A Seamless Layer-2 Handover Scheme for Mobile WiMAX based Wireless Mesh Networks Min Kim\ Il-kyeun Ra2 , Ji-sang Yoo3, Dong-wook Kim4, and Hwa-sung Kim5 15Dept. of Electronics and Communications Engineering, 3Dept. of Electronics Engineering 4Dept. of Electronic Material Engineering, Kwangwoon University, Seoul, Korea {lbeyond, 3jsyoo, 4dwkim, 5hwkim}@kw.ac.kr 2Dept. of CSE, University of Colorado Denver
[email protected]
Abstract - Wireless mesh networks have been studied as the next generation technology to solve problems of conventional wireless networks. Particularly, mobile WiMAX based wireless mesh networks are noticed due to many advantages. In this paper, we propose a layer-2 handover scheme for mobile WiMAX based wireless mesh networks. A layer-2 handover scheme based on mobile WiMAX causes many handover failures over mobile WiMAX based wireless mesh networks because it does not consider the characteristics of wireless mesh networks. The proposed scheme does not bring about packet losses achieving lower handover latency than a conventional handover scheme. Simulation results show that the proposed scheme achieves loss-free and low handover latency during the layer-2 handover. Keywords - Wireless Mesh Networks, Mobile WiMAX, Seamless handover.
1. Introduction In recent years, wireless mesh networks (WMNs) have widely been studied as an infinite potentiality 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 a region 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 a 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 through connecting the mesh router [1]. When the mesh client moves and re-associates with a different mesh router, a layer-2 handover event occurs and handover latency increases. If handover latency increases, the overall performance ofWMNs degrades, because the connectivity of real-time services is not maintained and many packet losses occur. There have been many works related to WMNs, which are mostly based on IEEE 802.11 MAC protocol. By the way, This research was supported by the Basic Research Program (Grant ROl-2006-000-10199-0) of the Korea Science & Engineering Foundation
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IEEE 802.11 based WMNs have a small capacity, compared to the wired networks. Besides, since several nodes share the small capacity again, an available capacity per a node further decreases. This small capacity is inappropriate for backhaul networking in WMNs that deal with both data among mesh routers and data of 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 over mobile WiMAX based WMNs, handover failure happens frequently, because it assumes that links between BSs (Base Stations) are wired links. The remainder of the paper is organized as follows. Section 2 provides a brief overview of related works. It presents a network model of mobile WiMAX based WMNs and the layer-2 handover procedure defined in mobile WiMAX. In section 3, we introduce the operation of our handover scheme. Section 4 explains the results of performance evaluation using the NS-2 network simulation tool. In section 5, we conclude our work and discuss future directions.
2. Related Works 2.1 Mobile WiMAX based WMNs 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 includes a wireless backbone between wireless BSs, the wireless BS and the ASN-GW (Access Service Network-Gateway). Mobile WiMAX based WMNs consist of wireless BSs that functions of the mesh router are appended, the ASN-GW that functions of the gateway are appended and MSs (Mobile Subscribers) which serve as the mesh client. Wireless BSs 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 MSs with the network access service. The ASN-GW needs an interface for WiMAX
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Mesh Mode and a separate interface to serve as the gateway. MSs are mesh clients based on mobile WiMAX and are identical to wireless mobile nodes used in mobile WiMAX networks.
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Figure 1. Mobile WiMAX based WMNs
2.2 Layer-2 handover procedure in mobile WiMAX In this section, we describe the layer-2 handover procedure and data integrity mechanisms to support seamless mobility defined in mobile WiMAX [3][4]. Fig. 2 shows the handover procedure specified in mobile WiMAX. The layer-2 handover procedure consists of 'scanning', 'handover preparation phase' and 'handover action phase' [4][5]. Serving
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The layer-2 handover is started by the request of MS or the network and can be changed according to various scenarios. Fig. 2 shows the layer-2 handover procedure started by MS's request. Once a MS triggers the handover by scanning, it notifies its handover intention by transmitting MOB_MSHO-REQ (Mobile Station HandOver REQuest) message, which contains the list of target BSs (t-BSs) and the MS's MAC identifier, to the serving BS (s-BS). The s-BS that received MOB_MSHO-REQ message sends HO Request (HandOver Request) message to the t-BSs through the backbone network to identify one t-BS that the handover is
ISBN 978-89-5519-139-4
2.3 Data integrity mechanisms during the handover When the MS is carrying out the handover procedure or re-establishes data path after handover completion, there are data packets that were not forwarded to the MS 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. Mobile WiMAX defines data path setup mechanism, data delivery synchronization mechanism and ARQ synchronization to support the data integrity considerations [3].
