An Index Structure Framework to Analyze HostMobility Supports for Integrated Networks

Yujia Zhai, Yue Wang, Jian Yuan, Yong Ren, Xiuming ShanDepartment of Electronic Engineering, Tsinghua University, Beijing, 100084, China

Email: dyj04@mails.tsinghua.edu.cn, {wangyue, jyuan, reny, shanxm}@tsinghua.edu.cn

Abstract— Convergence is a key design aspect for next gener-ation networks. Developing a general mobility managementmodel is an important requirement for the integrated mobilenetworks. This paper begins with a survey of mobility man-agement concept. Based on the analysis and comparison, wepresent a conceptual explanation of mobility managementlayer for mobility and its management. An index structureframework to analyze host mobility supports for integratedmobile networks is proposed in this paper. Our frame-work investigates the previous methods for mobility modelanalysis, and builds a general model to characterize andunify different mobility management schemes into an indexstructure. In our model, we construct the basic elementsof management mechanisms, i.e. node and edge, and definethe main operations, namely, update operation and queryoperation. At the same time, the fundamental performancemetrics and the expressions of the cost functions is obtained.The proposed framework is flexible in its elements andparameters, and could be applied for many scenarios. Wedemonstrate the utility of our framework by evaluatingvarious host mobility support schemes.

Index Terms— index structure, analysis framework, mobilitymanagement, host mobility support, integrated mobile IPnetworks

I. INTRODUCTION

As the availability and popularity of wireless networksincreases, the research community strives to device newcommunication systems (network architectures, network-ing protocols, services, etc.) that take into account theuser’s mobility [1]–[4]. Based on a global system offixed and wireless mobile service, the next generation ofwireless communication extends global service to includethe integration of heterogeneous services across networkproviders, network backbones and geographical regions.And the future mobile networks are expected to bring to-gether mobility – breaking up the geographical constrainsand the necessity to be tied to one particular backbonenetwork [5].

Regardless of network protocols, a real open, accom-modative, autonomous but still efficient mobility man-agement solution is called for. Some researches have

This paper is based on “An Index Structure Model for MobilityManagement of Integrated Mobile IP Networks,” by Yujia Zhai,YueWang, Jian Yuan, Yong Ren and Xiuming Shan, which appeared in Proc.IEEE Network Operations and Management Symposium Workshops(NOMS Workshops 2008), 7-11 April 2008, Salvador da Bahia, Brazil.c© 2008 IEEE.

This work is supported in part by the National Basic ResearchProgram of China (973 Program) under grants 2007CB307100 and2007CB307105, and the National Nature Science Foundation of Chinaunder grants 60672142 and 60772053.

attempted to give a complete solution to mobility man-agement for integrated mobile networks [6]–[8]. However,because of the differences in network organizations andprotocols, previous mobility support mechanisms in het-erogeneous networks are independent, and hence deny adirect and convenient unification [9], [10]. An efficient,flexible and comprehensive mobility support scheme isstill missing. Furthermore, this area still lacks standard-ization and evaluation criteria. Therefore, developing ageneral mobility management analysis framework forintegrated mobile networks appears important to designan efficient and unified mobility support scheme. Suchframework may help us to better understand the natureof mobility management and the necessity for mobilitysupport.

In this study, we focus on the impacts of mobilityand the essence of mobility management, and attemptto build up a general model to characterize and unifythe mobility management schemes. While doing so, wepropose an index structure framework to analyze hostmobility supports for integrated mobile networks. Thisanalysis attempts to answer the following questions:

1) Whether mobility support mechanisms in heteroge-neous networks can be integrated and unified?

2) If the answer to 1) is yes, why?3) If the answer to 1) is yes, how?

To answer Whether and Why, we analyze mobilityeffects and support mechanisms in heterogeneous net-works. It is shown that mobility causes uncertainty ofthe address, and the mobility management tries to reducethe uncertainty of address by managing the locationinformation of mobile users. We observe that the mobilitysupport mechanisms in heterogeneous networks may havethe similar management architectures. A Mobility man-agement layer, which builds upon physical topology layerand network (topology) layer, is presented. Based on themobility management layer, the management architectureof address information is the key issue for the mobilitymanagement. To answer How, we introduce the conceptof index structure into the mobility management, and pro-pose an index structure model for mobility managementof integrated mobile networks [11]. The managementarchitectures of address information in different categoriescan be characterized into the index structure.

In order to conduct our research and answer the abovequestions systematically, we present a mobility manage-

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ULC

USC

PLC

PSC

Figure 1. Index structure analysis framework.

ment analysis framework , and show how this frameworkcan be used to describe and analyze a host mobilitysupport protocol and evaluate its performance. As shownin Fig. 1, our framework focuses on the following as-pects: the analytical scheme of mobility models, the basicstructure, operations and elements of management mech-anisms, and the performance of host mobility supports.

The rest of this paper is organized as follows. Sec-tion II gives a brief description of the related work andour contribution. Section III overviews mobility manage-ment, and discusses the possibility of realizing a uniformmobility management mechanism. Section IV elaboratesthe three parts of our framework, especially the indexstructure model. The validation of our framework throughexample analysis is presented in Section V. Finally, ourconclusions from this study and planned future work arelisted in Section VI.

II. RELATED WORK

As mentioned in Section I, the demand for mobileservice has motivated research in updating existing high-speed wired (fixed) communication networks with wire-less communication techniques. In this section, we mainlydescribe current and proposed protocols and conceptualframework associated with mobility management so asto facilitate the identification of mobility managementrequirements for future mobile systems.

A. Mobility Management Protocols

Two types of backbone networks, among other alterna-tives, are person communication system (PCS) networksand mobile Internet protocol (Mobile IP), which underliemost current research activities. Many telecommunicationsystems such as the first and second generation wirelesscellular systems were designed mainly for voice services,and the integration with data networks becomes the majorpush for 3G and forthcoming 4G networks. The popularityof the Internet provides strong incentive for a more ubiq-uitous network, accessible anytime, anywhere.The future

integrated networks may be all IP-based heterogeneousnetworks.

