A Proxy Mobile IP based Layer-3 Handover Scheme for Mobile WiMAX based Wireless Mesh Networks Min-Kim, Jong-min Kim, Hwa-sung Kim Dept. of Electronics and Communications Engineering Kwangwoon University Seoul, Korea {beyond, sazemic, hwkim}@kw.ac.kr Il-kyeun Ra Dept. of Computer Science and Engineering University of Colorado Denver Denver, Colorado Ilkyeun.ra@ucdenver.edu Abstract In this paper, we present a proxy mobile IP based layer-3 handover scheme for mobile WiMAX based wireless mesh networks. Mobile WiMAX provides two types of layer-3 handover schemes: MIP based handover and PMIP based handover. MIP based handover incures the long handover latency because mobile nodes generate a lot of handover messages in wireless area. On the other hand, PMIP based handover decreases the handover latency by reducing the number of handover messages, because a mobile node does not participate in handover procedure. However, conventional PMIP based handover still has latency overhead, because of message exchanges between PBU and PBA after completing the layer-2 handover. Hence, we propose a new layer-3 handover method that does not bring about packet losses while achieving lower handover latency than conventional layer-3 handover scheme. Simulation results show that the proposed scheme achieves lossfree and low handover latency during the layer-3 handover. Keywords Wireless Mesh Networks, Mobile WiMAX, Proxy Mobile IP, Fast Handover. I. INTRODUCTION In recent years, Wireless Mesh Networks (WMNs) have widely been studied as an infinite potential emerges. WMNs are the next generation technology that expands conventional wireless networks to the wider area and enables the end-users to experience various wireless services. In WMNs, mesh routers are deployed to cover regions where wireless access is desired, much like the way access points are deployed in traditional single-hop networks. However, unlike access points in traditional single-hop networks, all mesh routers are not connected to wired infrastructure. They are rather interconnected via wireless links to configure a multi-hop network each other and form a wireless backbone. Mesh clients are wireless mobile nodes that accept the network access service by connecting to the mesh routers [1]. When the mesh client moves and re-associates with a different mesh router, both of layer-3 and layer-2 handover events occur and handover latency increases. If handover latency increases, the overall performance of WMNs degrades, because the connectivity of real-time services is not maintained and many packet losses occur. There have been many research works related to WMNs, which are mostly based on IEEE 802.11 MAC protocol. However, IEEE 802.11 based WMNs have a small capacity, compared to the wired networks. Also, an available capacity per a node further decreases because several nodes share the small capacity again. This small capacity is not sufficient for backhaul networking in WMNs that deal with both of data between mesh routers and data between many mesh clients which belong to mesh routers simultaneously. Also, IEEE 802.11 MAC protocol, CSMA/CA does not suit MAC protocol for mesh connectivity among mesh routers, because it is originally designed for single-hop WLAN environment, not for multi-hop wireless networks [2]. Hence, we notice WMNs based on WiMAX and mobile WiMAX. In this paper, mesh routers use WiMAX for multi-hop communication and mesh clients use mobile WiMAX as the network access technology over mobile WiMAX based WMNs. If mesh clients use the layer-2 handover defined in mobile WiMAX when mesh clients move to different mesh routers in mobile WiMAX based WMNs, handover failure happens frequently, because it assumes that links between s (Base Stations) are wired links. Also, layer-3 handover has failure in WMNs due to layer-2 handover. Mobile WiMAX provides two types of layer-3 handover schemes. These are MIP (Mobile IP) which is traditional layer-3 mobility support protocol and PMIP (Proxy Mobile IP) which is very popular recently as the next generation IP mobility protocol. Among the two ways of layer-3 handover schemes defined in mobile WiMAX, MIP based layer-3 handover can cause frequent handover failure resulted from the long handover latency because mobile node generates a lot of handover messages to wireless area in WiMAX based WMNs which has much broader wireless coverage than mobile WiMAX networks. Moreover, there is a fundamental problem that MIP protocol has to be deployed in mobile node which has limited performance and resource. Therefore, layer- 3 handover for mobile WiMAX based WMN has to be designed on PMIP basis which is network based mobility support protocol. If layer-3 handover is provided using PMIP, the handover performance in mobile WiMAX based WMNs, can be improved by reducing a number of handover messages and by saving the limited wireless resources. This is resulted from that mobile node does not participate to handover procedure [3]. 978-1-4244-8086-9/10/$26.00 2010 IEEE 33 ICUFN 2010
