WIRELESS MESH SENSOR NETWORKS: ADVANTAGES, DIFFICULTIES AND INTERCONNECTION METHODS Safak Durukan Odabasi
A.Halim Zaim
Computer Engineering Istanbul University Istanbul,Turkey
The Institute Of Graduate Studies In Science&Engineering Istanbul Commerce University Istanbul, Turkey
[email protected]
[email protected]
Abstract: A typical wireless sensor network (WSN) has lots of sensor nodes which connect each other with a wireless medium. These nodes have self-organization capability. Because of power constraints, sensor nodes mostly are on sleep mode or they process data. Especially, when a sensor network cannot provide QoS necessities because of node count increasing or when the sensor network works on an area where a mesh network is already deployed, we can talk about interconnection of sensor and mesh networks. Achieving such a a desirable network is only possible with a well-defined architecture of wireless mesh sensor. The important thing of this process is the strategy which is followed on interconnection of sensor and mesh networks that are the similar basically. In this work, we present sensor and mesh network architectures seperately and interconnection of them. Additionaly, we give an overview of existing strategies that are used to interconnect mesh and sensor networks with their advantages and challenges. Keywords: Wireless mesh sensor networks, methods, advantages, challenges.
interconnection
I. INTRODUCTION With the great progress at the communication technologies, Wireless Personal Communication Systems (WPCS) are now widespread all over the world. Mobile users require
communicating with each other and access the Internet anytime and anywhere, therefore wireless network services must accommodate mobile users with ubiquitous communication capability and information access regardless of user location. A typical wireless sensor network (WSN) has lots of sensor nodes which connect each other with a wireless medium. These nodes have self-organization capability. Because of power constraints, sensor nodes mostly are on sleep mode or they process data. They have a different approach from traditional and expensive wired sensor systems [1]. Wireless sensor networks can be used in military applications, observation of environment and health and similar areas for many aims. The basic component of a wireless sensor network is sensor node which has sensing, processing and communication abilities. As known, these nodes spill the media they watch without wire. Figure 1 shows a wireless sensor network structure.
Figure 1. Wireless Sensor Network Architecture
In daily life, the Internet is used many areas from banking transactions to online entertainment and its usage has been growing, so the number of Internet users has been growing in parallel. To facilitate the usage of these the Internet users, new ideas have emerged every day. Mesh networks are engaged at this point and with less need for new infrastructure, an improved the Internet access has been promised. Wireless mesh networks (WMNs) , consist of nodes which are in an ad hoc network automatically can be established and provide the connection. They have abilities such that selforganization and self-configuration. A wireless mesh network has two basic principles: There is no master node and all nodes
can be reached via the others. Each node is equal because of nonexistence of a control center. This property differs mesh architecture from the cellular and wireless LAN architecture. In wireless mesh networks [2], the network nodes are considered to be part of the infrastructure and are dedicated to the routing task. Mesh nodes’ processing, memory and bandwidth capacities generally exceed those of traditional ad hoc network nodes [3]. Additionally, the power consumption requirements are often less stringent than those of wireless ad hoc networks. Figure 2 shows a typical distributed multilayered wireless mesh network architecture [4].
Figure 2: Wireless Mesh Network Architecture
Wireless mesh sensor networks offer powerful new ways to monitor several values, and without the costs of major new installations or wire runs that are typically associated with these types of projects. These new systems can improve the quality of monitoring dramatically while also making entirely new monitoring applications possible. The benefits, like the sensors, reach far and wide - from improving human health, workforce efficiency, quality of life, and even the bottom line. In sensor applications which need more than thousands of nodes, hop counts amount towards sinks is high [3]. Adding new sinks solve this problem but at this time scalability becomes another issue. For interconnecting sink nodes, a wireless mesh network which has auto-deployment ability can be used. Thus, adding a mesh network to a sensor network reduces the load that is imposed on sensor network. Especially, when a sensor network cannot provide QoS necessities because of node count increasing or when the sensor network works on an area where a mesh network is already deployed, we can talk about interconnection of sensor and mesh networks. Currently, not much research that targets integration of sensor and mesh networks presents, although WMNs and
WSNs are not so different basically [3][5]. So, using interconnection of these two technology with together can come with a lot of advantages. II. WIRELESS MESH SENSOR NETWORKS (WMSN)
A. WMSN Architecture By reason of the fact that traditional WSNs have some limitations such as unbalance on energy consumption among nodes, poor scalability and robustness [6], wireless mesh sensor networks (WMSNs) which have a potential ability of improving scalability, reliability and throughput on sensor nodes, have become more populer in a wide area from industry to academic communities [7]. They also support node mobility. To achieve these expecting results, a well-defined architecture of wireless mesh sensor is needed. Tang et al. present such an architectural model for WMSNs [6]. Figure 3 shows wireless mesh sensor network architecture.
Figure 3. A wireless mesh sensor network architecture
In this model, every sensor network has a mesh router which equipped with gateways. While mesh routers deployed on every different sensor networks connect to the Internet via powerful base stations, they connect to form a mesh network. As seen on Figure 3, there are three logical layer [6]: • Wireless Sensor Network: Its aim is monitoring objects and reporting their information. • Wireless Mesh Network: It is in charge of transmitting sensed data to in long distance and secure way. •
The Internet: It provides accessing sensed data by users.
