A New Method to Support Umts/Wlan Vertical Handover Using Sctp Li Ma, Fei Yu, and Victor C

MOBILITY AND RESOURCE MANAGEMENT

A NEW METHOD TO SUPPORT UMTS/WLAN VERTICAL HANDOVER USING SCTP LI MA, FEI YU, AND VICTOR C. M. LEUNG, THE UNIVERSITY OF BRITISH COLUMBIA TEJINDER RANDHAWA, BRITISH COLUMBIA INSTITUTE OF TECHNOLOGY

ABSTRACT Many proposals to solve the mobility manage- Fixed serve ment problem in heterogeneous wireless networks 3.3.3.1 This article proposes a new method to facili- are found in the literature. Mobile IP (MIP) [3] tate seamless vertical handover between wide- from the Internet Engineering Task Force (IETF) area cellular data networks such as UMTS and is a network layer solution. By inserting a level of WLANs using the Stream Control Transmission indirection into the routing architecture, MIP Router 3 3.3.3.0 Protocol (SCTP). The multihoming capability provides transparent support for host mobility, and dynamic address configuration extension of including the maintenance of active Transmission Internet SCTP are applied in an UMTS/WLAN overlay Control Protocol (TCP) connections and User architecture to decrease handover delay and Datagram Protocol (UDP) port bindings. In this improve throughput performance. Unlike tech- scheme, a home agent and a foreign agent are used niques based on Mobile IP or Session Initiation to bind the home address of a mobile host (MH) r 1 Ro Protocol, the SCTP-based vertical handover to the care-of address at the visited network and 22 0 scheme does not require the addition of compo- provide packet forwarding when the MH is mov- Unlike techniques based nents such as home/foreign agents or a SIP serving between IP subnets. Triangular routing of all er to existing networks. Therefore, the proposed incoming packets to the mobile host via the home on MIP or SIP, the scheme provides a network-independent solution network can cause additional delays and waste of preferred by service providers. Performance bandwidth capacity. If the correspondent host has SCTP-based vertical evaluations are presented to demonstrate the knowledge of where the MH is located, it can effectiveness of the proposed scheme. send packets directly to the care-of address of the handover scheme does MH, thus enabling route optimization. The Ses- sion Initiation Protocol (SIP)-based approach [4] not require the addition INTRODUCTION aims to keep mobility support independent of the The complementary characteristics of third-gener- underlying wireless access technologies and net- of components such as ation cellular networks such as the Universal work layer elements. SIP is an application layer Mobile Telecommunications System (UMTS) and protocol. When an MH moves during an active home/foreign agents or 802.11 wireless local area networks (WLANs) session into a different network, it first receives a make integrating these two technologies attractive new network address, and then sends a new ses- SIP server to the [1, 2]. While UMTS networks provide always-on sion invitation to the correspondent host. Subse- wide-area connectivity with relatively low data quent data packets are forwarded to the MH existing networks. rates to users with high mobility, WLANs offer using this new address. much higher data rates to users with low mobility Although both MIP- and SIP-based approach- over smaller areas. Contemporary mobile devices es can provide some level of vertical handover are increasingly equipped with multiple (e.g., support between UMTS and WLANs, experi- UMTS/WLAN) network interfaces, which enable ments have shown that it is difficult to maintain This work is based in part the mobile user to access the Internet using the the continuity of ongoing data sessions during on a article presented at higher bandwidth offered by a WLAN whenever handover due to the long handover latency [5, IEEE VTCfall, Orlando, possible, and using UMTS service otherwise. 6]. Mobile users may experience quality of ser- FL, Oct. 2003. Since mobile users accessing the Internet via vice (QoS) degradation or session disruption/ter- UMTS/WLAN are free to move, an efficient mination during vertical handovers if these This work was supported mobility management scheme is crucial in this approaches are used. In this article we introduce by grants from Telus integration. Mobility management consists of sup- a novel transport-layer scheme to support Mobility and the port for roaming, which provides reachability of UMTS/WLAN vertical handovers. Unlike tech- Advanced Systems Insti- mobile users, and support for handover (also niques based on MIP or SIP, this approach fol- tute of BC, and by the referred to as handoff in the literature), which lows the end-to-end principle [7] in the Internet: Canadian Natural Sci- provides ongoing connection continuity in spite of anything that can be done in the end system ences and Engineering movements across and between UMTS and should be done there. Since the transport layer Research Council under WLANs. Handovers between UMTS and WLANs is the lowest end-to-end layer in the Internet grant CRD247855-01. are commonly referred to as vertical handovers. protocol stack, it is a natural candidate for verti-

