Published online 8 January 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.904
SPECIAL ISSUE PAPER
Next generation mobility management:
an introduction
F. Richard Yu1, Vincent W. S. Wong2, Joo-Han Song2, Victor C. M. Leung2∗and
Henry C. B. Chan3
1Carleton University, Ottawa, Canada
2The University of British Columbia, Vancouver, Canada 3The Hong Kong Polytechnic University, Hong Kong, China
ABSTRACT
Mobility management, which includes location management and handoff management, is essential in cellular wireless networks to provide service to mobile users. Location management enables call delivery to mobile users, while handoff management maintains the connectivity of ongoing calls while users move between cells. In next generation networks, mobile users will avail themselves with terminals capable of accessing wireless networks employing multiple technologies, thus making the task of mobility management more challenging. This paper reviews recent developments in location management, and surveys methods for handoff management between heterogeneous systems. Methods for inter-system handoffs in packet-switched inter-networks are discussed according to the protocol layer in which the handoffs take place, i.e., network layer, transport layer, and application layer. Open problems for mobility management in future wireless networks are also presented. Copyright © 2010 John Wiley & Sons, Ltd.
KEYWORDS
handoff management; location management; mobility management *Correspondence
Victor C. M. Leung, Department of Electrical and Computer Engineering, The University of British Columbia, 2332 Main Mall, Vancouver, BC, Canada V6T 1Z4.
E-mail: [email protected]
1. INTRODUCTION
In recent years, there has been tremendous growth in the use of wireless mobile communication services as they allow people to stay in touch while on the move. Currently, there exist disparate wireless systems that capture different requirements and needs of different users. Wireless local area networks (WLANs) offer high rates to users with low mobility over local areas. Wireless cellular networks provide relatively low data rates to user with high mobility over wide areas. Satellite networks are used extensively for worldwide coverage. Mobile ad hoc networks (MANETs) can be used to establish dynamic networks without the need of a fixed infrastructure due to their self-configuration and self-maintenance capabilities. In the next generation telecommunication network [1], these heterogeneous wireless networks are expected to be integrated to provide ubiquitous ‘always best connection’ to mobile users.
In cellular wireless networks (Figure 1) that support circuit-switched voice and packet-switched multimedia services, mobility management is an important function that ensures successful delivery of new calls to users and maintains ongoing calls with minimal disruptions, while users move between cells. There are two components in mobility management: location management and handoff (also referred to as handover in the literature) management.
Location management enables the network to deliver calls to mobile users by tracking their locations between calls. It involves two operations: location update and paging. When a location update occurs, e.g., when a mobile terminal (MT) or mobile node (MN) (we use these two terms interchangeably in this paper), moves from location area (LA) identified as LA1 to LA2 in Figure 1, it sends its location information, e.g., the identity of the LA in which it is located, to the network database. When a call arrives, the recipient MT may have already moved to another cell
Location Area LA 1 Location Cells VLR VLR HLR MSC MSC Location Area LA 1 Cells Location Area LA 1 Cells Location Area LA 1 Area LA 2 Cells VLR VLR HLR MSC MSC
Figure 1.Mobility management in cellular network architecture (MSC---mobile switching center; HLR---home location register;
VLR---visitor location register).
that is different from the one reported in the last location update, and hence the network needs to search (or page) a number of cells, e.g., all the cells constituting an LA in Figure 1, to locate the MT. There is a trade-off between the costs of location update and paging. If an MT updates its location more often, the network can have more precise information about the location of the MT, thus reducing the cost for paging when a call arrives; e.g., in Figure 1 this is realized by making each location area smaller. On the other hand, frequent location updates increase the costs of these updates. These tradeoffs led to different location update techniques, some of which dynamically adapt to user mobility.
Handoff management maintains service continuity by enabling an MT to keep its call connected when its point of connection to the network moves from one access point (or base station) to another. The handoff process can be intra-or intersystem. Intra-system handoffs (also referred to as horizontal handoffs) occur within a single network domain employing a homogeneous wireless access technology, and are usually handled in the link layer as an integral part of the wireless access method. Intra-system handoffs, e.g., due to movement of an MT between adjacent cells in Figure 1, have been widely investigated and a good survey can be found in [2]. In next generation networks that integrate several wireless access technologies, handoffs will need to support movements of terminals between networks employing different wireless access technologies. These are known as inter-system handoffs (also referred to as vertical handoffs), which will be surveyed in this paper. Several schemes have been proposed to solve the vertical handoff problem in heterogeneous wireless networks. Their operation scope varies from network to application layer. In packet-switched networks employing the Internet Protocol (IP), Mobile IP (MIP) based solutions can be employed in the network layer to provide transparent support for terminal mobility, including the maintenance of active Transmission Control Protocol (TCP) connections and User Datagram Protocol (UDP) port bindings. Transport layer solutions
for handoffs conform to the end-to-end principle in the Internet, i.e., anything that can be done in the end system should be done there. Since the transport layer is the lowest end-to-end layer in the IP protocol stack, it is a natural candidate for vertical handoff support. For multimedia connections set up using the application layer Session Initiation Protocol (SIP) for signaling, a SIP-based approach can be used to support vertical handoffs independent of the underlying wireless access technologies and network layer elements.
