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					      National Conference on Role of Cloud Computing Environment in Green Communication 2012                                           278


                                                              A.Arence Flemi
                          Asst. Prof. Vel Tech Multi Tech Dr.Rangarajan & Dr.Sakunthala Engineering College,
                                                          Avadi, Chennai, India

    Abstract – THE wireless mesh network (WMN) has recently emerged as a promising technology for next-generation wireless. In WMN each mesh
client has a Mobile Agent (MA) residing on its registered mesh router to handle the handoff signaling process. We proposed, handoff management for
IP-based WMNs remains largely unexplored. Conventional handoff mechanisms can cause significant performance degradation when directly applied to
WMNs due to overlooking the key features of WMNs. The proposed Planned Multicast Group (PMG)-based architecture can facilitate cross-layer
handoffs and hence reduce the total handoff delay caused from multiple layers. Extensive simulations are conducted to evaluate the feasibility and
efficiency of the proposed PMG approach.

  Keywords - Mobility, Wireless Mesh Networks, Planned Multicast Group.

                         I. INTRODUCTION

    IN THE last few years, the wireless mesh network (WMN)
    has drawn significant attention as a fast, easy, and Light-weight, gateway or bridge functions do not exist in
    inexpensive solution for broadband wireless access. mesh clients, and only a single wireless interface is needed
    However, there are still many technical challenges that we in a mesh client.
    have to overcome before the WMN can fully be deployed.
    Particularly, it is crucial to provide mobility support in the
                                                                              II. HANDOFF CHALLENGES IN WMNs
    WMN, because wireless users are free to move to
    anywhere at any time.In this paper, we propose a mobile
                                                                        A. WMNs
    agent (MA)-based handoff approach to address the issue of
    user mobility. Our approach aims to reduce handoff delay
                                                                        As shown in Fig. 1, a WMN consists of two types of
    and provide seamless handoff.
                                                                   nodes: mesh routers and mesh clients. The mesh routers
         Wireless mesh networks (WMNs) are dynamically
                                                                   form an infrastructure of the mesh backbone for mesh
    self-organized and self-configured, with the nodes in the
                                                                   clients. In general, mesh routers have minimal mobility
    network automatically establishing an ad hoc network and
                                                                   and operate just like a network of fixed routers, except that
    maintaining the mesh connectivity. WMNs are comprised
                                                                   they are connected by wireless links through wireless
    of two types of nodes: mesh routers and mesh clients.
                                                                   technologies such as IEEE 802.11. We observe from Fig. 1
    Other than the routing Capability for gateway/bridge
                                                                   that a WMN can access the Internet through a gateway
    functions as in a conventional Wireless router, a mesh
                                                                   mesh router, which is connected to the internet protocol
    router contains additional routing Functions to support
                                                                   (IP) core network with physical wires. In a WMN, every
    mesh networking. Through multi-hop Communications, the
                                                                   mesh router is equipped with a traffic aggregation device
    same coverage can be achieved by a Mesh router with
                                                                   (similar to an 802.11 access point) that interacts with
    much lower transmission power. To further improve the
                                                                   individual mesh clients. The mesh router relays aggregated
    flexibility of mesh networking, a mesh router is usually
                                                                   data traffic of mesh clients to and from the IP core
    equipped with multiple wireless interfaces built on either
                                                                   network. Typically, a mesh router has multiple wireless
    the same or different wireless access technologies. In
                                                                   Interfaces to communicate with other mesh routers, and
    spite of all these differences, mesh and conventional
                                                                   each wireless interface corresponds to one wireless
    wireless Routers are usually built based on a similar
                                                                   channel. These wireless channels have different
    hardware platform. Mesh routers have minimal mobility
                                                                   characteristics, because wireless interfaces are running on
    and form the mesh Backbone for mesh clients. Thus,
                                                                   different frequencies and built on either the same or
    although mesh clients can also work as a router for mesh
                                                                   different wireless access technologies, e.g., IEEE
    networking, the hardware platform and software for them
                                                                   802.11a/b/g/n. It is also possible that directional antennas
    can be much simpler than those for mesh routers. For
    example, communication protocols for mesh clients can be
    Department of CSE, Sun College of Engineering and Technology
 National Conference on Role of Cloud Computing Environment in Green Communication 2012                                    279

are employed on some interfaces to establish wireless        continue connectivity after registering to the new mesh
channels over long distances.                                router.

