Load-balancing and Inter-domain Mobility for ... - Semantic Scholar

Load-balancing and Inter-domain Mobility for Wireless Mesh Networks Bin Xie1, Yingbing Yu2, Anup Kumar1

Dharma Parkash Agrawal

Computer Engineering and Computer Science Dept.1 University of Louisville, Louisville, KY, 40292 Division of Natural and Mathematical Sciences2 LeMoyne-Owen College 807 Walker Avenue, Memphis, TN, 38126 (b0xie001, ak)@louisville.edu1, [email protected]

OBR Center for Distributed and Mobile Computing Department of ECECS University of Cincinnati Cincinnati, OH 45221-0030 [email protected]

Abstract—This paper proposes a scheme for domain partition to achieve the tradeoff between load-balancing and inter-domain mobility to reduce the negative impact of the host mobility. The load-balancing scheme for domains includes: an initialization procedure to divide a mesh network into domains, and a load adjustment procedure to rebalance the traffic load of neighboring domains when required. Moreover, the proposed scheme provides inter-domain mobility in support of multi-hop communication with the Multi-hop cellular IP (MCIP) mobility protocol. Our experimental results show that the proposed protocol effectively controls the migration of mesh routers as well as mobile stations.

enhancement of Multi-hop cellular IP further implements the inter-domain mobility of MSs in Section IV. In Section V, experiments illustrate the tradeoff between domain partition and inter-domain handoff. Section VI concludes the paper. II.

BACKGOUND AND RELATED WORK

A. Mesh Network Architecture Wired Link

Internet

Wireless Link

Keywords-Domain, Mobility, Load-balancing, Mesh Network HA/FA

Internet Gateway

I. INTRODUCTION Wireless mesh networks have been increasingly gaining attention in the last few years. Mesh networks not only provide high bandwidth Internet access, but also offer low-cost and flexible deployment from commercial market perspective. One of the typical applications is mesh community networks to provide Internet connections with shared broadband, in place of cables and DSLs with which the network service is expensive for users. Rather than a community, with complicated routing technologies, wireless mesh networks (i.e., enterprise networks) can be built in larger areas such as airports, hotels, shopping malls, or office buildings, etc. Different from pure ad hoc networks, mesh networks are tightly integrated with the Internet through Internet Gateways (IGWs) that act as Internet attachments for mesh routers as well as their associated mobile stations (MSs). The wireless link capacity of IGWs will be the bottleneck for a mesh network. Therefore, in order to reduce the domain congestion, a loadbalancing approach is needed to direct the router traffic toward a light-loaded IGW. When a router changes its IGW, it incurs the inter-domain mobility of its associated MSs [1]. The process of inter-domain mobility may seriously degrade network performance in the case of the inter-domain migration of mesh router frequently happens between two neighboring domains. The focus of this paper is to build a load-balancing approach that provides the tradeoff between load-distribution among IGWs and Internet mobility of MSs. The proposed approach for load-balancing not only balances the traffic load among domains, but also accordingly executes appropriate mobility operations to support the desirable inter-domain mobility. It effectively reduces the negative impacts caused by interdomain mobility. The rest of this paper is organized as follows: the background is discussed in Section II. Then, Section III describes our domain partition and adjustment algorithms. The