44 1536-1284/04/$20.00 © 2004 IEEE IEEE Wireless Communications • August 2004 cal handover support. Moreover, in the transport layer approach, no third party other than the endpoints participates in vertical handover, and Internet no modification or addition of network compo- server nents is required, which makes this approach universally applicable to all present and future network architectures. In addition, user mobility Internet in wireless networks has a significant impact on transport layer performance. A transport layer GGSN approach to vertical handover enables the end nodes to adapt the flow and congestion control Tight Loose parameters quickly, thus offering the potential coupling coupling SGSN for significant performance enhancements. This approach is used in [8], which proposes a new set of migrate options for TCP to support mobility. However, the approach in [8] requires glob- RNC WLAN gateway ally changing the widely deployed TCP, which is very difficult, if not impossible, in practice. A new transport layer protocol, Stream Con- trol Transmission Protocol (SCTP) [9], has recently been accepted by the IETF as a Request Node B Access point for Comments (RFC), joining TCP and UDP as a general-purpose end-to-end protocol above the IP layer. In this article we apply the multihoming feature and the latest dynamic address reconfig- WLAN coverage uration (DAR) extension [10] of SCTP, referred UMTS coverage to as the mobile extension of SCTP (mSCTP) [11], to support UMTS/WLAN vertical handover. SCTP was previously proposed to support Mobile client handover over homogeneous wireless networks [5]. However, experimental results in [5] show a GGSN:Gateway GPRS service node SGSN: Serving GPRS service node long interruption time during an SCTP han- RNC: Radio network controller dover. In this article we apply SCTP to support vertical handover between heterogeneous wire- Figure 1. Integrated UMTS/WLAN systems. less networks. We consider UMTS/WLAN vertical handover support via two types of SCTP configurations, single-homing asymmetric config- interworking [1, 2, 14]. Figure 1 shows the archi- uration [12] and dual-homing symmetric configu- tecture for UMTS/WLAN integration. ration [11], and apply SCTP message bundling In a tight coupling interworking architecture, [13] to reduce handover latency. The perfor- a WLAN is connected to an UMTS core net- mance of these configurations is evaluated by work in the same manner as other UMTS radio computer simulations. Results show that the access networks. The WLAN gateway imple- proposed scheme can overcome the problem of ments all the UMTS protocols (authentication, long interruption time during handover, espe- mobility management, etc.) required in the cially in the dual-homing SCTP configuration. UMTS radio access network. In this approach, The rest of this article is organized as follows. UMTS and WLAN would use the same authen- The next section describes the UMTS/WLAN tication, mobility, and billing infrastructures. The vertical handover problem. We present an main advantage of this solution is that the mech- overview of mSCTP. We describe the protocol anisms for mobility, QoS, and security in the architecture and procedures to support UMTS core network can be reused directly over UMTS/WLAN vertical handover using mSCTP. the WLAN. However, tightly coupled solutions We then present the simulation results to evalu- will be highly specific to the UMTS technology ate the handover latency and throughput perfor- and require extensive access interface standard- mance. Finally, we conclude the article. ization of WLANs beyond the existing standards. Moreover, the configuration and design of UMTS/WLAN VERTICAL HANDOVER UMTS network elements, such as the serving General Packet Radio Service (GPRS) support Since UMTS and WLANs will coexist to offer node (SGSN) and gateway GPRS support node Internet access to end users, the integration of (GGSN), have to be modified to sustain the these networks to allow seamless switchover of increased traffic from WLANs. services would be desirable from both the opera- In the loose coupling approach, the WLAN tor and end user perspectives. In this section we gateway does not have any direct connection to describe integrated UMTS/WLAN systems and UMTS network elements. Instead, it connects to several challenges in this integration, particularly the Internet. WLAN traffic would not go the issue of seamless vertical handover. through the UMTS core network. In this approach, UMTS and WLAN can use different INTEGRATED UMTS/WLAN SYSTEMS mechanisms and protocols to handle authentica- There are two different ways to design an inte- tion, mobility, and billing. Nevertheless, they can grated UMTS/WLAN network architecture, share the same subscriber database for functions defined as tight coupling and loose coupling such as security, billing, and customer manage-