The remainder of the paper is organized as follows. Section II explains mobile WiMAX based WMNs and layer-3 handover scheme which was defined in mobile WiMAX in detail. In section III, we propose an improved layer-3 handover scheme for mobile WiMAX based WMNs. Section IV explains the results of performance evaluation using the NS-2 network simulation. Section V is the conclusion. II. A. Mobile WiMAX based WMNs RELATED WORKS Fig. 1 shows the network model for mobile WiMAX based WMNs proposed in this paper. The network model is similar to the general architecture of WMNs, but it includes a wireless backbone between wireless s, the wireless and the ASN-GW (Access Service Network-Gateway). Mobile WiMAX based WMNs consist of wireless s that include appended functions of the mesh router. The ASN-GW that includes appended functions of the gateway and s (Mobile Subscribers) which serve as the mesh client. Wireless s basically use two types of network interfaces. The one is WiMAX Mesh Mode used as a wireless backhaul protocol to configure WMNs and the other is mobile WiMAX to provide s with the network access service. The ASN-GW needs an interface for WiMAX Mesh Mode and a separate interface to serve as the gateway. s are mesh clients based on mobile WiMAX and these are identical to wireless mobile nodes used in mobile WiMAX networks. AAA ASN-GW Internet ASN-GW : Mobile Station : Base Station ASN-GW: Access Service Network - Gateway AAA: Authentication, Authorization, and Accounting : Home Agent Wireless Link Figure 1. Mobile WiMAX based WMNs B. Layer-3 Handover Procedure in Mobile WiMAX Wired Link Layer-3 handover in mobile WiMAX is divided into MIP and PMIP based handover by mobility management. As discussed in the introduction, MIP based handover is a protocol performing the host based handover procedure that is executed by which has embedded MIP stack. PMIP based handover is a protocol performing the network based handover procedure that is executed by network instead of equipped with that has only generic IP stack without MIP protocol stack. PMIP based handover does not require additional signaling of layer-3 handover for because there is no requirement for [3]. In this section, we explain PMIP based handover procedure because a new layer-3 handover procedure, which is proposed in this paper, is based on PMIP. Layer-3 handover procedure based on PMIP, which is defined in mobile WiMAX, is started by the request of or the network and it can be changed according to various scenarios. Fig.2 shows the layer-3 handover procedure started by s request [4][5]. Once a triggers the handover by scanning, it notifies its handover intention by transmitting MOB_HO-REQ (Mobile Station HandOver REQuest) message, which contains the list of target s and the 's MAC identifier, to the serving. The serving that received MOB_HO-REQ message sends (HandOver Request) message to the target s through the backbone network to identify one target that the handover is possible. The target s performs Path Pre-Registration procedure for data integrity with the ASN-GW. At this moment, the ASN-GW buffers the packets destined to the which intends to initiate the handover. The target s which complete Path Pre- Registration procedure send (HandOver Response) message which informs the serving whether the handover is accepted or not. The serving sends MOB_HO-RSP message, which contains the list of the target s, to the and then sends (HandOver Acknowledgment) message that notify the target s of the receipt of message. The decides one target among the target s and sends MOB_HO-IND (HandOver Indication) message which implies the handover initiation. Subsequently, in order to connect the new wireless link, the starts the network reentry procedures. The serving that received MOB_HO-IND message sends HO Confirm (HandOver Confirm) message to the target and it sends message in response to HO Confirm message. Once the re-entry procedures are completed and the downlink service is resumed in new wireless link, the target starts Path Registration procedure to receive the several packets buffered at the ASN-GW. And then the target forwards the serving HO Complete message to report the completion of the layer-2 handover. Target, which has obtained information, sends PBU (Proxy Binding Update) message to (Home Agent) for the registration of location., which has received PBU, updates the location of in binding cache and sends PBA (Proxy Binding Acknowledgment) message. Target ASN-GW, which has received PBA, sends RA (Router Advertisement) unicast message to, can receive the transmitted data via new network. C. Data Integrity Mechanisms During the Handover When the is carrying out the handover procedure or reestablishes data path after handover completion, there are data packets that were not forwarded to the over the previous path in time. Some packet losses in packet data service such as Internet are restored by a retransmission of end-to-end protocol, but inevitable handover latency originates. Therefore, to guarantee a quality of various service classes during the handover, data integrity mechanisms that minimize packet losses, duplications and re-arrangements are needed. 34
