TAKEOVER: A New Vertical Handover Concept for Next-Generation Heterogeneous Networks

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1 TAKEOVER: A New Vertical Handover Concept for Next-Generation Heterogeneous Networks Hyun-Ho Choi and Dong-Ho Cho Department of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology (KAIST) TEL: , FAX: hopper@comis.kaist.ac.kr, dhcho@ee.kaist.ac.kr Abstract One of the major challenges for seamless mobility in next-generation heterogeneous networks is vertical handover, which is the process of maintaining a mobile user s active connections as it changes its point of attachment. For seamless vertical handover, we introduce a new concept,, which enables a neighbor node to process requests of a mobile node. We develop protocol and operation for the takeover and apply it to vertical handover for next-generation heterogeneous networks. The proposed handover scheme using the takeover protocol reduces average handover latency and so enables a fast and seamless handover between two different access technologies. The simulation results show that the takeover-based vertical handover scheme achieves better performances compared with conventional handover schemes, with respect to handover delay, packet loss, and power consumption. I. INTRODUCTION Next-generation communication requires universal wireless access and ubiquitous computing through seamless personal and terminal mobility. One of the major challenges for seamless mobility is vertical handover, which is the process of maintaining a mobile user s active connections as it moves between different types of network. In order to achieve seamless vertical handover in heterogeneous network environments, it is necessary to guarantee service continuity and quality-ofservice (QoS), which means low latency and low packet loss during handover [1]. Generally, conventional handover schemes have been proposed by using pre-registration or post-registration method, which can reduce the handover delay because they separate layer 2 (L2) and L3 handovers and perform the L3 handover earlier or later than the L2 handover [2]. However, these scheme can not be applied in heterogeneous network environments without modification, because they need pre-defined signaling between two access nodes involved in handover. Moreover, current pre-registration or post-registration schemes consider only registration process during handover, and so it is difficult to apply conventional schemes to the handover between heterogeneous networks. For vertical handover, not only the registration procedure but also the of authentication and connection setup with a new network are required, and these should be performed seamlessly to maintain the service continuity during handover [3]. However, it is difficult to satisfy these requirements in the case of vertical handover because a mobile node () suffers from communicating with two different access networks (ANs) concurrently, and cannot receive enough signal strength from a new AN in order to complete all handover before attachment with the new network [4]. Therefore, in this paper, we propose a new vertical handover concept for next-generation heterogeneous networks. In this concept, we use a neighbor node () to solve the problems of vertical handover. Since an is able to communicate with any node, anywhere and any time in the next-generation communication environments, an can search an and can communicate directly with it [5]. Thus, if an s neighbor node is connecting with the AN where the will move, it is possible to let the perform operations needed for vertical handover instead of the. Namely, the takes over the vertical handover from the and carries out handover requiring large latency such as registration and authentication before handover completion. By using this operation, fast and seamless handover can be achieved. In this paper, this concept is named because the takes over s handover operations. We define signaling messages and protocol for takeover operations and apply it to proposed vertical handover scheme. This paper is organized as follows. In Section II, we describe the proposed takeover protocol and operation. In Section III, we provide details of the proposed vertical handover scheme using takeover concept. In Section IV, we show simulation environments and discuss simulation results. We present concluding remarks in Section V. II. TAKEOVER PROTOCOL happens between two s that can communicate each other. Fig. 1 shows the basic operation of takeover protocol in which tries to request a takeover to. operation consists of four phases: neighbor node discovery, takeover initiation, takeover process, and takeover completion phases. First, the broadcasts Ready-to- message to discover an that can take over requests of the. This /05/$20.00 (c)2005 IEEE