                                                                  III. MA-BASED HANDOFF ARCHITECTURE IN WMNs

                                                                  In this section, we propose an MA-based handoff
                                                             architecture, which offers seamless and fast handoff to
                                                             support VoIP and other real-time applications. In our
                                                             approach, all the handoff logics are done by the MA, and
                                                             only the standard medium-access control protocol and IP
                                                             are used. Therefore, it is compatible with any 802.11
                                                             mobile devices, regardless of the vendor or architecture.

                                                             A. Introduction to an MA

                                                                  An MA is an executing program that can migrate
Fig.1: Wireless Mesh Backbone Network                        during execution from machine to machine in a
                                                             heterogeneous network. In other words, the agent can
B. Handoff Challenges
                                                             suspend its execution, migrate to another machine, and
                                                             then resume execution on the new machine from the point
        Mesh clients achieve Internet access through mesh
                                                             at which it left off. On each machine,
routers. A mesh client quite often moves from the coverage
of one mesh router to that of another. As a result, it
becomes an Urgent task in WMNs to maintain the ongoing
connections of roaming users. The mobile IP and related
protocols can be applied to WMNs to support user
mobility, but they only focus on the IP identity problem . In
this paper, we Investigate another important aspect of user
mobility support,i.e., the handoff process. Ideally, WMN
handoff should be accomplished with low computing cost
and short latency so that the handoff process can be
completely transparent to mesh clients. In this paper, we
define a WMN that offers the above handoff function as a
seamless handoff WMN. Most WMNs today require
specially modified clients to transfer connectivity from one
mesh router to another. Although some of them give the
appearance of continuous connectivity to a roaming client,
handoff delay can be as long as several seconds. This delay
is unacceptable for real-time applications, such as voice        Fig. 2: Architecture of MA-based handoff in WMNs.
over IP (VoIP) or videoconferencing. In the current 802.11
implementation, the handoff consists of two phases, i.e., The agent interacts with local resources to accomplish its
channel scanning and connection reestablishment. During task.
channel scanning, the mesh router scans all channels to
collect the information about neighboring mesh routers.            MAs have several advantages in developing
During connection reestablishment, the mesh client first distributed computing applications. By migrating to the
registers to the new mesh client through authentication       information resource, an agent can locally invoke resource
then proceeds to the post registration stage, which includes  operations, eliminating the network transfer of
reassociation, CAC, rerouting, and resource reservation to intermediate data. By migrating to the other side of an
meet the requirements of real-time applications. To reduce unreliable network link, an agent can continue executing,
handoff delay, previous studies mainly focused on even if the network link goes down, making MAs
shortening the channel scan latency. Different from particularly attractive in mobile computing environments.
previous works, in this paper, we propose an MA-based By choosing different migration strategies, an agent can
handoff architecture, where an MA takes care of the adapt itself to different tasks and network conditions,
handoff signaling process in the network layer and above. achieving full flexibility and customization. It is
Specifically, an MA accomplishes the tasks of the post appropriate to deploy MAs in a WMN, since a WMN is a
registration stage, such as reassociation, CAC, rerouting, typical distributed system with the feature of “mobility.”
and resource reservation, prior to the actual handoff. As a
result, the handoff mesh client is able to immediately
Department of CSE, Sun College of Engineering and Technology
 National Conference on Role of Cloud Computing Environment in Green Communication 2012                                           280