Wireless Mesh Router Mobile Station

R2

IGW1

R4

HA/FA

R5

R8

R13

Wireless Backbone

IGW2

R10

R12

R7

R1

R3

R6

R9

R11

MS3

MS2

Client MANETs

MS1 Local domain 1: D1

Local domain 2: D2

Figure 1. Three-layer Architecture of a Wireless Mesh Network

As can been seen in Figure 1, a wireless mesh network has three layers in its architecture [1]: IGWs connected to the Internet, wireless mesh backbone of routers, and client MANTETs (mobile ad hoc networks). In the top layer, the IGWs serve as Internet attachments and thus provide Internet connectivity for MSs. In the second layer, wireless mesh routers are interconnected with wireless links and configured to a wireless mesh backbone for MSs. A mesh network can be divided into local domains in terms of relationships between mesh routers and IGWs. As shown in Figure 1, a local domain (Di , the superscript i represents the ith domain) consists of a single IGW, mesh routers whose Internet traffic travels through the IGW, and the associated MSs. Figure 1 illustrates a mesh network with two local domains marked as D1 and D2. In the client MANET layer, all MSs are connected to mesh routers by a single hop or multi-hop path for Internet connectivity. When a MS is situated within the radio coverage of a mesh router, it connects to the wireless mesh backbone directly. Otherwise it operates in an ad hoc mode and connects to a mesh router through a multi-hop path. As shown in Figure 1, MS1 connects to a mesh router (i.e., R7) with a multi-hop route (path: R7-MS2-MS1). B. Load-balancing The load-balancing for mesh networks considers the issue of the flow distribution for avoiding network bottleneck and

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE GLOBECOM 2006 proceedings.

maximizing the network capability. Further, the load-balancing of mesh networks can be divided into: intra-domain and interdomain load-balancing. The intra-domain load-balancing optimizes the distribution of the traffic load over paths to avoid congestion inside a domain. For this purpose, Draves et al. Error! Reference source not found. consider a routing protocol that enables a mesh router to find a path to the IGW by evaluating the expected transmission time (ETT) over different wireless multi-radio paths. More information with respect to intra-domain load-balancing can be found at [1] [4] [6] [7]. On the contrary, the inter-domain load-balancing concerns with domain partitioning such that a mesh network appropriately allocates the traffic load to the IGWs of domains. The traffic in a mesh network typically goes through an IGW toward the Internet. When an IGW is highly loaded or congested, the interdomain load-balancing protocol has the capability to reallocate a part of traffic load in the overloaded domain to a neighboring free domain. Instead of redistribution of traffic load within a domain, the reallocation of data flow in inter-domain involves the inter-domain mobility of associated routers and MSs. If a mesh router changes its Internet attachment, all packets from the Internet cannot be redirected to the mesh router through the new IGW, and thus cannot be forwarded to the target MS due to lack of mobility support in existing load-balancing protocols. This paper develops an approach for domain partitioning for properly assigning mesh routers into domains. C. Mesh Router Migration As can be seen in Figure 1, each mesh router severs as a traffic aggregation node, which collects traffic from MSs and forwards it to an available IGW. A router migration occurs when a mesh router changes its Internet attachment (i.e., IGW) from its current one to another. As illustrated in Section II, the migration redirects data flow from a high traffic domain to a low traffic one. Thus, it achieves the load-balancing between two domains. For instance, in Figure 1, R7 migrates its aggregated traffic flow from IGW1 to IGW2 to reduce the load at IGW1. During the period of a router migration, the MSs associated with the router accordingly migrate from the current IGW to the new IGW. In Figure 1, if R7 migrates from IGW1 to IGW2, the mobility binding of MSs 1, 2 and 3 at their home networks should be changed to IGW2. Otherwise, the data packets from the Internet can no be redirected to R7 and will be dropped at D1. Therefore, besides the reassignment of routers between two neighboring IGWs, the migration involves the issue of inter-domain mobility of the MSs. As a result, every mesh router migration is not only the issue of domain repartition (router reassignment) but also involves inter-domain mobility of MSs that should be tackled by a mobility management protocol as discussed in the following subsection. D. Inter-domain Mobility Mobile IP handles the mobility of MSs, but is inadequate for multi-hop mesh networks because of its high update latency, large Internet signaling load, and lack of support of micro-mobility [2]. Recently a variety of enhancements have been proposed to overcome the shortcomings of the base mobile IP, e.g., MIP-RO, MIPv6, HMIPv6, IDMP, HAWAII, TeleMIP, Cellular IP, Fast handoffs. However, all above mobility protocols can not support heterogeneous multi-hop communication [2] that is needed to support the mobility of client MANETs as shown in Figure 1. Therefore, they are inapplicable for the mobility management in a mesh network. The recently proposed Multi-hop Cellular IP (MCIP) [2] [3] integrates mobile IP and cellular IP in support of

heterogeneous multi-hop communication for client MANETs. This protocol supports the migration for both single and multihop MSs in mesh networks. In the architecture of a MCIP mesh network, a local domain includes a home agent/foreign agent (HA/FA), a single IGW, routers, and single or multi-hop MSs. As shown in Figure 1, D1 and D2 are two MCIP local domains if the MCIP protocol is applied to the mesh network. The HA/FA address is used as the Internet identifier and the IGW address is the Care-Of-Address (CoA) for a local domain. A HA/FA in a local domain acts as the administrator for a local domain. Each MS in a local domain has a multi-hop paging/routing cache maintaining the necessary information for location management, connection management as well as multi-hop routing reconfiguration as needed. More details are described in Section IV. III.