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possible. The t-BSs perform Path Pre-Registration procedure for data integrity with the ASN-GW. At this moment, the ASN-GW buffers the packets destined to the MS which intends to initiate the handover. The t-BSs which complete Path Pre-Registration procedure send HO Response (HandOver Response) message which informs the s-BS whether the handover is accepted or not. The s-BS sends MOB_BSHO-RSP message, which contains the list of the t-BSs, to the MS and then sends HO Ack (HandOver Acknowledgment) message that notify the t-BSs of the receipt of HO Response message. The MS decides one t-BS among the t-BSs and sends MOB_HO-IND (HandOver Indication) message which implies the handover initiation. Subsequently, in order to connect the new wireless link, the MS starts the network re-entry procedures. The s-BS that received MOB_HO-IND message sends HO Comfirm (HandOver Comfirm) message to the t-BS and it sends HO Ack message in response to HO Comfirm message. Once the re-entry procedures are completed and the downlink service is resumed in new wireless link, the t-BS starts Path Registration procedure to receive the several packets buffered at the ASN-GW. And then the t-BS forwards the s-BS HO Complete message to report the completion of the MS' handover. The ASN-GW performs Path De-Registration procedure to eliminate the existing data path between the s-BS and the ASN-GW and data path between the t-BSs and the ASN-GW created by Path Pre-Registration.
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2.3.1 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 ofthe data path. The bi-casting is a particular case: traffic is bi-casted to the serving element and to only 1 target. 2.3.2 Data delivery synchronization mechanism Data delivery synchronization mechanism is to synchronize data which was buffered through different data paths during
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the handover. This synchronization can be achieved in 2 different ways: Using sequence number, Data retrieving. (1) Using sequence number scheme: A sequence number is attached to each SDU (Service Data Unit) in the data path. This sequence number shall be increased by 1 every time a SOU is forwarded in the data path. The t-BS receives the sequence number of SOU which the s-BS finally sends the MS during the handover from the s-BS or the MS attached to the t-BS. (2) Data retrieving scheme: Without creating sequence number for each SOU, the s-BS copies or buffers the data during handover preparation phase, when a final t-BS is identified through HO-IND, the s-BS is asked to push back all of its un-sentlun-acknowledged packets to the t-BS. 2.3.3 ARQ synchronization For ARQ enable traffic, mobile WiMAX MAC divides the SOUs onto logical parts called ARQ blocks. All blocks are of equal size except from the last one in the SOU (the block size is a per connection parameter). Each block is assigned a sequence number called BSN (Block Sequence Number). Because all the blocks belonging to one SOU are not delivered during the handover, there is a need to synchronize ARQ states between the s-BS and the t-BS about un-transmitted and un-acknowledged ARQ blocks. Upon completing the handover, the s-BS shares the information about the ARQ states and uplink/downlink SOUlARQ blocks buffers, with the t-BS.