Serving Serving ASN-GW (Previous MAG) Target ASN-GW (New MAG) Target s (LMA) Scanning MOB_HO-REQ MOB_HO-RSP HO Preparation Phase MOB_HO-IND Release of HO Confirm HO Confirm HO Confirm HO Action Phase Network Re-Entry HO Complete HO Complete Router Advertisement New Data Path HO Complete Proxy BU Proxy BA MAC-layer handover messages IP-layer handover messages Figure 2. Layer-3 handover in mobile WiMAX Mobile WiMAX defines data path setup mechanism, data delivery synchronization mechanism and ARQ synchronization to support the data integrity considerations [4]. 1) Data path setup mechanism: Data path setup mechanism for guaranteeing data integrity consists of the buffering mechanism and the bi/multi-casting mechanism. The buffering is that traffic of the services for which data integrity is required is buffered in the data path originator or in the terminator. This buffering might be done only during the handover or for simplicity it might be done within the lifetime of the session. The bi/multi-casting is to multi-cast downstream traffic at the originator of the data path. The bicasting is a particular case: traffic is bi-casted to the serving element and to only one target. 2) Data delivery synchronization mechanism: Data delivery synchronization mechanism is to synchronize data which was buffered through different data paths during the handover. This synchronization can be achieved in two different ways: Using sequence number, Data retrieving. a) Using sequence number scheme: A sequence number is attached to each SDU (Service Data Unit) in the data path. This sequence number will be increased by one every time a SDU is forwarded in the data path. The target receives the sequence number of SDU which the serving finally sends the during the handover from the serving or the attached to the target. b) Data retrieving scheme: Without creating sequence number for each SDU, the serving copies or buffers the data during handover preparation phase, when a final target is identified through HO-IND, the serving is asked to push back all of its un-sent/un-acknowledged packets to the target. 3) ARQ synchronization: For ARQ enable traffic, mobile WiMAX MAC divides the SDUs onto logical parts called ARQ blocks. All blocks are of equal size except from the last one in the SDU (the block size is a per connection parameter). Each block is assigned a sequence number called N (Block Sequence Number). Because all the blocks belonging to one SDU are not delivered during the handover, there is a need to synchronize ARQ states between the serving and the target about un-transmitted and un-acknowledged ARQ blocks. Upon completing the handover, the serving shares the information about the ARQ states and uplink/downlink SDU/ARQ blocks buffers, with the target. III. LAYER-3 NDOVER FOR MOBILE WIMAX BASED WMNS A. Considerations of Layer-3 Handover for Mobile WiMAX based WMNs In this section, we examine the problems when mobile WiMAX layer-3 handover scheme is applied to mobile WiMAX based WMNs, and considerations that are needed to solve the above problems. As described in section II-A, mobile WiMAX based WMNs is different from mobile WiMAX network because it has wireless backbone. Among the two ways of layer-3 handover scheme which is defined in mobile WiMAX, MIP based layer-3 handover scheme cannot be applied to mobile WiMAX based WMNs network because of following problems. As mentioned in introduction, MIP makes mobile node generate a lot of handover messages to wireless area. Therefore, in mobile WiMAX based WMNs which haves wider wireless coverage area than mobile WiMAX networks, long handover latency is inescapable, and it can cause frequent handover failure. Moreover, there is a fundamental problem that MIP has to be designed in MN which has limited performance and resource. Therefore, layer-3 handover for mobile WiMAX based WMNs has to be studied on PMIP basis. As discussed in section II-B, because mobile WiMAX provides PMIP based layer-3 handover, it can be applied to mobile WiMAX based WMNs. In conventional scheme, however, there is a message exchange between PBU and PBA after completing the layer-2 handover, and handover latency can be occurred. Since this handover latency can cause packet loss or out-of-sequence problem, it is very fatal to real-time multimedia service such as VoIP, Video streaming. Therefore, in this paper, we propose PMIP based layer-3 fast handover in order to reduce handover latency in PMIP. 35