The rest of the paper is organized as follows. Section 2 reviews some recently proposed location management schemes. Network layer handoff management schemes are presented in Section 3. Section 4 reviews transport layer handoff management schemes. Application layer handoff management schemes are presented in Section 5. Section 6 concludes the paper.
2. LOCATION MANAGEMENT
In this section, we summarize the latest results in recent publications on location update and paging with a particular focus on wireless cellular networks. Most of the results surveyed here are obtained in 2005--2008. The work in this section is complementary to [3]; together, they provide a more complete picture on location management for wireless cellular networks.
2.1. Location Update
In [4], a cost analysis model is presented for the dynamic movement-based location update scheme. The proposed model used the results from renewal theory. The location area residence time can follow hyper-exponential distribution and the cell residence time can follow any general distribution. In [5], an analytical model is proposed for the movement-based scheme for wireless cellular networks with the home location register (HLR) and visitor location register (VLR) architecture in Figure 1. Both the cell residence time and the location area residence time can follow general distribution. The cost for location update and paging is derived. This model can be considered as a generalization of the work in [4,6]. In [7], the statistics for the number of cell crossings for a mobile user in the movement-based location management scheme is derived. Based on these statistics, the optimal sequential paging sequences, which minimize the paging cost for any given movement threshold and call-to-mobility-ratio, are determined.
In [8], a hybrid location update scheme is proposed by combining the movement-based and time-based schemes. An MT updates its location whenever it has traversed a fixed number of cell-boundary crossings and a certain time interval has elapsed following the previous update. Simulation results showed that the proposed hybrid scheme performs better than the individual movement-based or
timer-based schemes when the coefficient of variation of the cell residence time is large.
For the implementation of the distance-based scheme, an approach is proposed in [9] by using the cell coordinates in calculating the physical distance traveled by the MT. Based on the speed and direction of each individual MT, the size and shape of the LA (see Figure 1) can also be selected. The paging cost can also be reduced when the LA is partitioned into multiple paging areas.
In [10], a combined dynamic location registration and update model is proposed. The registration decision depends on the time elapsed since the last call arrival and the distance that the MT has traveled since the last registration. For the distance-based update scheme, a single sample path-based ordinal optimization algorithm is proposed.
In [11], a mobility pattern-based scheme is proposed. It incorporates the time information in the mobility pattern profile of the terminal. The current location of the MT can be determined by its movement state and the current system time. Simulation results showed that the proposed scheme has a lower signaling traffic load when compared to the profile-based scheme [12].
In [13], two location tracking algorithms based on spatial quantization and temporal quantization are proposed. The movement sequence of the MT is first quantized into a smaller set of codewords and then a compressed representation of the codeword sequence is reported. Simulation results showed that the proposed algorithms can reduce the update cost to very low values with only a small increase in the paging overhead.
When a wireless cellular network is covered with both macrocells and microcells, the MT can communicate via the base stations from these two types of cells. In [14], a location management strategy is proposed for this kind of hierarchical cellular networks. To reduce the update cost, a location update is performed only in the macrocell tier. Depending on the paging load, an MT can be paged either in the macrocell tier or in the microcell tier. Results show that this scheme can provide a small paging delay and a low update cost.
The frequency of location updates can also be reduced by having some registration areas overlapped with each other [15]. Overlapping allows the MT to choose a registration area to send its update message. This feature is not available in environments where non-overlapping registration areas are deployed. Having overlapped registration areas can also reduce the ping-pong effect for users moving between the registration boundaries frequently. The work in [16] showed that the selection of the registration area can affect the number of subsequent registrations. It then formulated both deterministic (offline) and stochastic (online) optimization problems which aim to minimize subsequent registrations. In [17], four LA selection policies for overlapping LAs scenarios are proposed, namely MaxOL, Central, Random, and MinOL policies. An analytical model is used to determine the expected number of cell movements before an MT leaves a LA.
2.2. Paging
In [18], the problem of searching for multiple MTs concurrently is studied. Three concurrent search algorithms are proposed, namely: the brute force algorithm, the simple heuristic algorithm, and the conditional probability heuristic algorithm. These algorithms aim to concurrently locatekMTs withinktime slots based on the probabilistic information about the locations of MTs. Results showed that the average paging cost can be reduced significantly.
In [19], a pipeline paging scheme is proposed which can handle multiple paging requests by serving them in a pipeline manner in different paging areas. Results showed that the pipeline paging scheme outperforms both the blanket paging and sequential paging schemes in terms of a higher discovery rate and a lower total delay under certain conditions.
In [20], a simplified pipeline probability paging scheme is proposed. Based on the information on the location probabilities of individual MTs, the proposed scheme can handle paging requests in a pipeline manner with a paging delay constraint. It also has a better performance when compared with blanket paging and sequential paging schemes.
There are various works which studied location update and paging tradeoff. By using the results from majorization theory and Riesz’s rearrangement inequality, the work in [21] showed that jointly optimal paging and registration policies for either symmetric or Gaussian random-walk models are given by the nearest-location-first paging policies and distance threshold registration schemes.
In multi-system wireless networks or heterogeneous wireless networks (see Figure 2), an MT can be equipped with multiple interfaces and connect to multiple wireless access networks simultaneously. In [22], three location management strategies are proposed for multi-system wireless networks with loosely coupled architecture. Each strategy differs in the levels of coordination and trust required among the wireless access network providers. Among them, the centralized approach has the lowest paging cost but require all relevant history to be stored
in a central coordinating server. The distributed approach incurs a high cost in both paging and location update. The quasi-distributed approach reduces the MT’s update load by using a central coordinator which only maintains the aggregate statistics about the MT’s location information in each sub-network.