IEEE Wireless Communications • August 2004 45 OVERVIEW OF MOBILE SCTP

Fixed server STREAM CONTROL TRANSMISSION PROTOCOL 3.3.3.1 SCTP was originally designed as a specialized transport protocol for call control signaling in voice over IP (VoIP) networks and has been specified by the 3rd Generation Partnership Pro- ject (3GPP) to carry call signaling traffic in UMTS Router 3 3.3.3.0 [15]. Recognizing that other applications could use SCTP’s capabilities, the IETF has embraced Internet SCTP as a general-purpose transport layer protocol. Like TCP, SCTP offers a point-to-point connection-oriented reliable delivery service for applications communicating over an IP network. It inherits many TCP functions and at the same time incorporates many attractive new features. Router 1 Router 2 The most interesting new features of SCTP 1.1.1.0 2.2.2.0 are partial reliability and multihoming. Unlike B A TCP, which provides reliable deliveries, and UDP, which provides unreliable deliveries, SCTP has a partial reliability mechanism, by which it Mobile client Mobile client 2.2.2.1 can configure a reliability level. The reliability 1.1.1.1 level defines how persistent an SCTP sender should be in attempting to send a message to the receiver (e.g., never retransmit, retransmit up to a certain time, and retransmit until lifetime Figure 2. SCTP support of seamless handover. expires). The partial reliability mechanism bene- fits real-time traffic transferred during periods of poor QoS due to path failures or network con- ment as peer IP domains. This scheme allows gestion. One application of partial reliability is the independent deployment and traffic engi- the delivery of real-time telephony signaling. neering of UMTS and WLAN. Network opera- Another core feature of SCTP is multihoming, tors and service providers can operate these two which enables an SCTP session to be established networks separately through roaming agree- over multiple interfaces identified by multiple IP ments. addresses. SCTP normally sends packets to a desti- It is shown in [2] that loose coupling offers nation IP address designated the primary address, several advantages over tight coupling, such as but can reroute packets to an alternative secondary independent deployment and traffic engineering IP address if the primary IP address becomes of UMTS and WLANs. unreachable. Accordingly, the path between two SCTP hosts using the primary address(es) is the VERTICAL HANDOVER BETWEEN primary path, and a path between two SCTP hosts involving a secondary address is a secondary path. UMTS AND WLAN Note that two SCTP hosts can have only one pri- Vertical handover between UMTS and WLAN mary path, but more than one secondary path. This can be seen as the next evolutionary step from type of session is defined as an association in roaming in this integrated environment. Consid- SCTP. An SCTP association between two hosts, er, for example, a laptop/handheld that supports say, A and B, is defined as both UMTS and WLAN access capabilities. The {[a set of IP addresses at A] + [Port-A]} end user of this mobile device is connected to the Internet via a WLAN at a hot spot. As the + {[a set of IP addresses at B] + [Port-B]}. user moves out of the coverage of the hot spot, Any of the IP addresses on either host can be the mobile device detects the failing WLAN cov- used as a source or destination address in the IP erage and switches the connection to a UMTS packet. Before data can be exchanged, the two network. Similarly, when a mobile user connect- SCTP hosts must exchange the involved IP ed to a UMTS network travels to a hot spot, the addresses in the association establishment stage. device detects the coverage of an overlaid The multihoming mechanism is originally WLAN. The end user may want to switch to designed for fault-resilient communications WLAN access to enjoy the higher bandwidth. between two SCTP endpoints over wired net- Ideally, the end user would not be required to works. This powerful feature has been exploited intervene in the vertical handover between these to support IP mobility using SCTP. Specifically, two networks, and the QoS should not be the SCTP DAR extension [10], referred to as degraded due to this handover. Therefore, the mSCTP [11], can provide a simple but powerful objective of designing a UMTS/WLAN vertical framework for mobility support over IP networks. handover scheme is to make handover as seamless (with low latency and negligible loss of data) MOBILE SCTP and efficient as possible. We introduce a new scheme to support UMTS/WLAN vertical hand- In the base version of SCTP, the endpoints over using mSCTP, which is described in the fol- exchange all the IP addresses before the SCTP lowing sections. association is established, and these IP addresses