Extensive research has been done in designing practicalmobility support protocols for PCS and Mobile IP net-works. There are currently two commonly used standardsfor host mobility management in PCS networks: theElectronic and Telephone Industry Associations EIA/TIAInterim Standard 41 (IS-41) and the Global Systemfor Mobile Communications (GSM) Mobile ApplicationPart (MAP). Both standards are based on a two-leveldatabase hierarchy [12]. Related studies in this networkenvironment, including the mobility management of both3GPP and 3GPP2 standards, are generally based on sucharchitecture. Moreover, Mobile IP (MIP) is the mobility-enabling protocol developed by IETF to support globalhost mobility in IP networks [13], [14]. The IETF workinggroup in Mobile IP proposed Mobile IPv4 (MIPv4) [15]and Mobile IPv6 (MIPv6) [16] as the main protocolsfor supporting IP mobility. And various micro-mobilityprotocols have already been proposed for enhancing per-formance of the MIPv4 or MIPv6 based protocols, sincein certain cases Mobile IP could result in a high signalingload as well as high handoff latency and packet losses[17]–[21].

Though all these protocols mentioned above supporthost mobility, they behave differently for a given mobilitymodel. [10] presented that the provision of seamlessservice and mobility across heterogeneous systems hasbeen problematic due to four factors: 1) differences inradio access technologies; 2) differences in services andtheir non-portability; 3) differences in mobility manage-ment deployed protocols; and 4) lack of interoperabilitymechanism to resolve the differences between differentmobile systems. It is observed that no single instance ofthe existing mobility support protocols could support allof the mobility management requirements. Hence, we donot aim to provide a practical mobility support protocolwhen the environment of integrated mobile networksis not clear. Rather, it may be necessary to identify amobility management analysis framework and functionalarchitecture for future mobile systems.

B. Existing Conceptual Framework

Currently, a widely recognized conceptual frameworkof mobility management is based on location managementand handoff management [5], [22]. It is not unifiedin heterogeneous circumstances. The operations of onescheme is classified into location management or handoffmanagement according to whether the operation dealswith address information. Location management schemesdeal with querying and storing information in locationdatabases, which are not protocol-dependent and can beapplied to any mobile networks. However, in handoffmanagement, the operations rely on routing, resourcemanagement, and data delivery systems. Its interoperabil-ity between heterogeneous wireless networks is dependenton the local network interfaces and infrastructure. Thus,the handover schemes are network-protocol-dependent.

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Some operations, which are in the different networkcircumstances and have similar functions in deed, are notclassified uniformly.

On the other hand, relational databases use indexes tominimize the time it takes to find data. With the increasingnumber of wireless and mobile devices, several works ofexploiting the benefits of indexes in mobile environmentshave evolved, and index structures have been used inlocation-based services aiming at the efficient indexingmethod for future location queries [23]–[26]. There aresome similarities between mobility management and datamanagement of location-based services with regard totracking and managing moving objects. In our study, weintroduce the concept of index structure into the mobilitymanagement.

When we mainly consider mobility management layer,the management architecture, i.e. the index structure is thecore. And the common part of the schemes in heteroge-neous networks will show itself and can be described uni-formly by the index structure model. Our study is basedon the proposed framework which facilitates systematicalinvestigation of the effects of mobility in the case of hostmobile on the one hand, and the potential relationshipbetween mobility and mobility management performanceon the other. Specifically, the main contributions of ourproposed framework are multi-fold as follows:

1) Present a mobility management layer as a concep-tual explanation of mobility and its management.

2) Unify the results obtained in mobility model anal-ysis and present the general analytical scheme formobility model.

3) Describe management architectures in different cat-egories using index structure uniformly. Abstractbasic elements (i.e. node and edge) and definemain operations (i.e. update operation and queryoperation) of management mechanisms. Simplifythe mobility management between heterogeneoussystems.

4) Analyze basic performance metrics for mobilitymanagement mechanisms. Quantitate these metricsby using “cost functions” and propose the formalequations of the cost functions.

III. MOBILITY MANAGEMENT CONCEPTEXPLANATION

In this section, we mainly analyze the effects of mo-bility and attempt to gain a deep insight into mobilitymanagement by comparing current host mobility supportmechanisms in heterogeneous networks.

A. Mobility Effects on Networks

In communication networks, the ID and the address of anode, which represents a host or router, are two importantconcepts. The ID, which might be name or a kind ofphysical “address”, is used to identify a particular endsystem in the whole network and the address is used toprovide the location information of this endpoint.

In ordinary wired networks, like the telephone network,there is a fixed relationship between a terminal and itslocation. Within the wired IP network, an IP address hasthe function of both ID and address, i.e. the IP addressis used to identify a particular endpoint and also usedto find a route between the endpoints. Fixed terminalscommunicate differently depending on their sub-networklocations (i.e. the destination IP addresses).

In contrast, in environments where mobile nodes (MNs)are free to travel, an MN’s point-of-attachment changesfrequently as it moves around the network coverage area.The ID of an MN does not implicitly provide the locationinformation of the MN. The call delivery process becomesmore complex. If the current address of the MN isunknown, only with the original address, its correspondentnode which is the other end of the connection would notcommunicate with it successfully.

Therefore, compared with the fixed networks, the un-certainty of the address, which is caused by the mobility,is one of the most essential characteristics of the mobilenetworks. In order to reduce the uncertainty of address,the mobility management mechanism stores the locationinformation for each MN. Then the information can beretrieved for call delivery.

B. Mobility Management Layer

In our study, we conceptualize our research beyond the“whole protocol” level and attempt to answer whethermobility support mechanisms can be unified in integratedmobile networks. We compare many host mobility sup-port mechanisms in heterogeneous networks and focuson the management method of location information.Fig. 2 demonstrates mobility management schemes inPCS networks, and the corresponding schemes in MobileIP networks are given in brackets, where we use the“HLR” and “VLR” to differentiate the address storageunits on the different levels. Here, we use the comparisonbetween Hierarchical Mobile IPv6 (HMIPv6) in MobileIP networks and GSM MAP in PCS networks as anexample to illustrate our approach. As shown in Table I,we observe that although these two mechanisms differ innetwork organizations and protocols, they have the samemobility management structure.

