Load-aware cooperative hybrid routing protocol in hybrid wireless mesh networks

Accepted Manuscript Regular paper Load-aware cooperative hybrid routing protocol in hybrid wireless mesh networks Yuan Chai, Wenxiao Shi, Tianhe Shi PII: DOI: Reference:

S1434-8411(16)30295-3 http://dx.doi.org/10.1016/j.aeue.2017.02.002 AEUE 51794

To appear in:

International Journal of Electronics and Communications

Please cite this article as: Y. Chai, W. Shi, T. Shi, Load-aware cooperative hybrid routing protocol in hybrid wireless mesh networks, International Journal of Electronics and Communications (2017), doi: http://dx.doi.org/10.1016/ j.aeue.2017.02.002

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Load-aware cooperative hybrid routing protocol in hybrid wireless mesh networks Yuan Chai, Wenxiao Shi∗ , Tianhe Shi College of Communication Engineering, Jilin University, Changchun, China 130012

Abstract Hybrid wireless mesh networks (WMNs) combine the advantages of infrastructure and client mesh networks. Hybrid routing protocols which are the most adaptive types of routing protocols for hybrid WMNs all neglect load condition up till now. This paper proposes a load-aware cooperative hybrid routing protocol (LA-CHRP). LA-CHRP is not only adapted to cover the peculiarities of routers and clients, but also considers load in routing. Different load levels are set for mesh routers and clients respectively. Gateway and client oriented traffic are handled differently. For the former, in the cooperative mechanism, mesh routers can be used more reasonably by considering the load levels. For the latter, node-aware routing metric is used to choose mesh routers in priority, and state of load is also considered for mesh clients. When the energy is adequate, clients with less load are chosen to transport packets. The simulation results indicate that LA-CHRP can achieve high throughput with low latency and packet loss rate in hybrid WMNs. Keywords: Hybrid wireless mesh networks, Load-aware routing, Cooperative hybrid routing protocol, Node-aware routing metric

∗

Corresponding author E-mail address: [email protected] [email protected] (T. Shi).

(Y.

Chai),

[email protected]

Preprint submitted to AEU-International Journal of Electronics and Communications

(W.

Shi),

February 14, 2017

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Figure 1: Hybrid WMNs

1. Introduction With self-healing and self-configuring capabilities, wireless mesh networks (WMNs) can meet more communication requirements and attract much attention from scholars [1]. Two distinct types of nodes, which are mesh routers and mesh clients, form WMNs. Mesh routers 5

with multiple radio interfaces are generally static and can compose the multi-hop wireless backhaul networks. Mesh clients have a single radio interface with high mobility. Based on the deployment pattern, WMNs can be classified into client, infrastructure and hybrid WMNs [2]. Client WMNs are similar to pure ad hoc networks where mesh clients can forward packets to others. In infrastructure WMNs, mesh clients with no forward ability can correspond with

10

other nodes only through mesh routers. Hybrid WMNs which is illustrated in Fig. 1 include both infrastructure and client WMNs. Mobile mesh clients here can also perform as packet forwarders like mesh routers. According to the performance of wireless mesh networks, some research has been done

2

on different emphases. WMN-GA [3] and WMN-SA [4] have solved the connectivity and 15

coverage problem. Performance evaluation of mixed-bias scheduling schemes in [5] is applied to prioritise mesh routers near the gateways to handle different traffic. To the routing part, proper routing can extremely influence the network performance. For the energy constrained network, energy is considered as an essential factor [6]. Traffic-aware routing in [7] uses physical layer (PHY)/ media access control (MAC) parameters to improve the rout-

20

ing performance. Routing protocols can be broadly categorised into three types: proactive routing protocols [8], reactive routing protocols [9] and hybrid routing protocols [10]. The proactive ones are more adaptive for networks with static nodes, which are opposite to the reactive ones. Nowadays, existing proactive or reactive routing protocols designed for homogeneous networks such as infrastructure WMNs cannot function well in hybrid WMNs,

25

as these protocols do not consider typical features of hybrid WMNs. Hence, hybrid routing protocols including both proactive and reactive routing protocols can suit for hybrid WMNs. However, all of the few hybrid routing protocols do not consider load condition currently, and the load will be aggregate on the same node, causing poor network performance. A load-aware cooperative hybrid routing protocol (LA-CHRP) is first proposed in this paper.