3. NETWORK LAYER HANDOFF
MANAGEMENT
In contrast to circuit-switched networks, which generally support wireless handoffs within the access network (usually considered a function of the data link layer and below), packet-switched networks can support mobile handoffs in the network layer, by the ability to route packets successfully to the desired destination while the point of network attachment of the destination node has moved to a different part of the network or a different network domain. A domain is a common administrative entity that may include different access networks, such as WLAN and cellular networks of one service provider. Network layer handoff management solutions can be broadly classified into two categories:macro-mobilityand
micro-mobilitymanagement [23]. The movement of mobile nodes between two network domains is referred to as macro-mobility. On other hand, the movement of mobile nodes between two subnets within one domain is referred to as micro-mobility.
Network layer handoff management provides node mobility support at the IP layer. It does not depend on or make any assumption about the underlying wireless access technologies [24]. Signaling messages for handoff purposes are carried by IP datagrams. MIP [25] is an Internet Engineering Task Force (IETF) standard mobility management protocol that is designed to enable mobile nodes to move from one network to another while maintaining their own IP addresses that were pre-assigned by their home networks. Similarly, Mobile IPv6 (MIPv6) [26] provides node mobility support over IPv6.
3.1. Macro Mobility Solutions (Mobile IP) 3.1.1. Mobile IPv4 (MIPv4).
Mobile IP (see Figure 3) is a mobility management protocol for the global IP network (i.e., Internet). It introduces four new functional entities: home agent (HA), foreign agent (FA), MN, and correspondent node (CN).
An MN is able to detect whether it has moved into a new subnet by periodically receiving unsolicited Agent Advertisement messages broadcasted from each FA. An MN can also send Agent Solicitation messages to learn about the presence of any prospective FA. Each MN can have two addresses---a permanent IP address and a care of address (CoA), which is associated with the network a MN is currently visiting. When an MN discovers that it is in a foreign network, it obtains a new COA from FA (an MN can
Figure 3.Network mobility with mobile IP.
serve as its own FA). This CoA can be obtained by soliciting or listening for FA advertisements, or invoking Dynamic Host Configuration Protocol (DHCP) [27] or Point-to-Point Protocol (PPP) [28]. The MN registers the new CoA with its HA, and the HA updates the mobility binding by associating the IP address of the MN with its CoA.
Packets sent by a CN to an MN are intercepted by the HA in flight. The HA encapsulates the packets and tunnels them to the MN’s CoA. With an FA CoA, the encapsulated packet reaches the FA serving the MN, which decapsulates the packets and forwards them to the MN. With an associated CoA, the encapsulated packets reach the MN, which then decapsulates them.
3.1.2. Mobile IPv6.
The MIPv6 protocol has been specified by the IETF IP Routing for Wireless/Mobile Hosts Working Group [26]. MIPv6 uses many features of MIP, but it is integrated into IPv6 and presents many other enhancements. There are many important differences between MIP and MIPv6 as described below.
• There is no need for FA; it is not necessary to have a special local support to allow a MIPv6 node to operate correctly.
• MIP mobility defines the default mode of operation through the HA of the MN’s home network. This tends to burden the MN’s HA; the ability to redirect a CN to the MN’s current address is implemented only as an extension of the IPv4.
• MIPv6 incorporates route optimization as a fundamen-tal aspect of the protocol.
• Rather than encapsulating packets in tunnels as in MIP, most MIPv6 packets sent to a MN include IPv6
header extensions. Therefore, there is no need to encapsulate/decapsulate packets for tunnels.
• Use of IPv6 Neighbor Discovery eliminates the need to rely on Address Resolution Protocol or other link layer mechanisms; it also enables functions such as unreachability detection and agent discovery. Other differences include the way that authentication is accomplished between MNs and their agents or CNs, as well as how MNs can be accommodated even when they are attached to a network that is behind a firewall or Network Address Translation (NAT) device.
3.1.3. Proxy mobile IPv6 (PMIPv6).
To solve the mobile host-based global mobility chal-lenge, a Network-based Localized Mobility Management (NETLMM) protocol is being actively standardized by the IETF working group [29,30]. Proxy Mobile IPv6 (PMIPv6) [31] is a new standard currently being worked on at the IETF NETLMM working group.
Within a PMIPv6 network domain, the simplified signaling procedure for the handoff within a localized network can significantly reduce the handoff latency and the overhead of the control. PMIPv6 is designed to provide NETLMM to MNs with a standard IPv6 stack. The distinct functional entities of PMIPv6 are the Mobile Access Gateway (MAG) and the Local Mobility Anchor (LMA). The main job of the MAG is to detect the MN’s movements and initiate mobility-related signaling with the LMA on behalf of the MN. The serving network assigns a unique home network prefix to each MN, and theoretically this prefix always follows the MN wherever it moves within a PMIPv6 domain. From the viewpoint of the MN, the entire PMIPv6 domain appears as its home network. The MN can configure an address using any address configuration mechanism that is authorized in the PMIPv6 domain.
