Layer 3 wireless mesh networks: mobility management issues

IEEE Communications Magazine • July 2011156 0163-6804/11/$25.00 © 2011 IEEE

INTRODUCTION

Wireless communication has afforded remark-able convenience and benefits to human life,enabling communication at all times and fromany place. Cellular phone networks and wirelesslocal area networks (WLANs) are major vehiclesto provide wireless communication. The formeris categorized as a wide area network (WAN),while the latter is a local area network (LAN).WLANs can also be used to form access net-works to WANs. The wireless mesh network(WMN) is an emerging technology to extend theuse of wireless communication [1]. A WLANbackbone is composed of access points (APs) toaccommodate mobile hosts (MHs) and typicallywired LANs such as IEEE 802.3 to connect theAPs. On the other hand, a WMN backbone iscomposed of mesh routers (MRs) instead ofAPs. MRs generally have a capability similar toAPs in a WLAN for accommodating convention-

al MHs and are usually placed in a location simi-lar to that of APs in a WLAN. However, unlikeWLANs, neighboring MRs communicate witheach other via a wireless link. If two MRs arenot within direct wireless communication range,intermediate MRs may function to relay commu-nication (i.e., wireless multihop communication).Typically, no wiring is necessary to construct theWMN backbone. This is a significant advantageof the WMN over the WLAN, which requireswiring between APs; hence, cost and time todeploy a network service is saved using theWMN.

The WMN may be broadly classified into twocategories: Layer 2 and Layer 3 WMNs. In theformer, frame relaying (bridging) is performedin Layer 2 (the Media Access Control (MAC)layer) in the WMN backbone. A MAC address isused to deliver frames from one MH to anotherthrough the WMN backbone. Standardization ofthe Layer 2 WMN is under development inIEEE 802.11s. All wireless interfaces that formthe WMN backbone require use of the IEEE802.11s based devices. On the other hand, in thelatter type of WMN, Internet Protocol (IP)packet relaying is performed in Layer 3 (Net-work layer) in the WMN backbone [2]. An IPaddress is used to deliver IP packets from oneMH to another through the WMN backbone.There is no special requirement for the underly-ing wireless link systems used to connect theMRs. It is thus possible to select the most appro-priate wireless link system such as IEEE802.11a/b/g/n or IEEE 802.16 for each link interms of cost and performance to form a hetero-geneous wireless network for the WMN back-bone; each MR in the WMN backbone may havemultiple interfaces to different wireless link sys-tems. The broadcasting of Address ResolutionProtocol (ARP) request frames is necessary inthe Layer 2 WMN backbone, consuming pre-cious wireless bandwidth, while it can be avoidedin the Layer 3 WMN backbone. For these rea-sons, Layer 3 WMN is expected to be more scal-able than Layer 2 WMN.

Various approaches and technical challengesto realize the Layer 3 WMN have been proposedand discussed in the literatures, though stan-dardization is yet to begin. Mobility managementis one of the major technical issues and chal-lenges in the realization of the Layer 3 WMN. Itis necessary for MHs to continuously send and/or

ABSTRACT

Wireless Mesh Networks (WMNs) may bebroadly classified into two categories: Layer 2and Layer 3 WMNs. This article focuses on theLayer 3 WMN, which provides the same serviceinterfaces and functionalities to the conventionalmobile host (MH) as the conventional wirelesslocal area network. Three essential design issuesto realize seamless mobility management in theLayer 3 WMN are identified and systematicallydiscussed in this article. They are IP address res-olution, location management, and Media AccessControl (MAC) address resolution. The Layer 3WMN backbone requires systematic manage-ment of the IP and MAC addresses of each MH,which can be realized by four basic approaches:centralized management, home management,replication management, and distributed man-agement. It is shown that the address pair man-agement architecture is fundamental to realizingefficient packet forwarding and address resolu-tion. Design guidelines are provided to realizeLayer 3 WMN supporting seamless MH roam-ing, considering the applicability of address pairmanagement architectures with regard to WMNscale and client mobility; this applicability is con-sidered based on a qualitative evaluation of themanagement overhead and handover perfor-mance.

ACCEPTED FROM OPEN CALL

Kenichi Mase, Graduate School of Science and Technology, Niigata University

Layer 3 Wireless Mesh Networks:Mobility Management Issues

MASE LAYOUT 6/20/11 4:17 PM Page 156

IEEE Communications Magazine • July 2011 157

receive IP packets for the ongoing communica-tion sessions during roaming in WMN. Readersmay refer to [3, 4] for introductions and generaldiscussions of mobility management in WMNs.This article focuses on mobility management forLayer 3 WMN that supports client side trans-parency for conventional MHs. The contribu-tions of this article are:• The identification of the technical issues

involved in designing Layer 3 WMN to sup-port client side transparent mobility.