The similarities between the mechanisms in hetero-geneous networks on the mobility management layerindicate that it is practical to unify the mobility supportmechanisms in integrated mobile networks. By identifyingthe common parts, we construct a mobility managementlayer that builds upon physical topology layer and net-work (topology) layer (see Fig. 3), i.e. the managementarchitecture which is independent of network organiza-tions and protocols.

In addition, we give an explanation of mobility andits management based on the mobility management layer.The parts under the mobility management layer, whichinclude topology models, movement models and call ar-rival models. Combining these three kinds of models, weobtain mobility models, which can be seen as the import

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eg:HMIPv6

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VLR1

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VLR0

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VLR2 VLR3 VLR4

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VLR0

HLReg F-HMIPv6

MN

(f)

Called MN Calling MN

eg Cellular IP

(g)

Called MN Calling MN

(h)

Figure 2. Management architecture.

TABLE I.A MANAGEMENT MECHANISM COMPARISON DEMONSTRATION.

Sameness: mobility management structurePCS HMIPv6

ID (stable) Mobile Identification Number Home Address (HoA)One-lever Address (variable) Visitor Location Register’s ID Regional Care-of-Address (RCoA)Two-lever Address (variable) Temporary Local Directory Number Local Care-of-Address (LCoA)

One-lever Mapping Home Location Register (HLR) Home Agent (HA)Two-lever Mapping Visitor Location Register (VLR) Mobile Anchor Point (MAP)

Difference: network organization and protocolPCS HMIPv6

Addressing Mode Cell IP addressAddress Searching Mode SS7, Connection-Oriented IP routing, Connectionless

Location Registration Area Size & Update Time Agent Discovery & Movement Detection(Update) (MT moves into a new Location Area.) (MN moves into a new IP subnet.)

Mapping Precision Location Area IP SubnetCall Delivery Paging Routing & Tunneling

of mobility management layer. Moreover, on mobilitymanagement layer, an “index structure” is built up andmaintained by update operation, and is used and visited byquery operation. Furthermore, the fundamental function ofmobility management can be realized.

IV. INDEX STRUCTURE ANALYSIS FRAMEWORK

As mentioned in Section I, in order to systematicallyanalyze the impact of mobility, evaluate current hostmobility support protocols and design uniform mobilitysupport mechanisms for integrated mobile networks, wepropose a generic framework for analyzing host mobilitysupports, as shown in Fig. 1. Our framework mainlyinvolves the following parts: mobility model, managementmechanism and performance analysis.

A. Mobility Model

A mobility model is a set of rules used to generatetrajectories for mobile entities. Mobility models are usedin network simulations to generate network topologychanges due to node movement [27]. A large number of

related works have been done on mobility models [28]–[30]. By colligating the related works, we present thegeneral analytical scheme for the mobility model (seeFig. 4). This scheme includes three kinds of models:topology model, movement model, and call arrival model.

Firstly, a topology model describes the specific con-figuration used to divide the whole network into somecells, such as 1-D mesh configuration, 2-D mesh configu-ration, and hexagonal cell configuration, etc. Secondly,a movement model defines users’ movement patternsunder the assumption that the network is static. Randomwalk model, Markov model and movement-based modelall belong to movement models. Based on the topologymodel and the movement model, we can calculate the cellresidence time that is one of the parameters to determinethe mobility properties in a mobile network. The cellresidence time can be used to predict how long a user willstay in a certain cell before it moves into another. Finally,in an extreme case, if there is no call arriving whenthe user is moving, there is no need to track the user’smovement. So the call arrival model has a significantinfluence on the performance of mobility management.

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Internet

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RCoA LCoA

Binding Cache Binding Cache

RCoA LCoA

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Mobile Management Layer

Network Topology

Layer

Physical Topology Layer

(a) HMIPv6 in Mobile IP networks.

VLRVLR

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HLR

BSC

PSTN

MSCMSC

MSC

BSC BSC

BS

MT

CellPhysical Topology Layer

Network Topology Layer

Mobile Management Layer

(b) GSM MAP in PCS.

Figure 3. Mobility management layer.

Figure 4. The general analytical scheme for the mobility model.

Combining the mobility properties and the call arrivalproperty, we can estimate the call-to-mobility ratio ρ andthe average number ns of subnets for the MN to moveacross between two packet arrivals. Mobility managementis just to reduce the uncertainty of user’s address, whichis caused by the ns subnet boundary crossings.

B. Management Mechanism: Index Structure ModelIn this section, we focus on the mobility management

layer and attempt to build up a general model to charac-terize and unify the mobility management schemes.

1) Three Basic Management Architectures: As men-tioned in Section III-A, the uncertainty of the mobilenodes’ addresses is one of the most essential charac-teristics of the mobile networks. In order to reducethe uncertainty of address, the mobility managementmechanism stores the location information for each MN.Then the information can be retrieved for call delivery.Therefore, the key issue for mobility management layeris the management architecture of address information,i.e. how to organize the storage and distribution of theaddress information of mobile nodes.

Considering management architectures of current hostmobility supports in PCS and Mobile IP networks, weclassify the management architectures into three basiccategories:

1) Centralized architecture. In this architecture, a sin-gle database is used to store the current addressinformation of mobile nodes. Global mobility man-agement schemes are based on centralized databasearchitecture (see Fig. 2 (a)).

2) Hierarchical architecture. As shown in Fig. 2 (b)-(f), many local mobility management schemes, likeHMIPv6, use hierarchical architecture to handlethe user mobility. Based on centralized architec-ture, hierarchical foreign agents are used to handlethe micro-mobility that means the MN movementacross multiple subnets within a single network ofdomain whereas the higher levels of the hierarchyhandle the macro-mobility. When a call is initiated,the location database on the highest level is the uni-form ingress of address searching, and the packetsdelivered to an MN can be tunneled via the multiplelevels of foreign agents to the MN.

3) Distributed architecture. In this architecture, mul-tiple databases are distributed throughout the net-work coverage area. These location databases areorganized as a tree with the root at the top and theleaves at the bottom. The MNs are associated withthe location databases and each location databasecontains address information of the MNs that are re-siding in its sub-tree. Fig. 2 (g) and (h) demonstratea fully distributed registration scheme and a dis-tributed hierarchical database scheme respectively.When a call for an MN is initiated at a node onthe tree, the called MN can be located by graduallyfollowing the pointers from the residing leaf nodeof the MN to the root node.

