Abstract
Keywords
Introduction
Due to the features of low cost, highly robust, and high reliability, wireless mesh network (WMN) is one of the key technologies in the next generation of wireless network. The self-organizing and self-healing peculiarities can dramatically reduce the complexity in network deployment and maintenance. WMN can be widely used in many circumstances, such as company network, medical system, and broadband family network. In addition, because of the advantage of seamless broadband connection to the Internet, WMN has attracted increasing concentration from researchers.1–3
WMN is composed of infrastructure WMN, client WMN, and hybrid WMN. Infrastructure WMN includes static mesh routers with multiple radio interfaces, while client WMN is made up of mobile mesh clients which are always equipped with only one radio interface. Hybrid WMN combines advantages of infrastructure WMN and client WMN, which has stronger network connectivity, higher reliability, and larger coverage.4,5 Its structure is quite more flexible and closer to actual demand than infrastructure WMN or client WMN. 6 In hybrid WMN, mesh routers have high capacity and stability, while mesh clients have limited energy and high mobility. Mesh clients can not only communicate with each other by accessing mesh routers but they can also forward packets directly without the help of mesh routers. 7
Routing protocols aim to provide best routes for two communicating parties, which can greatly influence the network performance. They always centralize in the network layer. 8 However, current routing protocols mainly focus on infrastructure WMN or client WMN, and the routing protocols designed for hybrid WMN are few. As the multi-hop communication brought by mesh clients causes link diversity and complexity of network condition, 9 current routing protocols for infrastructure WMN are no longer adaptive for hybrid WMN. Besides, the features of mesh clients can cause the condition that links are unstable and network lifetime is short. Hence, designing a proper routing protocol considering special features of hybrid WMN is quite meaningful. Routing protocols designed for hybrid WMN nowadays have different emphases. 10 Some protocols consider congestion condition,6,11 and others also take interference and channel utilization into consideration. 12 In terms of the different features of mesh routers and clients, some protocols are node-aware routing protocols, 13 while some others just consider all the nodes in the network together. Although there are some routing protocols designed for hybrid WMN, the ones taking mobility and energy constraints of mesh clients into account are quite few.
Routing metric is the core measuring standard in routing protocol to choose the best route.14,15 It can contain a series of measurements by mathematical way. The measurements, which can be obtained from media access control (MAC) layer and physical layer, are often used in the routing protocol in network layer. This article concentrates on the routing metric improvements based on the cross-layer aware (CLA) routing protocol. 16 A regional energy- and mobility-aware (REMA) routing protocol is proposed. The main contributions of REMA are as follows:
The regional residual energy of neighbor mesh clients is taken into account in the routing metric. At the same time, the degree of dispersion of the remaining energy is computed. A region with more remaining energy and less degree of dispersion indicates that less packets are transmitted within this area. The node whose neighbor area has sufficient and balanced energy can be selected for network services. Energy can be balanced and the network lifetime can be prolonged.
The mobility of clients in a whole path is considered to enhance the stability and reduce network overhead. Besides the speed of a single mesh client, the mobility of clients in the whole path is also taken into account. The route with less average speed, less speed dispersion, and more stability will be chosen. In this case, frequent link breakup can be avoided and the overhead brought by discovering new route frequently can be reduced.
Related work
The number of routing protocols designed for hybrid WMN currently is not large. Ad hoc on-demand distance vector (AODV)17,18 is the most popular routing protocol that is used in hybrid WMN to adapt to dynamic changes. However, AODV only uses hop count as the routing metric, neglecting interference, load, mobility, and energy constraints, which overlooks typical features of hybrid WMN. High-performance AODV (HP-AODV)
19
and multi-link AODV (ML-AODV)
12
use multi-link and recommended channel approach to decrease interference. SafeMesh
13
considers the queue length at different interfaces and congestion to improve the performance of hybrid WMN. Congestion-adaptive AODV (CA-AODV)
6
based on AODV makes use of the routing metric Channel Diverse Congestion Aware (CDCA) to build a less load and interference route. CDCA gets queue length in MAC layer through the cross-layer method to measure the load. Intra-flow interference is also measured by channel diversity to improve the network performance. Mobility and energy constraints of mesh clients are neglected in CDCA, which bring less accuracy to evaluate the network condition. Also, on the basis of AODV, Adaptive Load-Aware Routing Metric (ALARM)
16
describes load and interference by queue length from MAC layer and the current data transmission rate. Cross layer Secure and Resource-aware On demand Routing (CSROR)
20
is designed to ensure routing security. Mobility is not considered in ALARM or CSROR either. Dynamic Weighted Cumulative Expected Transmission Time (D-WCETT)
21
modifies Weighted Cumulative Expected Transmission Time (WCETT)
22
to change the weight value of
CLA based on AODV applies hop count, channel utilization, speed, and residual energy to the routing metric. Network interference, load, mobility, and energy are considered synthetically in CLA by cross-layer method. The definition of the routing metric in CLA, that is
where
It can be seen that the metric of CLA considers channel condition, mobility, energy, and path length. The features of mesh routers and clients are also considered in CLA. Inter-flow interference, traffic load, stability, and energy validity can be measured effectively. However, CLA also have some disadvantages as follows:
The most remaining energy weight in the path is used into the routing metric, which neglects energy condition of neighbor nodes. The selected path can cause centralized consumption of energy in an area, and besides the importance of energy balance, network lifetime is also neglected.
The routing metric only considers the highest speed in the path and cannot take the influence of other nodes in the same path into overall consideration.
It is a source routing protocol, which is non-order-preserving and can cause high cost to store the path information in hybrid WMN.
REMA routing protocol
To solve the disadvantages of CLA, REMA measures the regional energy consumption and the mobility in the whole path respectively.
Regional energy-aware model
As shown in Figure 1, when source node