LOAD-BALANCING DOMAIN PARTITION

The basic idea of the proposed protocol is to attach each mesh router to a domain in such a way that the traffic of each router is proportionally directed to a specific IGW based on the capacity of domains. The following symbols are defined to simplify the presentation of the protocol.

Di: D:

the ith domain or the set of routers in domain i. the set of domain in a mesh network with N domains, D = {D1, D2, D3 … DN}. the number of routers in domain i. n or ni : the total available link capacity of the ith domain. Totali: i the current aggregated load or bandwidth Load n: requirement of the nth mesh router in the ith domain. the utilization of the bandwidth of the ith domain λi (n): with n routers. ϕ (i1 ,i2 ) ( n1 , n2 ) : the absolute difference of bandwidth utilization between two neighboring domains ( D i , D i ) and n1 and n2 are the number of routers in D i and D i . the load-balancing threshold in a mesh network. δbalance: the threshold for domain partition or δconvg: adjustment(e.g., 0.01). It runs as the convergence value to control the process of domain partition and adjustment. The process runs until ϕ (i1 ,i2 ) ( n1 , n2 ) has the value that is less than δconvg. i the set of the routers that have k hop count (e.g., Router k: 1-hop, 2-hop) to a domain Di. For example, in Figure 1, R2 has 1 hop to D1, thus, R2 is a member of Router11. Router (a, b) k: the set of routers at domain Da and having k-hop links to domain Db. Da is the domain with lower capability while Db is domain with higher capability: λa (n1)> λb (n2). r: a router in the set of Routerik or Router (a, b) k . If a router only belongs to one of the member of Routerik in domain i, this router is referred as a non-overlap router in the set of Routerik. On the hand, if a router belongs to two or more sets of Routerik, the router is called as an overlap router. ->: assigning a router to a domain and r-> Di means router r is allocated to Di. m_hop: the maximum hop for the farthest mesh router to reach the nearest IGW. All routers can reach at least one IGW with a path equal to or less than mhop links. 1

2

1

2

A. Load-balancing Metric In the proposed approach, the assignment of a mesh router to a domain is decided by evaluating two load metrics: hop

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ndwidth utilization on a domain. The hop count for er is the smallest number of hops from the mesh IGW in a domain. If a mesh router has multiple o operate multiple radios, the hop count is by using the interface that has the shortest range. For instance, if a router has both IEEE b interfaces, the hop count is determined by using a because of its shorter transmission range. Hop a router to be initially assigned to the nearest same time, in the process of domain adjustment it uter in an overloaded domain and close to a higher W to be adjusted with a higher priority. If a router e hop count to two IGWs, the router may be he higher capacity IGW.

nd used metric is the bandwidth utilization (λi (n)) with n routers, which is the percentage of the used ompared with the total available link capacity in a n

∑ Load

i

λi (n ) = x =1

(2)

x

Total i

dwidth utilization in Eq. (2) evaluates the current y of a domain: the higher value of λi (n) of Di, the ty left in the domain. If two neighboring domains nt capacity (Totali), it allows the load-balance distribute load to a domain after weighing the each domain. In the protocol, each domain λi (n) in real time and two neighboring domains mpute the absolute difference of the bandwidth olized by: (3) ϕ ( i ,i ) (n1 , n2 ) = λi (n1 ) − λi (n2 ) 1 2