2) Unified Index Structure: In this paper, we introducethe concept of index structure into the mobility man-agement because this structure acts as a catalog. All ofthe above categories of management architectures can becharacterized as an “index structure”. With the help ofindex structure, the extent of query can decrease accordingto the query request, and the paging in the whole networkmay be avoided. As a result, the time it takes to findaddress information can significantly be reduced. Here,the centralized architecture acts as a clustered index whichmaintains base table rows in the same physical sequenceas the index key; however, the hierarchical structure andthe distributed architecture have the function of a non-clustered index which maintenances a pointer chain, i.e.the previous cache is the catalog of the next cache therebygradually reducing the uncertainty of user’s address.

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3) Basic Elements: We characterize the basic elementsof management mechanism. Let N be the set of functionentities with regard to mobility management. Formally,we can describe a function entity n ∈ N as a node by aquadruple: 〈nid, ntype, nsign, {nprops}〉. nid is a uniquenode identifier. ntype is used for defining managementfunction of the node. nsign is a Boolean predicate fordistinguishing the data storing capability of the node,where nsign equals to 1 if the node has the ability tostore data and 0 otherwise. {nprops} is a set of propertiesabout the function entity, with reference to data processingcapability, cache size, energy storage, coverage area, etc.

Let E be the set of connection relations betweentwo function entities which can be denoted by edges.Similarly, we describe an edge e ∈ E by a quadruple:〈{nids}, etype, esign, {eprops}〉. {nids} is a set of iden-tifiers of the edge’s endpoints. etype is used for definingmanagement function of the edge. esign is a Booleanpredicate for distinguishing index connections and com-munication connections, where esign equals to 1 if theedge is an index connection or esign equals to 0 if theedge is a communication connection. {eprops} is a set ofproperties about the edge, including distance, link type,bandwidth, etc.

4) Basic Operations: With these administrable basicelements, we can design a practical mobility managementmechanism, which involves two main operations. Oneis update operation for building up and maintaining anindex structure. The other is query operation that enablesus to use and visit the index structure. The particularsof the mechanism are designed for confirming structureparameters and connection relations and defining messageformats and interaction processes in the update operationand the query operation.

C. Performance Analysis

Based on the structure parameters and connection re-lations, we can analyze the performance of a mobilitymanagement mechanism. It requires that the basic met-rics selected for the performance calculation can revealthe fundamental effects of the mobility management.Firstly, the mobility management should ensure the accesscontinuity which is one of users’ basic requirements.Therefore, the delay is a good metric, which may includehandoff latencies, processing delays, tunneling delays,etc. Secondly, the mobility management should consumenetwork resources as few as possible from the perspectiveof the network management. So, the overhead is anotherbasic performance metric and it may consist of signalingloads, memory consumptions, processing loads, routingoverheads and so on.

We use the “cost functions” to quantitatively representthe above metrics which can reveal the influence of themobility management mechanism on the performance. Amechanism could define N types of update operation andM types of query operation. Formally, the cost functionsare expressed as the following equations. Average update

TABLE II.COST FUNCTION PARAMETERS.

Parameter DescriptionN � types of update operationµi Frequency of update operation for type i

ni Time of update operation for type i

ULi Unit delay of update operation for type i

USi Unit overhead of update operation for type i

M � types of query operationνj Frequency of query operation for type j

mj Time of query operation for type j

PLj Unit delay of query operation for type j

PSj Unit overhead of query operation for type j

delay is estimated as:

CUL =N∑

i=1

µiULi. (1)

On the other hand, total update overhead is estimated as:

CUS =N∑

i=1

niUSi. (2)

Similarly, average query delay is estimated as:

CPL =M∑

j=1

νjPLj . (3)

And total query overhead is estimated as:

CPS =M∑

j=1

mjPSj . (4)

Table II shows the instructions about the parameters inthese cost functions. According to the definition of aparticular mechanism, we can get the concrete expressionsof these cost functions.

To sum up the insights from previous analysis, weprovide the index structure analysis framework for thehost mobile case, as shown in Fig. 1. This frameworkis composed of three parts. And in order to assure betterperformance, the management scheme can be dynamicallyadjusted according to the mobility model and the resultsof the performance analysis .

V. MOBILITY MANAGEMENT FRAMEWORKVALIDATION

The validation of the integrated mobility managementanalysis framework can be conducted either at the ex-ample analysis level or at the network simulation level.Where the former can verify the general utility andvalidity of our framework and the latter can validate theaccuracy of the analysis results.

The index structure model provides a new viewpointfor us to analyze mobility support schemes. In thissection, we will show the usability and flexibility of ourframework through instance analysis.

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AR

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Total Handover Latency

Registration (Binding Update to HA and CNs)

Registration to HA

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TMDn+1 TDADn

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I

Figure 5. Handover procedure and timing diagram in MIPv6.

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2tH

2tNmax{2tL, 2tN}tM

TDADn

tM

tM

Figure 6. Handover procedure and timing diagram in HMIPv6.

A. Unified Conceptual Framework

As mentioned in Section II-B, the widely recognizedconceptual framework of mobility management is notunified in heterogeneous circumstances. Some operations,which are in the different network circumstances and havesimilar functions in deed, are not classified uniformly. Inparticular, the terminal paging process belongs to loca-tion management in the PCS networks, and the routingand tunneling process in Mobile IP belongs to handoffmanagement. However, these processes are about addresssearching and call delivery, and they have the similarfunctions in deed.

In our conceptual framework, when we mainly considermobility management layer, the management architecture,i.e. the index structure is the core. And the common partof the schemes in heterogeneous networks will show itselfand can be described uniformly by the index structuremodel. The mobility management entities are described ina general manner that simplifies the mobility managementbetween heterogeneous systems. The basic operations willbe classified into update operation or query operation

according to whether is relative to building up andmaintaining an index structure. These two aforementionedprocesses are unified and both belong to query operation.

