A Group-based Handoff Scheme for Correlated Mobile Nodes in Proxy Mobile IPv6 Yong Li, Yurong Jiang, Haibo Su, Depeng Jin, Li Su, Lieguang Zeng State Key Laboratory on Microwave and Digital Communications Tsinghua National Laboratory for Information Science and Technology Department of Electronic Engineering, Tsinghua University, Beijing 100084, China Email:
[email protected]
Abstract—Proxy Mobile IPv6 (PMIPv6), a network-based IP mobility solution, is a promising approach for mobility management in all-IP wireless networks. How to enhance its handoffrelated performance, such as handoff delay and signaling cost, is an important issue. Current solutions rely on approaches such as fast handoff, routing optimization and paging extension. However, the case of many correlated Mobile Nodes (MNs) moving together and taking handoffs at the same time has not been considered. In this paper, we propose a group-based handoff scheme for correlated MNs to enhance the performance of PMIPv6. We first propose a correlated MNs detection algorithm to detect MNs as groups. Based on this algorithm, we propose a groupbased handoff procedure, and discuss its benefits and limitations. Furthermore, we evaluate the performance of PMIPv6 and our proposal through the analysis and simulation. The results show that the proposed scheme is very efficient in reducing both the handoff delay and signaling cost.
I. I NTRODUCTION With the rapid development of mobile devices and radio access technologies, wireless networks are evolving towards all-IP systems. It is important and urgent for the IP layer of networks to support efficient mobility management due to its transparency to applications and independence with regard to link layer access technology. Proxy Mobile IPv6 (PMIPv6) [1], an IP layer mobility solution, currently is being actively discussed and developed by Internet Engineering Task Force (IETF). Unlike the host-based mobility protocols such as Mobile IPv6 [2][3], PMIPv6 is a network-based approach to solve the challenges associated with IP layer mobility. By using a proxy mobility management agent to perform mobility-related signaling on behalf of the mobile node (MN), PMIPv6 achieves the function of mobility without the MN’s involvement. It does not require the MN to modify its protocol stack and get involved with mobility-related signaling exchange, which accelerates the deployment of IP mobility management. Therefore, PMIPv6 is a promising approach for realizing future all-IP wireless networks [1][4][5]. PMIPv6 introduces two functional entities, Local Mobility Anchor (LMA) and Mobile Access Gateway (MAG). The MAG maintains data paths for the MNs, and the LMA maintains a collection of routes for individual MNs while the MNs roam in the PMIPv6 domain. As all handoff-related signaling generated by the MAGs and packets to and from the MNs are concentrated into LMA, PMIPv6 is a centralized protocol
[6]. This architecture limits its scalability. Minimizing the signaling overhead and optimizing the handoff procedure are important issues. At the same time, reducing handoff delay is significant to the quality of service for applications. Solutions such as fast handoff [7], routing optimization [8] and paging extension [6] are proposed to address these issues. However, the case of many correlated MNs moving together and taking handoff process at the same time has not been considered. For example, passengers in a car are always located in the same network and move in the same direction [9]. In addition, family members and friends tend to spend a significant amount of time together [9]. When correlated MNs move together in a group and handoff from one MAG to another at the same time, handoff performance could be enhanced if the group information is used. In this paper, we propose a group-based handoff scheme for PMIPv6 to reduce handoff delay and signaling cost. According to MNs’ physical layer Signal-to-Noise Ratio (SNR) and history handoff information, we design a correlated MN detection algorithm to group MNs. Based on the detection algorithm, we propose a group-based handoff procedure relying on group information, which is the outcome of the detection algorithm. In order to shorten the handoff delay and reduce the signaling cost, this new handoff procedure transmits handoff information by group. Unlike Network Mobility (NEMO) solutions [10], our scheme can work in an ad hoc environment, where the infrastructures for NEMO such as mobile routers do not exist in a group. Analysis and results are given to verify the performance improvement of this new scheme. The remainder of this paper is organized as follows: In section II, we review the protocol details of PMIPv6. Then, in section III and IV, we present the group-based handoff scheme and discuss its beneficial features and limitations. In section V, we introduce the simulation framework and analyze the results. Finally we conclude the paper in section VI. II. PMIP V 6: P ROTOCOL OVERVIEW With respect to deployment, PMIPv6 does not need the MN to modify any software for mobility management [11], and with respect to performance, it tries to reduce the handoff related signaling cost and handoff delay by separating the MN from the involvement of handoff procedure [11]. Based
978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
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MAG (n-MAG). When the p-MAG detects the detachment of an MN from its link, it removes the binding and routing state for that MN, and sends a de-registration (DeReg PBU) message to LMA to end the packets delivery tunnel. Upon receiving this request, the LMA accepts the request and waits for the n-MAG to update the binding on a new link. Once the MN attaches to the n-MAG, it sends an RS message to the nMAG. Then, the n-MAG performs a binding update procedure and sets up a tunnel between the n-MAG and LMA for the MN to deliver packets. After the completion of this operation, the MN receives an RA message containing the MN’s home network prefix from n-MAG. This will ensure that the MN does not detect its change of IP layer attachment. III. C ORRELATED MN S D ETECTION A LGORITHM
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Signaling call flow for mobile node’s handoff.