46 IEEE Wireless Communications • August 2004 Since no addition or Applications Applications modification of network mSCTP mSCTP components is required, IPv4/IPv6 IPv4/IPv6 IPv4/IPv6 IPv4/IPv6 the proposed scheme UMTS WLAN Layer 2 Layer 2 Layer 2 layer 2 layer 2 and and and has a network and and layer 1 layer 1 layer 1 layer 1 layer 1 architecture that is Mobile clientNetwork nodes Fixed server much simpler than Figure 3. Protocol architecture. those required by cannot be changed during the session. However, in as it is possible for an interface to establish a con- network-layer or the integrated UMTS/WLAN environment, an MH nection to the Internet via an IP address, the may not have fixed, previously known IP addresses. interface can be added into the current associa- application-layer Therefore, the base version of SCTP cannot be tion. Particularly, mSCTP’s capabilities to add, used directly to support UMTS/ delete, and change the IP addresses dynamically solutions. WLAN vertical handover. Fortunately, the recently during an active SCTP association provides an proposed DAR extension [10] for SCTP enables end-to-end UMTS/WLAN vertical handover solu- the endpoints to add, delete, or change the IP tion. Since no addition or modification of network addresses during an active SCTP association using components is required, the proposed scheme has address configuration (ASCONF) messages. This a network architecture that is much simpler than forms the basis of mSCTP [11], the key address those required by network layer or application handling features of which are illustrated as follows. layer solutions. We describe the protocol architec- Without loss of generality, we use a client- ture and the procedure in the proposed vertical server model in the example, where a mobile handover scheme in the following subsections. client (MC) communicates with a fixed server (FS) using mSCTP, as shown in Fig. 2. In IP PROTOCOL ARCHITECTURE implementations, the outgoing interface of a Figure 3 shows the simplified protocol architec- multihomed host is often determined by the des- ture of the proposed scheme. Both the MC and tination IP address. The mapping of outgoing FS are assumed to implement mSCTP. In addi- source IP address and destination address is tion, we require both endpoints to implement done by a lookup in the host routing table main- SCTP message bundling. The MC supports both tained by the operating system. UMTS and WLAN at the physical and data link Assume that the MC uses IP address 1.1.1.1 at layers. There is no additional protocol require- location A. Traffic between the MC and FS is ment for other network nodes. To allow access routed through router 1. When the MC moves to any FSs over the Internet in general, and rec- from location A to location B, it detects the cover- ognizing that at the present time these FSs are age of router 2 and gets a new IP address, 2.2.2.1. likely to support TCP rather than mSCTP, the To add this new IP address to the SCTP associa- FS in Fig. 3 can in fact be a proxy server that tion, the MC sends an ASCONF(Add IP Address, provides mSCTP associations with MCs over 2.2.2.1) message to the FS. Note that the traffic is UMTS/WLAN while connecting to other FSs via still routed through router 1 since it is the primary TCP over the Internet. choice. During the overlap time, when the signal from router 2 becomes strong enough, the MC VERTICAL HANDOVER PROCEDURES sends an ASCONF(Set Primary Address, 2.2.2.1) Using the multihoming feature of SCTP, an MC message to the FS. Router 2 becomes the primary can have two IP addresses during vertical handover, router over which the MC’s traffic is routed. The one from the UMTS and the other from the routing tables are changed in the MC and FS WLAN. Similarly, an FS can also be configured for: accordingly. When the signal from router 1 • Single-homing: The FS provides only one IP becomes too weak to support communications, the address to support handover. MC deletes IP address 1.1.1.1 from the association • Dual-homing: The FS allows more than one by sending an ASCONF(Delete IP Address, 1.1.1.1) (usually two) IP addresses to support hand- message to the FS. over. Note that almost all servers in the current Inter- net are configured with only one IP address. SUPPORTING UMTS/WLAN Therefore, configuring each server with more VERTICAL HANDOVER USING MSCTP than one IP address is not an easy task. This is why the authors of [12] argue that it is natural to In this section we introduce a new scheme to sup- consider FS supporting handover with only one port UMTS/WLAN vertical handover using IP address as a fixed host should not add new IP mSCTP. The rationale behind the proposed addresses dynamically. However, the authors in scheme is that, due to the multihoming feature of [11] suggest that a server should use multiple IP mSCTP, from the association point of view it does addresses to provide the MC with multiple paths not matter whether an endpoint’s network inter- in order to fully take advantage of the existence faces belong to the same network or not. As long of a second interface at the MC for fault