B. Flexible Mechanism Description

The index structure model is applicable to various mo-bility management scenarios thanks to its comprehensivedesign and flexible parameters. In this paper, consideringthe IPv6-based mobile system and host mobility scenario,we utilize our index structure framework to analyzeMIPv6 and HMIPv6 as examples.

1) Basic Elements: Mobile IPv6 is a global mobilitymanagement scheme with centralized architecture. It usestwo addresses: 1) a home address (HoA) which is astable IP address in order to be stably identifiable toother network nodes and 2) a care-of-address (CoA)which is a temporary IP address for routing purpose.Clearly, an entity is needed that maps an HoA to thecorresponding currently valid CoA. In Mobile IPv6 thesemappings are exclusively handled by home agents (HAs)or corresponding nodes (CNs).

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Without regard to the properties of nodes, this mech-anism relates to four types of function entities: 〈MN,0〉, 〈AR, 0〉, 〈HA, 1〉 and 〈CN, 1〉, and four types ofconnection relations: 〈{MN-AR}, 0, {1 hop, wirelesslink}〉, 〈{AR-HA}, 1, {a hops, wired link}〉, 〈{AR-CN},1, {b hops, wired link}〉 and 〈{HA-CN}, 0, {c hops,wired link}〉, where for each edge, the properties {eprops}consist of distance parameter and link type.

On the other side, HMIPv6 is a localized mobilitymanagement scheme and adopts two-level index structure.The mobility management inside the local domain ishandled by a Mobility Anchor Point (MAP). Mobilitybetween separate MAP domain is handle by MIPv6.

On the same assumptions, HMIPv6 relates to fivetypes of function entities: 〈MN, 0〉, 〈AR, 0〉, 〈MAP, 1〉,〈HA, 1〉 and 〈CN, 1〉, and seven types of connectionrelations: 〈{MN-AR}, 0, {1 hop, wireless link}〉, 〈{AR-HA}, 0, {a hops, wired link}〉, 〈{AR-CN}, 0, {b hops,wired link}〉, 〈{HA-CN}, 0, {c hops, wired link}〉, 〈{AR-MAP}, 1, {d hops, wired link}〉, 〈{MAP-HA}, 1, {e hops,wired link}〉, and 〈{MAP-CN}, 1, {f hops, wired link}〉.

2) Basic Operations: The update operation and thequery operation correspond to handoff scheme and routingscheme respectively. Concretely, without soft handoversupport, reactive IPv6 handover process usually involvesthree steps of procedures: 1) movement detection (MD);2) address configuration (or duplicate address detection,DAD); and 3) registration (or binding update, BU). Fig. 5and Fig. 6 illustrate the handover procedure and timing di-agrams of MIPv6 and HMIPv6 respectively. The numbersof messages between the entities and their timing relationsare clearly described in these diagrams. The process ofrouting mainly refers to tunneling technology.

C. Effective Performance Analysis

We analyze the delays and the overheads for MIPv6and HMIPv6 using our mobility analysis framework.

1) Mobility Model and Assumptions: As the first step,we use the mobility model described in [29] to calculatethe import of mobility management layer.

There are two concepts: subnet and domain in networktopology layer. For an MN, let ts and td be indepen-dent and identically distributed (i.i.d.) random variablesrepresenting the subnet residence time and the domainresidence time, respectively. Let fs(t) and fd(t) be thedensity function of ts and td, respectively.

A two-dimensional random walk model for meshplanes is used to compute the density function of a domainresidence time. It is assumed that an MN resides in asubnet for a period and moves to one of its four neighborswith the same probability, i.e., with probability 1/4 andall subnets in a domain have the same shape and size. Adomain is referred to as an n-layer domain if it overlays4n2−4n+1 subnets. An n-layer domain overlays subnetsfrom layer 0 to layer n − 1. The subnets that surroundlayer x−1 subnets are called layer x subnets. The subnetsthat surround the layer n − 1 subnets are referred toas boundary neighbors, which are outside the domain.

2,0 2,1 2,2 2,3 2,0

2,3 1,0 1,1 1,0 2,1

2,2

2,32,0 2,2 2,1 2,0

2,31,01,11,02,1

2,21,10,01,1

Layer0

Layer1

Layer2

(a)

0,0

1,0

1,1

2,1

2,0

2,2

2,3

3,2

3,1

3,0

3,3

1/41

1

1

1

1

1/4

1/4

1/4

1/2

1/2

1/4

1/4

1/4

1/4

1/41/4

1/4

1/4

1/4

1/4

1/4

1/4

1/2

1/4

1/4

(b)

Figure 7. (a) Three-layer mesh domain structure. (b) State diagram ofthe two-dimensional random walk model.

Fig. 7(a) illustrates the three-layer domain and the typeof subnets for three-layer domain. A subnet type is of theform < x, y >, where x indicates that the subnet is inlayer x and y represents the y + 1st type in layer x. Thestate diagram of the random walk for three-layer domainis shown in Fig. 7(b). A state (x, y) represents that an MNis in one of the subnets of type < x, y >. The absorbingstate (n, j) represents that an MN moves out of domainfrom state (n − 1, j), where 0 ≤ j < 2n − 3.

Corresponding to the mobility model of our framework,the topology model is 2-D mesh domain structure andthe movement model is the 2-D random walk model. Thesubnet residence time ts is exponentially distributed withmean 1/λs. The call-to-mobility ratio(CMR) as the ratioof the packet arrival rate to the mobility rate, i.e. ρ =λ0/λs where λ0 is the packet arrival rate for each MN.

Thus, for an MN, the probabilities Πs(i) and Πd(j),which stand for the MN move across i subnets andj domains between two packet arrivals, can be finallyderived as follow [31]:

Πs (i) =

⎧⎪⎪⎨⎪⎪⎩

1 − 1ρ

(1 − f∗s (λ0)) , i = 0

1ρ

(1 − f∗s (λ0))

2 (f∗s (λ0))

i−1, i > 0

(5)

Πd (j) =

⎧⎪⎪⎨⎪⎪⎩

1 − 1ρΦ

(1 − f∗d (λ0)) , j = 0

1ρΦ

(1 − f∗d (λ0))

2 (f∗d (λ0))

j−1, j > 0

(6)

where f∗s (λ0) and f∗

d (λ0) is the Laplace transform offs(t) and fd(t) , respectively. And E[td] can be simplyrepresented as Φ/λs.