• Discussing the common frameworks andarchitectures for solving these issues andrealizing essential functions.

• Presenting the design guidelines, includingthe selection of architectures.

MOBILITY MANAGEMENT ISSUESAND RELATED WORKS

The architecture of WMNs can generally beclassified into three types: Infrastructure/Back-bone WMNs, Client WMNs, and Hybrid WMNs[1]. This article focuses on a primitive class ofInfrastructure/Backbone WMNs, where MRsform a communication backbone for convention-al IEEE 802.11 MHs to provide a service equiva-lent to that of WLAN (Fig. 1). Hereafter, thistype of WMN is referred to as “Layer 3 WMN”or simply “WMN.” An MH establishes a link-layer association with one of the MRs throughIEEE 802.11 infrastructure mode protocol andobtains an IP address through a Dynamic HostConfiguration Protocol (DHCP) service provid-ed in the WMN, when it first enters the WMN.It may roam within the service coverage ofWMN. The link-layer handover from one MR toanother follows the IEEE 802.11 handover pro-cedure. Instead of the IEEE 802.11 infra-structure mode, an ad hoc mode is used both forMRs and MHs in [5, 6]. However, such a restric-tion is not appropriate for meeting our objectiveof realizing a WMN that can be used instead ofWLAN and is hence out of the scope of thisarticle.

There are two approaches to supportingmobility management for the Layer 3 WMN. Inthe first approach, each MH has two IP address-es, one for representing MH identification andthe other for representing the location of theMH, which is the current point of attachment ofthe MH to the WMN (one of MRs), as in theMobile IP (two-tier addressing model). EachMR configures an individual Extended ServiceSet (ESS) composed of one Basic Service Set(BSS). When an MH roams from one MR toanother in the same WMN, Layer 2 handoveroccurs. In addition, it conducts a new DHCPrequest and response to configure its IP addressfor representing the location of MH (care-of-address). In the second approach, each MH hasa single IP address. All MRs cooperatively con-figure a single ESS. MHs can maintain their IPaddress constant (single addressing model) dur-ing MR-to-MR roaming. Just a link-layer han-dover is sufficient for MHs to realize seamlessroaming.

Since IP packet forwarding is performed inthe Layer 3 WMN backbone, the two-tier

addressing model is quite straightforward, whereMHs need to have functionalities to support theLayer 3 handover [7–9]. On the other hand, inthe single addressing model, no special function-alities are required for MHs (client side trans-parency in [4]). Since our goal is to provide thesame backbone service for conventional MHs asin the WMN, only the client side transparencyseems to be a promising approach. However, inthis approach, special care is required for IPpacket forwarding without any dependence onMH functionalities [10–13].

There are three essential design issues forsupporting mobility management for a singleaddressing model.

IP address resolution: Since an IP address isused to deliver IP packets from one MH toanother through the Layer 3 WMN backbone,each MR needs to recognize the IP addresses ofthe MHs that are associated to it. When an MHroams from one MR to another, it completes alink-layer association with a new MR. As aresult, the MR can recognize the MAC addressof the MH. However, it cannot freely obtain theIP address of the MH. In [14], DHCP servers onMRs assign IP addresses to the MHs based onthe MAC addresses of the MHs, so that the MRcan infer the IP address of the roamed MH fromits MAC address. However, this prevents effi-cient IP address assignment, for instance, thepossible use of address aggregation, as men-tioned in the next section. How then is the IPaddress of the roamed MH obtained?

Location management: Since each MH hasonly one IP address in the single addressingmodel and possibly roams from one MR toanother in the Layer 3 WMN backbone, this IPaddress represents MH identification but doesnot represent its location, unlike the care-of-address used in Mobile IP. How is the locationof an MH to which the packets are deliveredidentified? In MobileNAT, the address repre-senting MH identification is translated into theaddress representing the current point of theMH attachment to the Internet and is used forrouting packets to the MH [7, 8]. This techniquecan be used when an MH only communicateswith a corresponding node in the Internet butcannot be used for communications within theWMN. On the other hand, a routing protocoloperating in the WMN backbone can be used tocreate and maintain the routing table of eachMR, which includes the IP addresses of the MHs

Figure 1. An example of wireless mesh network structures.

Heterogenouswirelessnetwork

TheInternet

GW

MR: Mesh routerGW: GatewayMH: Mobile host

MH MH MH MH MH

MR MR

Wireless link

MR

MH MH MH

MRMR

IEEE802.11WLAN


IEEE Communications Magazine • July 2011158

as the destination (flat routing or host routing).The roaming of MHs updates the routing tablefor the related MRs, resulting in a possible highmessage overhead consuming wireless band-width. How then is efficient location manage-ment conducted?