30

The key contributions of this paper are presented as follows: • Setting different load levels for mesh routers and clients according to their different features in hybrid WMNs. The throughput tendency curves with the increasing load are fitted respectively for mesh routers and clients, which can indicate how load condition influences the network performance in terms of routers and clients separately. Then

35

according to the throughput tendency curves, different load levels can be set to measure

3

the extent of congestion. • Modifying the original cooperative routing mechanism to avoid excessive partial load. Based on the load levels, different traffic types are handled differently. For gateway oriented traffic, only when the load condition is good enough can the mesh router 40

returns route reply (RREP) message directly. If the mesh router has packets to forward, i.e. the load level is not zero, other nodes with lower load level can be chosen to forward packets. For client oriented data flows, the node-aware routing metric is applied and nodes with lower load can be selected. The remainder of this paper is organised as follows. Some current routing protocols

45

for hybrid WMNs are discussed in Section 2. Section 3 explains LA-CHRP in detail. The performance evaluation by using ns-3 [11] is given in Section 4. Section 5 concludes the paper.

2. Current routing protocols for hybrid WMNs At the present, according to load condition, some routing approaches are designed for 50

infrastructure WMNs. With the help of NOC, the network utilization can be maximized by balancing the traffic load [12]. A load balancing on-demand routing algorithm named LAM [13] solves the link load imbalance caused by inter-flow and inner-flow interference. Joint scheduling and routing algorithm with load balancing proposed in [14] maximizes the throughput and ensures fairness. LBM [15] balances the traffic load and reduces the

55

interference. NLR [16] considers load to further enhance routing efficiency for real-time communication applications. BPR [17] considers path length and routing to balance load.

4

C2WB [18] metric balances the traffic and improves the network capacity by avoiding routing the traffic through congested areas. The load-aware routing scheme proposed in [19] uses the dual decomposition method to maximize the utility. However, there are few routing protocols 60

specially designed for hybrid WMNs, and even fewer routing protocols consider load. For the reactive routing protocols, some of the existing ones which take load into consideration are improved on the basis of ad hoc on-demand distance vector (AODV) [20] protocol. The routing protocol proposed by Mohammad et al. [21] and AODV-HP [22] select recommended channels to decrease interference and load. SafeMesh [23] chooses the node with the least

65

time that a packet has to remain in the queue to forward packets, which can avoid the node with high load. The routing approaches above improve the routing metric based on AODV. In mechanism improvements of AODV, some also can help reduce and balance load. The routing protocol proposed by Alrayes et al. [24] improves the mechanism for route discovery and repairing by making use of local connectivity, which can decrease high load caused by

70

control packets effectively. M-AODV [25] exploits border mesh routers to strengthen the route maintenance so that the new route establishment can be done locally without sending so many control packets from source node. AODV-DF [26] uses a restricted directional flooding technique to avoid high overhead and load. Some other load-aware reactive routing protocols are based on DSR [27] or WCETT [28]. LCMR [29] modifies DSR to consider the

75

load of nodes when detects coding opportunities. D-WCETT [30] and WCETTT-LB [31] enhance the basic WCETT by incorporating load balancing into the routing metric. No more route discovery process is needed when the static routers have known the route to the destination in hybrid WMNs. Therefore, only reactive routing protocols cannot perform perfectly in hybrid WMNs, and hybrid routing protocols can perform better in hybrid 5

80

WMNs. Proactive and reactive routing protocols can be employed among routers and clients respectively. When static mesh routers have known the route to the destination by the proactive routing mode, the route discovery among mesh routers can be left out, and high load caused by unnecessary control packets can also be decreased. Nevertheless, the number of current hybrid routing protocols is quite limited. HDV [10] combines tree-based routing

85

strategy and reactive routing strategy to find routes. Only gateway oriented data flows are handled in HDV. HMesh [32] and the protocol proposed by Trivi˜ no et al. [33] both use OLSR [34] in infrastructure networks, and AODV in mobile client parts. The above hybrid routing protocols find routes only with smallest hop count. CHRP [35] improves the routing metrics in both proactive and reactive routing protocols, and gateway as well as client oriented data

90

flows are considered comprehensively. But all these existing hybrid routing protocols fail in specially taking the load condition into consideration. Thus, during the process of a mesh client accessing a mesh router, if this mesh router has known a route to the destination, it will transport packets unconditionally. If there are many clients near the router and they all want to access this same router, high load may be caused. So it is important to consider

95

load in hybrid routing protocols to improve the network performance of hybrid WMNs.