Wireless Link Wired Link Wireless Link Serving Serving ASN-GW (Previous MAG) Target ASN-GW (New MAG) Target s (LMA) Scanning MOB_HO-REQ MOB_HO-RSP Tunnel Setup between ASN-GWs HO Preparation Phase Buffering Start at Serving ASN-GW Data Forwarding Buffering Start at Target ASN-GW MOB_HO-IND Release of HO Confirm HO Confirm HO Confirm Fast Proxy BU HO Action Phase Fast Proxy BA HO Notification Handover Latency Network Re-Entry Router Advertisement Existing Router Advertisement HO Complete Existing Proxy BU Existing Proxy BA Latency Distinction between Conventional scheme and Proposed scheme HO Complete HO Complete Data Forwarding MAC-layer handover messages New Data Path IP-layer handover messages B. Operations of the Proposed Scheme In this section, we explain the PMIP based layer-3 handover scheme for mobile WiMAX based WMNs. Fig. 3 shows the handover procedure proposed in this paper. Proposed scheme is modified based on the consideration which was discussed in section III-A. Proposed handover scheme performs layer-3 handover using handover anticipation information prior to completion of layer-2 handover in order to reduce handover latency. Additionally, proposed handover scheme performs buffering in serving ASN-GW and target ASN-GW simultaneously in order to prevent packet loss. 1) Initiation of layer-2 handover: is served while periodically receiving the information about neighboring s from the present serving using message. scans signal of neighboring s if the strength of receiving signal from the serving falls below the predefined level. If finds better cell by cell searching, starts handover procedure by transmitting MOB_HO- REQ message to serving. Serving, which has received HO request message from, forwards a list of candidate s and MAC information of to serving ASN-GW. If candidates s belong to other ASN-GW, serving ASN-GW starts procedure for layer-3 handover. 2) Preparation for fast handover: If s requested by belong to new ASN-GW, serving ASN-GW starts buffering in order to prevent packet loss, and it transmits HO request message to candidates s which are going to handover. The candidate s forward HO response messages as the response of HO request message. Serving establishes traffic tunnel between serving ASN-GW and target ASN-GW for packet tunneling during the handover by sending as the response. Once the tunnel has established, serving ASN-GW transmits buffered packets to target ASN-GW. The target Figure 3. Layer-3 handover for mobile WiMAX based WMNs ASN-GW also performs buffering to prevent packet loss. Meanwhile, serving, which has received HO response message, informs the result of handover request using MOB_HO-RSP message., which has received MOB_HO-RSP message, chooses one among the candidate s who is going to perform handover, and sends MOB_HO-IND message to serving. Consequently, disconnects the existing link. At this time, serving forwards information about the to target using HO confirm message, and this result is reported to serving using message of target. Proposed scheme is different from existing handover scheme in that if serving ASN-GW receives HO confirm message from serving, it forwards FPBU (Fast Proxy Binding Update) message instead of ASN-GW and performs location registration for. This aims to reduce the handover latency by performing layer-3 handover in advance with the information about which is included in HO confirm message and the information about target that is determined to handover, while layer-2 handover. In the proposed scheme, serving ASN-GW registers to instead of target ASN-GW. Therefore, we need FPBU message that is different from PBU message used in existing PMIP., which has received FPBU message, registers the location of into binding cache, and it establishes tunnel to target ASN-GW, not to serving ASN- GW which sent FPBU message., which has finished all procedures, transmits FPBU message to serving ASN-GW, and the serving ASN-GW, which has received FPBU message, sends HO notification message to target ASN-GW in turn. HO notification, which has not existed in existing PMIP, is used because of two reasons. The first reason is to make target ASN-GW establishes tunnel with by notifying that serving ASN-GW registered the present location of to instead of target ASN-GW. The second reason is that target 36
ASN-GW would discard the packets that is forwarded from to through target ASN-GW on the assumption that these packets are aimless packet, because is not connected to the network. In order to prevent this problem, serving ASN- GW has to send HO notification message to target ASN-GW. 3) Connection with new ASN-GW and receiving packets: While layer-3 handover, started from MOB_HO-IND message sending, is performed, executes the procedure to connect to new in ASN-GW. As discussed in section III- B-2, network re-entry procedure for connecting to target is identical to the procedure which was defined in conventional mobile WiMAX. If the connection to target is established through the network re-entry procedure, target sends HO complete message, which means the completion of layer-2 handover, to target ASN-GW. Since target ASN-GW can send RA message to as soon as the layer-2 handover is completed due to the pre-performed layer-3 handover procedure, is able to use existing IP address which was used in serving ASN-GW. Lastly, target ASN-GW finishes handover procedure by transmitting the buffered packet. IV. SIMULATION In order to evaluate the performance of proposed handover scheme, we use the NS-2 network simulator. Fig. 4 shows the topology used for the simulation. In the given topology, the wired link transmission delay between and router R0, CN and router R0, ASN-GW1 and router R1, and ASN-GW2 and R1 is set to, and the delay between router R0 and router R1 is set to 10ms. 1 and 2 are mesh routers that belong to ASN-GW1, 3 and 4 are mesh