1

value (δconvg). Figure 2 gives the details of the algorithm. In the beginning, the domain set D of a mesh network initiates with N domains and the set of each domain is empty (Di = Ф). This means that each domain only has its correspondent IGW without any mesh router. N is also the total number of IGW in the mesh network. At the beginning, the utilization of each domain is zero (λi (0) =0). Assume that each mesh router already has discovered a path with k-hop count to the nearest IGW(s). Based on the hop-count information, for each K-hop from 1 to m_hop, each domain (Di ) in a mesh network initiates the set of Routeri k (line 1). If a mesh router can be connected to multiple IGWs with k-hop count, this mesh router is an overlap-router. On the contrary, non-overlap routers are the routers that only belong to one of the members of Routeri k. Then, for each K-hop from 1 to m_hop, the algorithm assigns each router to one of domains (lines 2-11). The non-overlap routers in of Routeri k are allocated to domain i (lines 4-5), and the overlap routers are selected to a domain that has higher capacity (lower λi (n)) (lines 8-9). After the assignment of all routers, by using Eq. (5) the network validates the convergence condition. If the load-balancing meets the requirement, the network enters a stable state otherwise the network starts the adjustment procedure as the illustration in Section III.D. 1 : Initialize ( D , D i , δ balance , δ convg , λ i ( 0), Router ki , m _ hop ) 2 : for ( k = 1, k ≤ m _ hop , k + + ){

3: For (each non-overlap router in Routerik) { 4: If the r is the member of Routerik , then r-> Di; 5: Update λi(n); 6: } 7: For (each overlap-router in Routerik) { 8: If the r is the member of several Routerik sets, then r-> Di \ with higher capacity; 9: Update λi(n); 10: } 11: }

2

Control posed protocol determines the assignment of a with a given load-balancing threshold: δbalance. The ntains a given balance by which the difference of tilizations of all pairs of neighboring domains ess than the load-balancing threshold: (4) 0 ≤ ϕ ( i ,i ) ( n1 , n2 ) ≤ δ balance ontrol parameter prevents mesh routers from gration because: if the traffic load in a domain the disequilibrium of the load with its neighboring till within the given threshold, the network has no ate any mesh router. A mesh network adjusts the f traffic load only when ϕ ( i1 ,i2 ) ( n1 , n 2 ) of two larger than the range set by Eq. (4). For this ch domain monitors its load in real-time and he load information with its neighboring domains or when required. the traffic load in a domain often changes nd randomly, it is hard for a load-balancing reach an absolute balance between two domains = 0). The protocol has a convergence value 2) e purpose of controlling the execution of domain ment. The execution of load adjustment will stop tem enters a stable state when the values of ) of all neighboring domains fall into the range: (5) 0 ≤ ϕ ( i ,i ) ( n1 , n2 ) ≤ δ convg

Figure 2. Initial Partition Algorithm IGW1 (Totail1:10) D1

IGW2 (Totail2:10)

D2

R1 (1)

IGW1 (Totail1:10)

D1

IGW2 (Totail2:10)

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1 2

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omain Partitioning Algorithml omain partition, the network provider specifies a ng threshold (δbalance) and an initial convergence

R2 (1) R3 (1)R4 (1) R7 (1)

R5 (1) R6 (1)

R8 (1)

R2 (1) R3 (1) R4 (1)

R9 (2)

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(b): Network after initialization φ (1, 2) (5, 5)= 0.1

(a) Original network IGW1 (Totail1:10)

R5 (1) R6 (1)

R10 (1)

(c): Load changes in R2, R4, R7, R9 φ (1, 2) (5, 5)= 0.3

IGW1 (Totail1:10)

IGW2 (Totail2:10) R1 (1)

D1

R2 (2) R3 (1) R4 (2) R7 (2)

D2 R5 (1) R6 (1)

R8 (1)

R9 (1)

R10 (1)

(d): Network Structure after Adjustment

Rx (y): Router x with current load of y.

Figure 3. An Example of Local Domain Partition

In Figure 3, we assume the load-balancing threshold (δbalance) is 0.25 and the convergence value is 0.15. Figure 3 (a) is the original network with the possible wireless links. At the beginning, each domain initiates its Routeri k (line 1) with a routing discovery process. In Figure 3, domain 1(D1 with IGW1) has: Router11 = {R1, R2, R3, R4} and Router12 = {R7, R8, R9}. Meanwhile, domain 2 (D2, IGW2) has Router21 = {R1, R5, R6} and Router22 = {R8, R9, R10}. The value of m_hop is 2 since every router can connect to at least one IGW at most 2 hops.