Schematic representation of regional energy.
The regional energy of node
where
Routing metric of REMA
The routing metric of REMA reserves the measurement of interference and load (i.e.
where
According to the regional energy-aware part of REMA, the process and the analysis of routing are shown in Figure 2. From the source node

Process of routing according to the regional energy-aware part of REMA.
For the mobility part, to measure client speed more accurately and attain the order preservation in the routing metric, a similar computing method of average degree of dispersion (AVDD) is adopted. AVDD is the sum of average speed and standard deviation of mesh clients in the whole path and is defined as
where
The mobility part of the routing metric (i.e.
where
where
According to the mobility part of REMA, the process and the analysis of routing are shown in Figure 3. From the source node

Process of routing according to the mobility part of REMA.
The routing metric of REMA in path
The variables which are used in REMA routing protocol are listed in Table 1.
Variable definition in REMA routing protocol.
From mentioned above, we can see that REMA routing protocol has several advantages:
Regional energy is considered in the route discovery process, which balances energy and load. The network’s lifetime can also be prolonged.
The measurement of mobility is improved, and the speed of mesh clients in the whole path is taken into account. The selected route is more stable.
The routing metric of REMA routing protocol has order preservation, decreasing the network cost.
After a node receives a route request (RREQ) message, the processing mechanism is shown in Figure 4.

RREQ processing mechanism.
Computation complexity of REMA
Assuming the number of nodes in a whole path is
Performance evaluation and simulated analysis
NS2 network simulator 25 is used to compare the performance of REMA, CLA, ALARM, and AODV.
Simulation parameters
At first, simulation models in NS2 are extended to multiradio–multichannel environment based on Roman scheme. 26 The backbone of hybrid WMN is deployed into a 5 × 5 grid or a random topology. The size of the simulation area is 1000 m × 1000 m. Detailed simulation parameters are shown in Table 2. The number of data flows and maximum speed of clients are changed to evaluate network performance.
Detailed simulation parameters.
Simulation parameters
Average network throughput: the bit number successfully received by destination nodes per second. The unit is kilobit per second (kbps).
Average packet loss rate: the ratio of packet number unsuccessfully received by the destination to the packet count sent by the source.
Average end-to-end delay: the ratio of the total time delay to the packet number successfully received by the destination. The unit is second per packet (s/p).
Average routing overhead: the ratio of control packet number to the count of data packets successfully received by the destination.
Energy consumption of every data packet: the ratio of the total consumed energy to the number of packets received by the destination. The unit is Joule per packet (J/p).
No-balance degree of remaining energy in mesh clients: the standard deviation of remaining energy of all mesh clients in the whole network when the first mesh client becomes invalid due to the exhausted energy.27,28
Network lifetime: the run time until the first client runs out of energy. The unit is seconds.
Simulated analysis
Performance comparison under different number of flows in hybrid WMN with grid backbone topology
The backbone composed of mesh routers in hybrid WMN is deployed into a 5 × 5 grid, and mesh clients are deployed randomly. The maximum speed of mesh clients is fixed to

Average network throughput versus the number of flows in hybrid WMN with grid backbone.

Average packet loss rate versus the number of flows in hybrid WMN with grid backbone.

Average end-to-end delay versus the number of flows in hybrid WMN with grid backbone.

Average routing overhead versus the number of flows in hybrid WMN with grid backbone.

Energy consumption of every data packet versus the number of flows in hybrid WMN with grid backbone.

No-balance degree of remaining energy in mesh clients versus the number of flows in hybrid WMN with grid backbone.