                                                                  protocol for network layer path reestablishment. Fourth,
B. MA-Based Handoff                                               once the backup connection is built up, the client MA will
                                                                  notify the mesh client that it is ready for handoff. Finally,
     To provide seamless handoff, we apply MA                     the mesh client receives the notification and waits for the
technology to WMNs. As shown in Fig. 2, in our solution,          fire of the handoff trigger to register to the new mesh
each mesh client is assigned a “client MA.” The mesh              router and complete the handoff. The foregoing
client places its client MA in the mesh router that it            illustrations show that before the actual handoff occurs in
registers with. If the mesh client moves from the coverage        the fifth step, the client MA has already constructed a
of one mesh router to that of another mesh router, the client     backup connection on the new mesh router in the third
MA also migrates. We study the scenario that a mesh client        step. As a result, overall handoff delay only involves
moves from the coverage of one mesh router to that of             registration delay, which is spent on the authentication
another mesh router during a call. To eliminate the overall       information exchange between the mesh client and the
handoff latency, we can employ a proactive scan scheme to         new mesh router. In addition to reducing handoff delay,
counteract channel scan delay and our MA approach to              MA-based handoff can also achieve high computing
counteract connection reestablishment delay.                      efficiency. The client MA executes handoff logics on the
Particularly, when the scan trigger of a proactive scan           mesh router where the network computing resource is
scheme is fired, the mesh client will actively probe              affluent and, thus releases the burden on the mesh client,
channels and choose the appropriate neighboring mesh              which is dedicated to running user applications.
router for handoff. Then, the client MA will move from the
current mesh router to the chosen mesh router and                                  IV.RELATED WORKS
complete the processes of reassociation, CAC, rerouting,
resource reservation, etc. Once the handoff trigger is fired,     A. PROPOSED PMG CROSS-LAYER HANDOFF DESIGN
the mesh client will register to the new mesh router and
resume all the connections using the facilities that have              We propose a PMG-based approach to position and
been prepared by the client MA earlier.                           configure mesh routers in order to form a scalable wireless
                                                                  mesh backbone for mobility assistance. The benefit of this
                                                                  approach is that the protocols used for address
                                                                  management and handoffs can be streamlined to take
                                                                  advantage of the resulting network architecture. Under the
                                                                  PMG approach, mesh routers are grouped into connected
                                                                  multicast groups rooted at gateway mesh routers. Special
                                                                  mesh routers, namely PMGs, are equipped with multiple
                                                                  IP addresses with each address corresponding to a
                                                                  different subnet. Note that a mesh router can use the
                                                                  Address Resolution Protocol (ARP) to map different IP
                                                                  addresses to the MAC address of the router.
                                                                  PMGs are the bridging nodes connecting different groups.
                                                                  They can facilitate information exchange between
                                                                  different groups during inter-gateway handoffs.
                                                                       Our PMG-based WMN architecture has the following
                                                                  major advantages:

                                                                  • By  planning multicast groups during the deployment,
            Fig.3:Process of MA-based handoff.                    each mesh router knows that which subnet it belongs to in
                                                                  advance. This design makes it straightforward for address
     Fig. 3 demonstrates that there are five steps in the joint   management and L3 handoff detection.
Handoff process of the proactive scan scheme and our MA           • Both multicast groups and PMG multicast messages can
approach. First, the scan trigger of the proactive scan
                                                                  be easily implemented in IPv6 as multicast addressing
scheme activates the channel scan, which locates the new
                                                                  is a required part of the IPv6 protocol? Therefore, we
mesh router for handoff. Second, the mesh client will
                                                                  believe that this solution is feasible and practical to be
inform its client MA on the current mesh router which one
                                                                  Implemented in future IPv6-based WMNs.
is the new mesh router. Third, the current mesh router
                                                                  • Information sharing for network management for
transfers the client MA to the new mesh router in the
                                                                  intrasubnet roaming is restrained to within a multicast
neighborhood, and the client MA will preset up backup
                                                                  group, instead of broadcasting to the whole mesh
connections on the new mesh router to prepare for seamless
                                                                  backbone, which saves signaling overhead. Information
handoff. The preset up of backup connections usually
                                                                  sharing between groups can be implemented using PMGs.
involves reassociation for context switching between the
                                                                  • Since the PMG-WMN architecture can facilitate the
old access point (AP) and the new AP by the inter access
Point protocol, interaction with the CAC module for               Cross-layer protocol design and PMGs are able to
resource reservation and negotiation with the routing             exchange handoff information between different subnets,

