurs when an MS detects a change in the RAI broadcast by the network, such as upon entering a new RA while in PS-IDLE or PS-CONNECTED state. During a PS attach, the MS registers its location at the RA level, enabling the network to route packet data efficiently to the MS without requiring CS domain involvement for data-only devices. If the RA change involves a different SGSN, the procedure includes context transfer and potential updates to the Home Location Register (HLR). This independence allows data sessions to persist seamlessly across RAs, supporting continuous IP connectivity for applications like web browsing or email in early mobile data environments.[69] A key advantage of RA-based mobility is the reduction in unnecessary CS signaling overhead, particularly for user equipment (UE) focused solely on packet data services. By decoupling PS tracking from CS location updates, the system avoids triggering Location Area Updates (LAU) for movements that do not affect voice services, thereby optimizing network resources and minimizing battery drain on data-centric devices.[69] However, the structural overlap between RAs and LAs—where each RA is confined to a single LA—introduces limitations, especially in Universal Mobile Telecommunications System (UMTS) deployments. When an MS crosses LA boundaries, it necessarily enters a new RA, often necessitating combined RAU/LAU procedures to synchronize both domains, particularly if the Gs interface links the MSC/VLR and SGSN for dual-mode operation. This coupling can increase signaling load and complexity compared to fully independent PS mobility.[70]

Evolved areas in 4G/5G

In 4G LTE networks, the Tracking Area (TA) serves as a fundamental unit for location management in idle mode, consisting of a group of cells under one or more eNodeBs that enables the Mobility Management Entity (MME) to track User Equipment (UE) without requiring constant updates.[71] Each TA is uniquely identified by a Tracking Area Identity (TAI), which combines the Mobile Country Code (MCC), Mobile Network Code (MNC), and Tracking Area Code (TAC) to facilitate paging and mobility procedures.[71] The MME assigns a Tracking Area List (TAL) to the UE during attachment or tracking area updates, allowing the device to roam across multiple TAs without triggering an update, thereby optimizing signaling overhead in high-mobility environments.[71] Building on LTE, 5G introduces refinements to TA concepts for enhanced efficiency in all-IP architectures, including UE-specific TALs that enable targeted paging by the Access and Mobility Management Function (AMF) based on the UE's mobility patterns and last known location.[72] Additionally, the RAN Notification Area (RNA) supports the RRC_INACTIVE state, defining a RAN-controlled set of cells or RAN areas where UEs can move without notifying the core network, preserving context at the last serving gNB for rapid resumption of connections.[73] RNA updates occur periodically or upon leaving the configured area, integrating with TA boundaries to minimize core network involvement while enabling features like small data transmission.[73] Enhancements in these networks include dynamic TA reconfiguration, where the MME or AMF reallocates TALs during tracking area updates, GUTI reallocation, or handovers to adapt to UE mobility, network load, or radio access technology changes, such as support for narrowband IoT.[71] In 5G, artificial intelligence and machine learning (AI/ML) further optimize area prediction by analyzing UE trajectories to proactively adjust TALs or RNAs, reducing update frequency and paging signaling as studied in 3GPP Release 18 for air interface enhancements.[74][75] Compared to earlier Location Areas (LAs) and Routing Areas (RAs) in pre-4G systems, TAs and their lists are typically larger and more flexible, encompassing multiple cells to decrease location update rates in scenarios with frequent handovers, while enabling overlapping configurations for seamless idle-mode mobility.[71] This evolution supports reduced core network signaling in high-speed or dense deployments, foundational to prior area concepts but tailored for packet-switched dominance.[76]