MAC address resolution: Since Layer 3WMN provides one ESS for MHs, ARP needsto be supported for it. An MH (source MH) ofone MR may send an ARP request to anotherMH (destination MH) belonging to a differentMR. For a straightforward emulation of ARP inthe Layer 3 WMN backbone, a flooding ARPrequest over all the MRs of the WMN backboneis required [12]; this ARP request consumeswireless bandwidth and should be avoided. In[13], a kind of proxy ARP was proposed. Uponreceiving the ARP request, the MR replies tothe ARP request from the MH with its ownMAC address. In [14], the MR replies with afake MAC address that is uniform on all MRs,and hence, the source MH does not need toupdate its ARP cache when it roams to anotherMR. Unfortunately, in both approaches, a prob-lem arises when the source and destination MHsmeet in the same MR later. How then is effi-cient ARP performed?

Several studies have been conducted onmobility management in WMNs. However, thethree design issues mentioned above have notyet been fully discussed. This article presentsframeworks to systematically address theseissues.

ADDRESS PAIRMANAGEMENT ARCHITECTURES

An MH needs to obtain an IP address when itnewly joins the WMN, that is, when it establish-es Layer 2 association with one of the MRs ofthe WMN. Two types of the DHCP service canbe considered. In type I, an address blockreserved for MHs is provided for the entireWMN and each MH is assigned an IP addressfrom the range of this address block. A DHCPserver located in the WMN performs IP address

assignment for MHs in cooperation with theDHCP relay servers located in the MRs. In typeII, the address block is divided into addressblock units with a pre-determined address range.Each MR requests an address block unit to theaddress management server (AMS) in the WMNbackbone. Upon receiving the request, the AMSassigns one or more address block units in theform of an address prefix to the requesting MR.An MH is assigned an IP address from the rangeof the address block units provided to the MR itassociates with. (This MR is hereafter referredto as the “home MR.”) Each MR may have aDHCP server to perform efficient DHCP ser-vice, although a single DHCP server arrange-ment is also possible. An MH may roam fromone MR to another, but it is expected that themajority of MHs remain in the original location(home MR). For these MHs, the prefixes oftheir IP addresses represent their locations.

Two MHs in the same Layer 3 WMN use theMAC address to identify the sender and receiverof the frame even if they connect different MRs,while the MRs in the WMN backbone use the IPaddress to identify the sender and receiver of thecorresponding IP packet included in the framein order to forward the IP packet. To performpacket forwarding in the WMN backbone, theMAC and IP addresses (an address pair) of eachMH and its location (the current MR) need tobe systematically managed in the Layer 3 WMN.This article presents four address pair manage-ment architectures, the first three architecturesfurther classified into Schemes A and B, to pro-vide a framework for the mapping between theMAC address and the IP address of each MH.This mapping is used to solve two design issues —IP address resolution and MAC address resolu-tion — mentioned in the previous section. Theaddress pair management architecture thus pro-vides a foundation for the packet forwardingmechanism based on the IP address resolution(see next section) and MAC address resolutionmechanism (see later sections).

CENTRALIZED MANAGEMENTIn Scheme A, a centralized server maintains theaddress pair records of all MHs in the WMN[13]. This server is also referred to as AMS forconvenience, but it may be different from theAMS used for IP address assignment. In SchemeB, the AMS additionally maintains the currentMR records of all MHs. When an IP address isassigned to an MH, the home MR obtains theaddress pair and sends it to the AMS. When anMH roams between MRs, the current MRobtains the MAC address of the MH and sendsit to the AMS, which returns the IP address ofthe MH to the current MR (see 1-3 in Fig. 2). Inaddition, in Scheme B the AMS updates the cur-rent MR of the MH (see 1-3 in Fig. 3).

HOME MANAGEMENTIn Scheme A, each MR maintains the addresspair records of the MHs for which it is the homeMR. In addition, a centralized server maintainsthe MAC address and the home MR pair recordsof all MHs in the WMN. This server is termedthe home server (HS). In Scheme B, each MRadditionally maintains the current MRs of the

Figure 2. Packet forwarding mechanism (centralized management-Scheme A).

AMS

7: Packet forwarding

4 4: IPaddressnotification

5: Route update

5: Route update

3: Response

2: IP addressinquiry

1: Roaming

6: Packet sending



MHs for which it is the home MR. When an IPaddress is assigned to an MH, the home MRreports the MAC address and the home MR(itself) of the MH to the HS. When an MHroams between MRs, the current MR obtainsthe MAC address of the MH and sends it to theHS, which returns the home MR of the MH tothe current MR. The current MR informs thehome MR of the MAC address of the MH andobtains the IP address of the MH in return. Inaddition, in Scheme B the home MR updatesthe current MR of the MH [11].

