Next generation mobility management: an introduction

WIRELESS COMMUNICATIONS AND MOBILE COMPUTINGWirel. Commun. Mob. Comput. 2011; 11:446–458

Published online 8 January 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.904

SPECIAL ISSUE PAPER

Next generation mobility management:an introductionF. Richard Yu1, Vincent W. S. Wong2, Joo-Han Song2, Victor C. M. Leung2∗ andHenry C. B. Chan3

1 Carleton University, Ottawa, Canada2 The University of British Columbia, Vancouver, Canada3 The Hong Kong Polytechnic University, Hong Kong, China

ABSTRACT

Mobility management, which includes location management and handoff management, is essential in cellular wirelessnetworks to provide service to mobile users. Location management enables call delivery to mobile users, while handoffmanagement maintains the connectivity of ongoing calls while users move between cells. In next generation networks,mobile users will avail themselves with terminals capable of accessing wireless networks employing multiple technologies,thus making the task of mobility management more challenging. This paper reviews recent developments in locationmanagement, and surveys methods for handoff management between heterogeneous systems. Methods for inter-systemhandoffs in packet-switched inter-networks are discussed according to the protocol layer in which the handoffs take place,i.e., network layer, transport layer, and application layer. Open problems for mobility management in future wirelessnetworks are also presented. Copyright © 2010 John Wiley & Sons, Ltd.

KEYWORDS

handoff management; location management; mobility management

*Correspondence

Victor C. M. Leung, Department of Electrical and Computer Engineering, The University of British Columbia, 2332 Main Mall, Vancouver,BC, Canada V6T 1Z4.E-mail: [email protected]

1. INTRODUCTION

In recent years, there has been tremendous growth inthe use of wireless mobile communication services asthey allow people to stay in touch while on the move.Currently, there exist disparate wireless systems that capturedifferent requirements and needs of different users. Wirelesslocal area networks (WLANs) offer high rates to userswith low mobility over local areas. Wireless cellularnetworks provide relatively low data rates to user withhigh mobility over wide areas. Satellite networks areused extensively for worldwide coverage. Mobile ad hocnetworks (MANETs) can be used to establish dynamicnetworks without the need of a fixed infrastructure due totheir self-configuration and self-maintenance capabilities.In the next generation telecommunication network [1],these heterogeneous wireless networks are expected to beintegrated to provide ubiquitous ‘always best connection’to mobile users.

In cellular wireless networks (Figure 1) that supportcircuit-switched voice and packet-switched multimediaservices, mobility management is an important functionthat ensures successful delivery of new calls to users andmaintains ongoing calls with minimal disruptions, whileusers move between cells. There are two componentsin mobility management: location management andhandoff (also referred to as handover in the literature)management.

Location management enables the network to delivercalls to mobile users by tracking their locations betweencalls. It involves two operations: location update and paging.When a location update occurs, e.g., when a mobile terminal(MT) or mobile node (MN) (we use these two termsinterchangeably in this paper), moves from location area(LA) identified as LA1 to LA2 in Figure 1, it sends itslocation information, e.g., the identity of the LA in whichit is located, to the network database. When a call arrives,the recipient MT may have already moved to another cell

446 Copyright © 2010 John Wiley & Sons, Ltd.

F. R. Yu et al. Next generation mobility management

LocationArea LA 1

Location

Cells

VLR VLR

HLR

MSC MSC

LocationArea LA 1

CellsLocationArea LA 1

CellsLocationArea LA 1

Area LA 2

Cells

VLR VLR

HLR

MSC MSC

Figure 1. Mobility management in cellular network architecture(MSC---mobile switching center; HLR---home location register;

VLR---visitor location register).

that is different from the one reported in the last locationupdate, and hence the network needs to search (or page)a number of cells, e.g., all the cells constituting an LA inFigure 1, to locate the MT. There is a trade-off betweenthe costs of location update and paging. If an MT updatesits location more often, the network can have more preciseinformation about the location of the MT, thus reducingthe cost for paging when a call arrives; e.g., in Figure 1this is realized by making each location area smaller. Onthe other hand, frequent location updates increase the costsof these updates. These tradeoffs led to different locationupdate techniques, some of which dynamically adapt to usermobility.

Handoff management maintains service continuity byenabling an MT to keep its call connected when its point ofconnection to the network moves from one access point (orbase station) to another. The handoff process can be intra-or intersystem. Intra-system handoffs (also referred to ashorizontal handoffs) occur within a single network domainemploying a homogeneous wireless access technology, andare usually handled in the link layer as an integral partof the wireless access method. Intra-system handoffs, e.g.,due to movement of an MT between adjacent cells inFigure 1, have been widely investigated and a good surveycan be found in [2]. In next generation networks thatintegrate several wireless access technologies, handoffs willneed to support movements of terminals between networksemploying different wireless access technologies. Theseare known as inter-system handoffs (also referred to asvertical handoffs), which will be surveyed in this paper.Several schemes have been proposed to solve the verticalhandoff problem in heterogeneous wireless networks. Theiroperation scope varies from network to application layer. Inpacket-switched networks employing the Internet Protocol(IP), Mobile IP (MIP) based solutions can be employed inthe network layer to provide transparent support for terminalmobility, including the maintenance of active TransmissionControl Protocol (TCP) connections and User DatagramProtocol (UDP) port bindings. Transport layer solutions

for handoffs conform to the end-to-end principle in theInternet, i.e., anything that can be done in the end systemshould be done there. Since the transport layer is the lowestend-to-end layer in the IP protocol stack, it is a naturalcandidate for vertical handoff support. For multimediaconnections set up using the application layer SessionInitiation Protocol (SIP) for signaling, a SIP-based approachcan be used to support vertical handoffs independent of theunderlying wireless access technologies and network layerelements.