3. Layer-2 Handover for mobile WiMAX based WMNs 3.1 The considerations of layer-2 handover for mobile WiMAX based WMNs In this section, we discuss the problems occurring when the conventional scheme of mobile WiMAX applies to mobile WiMAX based WMNs and the considerations to solve that problems. As discussed in section 2.1, the architecture of mobile WiMAX based WMNs is different from it of mobile WiMAX networks, because the links between BSs, the BS and the ASN-GW are multi-hop wireless links. Since the layer-2 handover of mobile WiMAX assumes that the backbone network is a wired link, many handover failures will originate over mobile WiMAX based WMNs. Moreover, mobile WiMAX networks are deployed in order that the handover messages go through the ASN-GW when the BS sends the neighbor BSs them. The reason why mobile WiMAX networks are deployed so is that to cable all BSs each other is very expensive and mobile WiMAX assumes that the wired backbone does not affect the handover performance highly. By the way, as shown in the fig. 3, BSs and the ASN-GW over mobile WiMAX based WMNs can directly communicate with each other due to the broadcast nature of the wireless link. Hence, the proposed scheme shall reduce the utilization of the wireless link by using direct communication between BSs and decreasing the number of handover messages between the BS and the ASN-GW during the handover.
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Data integrity mechanisms discussed in section 2.3 are defined in mobile WiMAX to minimize packet losses, out-of-sequence problem and handover latency during the handover. Due to the difference of network architecture as mentioned above, data integrity mechanisms cannot apply to mobile WiMAX based WMNs. In data path setup mechanism, the bi/multi-casting scheme is inappropriate to mobile WiMAX based WMNs, since it waste the bandwidth of wireless backbone. The buffering mechanism at the ASN-GW as another data integrity mechanism will cause frequent handover failures. When t-BSs perform Path Pre-Registration procedure with the ASN-GW by means of three-way handshake, the exchange of handover messages is not normally accomplished because of unexpected changes of wireless backbone. In the event, this situation will cause packet losses or out-of-sequence problem. Therefore, the proposed scheme selects the buffering mechanism as data path setup mechanism, but does not choose the ASN-GW as the buffering point. Meanwhile, there are data delivery synchronization mechanism and ARQ synchronization as data integrity mechanisms. Our paper does not consider two mechanisms, because they are separately achieved in the s-BS and the t-BS regardless of the network architecture and the handover procedure. 3.2 Operations of the proposed scheme In this section, we explain the layer-2 handover scheme for mobile WiMAX based WMNs. Fig. 3 shows the handover scheme proposed in this paper. The proposed scheme is entirely based on the layer-2 handover defined in mobile WiMAX including the messages for the context information transfer (via backbone) described in section 2. The operations of our handover scheme are as follows. Serving BS
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Handover preparation phase: Once the MS triggers the handover by scanning, it initiates the handover by sending MOB_MSHO-REQ message to its s-BS. And then the s-BS sends HO Request message containing the MS's session and
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establishment information, etc, to the t-BS using direct communication between BSs. The t-BSs which received HO Request message carry out Path Pre-Registration procedure which requests buffering for downlink data of the MS. Unlike the conventional scheme, t-BSs perform the procedure with the s-BS and bi/multi-casting mechanisms are not used. When t-BSs finish Path Pre-Registration procedure, they send the s-BS HO Response message which informs the s-BS of handover acceptance, along with Path Pre-Registration ACK message. The reason is that we minimize the number of signaling messages related to the handover over the wireless backbone. The s-BS responds with MOB_BSHO_RSP message including results of procedures that have performed with t-BSs and completes handover preparation phase by sending HO Ack message which is a reply of HO Response message.
Handover Action Phase: When the MS decides the t-BS and is ready to move, it informs the s-BS of handover action by sending MOB_HO-IND message and disconnects the previous link. The s-BS forwards HO Confirm message including session and authentication information related to the MS, to the t-BS and the t-BS responds with HO Ackmessage. The MS that moves into a new cell performs the network re-entry procedures. In order that the proposed scheme ensures the MS-side transparency and does not append any requirements to the MS, the network re-entry procedures are identical with them defined in mobile WiMAX. If the network re-entry procedures are successfully completed, the MS is offered the network service by the t-BS. When the network re-entry procedures are successfully completed, the t-BS informs the ASN-GW of the MS' handover completion by sending HO Complete message. If the t-BS sends the ASN-GW and the s-BS HO complete message after data forwarding, like the conventional scheme, HO Complete message would arrive too late due to variables of the wireless link. If HO Complete message does not arrive at the ASN-GW, it would continue to forward data destined to the MS to the s-BS, because the ASN-GW does not know whether MS' handover finishes or not. So, our scheme proposes that the t-BS informs the ASN-GW of handover completion by sending HO Complete message as soon as the network re-entry procedures are finished. The ASN-GW which receives HO Complete message carries out Path De-Registration procedure with the s-BS and eliminates the previously used data path. Following HO Complete message, the t-BS accomplishes Path Registration procedure to enable the s-BS to forward the MS's downlink data which are buffering at the s-BS. When data forwarding is finished, the t-BS sends the s-BS HO Complete message and the s-BS performs Path De-Registration with unselected t-BSs to delete data path established by Path Pre-Registration.