3. Load-aware cooperative hybrid routing protocol The LA-CHRP proposed in this paper is a load-aware hybrid routing protocol. The proactive routing part of LA-CHRP is used among mesh routers, and the reactive routing part is applied for both mesh routers and clients. When mesh routers cannot find route in 100

the proactive routing table, then the reactive routing protocol is used. LA-CHRP considers different features of mesh routers and clients in hybrid WMNs, and assigns different load 6

levels for each kind of nodes according to the load condition. In the cooperative routing mechanism, mesh routers only handle route request (RREQ) message and reply RREP when the load condition is good. In this case, the performance of network can be improved 105

dramatically, especially for gateway oriented traffic. In client oriented data flows, the nodeaware metric is used and nodes with less load will be selected. In contrast to mesh clients, mesh routers are more likely to be chosen.

3.1. Load levels design With the increasing of load, the network throughput is not always increasing. When the 110

load reaches a certain level, delay increases and the throughput starts to decline [36]. To fit the throughput curves with the load condition which can be indicated by the queue length [23], a large set of simulations have been done. In these simulations, the access protocol is CSMA/CA, and the MAC protocol is IEEE 802.11b with 2Mbps transmission rate. The antenna is omnidirectional. To make the communication successful, the test network is

115

connected. In other words, the neighbor nodes are within each other’s transmission distance. To avoid other factors such as interference also influencing the network throughput, the simple chain topology is considered. In addition, orthogonal channels are applied in adjacent two hops to avoid interference. Then the number of packets which are sent every second is changed in different experiments, so that the queue lengths at nodes are also changing. Data

120

flow is sent from one end to the other end. In each experiment, thousands of packets in one data flow are sent. To get the different throughput tendency curves for mesh routers and clients, experiments are made among routers and clients respectively. The queue lengths are detected at each mesh router or client every interval between two sent packets in the path.

7

Average network throughput

0

1

2

3

4 Queue length

5

6

7

Figure 2: Throughput tendency curve with queue length of mesh routers

These queue lengths are recorded together with the network throughput. The average queue 125

length corresponding to the throughput can be attained. Several hundred-time simulations have been done. Then according to the large amount of simulations, the average network throughput curve with different average queue lengths can be fitted. Static mesh routers with multiple radio interfaces have higher ability of handling packets than mobile clients. Thus, mesh routers can bear larger queue length and guarantee smooth communication. The

130

throughput tendency curve with queue length of mesh routers is shown in Fig. 2. Fig. 2 is fitted by thousands of samples, and the tendency of throughput is what we focus on. Fig. 2 shows that as the average queue length at mesh routers changes, throughput increases at the beginning but then drops. The peak value of throughput is appearing when the queue length is between three and four. According to this tendency, since the queue

135

length at a mesh router can only be integer, the load levels in mesh routers are set as illustrated in Table 1. Similarly, the throughput curve with queue length of mesh clients is shown in Fig. 3 which is also fitted by thousands of data.

8

Table 1: Load levels in mesh routers

0

1-3

4-6

7 and more

Load level

0

1

2

3

Average network throughput

Queue length

0

0.2

0.4

0.6

0.8

1 1.2 Queue length

1.4

1.6

1.8

2

Figure 3: Throughput tendency curve with queue length of mesh clients

From Fig. 3, according to the tendency, load levels in mesh clients can be assigned as 140

shown in Table 2.

3.2. Load-aware cooperative hybrid routing protocol design Different from all existing hybrid routing protocols for hybrid WMNs which have failed in considering load condition, LA-CHRP considers load factor to find route, avoiding traffic flows being too aggregate.