The rest of the paper is organized as follows. Section 2reviews some recently proposed location managementschemes. Network layer handoff management schemes arepresented in Section 3. Section 4 reviews transport layerhandoff management schemes. Application layer handoffmanagement schemes are presented in Section 5. Section 6concludes the paper.

2. LOCATION MANAGEMENT

In this section, we summarize the latest results in recentpublications on location update and paging with a particularfocus on wireless cellular networks. Most of the resultssurveyed here are obtained in 2005--2008. The work in thissection is complementary to [3]; together, they provide amore complete picture on location management for wirelesscellular networks.

2.1. Location Update

In [4], a cost analysis model is presented for thedynamic movement-based location update scheme. Theproposed model used the results from renewal theory. Thelocation area residence time can follow hyper-exponentialdistribution and the cell residence time can follow anygeneral distribution. In [5], an analytical model is proposedfor the movement-based scheme for wireless cellularnetworks with the home location register (HLR) and visitorlocation register (VLR) architecture in Figure 1. Boththe cell residence time and the location area residencetime can follow general distribution. The cost for locationupdate and paging is derived. This model can be consideredas a generalization of the work in [4,6]. In [7], thestatistics for the number of cell crossings for a mobile userin the movement-based location management scheme isderived. Based on these statistics, the optimal sequentialpaging sequences, which minimize the paging cost for anygiven movement threshold and call-to-mobility-ratio, aredetermined.

In [8], a hybrid location update scheme is proposed bycombining the movement-based and time-based schemes.An MT updates its location whenever it has traverseda fixed number of cell-boundary crossings and a certaintime interval has elapsed following the previous update.Simulation results showed that the proposed hybrid schemeperforms better than the individual movement-based or

Wirel. Commun. Mob. Comput. 2011; 11:446–458 © 2010 John Wiley & Sons, Ltd. 447DOI: 10.1002/wcm

Next generation mobility management F. R. Yu et al.

timer-based schemes when the coefficient of variation ofthe cell residence time is large.

For the implementation of the distance-based scheme, anapproach is proposed in [9] by using the cell coordinates incalculating the physical distance traveled by the MT. Basedon the speed and direction of each individual MT, the sizeand shape of the LA (see Figure 1) can also be selected. Thepaging cost can also be reduced when the LA is partitionedinto multiple paging areas.

In [10], a combined dynamic location registrationand update model is proposed. The registration decisiondepends on the time elapsed since the last call arrival and thedistance that the MT has traveled since the last registration.For the distance-based update scheme, a single sample path-based ordinal optimization algorithm is proposed.

In [11], a mobility pattern-based scheme is proposed. Itincorporates the time information in the mobility patternprofile of the terminal. The current location of the MT canbe determined by its movement state and the current systemtime. Simulation results showed that the proposed schemehas a lower signaling traffic load when compared to theprofile-based scheme [12].

In [13], two location tracking algorithms based on spatialquantization and temporal quantization are proposed.The movement sequence of the MT is first quantizedinto a smaller set of codewords and then a compressedrepresentation of the codeword sequence is reported.Simulation results showed that the proposed algorithms canreduce the update cost to very low values with only a smallincrease in the paging overhead.

When a wireless cellular network is covered with bothmacrocells and microcells, the MT can communicate viathe base stations from these two types of cells. In [14], alocation management strategy is proposed for this kind ofhierarchical cellular networks. To reduce the update cost,a location update is performed only in the macrocell tier.Depending on the paging load, an MT can be paged eitherin the macrocell tier or in the microcell tier. Results showthat this scheme can provide a small paging delay and a lowupdate cost.

The frequency of location updates can also bereduced by having some registration areas overlappedwith each other [15]. Overlapping allows the MT tochoose a registration area to send its update message.This feature is not available in environments wherenon-overlapping registration areas are deployed. Havingoverlapped registration areas can also reduce the ping-pongeffect for users moving between the registration boundariesfrequently. The work in [16] showed that the selection ofthe registration area can affect the number of subsequentregistrations. It then formulated both deterministic (offline)and stochastic (online) optimization problems which aimto minimize subsequent registrations. In [17], four LAselection policies for overlapping LAs scenarios areproposed, namely MaxOL, Central, Random, and MinOLpolicies. An analytical model is used to determine theexpected number of cell movements before an MT leaves aLA.

2.2. Paging

In [18], the problem of searching for multiple MTsconcurrently is studied. Three concurrent search algorithmsare proposed, namely: the brute force algorithm, thesimple heuristic algorithm, and the conditional probabilityheuristic algorithm. These algorithms aim to concurrentlylocate k MTs within k time slots based on the probabilisticinformation about the locations of MTs. Results showedthat the average paging cost can be reduced significantly.

In [19], a pipeline paging scheme is proposed whichcan handle multiple paging requests by serving them in apipeline manner in different paging areas. Results showedthat the pipeline paging scheme outperforms both theblanket paging and sequential paging schemes in terms of ahigher discovery rate and a lower total delay under certainconditions.

In [20], a simplified pipeline probability paging schemeis proposed. Based on the information on the locationprobabilities of individual MTs, the proposed schemecan handle paging requests in a pipeline manner with apaging delay constraint. It also has a better performancewhen compared with blanket paging and sequential pagingschemes.

There are various works which studied location updateand paging tradeoff. By using the results from majorizationtheory and Riesz’s rearrangement inequality, the work in[21] showed that jointly optimal paging and registrationpolicies for either symmetric or Gaussian random-walkmodels are given by the nearest-location-first pagingpolicies and distance threshold registration schemes.