2 Neighbor node Discovery fromto 1. Ready to 2. Clear to process (pre-authentication, pre-registration) Initiation Process Completion 3. Request 4. Reply 5. Stop 6. Completion 7. Thank takes over and processes the request of oan Request Handover Completion nan Fig. 1. Basic operations of takeover protocol. Fig. 2. Operation of proposed vertical handover based on takeover protocol. message contains the address of the and the purpose of takeover. As a response of the Ready-to- message, the sends Clear-to- message with its address. If the confirms its by this message, it starts takeover initiation procedure with the through exchange of Request and Reply messages. The Request message includes specific contents of the request and information needed according to a kind of request (e.g., s identity number, authentication method, security key, and user profile). At this time, the can process the takeover request instead of the. If the wants to interrupt the takeover operation, it can send Stop message to the. When the finishes the takeover operation, it delivers to the Completion message together with the final information created as a result of successful takeover execution. Finally, the transmits Thank message to the. III. PROPOSED VERTICAL HANDOVER SCHEME We apply the takeover protocol to vertical handover scheme in heterogeneous network environments. Fig. 2 and 3 illustrate operation and signaling procedure for the proposed vertical handover based on takeover protocol, respectively. First, an that hands over from old AN (oan) to new AN (nan) decides to start handover or takeover based on signal strength from two ANs. If the wants to initiate the takeover process, it broadcasts Ready-to- message to search a suitable which can help the during vertical handover period. The Ready-to- message contains s address, aim of the takeover, and identity of the nan where the wants to move. If an is able to take over the s request, it replies Clear-to- message with its address. If the discovers the, which is currently connected with the nan, it initiates the takeover. Through Request message, the demands the to perform preauthentication and pre-registration, except L2 attachment, among all needed for vertical handover. Since these authentication and registration take long time compared with total handover latency, if these are executed previously by the takeover process, faster vertical handover is possible. Here, the pre-authentication procedure observes authentication method specified according to the nan, and the preregistration uses the mobile IP (L3) registration procedure. In order to make the process these pre-operations, the Request message should deliver the s identity number and IP address, information for authentication, and security key to the. If the receives the Request message, it sends Reply and performs the pre-authentication and pre-registration in regular sequence. Here, if the pre-registration is executed not by normal process but by the takeover process, the network router can know this takeover context and can bi-cast data traffic into two ANs, in order to reduce the packet loss during the vertical handover. If the receives the Reply message from the, it sets a timer T 1. This timer T 1 means a maximum time that the can wait until takeover process is finished, and so it should be set to larger value than time required for takeover process. Possibly, if the timer T 1 is expired before the takeover is not completed, the interrupts current takeover process by sending Stop message to the. In this

3 oan HO & TO decision fromto Ready to ['s address, aim of TO, nan's ID] Clear to ['s address] Request ['s ID & IP, Auth. info. & key] Reply Set timer T1 Process Pre-authentication nan TABLE I SIMULATION PARAMETERS Name Value radius of AN 1000 m radius of 100 m number of per AN 100 distance power gradient (γ) 2(ad hoc) or 4(cellular) noise power -130 db target SNR 0dB transmission rate of 1 Mbps speed of (km/h) s position update duration 0.2 sec s direction update duration 10 sec time needed for authentication 1sec time needed for L3 registration 1sec time needed for L2 attachment 0.2 sec time needed for L2 detachment 0.2 sec T1 timer 2.5 sec No oan detachment T1 expire? Yes Stop Completion ['s new IP & auth. key] Thank nan attachment Pre-registration Release pre-processing Fig. 3. Signaling procedure of proposed vertical handover based on takeover protocol. case, the can retry the takeover to another or attempt the handover directly to the nan. This decision is based on current location of the and signal strength from the oan and nan. If the accomplishes the takeover mission, it sends Completion message to the to report takeover results and related information such as the s new IP address and authentication key, which will be used after the attaches to the nan. If the receives the Completion message, it replies Thank message. Then, the performs L2 detachment procedure with the oan and L2 attachment procedure with the nan. Accordingly, the can connect with the nan through only L2 attachment procedure without new authentication and registration procedure and thus fast vertical handover can be achieved by this takeover process. By simple analysis [6], we can obtain handover delay when the takeover is not used (D wot O ) and handover delay when the takeover is used (D wt O )asfollows. D wot O = T detach + T auth + T reg + T attach (1) D wt O = T detach + T attach (2) Assuming that the takeover operation is successful, the han- dover scheme with takeover can save authentication delay (T auth ) and registration delay (T reg ). When the takeover is successful with probability p and the handover succeeds at all times, average delay of takeover-based vertical handover scheme becomes D wt O p + D wot O (1 p). IV. SIMULATION RESULTS A. Simulation Environments To evaluate the performances of proposed vertical handover scheme, we carry out computer simulation [3]. Table I shows parameters used for simulation. The radius of an AN is 1km and the radius of is 100m, which means maximum distance that two s can communicate each other. The number of AN is unlimited and all ANs are placed to satisfy that distance between adjacent two ANs is 3 km. The number of per an AN is 100. The path loss is simply modelled as A d γ, where A is constant set to 1, d is a distance between two nodes, and γ is distance power gradient, which is set to 2 in the communication between two s or 4 in the communication between the and AN [7]. We assume that both the and AN consume transmission power to satisfy target SNR of 0 db for demodulation when noise power is -130 db in the whole frequency bandwidth [8]. To give mobility into s, we change the speed of the from 3 to 100 km/h. We assume that the s position is updated every 0.2 sec and the s direction is changed every 10 sec. Considering actual latency required for authentication and mobile IP registration used in UMTS or WLAN system, the times needed to perfrom pre-authentication and pre-registration are assumed to be 1 sec, respectively. Hence, the value of T 1 timer is set to 2.5 sec because it should be larger than the sum of times for two pre-. In addition, L2 attachment and detachment are assumed to require 0.2 sec, respectively. Moreover, to achieve simulation results, we use some following assumptions.