Table 2: Load levels in mesh clients

Queue length

0

1-2

3 and more

Load level

0

2

3

9

145

3.2.1. Client oriented traffic For client oriented traffic, load condition is considered to help find a proper route based on AODV. To distinguish mesh routers from clients, the weights of routers in LA-CHRP are lower than clients, which helps static routers to be selected in priority. The values of weights change around 1 and 4 respectively [23], constituting the node-aware routing metric. For

150

mesh routers equipped with three radio interfaces, different interfaces can be set to different weights. Some information can be obtained from MAC layer [37]. The cross-layer approach channel busy time (CBT) from MAC layer is an effective way to weigh the channel condition which can indicate the load status [38]. CBT is used here to set weights for different interfaces of mesh routers. The expression of CBT is [39] CBT =

155

Ttotal − Tidle Ttotal

(1)

where Ttotal is the total time of measuring channel condition. Tidle is the idle time during the whole detecting time. The less CBT, the better channel condition. Because each channel is banded with a radio interface of mesh router, CBT can reflect interface condition of mesh routers. Then the radio interface with less CBT can be chosen. The weight value of a router (i.e., Wr ) can be denoted as [35] Wr = CBT

160

(2)

For mesh clients, CHRP only considers the remaining energy of a mesh client. The remaining energy percent ( i.e., Ratioenergy ) of a client is Ratioenergy =

Elef t Etotal

(3)

where Elef t is the remaining energy of the current mesh client. Etotal is the total energy of a client at the beginning. 10

However, when the energy is adequate, energy is not the most important factor to select 165

route. In this case, the load condition should also be taken into account. As mesh clients are equipped with a single radio interface, and the interface is banded with the same channel. CBT which can reflect the channel condition and load is not a good way to distinguish the differences between different mesh clients interfaces. So we use a load extent to reflect the load condition of a client node directly. The load extent considering the load level is defined

170

as loadextent =

load level level max

(4)

where load level is the load level of the current client. level max is the maximum load level which has been defined. According to the previous assignment, the value of level max is 3. Energy levels are classified into three ranges [40]. When the energy of a client is adequate and the Ratioenergy is over 20%, the weight value of a client (i.e., Wc ) is Wc = (1 − Ratioenergy ) + loadextent + 3

(5)

= 4 − Ratioenergy + loadextent 175

A client with adequate energy and less load will be chosen. When the left energy is not quite sufficient, the client should not be used as far as possible. The weight value is set to 10 for clients with Wc lower than 10%. 7 is set for those clients with Wc between 10% and 20% [35]. The routing metric (i.e., Mpath ) in LA-CHRP is Mpath =

p ∑

Wi

i=1

=

m ∑ a=1

Wr

a

+

n ∑

(6) Wc

b

b=1

where Wi is the weight value of node i. p is the total hop count in the path. m and n are the 180

hop counts of routers and clients in the path separately. Wr 11

a

is the weight value of router

a, and Wc

b

denotes the weight value of client b. Channel state, load condition and energy

are considered in this node-aware routing metric, and mesh routers with less weight value are selected in priority.

3.2.2. Gateway oriented traffic 185

For gateway oriented traffic, mesh routers equipped with the proactive routing protocol can know the way to gateway [35]. However, border mesh routers using LA-CHRP will not forward data packets unconditionally like original hybrid routing protocols. As both mesh routers and mesh clients are equipped with the modified reactive routing protocol based on AODV, routers and clients can communicate as the discipline of this reactive protocol.

190

In the mechanism of AODV, each node periodically broadcasts hello packets to maintain neighbor information. In LA-CHRP, hello packets also contain the load level information, which will not bring extra cost [41]. The frequency of hello packet exchange depends on the magnitude of average node mobility and the time cost by sending a packet in the queue. In simulations, hello packets are exchanged every second. During the routing process, when a

195

RREQ is received by a mesh router which has known the route to gateway, this router will check firstly whether itself is already in congestion or has packets to forward. If the load level of this mesh router is not zero and other neighbor nodes which can receive the RREQ have lower load levels, the router will drop the RREQ directly without replying a RREP. The pseudocode of processing a RREQ at the mesh router i is shown in Fig. 4.

200

load level i is the load level of the mesh router i. load level j is the load level of another neighbor node j which are able to receive the RREQ. min load level is the minimal load level of the other neighbor nodes.

12

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Figure 4: Pseudocode of processing a RREQ

Fig. 5 shows how the node handles the received RREQ clearly. To understand the routing mechanism of LA-CHRP more intuitively, an corresponding example is shown in Fig. 6. 205

In Fig. 6, mesh routers know the way to gateway by the proactive routing protocol. If the client S wants to communicate with gateway D, a RREQ will be broadcasted. Mesh router A and mesh client B will receive the RREQ. In original cooperative hybrid routing protocols, as A knows the way to D, RREP will be returned to S directly. However, if A is congested, the performance may not be good. In LA-CHRP, A will firstly check its load

210

level. If the value is not zero, and the load level of B is lower than A, A will drop the RREQ without replying RREP. If B does not know the way to D, it will forward the RREQ until this message is received by a mesh router C that is not congested. If C still has packets to send, it will do the same as A. However, if both A and B have packet queues to forward but are with the same load level, mesh router A will be used in priority.