Modern enhancements

Mobility in LTE networks

Mobility management in Long-Term Evolution (LTE) networks, part of the Evolved Packet System (EPS), relies on key entities such as the Mobility Management Entity (MME) for core network control and the evolved Node B (eNB) for radio access mobility, primarily facilitated through the X2 interface between eNBs. The MME handles signaling for user equipment (UE) attachment, authentication, and location tracking, while eNBs manage direct handovers to minimize core involvement and reduce latency.[77] This architecture supports seamless transitions across cells, enabling high-speed data services in urban and mobile environments. In idle mode, LTE employs Tracking Area Updates (TAU) to maintain UE location at a granular level without excessive signaling. When a UE moves to a new Tracking Area (TA) not included in its assigned TA List, it initiates a TAU procedure by sending a TAU Request to the MME via the serving eNB, updating its location and potentially receiving a refreshed TA List for camping without further updates.[78] Paging for incoming calls or data occurs over the entire TA List, allowing the network to reach the UE efficiently across multiple TAs, which optimizes battery life and reduces overhead compared to cell-level tracking in prior systems.[79] In connected mode, mobility is achieved through handovers, where the source eNB measures signal quality and prepares the target eNB via X2 or S1 interfaces, followed by a path switch to redirect downlink traffic from the Serving Gateway (SGW) to the target eNB.[80] This procedure ensures low interruption times, typically under 50 ms, supporting Voice over LTE (VoLTE) continuity by preserving IMS bearers during intra-LTE transitions, thus maintaining real-time voice sessions without fallback to circuit-switched networks.[81] LTE introduces significant mobility improvements over 3G systems, including reduced end-to-end latency to 20-50 ms, enabling faster handovers and responsive applications.[82] The always-on data capability in RRC connected mode allows persistent connectivity without frequent state transitions, minimizing signaling storms during mobility. Carrier aggregation further enhances performance by combining multiple frequency bands, supporting seamless handover across aggregated carriers in high-mobility scenarios like vehicular travel.[40] From 3GPP Release 8 (Rel-8) to Rel-15, mobility enhancements progressively address Heterogeneous Networks (HetNets), incorporating small cells and macro overlays. Rel-8 establishes baseline intra-LTE handover and TAU procedures, while Rel-10 introduces carrier aggregation for inter-eNB mobility. Subsequent releases, such as Rel-12 and Rel-13, optimize load balancing and dual connectivity in HetNets to handle dense deployments, significantly improving handover success rates in urban settings. Rel-15 includes support for high-speed mobility up to 500 km/h in applicable scenarios and enhances integration with non-3GPP accesses.[83] These evolutions pave the way for 5G extensions in later releases.

Mobility in 5G networks

In 5G networks, mobility management is anchored in the 5G Core (5GC) architecture, where the Access and Mobility Management Function (AMF) serves as the central control plane entity responsible for registration, connection management, and mobility handling for user equipment (UE). The AMF replaces the Mobility Management Entity (MME) from LTE, enabling a more modular and service-based architecture that supports seamless transitions across access networks. Handovers in 5G occur primarily through the Next Generation Radio Access Network (NG-RAN), utilizing the NG Application Protocol (NGAP) for N2-based procedures between gNBs or between NG-RAN nodes and the AMF, ensuring continuity during inter-node mobility without core network involvement in many cases. This design future-proofs the system for ultra-reliable low-latency communications (URLLC) by decoupling user plane and control plane functions, allowing efficient resource allocation during movement. Key innovations in 5G mobility include network slicing tailored to specific mobility profiles, which allows operators to provision logical networks with customized policies for different UE types, such as high-mobility vehicular users or stationary IoT devices. For low-latency applications, edge relocation procedures enable the dynamic migration of user plane functions (UPF) closer to the UE's location, minimizing end-to-end delay during handovers in edge computing scenarios. Additionally, predictive handover mechanisms leverage machine learning (ML) algorithms to anticipate UE movement based on historical data and radio measurements, optimizing trigger timing and target selection to reduce failure rates and latency. These features, integrated via the AMF and session management function (SMF), enhance overall network efficiency and support diverse service requirements.[14] In idle and inactive modes, 5G introduces the RAN Notification Area (RNA), a configurable set of cells or tracking areas where the UE monitors for paging without frequent registration updates; if the UE moves outside the RNA, it performs an RNA update procedure to notify the network, balancing power consumption and reachability. Paging is enhanced through beamforming in the NG-RAN, where directional beams are used to deliver paging messages efficiently in millimeter-wave deployments, improving coverage and reducing UE battery drain compared to omnidirectional broadcasting. This beam-based approach, combined with the RRC_INACTIVE state, allows quick resumption of connections for UEs that were previously active, minimizing signaling overhead. 5G addresses mobility challenges for massive IoT deployments by supporting optimized procedures for low-power wide-area (LPWA) devices, such as extended discontinuous reception (eDRX) and mobility restrictions that limit unnecessary handovers for stationary or slow-moving sensors, enabling scalability to millions of connections per cell. For Vehicle-to-Everything (V2X) communications, 5G provides enhanced sidelink and network-assisted mobility, including mode switching between direct (PC5) and Uu interfaces to maintain connectivity at high speeds, with low-latency handovers tailored for cooperative intelligent transport systems. From 3GPP Release 15 onward, dual connectivity options like E-UTRA NR Dual Connectivity (EN-DC) allow non-standalone (NSA) 5G deployments where LTE serves as the master node and NR as the secondary, facilitating smooth mobility integration by leveraging LTE's coverage for control signaling while boosting throughput with 5G data bearers. This builds on LTE foundations as a baseline for initial 5G rollouts, enabling inter-RAT handovers with minimal disruption. Subsequent releases further refine these mechanisms for standalone (SA) operations and advanced scenarios: Rel-17 introduces conditional handovers and enhanced mobility for extended reality (XR) services, while Rel-18 specifies AI/ML integration for predictive mobility optimization in NG-RAN and support for non-terrestrial networks (NTN).[84]

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