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Let consider the situation of k =1 (line 2). R2, R3 and R4 are non-overlap routers of Router11 and thus assigned to D1 (line 3-4). With the same logic, R5 and R6 are assigned to D2. Each domain then calculates its λi (n) (line 5). At this point, domain 1 has D1 = {R2, R3, R4} and λ1 (3) = 0.3. Domain 2 has D2 = {R5, R6} and λ2 (2) = 0.2. R1 is an overlap router in both Router11 and Router21, and it should be assigned to the lightloaded domain (line 7-10). If R1 is selected to D1, it has: λ1 (4) = 0.4, λ2 (2) = 0.2, then φ (1,2)(4,2) = 0.2. On the other hand, if R1 is selected to D2, it has λ1 (3) = 0.3, λ2 (3) = 0.3, then φ (1,2) (3,3) = 0. Thus, R1 should be assigned to D2 based on above calculation. After the assignment of k =1, the two domains have: D1 = {R2, R3, R4}, D2 = {R1, R5, R6}. Then, the routers in Router12 and Router22 will be considered with k = 2 in the second iteration (line 2-11). In Figure 3, the non-overlap router R7 in Router12 will be assigned to D1 (line 4). Meanwhile, R10 is assigned to D2 because it is a non-overlap router in Router22. Meanwhile, each domain updates its λi (n) again (line 5). In the example, in domain 1, λ1 (3) is updated to λ1 (4) = 0.4, and in domain 2 λ2 (4) is updated to λ2 (4) =0.4. R8 and R9 are two overlap routers both in Router12 and Router22 (line 7-10). Since φ (1,2)(4,4) = 0, R8 can be assigned to any one of the domains, say, D1. After the allocation, domain 1 has λ1 (5) = 0.5. Obviously, R9 should be assigned to D2 because λ2 (4) is less than λ1 (5) after the assignment of R8. After this step, λ1 (5) keeps 0.5 and λ2 (4) is updated to λ2 (5) = 0.6. When k =2 (m_hop), all mesh routers have been assigned and Figure 3 (b) shows the result. For D1 and D2, it has φ (1,2)(5, 5) = λ1 (5) – λ2 (5) = 0.1 < δconvg. The network enters a stable state. D. Load Adjustment Algorithm The adjustment procedure reallocates one or more routers from an overloaded domain to a neighboring domain in response to the load changes. Figure 4 depicts the adjusting algorithm. In real-time, each domain updates its traffic load information and calculates the difference of bandwidth utilization ( ϕ ( i1 ,i2 ) ( n1 , n 2 ) ) with its neighboring domains (line 1). If the disequilibrium of traffic load between two neighboring domains is higher than the given threshold (Eq. (4)), the two domains start the procedure of load adjustment (line 2). For each value of k and starting from k = 1 (line 3), the procedure generates the set of Router (a, b) k (line 5) and reassigns one or more routers in Router (a, b) k to the lower loaded domain Db (lines 6 and 7). The process continues until the load-balancing of the two domains is less than the given convergence value (line 4). During the process of adjustment, it increases the load in the higher capable domain Db and decreases the load in the lower capacity domain Da. Two factors should be considered with respect to line 6 (i) the assigned router(s) minimize the imbalance between two domains; (ii) the addition of the router(s) doesn’t cause congestion in the new domain. With regard to the first factor, it may need to calculate the difference of bandwidth utilization ( ϕ ( i1 ,i2 ) ( n1 , n 2 ) ) of all alternative reassignments. As for the second factor, adjusting mesh routers evaluate the link capacity of the path(s) to the new IGW. If the path(s) cannot afford the traffic load imposed by a mesh router, this router should be kept in the current domain.