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When the mesh client S’ wishes to send data to the mesh client D’, S’ broadcasts RREQ

13

Figure 5: RREQ processing mechanism in hybrid WMNs

Figure 6: A simple route discovering process example in hybrid WMNs

14

firstly. In addition to remaining energy, load condition is considered in a mesh client. Due to the node-aware routing metric, a route with more mesh routers will be selected in priority. Finally, the route S’-E-F-D’ containing clients with less load and more energy will be chosen.

4. Performance evaluation

220

4.1. Simulation environment A square of 1000m by 1000m is set as the simulation area. 25 mesh routers and 50 mesh clients are located in this area. Mobile mesh clients are set with the random mobility model [42]. Constant bit rate (CBR) data stream and variable bit rate (VBR) data stream are sent here. Table 3 shows the detailed simulation parameters.

225

4.2. Performance metrics We use average packet loss rate, latency and network throughput to assess the performance of LA-CHRP. The definition of these performance metrics are as follows. • Average packet loss rate Lr is Lr =

Ntotal − Nreceived Ntotal

(7)

where Ntotal is the total number of packets sent by the original node. Nreceived is the 230

number of packets successfully received by the destination. • Average Latency De is

N ∑

De =

Dej

j=1

N

(8)

where Dej is the delay of packet j. N is the total number of packets successfully received by the destination node. 15

Table 3: Simulation parameters

Simulation Parameters

Values

Simulation time

100 s

Traffic type

UDP

MAC protocol

IEEE 802.11b

Packet size

1024 bytes

Packet rate

80 kbps

Number of radio interfaces in each router

3

Number of channels in each router

3 channels (1, 6, and 11)

Number of radio interfaces in each client

1

Number of channels in each client

1 channel (1)

Transmission range

250 m

Interference range

550 m

Propagation model

Two Ray Ground

Antenna

Omnidirectional

• Average network throughput Th is Th =

235

Nreceived × Byte × 8 (Tend − Tstart ) × 1024

(9)

where Nreceived is the number of packets received by the destination successfully. Byte is the number of bytes contained in one packet. Tend is the time when the last packet is received. Tstart is the time when the first packet starts sending. The unit of Th is kbps, which indicates how many thousands of bits are successfully received per second. 16

4.3. Simulation results and analyses 240

4.3.1. Hybrid WMNs with grid backbone networks In this part, mesh routers compose a grid backbone network. Mesh clients compose a mobile access network with random topology. Two kinds of simulations are done. One case is evaluating the performance with different flow number and fixed client speed. The speed is 2m/s here. The other case is with different client speed and fixed flow number 8. When

245

data stream is CBR, the obtained average packet loss rate, latency and network throughput under the two cases are illustrated in Fig. 7 - Fig. 9. As mesh clients can move randomly, the topology of network is changing all the time. In addition, the source and destination nodes are also chosen randomly. Thus, with different number of flows or client speeds, the

80

90

70

80

60 50 40 30 20

LA−CHRP CHRP HMesh

10 0

2

4

6 8 Number of flows

10

Average packet loss rate(%)

Average packet loss rate(%)

simulation results may fluctuate irregularly.

70 60 50 40 30 LA−CHRP CHRP HMesh

20 10 0

12

(a)

2

4

6 8 Speed(m/s) (b)

10

12

Figure 7: Average packet loss rate in hybrid WMNs with grid backbone topology. (a) Different number of flows. (b) Different speed of clients

250

Fig. 7 shows the average packet loss rate of LA-CHRP always performs better than CHRP and HMesh under different cases. In addition to the different features of different kinds of nodes and limited energy of mesh clients, the proposed LA-CHRP considers load

17

1200

450 LA−CHRP CHRP HMesh

Average latency(ms/p)

Average latency(ms/p)

1000

LA−CHRP CHRP HMesh

400

800 600 400

350 300 250 200 150 100

200 50 0

2

4

6 8 Number of flows (a)

10

12

0

2

4

6 8 Speed(m/s) (b)

