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  1. 1. Computer Communications 29 (2006) 1363–1385 www.elsevier.com/locate/comcom Mobility management across hybrid wireless networks: Trends and challenges Farhan Siddiqui, Sherali Zeadally * High-Speed Networking Laboratory, Department of Computer Science, Wayne State University, Detroit, MI 48202, USA Received 14 July 2005; revised 31 August 2005; accepted 9 September 2005 Available online 6 October 2005Abstract Future generation wireless networks are envisioned to be a combination of diverse but complementary access technologies. Internetworkingthese types of networks will provide mobile users with ubiquitous connectivity across a wide range of networking environments. The integrationof existing and emerging heterogeneous wireless networks requires the design of intelligent handoff and location management schemes to enablemobile users to switch network access and experience uninterrupted service continuity anywhere, anytime. Real deployment of such mobilitystrategies remains a significant challenge. In this article, we focus on handoff management. We discuss in detail handoff decision andimplementation procedures and present recent handoff techniques that aim at providing mobility over a wide range of access technologies. Wealso discuss some of the capabilities of mobile terminals that are necessary to implement seamless mobility over hybrid wireless networks.Furthermore, we also present and discuss limitations of recent handoff design architectures and protocols as well as outstanding challenges thatstill need to be addressed to achieve portable and scalable handoff solutions for continuous connectivity across wireless access networks.q 2005 Elsevier B.V. All rights reserved.Keywords: Wireless; Heterogeneous; Overlay; Mobility; Handoff; Multimode; Protocol1. Introduction providing a low latency, high bandwidth, and wide area data service to a large number of mobile users. Wireless Overlay Mobile wireless technology has gained tremendous popu- Networks [1]—a hierarchical structure of room-size, building-larity due to its ability to provide ubiquitous information access size, and wide area data networks solve the problem ofto users on the move. However, presently, there is no single providing network connectivity to a large number of movingwireless network technology that is capable of simultaneously consumers in an efficient and scalable way. In an overlay Abbreviations 3G, third generation; 3GPP, third generation partnership project; AAA, authentication, authorization and accounting; ACK, acknowledgement;ADSL, asymmetric digital subscriber line; AR, access router; ASCONF, address configuration; BS, base station; CA, congestion avoidance; CAR, candidate accessrouter; CARD, candidate access router discovery; CD, communication daemon; CDMA, code division multiple access; CN, correspondent node; cSCTP, mobilestream control transmission protocol; CTAR, context transfer activation request; CTD, context transfer data; CTP, context transfer protocol; CTR, context transferrequest; DAB, digital audio broadcasting; DCCP, datagram congestion control protocol; DECT, digital enhanced cordless telephone; DHCP, dynamic hostconfiguration protocol; DMT, discrete multitone; DNS, domain name service; DSP, digital signal processors; DSSS, direct sequence spread spectrum; DVB, digitalvideo broadcasting; EDGE, enhanced data rates for GSM evolution; FA, foreign agent; FDD, frequency division multiplexing; FHSS, frequency hopping spreadspectrum; FS, fixed server; GPRS, general packet radio service; GSM, global system for mobile communication; GWFA, gateway foreign agent router; HA, homeagent; HiperLAN, high performance radio local area network; HOPOVER, handoff protocol for overlay networks; HP, handoff-prepare; HSCSD, high-speed circuit-switched data; IETF, internet engineering task force; IMT, international mobile telecommunications; IP, internet protocol; L2, layer 2; LSS, location-service server;MN, mobile node; MR, multicast router; MSC, mobile switching center; mSCTP, mobile stream control transmission protocol; NACK, negative acknowledgement;NAR, next access router; OFDM, orthogonal frequency division multiplexing; PSTN, public switched telephone network; QAM, quadrature amplitude modulation;QoS, quality of service; QPSK, quadri-phase shift keying; RNC, radio network controller; RRC, radio resource control; SCTP, stream control transmission protocol;SDR, software-defined radios; SIP, session initiation protocol; SS, slow start; TCP, transmission control protocol; TDD, time division multiplexing; TDMA, timedivision multiple access; UMTS, universal mobile telecommunications system; VDC, virtual domain controller; VoIP, voice over internet protocol; WG, workinggroup; WLAN, wireless local area network. * Corresponding author. Tel.: C1 3135770731; fax: C1 3135776868. E-mail address: zeadally@cs.wayne.edu (S. Zeadally).0140-3664/$ - see front matter q 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.comcom.2005.09.003
  2. 2. 1364 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385network, lower levels are comprised of high bandwidth Division Multiple Access (CDMA) transmitted power is muchwireless cells that provide a small coverage area. Higher higher as compared to WLANs.levels in the hierarchy provide a lower bandwidth but a much Currently, several wireless technologies and networks existwider access network. A mobile device with multiple wireless that capture different needs and requirements of mobile users. Fornetwork interfaces can access these networks as it moves high-data-rate local-area access, WLANs are satisfactorybetween different network environments. Next generation solutions [8]. For wide-area communications, traditional cellularwireless systems typically constitute different types of access networks may provide voice and data services. For worldwidetechnologies [15]. The heterogeneity that will characterize coverage, satellite networks have been used extensively infuture wireless systems instigates the development of military and commercial applications. Since, different wirelessintelligent and efficient handoff management mechanisms networks are complementary to each other, their integration willthat can provide seamless roaming capability to end-users empower mobile users to be connected to the system using themoving between several different access networks. ‘best available’ access network that suits their needs. Next generation wireless systems are envisaged to be a combination of a plethora of networking technologies. Fig. 2 illustrates a typical2. Wireless overlay networks network architecture for next generation heterogeneous systems. Some key features [3] of next generation heterogeneous Fig. 1 shows a typical structure [2] of wireless overlay access networks include:networks. First, the networks’ service areas are overlapped. Forexample, the General Packet Radio Service (GPRS) network † High usability with anytime, anywhere connectivity.acts as an umbrella network to the Wireless Local Area † Support for multimedia services with low transmission cost.Network (WLAN) network. Even the different cells of the † Integrated access networks with a common IP-based core.same network overlap. This overlapping can be utilized to † Use of multimodal devices (capable of supporting variousreduce service disruption, by simultaneously connecting to types of network access technologies).different subnets of the same access technology during † Support for telecommunication, data and multimediatransition from one network to another. Second, the networks services.support different data rates and cell sizes. For instance, IEEE † Support for personalized services.802.11b WLAN supports a data rate of 11 Mbps and GPRS a † Support for integrated service access from various servicemuch lower data rate of about 9.6 Kbps. Third, because of the providers.different characteristics of the networks involved, it is notpossible to compare the signal powers received from the base Table 1 shows some of the technologies, which will be part ofstations of different networks to decide which network to future heterogeneous systems along with their characteristics.connect to. Fourth, each network may offer a different level ofreliability, security, quality of service etc. As mobile hosts 3. Handoffs in wireless overlay networksmove across different networks, a mechanism for conveyingthe new IP address to the correspondent nodes is required. Handoff is the process by which a mobile terminal keeps itsAlso, the power consumed by the network interfaces is connection active when it migrates from the coverage of onedifferent for each network technology and is directly network access point to another. Different types of handoffsproportional to the transmitted power. For example, the Code can occur in wireless overlay networks. 3.1. Horizontal vs. vertical handoff Handoffs that occur between the access-points of the same network technology and are termed horizontal handoffs or intra-system handoffs [4]. In other words, horizontal handoffs occur between homogeneous cells of a wireless access system, example between two cells of a cellular system, etc. Handoffs that occur between different access-points belonging to different networks (example WLAN to GPRS) are referred to as vertical handoffs or inter-system handoffs. Thus, vertical handoffs are implemented across heterogeneous cells of access systems, which differ in several aspects such as bandwidth, data rate, frequency of operation, etc. The different character- istics of the networks involved make the implementation of vertical handoffs more challenging as compared to horizontal handoffs. The terms horizontal and vertical follow from the overlay network structure that has networks with increasing Fig. 1. Wireless Overlay Networks. cell sizes at higher levels in the hierarchy. Vertical handoffs are