4 Fig. 4. Successful probability of takeover vs. speed of. Fig. 5. Average handover delay vs. successful probability of handover. Every is located randomly with uniform distribution in whole AN coverage. Every can give and take takeover request. All handovers are vertical handover generated between two different ANs. If enters into handover area, every prefers to perform takeover process rather than direct handover process. Every is active node which is receiving downlink packets during vertical handover. When T 1 timer is expired, the carries out handover process directly only if signal strength of nan is greater than that of oan. Otherwise, the retries the takeover process. B. Results and Discussions We investigate probability of successful takeover execution, average handover delay, packet loss rate, and power consumption as performance measures in two cases that the takeover operation is applied or not. Fig. 4 shows successful probability of takeover versus speed of according to time consumed takeover execution. failure occurs when the connection between the and is broken or the timer T 1 is expired before the receives the Completion message. Here, we define takeover execution time (T takeover )asaduration from the Ready-to- message to the Thank message transmission. We assume that it takes from 2 to 4 sec for takeover execution, taking account of an searching, transmissions of signaling messages, takeover process (i.e, preauthentication and pre-registration), and processing delays at each node. As the speed of increases, the successful probability of takeover decreases. If the s speed is larger, i.e., the mobility is increased, it is difficult to maintain the connection Fig. 6. Expected number of dropped packets vs. playout time. between the and during the takeover process, and so the successful probability of takeover is decreased. Moreover, successful probability of takeover is reduced by the same reason as the takeover execution time is increased. Fig. 5 exhibits average handover delay versus successful probability of handover. When the takeover protocol is not used, the average handover delay is worst. If the takeover is applied, the handover delay is decreased because delay by both authentication and registration is removed by the takeover operation. In addition, the handover delay becomes better as the speed of is slower. This is because the successful probability of takeover is larger as the s speed is slower. Finally, the total vertical handover delay is reduced by using the proposed takeover operation. Fig. 6 shows expected number of dropped packets versus playout time. For the packet loss performance during handover, all ANs are modeled as simple M/M/1 queues [9]. The service

5 consider s energy consumption, the proposed takeover process induces more energy consumption as the s speed increases and the successful probability of takeover decreases. However, if we assume that the is a plugged terminal existed in handover region as a special case, we can ignore the s energy consumed by takeover process. Although the proposed vertical handover scheme requires more signaling cost and processing overhead of s, it can achieve better performances than conventional handover schemes that do not use the takeover protocol, with respect to handover delay, packet loss rate, and power consumption. Especially, the proposed handover scheme shows better performances as the takeover execution time is smaller and the velocity of s is slower, because these factors increase the successful probability of takeover operation. Fig. 7. Average energy consumed for handover per a node vs. speed of. time of a packet is assumed to be exponentially distributed, including the processing time and the transmission time. We assume that the AN transmits 100 byte packets every 100 ms to the, and the service rate is 1 Mbps in all ANs. In addition, all link delays between two nodes and processing delays are assumed to be 10 ms [10]. The playout time is the maximum allowed end-to-end delay between the AN and. Hence, the number of dropped packets falls as the playout time is increased. If the takeover is not used, more packet loss occurs compared with scheme using takeover method because packet loss duration out of overall vertical handover delay is increased. As the speed of is slower, the number of dropped packet shows better performance since the successful probability of the takeover increases and so the average packet loss duration is reduced. Therefore, the proposed vertical handover scheme using takeover can guarantee a low packet loss rate because fast handover is achieved by the takeover process. Fig. 7 displays average energy consumed for handover per a node versus speed of. We measure the energy consumption when the takeover process is used or not, according to speed of. When the takeover process is applied, the energy consumed in the or is measured. An s energy required for handover without the takeover and an s energy required for takeover process have almost constant though the speed of is varied, because these values are not affected generally by s mobility. However, in the case of handover scheme with takeover, energy consumption per an is increased if the speed of is larger. That is, s energy consumption is increased by more takeover failure as the speed of is greater. Considering only s energy consumption, the scheme with takeover shows better performance than the scheme without takeover in every speed of. However, the sum of energy consumption of both and is not better when the speed of is more than about 25 km/h. If we V. CONCLUSIONS For seamless vertical handover, we introduce a new concept, takeover, which enables an to process requests of an. We develop protocol and operation for the takeover and apply it to vertical handover for next-generation heterogeneous networks. The simulation results show that the takeoverbased vertical handover scheme achieves faster and seamless handover compared with conventional handover schemes without the takeover. Therefore, our proposed vertical handover scheme guarantees seamless mobility and service continuity by using a novel concept of takeover in heterogeneous networks. This takeover concept is also applicable to various situations in which an is restricted by its coverage, battery power, resource, and so on. REFERENCES [1] J. 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