10

12

Figure 8: Average latency in hybrid WMNs with grid backbone topology. (a) Different number of flows. (b) Different speed of clients

condition in the gateway and client oriented data flows respectively. As congestion is further avoided in hybrid routing protocol, average packet loss rate becomes lower. The value of 255

LA-CHRP is decreased by 22.2% and 38.4% compared to CHRP and HMesh respectively. Fig. 8 shows that the performance of LA-CHRP in average latency is always better than CHRP and HMesh. The value of LA-CHRP is smaller than the other two at all times. In LA-CHRP, when mesh routers receive RREQ, they will check whether other nodes which are able to receive the RREQ have lower load levels. If other neighbor nodes with lower load

260

levels exist, this mesh router just discards the RREQ. The same router with congestion is not always used because neighbor mesh clients no longer access it unconditionally, which can reduce the delay. The average latency of LA-CHRP is 38.2% and 45.4% lower than CHRP and HMesh respectively. Fig. 9 shows that the average network throughput of LA-CHRP is consistently larger than

265

CHRP and HMesh in different cases. Two different kinds of data flows in the network are considered, and in both cases, load condition are taken into account to avoid high load and

18

300 LA−CHRP CHRP HMesh

250

Average network throughput(kbps)

Average network throughput(kbps)

300

200 150 100 50 0

2

4

6 8 Number of flows (a)

10

12

250 200 150 100 LA−CHRP CHRP HMesh

50 0

2

4

6 8 Speed(m/s) (b)

10

12

Figure 9: Average network throughput in hybrid WMNs with grid backbone topology. (a) Different number of flows. (b) Different speed of clients

congestion. For gateway oriented traffic, although mesh routers know the way to gateway, they will only forward packets when they are not congested. For client oriented traffic, in the node-aware routing metric, load condition is also considered to find a less congested way. 270

Avoiding the load being excessive successfully, the throughput is developed dramatically. The average network throughput of LA-CHRP is 25.7% and 43.6% higher than CHRP and HMesh respectively. Then VBR data stream is used, and the obtained average packet loss rate, latency and network throughput versus different number of flows are shown in Fig. 10 - Fig. 12.

275

From Fig. 10 - Fig. 12, we can see that when the data stream is VBR, the performance of LA-CHRP is also better than other two routing protocols. In this case, the rate of data packet transmission is changed, and load may be caused. As LA-CHRP avoids high load in both gateway and client oriented traffic, it brings better performance for the whole network.

19

Average packet loss rate(%)

80

60

40 LA−CHRP CHRP HMesh

20

0

2

4

6 8 Number of flows

10

12

Figure 10: Average packet loss rate in hybrid WMNs with grid backbone topology

Average latency(ms/p)

300 250 200 150 100

LA−CHRP CHRP HMesh

50 0

2

4

6 8 Number of flows

10

12

Average network throughput(kbps)

Figure 11: Average latency in hybrid WMNs with grid backbone topology

160

120

80 LA−CHRP CHRP HMesh

40

0

2

4

6 8 Number of flows

10

12

Figure 12: Average network throughput in hybrid WMNs with grid backbone topology

20

90

80

80

Average packet loss rate(%)

Average packet loss rate(%)

90

70 60 50 40 30 LA−CHRP CHRP HMesh

20 10 0

2

4

6 8 Number of flows (a)

10

LA−CHRP CHRP HMesh

70 60 50 40 30 20 10 0

12

2

4

6 8 Speed(m/s) (b)

10

12

Figure 13: Average packet loss rate in hybrid WMNs with general backbone topology. (a) Different number of flows. (b) Different speed of clients

4.3.2. Hybrid WMNs with general backbone networks In this part, 25 mesh routers and 50 mesh clients are randomly deployed together. CBR data stream is employed here. When the distribution of nodes is uniform distribution, the simulation results are shown in Fig. 13 - Fig. 15. 700

200 LA−CHRP CHRP HMesh

500 400 300 200 100 0

LA−CHRP CHRP HMesh

180

Average latency(ms/p)

600

Average latency(ms/p)

280

160 140 120 100 80 60 40 20

2

4

6 8 Number of flows (a)

10

12

0

2

4

6 8 Speed(m/s) (b)

10

12

Figure 14: Average latency in hybrid WMNs with general backbone topology. (a) Different number of flows. (b) Different speed of clients

Fig. 13 - Fig. 15 show the results in general hybrid WMNs with uniform distribution. The results are similar to that in hybrid WMNs with grid backbone topology. When the