REPLICATION MANAGEMENTIn Scheme A, each MR maintains the addresspair records of all MHs in the WMN [10, 11]. InScheme B, each MR additionally maintains thecurrent MRs of all MHs. When the IP address isassigned to an MH, the home MR obtains theaddress pair and sends it to all the other MRsusing flooding or multicast protocols. When anMH roams between MRs, the current MRobtains the MAC and IP addresses of the MHusing its own address pair records. In Scheme B,when an MH roams between MRs, the currentMR obtains the MAC address of the MH andsends it (or its IP address) to all the other MRsto notify the current MR of the MH and eachMR updates the current MR of the MH (see 1-3in Fig. 4).

DISTRIBUTED MANAGEMENTEach MR maintains the address pair records ofMHs with which it has or had an association[15]. When an MH roams between MRs, thecurrent MR obtains the MAC address of theMH. If the current MR does not know the cor-responding IP address, it sends an IP addressrequest that includes the MAC address of theMH to the neighboring MRs based on theexpanded ring search. In this ring search, themaximum number of searches is given and themaximum number of hops per search increaseswith each search trial [15]. The neighboring MRthat maintains the address pair record of therequested MH returns the IP address of the MHto the requesting MR (see 1–3 in Fig. 5). Assum-ing the MH’s typical roaming speed from oneMR to another, it is highly probable that one ofthe one-hop neighboring MRs has the requestedaddress pair record. The maximum number ofhops in the first search may thus be limited toone or two hops, and the first search is expectedto almost always succeed.

PACKET FORWARDING MECHANISMConsider IP packet forwarding between MHs ina Layer 3 WMN backbone. The MH that sendsthe packets and its current MR are termed thesource MH and source MR, respectively. TheMH that receives the packets and its currentMR are termed the destination MH and destina-tion MR, respectively. The source and destina-tion MHs may be accommodated in the sameMR or in different MRs. In the former case,each frame that conveys an IP packet is relayedto the destination MH at Layer 2 using the APcapability of the MR, as is usual in WLAN. Inthe latter case, packet forwarding is performed

in the manner of either flat or hierarchical rout-ing. Flat routing is employed in centralized man-agement-Scheme A, home management-SchemeA, replication management-Scheme A, and dis-tributed management. MRs have route entriesfor destination MHs in their routing table. Effi-cient location management in these schemes isone of the design issues mentioned in the secondsection. Hierarchical routing is employed in cen-tralized management-scheme B, home manage-ment-Scheme B, and replication management-Scheme B. The source MR first identifies thedestination MR of the destination MH and thensends the packets to the destination MR bymeans of IP-in-IP encapsulation or IPv6 routingheader (tunneling). The destination MR thenforwards the received packets to the destinationMH. MRs do not have the route information fordestination MHs but for destination MRs intheir routing table.

In both flat and hierarchical routing, rout-ing protocol runs in the WMN backbone. Inflat routing, MH information (IP address) isexplicitly included in the routing messages forroute creation and update, while in hierarchi-cal routing, it is not. The mobile ad hoc net-work (MANET) routing protocol is areasonable candidate to be used for the WMNbackbone, since it supports the wireless multi-hop packet forwarding capability. MANETrouting protocols are generally classified intoproactive and reactive routing protocols. Eitherof these can be used in the WMN backbone.The choice of the appropriate routing dependson the scale of the WMN, traffic characteris-tics, and other conditions.

Next, the packet forwarding mechanism foreach of the address pair management approach-es is described.

CENTRALIZED MANAGEMENTScheme A — Each MR periodically notifies theIP address information of the MHs that associ-ate with it to other MRs in routing messages(proactive routing) or in response to the routerequest (reactive routing). If IP address assign-ment type II is used in proactive routing, it does

Figure 3. Packet forwarding mechanism (centralized management-Scheme B).

AMS


4: Packet sending

3: Response

6: Response

2: IP address inquiry

3: Current MRupdate

3: Current MRinquiry

1: Roaming


not have to advertise the individual IP addressinformation of the associating MHs for which itis the home MR. Instead, it advertises the prefixinformation of the address block units that itowns. As a result, the amount of IP addressinformation of the MHs included in the routingmessages is substantially reduced to performefficient location management. A source MRcan then calculate or discover a route to theAMS and the destination MH by means of arouting protocol that runs in the WMN back-bone and forwards the packet to the destinationMH (see 4–7 in Fig. 2).