In multi-system wireless networks or heterogeneouswireless networks (see Figure 2), an MT can be equippedwith multiple interfaces and connect to multiple wirelessaccess networks simultaneously. In [22], three locationmanagement strategies are proposed for multi-systemwireless networks with loosely coupled architecture. Eachstrategy differs in the levels of coordination and trustrequired among the wireless access network providers.Among them, the centralized approach has the lowestpaging cost but require all relevant history to be stored

Figure 2. A multi-system wireless network.

448 Wirel. Commun. Mob. Comput. 2011; 11:446–458 © 2010 John Wiley & Sons, Ltd.DOI: 10.1002/wcm


in a central coordinating server. The distributed approachincurs a high cost in both paging and location update. Thequasi-distributed approach reduces the MT’s update loadby using a central coordinator which only maintains theaggregate statistics about the MT’s location information ineach sub-network.

3. NETWORK LAYER HANDOFFMANAGEMENT

In contrast to circuit-switched networks, which generallysupport wireless handoffs within the access network(usually considered a function of the data link layerand below), packet-switched networks can support mobilehandoffs in the network layer, by the ability to routepackets successfully to the desired destination while thepoint of network attachment of the destination node hasmoved to a different part of the network or a differentnetwork domain. A domain is a common administrativeentity that may include different access networks, suchas WLAN and cellular networks of one service provider.Network layer handoff management solutions can bebroadly classified into two categories: macro-mobility andmicro-mobility management [23]. The movement of mobilenodes between two network domains is referred to as macro-mobility. On other hand, the movement of mobile nodesbetween two subnets within one domain is referred to asmicro-mobility.

Network layer handoff management provides nodemobility support at the IP layer. It does not depend onor make any assumption about the underlying wirelessaccess technologies [24]. Signaling messages for handoffpurposes are carried by IP datagrams. MIP [25] isan Internet Engineering Task Force (IETF) standardmobility management protocol that is designed to enablemobile nodes to move from one network to anotherwhile maintaining their own IP addresses that were pre-assigned by their home networks. Similarly, Mobile IPv6(MIPv6) [26] provides node mobility support over IPv6.

3.1. Macro Mobility Solutions (Mobile IP)

3.1.1. Mobile IPv4 (MIPv4).

Mobile IP (see Figure 3) is a mobility managementprotocol for the global IP network (i.e., Internet). Itintroduces four new functional entities: home agent (HA),foreign agent (FA), MN, and correspondent node (CN).

An MN is able to detect whether it has moved into anew subnet by periodically receiving unsolicited AgentAdvertisement messages broadcasted from each FA. AnMN can also send Agent Solicitation messages to learnabout the presence of any prospective FA. Each MN canhave two addresses---a permanent IP address and a care ofaddress (CoA), which is associated with the network a MNis currently visiting. When an MN discovers that it is in aforeign network, it obtains a new COA from FA (an MN can

Figure 3. Network mobility with mobile IP.

serve as its own FA). This CoA can be obtained by solicitingor listening for FA advertisements, or invoking DynamicHost Configuration Protocol (DHCP) [27] or Point-to-PointProtocol (PPP) [28]. The MN registers the new CoA with itsHA, and the HA updates the mobility binding by associatingthe IP address of the MN with its CoA.

Packets sent by a CN to an MN are intercepted by the HAin flight. The HA encapsulates the packets and tunnels themto the MN’s CoA. With an FA CoA, the encapsulated packetreaches the FA serving the MN, which decapsulates thepackets and forwards them to the MN. With an associatedCoA, the encapsulated packets reach the MN, which thendecapsulates them.

3.1.2. Mobile IPv6.

The MIPv6 protocol has been specified by the IETF IPRouting for Wireless/Mobile Hosts Working Group [26].MIPv6 uses many features of MIP, but it is integrated intoIPv6 and presents many other enhancements. There aremany important differences between MIP and MIPv6 asdescribed below.

• There is no need for FA; it is not necessary to have aspecial local support to allow a MIPv6 node to operatecorrectly.

• MIP mobility defines the default mode of operationthrough the HA of the MN’s home network. This tendsto burden the MN’s HA; the ability to redirect a CNto the MN’s current address is implemented only as anextension of the IPv4.

• MIPv6 incorporates route optimization as a fundamen-tal aspect of the protocol.

• Rather than encapsulating packets in tunnels as inMIP, most MIPv6 packets sent to a MN include IPv6



header extensions. Therefore, there is no need toencapsulate/decapsulate packets for tunnels.

• Use of IPv6 Neighbor Discovery eliminates the needto rely on Address Resolution Protocol or other linklayer mechanisms; it also enables functions such asunreachability detection and agent discovery.

Other differences include the way that authentication isaccomplished between MNs and their agents or CNs, aswell as how MNs can be accommodated even when they areattached to a network that is behind a firewall or NetworkAddress Translation (NAT) device.

3.1.3. Proxy mobile IPv6 (PMIPv6).

To solve the mobile host-based global mobility chal-lenge, a Network-based Localized Mobility Management(NETLMM) protocol is being actively standardized bythe IETF working group [29,30]. Proxy Mobile IPv6(PMIPv6) [31] is a new standard currently being workedon at the IETF NETLMM working group.

Within a PMIPv6 network domain, the simplifiedsignaling procedure for the handoff within a localizednetwork can significantly reduce the handoff latency andthe overhead of the control. PMIPv6 is designed to provideNETLMM to MNs with a standard IPv6 stack. The distinctfunctional entities of PMIPv6 are the Mobile AccessGateway (MAG) and the Local Mobility Anchor (LMA).The main job of the MAG is to detect the MN’s movementsand initiate mobility-related signaling with the LMA onbehalf of the MN. The serving network assigns a uniquehome network prefix to each MN, and theoretically thisprefix always follows the MN wherever it moves within aPMIPv6 domain. From the viewpoint of the MN, the entirePMIPv6 domain appears as its home network. The MNcan configure an address using any address configurationmechanism that is authorized in the PMIPv6 domain.