  3. 3. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 1365 Fig. 2. A typical Architecture of Future Heterogeneous Access Systems.generally of two types namely, upward and downward of a new WLAN cell and the loss of coverage from the previoushandoffs. WLAN cell. In this case, the mobile device will definitely want to make a handoff to the new cell to continue the service.3.2. Upward-vertical handoff vs. downward vertical handoff Similarly, all upward vertical handoffs can be anticipated, which means that a mobile device will always want to handoff An Upward-vertical handoff is a handoff to a wireless to a network higher in the overlay (example switching to GPRSoverlay with a larger cell size and generally lower bandwidth after receiving a link layer trigger indicating weak coverage ofper unit area. So, an upward-vertical handoff makes a mobile a WLAN). However, in the case of downward verticaldevice disconnect from a network providing faster but smaller handoffs, a link layer trigger can indicate to a mobile devicecoverage (example WLAN) to a new network providing slower that it is now under the coverage of a new network (examplebut broader coverage. A downward vertical handoff is a WLAN) and the mobile node may wish to execute the handoff.handoff to a wireless overlay with a smaller cell size, and Downward vertical handoffs may be anticipated or unantici-generally higher bandwidth per unit area. A mobile device pated, such that a mobile device may already be under theperforming a downward vertical handoff disconnects from a coverage of the new network but may prefer to postpone acell providing broader coverage to one providing limited handoff based on requirements of the applications running oncoverage but higher access speed [5]. A vertical handoff may the mobile node, and may execute a handoff later, beingbe to an immediately higher or lower overlay, or the mobile already aware of the coverage status of the new network [4].host may ‘skip’ an overlay. For example, a mobile may handofffrom an in-room network directly to a wide-area network, or 3.4. Hard vs. soft handoffvice versa. A handoff is hard if the MN can be associated with only one3.3. Anticipated vs. unanticipated handoff access point at a time. A soft handoff occurs if the MN can communicate with more than one access points during handoff. Anticipated handoffs are those which the mobile device will For example, if the MN is equipped with multiple networkalways want to perform, such as horizontal handoffs and interfaces, it can simultaneously connect to multiple access-upward vertical handoffs. For example, in the case of Wireless points in different networks during soft handoff [6]. SoftLANs (WLANs), a link layer trigger may indicate the presence handoff may also be referred to as make before breakhandoff in
  4. 4. 1366 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385Table 1Characteristics of heterogeneous wired and wireless access systems Access network type Frequency Data rate Coverage Cost TechnologyWireless technologies Bluetooth 2.4 GHz ISM band Max. 721 kbps 0.1–10 m Low DSSS, FHSS IEEE 802.11g 2.4 GHz 54 Mbps 30–150 m Low OFDM IEEE 802.11b 2.4 GHz 11 Mbps Up to 100 m Low DSSS IEEE 802.11a 5 GHz 20 Mbps 50–300 m Low OFDM, TDD HiperLAN2 5 GHz 54 Mbps 150 m max. Low OFDM IMT2000, UMTS 2 GHz Max. 2 Mbps 30 m–20 Km High FDD, TDD IEEE 802.20 Below 3.5 GHz Up to 9 Mbps 20 Km High OFDM IEEE 802.16 10–66 GHz Max 70 Mbps Over 50 Km Low OFDM GSM, GPRS, 900,1800,1900 MHz 9.6–384 Kbps Up to 35 Km High TDMA, FDD HSCSD, EDGE Satellite Up to 14 GHz Max 144 Kbps Several Kilometers High DAB 176–230 MHz; 1452– 1.5 Mbps Up to 100 Km Low OFDM 1467.5 MHz DVB-T !860 MHz 5–31 Mbps Up to 100 Km Low OFDM DECT/DECT Link 1880–1900 MHz Up to 2 Mbps Up to 50 m Low TDMA/TDDWired technologies ADSL Baseband !Z6.144 Mbps 2–6 Km Low DMT (down) 0.640 Mbps (up) Cable modem 60–860 MHz (down) !40 Mbps (down)! 5–20 Km Low FDD/QAM/ 10–40 MHz (up) 10 Mbps (up) QPSKIMT, International Mobile Telecommunications; HSCSD, High-Speed Circuit-Switched Data; UMTS, Universal Mobile Telecommunication System; DAB, DigitalAudio Broadcasting; GSM, Global System for Mobile Communication; DVB, Digital Video Broadcasting; EDGE, Enhanced Data Rates for GSM Evolution; ADSL,Asymmetric Digital Subscriber Line; DECT, Digital Enhanced Cordless Telecommunications; GPRS, General Radio Packet Service; HiperLAN, High PerfprmanceRadio Local Area Network; OFDM, Orthogonal Frequency Division Multiplexing; TDMA, Time Division Multiple Access; FDD, Frequency DivisionMultiplexing; DSSS, Direct Sequence Spread Spectrum; TDD, Time Division Duplex; QAM, Quadrature Amplitude Modulation; QPSK, Quadri-Phase ShiftKeying; FHSS, Frequency Hopping Spread Spectrum; DMT, Discrete Multitone.which the mobile node’s connection may be created at the which are broadcasted by different wireless technologies. Thetarget base station before the old base station connection is MN will know a wireless network is reachable if its servicereleased. On the other hand, in the case of hard handoff or advertisements can be heard. The simplest way to discoverbreak before makehandoff the new connection may be set up reachable wireless networks is to always keep all interfaces on.after the old connection has been torn down. However, keeping an interface active all the time consumes the battery power even without receiving/sending any packets (as3.5. Mobile-controlled handoff vs. network-controlled handoff shown in Table 2). Therefore, to avoid keeping the idlevs. mobile-assisted handoff interfaces always on is critical. Also the discovery time should be low so that the MN can benefit faster from the new wireless Under network-controlled handoff, the network makes the network.decision for handoff, while under mobile-controlled handoff, The power efficiency and the system discovery time are thethe mobile node must take its own signal strength measure- most critical considerations for system discovery methods’ments and make the handoff decision on its own. Under performance. The interface may be activated periodically tomobile-assisted handoff, the decision to handoff is made by the receive service advertisements. The activating frequencymobile node in cooperation with the network. directly affects the system discovery time. The MN that activates the interfaces with high frequency may discover the4. Vertical handoff process reachable network quickly but its battery may run out very soon. The MN that activates the interface with low frequency The vertical handoff process may be divided into three may increase the power efficiency, but it may discover thephases [5]: reachable wireless networks slowly. There exists a tradeoff between the power efficiency and the system discovery time1. Network Discovery [5].2. Handoff Decision Table 23. Handoff Implementation Power consumption by 3G-CDMA and WLAN NICs Technology Transmit mode Receive mode Idle mode4.1. Network discovery (W) (mW) (mW) 3G: CDMA 1X wireless 2.8 495 82 This is the process where a Mobile Node (MN) searches for modem NIC ORINOCO IEEE 1.3 900 740reachable wireless networks. A MN with multiple interfaces 802.11B NICmust activate the interfaces to receive service advertisements,
  5. 5. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 13674.2. Handoff decision: traditional vs. next generation handoff 4.3. Handoff metrics in heterogeneous networksstrategies Handoff metrics are used to indicate whether or not a Handoff decision is the ability to decide when to perform handoff is needed. In traditional handoffs, only signal strengththe handoff. A decision for vertical handoff may depend on and channel availability are considered. In next generationseveral issues relating to the network to which the mobile heterogeneous wireless environment, new handoff metrics willnode is already connected and to the one that it is going to have to be considered in connection with signal strength. Somehandoff. For example, the decision to perform mobile- of these metrics [7] include:controlled handoffs may be made by a vertical handoff agent,sitting in the mobile device based on policies such as 4.3.1. Costnetwork bandwidth, load, coverage, cost, security, QoS, or Cost is a major consideration to users since differenteven user preferences. User preference is important when network operators may employ different billing strategies.performing vertical handoffs. For instance, if the new These variations in billing plans may affect the user’s choice ofnetwork to which a mobile device performs a handoff does handoff.not offer security; the user may still decide to use the oldnetwork. Depending upon coverage, a user may wish to use a 4.3.2. Network conditionssecure and expensive link for his official email traffic Network-related parameters such as traffic, available(example using GPRS) but may