Scheme B — Each MR can calculate or dis-cover a route to AMS and other MRs by meansof a routing protocol that runs in the WMNbackbone. If the source MR does not hold thecurrent MR of a received packet, it sends thelocation request that includes the IP address ofthe destination MH to the AMS, which thenreturns the current MR of the destination MH.The source MR then forwards the packet tothe current MR through tunneling (see 4–7 inFig. 3).

HOME MANAGEMENTScheme A — The route update is performed ina manner similar to that in centralized manage-ment-Scheme A.

Scheme B — Each MR can calculate or discov-er a route to other MRs by means of a routingprotocol that runs in the WMN backbone. InMethod 1, if the source MR does not hold thecurrent MR of a received packet, it sends thelocation request that includes the IP address ofthe destination MH to its home MR, whichreturns the current MR of the destination MH.The source MR then forwards the packets to thecurrent MR through tunneling. In Method 2, thesource MR sends packets to the home MR ofthe destination MH, after which the home MRforwards the packets to the current MR; boththe sending and forwarding of the packetsemploy tunneling. To directly resolve the home

MR from the IP address of the destination MH,IP address assignment-type II is assumed above.A kind of home management-Scheme B-Method 2is termed the transparent MIP (Mobile IP) in[11].

REPLICATION MANAGEMENTScheme A — The route update is performed ina manner similar to that in centralized manage-ment-Scheme A.

Scheme B — Each MR can calculate or discov-er a route to other MRs by means of a routingprotocol that runs in the WMN backbone. Thesource MR owns the record of the current MRof the destination MH and forwards the packetto the current MR through tunneling (see 4–5 inFig. 4).

DISTRIBUTED MANAGEMENTThe route update is performed in a manner sim-ilar to that in centralized management-SchemeA (see 4-6 in Fig. 5).

MAC ADDRESSRESOLUTION MECHANISM

In WLAN, a source MH broadcasts an ARPrequest that includes its MAC and IP addressesand the IP address of the destination MH, whenthe MAC address of the destination MH doesnot exist in its ARP table. Upon receiving theARP request, the destination MH sends back anARP reply, which includes its MAC and IPaddresses, to the source MH and updates itsARP table with regard to the source MH. Thesource MH updates its ARP table when itreceives the ARP reply.

Similarly, ARP needs to be supported in theLayer 3 WMN to provide the ESS service toMHs. The receiver MAC address of the frameoriginating from the source MH and conveyingan IP packet to be sent to the destination MH isthus set to the MAC address of the destinationMH. Consider the case that the source and des-tination MRs are different. In such a case, IPpackets received from the source MH are for-warded from the source MR to the destinationMR without using the MAC address of the des-tination MH set in the receiver MAC address ofthe frames originating from the source MH.Thus, one may consider that faithful (correct)address resolution is not necessary and thatinstead of returning the actual MAC address ofthe destination MH, a dummy MAC address isreturned by the source MR [13, 14]. However,unfortunately, this consideration is inaccurate,because if the source MH and destination MHlater meet in the same MR, the frames cannotbe relayed (bridged) in the Layer 2 of the MR;therefore, Layer 3 forwarding is required, whichunnecessarily increases the Layer 3 processingload of the MR.

Next, the ARP mechanism for all the central-ized and distributed management approaches isdescribed under the assumption that the routeinformation to the destination MH or the desti-nation MR is proactively calculated or reactivelydiscovered. Due to space constraints, the ARP


Figure 4. Packet forwarding mechanism (replication management-Scheme B).


4: Packet sending

2: CurrentMR

notification

3: Current MRupdate

3: CurrentMR

update

1: Roaming

2



mechanism for other approaches is not dis-cussed. The proposed ARP mechanism performsfaithful ARP replies, while the ARP requestmulticast is not necessary in the WMN back-bone.

CENTRALIZED MANAGEMENTScheme A — When a source MR receives anARP request and contains the reply information(MAC address of the destination MH) in itsARP table, it returns the ARP reply on behalf ofthe destination MH. Otherwise, it sends an ARPrequest packet (IP packet that includes the ARPrequest message) to the AMS. Upon receivingthis request, the AMS returns an ARP replypacket (IP packet that includes the ARP replymessage) to the source MR. When the sourceMR receives this reply, it updates its ARP tableand returns the ARP reply to the source MH onbehalf of the destination MH. It also sends agratuitous ARP reply packet (IP packet thatincludes the gratuitous APR message) to thedestination MH. The destination MR interceptsthis ARP reply packet, updates its ARP table,and sends the gratuitous ARP reply to the desti-nation MH on behalf of the source MH. Thesource and destination MHs update their ARPtables when they receive the ARP and gratuitousARP replies, respectively.