3.1.4. Optimized mobile IP schemes.

The problem of triangular routing has been alleviatedby using a route optimization scheme [32]. The basic ideabehind route optimization is to use a direct routes betweenMNs and their CNs to bypass the HA. CNs maintain abinding cache of the CoAs of MNs. When a CN sendspackets to an MN, it first checks if it has a binding cacheentry for the MN. If it has a binding cache entry, theCN tunnels the packets directly to the CoA of the MN.If no binding cache entry is available, the CN sends thepackets following the basic Mobile IP procedure, that is,via the HA of the MN. Route optimization also takescare of the packets already tunneled to the old CoA andon the way. When an MN registers with a new FA, itrequests the new FA to notify the previous FA about themovement. This ensures that packets on the way to theold CoA are successfully forwarded. It also ensures thatpackets from the CN with obsolete binding cache entriesfor the MN are successfully delivered to the MN’s new

CoA. Moreover, route optimization also ensures that anyresources consumed by the MN at the old FA are releasedimmediately, rather than waiting for the registration time toexpire.

Enhanced Mobile IP (EMIP) [33] has been developed toeliminate tunneling between the HA and the FA. It differsfrom MIP in the way packets are redirected from the HAto the FA. A concept built on NAT [34] is used in place oftunnels. Instead of creating a tunnel, a mapping is createdbetween the HA and the FA when each connection the MNcommunicates across is established. Mappings are creatingby intercepting packets to and from the MN at the HA andthe FA. The HA and FA then exchange mapping requestand mapping reply messages containing the source anddestination IP and port address of the MN and the CN.The mobility agent that intercepts the packet also suppliesa care of port (CoP) that is used by the HA and the FA toidentify the mapping. Once the mapping between the HAand the FA is established for a communication session, themobility agents can redirect packets to and from the MN bymodifying the IP and TCP or UDP packet headers insteadof using a tunnel.

3.2. Micro Mobility Solutions

MNs may move frequently between subnets of one domain.To reduce signaling load and delay to the home networkduring movements within one domain, many micro-mobility solutions have been proposed. They can be broadlyclassified into two groups: tunnel-based and routing-basedmicro-mobility schemes [35].

3.2.1. Tunnel-based schemes.

Tunnel-based schemes use local or hierarchical reg-istration and encapsulation concepts to limit the scopeof mobility-related signaling messages, thus reducingthe global signaling load and handoff latency. In MIPwith regional registration (MIP-RR) [36], the regionalregistrations are performed via a new network entity calledthe Gateway Foreign Agent, which introduces a layer ofhierarchy in the visited domain. Regional registrationsreduce the number of signaling messages to the homenetwork, and reduce the signaling delay when a MNmoves from one FA to another within the same domain.Hierarchical MIPv6 (HMIPv6) [37] is an enhancement ofMIPv6, which is designed to reduce handoff latency and theamount of signaling by managing local mobility for mobileconnections. HMIPv6 adds an extra level that separateslocal from global mobility. In HMIPv6, global mobilityis managed by the MIPv6 protocols, while local handoffsare managed locally. A new node in HMIPv6 called theMobility Anchor Point (MAP) serves as the local entity toaid in mobile handoffs. The MAP, which replaces the FAin MIPv4, can be located anywhere within a hierarchy ofrouters. Unlike FA, there is no need to have a MAP residingin each subnet. The MAP helps to decrease handoff-related



latency because a local MAP can be updated more oftenthan a remote HA. The intra-domain mobility managementprotocol (IDMP) [24] is a two-level hierarchical approach.The first hierarchy consists of different mobility domains,and the second hierarchy consists of IP subnets within onedomain. This hierarchical approach localizes the scope ofintra-domain location update messages and reduces boththe global signaling load and update latency.

3.2.2. Routing-based schemes.

Routing-based host mobility schemes maintain host-specific routes in the routers to forward packets. Thehost-specific routes are updated based on host mobility.Cellular IP [38] maintains distributed caches for locationmanagement and routing purposes. A distributed pagingcache roughly maintains the positions of idle MNs in aservice area. Cellular IP uses this paging cache to quicklyand efficiently pinpoint idle MNs that wish to engagein active communications. Handoff aware wireless accessInternet infrastructure (HAWAII) [39] is another domain-based scheme. The edge gateway, connecting the accessnetwork to the Internet core, is called the domain rootgateway. Each gateway has a default route inside the domainpointing toward the domain root gateway. For every mobilenode setting up its path, an entry for its IP address is added inthe domain root gateway and associated with the appropriateinterface.

4. TRANSPORT LAYER HANDOFFMANAGEMENT

Handoff management can be done in the transportlayer. Transport layer solutions follow the end-to-endprinciple [40] in the Internet, i.e., anything that can be donein the end system should be done there. Since the transportlayer is the lowest end-to-end layer in the Internet protocolstack, it is a natural candidate for handoff management.Moreover, in the transport layer solutions, no third partyother than the endpoints participates in handoffs, and nomodification or addition of network components is required.TCP and UDP are the two most commonly used transportlayer protocols in the Internet. Stream Control TransmissionProtocol (SCTP) is a new IETF standardized transport layerprotocol. We classify transport layer handoff managementscheme as TCP/UDP-based handoff management schemesand SCTP-based handoff management schemes.