Department of CSE, Sun College of Engineering and Technology
 National Conference on Role of Cloud Computing Environment in Green Communication 2012                                    281

both intra and inter-gateway mobility can be improved.            the multimedia session on layer-5;
  The basic idea of the proposed cross-layer handoff design   25.}
is to take advantages of the PMG-based architecture and
utilize the information obtained from the L2, such as the
link quality of the new channel and the IP address of the
new AP after a handoff, to predict the L3 and L5 handoffs     B. L3 Handoff Preparation to Eliminate the L3 Handoff
in advance so that part of the handoff procedures can be      Detection Delay & Routing Path Discovery Delay
carried out                                                         When the RCUR of the MN reaches the RL3T, it
                                                              triggers the oAP to notify the PMG to prepare for the L3
                                                              handoff. The PMG first checks whether the cAP is located
                                                              in the current subnet of the MN or not. For the intra-
                                                              gateway case, the IP address of the MN does not need to
                                                              change. The cAP will be notified by the PMG to prepare
                                                              the LHRD path between the cAP and PGMMR) for the
                                                              MN in advance, while the PMG takes care of the UHRD to
                                                              the old gateway. For the inter-gateway case, the
                                                              corresponding PMG which belongs to both the old and
                                                              new subnets first formulates an IP address for the MN by
                                                              using the cAP’s network ID and MN’s interface ID via the
                                                              PMGM it receives. This IP address is stored in the
                                                              PMGMR and cAP’s routing table before MN’s L3 handoff
                                                              starts. Furthermore, the PMG prepares for the LHRD to
Fig.4:Hand-off Delay using PMG based cross-layer              the cAP and the UHRD to the new gateway. By doing so,
handoff design                                                the routing path for both the binding update to the HA and
                                                              binding acknowledgement from the HA are prepared
Parallel before an MN completes a L2 handoff. Fig. 5          for the MN in advance. Nevertheless, since the PMGMR
shows the sequence of the handoff delays of the proposed      could be multiple hops away from the cAP and the
PMG based cross-layer handoff scheme and the complete         gateway, the preparation time for both the LHRD and
handoff procedure is shown in Algorithm 1.                    UHRD increases with the number of hops. After the MN
                                                              finishes the L2 handoff, the cAP sends the MN the IP
Algorithm 1                                                   address of the MN. Our objective in this stage is to
PMG-based algorithm for cross-layer handoffs                  eliminate both the L3 handoff detection delay and routing
1. While (true){                                              path discovery delay which are significant handoff delays
2. If (RL3T < RCUR ≤ RL2T)                                    in the L3 handoff.
3. MN sends an HOM to its oAP to retrieve nAP list;
4. oAP informs nAPs to activate additional channel;                       V. PERFORMANCE EVALUATION
5. MN sends Probe Request & waits for Probe Response;
6. MN sorts nAPs & obtains the cAP;                                 In this section, we conduct simulations to
7. MN sends an HOM which contains the preferred           evaluate the performance of the proposed PMG-based
   cAP’s network ID to its oAP;                           cross layer handoff scheme. Since the current modeler
8. oAP sends a multicast PMGM to locate the PMGMR;        does not provide the WMN handoff support, we
9. If (L2HT < RCUR ≤ RL3T)                                implement new models for mesh routers with both Mobile
10. oAP unicasts to the PMGMR for handoff preparations;   IPv6and AODV routing functionalities activated so as to
11. If (cAP belongs to another subnet)                    realize the handoff support in IP-based infrastructure
12. PMGMR formulates an address for the MN;               WMNs.
13. PMGMR prepares the UHRD to the new gateway            A. Simulation Setup
    & the LHRD to the cAP;                                       We developed two default handoff scenarios in
14. SIP message exchanges;                                WMNs in order to compare with our proposed PMG-based
15. else                                                  WMN architecture. One is the default-based handoff
16. PMGMR prepares the UHRD to the old gateway            scheme which depends on RA messages to trigger an
    & the LHRD to the cAP;                                MN’s L3 handoff, as explained in Section III-A. The other
17. If (RCUR ≤ L2HT)                                      is the gateway-based handoff scheme under which an MN
18. If (subnet changes)                                   detects a L3 handoff by receiving a reply message from
19. MN associates to the cAP;                             the gateway. In our handoff simulation, the WMN is
20. MN obtains a new IP address & uses the obtained       composed of two gateways, a few regular mesh routers,
     routing path for address binding with the HA;        and one PMG. All mesh routers and gateways’ wireless
21. MN resumes the multimedia session on layer-5;         interfaces use both AODV and RIPng IPv6 routing
22. else                                                  protocols for delivering multihop IPv6 traffic. Only the
23. MN associates to the cAP;                             PMG has multiple IP addresses (two IP addresses in our
24. MN uses the obtained routing path for resuming        simulation) with each IPv6 address belonging to a
Department of CSE, Sun College of Engineering and Technology
 National Conference on Role of Cloud Computing Environment in Green Communication 2012                                        282