3.1.4. Optimized mobile IP schemes.
The problem of triangular routing has been alleviated by using a route optimization scheme [32]. The basic idea behind route optimization is to use a direct routes between MNs and their CNs to bypass the HA. CNs maintain a binding cache of the CoAs of MNs. When a CN sends packets to an MN, it first checks if it has a binding cache entry for the MN. If it has a binding cache entry, the CN tunnels the packets directly to the CoA of the MN. If no binding cache entry is available, the CN sends the packets following the basic Mobile IP procedure, that is, via the HA of the MN. Route optimization also takes care of the packets already tunneled to the old CoA and on the way. When an MN registers with a new FA, it requests the new FA to notify the previous FA about the movement. This ensures that packets on the way to the old CoA are successfully forwarded. It also ensures that packets from the CN with obsolete binding cache entries for the MN are successfully delivered to the MN’s new
CoA. Moreover, route optimization also ensures that any resources consumed by the MN at the old FA are released immediately, rather than waiting for the registration time to expire.
Enhanced Mobile IP (EMIP) [33] has been developed to eliminate tunneling between the HA and the FA. It differs from MIP in the way packets are redirected from the HA to the FA. A concept built on NAT [34] is used in place of tunnels. Instead of creating a tunnel, a mapping is created between the HA and the FA when each connection the MN communicates across is established. Mappings are creating by intercepting packets to and from the MN at the HA and the FA. The HA and FA then exchange mapping request and mapping reply messages containing the source and destination IP and port address of the MN and the CN. The mobility agent that intercepts the packet also supplies a care of port (CoP) that is used by the HA and the FA to identify the mapping. Once the mapping between the HA and the FA is established for a communication session, the mobility agents can redirect packets to and from the MN by modifying the IP and TCP or UDP packet headers instead of using a tunnel.
3.2. Micro Mobility Solutions
MNs may move frequently between subnets of one domain. To reduce signaling load and delay to the home network during movements within one domain, many micro-mobility solutions have been proposed. They can be broadly classified into two groups:tunnel-basedandrouting-based
micro-mobility schemes [35].
3.2.1. Tunnel-based schemes.
Tunnel-based schemes use local or hierarchical reg-istration and encapsulation concepts to limit the scope of mobility-related signaling messages, thus reducing the global signaling load and handoff latency. In MIP with regional registration (MIP-RR) [36], the regional registrations are performed via a new network entity called the Gateway Foreign Agent, which introduces a layer of hierarchy in the visited domain. Regional registrations reduce the number of signaling messages to the home network, and reduce the signaling delay when a MN moves from one FA to another within the same domain. Hierarchical MIPv6 (HMIPv6) [37] is an enhancement of MIPv6, which is designed to reduce handoff latency and the amount of signaling by managing local mobility for mobile connections. HMIPv6 adds an extra level that separates local from global mobility. In HMIPv6, global mobility is managed by the MIPv6 protocols, while local handoffs are managed locally. A new node in HMIPv6 called the Mobility Anchor Point (MAP) serves as the local entity to aid in mobile handoffs. The MAP, which replaces the FA in MIPv4, can be located anywhere within a hierarchy of routers. Unlike FA, there is no need to have a MAP residing in each subnet. The MAP helps to decrease handoff-related
latency because a local MAP can be updated more often than a remote HA. The intra-domain mobility management protocol (IDMP) [24] is a two-level hierarchical approach. The first hierarchy consists of different mobility domains, and the second hierarchy consists of IP subnets within one domain. This hierarchical approach localizes the scope of intra-domain location update messages and reduces both the global signaling load and update latency.
3.2.2. Routing-based schemes.
Routing-based host mobility schemes maintain host-specific routes in the routers to forward packets. The host-specific routes are updated based on host mobility. Cellular IP [38] maintains distributed caches for location management and routing purposes. A distributed paging cache roughly maintains the positions of idle MNs in a service area. Cellular IP uses this paging cache to quickly and efficiently pinpoint idle MNs that wish to engage in active communications. Handoff aware wireless access Internet infrastructure (HAWAII) [39] is another domain-based scheme. The edge gateway, connecting the access network to the Internet core, is called the domain root gateway. Each gateway has a default route inside the domain pointing toward the domain root gateway. For every mobile node setting up its path, an entry for its IP address is added in the domain root gateway and associated with the appropriate interface.
4. TRANSPORT LAYER HANDOFF
MANAGEMENT
Handoff management can be done in the transport layer. Transport layer solutions follow the end-to-end principle [40] in the Internet, i.e., anything that can be done in the end system should be done there. Since the transport layer is the lowest end-to-end layer in the Internet protocol stack, it is a natural candidate for handoff management. Moreover, in the transport layer solutions, no third party other than the endpoints participates in handoffs, and no modification or addition of network components is required. TCP and UDP are the two most commonly used transport layer protocols in the Internet. Stream Control Transmission Protocol (SCTP) is a new IETF standardized transport layer protocol. We classify transport layer handoff management scheme as TCP/UDP-based handoff management schemes and SCTP-based handoff management schemes.
4.1. TCP/UDP-based Handoff Management Schemes
A transport layer mobility architecture called MSOCKS [41] is proposed to split a TCP connection that allows MNs to not only change their points of attachments to the Internet but also control which network interfaces are used for the different kinds of data leaving from and
arriving at the MNs. MSOCKS is built around a proxy that is inserted into the communication path between a MN and its correspondent hosts. The proxy can maintain one stable data stream, isolating the correspondent host from any mobility issues. Meanwhile, the proxy can simultaneously make and break connections to the MN as needed to migrate data streams between network interfaces or subnets.