4.1. TCP/UDP-based Handoff ManagementSchemes

A transport layer mobility architecture calledMSOCKS [41] is proposed to split a TCP connection thatallows MNs to not only change their points of attachmentsto the Internet but also control which network interfacesare used for the different kinds of data leaving from and

arriving at the MNs. MSOCKS is built around a proxythat is inserted into the communication path between aMN and its correspondent hosts. The proxy can maintainone stable data stream, isolating the correspondent hostfrom any mobility issues. Meanwhile, the proxy cansimultaneously make and break connections to the MN asneeded to migrate data streams between network interfacesor subnets.

Indirect TCP (I-TCP) is proposed in [42] to solve thehandoff management problem. In this scheme, a TCPconnection between the CN and gateway as well asan I-TCP connection between the gateway and MN isestablished to provide the end-to-end communication. TheTCP portion remains unchanged during the lifetime of thecommunication and remains unaware of the mobility ofMNs. In the I-TCP portion, when the MN moves fromone subnet to another one, a new connection between theMN and the gateway is established and the old one isreplaced by the new one. The transport layer TCP needsto be modified in this scheme. Mobile TCP (M-TCP) [43]is an enhanced version of I-TCP. The gateway maintainsa regular TCP connection with the CN and redirects allpackets coming from the CN to the MN. Compared to I-TCP,M-TCP requires less complexity in the wireless part of theconnection. Mobile UDP (M-UDP) is proposed in [44] tosupport user mobility in traditional UDP protocol. Similarto M-TCP, M-UDP uses a gateway to split the connectionsbetween MN and CN to ensure continuous communicationbetween them when a MN changes subnets.

A new set of migrate options for TCP are proposedin [45] to support handoff management. In this protocol,the MN and CN jointly determine a unique token numberto identify the TCP connection when the connection is setup. Such token numbers are calculated according to theaddresses/ports number of the peer nodes. Upon moving toa new location, MN will notify CN of its new IP address/portalong with the token number so that the CN can update theinformation in the correspondent connection.

Freeze-TCP proposed in [46] lets the MN ‘freeze’ or stopan existing TCP connection during handoff by advertisinga zero window size to the CN, and unfreezes the connectionafter handoff. This scheme reduces packet losses duringhandoff. However, the handoff delay is high.

A mapping layer between the IP layer and the TCP layeris proposed in References [47,48] to intercept all addressupdate messages. The data packets will be replaced withthe initial connection information before submitting to theupper layer. It also considers the situation that two peernodes make handoffs at the same time. During dual roaming,MNs cannot deliver the address update messages to eachother since the destination addresses are not in use any more.It is proposed in References [47,48] to address this problemby setting up a third party subscription/notification server.When a MN roams into a new network, it will send the newaddress to both the peer node and the notification server. IfMNs cannot find each other after handoffs, they will trackpeer node’s new address from the notification server so thatthe connection can be maintained.



One critical issue with these approaches is that TCPand UDP have become the de facto standard used inmillions of hosts for most Internet applications ranging frominteractive sessions such as voice over IP (VoIP) calls andweb browsing, to bulk data transfer. It is very difficult, if notimpossible, to modify them according to the characteristicsof specific wireless networks, which are merely the accessnetworks in the global Internet.

Radial Reception Control Protocol (R2CP) [49] is basedon Reception Control Protocol (RCP), which is a TCP clone.In this protocol, the congestion control and reliability issuesare moved from sender to receiver based on the assumptionthat the MN is the receiver and should be responsible forthe network parameters control. Heterogeneous wirelessconnections and handoff between them are supported inR2CP.

4.2. SCTP-based Handoff ManagementSchemes

In recent years, a new IETF-standardized general-purposetransport layer protocol called SCTP [50] has gainedsignificant attention as a candidate transport protocol forthe next generation Internet. While it inherits many TCPfunctions, it also incorporates many attractive new featuressuch as multi-homing, multi-stream, and partial reliability.Unlike TCP, which provides reliable in-sequence deliveryof a single byte stream, SCTP has a partial orderingmechanism whereby it can provide in-order delivery ofmultiple message streams between two hosts. This multi-stream mechanism benefits applications that require reliabledelivery of multiple, unrelated data streams, by avoidinghead-of-line blocking. The multi-homing feature enablesan SCTP session to be established over multiple interfacesidentified by multiple IP addresses. SCTP normally sendspackets to a destination IP address designated as the primaryaddress, but can redirect packets to an alternate secondaryIP address if the primary IP address becomes unreachable.

The SCTP multi-homing feature and dynamic addressreconfiguration extension [51], collectively referred asmobile SCTP (mSCTP) [52], can be used to solve thehandoff management problem in next generation wirelessnetworks in the transport layer by dynamically switchingbetween alternate network interfaces. A vertical handoffscheme between WLANs and cellular networks usingmSCTP is proposed in [53], in which an MN can beconfigured with multiple addresses in a connection withone address as the primary address. Before the MN makesa handoff, it will acquire another address from the newdomain and set it as the secondary address in the connection.The handoff is then accomplished by asking the CN toset this address as the primary address and then givingup the old primary address. Using mSCTP to enablehandoffs has many advantages, including simpler networkarchitecture, improved throughput and delay performance,and ease of adapting flow/congestion control parameters tothe new network during and after handoffs [53]. Figure 4

Figure 4. Protocol architecture of the mSCTP-based handoffmanagement scheme.

shows the simplified protocol architecture of the mSCTP-based handoff management scheme. Assuming that boththe mobile client and fixed server implement mSCTP, thenthere is no additional requirement to modify the protocolsin other network nodes.