different subnet. The PMG message interval on PMGMR is
uniformly distributed from 0.5s to 1s. The Internet
backbone network has a constant latency of 0.1 second.

B. Results Analysis
        Fig.5: shows the detailed delay elements incurred in
L2, L3,and L5 handoffs versus the number of wireless hops
between the AP of the MN and the gateway, under the
three considered scenarios (default-based, gateway-based,
and PMG-based). In the figure, L2SD is the L2 channel
scanning delay, L3DD is the L3 handoff detection delay,
L3UD is the L3 binding update delay, L3AD is the L3
binding acknowledgement delay,L5RD is the L5
multimedia session RE-INVITE delay, and THD is the
total handoff delay. Under our proposed handoff scheme,
the MN is notified the potential channel information before
being associated to the cAP due to the exchange of handoff     Fig. 5: Total handoff delay under different percentage of
information between the PMG and MN via L3 messages;            background traffic.
For L3DD, unlike the case of default-based and gateway-
based handoff schemes in which the MN needs to wait for
either a RA message or the reply message from a gateway
to determine whether it has changed a subnet, the MN in
our PMG-based architecture can start a L3 handoff                   On one hand, from the figure, we can see that the
immediately after a L2 handoff finishes. So the L3DD in        overhead is large if the MN is triggered to start the handoff
our proposed scheme can be reduced to almost zero.             preparation early. On the other hand, the total delay
           There is no major difference in the other three     increases if the handoff preparation is triggered late.
delays (L3UD, L3AD, and L5AD) between the default-             Therefore, it is vitally important to choose an appropriate
based and gateway base scenarios, since after the MN           handoff threshold in order to balance the tradeoff between
detects its subnet change; it starts the L3 and L5 handoffs    the overhead generated during a handoff and the
sequentially. In our PMG based handoff scheme, as the          corresponding handoff delay.
PMGMR triggers the route path preparation in the target
subnet prior to the MN’s arrival, the L3UD, L3AD, and
L5RD can be reduced to a level only depending on the
multi-hop signaling message traversal time.In Fig. 6, the
total handoff delay is much lower in our PMG based
handoff scheme as compared to the other two schemes,
because our proposed scheme employs a cross-layer design
and eliminates L3 handoff detection and route discovery

                                                                           Fig.6: Total handoff delay and number of
                                                                       overhead messages

                                                                                 VI. CONCLUSION

                                                               In this paper, we introduced a novel Planned Multicast
                                                               Group (PMG)-based architectural design to facilitate
                                                               cross-layer handoffs in WMNs. By implementing PMG
                                                               mesh routers (PMGs) which are strategically placed in the
                                                               mesh backbone to cover target subnets, inter-gateway
                                                               handoff preparations can be proactively prepared before an
                                                               MN loses its connection with the old subnet. We designed
                                                               the detail procedure of the proposed PMG-based cross-
Department of CSE, Sun College of Engineering and Technology
 National Conference on Role of Cloud Computing Environment in Green Communication 2012                                        283

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Department of CSE, Sun College of Engineering and Technology

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