Scheme B — The procedure is the same as thatemployed in Scheme A. The difference is in thesender and receiver of the gratuitous ARP. InScheme A, the source MR sends a gratuitousARP reply packet to the destination MH, whilein Scheme B, the AMS sends a gratuitous ARPreply packet to the destination MR, since theAMS has the current MR record of the destina-tion MH. Upon receiving the ARP reply, thedestination MR updates its ARP table and sendsthe gratuitous ARP reply to the destination MHon behalf of the source MH.

DISTRIBUTED MANAGEMENTWhen a source MR receives an ARP requestand it contains the information in its ARP table,it returns the ARP reply on behalf of the desti-nation MH; otherwise, it sends an ARP requestpacket to the destination MH. The destinationMR intercepts this ARP request, updates itsARP table, and returns an ARP reply packet tothe source MR. Upon receiving this reply, thesource MR updates its ARP table and returnsthe ARP reply to the source MH on behalf ofthe destination MH. The destination MR alsosends a gratuitous ARP reply to the destinationMH on behalf of the source MH. The sourceand destination MHs update their ARP tableswhen they receive the ARP and gratuitous ARPreplies, respectively. It should be noted that thismethod can also be employed in centralizedmanagement-Scheme A and home management-Scheme A.

DESIGN GUIDELINESThe mechanisms and features of address pairmanagement architectures for realizing Layer 3WMN that supports client side transparentmobility management are summarized in Table

1. The requirements and guidelines in designingand selecting the Layer 3 WMN architectureswith consideration of their applicability to WMNscale and client mobility are given below.

BASIC REQUIREMENTThe mobility management architectures shouldsatisfy the following two requirements:• Data frames that are sent from one MH to

another belonging to the same MR shouldbe relayed in Layer 2. Note that this condi-tion is always satisfied in the conventionalWLAN, where AP is used to bridge dataframes in the BSS. If Layer 3 processing isrequired to relay data frames within theMR of WMN, the MR requires highercomputing power than the AP of theWLAN and may act as a bottleneck to pro-cessing.

• ARP request message flooding or multicastshould be avoided to preserve the preciouswireless bandwidth of the WMN.All of the four basic architectures presented

in this article satisfy the requirements mentionedabove and recommended as the basic architec-ture options in designing Layer 3 WMN.

ARP MESSAGE OVERHEADThe ARP message overhead within each BSS iscommon in all architectures. In the WMN back-bone, the ARP request and reply messagesrequire two-way unicast message propagation,while the gratuitous ARP reply requires one-way unicast message propagation; the formermessage overhead is approximately double thelatter one. The relative cost for the ARP mes-sage overhead is shown in Table 1, assumingthat the number of hops from the source MRto the AMS, to the home MR, or to the desti-nation MR is the same, and that the two-wayunicast message propagation cost is 1 unit.Since the ARP message traffic is usually small-er than the data packet traffic, the ARP costdifference between different architectures maynot be the primary factor for selecting thearchitecture.

Figure 5. Packet forwarding mechanism (distributed management).

Packet forwarding4 4: IP

addressnotification

5: Route update

5: Routeupdate

3: Response

2: IP address inquiry

1: Roaming

6: Packet sending



CENTRALIZED MANAGEMENT VERSUSHOME MANAGEMENT

Among the four basic architecture approaches,centralized management and home managementare similar since both these approaches require acentralized sever, AMS, and HS, respectively.Two successive inquiries for resolving the homeMR and IP address are required in the case ofroaming in home management, which is its maindrawback as compared to centralized manage-ment. Home management may thus be excludedfrom the architecture options.

SCHEME B OPTION INMANAGEMENT ARCHITECTURES

In Scheme B, the current MR of each MH isadditionally maintained in the AMS in case ofcentralized management and in all the MRs incase of replication management. In centralizedmanagement-Scheme B, current MR notificationis performed together with IP address inquiry tothe AMS, which is also necessary in Scheme A.Therefore, Scheme B is almost the same asScheme A in terms of message overhead. On theother hand, in replication management, IPaddress inquiry is not necessary and current MRnotification needs to be independently providedfor Scheme B. Therefore, Scheme B is inferior

to Scheme A in terms of message overhead, andthe advantage of Scheme B over Scheme A isdebatable. Replication management-Scheme Bmay thus be excluded from the architectureoptions.

SCHEME A VERSUS SCHEME B INCENTRALIZED MANAGEMENT

In Scheme A, routing overhead and handoverdelay significantly depend on inter-MR mobility(the number of MHs that roam from one MR toanother with respect to time). With high inter-MR mobility, route update needs to be fre-quently performed to quickly follow the changein MH locations, thereby increasing the routingoverhead. Otherwise, the route update isdelayed, resulting in increasing handover delay.On the other hand, in Scheme B, routing over-head and handover delay do not depend oninter-MR mobility, since route update is notnecessary. Therefore, Scheme B can be superiorto Scheme A when inter-MR mobility is signifi-cantly high.