on the fact that MIPv6 is a mature protocol with several implementations, PMIPv6 reuses the home agent functionality and message format of MIPv6 [1]. Once an MN enters a PMIPv6 domain and attaches to an access link, the MN attachment procedure described in Fig. 1 is performed. After the MAG receives a Router Solicitation (RS) message from the MN, it will perform an access authentication using the MN’s link layer identifier by sending a Query message to the AAA Auth server. After successful authentication, the MAG gets the MN’s profile and then sends a Proxy Binding Update (PBU) message containing the identifier of the MN to the LMA on behalf of the MN. Once the LMA receives the PBU message, it performs access authentication to make sure that the PBU message is authorized. If the MAG and MN are trustworthy, the LMA will accept the PBU message, store a binding cache for the MN and send a Proxy Binding Acknowledgment (PBA) message including the MN’s home network prefix. The MAG sends a Router Advertisement (RA) messages to the MN in order to advertise the MN’s home network prefix. Then, the MN is able to configure an IP address and use the tunnel between the MAG and the LMA to send or receive packets. After the attachment procedure is completed, the MN’s IP address will stay the same while it moves within the PMIPv6 domain. Fig. 2 shows the signaling call flow for MN’s handoff from previously attached MAG (p-MAG) to the newly attached
In this section, we give the system model and propose an algorithm to detect correlated MNs. The goal of this algorithm is to group correlated MNs, and the proposed group-based handoff procedure relies on this algorithm. A. System Model and Problem Statement We assume that there is one centralized LMA in the PMIPv6 domain, which is the home agent for all MNs, and each MAG in this domain has a coverage area. The topology of MAGs is modeled as an N × N mesh network. For example, Fig. 3(a) shows a 5 × 5 mesh PMIPv6 domain. The MN’s Signal-to-Noise Ratio (SNR), which is widely used as handoff trigger [12], is considered as an important parameter to detect correlated MNs. We use two technologies to eliminate fading effects such as fast fading and slow fading to SNR. One is that the SNRs used in the detection algorithm are from both the current serving MAG and the adjacent MAGs. The other is that the average SNR is used. The average SNR of MN i from MAG ∂ at the time t, denoted as Si,∂ (t) , is defined as follows: Si,∂ (t) =
1 Tavg
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[12]
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where Tf is the frame length, Tavg is the duration time used to computer the average SNR, and SN Ri,∂ is the instantaneous SNR of MN i from MAG ∂. The values of SNRs from the serving MAG and from the adjacent MAGs have different impacts. To address this,
978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
an impact factor vector θ is considered. If the MN is in the MAG ∂0 , the number of adjacent MAGs is 8, from in ∂1 to ∂8 , which√ is shown √ √ Fig.√ 3(a). The θ is set to {1, 1, 1, 1, 1, 1/ 2, 1/ 2, 1/ 2, 1/ 2} according to their relative position to the current serving MAG ∂0 . Then, we define SNR correlation factor for MN i and j at the time t, denoted as Ci,j (t), as follows: 8
Ci,j (t) =
k=0
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3) If A \ B = ∅, go to step 2). Otherwise, select an MN in A \ B, named as j, and let B = B ∪ {j}. 4) Select any MN in Gm , named as k. If Equation (1) is satisfied, let Gm = Gm ∪ {j}, G = G ∪ Gm , and go back to step 3). Otherwise, go to step 3) directly. Ck,j (t) ≤ Dk (n − 1) = Dj (n − 1) Sk (n − 1) = Sj (n − 1)
|Si,∂k (t − nΔT ) − Sj,∂k (t − nΔT )|