still opt for a cheaper link bandwidth, network latency, and congestion (packet loss)to access web information (example WLAN) [4] (Table 3). may need to be considered for effective network usage. Use of Traditionally, handoff decision has been performed based network information in handoff decisions can also be useful foron the channel quality indicated by the received signal strength load balancing across different networks, possibly relievingand other measurements, and the availability of resources in the congestion in certain systems.new cell. This is done periodically so that degradations insignal strength below a prescribed threshold can be detected 4.3.3. Battery powerand handoff to another radio channel or cell can be initiated. Battery power may be a significant factor for handoff inHowever, the use of the signal strength as the only criteria to some cases. For example, when the battery level is low, themake a handoff decision limits the ability of the terminal to user may choose to switch to a network with lower powerinitiate a handoff for other reasons such as security requirements, such as an ad hoc Bluetooth network.requirements, etc. Also, traditional handoff does not allowuser selection of networks, and assumes that there is only one 4.3.4. Application typeschoice for access technology. In a heterogeneous environment, Different types of applications require different levels ofuser choice is a desirable amenity. Traditional handoff link reliability, latency, and data rate. The user applications runningtransfer only concerns the delivery of packets to the new point on a mobile device may also influence the handoff decision.of attachment. On the contrary, handoff in a heterogeneousenvironment demands the delivery of the context of the 4.3.5. Mobile node conditionsinformation flow between the mobile node and the network as Mobile node conditions include dynamic factors such aswell. Context may include security associations, QoS velocity, moving pattern, and location information.guarantees, authentication headers, and so on. Traditionalhandoff protocols are developed for homogeneous systems that 4.3.6. User preferencesrely on a common signaling protocol, routing technique, and User preferences can be used to cater special requests formobility management standard. In heterogeneous environ- one type of system over another.ments, mobile nodes and network routers must be able to Next generation handoff will have to incorporate many ofinteroperate with different networks, and corresponding the above criteria rather than only signal strength in order toprotocols and standards. perform a handoff decision. Developing a handoff decisionTable 3Comparison between traditional and next generation handoff strategies Traditional handoff Next generation handoffHandoff metrics Signal strength Bandwidth, cost, user-preference, network conditions, security, etc.Radio link transfer Delivery of packets to the new point of attachment Delivery of packets as well as contextual information (such as securityrequirements associations, QoS guarantees, authentication headers, etc.)Protocols Relies on common signaling protocols, routing Requires mobile nodes and network routers to interoperate between different techniques, and mobility management standards networks with various protocols and standardsNetwork types Handoff occurs between homogeneous cells/net- Handoff occurs between heterogeneous cells/ networks (e.g. handoff between works (e.g. a handoff between two WLANs) a WLAN and a GPRS network)Terminal types Terminal equipped with a single access technology Multimode terminal (equipped with multiple access interfaces required) interface suffices
  6. 6. 1368 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385function that takes into account numerous factors remains a re-establishment of routing information during host mobilityresearch challenge. is handled by mobility protocols (example Mobile IPv4 [35] and IPv6 [36]). The access networks may also need to establish4.4. Handoff implementation and keep service state information (referred to as service context [37]) necessary to process and forward packets in a way Handoff implementation requires the actual transfer of data that suits specific service requirements. A context is initiallypackets to a new wireless link in order to reroute a mobile established in a network through protocol exchanges with theuser’s connection path to the new point of attachment. It mobile node. However, when mobile nodes change theirrequires the network to transfer routing information about the routing path, we need to ensure that nodes along the new pathmobile user to the new (or target) access router for the proper continue to provide similar treatment to IP packets as wasforwarding of packets. Since, next generation heterogeneous provided along the old routing path. To provide seamlessnetworks will operate in an environment of multiple standards mobility, both the IP level connectivity and relevant contextand networks, transfer of packets to a new wireless link will information needs to be quickly re-established.also involve transfer of additional contextual information in If the mobile node is required to re-establish these serviceorder to enable the mobile node to move through different contexts using the same signaling process it used to initiallynetworks, while maintaining its data flows. The desired goal of establish them, delay-sensitive real time traffic (VoIP, video,transferring the context of a mobile node to the new network is etc.) as well as TCP-based applications may be seriouslyto minimize the delay in re-establishing the mobile node’s impacted. This impact is due to the large delay that would betraffic flows. However, if the context transfer delay is so large introduced because of protocol exchanges with the mobileas to have the same effect of the complete re-establishment, or node. Thus, it is not possible to re-establish IP connectivity andlarge enough to increase the overall handoff call dropping rate, service context information within the stringent time-constrainthe advantages of context transfer are lost. A mechanism to imposed by time-sensitive applications [39].allow for inter-network and/or inter-service-provider agree- Context transfer [38] is a solution proposed by the Seamobyments to support fast intersystem handoffs while avoiding an Working Group (WG) to reduce the handoff time byunreasonable amount of inter-network signaling exchanges to transferring the information related to the mobile node fromvalidate or institute the adjustment in services is presently a the current access router to the next access router over thecrucial research problem [7]. wired network, avoiding using the limited wireless bandwidth Some of the motivations behind performing context transfer resources. An alternative to re-establishing service contextsbetween nodes in an IP access network are: when the mobile node moves to a new subnet is to transfer the existing service contexts from the host’s Current Access Router1. The success of a time sensitive service such as VoIP, video, to the New Access Router (NAR) of the destination subnet. etc. in a mobile environment depends heavily on the Such services whose contexts may be transferred from one minimization of the impact of the traffic redirection in order router to another are referred to as context transfer candidate to deliver the host’s IP traffic to the new point of access. services [37]. Examples of such services include Quality of2. During the establishment of the new forwarding path, nodes Service (QoS), Authentication, Authorization and Accounting along the new path must be prepared to provide similar (AAA), and header compression state established and forwarding treatment to IP packets. maintained between the mobile node and the Access Router3. In order to replicate the context of one forwarding node to (AR) to reduce the large IP header overhead of short (example another forwarding node, an open, standard solution for VoIP) packets over bandwidth-limited wireless links. context transfer must be developed. Earlier works on maintaining context associations while moving across different wireless networks were mainly Context transfer is further discussed in detail in the focused on setting up protocol state after handoff by signalingfollowing section. new state information over the wireless link. With context transfers, information is transferred from the old AR to the new4.4.1. Context transfer for seamless handoffs in heterogeneous AR through wired links. This approach results in faster andwireless networks more secure transfer than signaling over the slow and error- Handoff may result in a change in the access router to which prone wireless links. With context transfer, it would be possiblethe mobile node is connected to. In many cases (when the to keep practically all handoff-related signaling within wiredmobile node moves across the access networks), the change in links of the access network [40].the communication path may also include a change in the To achieve seamless mobility across various networks andcommunications network. For example, a host may move from access technologies, the mobile node needs to have thea WLAN to a cellular network resulting in a change in the type information about access routers to which the mobile nodeof the wireless link. could make a handoff to. These access routers are called Whenever the routing path of a mobile node changes due to Candidate Access Routers (CARs). The information that thehandoff, the data flows (IP packets) of the host need to be re- mobile node needs to have in order to select a CAR androuted by the access network to the mobile node’s new point perform a handoff includes the identities (IP addresses) ofof attachment. This change of the routing path and CARs as well as their capabilities for supporting newly routed