Indirect TCP (I-TCP) is proposed in [42] to solve the handoff management problem. In this scheme, a TCP connection between the CN and gateway as well as an I-TCP connection between the gateway and MN is established to provide the end-to-end communication. The TCP portion remains unchanged during the lifetime of the communication and remains unaware of the mobility of MNs. In the I-TCP portion, when the MN moves from one subnet to another one, a new connection between the MN and the gateway is established and the old one is replaced by the new one. The transport layer TCP needs to be modified in this scheme. Mobile TCP (M-TCP) [43] is an enhanced version of I-TCP. The gateway maintains a regular TCP connection with the CN and redirects all packets coming from the CN to the MN. Compared to I-TCP, M-TCP requires less complexity in the wireless part of the connection. Mobile UDP (M-UDP) is proposed in [44] to support user mobility in traditional UDP protocol. Similar to M-TCP, M-UDP uses a gateway to split the connections between MN and CN to ensure continuous communication between them when a MN changes subnets.
A new set of migrate options for TCP are proposed in [45] to support handoff management. In this protocol, the MN and CN jointly determine a unique token number to identify the TCP connection when the connection is set up. Such token numbers are calculated according to the addresses/ports number of the peer nodes. Upon moving to a new location, MN will notify CN of its new IP address/port along with the token number so that the CN can update the information in the correspondent connection.
Freeze-TCP proposed in [46] lets the MN ‘freeze’ or stop an existing TCP connection during handoff by advertising a zero window size to the CN, and unfreezes the connection after handoff. This scheme reduces packet losses during handoff. However, the handoff delay is high.
A mapping layer between the IP layer and the TCP layer is proposed in References [47,48] to intercept all address update messages. The data packets will be replaced with the initial connection information before submitting to the upper layer. It also considers the situation that two peer nodes make handoffs at the same time. During dual roaming, MNs cannot deliver the address update messages to each other since the destination addresses are not in use any more. It is proposed in References [47,48] to address this problem by setting up a third party subscription/notification server. When a MN roams into a new network, it will send the new address to both the peer node and the notification server. If MNs cannot find each other after handoffs, they will track peer node’s new address from the notification server so that the connection can be maintained.
One critical issue with these approaches is that TCP and UDP have become the de facto standard used in millions of hosts for most Internet applications ranging from interactive sessions such as voice over IP (VoIP) calls and web browsing, to bulk data transfer. It is very difficult, if not impossible, to modify them according to the characteristics of specific wireless networks, which are merely the access networks in the global Internet.
Radial Reception Control Protocol (R2CP) [49] is based
on Reception Control Protocol (RCP), which is a TCP clone. In this protocol, the congestion control and reliability issues are moved from sender to receiver based on the assumption that the MN is the receiver and should be responsible for the network parameters control. Heterogeneous wireless connections and handoff between them are supported in R2CP.
4.2. SCTP-based Handoff Management Schemes
In recent years, a new IETF-standardized general-purpose transport layer protocol called SCTP [50] has gained significant attention as a candidate transport protocol for the next generation Internet. While it inherits many TCP functions, it also incorporates many attractive new features such as multi-homing, multi-stream, and partial reliability. Unlike TCP, which provides reliable in-sequence delivery of a single byte stream, SCTP has a partial ordering mechanism whereby it can provide in-order delivery of multiple message streams between two hosts. This multi-stream mechanism benefits applications that require reliable delivery of multiple, unrelated data streams, by avoiding head-of-line blocking. The multi-homing feature enables an SCTP session to be established over multiple interfaces identified by multiple IP addresses. SCTP normally sends packets to a destination IP address designated as the primary address, but can redirect packets to an alternate secondary IP address if the primary IP address becomes unreachable. The SCTP multi-homing feature and dynamic address reconfiguration extension [51], collectively referred as mobile SCTP (mSCTP) [52], can be used to solve the handoff management problem in next generation wireless networks in the transport layer by dynamically switching between alternate network interfaces. A vertical handoff scheme between WLANs and cellular networks using mSCTP is proposed in [53], in which an MN can be configured with multiple addresses in a connection with one address as the primary address. Before the MN makes a handoff, it will acquire another address from the new domain and set it as the secondary address in the connection. The handoff is then accomplished by asking the CN to set this address as the primary address and then giving up the old primary address. Using mSCTP to enable handoffs has many advantages, including simpler network architecture, improved throughput and delay performance, and ease of adapting flow/congestion control parameters to the new network during and after handoffs [53]. Figure 4
Figure 4. Protocol architecture of the mSCTP-based handoff management scheme.
shows the simplified protocol architecture of the mSCTP-based handoff management scheme. Assuming that both the mobile client and fixed server implement mSCTP, then there is no additional requirement to modify the protocols in other network nodes.