routers that belong to ASN-GW2. We used WiMAX Mesh Mode as the physical layer and the MAC layer and OLSR [6] as the routing protocol. In this topology, starts from 2 and moves at fixed speed to 3 while receiving TCP traffic transmitted from CN. During the movement of, layer-3 handover is required. We measure the throughput, the TCP sequence number and the handover latency on TCP traffic during the layer-3 handover. The simulation parameters are listed in Table 1. Parameter Simulation TABLE I. SIMULATION PARAMETER Value 300 secs The number of nodes 11 Distance between nodes Transmission range PHY, MAC protocols Routing protocol Traffic 100m 100m WiMAX Mesh Mode OLSR TCP Traffic Fig. 5 shows the simulation result on TCP throughput of mobile WiMAX layer-3 handover scheme and the proposed scheme in mobile WiMAX based WMNs. As shown in Fig. 5, TCP throughput of the proposed scheme begins to degrade earlier than conventional scheme, but it does not degrade as much as conventional scheme, because buffering is performed at both of serving ASN-GW and target ASN-GW in proposed scheme. On the other hand, conventional scheme experiences much more performance degradation in terms of TCP throughput due to the long handover latency. And it takes considerable time to recover the TCP throughput level before the handover occurred. CN Figure 5. Simulation result on TCP throughput R0 10ms ASN- GW1 R1 ASN- GW2 1 2 3 4 Figure 4. Simulation topology for performance evaluation Figure 6. TCP sequence number of Mobile WiMAX 37
Figure 7. TCP sequence number of the proposed scheme Fig. 6 and 7 show the simulation results on TCP sequence number of the proposed scheme and the conventional scheme in mobile WiMAX based WMNs. Fig. 6 shows that conventional scheme experiences about 1600ms latency during the layer-3 handover. This latency can t be tolerable for the handover latency defined in mobile WiMAX. Therefore, mobile node can t receive seamless service. Also, conventional scheme can t avoid the packet loss during the handover due to the long handover latency. Additionally, mobile node which moves to new network, experience out-of-sequence problem because it receives retransmitted packets while receiving the newly incoming stream over the new network. On the other hand, the proposed scheme shows about 800ms handover latency, so it satisfies the requirement for handover latency. And it does not have the packet loss and out-of-sequence problem because it adopts the buffering mechanism and the fast handover procedure. Finally, Fig. 8 shows the simulation result on handover latency according to the changes of the traffic load. Even though both of proposed and conventional schemes showed more handover latency as the traffic load increases in mobile WiMAX based WMNs, the proposed scheme shows much less handover latency. The conventional scheme which abuses the wireless backbone of mobile WiMAX based WMNs during handover shows that increases, the handover latency increases largely because of the lacking bandwidth and the long delay as the traffic load increases. Moreover, if the traffic load is more than 2.5packets/s, the conventional scheme does not satisfy the requirement on handover latency. Meanwhile, although the traffic load is increased up to 5packets/s, the proposed scheme satisfies the requirement, so it can offer seamless service to the mobile nodes. V. CONCLUSION In this paper, we present a PMIP based layer-3 handover scheme for mobile WiMAX based WMNs. As a related research, we introduce a network model for mobile WiMAX based WMNs and we also describe the layer-3 handover procedure and data integrity mechanisms defined in mobile WiMAX. Mobile WiMAX defines two handover schemes for layer-3 handover: MIP based and PMIP based handover. MIP based handover is not suitable for mobile WiMAX based WMNs because of many problems. PMIP based handover is suitable for mobile WiMAX based WMNs because it reduces the traffic in wireless backbone, however, it still has handover latency. Therefore, we propose a PMIP based fast handover scheme that prevents the packet loss resulted from handover latency and eliminates handover delay due to the handover latency. Through the simulation, we show that proposed handover scheme outperforms the conventional PMIP based scheme in terms of TCP throughput, packet loss and handover latency in mobile WiMAX based WMNs. REFERENCES Figure 8. Simulation result on handover latency [1] Ian F. Akyildiz and Xudong Wang, A Survey on Wireless Mesh Networks, IEEE Communications Magazine, vol. 43, No. 9, pp. 23-30, September 2005. [2] Tzu-Jane Tsai and Ju-Wei Chen, IEEE 802.11 MAC Protocol over Wireless Mesh Networks: Problems and Perspectives, 19th International Conference on AINA, Vol. 2, pp. 60-63, March 2005. [3] S. Gundavelli, K. Leung, V. Devarapalli, K. Chowdhury, B. Patil, Proxy Mobile IPv6, IETF RFC 5213, Aug. 2008. [4] WiMAX Forum NWG, Stage-2: Architecture Tenets, Reference Model and Reference Points, 2007. [5] WiMAX Forum NWG, Stage-3: Detailed Protocol and Procedures, 2007. [6] T. Clausen and P. Jacquet, Optimized Link State Routing Protocol, RFC 3626, October 2003. 38