  7. 7. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 1369data flows. This procedure of finding the access routers and of the messages exchanged. The applications that will maketheir capabilities for the purpose of making a handoff is called use of CARD and context transfer can have requirements onCandidate Access Router Discovery (CARD) [41]. The the minimum interpacket delay and packet losses. TimelinessSeamoby WG is involved in the development of a protocol can be achieved by properly optimizing the message exchange,that provides fast mobility within an access network. This is avoiding unnecessary messages, especially on the wirelessachieved by transferring the context of all IP-flows from one link, and minimizing the number of entities involved inaccess router to another during handoff and eliminating the signaling. However, there are limits to the applicability ofneed to repeat the initial signaling process when the mobile CARD and context transfer. Although CARD is especiallynode moves across IP subnets. useful for those network configurations where several ARs are close to each other, attention must be paid to avoid over-4.4.2. Candidate access router discovery and context transfer whelming the wireless link with messages aiming to performprotocol address translation [40]. Handoffs in IP-based networks involve changes of the Context transfer is not always possible, even though theaccess point at the link layer and routing changes at the IP CAR can support the transferred contexts. An example couldlayer. For example a mobile node can initiate the handoff as be when the mobile node crosses the border between twosoon as it detects a layer 2 (L2) id of a new access point during administrative domains: in such a case, the new networkthe link layer scan. A reverse address translation mechanism is provider might require the mobile node to be re-authenticatedneeded to retrieve the IP address of the candidate access router from the beginning rather than to accept its transferredbased on its L2 id. Support for address translation is provided contexts. Security is a major concern in this environment; itby the CARD protocol. The mobile node communicates to its is especially important that the entities exchanging context orcurrent access router the L2 ids of the CARs; the AR performs router identity have authenticated each other, and thethe translation and sends the required information back to the communication is encrypted. In the following sections, wemobile node. After a CAR has been found, the handoff process discuss the main features and operation of the Candidatemust be fast in order to support time-sensitive services. Access Router Discovery and Context Transfer protocols. To avoid delays in re-initializing all service states on thenew access router, previously stored protocol state informationcan be transferred to the new subnet in advance through the 4.4.3. CARD protocolwired network in order to quickly re-establish the service. CARD [35] and Context Transfer Protocol (CTP) [42] wereContext is the information required for re-establishing the designed by the Internet Engineering Task Force (IETF)service at the new AR. Context transfer is the movement of the Seamoby WG to enable context information to be seamlesslycontext from one router to another as a means of re-establishing exchanged during handoffs. The CARD protocol allows mobilecontext transfer candidate services on a new subnet without nodes to discover information about the AR candidates for thehaving to perform entire protocol exchanges with the mobile handoff. The two main functionalities of the CARD protocolnode from the beginning. In order for context transfer to be are support for reverse address translation from the link layersuccessful, the new AR must be capable of supporting the ID of the AR to the IP address and discovery of the capabilitiesrerouted flow (must support the same context-transfer of the candidate AR. The actual mapping between link layercandidate services as the sending router); otherwise, even a and IP layer addresses is done at the current AR. The CARDtimely transfer of flow state can lead to service disruption due protocol carries the needed information between the mobileto a lack of support at the NAR. The mobile node should have node and AR. The ARs maintain a local CAR table where thethe means to discover the capabilities of the AR to which it is link layer IDs of the CARs, with the corresponding IPgoing to attach after handoff to be sure to receive the desired addresses, are mapped. In the table each AR is associatedsupport. Such means are provided by the CARD protocol. In with a set of capabilities, refreshed periodically. The CARDaddition to mapping link layer IDs to IP addresses, the protocol specification does not mandate any mechanism for populatingcan optionally be used to request from the current AR the the local CAR table; two dynamic approaches, one relaying oncapabilities of the CARs to eventually decide, whether it is a centralized entity coordinating the IP address and anotherworthwhile to perform a handoff to that AR. Capabilities may distributed one, are mentioned. The table is updated using theinclude flow related parameters, such as, QoS support or CARD Request and CARD Reply messages, which aresecurity and authorization information. exchanged between the mobile node and its current AR or Context transfer is most useful when the context can be between the current AR and the CARs. Capabilities are carriedtransferred in advance so that the new AR can establish quickly as options in the CARD messages and expressed as a list ofthe service and make it available to the mobile node once it has attribute-value pairs. The mobile node can specify a set ofperformed the handoff. To initiate proactive context transfer, capabilities in which it is interested. The current AR canthe context transfer solution must define the events that trigger perform filtering of information and can provide only the CARsthe transfer of the context to the new subnet, so that all the matching the preferences with subsequent saving of bandwidth.involved network entities can receive and correctly process the Examples of capabilities are resource availability information,information necessary to guarantee seamless handoff. An wireless link layer technologies supported, and existingimportant feature in the common framework is the timeliness security mechanisms. The request for capabilities is optional;
  8. 8. 1370 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385the only necessary request instantiated by the mobile node is the new AR. Scanning for other access points from the mobileaddress translation. node or listening for potential mobile nodes from access points The CARD process is initiated by the mobile node upon may not be possible with all wireless links. Moreover, thereceipt of a CARD trigger, such as the link layer ID of a handoff may be forced suddenly due to access point failure, forwireless access point; the mobile node sends to its current AR example. Second, context information that is updated at highthe MN-AR CARD Request message, specifying one or more rates cannot be sent proactively, as it would be obsolete at theaccess point ID and a list of desired capabilities. The mobile new AR after mobile node handoff. When proactive contextnode informs the current AR about the need for CAR transfer cannot be used, having reactive context transfer, as incapabilities by raising a flag in the header of the option field. the second case, is still better than having no context transfer atIf no link layer IDs are provided, the current AR will reply with all, because services need not be re-established from thethe IP addresses of the CARs currently present in its local table. beginning.If, for example, the mobile node requests a list of the For proactive context transfers initiated by the mobile node,capabilities supported by the CAR by specifying an access the CTAR message contains the IP addresses of the new ARpoint’s link layer ID in the Request message, then the AR and the mobile node on the current AR, the list of the contextsresolves the IP address of the CAR based on the L2 ID received to be transferred, and a token that authorizes the transfer. Theand sends to the specified CAR an AR-AR CARD Request CTD message contains the transferred contexts and the old IPindicating all the CAR capabilities whose timers have expired address of the mobile node, as well as parameters for the newin the current AR local table. The CAR responds with the AR- AR to verify the authorization token. The CTAR message isAR CARD Reply reporting the requested capabilities. The always sent by the MN to the new AR, even if the proactivecurrent AR updates the capabilities entries in its table and sends context transfer is initiated by the current AR based on aback