In [54], an improvement to mSCTP called Sending-bufferMulticast-Aided Retransmission with Fast Retransmission(SMART-FRX) is proposed. This scheme consists oftwo sub-schemes that perform different functionalities toenhance the mSCTP performance during WLAN to cellularforced vertical handoffs, namely: 1) The Sending-bufferMulticast-Aided Retransmission (SMART) sub-schemethat forces mSCTP to immediately enter into slow-startover the cellular link at the beginning of the handoff period,by multicasting the data buffered in the primary WLANlink on both the cellular and WLAN links; 2) The FastRetransmission (FRX) sub-scheme that enables mSCTP torecover error losses on the cellular link by retransmittingover the same link, thus avoiding potentially long delaysof error loss retransmissions over the possibly unreachableWLAN link. A new analytical model for SCTP is alsoproposed in [54] that takes into account the dynamicchanges of the congestion window, the round trip time(RTT), the slow-start and congestion avoidance processes,and other factors that may affect the SCTP performanceduring vertical handoff in heterogeneous wireless networks.

5. APPLICATION LAYER MOBILITYMANAGEMENT

In this section, we discuss application layer mobilitymanagement in general and handoff management inparticular. The major advantages of handling mobility at theapplication layer are that it is flexible and does not requirethe installation of additional network equipment, althoughit generally does require more overheads and longerprocessing time [55]. Currently most application layermobility management protocols have been implementedusing SIP. Defined in RFC 3261 [56], SIP aims tomanage voice/multimedia communications sessions overthe Internet (e.g., for VoIP calls). According to RFC 3261,a SIP system comprises the following key components:

• User agent client for interfacing with a user;• User agent server for serving a client request;



• Proxy server for forwarding and re-processingfunctions;

• Redirect server for redirecting a request to anotherlocation; and

• Registrar for processing REGISTER requests typicallyfor location management purposes.

SIP users are identified by uniform resource indicators(URIs) and SIP terminals/servers communicate with eachother by exchanging text-based messages that are similar tothe Hypertext Transfer Protocol (HTTP) messages. A SIPsession is generally set up as follows. First a caller sendsan INVITE message to the callee through the caller’s andcallee’s proxy servers. If the callee can accept the call, itreplies to the caller with a 200 OK message through theproxy servers. The caller then sends an ACK message to thecallee. Subsequently the call session can be set up. When,for example, the caller wants to end the call, it sends a BYEmessage to the callee. Finally, to end the session, the calleesends a 200 OK message to the caller.

SIP can support different types of mobility management(see [57] for details). For pre-call mobility, a MN can updateits current location to the home domain whenever it entersa new domain. When a CN wants to communicate with theMN, it first sends an INVITE message to the home domain.If the MN has moved, the CN then receives a 302 MOVEDTEMPORARILY message, which includes the new locationof the MN. After receiving the message, it sends anotherINVITE message to the MN. The MN finally sends a 200OK message to set up the session. For mid-call mobility(i.e., handoff), whenever a MN moves to a new domain,it can initiate a handoff by sending a re-INVITE messageto the CN to set up another session and to terminate theprevious session. Some fast-handoff techniques have alsobeen proposed [58] to reduce handoff delay. In general,these techniques allow a MN to receive data from an agent ora special component in the network when the new session isbeing set up during the handoff process. SIP can also supportsession mobility and personal mobility. For session mobility(i.e., session handoff), a mobile user can switch an ongoingsession from one terminal to another terminal. This can beimplemented using, for example, a call transfer approach.In this approach, a terminal can send a REFER messageto the CN, providing information on the new terminal thatshould handle the session. The CN then sends an INVITEmessage to set up the session with the new terminal. Finally,the old terminal ends the previous session by sending aBYE message. For personal mobility, different cases canbe supported by SIP. For example, a user address can belinked to multiple terminals or multiple user addresses canbe linked to one terminal. Details can be found in [57] andits references.

In recent years, there has been considerable interest inusing SIP to support mobility management, particularlyhandoff management for different networking applicationsor cases, as illustrated in Figure 5. In the following, wegive an overview of three examples: integrated mobilitymanagement using SIP and MIP, SIP-based handoff

management for Universal Mobile TelecommunicationsSystem (UMTS)/WLAN-based systems, and SIP-basedmobility management for peer-to-peer (P2P) networks.

In general, MIP and SIP are well suited for supportingTCP-based and UDP-based communications sessions,respectively. It is of interest to integrate them (e.g., to betterserve a MN running both TCP-based data and UDP-basedmultimedia applications). In early proposals such as theEVOLUTE architecture [59,60], MIP and SIP are executedseparately with almost no interaction. In [60], an integratedMIP and SIP protocol has been proposed to streamlinethe protocol operations using either a tightly integratedor a loosely integrated approach. In the tightly integratedapproach, a common home mobility server is set up for MIPand SIP. This allows a MN to update its location for bothMIP and SIP operations using a single MIP registrationrequest. MIP and SIP are then used for handling handoffsfor TCP-based and UDP-based communications sessions,respectively. Note that if both TCP-based and UDP-basedsessions are used, SIP can be employed to enable routeoptimization for MIP (see [60]). If the tightly integratedapproach cannot be implemented, a loosely integratedapproach is proposed as an interim solution. In this case,it is assumed that the MIP HA can communicate with theSIP home registrar to facilitate location update. Again,the purpose of this is to eliminate duplication of effort inlocation update. When a MN enters a new domain, it updatesits location to the MIP HA through a MIP registrationrequest. The MIP HA then updates the SIP home registraras well. Similar to the tightly integrated approach, handoffsfor TCP-based and UDP-based communications sessionsare handled by MIP and SIP, respectively.