Consequently, between two packet arrivals, an MNmoves across the following average numbers of subnetsand domains, respectively:

ns =∞∑

i=0

iΠs (i) , (7)

nd =∞∑

j=0

jΠd (j) . (8)

For simplicity, our analysis are based on the follow-ing assumptions. Firstly, we assume that the processing

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abilities of entities are relatively strong. Therefore, theprocessing delays and overheads can be ignored and wemerely consider the packet delivery delays and overheads.Secondly, we assume that the delays and the overheadsfor delivering signaling messages through a particularpath are available. While the delay values are simplyobtained by empirical measurements, the cost valuesshould be determined based on the nodes’ and the edges’properties such as average bandwidth and resource on theparticular wireless and wired path. On the other hand,the distance parameters between any network nodes arethe number of hops packets travel. Finally, although allsignaling messages defined in a management mechanismhave different bytes of their sizes, we assume that they allhave the same delays and overheads if their destinationand source are identical.

2) Analytical Results: In MIPv6, when the BU mes-sages are send to HA and CN simultaneously for eachupdate, N = M = 1. And in HMIPv6 N = 2, M = 1.The µ, n, ν, m depend on movement patterns and callarrival characteristics. For MIPv6 and HMIPv6, the querycosts mostly include the delays and the overheads causedby one-level tunnel and two-level tunnels respectively, soPL and PS can be directly determined by parameters ofthe tunneling technology.

In this paper, we mainly focus on the update costscaused by handover process. Here, delay (i.e. handoverlatency) is defined for an MN as the time that elapsesbetween the connection reestablishment with a new accesspoint (AP) and the arrival of the first packet on the newsubnet, and overhead (i.e. signaling cost) is defined bythe cost of network-layer signaling messages necessaryto complete an IPv6 handover.

And for the latencies and the signaling costs listed inTable III, we will use the same parameter symbols forthe sake of simplicity. Here, a, b, c, d are the distanceparameters defined in section V-B, and Rm, RM , RL, RTand DTimes are the procedure parameters the router’sconfigured MinRtrAdvInterval and MaxRtrAdvInterval,the provious AR’s MaxRtrAdvInterval, RetransTimer, andDupAddrDetectTransmits, respectively [19], [32]. Basedon the reference system architecture and performanceparameters, we obtain unit handover latency and unitsignaling cost for MIPv6, respectively:

UL = TMDn+l+TDADn

+max{2tL, 2tN}+2tN , (9)

US = 3tR + 2tL + 2tH + 3tN . (10)

For HMIPv6, during the intra-domain handover proce-dure, unit update delay and unit update overhead are asfollows, respectively:

UL1 = TMDn+l+ TDADn

+ 2tM , (11)

US1 = 3tR + 2tM . (12)

On the other hand, during the inter-domain handoverprocedure, unit update delay and unit update overheadare as follows, respectively:

UL2 = TMDn+l+2TDADn

+2tM+max{2tL, 2tN}+2tN ,

TABLE III.LISTS OF PERFORMANCE PARAMETERS.

Symbol Description

αLatency or signaling cost of a packet deliverythrough a wireless medium between MN andAP

βLatency or signaling cost of a packet deliverybetween AP and AR

γLatency or signaling cost of a packet’s hopdelivery in wired medium

tR = α + βLatency or signaling cost of a packet deliverybetween MN and AR

tH = tR + aγLatency or signaling cost of a packet deliverybetween MN and HA

tL = tR + (a + c) γLatency or signaling cost of a packet deliverybetween MN and CN via HA

tN = tR + bγLatency or signaling cost of a packet deliverybetween MN and CN

tM = tR + dγLatency or signaling cost of a packet deliverybetween MN and MAP

TMDn+l

Average period to complete the MD procedurewith both network and link-layer notificationdepending on the values of Rm, RM , and RL

TDADn

Average of DAD latency that is defined a timeinterval between the MD completion and thecompletion of address configuration and DAD,depending on the values of RT and DTimes

(13)

US2 = 4tR + 2tM + 2tL + 2tH + 3tN . (14)

By using (7), (9) and (10), the average update delayand the total update overhead of MIPv6 are as follows:

CULMIP v6 = UL, (15)

CUSMIP v6 = nsUS . (16)

Then, by using (7), (8), (11), (12), (13) and (14), we canobtain the cost functions of average update delay and totalupdate overhead of HMIPv6 as follows, respectively:

CULHMIP v6 =(ns − nd) UL1 + ndUL2

ns, (17)

CUSHMIP v6 = (ns − nd) US1 + ndUS2. (18)

For our analysis, the following fixed parameter areused: α = 3ms, β = 2ms, γ = 1ms, c = 5(#hopsbetween HA and CN), d = 5(#hops between AR andMAP), TMD = 0.25s, and TDAD = 1.0s.

Fig 8 (a) and (b) respectively show the 3-Dgraphs of the total update overheads, CUSMIP v6 andCUSHMIP v6 , and the average update delays, CULMIP v6

and CULHMIP v6 , for different values of the MN-HA andMN-CN distances, where ρ = 0.2. As shown in thefigure, the update delays and overheads increase as eitherthe MN-HA or MN-CN distance increases. And the totalupdate overhead and the average update delay of HMIPv6increase slower than those of MIPv6, i.e. the effects onthe update performances of HMIPv6 is smaller than thoseof MIPv6.

Fig 9 shows the total update overheads, CUSMIP v6 andCUSHMIP v6 , as a function of CMR for the two schemeswhen a = 10(#hops between AR and HA), b = 10(#hops

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0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

10

15

20

25

30

35

40

1015

2025

3035

Tota

lU

pdate

Overh

ead

Dis

tanc

ebe

twee

nM

Nan

dH

A

Distance between MN and CN

MIPv6

HMIPv6

(a) Effects on update overhead.

1.30

1.35

1.40

1.45

1.50

1.55

10

15

20

25

30

35

40

1015

2025

3035

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rage

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Dela

y

Dis

tanc

ebe

twee

nM

Nan

dH

A

Distance between MN and CN

MIPv6

HMIPv6

(b) Effects on update delay.