4. Simulation In order to evaluate the performance of handover mechanism proposed in this paper, we use the NS-2 network simulator. Fig. 4 is a topology used for simulation. As shown in Fig. 4, the topology is composed of the BS1-BS8, the
ISBN 978-89-5519-139-4
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ASN-GW, the MS and a source node. The wired link between the source node and the ASN-GW is full-duplex link and the delay of the link is established by 2msec. WMNs consist of 8 BSs and 1 ANS-GW. The simulator uses WiMAX Mesh Mode as the physical layer and the MAC layer and OLSR [6] as the routing protocol. Since OLSR is proactive routing protocol, the time exchanging control messages to make routing tables based on them is needed. Accordingly, the source node generates TCP traffic and forwards it to the MS as of 25 sec after the simulation is started. Total simulation time is 300s. Under this topology, the MS performs the layer-2 handover procedure, while it moves from BS 1 to BS2 receiving TCP traffic being transmitted from the source node. We measure the throughput, the sequence number and the handover latency on TCP traffic during the layer 2 handover. The simulation parameters are listed in Table 1.
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Figure 4. Simulation topology for performance evaluation Table 1. Simulation Parameters Parameter Value Simulation topology 3x3 Grid Topology Simulation time 300 secs The number of nodes 9 100m Distance between nodes Transmission range 100m PHY, MAC protocols WiMAX Mesh Mode Routing protocol OLSR Traffic TCP traffic
Fig. 5 shows the simulation result on TCP throughput of mobile WiMAX scheme and the proposed scheme over mobile WiMAX based WMNs. In this simulation, the conventional scheme only makes use of the buffering mechanism for data integrity. As shown in fig. 5, the simulation result shows that when the layer-2 handover happens, throughput is zero because of the buffering mechanism. However, in the conventional scheme in comparison with the proposed scheme, the buffering occurs late and TCP throughput decreases late, because it buffers traffic at the ASN-GW. Also, since the MS receives the
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buffered packets via the wireless link in the conventional scheme, the time that the mobile node moving into the new cell performs Path Registration procedure with the ASN-GW to receive the buffered data is even longer than it of the proposed scheme and the time that rep throughput is zero is longer too. If the time that TCP throughput is zero is too long, handover latency increases and consequently, the end-user devices cannot receive the user-traffic seamlessly. The proposed scheme shows that handover latency is much shorter than the conventional scheme, since it buffers traffic at the s-BS.
Fig.6 and 7 show the simulation results on rcp sequence number of two schemes over mobile WiMAX based WMNs. In this simulation, the conventional scheme makes use of the buffering mechanism for data integrity. The conventional scheme introduces about 200ms handover latency during the handover. This latency is not adequate for the requirement on handover latency defined in mobile WiMAX. In mobile WiMAX specification, the MS moving at 60Kmlh shall content below 150ms handover latency and at 120Kmlh, below 50ms. Also, the MS receives retransmitted packets while receiving the newly incoming stream over the new network. The reason is that the situation that the MS disconnects the s-BS and moves into the new network before data destined to the MS is buffered at the ASN-GW happens frequently. Because the mobile node which already moved into the new network cannot receive the packets which the ASN-GW sends the s-BS before the buffering, it loses the packets and recovers them by TCP retransmission algorithm. On the other hand, the proposed scheme satisfies the requirement on handover latency as 40ms and does not cause packet losses and out-of-sequence problem, since it modifies the buffering mechanism and the handover procedure considering WMNs. 400
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