In [54], an improvement to mSCTP called Sending-buffer Multicast-Aided Retransmission with Fast Retransmission (SMART-FRX) is proposed. This scheme consists of two sub-schemes that perform different functionalities to enhance the mSCTP performance during WLAN to cellular forced vertical handoffs, namely: 1) The Sending-buffer Multicast-Aided Retransmission (SMART) sub-scheme that forces mSCTP to immediately enter into slow-start over the cellular link at the beginning of the handoff period, by multicasting the data buffered in the primary WLAN link on both the cellular and WLAN links; 2) The Fast Retransmission (FRX) sub-scheme that enables mSCTP to recover error losses on the cellular link by retransmitting over the same link, thus avoiding potentially long delays of error loss retransmissions over the possibly unreachable WLAN link. A new analytical model for SCTP is also proposed in [54] that takes into account the dynamic changes of the congestion window, the round trip time (RTT), the slow-start and congestion avoidance processes, and other factors that may affect the SCTP performance during vertical handoff in heterogeneous wireless networks.
5. APPLICATION LAYER MOBILITY
MANAGEMENT
In this section, we discuss application layer mobility management in general and handoff management in particular. The major advantages of handling mobility at the application layer are that it is flexible and does not require the installation of additional network equipment, although it generally does require more overheads and longer processing time [55]. Currently most application layer mobility management protocols have been implemented using SIP. Defined in RFC 3261 [56], SIP aims to manage voice/multimedia communications sessions over the Internet (e.g., for VoIP calls). According to RFC 3261, a SIP system comprises the following key components:
• User agent client for interfacing with a user;
• Proxy server for forwarding and re-processing functions;
• Redirect server for redirecting a request to another location; and
• Registrar for processing REGISTER requests typically for location management purposes.
SIP users are identified by uniform resource indicators (URIs) and SIP terminals/servers communicate with each other by exchanging text-based messages that are similar to the Hypertext Transfer Protocol (HTTP) messages. A SIP session is generally set up as follows. First a caller sends an INVITE message to the callee through the caller’s and callee’s proxy servers. If the callee can accept the call, it replies to the caller with a 200 OK message through the proxy servers. The caller then sends an ACK message to the callee. Subsequently the call session can be set up. When, for example, the caller wants to end the call, it sends a BYE message to the callee. Finally, to end the session, the callee sends a 200 OK message to the caller.
SIP can support different types of mobility management (see [57] for details). For pre-call mobility, a MN can update its current location to the home domain whenever it enters a new domain. When a CN wants to communicate with the MN, it first sends an INVITE message to the home domain. If the MN has moved, the CN then receives a 302 MOVED TEMPORARILY message, which includes the new location of the MN. After receiving the message, it sends another INVITE message to the MN. The MN finally sends a 200 OK message to set up the session. For mid-call mobility (i.e., handoff), whenever a MN moves to a new domain, it can initiate a handoff by sending a re-INVITE message to the CN to set up another session and to terminate the previous session. Some fast-handoff techniques have also been proposed [58] to reduce handoff delay. In general, these techniques allow a MN to receive data from an agent or a special component in the network when the new session is being set up during the handoff process. SIP can also support session mobility and personal mobility. For session mobility (i.e., session handoff), a mobile user can switch an ongoing session from one terminal to another terminal. This can be implemented using, for example, a call transfer approach. In this approach, a terminal can send a REFER message to the CN, providing information on the new terminal that should handle the session. The CN then sends an INVITE message to set up the session with the new terminal. Finally, the old terminal ends the previous session by sending a BYE message. For personal mobility, different cases can be supported by SIP. For example, a user address can be linked to multiple terminals or multiple user addresses can be linked to one terminal. Details can be found in [57] and its references.
In recent years, there has been considerable interest in using SIP to support mobility management, particularly handoff management for different networking applications or cases, as illustrated in Figure 5. In the following, we give an overview of three examples: integrated mobility management using SIP and MIP, SIP-based handoff
management for Universal Mobile Telecommunications System (UMTS)/WLAN-based systems, and SIP-based mobility management for peer-to-peer (P2P) networks.
In general, MIP and SIP are well suited for supporting TCP-based and UDP-based communications sessions, respectively. It is of interest to integrate them (e.g., to better serve a MN running both TCP-based data and UDP-based multimedia applications). In early proposals such as the EVOLUTE architecture [59,60], MIP and SIP are executed separately with almost no interaction. In [60], an integrated MIP and SIP protocol has been proposed to streamline the protocol operations using either a tightly integrated or a loosely integrated approach. In the tightly integrated approach, a common home mobility server is set up for MIP and SIP. This allows a MN to update its location for both MIP and SIP operations using a single MIP registration request. MIP and SIP are then used for handling handoffs for TCP-based and UDP-based communications sessions, respectively. Note that if both TCP-based and UDP-based sessions are used, SIP can be employed to enable route optimization for MIP (see [60]). If the tightly integrated approach cannot be implemented, a loosely integrated approach is proposed as an interim solution. In this case, it is assumed that the MIP HA can communicate with the SIP home registrar to facilitate location update. Again, the purpose of this is to eliminate duplication of effort in location update. When a MN enters a new domain, it updates its location to the MIP HA through a MIP registration request. The MIP HA then updates the SIP home registrar as well. Similar to the tightly integrated approach, handoffs for TCP-based and UDP-based communications sessions are handled by MIP and SIP, respectively.