to the mobile node the IP address of the CAR, along with network decision. This allows the new AR to have means tothe requested capabilities with the MN-AR CARD Reply compare the authorization token received from the mobile nodemessage. with the CTAR message with the parameters received from the current AR with the CTD message. Optionally, the new AR can4.4.4. Context transfer protocol acknowledge the received context with a CTD Reply message A context transfer protocol has been developed by the IETF (CTDR), reporting the outcome of the context transfer process.Seamoby working group [25]. According to this protocol, as For the reactive context transfer case, the mobile nodethe mobile node moves from its previous access router to the solicits the new AR to start the context transfer with the CTARnew access router, the corresponding information about each of message, providing the parameters described above. The newthe mobile node’s data-flows is forwarded between the access AR requests context transfer from the old AR with the CTRrouters. Each data-flow is categorized into feature contexts, message, in which it provides the mobile node’s current IPwhich allow the network to indicate and provide the particular address, the contexts to be transferred, and the authorizingcontext information needed per data-flow. For example, a token generated by the mobile. The context is transferred withparticular mobile user may be downloading streaming video the CTD message and acknowledged with the CTDR message.while conducting a voice transmission. The context required to The signaling diagrams show that the context transfer protocolcontinue the voice call may be authentication information only, creates only a few messages to be exchanged. Moreover, mostwhile the context needed for the video service may be QoS and of the signaling exchange happens on the wired links betweenheader compression, in addition to authentication. ARs, and only one or two messages are sent through the CTP defines two ways to initiate context transfer operations: wireless links. The size of messages can be kept low,proactive and reactive. In the former case, the current AR optimizing the use of bandwidth in the wired link too. In theinitiates the context transfer before the actual handoff after CTAR message, in fact, the mobile node can specify thereceiving a Context Transfer Activate Request (CTAR) context types the current AR must transfer, the default beingmessage from the mobile node or due to a network decision. all, leading to smaller message size. Context transfer can alsoThe current AR sends a Context Transfer Data (CTD) message be network initiated; in the proactive case, the current AR afterto the new AR to start the context transfer. In the latter case, the a proper trigger (not specified by the CTP specification) cannew AR receives the CTAR message from a new mobile node begin the transfer, sending a CTD to the new AR. In thethat includes the IP address of the old AR and information reactive case, the new AR, again after a trigger, sends a CTRabout the contexts to be transferred. The new AR sends a request to the current AR.Context Transfer Request (CTR) message to the old AR, whichreplies with a CTD message that starts the transfer of context 5. Terminal requirements for future heterogeneous wirelessinformation. networks In the first case, context transferred before actual handoffexecution, implies proactive transfer of context information, As future wireless systems are expected to support severaland it is clearly more desirable than reactive transfer. However, heterogeneous access networks, we need to develop terminalsproactive context transfer is not always possible. First, the that that have the capability to access these diversecurrent AR must know in advance that a handoff is going to be technologies. In this context, terminals and devices capableperformed so that it can transfer context information a priori to of supporting different types of access technologies are being
  9. 9. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 1371designed. Current multimode devices are of two different 5. Traffic should be smoothly transferred while changing thetypes: active interface in a way that is transparent to the user, that is, as seamlessly as possible.† The first type includes those devices capable of supporting multiple access systems by incorporating several network interface cards and the appropriate software for switching 5.1.2. Deployment issues for multimode terminals between these network interfaces. These devices are often referred to as ‘multimode terminals’. Network detection. The multimode terminal must be† The second type includes those devices which use adaptable able to detect the availability of a new network (example software modules that operate on a generic, reconfigurable WLAN connectivity). For some interface technologies, hardware platform consisting of Digital Signal Processors resources are not exhausted even if the interface is permanently (DSPs) and general purpose microprocessors, used to on and scanning for network access coverage. In the case of implement radio functions such as generation of transmitted IEEE 802.11 WLANs, this approach is infeasible due to the signal (modulation) at transmitter and tuning/detection of amount of power needed to keep the interface actively received radio signal (demodulation) at receiver. Such scanning for access points. So, the issue that arises is how to devices are referred to as Software-defined Radios (SDRs). activate the WLAN interface when in the vicinity of a hotspot. Currently, this activation of the interface is left to the user who manually enables their WLAN card when in a hotspot.5.1. Multimode wireless terminals However, since one of the multimode terminal requirements is to minimize user interaction, a desirable goal in this case, Multimode wireless terminals [10] are devices supporting would be however, to automate this process. One solution thatmultiple radio access technologies and allow reception of would achieve this result would be to advertise the presence ofdata over multiple system bearers with different character- nearby services (or coverage of other technologies) via theistics. An intelligent multimode terminal should be able to currently active interface. For example, the presence of adecide autonomously the active interface that is best for an hotspot could be sent via GSM, which would then activate theapplication session and to select the appropriate radio WLAN interface automatically.interface as the user moves in and out of the vicinity of aparticular technology. The decision regarding the switching Network information. Finding out information about theof the interface and the handoff of the active sessions to the network to support interface selection decisions. For example, ifnewly active interface may be decided based on user the user specified that the mobile terminal should default to usingpreference setup (may also be referred to as ‘user policy’) the cheapest connection, then for the terminal to be able to decideon the multimode terminal regarding the interface usage. For which connection is the cheapest, some charging informationexample, if the user has setup some preference to indicate must be made available by the network. Service providers maythat the PDA should default to using the fastest and cheapest choose to advertise certain pricing information allowing theinterface available, the terminal will switch to the WLAN prices of the different networks to be compared. This informationinterface whenever the PDA enters into the WLAN coverage also needs to be formatted or standardized between operators soarea. However, the multimode terminal must also take into that it is easy to compare. Other information such asaccount other factors such as the QoS requirements of the authentication methods which allow the terminal to determinerunning applications, etc. in advance if switching access networks is required may also be advertised. We need to investigate how to make this information5.1.1. Requirements for multimode terminal operation available to mobile users. Some of the requirements that need to be fulfilled in order tobuild intelligent multimode devices include: 5.2. Software-defined radios1. The terminal should operate with minimal inputs from the Software-Defined Radio (SDR) [20–22,26] is a technology user. From a user experience, it is preferable to carry out that uses adaptable software and flexible hardware platforms to these decisions in an automated way rather than having to address the problems that arise from the constant evolution and query the user every time a new interface becomes technical innovation in the wireless industry, particularly as available, or an old interface disappears. waveforms, modulation techniques, protocols, services and2. The terminal should be able to handle session handoffs from standards change [16]. It is receiving enormous recognition and one interface to the other based on user policy. generating widespread interest among wireless users and3. Radio access interfaces should be selected based on user developers as it promises to solve these problems by preferences, application QoS requirements, and, infor- implementing the radio functionality as software modules mation about the network. running on a generic hardware platform. SDR technology aims4. The requirements of applications should be determined and to take advantage of re-programmable hardware modules to then decide whether an application could benefit from build an open-architecture based on radio system software. changing interfaces. SDR facilitates the software implementation of some of the