1: Update all 2: If ( ϕ 3: 4:

5: 6: 7: 8: 10: }

( a ,b )

ϕ ( i1 ,i2 ) ( n1 , n 2 ) ;

(n a , n b ) > δ balance ) { k=1; while ( ϕ ( a ,b ) ( n , n ) > δ ){ a b convg Generate Router (a,b) k ; One or more routers in Router (a,b) k -> Db to minimize the loadbalancing; Update λa(na), λb(nb); Update ϕ ( a ,b) (n , n ) ; k++; a

b

}

Figure 4. Adjusting Algorithm

Figure 3 (c) is the network after load change at R2, R4, R7 and R9. In this case, the load changes of R2, R7, and R9 cause the imbalance of D1 and D2. The disequilibrium is larger than the given threshold (δbalance): λ2 (5) = 0.8, λ1 (5) = 0.5 and φ (2, 1) (5, 5) = 0.3 (line 1). Then the network adjusts the network structure. When k=1, Router (1, 2) 1 is empty because no router in D1 has 1 hop link to D2. In this case, no router can be reallocated (line 3-7). When k = 2, Router (1, 2) 2 is the set of {R8} (line 5). The network reallocates R8 from D1 to D2 (line 6). Then, λ2 (5) is changed to λ2 (6) =0.6, λ1 (5) is updated to λ1 (4) = 0.7, and φ (1, 2) (4, 6) = 0.1 (line 7). After the loadrebalancing, the network comes into a stable state because φ (1, 2) (4, 6) is less than δconvg. Figure 3 (d) gives the result of network adjustment. IV.

INTER-DOMAIN MOBILITY

When a mesh router migrates to a new domain, the data packets of its associated MSs cannot be directly forwarded from their HAs to the new IGW until the MSs have accordingly migrated to the new IGW and updated the mobility bindings at their HAs. Our MCIP [2] [3] deals with the mobility of MSs for both single hop MSs (e.g., MSs 2 and 3 in Figure 5) and multi-hop MSs (e.g., MS1 in Figure 5). Each MS has a permanent IP home address. For each mesh domain, the IGW address acts as the CoA address for all MSs that are associated with a mesh router in the IGW domain. In MCIP, Multi-hop paging caches maintain the MSs’ location information while multi-hop routing caches keep the routes for data packet transmission. A multi-hop paging/routing cache stores a single or multi-hop route between the IGW and a single or multi-hop MS. The route for the MS toward the IGW includes two parts, the path from the MS to its associated MS and the path from the associated MS to the IGW. The IGW maintains a paging cache with a route to each of its associated MSs even when the MSs are in the idle state. The IGW also maintains a routing cache for each active MS (sending or receiving packets) where this route is used to send data packets. At the same time, each MS also keeps paging and/or routing cache information. Figure 5 illustrates multi-hop paging and routing caches stored at the IGW for MS1. As shown in Figure 5, when MS1 is idle and R4 is a router member of D1, the corresponding multi-hop paging cache stored by the IGW1 has the path: IGW-R3-R4-MS2-MS1. When MS1 has data packet to receive or send, it moves to active state and creates a multi-hop routing cache. The data packets of MS1 will be delivered between IGW1 and MS1 by following the multi-hop routing cache. The original path in Figure 5 shows the path for communication. Next, we discuss the process of multi-hop paging/routing cache update for supporting inter-domain mobility.

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MCIP D1 : IGW1 R1

R2

FA1

FA2

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MCIP D2 : IGW2 R6

R7

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R4

R4 leaves MCIP D1 and joins MCIP D2, MS2 and thus MSs 1-3 accordingly migrate from D1 to D2 by MCIP protocol.

MS3

: R4 migrates from D1 to D2.

MS1

Figure 5. MCIP for the Mobility of MSs

While migrating to a new domain, each MS has to bind its home address with the CoA address at the MS’s HA so that the HA creates a new mobility binding by using the new CoA address. In the domain load-balancing algorithms of Sections III.C and III.D, there are three instructions that start the process of MS migration: r-> Di (line 4) in Figure 2, r-> Di with higher capacity (line 8) in Figure 2, and One or more routers in Routerk(a,b) -> Db to minimize the load-balance (line 6) in Figure 4. The inter-domain mobility for a MS performs two functions using MCIP [2] [3]: (i) Mobility binding at FA/HA, and (ii): Multi-hop routing/paging update. By the process of registration, the HA updates the current address of the MS so that data packets can be delivered to the new IGW, which in turn can further forward the data packets to the destination MS. The multi-hop paging/routing cache of MS1 is then updated to the path: IGW2-R5-R4-MS2-MS1. The earlier multi-hop paging/routing cache of MS1 in D1 will be cleared after the expiration. V.

