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Issues in Wireless Mesh Networks. Abstract- This paper states the issues related to wireless mesh networks, including problems and solutions. Some principal security issues for to wireless mesh networks are investigated. The threats and the security goals are studied, to design a network which has the vulnerability to security attacks . Most mobile nodes in a wireless ad hoc network (wireless mesh networks) are powered by batteries. The limited lifetime/energy of a battery has constraint the network performance. Therefore, power consumption of network operations is a critical issue . The planning of a network is important to minimize the system cost and to provide performance . Location is considered as an important issue in the design of wireless mesh networks.
Index Terms- wireless mesh network, issues, security, planning.
A wireless mesh network (WMN) is a communications network that employs the arrangement of nodes in a mesh topology. There are two connection arrangements, which are full mesh topology (each node is connected to the others) and partial mesh topology (nodes are connected to only some of other nodes). Wireless mesh networks often consist of the mesh clients, mesh routers and gateways . The mesh clients are wireless devices such as laptops, cell phones while the mesh routers forward traffic to and from the gateways with or without Internet. The coverage area of nodes in a single network is called a mesh cloud. A wireless mesh network is one of the wireless ad-hoc networks. Usually, all nodes in a wireless mesh network are assumed to be in immobile status but the routers can be highly mobile.
Wireless mesh network provides a good solution for wireless Internet connectivity in a sizable geographic area. It allows the network to be designed in such a way that is fast, easy, reliable, and has a lower cost that other classic wireless networks, by decrease the number of wireless routers needed. Therefore, it is also suitable for areas that do not have existing data cabling or for the deployment of a temporary wireless network .
Section II describes some characteristics of wireless mesh network. Section III is the study of the security issues while Section IV is is the study of the power issue in wireless mesh network. Section V is the planning of the design of the network. Section VI is the conclusion for this paper.
Characteristics of WMN
Wireless mesh network is very reliable. It can do self healing if one node has failure, in which its neighbor's nodes find another route automatically. The capacity can be increased by adding additional nodes. A transmission data will hop from one node to another until it reaches the destination. Therefore, more devices (nodes) in the networks, the higher the available bandwidth is.
There are a few constraints in wireless mesh network, such as CPU, battery, mobility and bandwidth. The computations at the end nodes are large. It becomes slow since the computing power of the processor is small. A battery has limited energy and lifetime. It is not suitable for large consumption for transmission. A mobile node can produce latency in the convergence of the network. Total bandwidth in the mobile nodes is limited .
Figure 1: An example of Wireless Mesh Network
The security issues for wireless mesh network are as important as other communication networks, and they are basically the same. It is important that a network to be available only to authenticated users. Besides that, we need to keep the data confidential by using cryptography.
Availability of a network means despite attacks, a network services still survives and continues to work. The authorized users will be able to gain uninterrupted access to the information in the system. There are a few types of attacks can be launched to interface the availability of a network.
On the physical layer, an attacker can employ signal jamming to interface with communication on physical channel . By using spread spectrum and frequency hopping, the jamming range is widening for the attacker.
Denial of Service (DoS) can be launched at any layer of wireless mesh network . Long-term Denial of Service attacks can discontinue a network's ability. A technique to instigate DoS is by flooding the network system with many requests. The traffic in the system becomes overwhelmed and thus cannot perform transmissions normally. We can use firewall to stop requests from certain users, assuming they are attackers when they send lots of requests. But nowadays, there is an attack that uses 'zombies' systems, so-called distributed DoS , which is almost impossible to counter. Intrusion Detection and Prevention system are deployed to monitor distributed DoS attacks .
Battery life can be a critical problem in a wireless mesh network. Battery exhaustion attack also known as 'sleep deprivation attack' and it does more damage than denial of service attacks. There are some battery management and monitoring system which can help by predicting the amount of usable energy remaining. Such system does not consume too much of its own battery life .
Authenticity is required to the identity of the neighboring nodes to allow communication. Without authenticity, a node can be masqueraded, thus unauthorized users can gain access to the system's information or interfere the system's operation. The usual authentication mechanisms involve using a central system as admin to control the communication within the network. In a mesh network, such server is impossible sometimes, but we can still use some other ways, such as Secure Transient Association . A device acts as universal controller is used to control only one's all devices. A user uses the controller to command owned devices only. If the device is no longer owned, it should not obey the command from the user's controller. One thing to make sure is that no user have similar controller that can control another user's devices. This is done by suing the concept of public key cryptography.