9N where ΔT is the length of the time slot, and N is the number of time slots. If Ci,j (t) ≤ ,where is the predefined threshold, MN i and j may be correlated MNs, and they may be grouped into a handoff group. But using only the SNRs of MNs, the group information may still be incorrect. For example, MN handoff from ∂0 to ∂1 and MN handoff from ∂0 to ∂3 may be detected as one group because their correlation factor is small enough. But in fact their mobility patterns may not be correlated. Therefore, the position of the MN must be considered. When an MN i moves from MAG ∂ to MAG β at the time t in the nth handoff, we save this as history handoff information for future use, as follows: Si (n) = ∂, Di (n) = β, Ti (n) = t. where Si (n), Di (n) and Ti (n) denote the source MAG, the destination MAG and the time when nth handoff of MN i takes place, respectively. As the nth handoff occurs, we can use the last time, (n−1)th handoff information to decide whether their mobility patterns are correlated. If two MNs handoff from the same MAG to another, and the interval between the moments when their handoffs takes place is less than the predefined threshold, says TΔ , we assume their mobility patterns are correlated. B. Correlated MNs Detection Algorithm (CMDA) In this subsection, we propose a Correlated MNs Detection Algorithm (CMDA) to group MNs according to their SNR correlation factor and the last handoff information. The CMDA is executed in the LMA. The LMA is a centralized entity in the PMIPv6 domain, thus it can obtain related information easily from its domain. The LMA executes CMDA for each MAG and classifies MNs in each MAG into groups. Details of the algorithm are as follows. 1) Let i denote the number of MNs in the current MAG. Let M represent the MN set, and initialize it as M = {1, 2, 3, · · ·, i}. Let G represent the set of MNs, which have been grouped, and initialize it as G = ∅. Let the group index m = −1. Let Gm represent the mth group MN set, and initialize it as Gm = ∅. 2) Let A = M \ G (set M excepts set G). If A = ∅, go to step 5). Otherwise, select an MN i ∈ A, let m = m + 1, add i to Gm , let G = G ∪ Gm , and let the temporary MN set B = ∅.
,
(1)
|Tk (n − 1) − Tj (n − 1)| ≤ TΔ 5) end The outcome of this algorithm is a list of handoff groups, named G0 , G1 , G2 · · · Gm , and this information is located in the LMA. To take advantage of this information, we propose a new handoff procedure for PMIPv6 in next section. IV. G ROUP - BASED H ANDOFF S CHEME FOR C ORRELATED MN S A. Group-based Handoff Procedure In every time slot, the MAGs calculate and send SNRs of all MNs in both its current and adjacent domains to the LMA. When a handoff procedure of an MN is performed, the LMA will classify MNs into groups according to CMDA. Using these handoff groups, we can enhance the handoff performance of PMIPv6 in two ways. One is to reduce the handoff signaling cost by sending the PBA (Proxy Binding Acknowledgment) message per group, not per MN, and saving some additional unnecessary handoff messages such as PBU (Proxy Binding Update) and DeReg PBU (de-registration PBU). The other is to decrease the handoff delay by simplifying the handoff procedure. Based on those considerations, we propose a groupbased handoff procedure for correlated MNs. Fig. 4 shows the handoff procedure for a group of MNs moving from previous MAG (p-MAG) to the new MAG (nMAG). The handover procedure is performed as follows: Step 1 The p-MAG sends a DeReg PBU message to the LMA. When the p-MAG detects an MN’s detachment from its link, it sends a DeReg PBU message, including the link layer identifier of the MN, to the LMA. Step 2 The LMA searches the current handoff group and sends a group-based PBA message to p-MAG. Once the LMA gets the current handoff MN by the DeReg PBU message, it uses the CMDA algorithm to group MNs for the current serving MAG. Then, it searches the group in which the current handoff MN is. Based on the searching results, it sends a group-based PBA message to p-MAG as follows: If the MN is alone, which means there is only one MN in its handoff group, the PBA message contains only the current MN link layer identifier. Otherwise, the PBA message contains link layer identifiers of all MNs in its group. Then, the LMA waits for updating the binding of this group. When the p-MAG receives the PBA message, it knows all MNs in the current handoff group. Therefore, it will not send other DeReg PBU
978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
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The group-based Handoff procedure for PMIPv6.