21

250

Average network throughput(kbps)

Average network throughput(kbps)

350 LA−CHRP CHRP HMesh

300 250 200 150 100 50 0

2

4

6 8 Number of flows (a)

10

12

200

150

100 LA−CHRP CHRP HMesh

50

0

2

4

6 8 Speed(m/s) (b)

10

12

Figure 15: Average network throughput in hybrid WMNs with general backbone topology. (a) Different number of flows. (b) Different speed of clients

285

number of flows or the speed of clients are changed, LA-CHRP always perform better than CHRP and HMesh in terms of average packet loss rate, average latency and average network throughput. Firstly, LA-CHRP is a hybrid routing protocol which is quite adaptive for features of hybrid WMNs. Owing to the coordination of proactive and reactive routing protocols, some extra load caused by control packets can be avoided. Because when mesh

290

clients inquire mesh routers about routes, mesh routers can reply the request directly if mesh routers know the paths to destination in their proactive routing tables. For mesh routers, if one route is existing in both proactive and reactive routing table, the route in proactive routing table is used to avoid conflict. Then, in addition to the cooperation of proactive and reactive routing protocols, load condition is further considered in LA-CHRP. When mesh

295

clients access mesh routers, mesh routers only handle these requests if they are not with high load. In the node-aware routing metric, except for channel condition and residual energy of mesh clients, load is also taken into consideration. For these reasons, although because of the changing of network topology and the uncertainty of link location, the network performance

22

80 LA−CHRP CHRP HMesh

60

Average packet loss rate(%)

Average packet loss rate(%)

80

40

20

0

2

4

6 8 10 Number of flows (a)

60

40

20

0

12

LA−CHRP CHRP HMesh

2

4

6 8 10 Number of flows (b)

12

Figure 16: Average packet loss rate in hybrid WMNs with general backbone topology. (a) Exponential

150

100

120

80

Average latency(ms/p)

Average latency(ms/p)

distribution. (b) Normal distribution

90 60 LA−CHRP CHRP HMesh

30 0

2

4

6 8 10 Number of flows (a)

12

LA−CHRP CHRP HMesh

60 40 20 0

2

4

6 8 10 Number of flows (b)

12

Figure 17: Average latency in hybrid WMNs with general backbone topology. (a) Exponential distribution. (b) Normal distribution

is unpredictable and unstable, the performance of LA-CHRP is always better. 300

Then the distribution of nodes is changed into exponential and normal distribution respectively. The simulation results are shown in Fig. 16 - Fig. 18. Fig. 16 - Fig. 18 show that the performance of LA-CHRP is also better than that of CHRP and HMesh. We can see that whatever the distribution of nodes is, the simulation results are similar. Considering load condition, the hybrid routing protocol LA-CHRP helps

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to improve the network performance greatly in different cases. 23

450

Average network throughput(kbps)

Average network throughput(kbps)

300 250 200 150 100 LA−CHRP CHRP HMesh

50 0

2

4

6 8 10 Number of flows (a)

12

400 350 300 250 200 150 LA−CHRP CHRP HMesh

100 50 0

2

4

6 8 10 Number of flows (b)

12

Figure 18: Average network throughput in hybrid WMNs with general backbone topology. (a) Exponential distribution. (b) Normal distribution

5. Conclusion Hybrid WMNs include both static mesh routers with multiple radio interfaces and mobile mesh clients with a single radio interface. Because of mobility features in disparate nodes, hybrid routing protocols are the most adaptable ones for hybrid WMNs. However, current 310

hybrid routing protocols do not consider load condition, which may cause high congestion and poor performance. This paper proposes a routing protocol LA-CHRP to avoid the partial load being excessive. Gateway and client oriented data flows are both taken into account. For gateway oriented traffic, when a mesh router which knows the way to gateway receives a RREQ, it will never handle the RREQ unconditionally. Only when the load condition of

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the router is good enough can a RREP be replied. If other neighbor nodes which are able to receive the RREQ have lower load levels, the router will drop the RREQ and other nodes will respond. For client oriented data flows, a node-aware routing metric is used to select mesh routers in priority, and the load condition is also taken into consideration. In general, LA-CHRP can balance the load and simulation results have shown that LA-CHRP can make

24

320

significant performance improvements for hybrid WMNs.

Acknowledgements The work of this paper is supported by the National Natural Science Foundation of China (No.61373124).

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