IEEE Wireless Communications • August 2004 47 Note that almost all MC UMTS_IP MC WLAN_IP FS FS_IP servers in the current Data Internet are configured ASCONF (Add IP Address, WLAN_IP) with only one IP ASCONF_ACK address. Therefore, UMTS->WLAN ASCONF (Set Primary Address, WLAN_IP) configuring each server ASCONF_ACK with more than one Data

IP addresses is not ASCONF (Set Primary Address, WLAN_IP) an easy task. ASCONF_ACK WLAN->UMTS Data

ASCONF (Delete IP Address, WLAN_IP)

ASCONF_ACK

Figure 4. The vertical handover procedure (the FS is in a single-homing configuration).

resilience. Which configuration (single-homing or through the WLAN. The WLAN-to-UMTS hand- dual-homing) should be used in an FS supporting over is triggered by the MC sending an ASCONF handover is still an ongoing research topic. In message with parameters set to “set primary this article the detailed handover procedures of address” and UMTS_IP. After the MC receives both single-homing and multihoming configura- an ACK from the FS, the UMTS becomes the tions are presented, and the handover perfor- primary choice, and the traffic between the MC mance of the two configurations are compared. and the FS is routed though the UMTS. The vertical handover procedures of the sin- If the MC loses the signal from the WLAN gle-homing and dual-homing configurations are cell, it starts the delete IP address process. The shown in Figs. 4 and 5, respectively. For each of MC sends an ASCONF message with parame- these configurations, the handover procedure ters set to “delete IP address” and WLAN_IP to has three basic steps: request that the FS release the address • Add IP address WLAN_IP from its host routing table. After the • Vertical handover triggering MC receives an ACK from the FS, it deletes • Delete IP address WLAN_IP from its address list, and WLAN_IP Note that UMTS-to-WLAN handover is shown is released from the association. in the upper part, and handover in the reverse In this configuration, because of the hand- direction is in the lower part of each figure. The shake process, the overall handover delay can be handover procedures are described as follows. calculated as Delay = T + T , Single-Homing FS — In this case, an FS is config- overall ASCONF handover ured with only one IP address, say, FS_IP. where TASCONF, the ASCONF and ASCONF_ Assume that an MC has been allocated with an ACK transmission time, is IP address, UMTS_IP, in a UMTS cell and using   this IP address to communicate with the FS via ASCONF__ Chunk Size ×   mSCTP. When the MC moves into a WLAN cell 2  Bandwidth , covered by a UMTS cell, it gets a new IP  + Propagation_ Delay  address, WLAN_IP, and starts the add IP address process. The MC informs the FS of its and Thandover is the change-over command delay new IP address by sending an ASCONF message and buffered data transfer time. to the FS with parameters set to “add IP address” and WLAN_IP. Dual-Homing FS — In this case the FS is configured The vertical handover triggering process with two IP addresses, say, FS_IP_1 and FS_IP_2, allows the MC to trigger a handover based on as shown in Fig. 5. At the beginning of the proce- some decision rules. The UMTS-to-WLAN hand- dure, UMTS_IP and FS_IP_1 are the primary IP over is triggered by the MC sending an ASCONF addresses of the MC and FS, respectively. There message with parameters set to “set primary are two differences between this procedure and address” and WLAN_IP. After the MC receives that for a single-homing FS. The first difference an acknowledgment (ACK) from the FS, the is the add/delete IP address processes. In the dual- WLAN becomes the primary choice, and the homing configuration, when the FS responds to traffic between the MC and the FS is routed the MC’s add/delete IP address request with an

48 IEEE Wireless Communications • August 2004 The Vertical Handover MC UMTS_IP MC UMTS_IP FS FS_IP_1 FS FS_IP_2 Triggering process allows Data the MC to trigger a

ASCONF (Add IP Address, WLAN_IP) handover based on some decision rules. ASCONF_ACK bundles with ASCONF (Add IP Address, FS_IP_2) The UMTS to WLAN UMTS->WLAN ASCONF_ACK handover is triggered Data by the MC sending an Data ASCONF message with ASCONF (Delete IP Address, WLAN_IP) parameters set to

WLAN->UMTS ASCONF_ACK bundles with ASCONF (Delete IP Address, FS_IP_2) “Set Primary Address” and WLAN_IP. ASCONF_ACK

Figure 5. The vertical handover procedure (the FS is in a dual-homing configuration).