PERFORMANCE E VALUATION

A. Experimental Configuration By using ns-2 [8], the load-balancing scheme and interdomain mobility protocol are implemented in mesh networks with different traffic patterns and different load-balancing threshold (δbalance). In the tested mesh networks, twenty-five mesh routers are located in an experimental area (1500m x 1500m) and form a square grid network (5x5). Each router can directly communicate with up to four neighboring routers. Two domains are connected to the Internet, which is represented by a wired router. Two mesh routers at the upper-left and upperright corners of the grid mesh network are also configured as IGWs with wired links that connect the Internet. In order to drive the two domains to saturation, the maximized physical bandwidth of each IGW is set as 1Mbps. Each IGW in the domain collects its traffic load information and updates its bandwidth utilization (λi (n)). Five different load-balancing thresholds (δbalance: 0, 0.01, 0.02, 0.05, and 0.1) are used. The convergence value (δconvg) for each adjustment is set as 0.5% in all experiments. Each data point shown in the following figures is averaged over five runs with different random seeds. In all experiments the simulation runs for 1000 seconds. In order to check the performance of the load adjustment algorithm, the experiments uses two different traffic patterns in each mesh router with different traffic variances: • Traffic pattern-1, a smaller range of traffic change per second (e.g., [-5, 5 packets/second]), and

Traffic pattern-2, a larger range of traffic change per second (e.g., [-10, 10 packets/second]). For each pattern, a router is initiated with the same traffic rate (e.g., 20 packets per second). In a given second afterward, the rate of traffic of a router will either increase or decrease by an additional rate, which is randomly selected from the traffic change range (e.g., -8 packets/second in traffic pattern-2). For instance, in the 5th second of the simulation, a router with traffic pattern-2 has the rate of the 4th second (e.g., 45 packets per second), plus/minus a randomly selected rate (e.g., -6 packets per second, which is 39 packets per second). Traffic patterns make the overall traffic in the tested domains vary in a different degree. All data packets of the traffic patterns are directed toward the Internet. The size of each data packet is 1024 bytes without including the IP header. The traffic rate serves as the current load of the router and is reported to its associated IGW. Moreover, in order to evaluate the impact of inter-domain mobility of MSs, experiments set two values for the wired router to represent Internet delays: 0.25 and 0.3 second. Based on different Internet delays, the performance of inter-domain mobility of MSs is evaluated by Data delivery rate (DDR) of data packets: For a specific router, the average percentage of data packets received by the destination MS in comparison with the number of data packets sent by the source CN on the Internet. B. Frequency of Router Migration Av erage Fre que ncy of Mesh Route r Migration (times/ second)

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0.01 0.02 0.05 Thre shold of Loa d Ba lance

0.1

(b) Figure 6. Average Router Migration Frequency The first goal of these experiments is to observe the average frequency of router migration in a domain under different loadbalancing thresholds (δbalance). It can be observed from Figures 6 (a) (b) that the router migration is affected by two factors: the variance of traffic pattern and load-balancing threshold. Figure 6 shows that traffic pattern-2 has the higher frequency of router migrations. When the traffic in mesh routers changes in a larger range, the traffic imbalance between domains varies more wildly. Therefore, more mesh router migrations occur at each adjusting procedure. Even if the traffic varies in a small range, the experiments in Section V.C show that its traffic fluctuation may cause a lower DDR due to mobility of mesh routers as well as their associated MSs. In each traffic pattern, as can been seen from Figure 6, the load-balancing threshold (δbalance) has a