Figure 8. Effects of MN-HA and MN-CN distances on update performances.

0.1 1 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

To

tal U

pd

ate

Ove

rhe

ad

CMR

MIPv6

HMIPv6

Figure 9. Effects of CMR on update overheads.

between AR and CN), and domain layer is 3. As shown inthe figure, the update overheads become low when CMRincrease. When CMR is high, the mobility rate is lowcompared with the packet arrival rate and the overheadfor update decreases. When CMR is low, the mobilityrate dominates and the related update overhead increases.And HMIPv6 performs better than MIPv6 in terms of theupdate overhead, especially when the CMR is low i.e. thefrequency of subnet boundary crossings is high.

Fig 10 plots the total update overheads, CUSMIP v6

and CUSHMIP v6 , as a function of domain size whena = 10(#hops between AR and HA), b = 10(#hopsbetween AR and CN), and ρ = 0.2. It is observed inthe figure that the total update overheads of HMIPv6 ismuch less than that of MIPv6. Up to 40% ∼ 57% of theupdate overhead can be saved when using HMIPv6. Inlarge domain, the interdomain movement occurs less, sothat the total update overhead of HMIPv6 becomes low,and the advantage of it becomes high.

2 3 4

0.2

0.3

0.4

0.5

0.6

0.7

0.8

To

tal U

pd

ate

Ove

rhe

ad

Domain Layer

HMIPv6

MIPv6

Figure 10. Effects of Domain Layer on update overheads.

All of the above analysis results are consistent withthe optimization goals of HMIPv6. It demonstrates thevalidity of our framework. Similarly, a rich set of pa-rameterized mobility models can be introduced, and theperformance of other mobility management schemes canbe described and analyzed simply and effectively usingour model and method.

VI. CONCLUSIONS AND FUTURE WORK

In this article, we presented a mobility managementlayer as a conceptual explanation of mobility and its man-agement, and proposed a generic index structure frame-work to analyze host mobility supports for integratedmobile networks in a systematic manner. This frameworkis consistent with the observation of previous studies.But unlike previous studies that discussed a certain hostmobility support protocol or compared different mobilitymanagement mechanisms, there is no clear winner amongthe current protocols and no practical integrated mobility

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support mechanism in our study, since the environmentof integrated mobile networks is not clear. We hopethat our framework can be incorporated into the currentscenarios to test the host mobility support protocols andproved helpful for designing uniform mobility supportmechanisms for future integrated mobile networks.

In our framework, we first investigated the previousmethods in the area of mobility model analysis. Thenwe introduced the index structure concepts into mobilitymanagement, and built a general model to unify theseschemes into an index structure that simplified the mobil-ity management between heterogeneous systems. At thesame time, we characterized the basic elements of themanagement mechanisms, and proposed the fundamentalperformance metrics and the expressions of the costfunctions. Our framework is flexible in the definitions ofthe models and their parameters, and is independent ofspecific network organization and protocol. Thus it is auniversal framework, and is suitable for many applicationenvironments. The universality, simplicity and flexibilityof our model seem to be significant when compared withthe other conceptual framework and used to describe andanalyze real-world schemes.

Our study is an attempt to meet the challenges ofefficient, flexible and comprehensive mobility manage-ment in integrated mobile networks. The potential benefitsof bringing index structure and mobility managementtogether are important, and need to be further exploredin the future to design efficient and unified mobilitysupport mechanisms for future integrated mobile net-works. Moreover, based on the understanding of mobilityand mobility management, we could further extend ourframework for the network mobility case and designnetwork mobility support mechanisms. We believe thatseveral parameters such as node density, topology stabilitymetrics and relative speed metrics may affect the mobilitymanagement performance and thus are worthy of furtherinvestigation. Qualitative analysis of network mobility iscurrently being carried out in our working group.

REFERENCES

[1] S. Y. Hui and K. H. Yeung, “Challenges in the migrationto 4G mobile systems,” IEEE Commun. Mag., vol. 41, pp.54–59, Dec. 2003.

[2] J. F.Huber, “Mobile next-generation networks,” IEEE Mul-timedia, vol. 11, pp. 72–83, Jan./Mar. 2004.

[3] I. F. Akyildiz, J. Xie, and S. Mohanty, “A ubiquitous mo-bile communication architecture for next-generation het-erogenous wireless systems,” IEEE Radio Communication,vol. 43, no. 6, pp. 29–36, 2005.

[4] Y.-J. Choi, K. B. Lee, and S. Bank, “All-IP 4G networkarchitecture for efficient mobility and resource manage-ment,” IEEE Wireless Commun. Mag., vol. 14, pp. 42–46,Apr. 2007.

[5] I. F. Akyildiz and J. Mcnair, “Mobility management innext-generation wireless systems,” Proc. IEEE, vol. 87, pp.1347–1384, Aug. 1999.

[6] F. M. Chiussi, D. A. Khotimsky, et al., “Mobility manage-ment in third-generation all-IP networks,” IEEE Commun.Mag., vol. 40, pp. 124–135, Sept. 2002.

[7] V. K. Varma, S. Ramesh, et al., “Mobility managementin integrated UMTS/WLAN networks,” in Proc. IEEEthe International Conference on Communication(ICC’03),vol. 2, May 11–15, 2003, pp. 1048–1053.

[8] C. Makaya and S. Pierre, “An architecture for seamlessmobility support in IP-based next-generation wireless net-works,” IEEE Trans. Veh. Technol., vol. 57, pp. 1209–1225,Mar. 2008.

[9] I. F. Akyildiz, J. Xie, and S. Mohanty, “A survey ofmobility management in next-generation all-IP-based wire-less systems,” Wireless Communications, IEEE [see alsoIEEE Personal Communications], vol. 11, no. 4, pp. 16–28, 2004.

[10] B. Odadzic and M. Jankovic, “Network to network mobil-ity management requirements for new generation mobilesystems,” in Proc. IEEE the 7th International Conferenceon Telecommunications in Modern Satellite, Cable andBroadcasting Services ’05, Sept. 28–30, 2005, pp. 233–236.