ACK, the FS bundles an ASCONF to request the Figures 6 and 7 show the delay performance MC to add/delete the FS’s secondary IP address for vertical handover from UMTS to WLAN and into/from the association. The MC then sends an in the reverse direction, respectively. When the ACK to confirm the completion of the add/delete FS is in single-homing configuration, the hand- IP address process. The second difference is in over delay is the time interval in which the FS the handover triggering process. Since both the receives the first packet on the new primary link MC and FS are in dual-homing configuration, and the last packet on the old primary link. the MC can directly set the FS’s secondary According to the simulation results, the UMTS- address as the primary destination in its host to-WLAN handover delay is 533 ms in Fig. 6a, routing table and start to send data on the new and WLAN-to-UMTS delay is 513 ms in Fig. 7a. link. In this case, the handover delay becomes When FS is in dual-homing configuration, the handover delay is the time interval in which the Delay = T . overall handover FS receives the same transmission sequence num- Since in the dual-homing configuration there ber on both links. These two handover delays are is no handshake process in the vertical handover reduced to 234 ms in Fig. 6b and 212 ms in Fig. triggering process, the handover delay is smaller 7b, respectively. This is because when the FS is in than that in the single-homing case. single-homing configuration, the MC sends a set primary address request to trigger a handover, SIMULATION RESULTS AND DISCUSSIONS thus increasing the overall delay with a handshake processing time. However, when the FS is in dual- In this section we present and discuss the simu- homing configuration, the MC can trigger a han- lation results of the proposed scheme. The objec- dover by directly setting the FS’s secondary tive of the simulations is to evaluate two critical address; therefore, the handover delays in both performance metrics, UMTS/WLAN handover directions are reduced significantly. delay and overall throughput for each of the two Figure 8 shows the throughput performance configurations described earlier. We use network for vertical handover in both directions. We can simulator ns-2 to perform the simulations and see that the throughput (bits per second) of an obtain the results reported in this article. We FS in a dual-homing configuration is much high- extend the SCTP module in ns-2 so that the er than that of an FS in a single-homing configu- multihoming feature can work over wireless ration. This is because, besides the delay links. The IEEE 802.11 WLAN model in ns-2 is advantage, a dual-homing FS allows both the used to represent the medium access control MC and FS to operate in a symmetric multi- (MAC) layer. The bandwidths are set to be 384 homed configuration. This configuration enables kb/s for the UMTS link and 2 Mb/s for the easy distinction of the two paths between the WLAN link. The network propagation delay is MC and FS, so the redundant path can help pro- set to 100 ms. FTP traffic is started at the MC at vide fault tolerance to data transmission during time 1 s. The handover triggering process is acti- handover. In the simulations buffered data are vated at time 5 s. We examine the impacts of the sent over both old and new connections when a different FS configurations on the delay and changeover of primary and secondary paths throughput performance. occurs. In this way, packet loss and retransmis-

IEEE Wireless Communications • August 2004 49 600 600

500 500

400 400 Packets on WLAN link Packets on WLAN link

300 300

200 200 Packets on UMTS link Packets on UMTS link Transmission sequence number Transmission sequence number 100 100

Handover time Handover time 0 0 0 1 2 3 4 5 6 78 0 1 2 3 4 5 6 78 (a) Time (s) (b) Time (s)

Figure 6. Delay performance of the proposed vertical handover scheme (from UMTS to WLAN) with the FS in a) single-homing; b) dual-homing configuration.