1-4244-0357-X/06/$20.00 ©2006 IEEE


significant impact on the router migration. The average frequency of router migration in a domain increases when the load-balancing threshold decreases. According to Eq. (4) in Section III.B, in order to maintain a given load-balancing threshold (δbalance), the network initiates more adjustments in the case of a small δbalance. When the load-balancing threshold is set as 0, domains pursue the load-balance to the greatest degree. In this case, the network adjusts the mesh routers even the imbalance of load-balancing varies small. The adjustments cause a high frequency of router migrations and degrade the network performance as shown in the next section. As can been seen from Figure 6, the average frequency is 2.327 times per second in traffic pattern-2 when the load-balancing threshold is 0. C. Inter-domain Mobility In these experiments we observe the performance of packet transmission under different frequencies of router migration. In our experiments, as illustrated in Figure 5, a router (e.g. R4 in Figure 5) is configured with different frequencies of migration between two domains. To remove the effects of MS’s movement and focus on the impact of the router migration, all MSs are kept stationary in the experiments. MSs with different wireless hops toward the router (1 -5 hops) are configured to receive packets from the CN on the Internet as shown in Figure 5. Data packets are originated at the CN on the Internet and directed to the destination MS associated with the testing router.

migration varies in the range from 0.008 to 2.327. Also it is can be seen from Figure 7, when the frequency is larger than 0.5, router migration degrades the DDR to less than 80%. When the migration frequency of a router is less than 0.125, the associated MSs perform well in which the throughput is higher than 95%. At the same time, it can also be observed from Figures 7 (a) (b) that the DDR degrades when the internet delay increases and the hop number increases. It is because these two parameters increase the handoff delay. From the frequency of router migration and the performance of DDR as shown in Figures 6 and 7, we can find a tradeoff between load-balancing and inter-domain mobility. The tradeoff is used to choose a load-balancing threshold (δbalance) for a mesh network. In the algorithms of Figure 2 and Figure 4, the frequency is controlled by E.q (4) with different load-balancing thresholds. In traffic pattern-1, for example, when the load-balancing threshold is set as 0.05, the migration frequency is 0.047 times per second in which the DDR is larger than 97%. When the load-balancing threshold is less than 0.05, the DDR is lower than 97%. On the contrary, the DDR is higher than 97% when the load-balancing threshold is higher than 0.05. With the same logic, the tradeoff between load-balance and inter-domain mobility for traffic pattern-2 can be achieved by setting the threshold as 0.01. The tradeoff enables the network to optimize the load-balancing by router migration while maintaining a high performance for data transmission. VI.

Data Packet Ratio (Internet Delay: 0.25s) 1 Data Delivery Ratio %

0.9

1 hop

0.8

2 hop

0.7

3 hop

0.6

4 hop

0.5

5 hop

0.4 0.3 0.2 0.1 0.0625

0.125

1.25

0.5

1

2

Fre que ncy of Me sh Router Migra tion (time s/second)

(a)

REFERENCES

Data Packet Ratio (Internet Delay: 0.3s)

[1]

Data Delivery Ratio %

1 0.9 0.8

1 hop

0.7 0.6

2 hop

0.5 0.4

3 hop

[2]

4 hop

0.3 0.2

5 hop

0.1 0.0625

0.125

1.25

0.5

1

CONCLUSION

When a mesh router changes its domain, it forces MSs to update the mobility binding at their home networks. The process of inter-domain mobility degrades the performance of network. The proposed algorithms consider the load-balance and the DDR of MSs to achieve the tradeoff between the loadbalance and inter-domain mobility. The experiments in this paper show the trade-off in controlling the load-balance and inter-domain mobility of MSs.

2

Freque ncy of Me sh Route r Migra tion (tim es/second)

[3]

(b) Figure 7. DDR with Different Router Migration Frequencies

Figures 7 (a) (b) show that the DDR of data packet is significantly affected by the mesh router migration. When the frequency of router migration between domains increases, the router greedily pursues the load-balancing but degrades the DDR dramatically. When a router migrates to new domain, it leaves the old domain and the path for data transmission is disconnected. The migration of the mesh router forces the associated MSs to initiate the inter-domain mobility. Before creating a new mobility binding, the data packets can’t be forwarded to the new IGW due to lack mobility binding with the new domain. As shown in Figure 6, the frequency of router

[4] [5]

[6] [7]

[8]

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