[11] Y. Zhai, Y. Wang, J. Yuan, et al., “An index structuremodel for mobility management of integrated mobile IPnetworks,” in Proceedings of IEEE Network Operationsand Management Symposium Workshops (NOMS Work-shops 2008), Apr.7-11, 2008, pp. 32–38.

[12] S. Mohan and R. Jain, “Two user location strategies forpersonal communications services,” IEEE Personal Com-mun. Mag., vol. 1, no. 10, pp. 42–50, 1994.

[13] C. E. Perkins, “Mobile IP,” IEEE Commun. Mag., vol. 35,pp. 84–99, May 1997.

[14] C. Perkins, “IP mobility support,” RFC 2002, Oct. 1996.[15] C. Perkins, “IP mobility support for IPv4,” RFC 3344,

Aug. 2002.[16] D. Johnson, C. Perkins, and J. Arkko, “Mobility support

in IPv6,” RFC 3775, June 2004.[17] R. Ramjee et al., “HAWAII: a domain-based approach

for supporting mobility in wide-area wireless networks,”IEEE/ACM Trans. Networking, vol. 10, pp. 396–410, June2002.

[18] A. Campbell, J. Gomez, S. Kim, et al., “Design imple-mentation and evaluation of cellular IP,” IEEE PersonalCommun. Mag., vol. 7, pp. 42–49, Aug. 2000.

[19] H. Soliman, C. Castelluccia, et al., “Hierarchical mobileIPv6 mobility management (HMIPv6),” RFC 4140, Aug.2005.

[20] R. Koodli, “Fast handovers for mobile IPv6,” RFC 4068,July 2005.

[21] W. Ma and Y. Fang, “Dynamic hierarchical mobility man-agement strategy for mobile IP networks,” IEEE J. Select.Areas Commun., vol. 22, pp. 664–676, May 2004.

[22] J. zhao Sun and J. Sauvola, “Mobility and mobilitymanagement: A conceptual framework,” in Proc. IEEEthe 10th International Conference on Networks(ICON’02),Aug. 27–30, 2002, pp. 205–210.

[23] H. D. Chon, D. Agrawal, and A. E. Abbadi, “Storage andretrieval of moving objects,” in Proceedings of the Int.Conf.on Mobile Data Management 2001 (MDM 2001),2001, pp. 173–184.

[24] H. K. Park, J. H. Son, and M. H. Kim, “An efficientspatiotemporal indexing method for moving objects inmobile communication environments,” in Proceedings ofthe Int. Conf.on Mobile Data Management 2003 (MDM2003), 2003, pp. 78–91.

[25] H. K. Park, J. H. Son, and M. H. Kim, “Adaptive indexmanagement for future location-based queries,” The Jour-nal of Systems and Software, vol. 74, pp. 313–324, 2005.

[26] L. Xiaofeng, C. Chuanbo, and L. Yunsheng, “Processingglobal nearest neighbor query,” in Eighth ACIS Inter-national Conference on Software Engineering, ArtificialIntelligence, Networking, and Parallel/Distributed Com-

JOURNAL OF NETWORKS, VOL. 4, NO. 1, FEBRUARY 2009 63

© 2009 ACADEMY PUBLISHER

puting, 2007 (SNPD 2007), vol. 1, July 30, 2007, pp. 458–462.

[27] C. Tuduce and T. Gross, “A mobility model based onWLAN traces and its validation,” in Proc. IEEE INFO-COM’05, vol. 1, Mar. 2005, pp. 664–674.

[28] A. Bar-Noy and I. Kessler, “Mobile users: To update or notto update?” ACM-Baltzer Journal of Wireless Networks,vol. 1, no. 2, pp. 175–185, 1995.

[29] I. F. Akyildiz, Y. B. Lin, et al., “A new random walk modelfor PCS neworks,” IEEE J. Select. Areas Commun., vol. 18,no. 7, pp. 1254–1260, 2000.

[30] Y. Xiong and Z. Bu, “A general distribution formula of thecount of mobiles crossing location areas between two datatraffic arrivals,” ACM SIGMOBILE Mobile Computing andCommunications Review, vol. 8, no. 3, pp. 68–72, 2004.

[31] Y.-H. Han, JinHyeock, and S.-H. Hwang, “Reactive han-dover optimization in IPv6-based mobile networks,” IEEEJ. Select. Areas Commun., vol. 24, pp. 1758–1772, Sept.2006.

[32] S. Thomson and T. Narten, “IPv6 stateless address auto-configuration,” RFC 2462, Dec. 1998.

Yujia Zhai received her B.S. degree in Electronic Engineeringand Information Science from University of Science and Tech-nology of China in 2004. She is currently pursuing her PhDdegree in the Department of Electronic Engineering, TsinghuaUniversity. Her research interests include mobility management,wireless and mobile networks, and complex networks.

Yue Wang received his B.S. and PhD degrees from the Elec-tronic Engineering Department of Tsinghua University in 1999and 2005, respectively. He is now an assistant professor atTsinghua University. His research interests include computernetworks, data fusion, and complex networks.

Yuan Jian received his B.S. and M.S degrees from SoutheastUniversity in 1986 and 1989, respectively, and PhD degree fromUniversity of Electronic Science and Technology of China in1998. He then did research job of postdoctoral in the ElectronicEngineering Department of Tsinghua University. He was avisiting researcher at NIST, USA from 2000 to 2003. He isnow an associate professor with the Department of ElectronicEngineering, Tsinghua University. His research interests includenetwork theory and technology, complex network dynamics, andinformation and network security.

Yong Ren received his B.S., M.S and PhD degrees from HarbinInstitute of Technology in 1984, 1987 and 1994, respectively.He then did research job of postdoctoral in the ElectricalEngineering Department of Tsinghua University from 1995 to1997. He is currently a professor and Ph. D. supervisor in theDepartment of Electronic Engineering at Tsinghua University.His research interests include information sharing networks andwireless networks.

Xiuming Shan received his B.S. degree from the ElectronicEngineering Department of Tsinghua University in 1970. He isthe chair professor of Institute of High-speed Signal Processingand Network Transmission, Electronic Engineering Department,Tsinghua University. His research interests include radar signalprocessing, computer networks, and complex systems.

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