SIP can be used to support mobility management for WLANs [61] interworking with UMTS [62,63]. Here as an example, we give an overview of the protocol presented in [63]. In future wireless networks, UMTS, and WLAN can be integrated to provide the so-called ‘always best connected’ services. Basically, UMTS can provide wide area services at moderate data rates whereas WLAN can provide local area services at higher data rates. When a mobile user enters the coverage area of a WLAN, an ongoing multimedia session can be handed over from UMTS to the WLAN through a vertical handoff process. Note that unlike a conventional horizontal or intra-technology handoff, this vertical handoff is optional and aims to provide better services. In [63], a SIP-based seamless handoff scheme has been proposed to facilitate the aforementioned vertical handoff using a soft handoff approach. Consider an MN that is equipped with both UMTS and WLAN interfaces. Suppose that the MN has a SIP session with a CN through the UMTS interface. After entering the coverage of a WLAN, it can initiate the vertical handoff by sending an INVITE message to the CN through the WLAN interface to set up another session. The INVITE message contains a special JOIN header, which provides the relevant information of the ongoing session. Note that during the handoff process, two sessions are maintained and it is assumed that duplicate packets can be deleted from the terminals. Having set up the session
Figure 5.Application scenarios of SIP-based mobility support.
through the WLAN interface, the previous session can then be terminated by sending a BYE message through the UMTS interface. Furthermore, the MN can also update its new location to the home domain. To reduce handoff delay, it is possible to pre-establish a new connection near a UMTS/WLAN boundary based on location information and movement prediction.
SIP can also support mobility management for P2P networks. Unlike other networks, P2P networks are based on a fully distributed architecture so that there are new technical challenges. In [64], a SIP-based mobility management protocol called SAMP is proposed to tackle these technical challenges. For location registration, an MN first needs to find an anchor SIP server on the P2P network by sending a REGISTER message to a SIP multicast address. After receiving the qualified replies (i.e., replies received within a certain period), the MN randomly chooses one of them as the anchor server. Note that the random choice is for load balancing. Having connected to an anchor server, the MN then updates its location by sending a REGISTER message to the home SIP server through the anchor server. A SIP terminal can set up a session with the MN by sending an INVITE message to the MN through the home SIP server and the anchor server. In [64], two optimization techniques are also proposed to enhance the system efficiency. First, a hierarchical registration scheme is used to allow an MN to register with an anchor server rather than the home server to reduce handoff delay. Second, a scheme employing a two-tier cache is proposed to reduce session setup delays. In this scheme, a CN first tries to set
up a session with the MN based on its cached address. Having received the request, the CN’s anchor server also tries to forward the message based on the cached location information for the MN. If the session cannot be set up using the cached information, the more time consuming standard session set up process will be executed based on the P2P network protocol.
6. RESEARCH CHALLENGES
In this paper, we have introduced recent advances in mobility management schemes, including location manage-ment and handoff managemanage-ment schemes that are applicable to future generation wireless mobile networks. We have reviewed many existing solutions that have been proposed for conventional circuit-switched cellular networks as well as contemporary packet-switched networks.
In general, location management is needed to locate the MT that is the recipient of a call, and if the location information is imprecise, then paging is used as a location management technique to find the MT within a wider area in which the MT may be located. While existing schemes are designed for cellular networks with centralized base stations in each cell, the current trend of extending cellular networks into mesh networks create new challenges in location management; e.g., what is the location information that needs to be reported, which network entities to report this information to, and how to utilize this information to efficiently deliver calls to the MTs? While flooding is widely
used as a method to establish initial routing in MANETs, in mesh networks where most relay nodes are fixed and terminals are mobile, and where a large number of MTs need to be served, unconstrained flooding over the network domain may not be efficient.
For intra-system handoff management between access points or base stations supporting a common wireless access technology, usually technology-specific handoff methods operating at the link layer and below are the most effective. On the other hand, inter-system handoff management is more involved due to possibly different wireless access technologies between different systems. Transport-layer handoff solutions can be very effective in such situations, as they conform to the end-to-end principal of the Internet; however, the nodes at both ends of the connections need to support the same version of the transport protocol that is enhanced to support vertical handoffs. In particular, it is clear that the new generation transport protocol, SCTP, includes features that would make it quite effortless to support inter-system handoffs. For multimedia connections that are set up using SIP, it is logical to use the same protocol to maintain continuity of a connection during a handoff. This is effective if the MN can switch network access in a rather seamless manner. Hence it is necessary to develop cross-layer techniques that coordinate the switching of access networks and the connection maintenance via SIP. Some researchers and practitioners suggest that future generation wireless networks will have an open architecture where access points and base stations function just like any other IP node that is capable of forwarding packets. Such an architecture naturally points to the use of MIP or MIPv6 for terminal mobility support. This paradigm is limited, however, by the assumption that each terminal has a permanent IP address assigned by its home network. In future wireless networks, a good portion of the MNs may in fact be nomadic in nature in that they do not have a fixed home network, but they still need fast and efficient mobility management when they move within a network domain or across different network domains. Development of new mobility management methods for such scenarios will be a new challenge. Finally, it is quite possible that a large number of MNs will be equipped with global positioning system (GPS) receivers, allowing them to determine their own geographic locations accurately. How geographical location information can be effectively used to enable efficient call delivery and seamless handoffs remains an open question. Furthermore, as not all MNs will be equipped with GPS, it would be of interest to develop mobility management techniques that take advantage of the GPS capability of some MNs to aid call delivery and handoffs for other less capable terminals.