A message that is transmitted over a network must have integrity. Integrity guarantees that a message is not altered during the transmission. The usual mechanism to ensure integrity of data is using hash functions, message digestion  and encryption. Using key cryptography, nodes will establish a common secret key. When a protected message (encrypted using the secret key) arrives at those nodes, they can decrypt the message using the secret key too. There are other cryptographic techniques, symmetric or asymmetric cryptographic. Diffie-Hellman (D-H) key exchange allows two parties to jointly establish shared keys over an insecure communications channel .
Since most mobile nodes are powered by energy limited batteries, which constraints the performance of the network, the batteries power has to be optimize. Hybrid Particle Swarm Optimization (HPSO)  was proposed for solving the minimum power multicast problem in wireless mesh networks. Hybrid PSO algorithm, PSO and Genetic Algorithm (GA) were compared for their performance. The result had shown better global optimization performance and efficiency for HPSO.
Figure 2: HPSO 
Figure 3: Multicast tree power with swarm scale 80 
Figure 4: The success rate 
Table 1: Comparison of running time (sec) 
Planning of a network is an important issue to minimize the system cost. It is not an easy task since we need to consider many variables: the network topology, building heights, antenna types to be used and their orientations, and radio transmit powers . These variables have constraint the network performance. Lack of planning implies that the network may be undesirable due to the loss bandwidth. This is especially true when doing the long distance planning.
We need to consider the capacity required, power constraints, signal strength and some physical variables such as tower heights, antenna types, and transmit powers are dependent on the network topology. Any obstacle in the planning area which can affect the transmission of signal needs to be observed.
Determining the topology in multi-hop wireless mesh network has been a topic of significant research [11, 12, 13, 14]. The work in  showed that the problem of constructing network topology which minimizes the maximum transmitter power allocated to any node is polynomial time solvable. In  the authors propose a cone based distributed algorithm for topology control, while  describes a distributed protocol which is designed for sector antennas. The work in  uses the concept of range neighborhood graphs for topology control . There are many other algorithm that were proposed, which each of them have different advantages over ohers.
First, let's consider the tower. The higher a tower, the higher it cost. The tower height required is depends on the link length since we need to achieve line-of-sight (LOS) clearance . A single tall tower can support several neighbor's nodes, thus affect the topology of the network. Therefore, the tower height and the mesh network topology determine the cost of the planning. Furthermore, we wish to have a certain level of network performance, by altering capacity of network. The capacity required depends on the population/area and the speed for users. This is the throughput constraint which also has a dependence on the antenna type
The tower heights, antenna types, and transmit powers are set of links that formed the network topology. Therefore, a network topology should be made in the first place. The tower heights is merely important to get a LOS, while the antenna types and transmit powers are directly dependent to each other.
In , they did a Topology Search, by checking the throughput constraint, search pruning strategies, eliminating long links, tree depth restriction and Dynamic Cost Bounding (DCB). Later, the heights of the towers were determined such that the overall cost is minimized at the various nodes of the network topology. Figure 5 shows the LOS of a tower, with a tree as an obstacle. Antenna is easier to decide. We just need to consider the cost reduction, and interference reduction. At the end, the planning made in  was success since they manage to get the cost of the generated topology within about 2% of a lower bound and also shown their search pruning strategies to be more effective.
Figure 5: The LOS constraint on the tower height 
A network should provide fair information sharing and minimize the end-to-end delay in wireless mesh networks . To get such network performance, the network topology designed must have a good routing strategy and hop-count distribution. Since wireless mesh networks are generally considered robust and highly connected [17, 20], it is assumed that each node can always find one or more paths to the gateway. Throughput and delay can be analyzed by doing some calculations base on certain formula in .
The fairness problem is caused by the location of the nodes (the distance between a node and gateway). The nodes that are more distance with the gateway may experience a lower throughput than the nodes closer to the gateway. Successful channel-access probability and forwarding probability are determined so that each node will not suffer end-to-end delay and causes unsuccessful transmission.
As conclusion, there are many issues in wireless mesh network to bring a designed network become practical. The physical variables such as area and buildings that act as each node are planned first. The power supplied to each node need to be predicted accurately to make sure a network's availability. The transmission rate need to be calculated well to make sure the whole network has a fair transmission. Lastly, when put into real usage, some securities need to be implied into the network so that only authenticated users are able to use the network.