ADDRESS PAIR MANAGEMENT OVERHEADThe address pair management overhead signifi-cantly depends on the MH changing rate (thenumber of new MHs joining WMN with time).Consider the following two cases:

Table 1. Comparison between address pair management architectures.

Addresspair

location

CurrentMR

location

Address pair & Current MRmanagement message

overhead Routingmessageoverhead

ARP message overhead

Forwardingincrease Handover delay

Joining RoamingARP request &

reply(Relative cost)

GratuitousARP reply

Cen

tral

ized

man

agem

ent

SchemeA AMS N/A

Addresspair toAMS

IP addressinquiry to AMS Route update

From sourceMR to AMS (1)

From sourceMR to destina-tion MH (0.5)

N/AIP address inquiry

to AMS Routeupdate

SchemeB AMS AMS

IP addressinquiry to AMS

Current MRnotification

Current MRinquiry to

AMS Tunnel-ing

From AMS todestination

MR (0.5)

Current MRinquiry to

AMS

IP address inquiryto AMS

Hom

em

anag

emen

t

SchemeA

HomeMR N/A

MACaddress &home MR

to HS

Home MRinquiry to HS IPaddress inquiry

to home MR

Route update

From sourceMR to Home

MR (1)


N/A

Home MR inquiryto HS IP address

inquiry to home MRRoute update

SchemeB

HomeMR

HomeMR

Home MRinquiry to HS IPaddress inquiry

to home MRCurrent MR

notification tohome MR

Current MRinquiry tohome MR

(method 1)Tunneling

(Method 1 &Method 2)

From homeMR to destina-tion MR (0.5)

Current MRinquiry tohome MR

(Method 1)Triangular

routing(Method 2)

Home MR inquiryto HS IP address

inquiry to home MRCurrent MR notifi-cation to home MR

Repl

icat

ion

man

-ag

emen

t

SchemeA Each MR N/A

Addresspair to all

MRs

N/A Route update

N/A


N/A

Route update

SchemeB Each MR Each MR

Current MRnotification to

all MRsTunneling

From sourceMR to destina-tion MR (0.5)

Current MR notifi-cation to all MRs

Distributedmanagement

Currentand past

MRsN/A N/A

IP addressinquiry to

neighbor MRsRoute update

From sourceMR to destina-

tion MH (1)N/A N/A

IP address inquiryto neighbor MRs

Route update



Case 1 — Low MH changing rate: Roamingoverhead is of primary concern since joiningoverhead is not significant. In replication man-agement-Scheme A the roaming overhead iszero, while in other managements, it increaseswith inter-MR mobility. Replication manage-ment-Scheme A is thus an attractive choice inthe case of high inter-MR mobility. However,the joining overhead may still become significantwhen the WMN scale is quite high in replicationmanagement-Scheme A. In such a case, dis-tributed management may also be considered asuitable option.

Case 2 — High MH changing rate: The join-ing overhead is of considerable concern. It iszero in case of distributed management, while itincreases with MH changing rate in case of othermanagement approaches. In distributed manage-ment, the roaming overhead remains almostunchanged with the WMN scale, since the IPaddress inquiry is targeted only at the neighborMRs. Distributed management is thus superiorto other management options.

CONCLUSIONSWireless mesh network (WMN) is an emergingmobile network technology, and extensive stud-ies are being conducted to realize scalable andhigh-performance WMNs. This article identi-fied the essential technical issues in designingLayer 3 WMN that supports client side trans-parent mobility management. In this design, theLayer 2 system for MHs and the Layer 3 systemfor MRs need to be seamlessly combined inMRs. The most fundamental issue in terms ofarchitecture is to efficiently maintain theaddress pair (MAC and IP addresses) and thelocation information of each MH that newlyjoins and roams in a WMN. Four address pairmanagement architectures — centralized man-agement, home management, replication man-agement, and distributed management — arepresented. The first three management archi-tectures are further classified into Scheme Aand Scheme B, which employ flat and hierarchi-cal routing, respectively. This article discussesthe use of these management architectures asthe frameworks for realizing the essential mech-anisms of the Layer 3 WMN — packet forward-ing and address resolution. Finally, therequirements and guidelines in designing andselecting Layer 3 WMN architectures support-ing client side transparent mobility manage-ment are given, considering their applicabilityto WMN scale and client mobility based on thequalitative evaluation of message overhead andhandover performance. This article can be usedto lay a foundation and provide frameworks fordesigning and developing the practical Layer 3WMN.