  10. 10. 1372 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385functional modules in a radio system such as modulation/de- network operators face problems during migration of themodulation, signal generation, coding, and link-layer proto- network from one generation to next due to presence ofcols. This helps in building reconfigurable software radio large number of subscribers using legacy handsets that maysystems where dynamic selection of parameters for each of the be incompatible with a newer generation network. Theabove-mentioned functional modules is possible. A complete network operators also need to incur high equipment costshardware based radio system has limited utility since when migrating from one generation to next. Equipmentparameters for each of the functional modules are fixed. A vendors face problems in rolling out newer generationradio system built using SDR technology extends the utility of equipment due to the short time-to-market requirements.the system for a wide range of applications that use different † The air interface and link-layer protocols differ acrosslink-layer protocols and modulation/demodulation techniques. various geographies (for example, European wirelessThe SDR system can be reconfigured depending on the networks are predominantly GSM/TDMA based while insoftware module being used. Also, the software modules that USA the wireless networks are predominantly IS94/CDMAimplement new services/features can be downloaded over-the- based). This problem has inhibited the deployment of globalair onto the handsets. This kind of flexibility offered by SDR roaming facilities causing great inconvenience to subscri-systems helps in dealing with problems due to differing bers who travel frequently from one continent to another.standards and issues related to deployment of new services/- Handset vendors face problems in building viable multi-features. In a nutshell, Software-Defined Radio (SDR) refers to mode handsets due to high cost and bulky nature of suchthe technology wherein software modules running on a generic handsets.hardware platform consisting of DSPs and general purpose † Wireless network operators face deployment issues whilemicroprocessors are used to implement radio functions such as rolling-out new services/features to realize new revenue-generation of transmitted signal (modulation) at transmitter streams since this may require large-scale customizationsand tuning/detection of received radio signal (demodulation) at on subscribers’ handsets. SDR technology enablesreceiver. SDR technology can be used to implement military, implementation of radio functions in networking infra-commercial and civilian radio applications. A wide range of structure equipment and subscriber terminals as softwareradio applications likes Bluetooth, WLAN, GPS, Radar, modules running on a generic hardware platform. ThisWCDMA, GPRS, etc. can be implemented using SDR significantly eases migration of networks from onetechnology. generation to another since the migration would involve only a software upgrade. Furthermore, since the radio5.2.1. Motivation for SDR development functions are implemented as software modules, multiple As mobile communications networks evolve to accommo- software modules that implement different standards candate accelerating demand for wireless voice and data services, co-exist in the equipment and handsets. An appropriateequipment manufacturers and network operators are being software module can be chosen to run (either explicitlyforced to confront many difficult, and simultaneous, chal- by the user or implicitly by the network) depending onlenges. Those challenges include: the support for an explosive the network requirements. This helps in building multi-subscriber growth using multiple standards, modes and mode handsets and equipment resulting in ubiquitousfrequencies, the proliferation of equipment and service connectivity irrespective of the underlying networkplatforms, meeting the demand for wireless Internet and technology used. SDR technology supports over-the-airinformation services; supporting the higher cost and longer upload of software modules to subscriber handsets. Thislifetime of third-generation (3G) wireless terminals; adapting helps both network operators as well as handsetto the increasing competition between service providers, and manufacturers. Network operators can perform massovercoming the cost and scarcity of the wireless spectrum. customizations on subscriber’s handsets by simply To address the above challenges, SDR offers the wireless uploading appropriate software modules resulting in fasterindustry an alternative vision where every consumer has a deployment of new services. Manufacturers can performsingle, personalized terminal that works everywhere and towhich new services are instantly downloaded. SDR has remote diagnostics and provide defect fixes by justgenerated tremendous interest in the wireless communication uploading a newer version of the software module toindustry for the wide-ranging economic and deployment consumers’ handsets as well as network infrastructurebenefits it offers. Some of the factors that are fueling such a equipment. However, SDR technology has some draw-strong interest in SDRs include: backs such as higher power consumption, higher processing power requirement, and higher initial costs.† Commercial wireless network standards are continuously SDR technology may not be suitable for all kinds of radio evolving from 2 to 2.5G/3G and then further onto 4G. Each equipment due to these factors. Hence, these factors generation of networks differ significantly in link-layer should be carefully considered before using SDR protocol standards causing problems to subscribers, technology in place of a complete hardware solution. wireless network operators and equipment vendors. For example, SDR technology may not be appropriate in Subscribers are forced to buy new handsets whenever a pagers while it may offer great benefits when used to new generation of network standards is deployed. Wireless implement base-stations [18,19].
  11. 11. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 13735.2.2. Key features of SDR technology innovative third-party applications on their handsets as in a personal computer system. This enhances the appeal and utility5.2.2.1. Reconfigurability. SDR allows co-existence of mul- of such handsets.tiple software modules implementing different standards on thesame system allowing dynamic configuration of the system by 6. Recent vertical handoff management techniquesselecting the appropriate software module to run. This dynamic proposed for heterogeneous wireless networksconfiguration is possible both in handsets as well asinfrastructure equipment. The wireless network infrastructure Several techniques have been proposed for performingcan reconfigure itself to the subscriber’s handset type or the handoffs while roaming across heterogeneous wireless accesssubscriber’s handset can reconfigure itself to network type. networks. These approaches operate at different layers of theSDR technology facilitates the implementation of future-proof, network protocol stack. Most of these approaches are based onmulti-service, multi-mode, multi-band, multi-standard term- a modification to the Mobile IP protocol and are implementedinals and infrastructure equipment. Reconfiguration also helps at the network layer [7,9,11,23,28]. Other handoff strategiesin debugging errors on a mobile terminal or a base station by include SIP-based handoff (operating at the application layer)downloading appropriate software to reconfigure the hardware. [29], and SCTP-based handoff (operating at the transport layer)Also, hardware equipment needs to be replaced when new [27]. These are also some recent schemes for handofffunctions are added to a particular device. SDR’s ability to management proposed to achieve seamless mobility for TCPreconfigure solves this problem. connections [30,33]. Other types of handoff schemes are based on efficient energy consumption of network interfaces [5,24]. Multimode operation. SDR provides multimode In the following section, we briefly describe these approachesoperation [17], which is essential for next generation wireless as well as their benefits and limitations (Table 4).systems. It helps in saving infrastructure costs. For example, ifone system is suitable for covering outdoor area and another 6.1. Mobile IPsystem is suitable for indoors; coverage can be expandedwithout additional investment in either system if the systems Mobile IP [7,11] is a mobility management protocolcooperate and dual-mode terminals become available. proposed to solve the problem of node mobility by redirecting packets to the mobile node’s current location. The Mobile IP5.2.2.3. Ubiquitous connectivity. SDR enables the implemen- architecture is shown in Fig. 3. Its main components include atation of air interface standards as software modules and Home Agent (HA) and a Foreign Agent (FA). HA is a router onmultiple instances