Network lifetime versus the number of flows in hybrid WMN with grid backbone.
Figure 5 shows that the average network throughput of REMA is higher than other routing protocols. Figures 6 and 7 illustrate that the average packet loss rate and average end-to-end delay of REMA are always lower than CLA, ALARM, and AODV. As more energy consumption indicates more load, REMA also measures the regional load along with regional energy. The selected route can effectively avoid an area with heavy load, so the average packet loss rate and end-to-end delay are declined. Besides, the mobility of mesh clients is also considered in REMA. The stability of the path is improved and frequent breakup caused by fast speed is averted. The packet loss rate and delay are further reduced, and the throughput is improved.
The advantage of REMA in low overhead and high energy efficiency can be seen in Figures 8 and 9. Besides the measurement of load, energy, and mobility in the routing metric of REMA, REMA also has order preservation. However, CLA is non-order-preserving, which brings higher overhead than REMA. The overhead in ALARM is also higher resulting from multiple copies of the route request packet are forwarded to build multiple links. In terms of energy efficiency, REMA can measure intensity of energy consumption in an area more accurately, and high energy efficiency can be obtained due to the energy balance. In addition, low network overhead also contributes to the energy saving in REMA.
Figure 10 shows that the no-balance degree of remaining energy of REMA is lower than CLA, ALARM, and AODV. Because of the multi-link metric in ALARM, the load can be balanced in some extent and the performance of ALARM is better sometimes. The average performance of REMA is still better than that of CLA and ALARM. Figure 11 illustrates that the lifetime of REMA is longer than CLA, ALARM and AODV. REMA describes the regional energy, which can achieve energy balance in the whole network. The energy balance can avoid large differences among energy consumption of mesh clients and then the network lifetime can be prolonged.
Performance comparison under different maximum speed of mesh clients in hybrid WMN with grid backbone topology
The hybrid WMN is also with a 5 × 5 grid backbone topology, and mesh clients are deployed randomly. The number of data flows is fixed as 11, and the maximum speed of mesh clients gradually increases. The performance comparisons between REMA, CLA, ALARM, and AODV in terms of the seven performance metrics are shown in Figures 12–18.

Average network throughput versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

Average packet loss rate versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

Average end-to-end delay versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

Average routing overhead versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

Energy consumption of every data packet versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

No-balance degree of remaining energy in mesh clients versus the maximum speed of mesh clients in hybrid WMN with grid backbone.

Network lifetime versus the maximum speed of mesh clients in hybrid WMN with grid backbone.
Figures 12–18 illustrate that REMA can maintain better performance with increasing maximum speed of mesh clients. REMA can select a path with less average mobility, which enhances the stability of the route. Although CLA uses the maximum speed in a path into the routing metric and aims to decrease breaking probability, CLA neglects the mobility condition of mesh clients in the whole path. ALARM does not consider the mobility features of mesh clients. AODV only considers the hop count during the process of selecting routes, which neglects the features of mesh routers and mesh clients in hybrid WMN. Therefore, REMA can choose path with higher stability. Meanwhile, regional energy-consumed condition is considered in REMA to balance energy and load. This is the reason why REMA has more advantages in hybrid WMN.
Performance comparison under different number of flows in hybrid WMN with random backbone topology
To further verify the effectiveness of REMA, the performance of REMA is also analyzed in hybrid WMN with random backbone topology. In this part, mesh clients and mesh routers are both deployed randomly. The maximum speed of mesh clients is fixed to 10 m/s, and the number of data flows is changed. The performance comparisons between REMA, CLA, ALARM, and AODV in terms of the six performance metrics are shown in Figures 19–24.

Average packet loss rate versus the number of flows in hybrid WMN with random backbone.

Average end-to-end delay versus the number of flows in hybrid WMN with random backbone.

Average routing overhead versus the number of flows in hybrid WMN with random backbone.

Energy consumption of every data packet versus the number of flows in hybrid WMN with random backbone.

No-balance degree of remaining energy in mesh clients versus the number of flows in hybrid WMN with random backbone.

Network lifetime versus the number of flows in hybrid WMN with random backbone.
It can be seen from Figures 19–24 that the performance of REMA remains better than other routing protocols in hybrid WMN with random backbone. Based on CLA, REMA measures regional energy to balance energy and load. The packet loss rate and delay can be declined. Besides, REMA measures the mobility of mesh clients more accurately, which makes the chosen route more stable. The routing metric of REMA has order preservation to reduce the occupation of network resource. Network overhead and energy consumption are reduced. With the comprehensive consideration of load, energy constraints, and mobility, REMA can balance energy, prolong network lifetime, and improve the whole network performance.
Conclusion
To adapt the features of energy constraints and mobility in hybrid WMN, a REMA routing protocol is proposed in this article. Based on CLA, REMA measures energy and mobility more accurately. In the regional energy part of REMA, energy-consumed intensity and the discretization degree of neighbors are described. The energy and load can be balanced better for the whole network, and the network lifetime is prolonged. At the same time, the mobility of mesh clients in the whole path is measured to strengthen stability. Simulation results show that REMA can achieve better network performance and obtain the goal of prolonging network lifetime, balancing energy and load.
The single-radio mesh clients are taken into account currently. We plan to extend the mesh clients to be multiple-radio, improving their data forwarding ability. Then, the performance of current routing protocols designed for hybrid WMN can be evaluated in the future.