messages to the LMA if it detects the detachments of other MNs in the same group. Step 3 The n-MAG sends a PBU message to the LMA after receiving an RS message from the first handoff MN in the group. When an MN attaches to the n-MAG, it will send an RS message to the n-MAG to ask for the network prefix. Then, the n-MAG sends a PBU message with the link layer identifier of this MN. Step 4 The LMA sends a group-base PBA messages to the n-MAG according to the handoff group. After the LMA receives the PBU message, it is able to identify the current handoff group because of step 2. According to the group information, it sends a group-based PBA message with link-layer identifier and home network prefix of all MNs in the current handoff group. Step 5 Handoff for the rest MN of the group. When other MNs in the group attach to the n-MAG, they send an RS message to n-MAG to ask for the home network prefix directly. The n-MAG responds with an RA message at once because the network prefix of the current handoff MN has been obtained after the first MN’s handoff. Therefore, the handoff delay of MNs, except for the first handoff MN, is significantly decreased. B. Discussions 1) Protocol related consideration: In the design of CMDA, we try to classify the MNs into a handoff group correctly. However, since the real condition is complex, it may get wrong results. For example, users in a group may not be detected by the CMDA, or users not in a group are detected as a group. Fortunately, the proposed handoff procedure is compatible with those situations. For example, when an MN has not been detected into a group, it will take the handoff procedure of normal PMIPv6 described in section II. After it moves into the n-MAG, it sends an RS message to the nMAG, and the n-MAG will not find its handoff information and sends a PBU message to the LMA. After the n-MAG receives an FBA message from the LMA, it sends an RA message with home network prefix to this MN. For another
example, when an MN is detected into a group by mistake, that means its handoff information, which includes its linklayer identifier and home network prefix, is sent to a n-MAG but the MN will not move to this n-MAG, the LMA will find that it is still in the p-MAG, the binding information will not be deleted, and the communication will continue. In order to support our proposal, the only change to the message format of PMIPv6 standard [1] is the PBA message. The PBA in PMIPv6 is used to acknowledge the binding update of one MN, but it is used to acknowledge the binding update of a group of MNs in the group-based handoff procedure. Therefore, the PBA must be allowed to take along more than one link-layer identifiers and home network prefixes. In the PBA message, the link-layer identifier and home network prefix are sent by Home Network Prefix option and Mobile Node Link-layer Identifier option, respectively. Therefore, the group-based PBA message will contain more than one Home Network Prefix options and Mobile Node Link-layer Identifier options in the group-based handoff scheme. 2) The Group-based handoff scheme VS NEMO solutions: The Network Mobility (NEMO) is concerned with managing the mobility of an entire network, which includes one or more mobile routers and a group of MNs. It is mainly used in automotive and aviation communities, where infrastructures such as mobile routers of a group exist. However, in a more general scenario, the MNs are in an ad hoc structure, and it is impossible to deploy the mobile routers. For example, when family members or friends walk together as a group, our group-based handoff scheme works well. There is no need to install infrastructure-related devices such as mobile router in the group. This is the main difference between our approach and NEMO solutions. Furthermore, the handoff group in our scheme is dynamically, as opposed to statically, detected by the CMDA algorithm to adopt the changes of MNs in a group. In the NEMO, none of the MNs behind the mobile routers need to be aware of the network’s mobility. This is similar to our group-based handoff scheme for PMIPv6, where the MN cannot detect its IP layer handoff when it moves from one MAG to another. 3) Benefits analysis: The improvement of our group-based handoff scheme includes two aspects. The first is that the signaling cost is reduced significantly. In the group-based handoff scheme, the correlated MNs information is used to reduce mobility-related signaling exchange. For the first handoff MN, the MAG needs to exchange the PBA/PBU messages with the LMA. But the rest of MNs in the group only need to exchange the RS/RA messages to complete the handoff procedure. The second is that the average handoff delay is reduced. From the handoff procedure of PMIPv6, we know that the IP layer handoff delay of each MN is the sum of one round trip time (RTT) between the MN and MAG, one RTT between the MAG and LMA, and the packets process time in the MAG and LMA. However, in our group-based handoff scheme, only the first handoff MN needs such long delay. The rest of the MNs in the group only need one RTT between the MN and the MAG to complete the handoff process. In the next section,