of such modules that implement different a mobile node’s home network, which encapsulates datagramsstandards can co-exist in infrastructure equipment and for delivery to the mobile node when it is away from home, andhandsets. Such benefits enable the global roaming facility. If maintains current location information for the mobile node. FAa terminal is incompatible with the network technology in a is a router on a mobile node’s visited network (foreignparticular region, an appropriate software module needs to be network) that provides routing services to the mobile nodeinstalled onto the handset (possibly over-the-air) resulting in when registered. The FA decapsulates and delivers datagrams,seamless network access across various geographies. Further, tunneled by the mobile node’s home agent, to the mobile node.if the handset used by the subscriber is a legacy handset, the When a mobile node moves out of its home network it mustinfrastructure equipment can use a software module imple- obtain another IP address because according to the traditionalmenting the older standard to communicate with the handset. IP protocol a node’s address is fixed by geographical location. So, in mobile IP, a mobile host uses two IP addresses: a fixed5.2.2.4. Interoperability. SDR facilitates implementation of home address (a permanent IP address assigned to the host’sopen architecture radio systems. End-users can seamlessly use network) and a care-of-address—a temporary address from theTable 4Emerging technologies for seamless mobility across heterogeneous wireless access networksTechnology DescriptionInternet protocol (IP) Provides a backbone and a scalable solution to interconnect different networksMobile IP Enables mobile devices to maintain their IP addresses while moving between different networksMobile IPv6 Provides enhanced features to Mobile IP for optimizing the mobility processMobile stream control transmission protocol Transport layer solution that provides seamless roaming capability across different networks(mSCTP)Session initiation protocol (SIP) Application layer solution that provides both, personal and terminal mobility across different networks4G networks An all-IP architecture that integrates several different networks with a plethora of services and capabilitiesIntelligent software Enables the creation of smart terminals that can make network selection decisions with minimal user interventionSoftware-defined radio (SDR) Elements of a wireless network whose operational modes and parameters can be changed via specialized software inside the DSPs
  12. 12. 1374 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 Fig. 3. Mobile IP Network Architecture.new network (i.e. foreign network) that changes at each new problem results in communication routes significantlypoint of attachment. When the mobile node moves, it has to longer than the optimum routes and introduces additionalfirst discover its new care-of-address. The care-of-address can delay for packet delivery [12].be obtained by periodic advertising from the foreign agent 2. When the MN moves form one subnet to another, the newthrough broadcasting. The mobile node then registers its care- FA cannot inform the old FA about the movement of theof-address with its home agent by sending a registration request MN. Hence, packets already tunneled to the old care-of-to its home agent via the foreign agent. The home agent then address are lost.sends a registration reply either granting or denying the 3. Mobile IP is not a satisfactory solution for highly mobilerequest. If the registration process is successful, any packets users. Mobile IP requires a MN to send a location update todestined for the MN are intercepted by the HA, which it’s HA whenever it moves from one subnet to another. Thisencapsulates the packets and tunnels them to the FA where location registration is required even though the MN doesdecapsulation takes place and the packets are then forwarded to not communicate with others while moving. The signalingthe appropriate MN. Tunneling involves doing IP-within-IP or cost associated with location updates may become veryIP encapsulation. The home agent encapsulates the packets into significant as the number of MNs increase. Moreover, if theits IP packet (IP inside IP). The inner packet is the original, distance between the visited network and the home networkuntouched packet; the outer packet is addressed to the care-of is large, the signaling delay is long.address and sent through the usual Internet routing mechan-isms. Once the foreign agent receives packets, they are strippedof their outer layer and the original packet is extracted. Thus, 6.2. Mobile IPv6the Mobile IP process can be summarized in three steps:† Discovering the care-of address Improvements have been proposed and adopted for Mobile† Registering the care-of address IP under the title of Mobile IP version 6. Mobile IPv6† Tunneling to the care-of address eliminates triangular routing and enables the correspondent node to reroute packets on a direct path to the mobile node. There are certain routing inefficiencies in Mobile IP This process is referred to as route optimization. In addition, Mobile IPv4 also suffers from a lack of security constructs for1. Packets sent from a CN to an MN are first intercepted by the authorization, authentication, and accounting, as well as for HA and then tunneled to the MN. However, packets from source routing [11]. Mobile IPv6 includes embedded binding the MN are sent directly to the CN. This triangular routing updates and care-of address configuration for the execution of
  13. 13. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 1375location updates and processing the change in the mobile packet. Each BS forwards the packet to its gateway router.node’s address. The newer version also includes authentication The BS selection is based on signal strength, bandwidth,header processing to provide validation of mobile nodes. pricing and other factors, assuming such information isFinally, IPv6 has a fourfold increase in IP address space, which available from BS beacons. When such information ismay be useful for developing new mobile node addressing unavailable, the MN simply makes a random selection.schemes. Regardless of the type of network, issues such as † The authentication server of the new network verifies theaddressing, route optimization, and authentication are part of validity of the authentication information included in thethe challenging overall problem of creating an efficient handoff HP packet. If it is invalid, a HP_NACK (negative Ack of themechanism that satisfies the seamless mobility needs of the HP) is sent to the MN, and no further handoff preparationuser population while enabling advanced processing and work is performed. If it is valid, the routing state along theoptimization operations at the network. An improvement path from the gateway router to the chosen BSs is set.over Mobile Ipv6 is Hierarchical Mobile Ipv6 [13] which is † Based on the resource reservation information included inproposed to minimize the amount of signaling between the the HP packet, resources are reserved in target cells andcorrespondent node and the HA. along the path from the MN’s current sender to the target cell. Each of the chosen BS allocates a buffer for the MN6.3. Network layer handoff approaches and prepares to buffer packets that are in transit to the MN. † Each of the new BSs sends the old BS a HP_ACK packet.6.3.1. HOPOVER Upon receiving such a packet, the old BS adds the HandOff Protocol for OVERlay networks (HOPOVER) [9] corresponding new BS to the forward list for that MN andis a Mobile-IP based approach that handles both macro-level begins forwarding packets to the new BS. On completing(vertical handoffs) as well as micro-level (horizontal handoffs) these signaling procedures, routing information along themobility. It is designed to address the problems of high handoff path from the new gateway routers to the new BSs is set upfrequency due to fast moving mobile nodes, which cannot be and packets in transit have been buffered for the MN.handled by the traditional Mobile-IP protocol due to the highsignaling overhead that is incurred. HOPOVER is scalable for alarge number of mobile devices and expanding networks. It Handoff. When the MN decides, it is actually movingalso incurs low signaling overhead. Fig. 4 shows the to another cell, the actual handoff process is performed usingHOPOVER architecture. the following procedures. The Handoff procedure in the HOPOVER scheme consistsof three steps: † The MN sends a Handoff message to the BS of the target cell. The new BS then begins forwarding packets to the MN6.3.1.1. Handoff Preparation. When a Mobile Node (MN) including the buffered ones.decides it may encounter a handoff (based on the comparison of † The new BS sends a Leave message to the old BS and all thebase station beacons it receives), it starts with the following ‘handoff preparing’ BSs.handoff-preparation processes. † The old BS records the MN’s current network so that it can forward packets there. It stops forwarding packets to other† The MN chooses a small group of neighboring Base BSs, which are on the forwarding list. Stations (BSs) and sends them a Handoff-Prepare (HP) † The old BS removes the MN from its ‘current MN’ list. † Other handoff-preparing BSs remove the related routing information, allocated buffers and buffered packets. † The old BS and other handoff-preparing BSs delete themselves from the resource reservation tree they joined for that MN by sending a Release message. Thus, the network releases those reserved resources. Updating mobile IP information. After handoff, the previous BS maintains the forwarding address for the MN. To avoid wasted resources and additional delay, sometimes, the MN’s Mobile IP FA information should be modified to reflect the MN’s current address. Timers are used to make sure that the Home Agent is contacted only if the MN stays in the new cell for long time. After a handoff, both the old and new BSs set up a timer for the MN. If after the timer expires, the MN is still in the new network then the new BS sends a Mobile IP update packet to Fig. 4. HOPOVER Architecture. the MN’s HA, so that the new BS becomes the new FA for the