sion delay can be avoided. Duplicated packets been presented for current interest, the pro- are dropped by the receiver, and different strate- posed method is useful for supporting vertical gies may be employed by the sender and receiver handover between any heterogeneous wireless to adapt flow, congestion, and other QoS control networks in general and is not limited to UMTS parameters easily and quickly during and after and WLAN. We have studied different scenarios handover. In Fig. 8 we also observe that SCTP employing single-homing and dual-homing fixed readily copes with the sudden change of link servers to support handover. Simulation results bandwidth during a vertical handover. Going show that delay and throughput performance from low bandwidth to high bandwidth in a can be improved significantly using the dual- UMTS-to-WLAN handover results in SCTP homing configuration with message bundling. In going into slow start, whereas going in the the dual-homing configuration, duplicated reverse direction from high-bandwidth WLAN buffered data transmission over both old and to low-bandwidth UMTS, SCTP congestion new paths may help the receiver and sender to avoidance control is activated. adapt to a sudden change in link characteristics easily and quickly during and after a vertical CONCLUSIONS handover. A new method to support UMTS/WLAN vertical handover using SCTP, more specifically a REFERENCES dynamic address reconfiguration extension called [1] A. K. Salkintzis, C. Fors, and R. Pazhyannur, “WLAN- GPRS Integration for Next-generation Mobile Data Net- mSCTP, has been proposed in this article. works,” IEEE Wireless Commun., vol. 9, no. 5, Oct. Although UMTS/WLAN vertical handover has 2002. pp. 112–24.

600 600 Packets on UMTS link Packets on UMTS link 500 500

400 400 Packets on WLAN link Packets on WLAN link

300 300

200 200

Transmission sequence number 100 Handover time Transmission sequence number 100 Handover time

0 0 1 2 3 4 5 6 78 1 2 3 4 5 6 78 (a) Time (s) (b) Time (s)

Figure 7. Delay performance of the proposed vertical handover scheme (from WLAN to UMTS) with the FS in a)single-homing; b) dual-homing configuration.

50 IEEE Wireless Communications • August 2004 × 105 × 105 9 12

8 10 7 FS is in dual homing 6 8 FS is in dual homing 5 6 4 FS is in single homing

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2 FS is in single homing Overall throughput at receiver (b/s) Overall throughput at receiver (b/s) 2 Handover time 1 Handover time

0 0 2 3 4 5 6 78 2 3 4 5 6 78 (b) Time (s) (a) Time (s)

Figure 8. Throughput performance of the proposed vertical handover scheme; handover a) from UMTS to WLAN; b) from WLAN to UMTS.