REFERENCES[1] I. F. Akyildiz, X. Wang, and W. Wang, “Wireless Mesh

Networks: A Survey,” Computer Networks, vol. 47, no.4, pp. 445–87, 2005.

[2] K. Mase et al., “A Testbed-based Approach to DevelopLayer 3 Wireless Mesh Network Protocols,” Trident-com2008.

[3] J. Xie and X. Wang, “A Survey of Mobility Managementin Hybrid Wireless Mesh Network,” IEEE Network, vol.22, no. 6, 2008, pp. 34–40.

[4] V. Mirchandani and A. Prodan, “Mobility Managementin Wireless Mesh Networks,” Comp. Commun. and Net-works, 2009, pp. 349–78.

[5] S. Speicher, “OLSR-FastSync: Fast Post-Handovers RouteDiscovery in Wireless Mesh Networks,” Proc. IEEE Vehic.Tech. Conf. (VTC’06-Fall), 2006, pp. 1–5.

[6] Y. Amir et al., “Fast Handover for Seamless WirelessMesh Networks,” Proc. ACM Int’l. Conf. Mobile Syst.,Apps., Services (MobiSys), 2006, pp. 83–95.

[7] M. Buddhikot et al., “MobileNAT: A New Technique forMobility Across Heterogeneous Address Spaces,” ACMMobile Networks and Applications, vol. 10, no. 3,2005, pp. 289–302.

[8] K. N. Ramacandran et al., “On the Design and Imple-mentation of Infrastructure Mesh Networks,” Proc. 1stIEEE Wksp. Wireless Mesh Networks, 2005.

[9] A. M. Srivatsa and J. Xie, “A Performance Study ofMobile Handover Delay in IEEE 802.11-Based WirelessMesh Networks,” Proc. IEEE Int’l. Conf. Commun. (ICC),2008, pp. 2485–89.

[10] Y. Owada and K. Mase, “A Study on Protocol, Imple-mentation and Throughput Evaluation for MultihopWireless LAN,” IEEE Vehic. Tech. Conf. (VTC 2003-Spring), vol. 3, 2003, pp. 1773–77.

[11] V. Navda, A. Kashyap, and S. R. Das, “Design and Eval-uation of iMesh: An Infrastructure-Mode Wireless MeshNetwork,” IEEE Int’l. Symp. A World of Wireless, Mobileand Multimedia Networks (WoWMoM), 2005, pp.164–70.

[12] L. Iannone and S. Fdida, “MeshDV: A Distance VectorMobility-Tolerant Routing Protocol for Wireless MeshNetworks,” IEEE ICPS Wksp. Multi-hop Ad Hoc Net-works: From Theory to Reality (REALMAN), 2005.

[13] H. Wang et al., “A Network-Based Local Mobility Man-agement Scheme for Wireless Mesh Networks,” Proc.IEEE Wireless Commun. Net. Conf. (WCNC), 2007, pp.3795–800.

[14] M. Ren et al., “MEMO: An Applied Wireless Mesh Net-work with Client Support and Mobility Management,”Proc. IEEE Global Telecom. Conf. (GLOBECOM), 2007,pp. 5075–79.

[15] N. Azuma, K. Mase, and H. Okada, “A Proposal ofLow-Overhead Routing Scheme for Layer 3 WirelessMesh Networks,” Int’l. Symp. Wireless Pers. MultimediaCommun., 2009.

BIOGRAPHYKENICHI MASE [F] ([email protected]) received the B.E., M. E., and Dr. Eng. Degrees in Electrical Engineeringfrom Waseda University, Tokyo, Japan, in 1970, 1972, and1983, respectively. He joined Musashino Electrical Commu-nication Laboratories of NTT Public Corporation in 1972.He was Executive Manager, Communications Quality Labo-ratory, NTT Telecommunications Networks Laboratoriesfrom 1994 to 1996 and Communications Assessment Labo-ratory, NTT Multimedia Networks Laboratories from 1996to 1998. He moved to Niigata University in 1999 and isnow Professor, Graduate School of Science and Technolo-gy, Niigata University, Niigata, Japan. He received IEICEbest paper award for the year of 1993 and the Telecom-munications Advanced Foundation award in 1998. Hisresearch interests include communications network designand traffic control, quality of service, mobile ad hoc net-works and wireless mesh networks. He is an IEICE Fellow.

The most

fundamental issue in

terms of architecture

is to efficiently

maintain the

address pair

(MAC and

IP addresses) and

the location

information of each

MH that newly joins

and roams in a

WMN.


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Layer 3 wireless mesh networks: mobility management issues