978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
In this section, we develop an simulation framework to evaluate the performance improvement of our group-based handoff scheme. The performance parameters include handoff delay and signal cost, which are the most important two performance metrics in the mobility management protocols. The handoff delay is defined as the time interval between the time when MN can not receive packets in the p-MAG and the time when it receives first packet in the n-MAG, and the signaling cost is defined as the number of packets involved in handoff procedure. From the handoff procedure described in Fig. 2, we can obtain the handoff delay of one MN in PMIPv6, denoted by T1 , as follows: T1 = tlink + 2tM N −M AG + 2tLM A−M AG , where tlink is the link layer handoff delay, tM N −M AG is the delay time for transmitting a message between the MN and MAG, and tLM A−M AG is the delay time for transmitting a message between LMA and MAG. Similar to the above analysis, from Fig. 4, we can get the handoff delay of one MN (except for the first handoff MN) in the group-based handoff scheme, denoted by T2 , as follows: T2 = tlink + 2tM N −M AG . Similar to paraments analysis in [13], tM N −M AG includes the time spent on generating the message and waiting (twait ), the time needed to access the wireless channel (taccess ), and the time for transmitting the message (ttran ). Therefore, we can obtain that: tM N −M AG = twait + taccess + ttran . Similar to the above analysis, tLM A−M AG can be expressed as follows: tLM A−M AG = n(twait + ttran ), where n is the hops between the LMA and MAG, and taccess is deleted since the links between LMA and MAG are wired. We use the M/M/1 queuing model to represent the process of sending packets. Therefore, twait = 1/(μ − λ) where μ is the packet transmission rate and λ is the packet arrival rate. We assume that the wireless channel access follows IEEE 802.11 CSMA mechanism. Therefore, taccess = 20 ∗ CW us [13] where CW is a random choose value from 2i − 1, i = 5, 6, ..., 10. Furthermore, we assume that the wireless transmission delay is 1 us and wired transmission delay is d/(2 ∗ 108 ) m/s where d is the wired link distance of one hop. The parameter settings in the simulation are as follows: tlink is chosen from the uniform distribution on [20, 22] ms, n is set to 10, λ is chosen from the uniform distribution on [3220, 4500] pkt/sec, μ is chosen from the uniform distribution on [4600, 5000] pkt/sec, d is set to 1000 m, the number of MNs
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is set to 100000, the ratio of the MNs travel in group is set to 80%, and the group size is set to vary from 10 to 50. In order to evaluate the effect of CMDA’s false detection, denoted by P , we set its false detection varies from 5% to 20% in the simulation. The outcome includes the likelihood (histogram) of handoff delay occurring within the whole simulation scenario, the average handoff delay, and the average signaling cost of one MN, which are shown in Fig. 5, Fig. 6, and Fig. 7. A. Average Handoff Delay and Signaling Cost In this section, we analyze the enhancement of our proposal and the impact of CMDA on the mobility-related performance including signaling cost and handoff delay. Fig. 5(a) shows the average handoff delay of one MN according to the group size. As the values of false detection P becomes larger, the average handoff delay is increased. However, even when the false detection P equals 20%, the average handoff delay of the group-based handoff scheme is reduced by 23% comparing with that of PMIPv6. This is because in our group-based handoff procedure, only the first handoff MN needs a long delay. The rest of the MNs in the group only need one RTT between MN and MAG to complete the IP layer handoff procedure. Therefore, the average handoff delay is reduced. Fig. 5(b) shows the average mobility-related signaling cost of one MN according to the group size. With the increase of the group, the system signaling cost decreases. As the values of false detection P becomes larger, the average signaling cost is increased. When the false detection P equals 20%, the average signaling cost of group-based handoff scheme is about 3 packets, approximately 50% of PMIPv6, which is 6 packets. The reason is that for the first handoff MN, the MAG needs to exchange PBA/PBU messages with the LMA. But the rest of the MNs in the group only need to exchange RS/RA messages with MAG. B. Handoff Delay Distribution In this section, we investigate the distribution of the handoff delay of one MN in the simulation. Fig. 6 shows the histogram of the PMIPv6 protocol’s handoff delay for each MN. This simulation result shows that in more than 80% of cases, the handoff delay of PMIPv6 is in the range between 40 and 65 ms. The time for link layer
978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.