[2] M. Buddhikot et al., “Integration of 802.11 and Third- national SS7 and GSM networks. From 2002 to 2004 he generation Wireless Data Networks,” Proc. IEEE INFO- was a research and development engineer at Ericsson COM ’03, San Francisco, CA, Apr. 2003. Mobile Platforms, Sweden, where he worked on dual-mode [3] C. E. Perkins, “IP Mobility Support,” RFC 2002, Oct. 1996. UMTS/GPRS handsets. He is currently a postdoctoral [4] H. Schulzrinne and E. Wedlund, “Application-Layer research fellow at UBC. His research interests are quality of Mobility Using SIP,” ACM Mobile Comp. and Commun. service, cross-layer design, and mobility management in Rev., vol. 4, no. 3, July 2000, pp. 47–57. wireless networks. [5] W. Xing, H. Karl, and A. Wolisz, “M-SCTP: Design and Pro- totypical Implementation of an End-to-End Mobility Con- VICTOR C. M. LEUNG [S’75, M’89, SM’97, F’03] (vleung@ece. cept,” Proc. 5th Int’l. Wksp., Berlin, Germany, Oct. 2002. ubc.ca) received a B.A.Sc. (Hons.) degree in electrical engi- [6] P. A. Pangalos et al., “End-to-end SIP based Real Time neering from UBC in 1977, and was awarded the APEBC Application Adaptation During Unplanned Vertical Gold Medal as head of the graduating class in the Faculty Handovers,” Proc. IEEE GLOBECOM ’01, San Antonio, of Applied Science. He attended graduate school at UBC TX, Nov. 2001. on a Natural Sciences and Engineering Research Council [7] J. H. Saltzer, D. P. Reed, and D. D. Clark, “End-to-end Postgraduate Scholarship and obtained a Ph.D. degree in Arguments in System Design,” ACM Trans. Comp. Sys., electrical engineering in 1981. From 1981 to 1987 he was vol. 2, no. 4, Nov. 1984, pp. 278–88. a senior member of technical staff at Microtel Pacific [8] A. C. Snoeren and H. Balakrishnan, “An end-to-end Research Ltd. (later renamed MPR Teltech Ltd.), specializing Approach to Host Mobility,” Proc. ACM Mobicom ’00, in the planning, design, and analysis of satellite communi- Boston, MA, Aug. 2000. cation systems. He also held a part-time position as visiting [9] R. Stewart et al., “Stream Control Transport Protocol,” assistant professor at Simon Fraser University in 1986 and IETF RFC 2960, Oct. 2000. 1987. In 1988 he was a lecturer in the Department of Elec- [10] R. Stewart et al., “Stream Control Transmission Protocol tronics at the Chinese University of Hong Kong. He joined (SCTP) Dynamic Address Reconfiguration,” draft-ietf-tsvwg- the Department of Electrical Engineering at UBC in 1989, addip-sctp-08.txt, Sept. 2003, work in progress. where he is a professor, holder of the TELUS Mobility [11] M. Riegel and M. Tuexen, “Mobile SCTP,” draft-riegel-tuex- Industrial Research Chair in Advanced Telecommunications en-mobile-sctp-03.txt, Aug. 2003, work in progress. Engineering, and a member of the Institute for Computing, [12] S. J. Kohet al., “Mobile SCTP for Transport Layer Information and Cognitive Systems. His research interests Mobility,” draft-sjkoh-sctp-mobility-03.txt, Feb. 2004, are in the areas of architectural and protocol design and work in progress. performance analysis for computer and telecommunication [13] R. Stewart and Q. Xie, Stream Control Transmission Proto- networks, with applications in satellite, mobile, personal col, a Reference Guide, Addison Wesley Longman, 2001. communications, and high-speed networks. He is a Fvoting [14] A. K. Salkintzis, “The EAP-GPRS Protocol for Tight Inte- member of ACM. He is an editor of IEEE Transactions on gration of WLANs and 3G Cellular Networks,” Proc. Wireless Communications and an associate editor of IEEE IEEE VTC ’03 Fall, Orlando, FL, Oct. 2003. Transactions on Vehicular Technology. He is the Technical [15] 3GPP, “UTRAN Iub Interface: Signaling transport,” Programming Committee (TPC) Co-Chair in networking for 3GPP TS 25.432, v. 6.0.0, Dec. 2003. IEEE WCNC 2005, New Orleans, Louisiana, and has served on the TPCs of numerous international conferences.

BIOGRAPHIES TEJINDER S. RANDHAWA ([email protected]) until LI MA ([email protected]) received a B.Eng. degree in recently was a research scientist at the New Media Innova- applied mathematics from Beijing University of Aeronautics tion Center (NewMIC), Vancouver, Canada, where he led and Astronautics in 1991 and an M.A.Sc. degree in electri- research and development of software defined radios, ver- cal engineering from the University of British Columbia tical handoffs, and mobile ad hoc networks in the Wireless (UBC), Canada, in 2004. From 1991 to 2000 she was a net- group. Prior to NewMIC, he worked in industry for 13 work engineer in the Technical Center of Guangdong Post years, holding senior positions with Acterna, Microtel Pacif- and Telecom, a software engineer in Singapore Telecom, ic Research (MPR) Teltech, MacDonald Dettwiler & Associ- and a network planner in C1 Communications Inc. Her ates, and Atomic Energy of Canada Ltd. He is a faculty research interests are UMTS/WLAN integration using member at the British Columbia Institute of Technology Stream Control Transmission Protocol (SCTP) and modeling and an adjunct professor at Simon Fraser University, and SCTP in wireless networks. has taught graduate and senior undergraduate level cours- es in wireless network protocols, data network protocols, FEI YU [S’00, M’04] ([email protected])received an M.S. distributed systems, network security, and database sys- degree in computer engineering from Beijing University of tems for several years. He received his Ph.D. in engineering Posts and Telecommunications in 1998, and a Ph.D. degree science from Simon Fraser University (2000). He has Mas- in electrical engineering from UBC in 2003. From 1998 to ter’s degrees from both Simon Fraser University (1997) and 1999 he was a system engineer at China Telecom, working the University of Saskatchewan (1988). He has co-authored on the planning, design, and performance analysis of a book and more than 25 IEEE technical papers.

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