SIP can be used to support mobility management forWLANs [61] interworking with UMTS [62,63]. Here as anexample, we give an overview of the protocol presented in[63]. In future wireless networks, UMTS, and WLAN can beintegrated to provide the so-called ‘always best connected’services. Basically, UMTS can provide wide area services atmoderate data rates whereas WLAN can provide local areaservices at higher data rates. When a mobile user enters thecoverage area of a WLAN, an ongoing multimedia sessioncan be handed over from UMTS to the WLAN througha vertical handoff process. Note that unlike a conventionalhorizontal or intra-technology handoff, this vertical handoffis optional and aims to provide better services. In [63], aSIP-based seamless handoff scheme has been proposed tofacilitate the aforementioned vertical handoff using a softhandoff approach. Consider an MN that is equipped withboth UMTS and WLAN interfaces. Suppose that the MNhas a SIP session with a CN through the UMTS interface.After entering the coverage of a WLAN, it can initiate thevertical handoff by sending an INVITE message to the CNthrough the WLAN interface to set up another session. TheINVITE message contains a special JOIN header, whichprovides the relevant information of the ongoing session.Note that during the handoff process, two sessions aremaintained and it is assumed that duplicate packets canbe deleted from the terminals. Having set up the session



Figure 5. Application scenarios of SIP-based mobility support.

through the WLAN interface, the previous session canthen be terminated by sending a BYE message throughthe UMTS interface. Furthermore, the MN can also updateits new location to the home domain. To reduce handoffdelay, it is possible to pre-establish a new connection neara UMTS/WLAN boundary based on location informationand movement prediction.

SIP can also support mobility management for P2Pnetworks. Unlike other networks, P2P networks are basedon a fully distributed architecture so that there arenew technical challenges. In [64], a SIP-based mobilitymanagement protocol called SAMP is proposed to tacklethese technical challenges. For location registration, an MNfirst needs to find an anchor SIP server on the P2P networkby sending a REGISTER message to a SIP multicastaddress. After receiving the qualified replies (i.e., repliesreceived within a certain period), the MN randomly choosesone of them as the anchor server. Note that the randomchoice is for load balancing. Having connected to an anchorserver, the MN then updates its location by sending aREGISTER message to the home SIP server through theanchor server. A SIP terminal can set up a session with theMN by sending an INVITE message to the MN throughthe home SIP server and the anchor server. In [64], twooptimization techniques are also proposed to enhance thesystem efficiency. First, a hierarchical registration schemeis used to allow an MN to register with an anchor serverrather than the home server to reduce handoff delay. Second,a scheme employing a two-tier cache is proposed to reducesession setup delays. In this scheme, a CN first tries to set

up a session with the MN based on its cached address.Having received the request, the CN’s anchor server alsotries to forward the message based on the cached locationinformation for the MN. If the session cannot be set up usingthe cached information, the more time consuming standardsession set up process will be executed based on the P2Pnetwork protocol.

6. RESEARCH CHALLENGES

In this paper, we have introduced recent advances inmobility management schemes, including location manage-ment and handoff management schemes that are applicableto future generation wireless mobile networks. We havereviewed many existing solutions that have been proposedfor conventional circuit-switched cellular networks as wellas contemporary packet-switched networks.

In general, location management is needed to locatethe MT that is the recipient of a call, and if the locationinformation is imprecise, then paging is used as a locationmanagement technique to find the MT within a wider areain which the MT may be located. While existing schemesare designed for cellular networks with centralized basestations in each cell, the current trend of extending cellularnetworks into mesh networks create new challenges inlocation management; e.g., what is the location informationthat needs to be reported, which network entities to reportthis information to, and how to utilize this information toefficiently deliver calls to the MTs? While flooding is widely



used as a method to establish initial routing in MANETs,in mesh networks where most relay nodes are fixed andterminals are mobile, and where a large number of MTsneed to be served, unconstrained flooding over the networkdomain may not be efficient.

For intra-system handoff management between accesspoints or base stations supporting a common wireless accesstechnology, usually technology-specific handoff methodsoperating at the link layer and below are the most effective.On the other hand, inter-system handoff management ismore involved due to possibly different wireless accesstechnologies between different systems. Transport-layerhandoff solutions can be very effective in such situations,as they conform to the end-to-end principal of the Internet;however, the nodes at both ends of the connections needto support the same version of the transport protocol thatis enhanced to support vertical handoffs. In particular, itis clear that the new generation transport protocol, SCTP,includes features that would make it quite effortless tosupport inter-system handoffs. For multimedia connectionsthat are set up using SIP, it is logical to use the same protocolto maintain continuity of a connection during a handoff.This is effective if the MN can switch network access in arather seamless manner. Hence it is necessary to developcross-layer techniques that coordinate the switching ofaccess networks and the connection maintenance via SIP.Some researchers and practitioners suggest that futuregeneration wireless networks will have an open architecturewhere access points and base stations function just likeany other IP node that is capable of forwarding packets.Such an architecture naturally points to the use of MIPor MIPv6 for terminal mobility support. This paradigmis limited, however, by the assumption that each terminalhas a permanent IP address assigned by its home network.In future wireless networks, a good portion of the MNsmay in fact be nomadic in nature in that they do nothave a fixed home network, but they still need fast andefficient mobility management when they move withina network domain or across different network domains.Development of new mobility management methods forsuch scenarios will be a new challenge. Finally, it is quitepossible that a large number of MNs will be equipped withglobal positioning system (GPS) receivers, allowing themto determine their own geographic locations accurately.How geographical location information can be effectivelyused to enable efficient call delivery and seamless handoffsremains an open question. Furthermore, as not all MNs willbe equipped with GPS, it would be of interest to developmobility management techniques that take advantage ofthe GPS capability of some MNs to aid call delivery andhandoffs for other less capable terminals.