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The authors would like to thank the anonymous reviewers for providing valuable comments. This work is supported by the National Basic Research Program (No. 2007CB310701), National Natural Science Fund (NNSF-90607009) and National High Technology Research and Development Program (No. 2008AA01Z107 and No. 2008AA01A331).
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almost 50% of MNs’ delay is reduced about 23 ms, and with respect to signaling cost, there is a reduction of approximately 50% compared with PMIPv6.
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R EFERENCES handoff and the time consumed by the RS and RA messages exchange are necessary for the handoff procedure. However, the time consumed by the PBU and PBA messages exchange can be eliminated by the group-based handoff scheme for some MNs, which move together as a group. Fig. 7 shows the histogram of handoff delay of our group-based handoff scheme for PMIPv6. From the result, we can see that in more than 50% of cases, the handoff delay varies in the range between 20 and 40 ms, in about 25% of cases, the handoff delay varies in the range between 40 and 65 ms. This is because the MNs’ group information is used, and the time consumed by the PBU and PBA messages exchange is saved for some MNs. From the simulation scenario, we can see that about 80% of MNs move in groups. As the result shows that about half of the MNs’ handoff delay is reduced by about 23 ms compared with that of PMIPv6. VI. C ONCLUSION In this paper, we propose a group-based handoff scheme for PMIPv6 in order to reduce its handoff delay and signaling cost. A group-based handoff procedure based on the correlated MNs detection algorithm is designed. Simulation shows that the proposed procedure reduces both the mobility-related signaling cost and handoff delay. With respect to handoff delay,
[1] S. Gundavelli, “Proxy Mobile IPv6,” RFC 5213, IETF, August 2008. [2] D. Johnson, “Mobility Support in IPv6,” RFC 3775, IETF, June 2004. [3] H. Soliman, C. Castelluccia, and K. El Malki, “Hierarchical Mobile IPv6 Mobility Management (HMIPv6),” RFC 4140, IETF, August 2005. [4] K. Kong, W. Lee, Y. Han, M. Shin, and H. You, “Mobility management for all-IP mobile networks: mobile IPv6 vs. proxy mobile IPv6 ,” IEEE Wireless Communications, vol. 15, no. 2, pp. 36–45, 2008. [5] K. Kong, W. Lee, Y. Han, and M. Shin, “Handover Latency Analysis of a Network-Based Localized Mobility Management Protocol,” IEEE International Conference on Communications, 2008., pp. 5838–5843, 2008. [6] J. Lee, T. Chung, S. Pack, and S. Gundavelli, “Shall we apply paging technologies to proxy mobile IPv6?” in MobiArch Workshop of Sigcomm 2008. ACM New York, USA, 2008, pp. 37–42. [7] J. Lee and J. Park, “Fast Handover for Proxy Mobile IPv6 based on 802.11 Networks,” in IEEE ICACT 2008., vol. 2, 2008. [8] S. Park, N. Kang, and Y. Kim, “Localized Proxy-MIPv6 with Route Optimization in IP-Based Networks,” IEICE transaction on Communications, vol. 90, no. 12, p. 3682, 2007. [9] R. Gau and C. Lin, “Location Management of Correlated Mobile Users in the UMTS,” IEEE transaction Mobile Computing., vol. 4, no. 6, pp. 641–651, 2005. [10] T. Ernst, “Network Mobility Support Goals and Requirements,” RFC 4886, IETF, July 2007. [11] J. Kempf et al., “Goals for Network-Based Localized Mobility Management (NETLMM),” RFC 4831, IETF, April 2007. [12] H. Lee, D. Kim, B. Chung, and H. Yoon, “Adaptive Hysteresis Using Mobility Correlation for Fast Handover,” IEEE Communications Letters, vol. 12, no. 2, pp. 152–154, 2008. [13] J. Xie, I. Howitt, and I. Shibeika, “IEEE 802.11-based Mobile IP fast handoff latency analysis,” in IEEE International Conference on Communications, 2007, pp. 6055–6060.
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