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AUTHOR’S BIOGRAPHIES
F. Richard Yu received the Ph.D. degree in Electrical Engineering from the University of British Columbia in 2003. From 2002 to 2004, he was with Ericsson (in Lund, Sweden), where he worked on the research and development of 3G cellular networks. From 2005 to 2006, he was with a start-up in California, USA, where he worked on the research and development in the areas of advanced wireless communication technologies and new standards. He joined Carleton School of Information Technology and the Department of Systems and Computer Engineering at Carleton University, in 2007, where he is currently an Assistant Professor. He received the Leadership Opportunity Fund Award from Canada Foundation of Innovation in 2009 and best paper awards at IEEE/IFIP TrustCom 2009 and Int’l Conference on Networking 2005. His research interests include cross-layer design, security and QoS provisioning in wireless networks.
He has served on the Technical Program Committee (TPC) of numerous conferences and as the co-chair of ICUMT-CWCN’2009, TPC co-chair of IEEE INFOCOM-CWCN’2010, IEEE IWCMC’2009, VTC’2008F Track 4, WiN-ITS’2007. He is a senior member of the IEEE.
Vincent W. S. Wong received the B.Sc. degree from the University of Manitoba, Winnipeg, MB, Canada, in 1994, the M.A.Sc. degree from the University of Waterloo, Waterloo, ON, Canada, in 1996, and the Ph.D. degree from the University of British Columbia (UBC), Vancouver, BC, Canada, in 2000. From 2000 to 2001, he worked as a systems engineer at PMC-Sierra Inc. He joined the Department of Electrical and Computer Engineering at UBC in 2002 and is currently an Associate Professor. His research areas include protocol design, optimization, and resource management of communication networks, with applications to the Internet, wireless networks, RFID systems, and intelligent transportation systems. Dr Wong is an Associate Editor of the IEEE Transactions on Vehicular Technology and an Editor of KICS/IEEE Journal of Communications and Networks. He serves as TPC member in various conferences, including
IEEE Infocom, ICC, and Globecom. He is a senior member of the IEEE and a member of the ACM.
Joo-Han Song received the M.S. degree in Electrical Engineering from the Hongik University, Seoul, Korea, in 2001, and the Ph.D. degree from the University of British Columbia (UBC), Vancouver, BC, Canada, in 2005. He worked as a senior engineer in the 4G System Laboratory at Samsung Electronics, Korea from 2005 to 2007. In 2007, he obtained a postdoctoral fellowship at UBC studying security issues in vehicular ad hoc networks (VANETs). His research interests include routing and security for mobile ad hoc networks, design of MAC algorithms for 4G system, and performance evaluation and modeling of wireless networks.
Victor C. M. Leung received the B.A.Sc. (Hons.) degree in Electrical Engineering from the University of British Columbia (UBC) in 1977, and was awarded the APEBC Gold Medal as the head of the graduating class in the Faculty of Applied Science. He attended graduate school at UBC on a Natural Sciences and Engineering Research Council Postgraduate Scholarship and completed the Ph.D. degree in Electrical Engineering in 1981.
From 1981 to 1987, Dr. Leung was a senior member of Technical Staff at Microtel Pacific Research Ltd. (later renamed MPR Teltech Ltd.), specializing in the planning, design and analysis of satellite communication systems. In 1988, he was a lecturer in the Department of Electronics at the Chinese University of Hong Kong. He returned to UBC as a faculty member in 1989, where he is currently a Professor and the holder of the TELUS Mobility Research Chair in Advanced Telecommunications Engineering in the Department of Electrical and Computer Engineering. He is a member of the Institute for Computing, Information and Cognitive Systems at UBC. Dr Leung has co-authored more than 400 technical papers in international journals and conference proceedings. His research interests are in the areas of architectural and protocol design and
performance analysis for computer and telecommunication networks, with applications in satellite, mobile, personal communications, and high speed networks.
Dr Leung is a registered professional engineer in the Province of British Columbia, Canada. He is a fellow of IEEE, a fellow of the Engineering Institute of Canada, and a fellow of the Canadian Academy of Engineering. He has served on the editorial boards of the IEEE Journal on Selected Areas in Communications—Wireless Communications Series, the IEEE Transactions on Wireless Communications, the IEEE Transactions on Vehicular Technology, the IEEE Transactions on Computers, the Journal of Communications and Networks, Computer Communications, as well as several other journals. He has guest-edited special issues of several journals, and served on the technical program committee of numerous international conferences. He is the general chair of Adhocnets 2010 and general co-chair of BodyNets 2010 and CWCN 2010. He chaired the TPC of the wireless networking and cognitive radio track in IEEE VTC-fall 2008. He was the general chair of QShine 2007, and symposium chair for Next Generation Mobile Networks in IWCMC 2006-2008. He was a general co-chair of IEEE EUC 2009 and ACM MSWiM 2006, and a TPC vice-chair of IEEE WCNC 2005.
Henry C. B. Chanreceived his B.A. and M.A. degrees from the University of Cambridge, England, and his Ph.D. degree from the University of British Columbia, Canada.
From October 1988 to October 1993, he worked with Hong Kong Telecom-munications Limited primarily on the development of networking services in Hong Kong. Between October 1997 and August 1998, he worked with BC TEL Advanced Communications on the development of high-speed networking technologies and ATM-based services. Currently, he is an Associate Professor in the Department of Computing at The Hong Kong Polytechnic University. His research interests include networking/communications, wireless networks, Internet technologies and electronic commerce.
Dr Chan is a member of IEEE and ACM. He is currently serving as an executive committee member of the IEEE Hong Kong Section Computer Chapter.