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AUTHOR’S BIOGRAPHIES

F. Richard Yu received the Ph.D.degree in Electrical Engineering fromthe University of British Columbiain 2003. From 2002 to 2004, hewas with Ericsson (in Lund, Sweden),where he worked on the research anddevelopment of 3G cellular networks.From 2005 to 2006, he was with astart-up in California, USA, where he

worked on the research and development in the areasof advanced wireless communication technologies andnew standards. He joined Carleton School of InformationTechnology and the Department of Systems and ComputerEngineering at Carleton University, in 2007, where he iscurrently an Assistant Professor. He received the LeadershipOpportunity Fund Award from Canada Foundation ofInnovation in 2009 and best paper awards at IEEE/IFIPTrustCom 2009 and Int’l Conference on Networking 2005.His research interests include cross-layer design, securityand QoS provisioning in wireless networks.

He has served on the Technical Program Committee(TPC) of numerous conferences and as the co-chair ofICUMT-CWCN’2009, TPC co-chair of IEEE INFOCOM-CWCN’2010, IEEE IWCMC’2009, VTC’2008F Track 4,WiN-ITS’2007. He is a senior member of the IEEE.

Vincent W. S. Wong received theB.Sc. degree from the University ofManitoba, Winnipeg, MB, Canada,in 1994, the M.A.Sc. degree fromthe University of Waterloo, Waterloo,ON, Canada, in 1996, and the Ph.D.degree from the University of BritishColumbia (UBC), Vancouver, BC,Canada, in 2000. From 2000 to 2001,

he worked as a systems engineer at PMC-Sierra Inc.He joined the Department of Electrical and ComputerEngineering at UBC in 2002 and is currently an AssociateProfessor. His research areas include protocol design,optimization, and resource management of communicationnetworks, with applications to the Internet, wirelessnetworks, RFID systems, and intelligent transportationsystems. Dr Wong is an Associate Editor of the IEEETransactions on Vehicular Technology and an Editor ofKICS/IEEE Journal of Communications and Networks. Heserves as TPC member in various conferences, including



IEEE Infocom, ICC, and Globecom. He is a senior memberof the IEEE and a member of the ACM.

Joo-Han Song received the M.S.degree in Electrical Engineering fromthe Hongik University, Seoul, Korea,in 2001, and the Ph.D. degree from theUniversity of British Columbia (UBC),Vancouver, BC, Canada, in 2005. Heworked as a senior engineer in the4G System Laboratory at SamsungElectronics, Korea from 2005 to 2007.

In 2007, he obtained a postdoctoral fellowship at UBCstudying security issues in vehicular ad hoc networks(VANETs). His research interests include routing andsecurity for mobile ad hoc networks, design of MACalgorithms for 4G system, and performance evaluation andmodeling of wireless networks.

Victor C. M. Leung received theB.A.Sc. (Hons.) degree in ElectricalEngineering from the University ofBritish Columbia (UBC) in 1977, andwas awarded the APEBC Gold Medalas the head of the graduating class inthe Faculty of Applied Science. Heattended graduate school at UBC ona Natural Sciences and Engineering

Research Council Postgraduate Scholarship and completedthe Ph.D. degree in Electrical Engineering in 1981.

From 1981 to 1987, Dr. Leung was a senior memberof Technical Staff at Microtel Pacific Research Ltd. (laterrenamed MPR Teltech Ltd.), specializing in the planning,design and analysis of satellite communication systems. In1988, he was a lecturer in the Department of Electronicsat the Chinese University of Hong Kong. He returned toUBC as a faculty member in 1989, where he is currently aProfessor and the holder of the TELUS Mobility ResearchChair in Advanced Telecommunications Engineering in theDepartment of Electrical and Computer Engineering. Heis a member of the Institute for Computing, Informationand Cognitive Systems at UBC. Dr Leung has co-authoredmore than 400 technical papers in international journalsand conference proceedings. His research interests arein the areas of architectural and protocol design and

performance analysis for computer and telecommunicationnetworks, with applications in satellite, mobile, personalcommunications, and high speed networks.

Dr Leung is a registered professional engineer in theProvince of British Columbia, Canada. He is a fellow ofIEEE, a fellow of the Engineering Institute of Canada,and a fellow of the Canadian Academy of Engineering.He has served on the editorial boards of the IEEEJournal on Selected Areas in Communications—WirelessCommunications Series, the IEEE Transactions on WirelessCommunications, the IEEE Transactions on VehicularTechnology, the IEEE Transactions on Computers, theJournal of Communications and Networks, ComputerCommunications, as well as several other journals. He hasguest-edited special issues of several journals, and served onthe technical program committee of numerous internationalconferences. He is the general chair of Adhocnets 2010 andgeneral co-chair of BodyNets 2010 and CWCN 2010. Hechaired the TPC of the wireless networking and cognitiveradio track in IEEE VTC-fall 2008. He was the general chairof QShine 2007, and symposium chair for Next GenerationMobile Networks in IWCMC 2006-2008. He was a generalco-chair of IEEE EUC 2009 and ACM MSWiM 2006, anda TPC vice-chair of IEEE WCNC 2005.

Henry C. B. Chan received his B.A.and M.A. degrees from the Universityof Cambridge, England, and his Ph.D.degree from the University of BritishColumbia, Canada.

From October 1988 to October 1993,he worked with Hong Kong Telecom-munications Limited primarily on thedevelopment of networking services in

Hong Kong. Between October 1997 and August 1998,he worked with BC TEL Advanced Communications onthe development of high-speed networking technologiesand ATM-based services. Currently, he is an AssociateProfessor in the Department of Computing at The HongKong Polytechnic University. His research interests includenetworking/communications, wireless networks, Internettechnologies and electronic commerce.

Dr Chan is a member of IEEE and ACM. He is currentlyserving as an executive committee member of the IEEEHong Kong Section Computer Chapter.


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