  14. 14. 1376 F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385MN. Also, the new BS notifies the old BS to remove the micro cells and uses an overlay network. The BS is a network-forwarding information for that MN. layer router having two wireless interfaces: wireless and wired. If before the timer expires, the MN moves back to the MN accesses the Internet via the BSs over the wireless links.previous network, then in that case, no contact with the Mobile The BSs can buffer the last few IP packets sent to the MN.IP HA is performed. Both timers are removed. Fig. 5 shows the network architecture of the Hierarchical If before the timer expires, the MN moves to a third approach.network. The second BS sends Mobile IP update packet to the When the MN is away from its home network, the HAMN’s HA to make itself the new FA of the MN. At the same intercepts the packets addressed to it and sends these packetstime, the second BS begins monitoring the MN’s stay in the through a tunnel to the current mobile node’s attachment. Thethird network. MN would previously have registered its GW router care-of- address with it’s HA. The GWFA router decapsulates packets6.3.1.4. Limitations of HOPOVER. The HOPOVER handoff sent by the HA and forwards those packets to the MN as aprocess requires base stations to maintain excessive infor- multicast stream using the multicast address assigned to thismation about mobile nodes. This information includes a list of MN. A small group of BSs are selected by the MN to listen toBSs to forward packets to, buffering queues for packets in its multicast address for packets encapsulated and sent by thetransit, routing and resource reservation parameters, as well as GWFA router. One of the BSs is selected by the MN to be theinformation regarding current network characteristics. The forwarding BS; it decapsulates the packets sent by the GWFAhandoff process using the HOPOVER scheme also calls for router and forwards those packets to the MN. The other BSs arecomplete standardization among different network service buffering BSs; they hold a small number of packets from theproviders so that the various network entities can exchange GWFA router in a circular buffer. BSs send out periodicsignaling messages among themselves. This requires major beacons similar to Mobile IP foreign agent advertisements. Themodifications to the existing infrastructure. MN listens to these packets and determines which BS should forward packets for the MN, which BSs should buffer packets6.3.2. Hierarchical approach in anticipation of a handoff, and which BSs should not belong The Hierarchical approach [23] is a Mobile-IP based to the multicast group at all. When the MN initiates a handoff,scheme proposed to improve vertical handoffs in hetero- it instructs the old BS to move from forwarding to bufferinggeneous systems. It defines five network entities: Home Agent mode, and the new BS to move from buffering to forwarding(HA), Foreign Agent (FA), Gateway Foreign Agent Router mode. The new forwarding BS forwards the buffered packets(GWFA), a Base Station (BS), Multicast Router (MR) and a that the MN has not yet received.Mobile Node (MN). The HA is a router in the mobile node’shome network. The GWFA is a router implementing the role of Limitations of the hierarchical approach. Thea FA and a multicast router in order to manage the visitor Hierarchical approach involves registration and packet for-mobile nodes. It can also implement the role of an HA if its warding mechanisms similar to the Mobile IP protocol and thisdomain is the home network. The GWFA router periodically incurs additional delay as well as signaling overhead sincebroadcasts agent advertisement messages containing its IP packets are intercepted by the HA and then forwarded to theaddress and assigns a multicast address unique within its mobile node.domain to the MN. The GWFA router manages macro and 6.3.3. OmniCon The OmniCon approach [28] is a modification to the Mobile IP protocol and is designed to support vertical handoffs in heterogeneous networks. It provides support for packet scheduling and buffering mechanisms to accommodate different transmission characteristics of the networks involved. Fig. 6 shows the OmniCon architecture. The OmniCon approach modifies the MN and the GPRS FA with two new modules: (i) Decision Module (ii) Communication Daemon The Decision Module is a WLAN link availability monitoring system incorporated into the MN to constantly monitor the WLAN signal strength, quality and noise level. The MN and the FA are modified to support TCP tunneling. The MN and the GPRS FA are also enhanced with OmniCon Communication Daemons (CDs), which establish a TCP Fig. 5. Architecture of Hierarchical Handoff Scheme. connection with each other over the GPRS link.
  15. 15. F. Siddiqui, S. Zeadally / Computer Communications 29 (2006) 1363–1385 1377 Fig. 6. OMNICON Network Architecture. For vertical handoffs, OmniCon implements a virtual Limitations of the Omnicon Approach. The Decisionnetwork interface called tcptun. This virtual interface Module in the OmniCon approach makes the handoff decisioncommunicates with the Mobile IP software and the CDs. based only on the detection of the WLAN signal performed byDuring system initialization, the tcptun is assigned the same IP the WLAN link availability monitoring system. It does notaddress as the WLAN network interface. The Mobile IP consider the requirements of the running applications or usersoftware listens on this interface for mobile agent advertise- preferences to switch between access networks.ments. On reception of GPRS foreign agent advertisements ontcptun, the Mobile IP software registers with the FA using the 6.4. Transport layer handoff approachessame interface. Once the registration is successful, the routingtable entries are updated to set the tcptun interface as the Implementation of handoff techniques at the transport layerdefault interface for all outbound packets bearing the Mobile IP requires a means to detect and reconfigure mobile hosts as theyaddress as the source address. These outbound packets are read move from one network type to another. This includes theby the CD and are tunneled over the TCP connection to the detection of new networks and the allocation of new IPGPRS FA, which takes care of further routing. addresses. These tasks are often handled by Dynamic Host Whenever, the MN detects that the WLAN signal Configuration Protocol (DHCP) or Router/Neighbor Discoverystrength has fallen below a particular threshold, OMNICON methods. A handoff protocol at the transport layer is required totriggers a handoff from the WLAN interface to the virtual implement a method that can dynamically rebind a connec-interface. The handoff triggering is carried out by sending tion’s IP address. Transport layer handoff techniques operatemultiple foreign agent advertisements on behalf of the GPRS by exploiting services from network and data-link layers forFA through the virtual interface, up to the TCP/IP stack of network detection and IP address management. Hence, handoffthe MN. This enables Mobile-IP to carry out a network layer techniques implemented at the transport layer are often referredhandoff and start using the virtual interface. Mobile IP to as cross-layer approaches [43]. Recent transport-levelswitches back to the WLAN interface by holding off the handoff management protocols that have been proposedforeign agent advertisements on the GPRS link. In order to include the Stream Control Transmission Protocol (SCTP)avoid multiple handoffs between the two networks, the [45] and the Datagram Congestion Control Protocol (DCCP)decision module uses two signal-strength threshold values, [46].with the maximum value referred to as the high-watermark SCTP provides a connection-oriented reliable service. Aand the minimum value referred to as the low-watermark. A connection between two SCTP end-points is called anhandoff from the WLAN to the GPRS is triggered only if association. Multi-homing is a prominent feature of SCTP.the signal strength falls below the low-watermark. On the Multi-homing allows an association to maintain multiple IPother hand, a handoff in the reverse direction (GPRS to addresses. In the case of SCTP, an end-point of an SCTPWLAN) is triggered only when the signal strength improves association can be mapped to several IP addresses. Amongabove the high-watermark. those addresses, one address is used as the primary address for