As a result of this layered architecture, there are also three types of nodes in WMSNs. • Sensor nodes: They detect objects and send data which are collected from this objects to most proper wireless mesh gateway (WMG) based on specified routing policies. • Wireless Mesh Gateways (WMGs): They work as sink nodes, low-level WSNs’ gateways and also WMNs’ routers. • Wireless Mesh Routers (WMRs): They just work as routers of WMNs. These different networks in the architecture use different medium access control (MAC) and routing protocols [6]. WSNs use short-distance communication protocols like 802.15.4 while WMNs use long-distance protocols like 802.11. B. Advantages of WMSNs As we mentioned in the previous section, to solve problems cause by inability of WSNs, a layered architecture include WMNs and WSNs is needed. Interconnecting of these two technologies has lots of significant advantages [8].
• Self forming: As nodes are powered on, they automatically enter the network. This simplifies installation and deployment of network. • Self-healing: As a node leaves the network the remaining nodes automatically rerouted their signals around the out-of network. Node to ensure a more reliable communication path. So, this feature makes WMSNs highly reliable. • Ability of multi hop routing: This means that data from a node can jump through multiple nodes before delivering its information to a host gateway (or) controller that may be monitoring the network. • Mobility Support: WMSNs are highly scalable. Whenever sensor nodes or WMGs (WMRs) enter networks, new network topology is automatically set up. Also, multiple gateways are deployed in a sensor network in the energyefficient way; and sensor nodes send data to the best gateway, increasing network throughput and avoid the single point of failure. Long- distance transmission among WMGs (WMRs) further reduces the average number of hops of transmission among different sensor networks. Thereby, by adapting the dynamic change of topology, WMSNs support both sink and node mobility. • Interconnection of heterogeneous sensor networks: By communicating with WMGs, it is possible to interconnect different types of sensor networks. • Interconnection of sensor networks with the Internet: Providing interconnection of sensor networks with the Internet extend application area of WSNs.
C. Challenges of WMSNs There are some challenges to when interconnecting WMNs and WSNs [3]:
come over
• Differences in radio technology: Gateway nodes need to have both sensor and mesh technology interface. WMNs and WSNs can operate in the same frequency range. This causes radio interference. Getting these two different technologies with together in a successful way wants for avoiding with radio failures. • Backwards compatibility: While adding these two types networks to each other, backwards compatibility need to be tackled. Because, if a WMN is added to a WSN, sensor nodes do not need any adjustment. Also, using mesh network to interconnect sensor networks can reduce deployment costs. • Translation of metrics: By using WMN support to increase efficiency in a sensor network, sensor nodes need to know alternative ways to route packets over WMN. This mean is that WMN metrics should translate to WSN metrics. • Addressing schemes: WMN and WSN can use different addressing schemes. In this case, they need a mapping of address types to communicate with each other. • Routing strategies: Although WMN and WSN use the same IP version, routing schemes they used might be different. • Synchronization strategies: In WSNs, sometimes a sleeping schemes is implemented because of power constraint. To send a packet from WMN to WSN, WSN needs to be aware of synchronization strategies. Achieving a successful interconnection of WMN and WSN requires to be taken into consideration.
B. Supernode Repeater backbone solution is not useful because of that backbone is just forwarding packets in an unintelligent way. It is possible to restore repeater solution by combining an intelligent sensor interface. For this, the simple solution is that the gateway which receives the packet process it and then forward the same packet to the other gateways. C. Termination gateway The cause of repeater backbone method is using an unintelligent backbone. This mechanism supports communication between the sensor nodes which are already a part of the network. Adding intelligent WMN to the configuration makes intelligent routing possible. In this model, sink nodes called termination gateways. Because, from side of sensor nodes network paths are accomplished on sinks. If we talk about the communication between sensor nodes, they must choose WMN or gateway node to route their packets over. This choice depends on some metrics like potential of routes or performance. If in the network, there are more than one subnet per gateway, communication of subnets pass over the termination gateways. This type networks, subnet based, count of routes in the routing table is less. Therefore, sensor network’s scalability increases. There is also no need to store routes of individual nodes. So, WMN’s routing task is simply. This solution is really different from supernode by using termination gateway. Adding sink nodes to network results additional overhead for signaling. Also, for using temination gateway nodes, WSN must be designed properly.
III. WMN-WSN INTERCONNECTION METHODS Sending packets of a WSN over a mesh backbone and using mesh architecture’s advantages such as having more bandwidth, energy flexibility provide great convenience to network. Currently, there are not many research about interconnection of WSNs and WMNs. To minimize energy use or reduce delay, a systematic overview needs to presented. In this section we analyze several methods for routing on a WMSN with their advantages and disadvantages. A. Designing of Repeater Backbone In this method, mesh backbone is used as a invisible repeater to send a sensor packet from WSN to all other gateways through sensor interfaces of all gateways [3]. It is a simple solution, because actually packets are just sent over backbone without any changes. Besides this method has some disadvantages like multiplying sensor packets while sending to other gateways, increasing interference. Furthermore, repeaters should be able to detect packets from other repeaters to avoid sending the same packet again and again. Using multi-channel sensor protocols makes gateway nodes need to have additional sensor interface.
D. Translation gateway On the termination gateway technique, the responsibility of a gateway node is tunneling sensor packets over mesh networks to correct destination gateway. For reducing signalization between sensor and gateway nodes, in other words for connecting WSN and WMN directly, a wellarounded approach is needed. Translation of sensor packets to mesh packets can be helpful for gateways. These relaxed gateway nodes are called translation gateways. At every networks local protocols are used for routing packets . Hence, gateways should provide some operations: protocol translation, metric translation, address translation and packet translation. By these processes, intermediate nodes cannot know whether packets are originated from a sensor node or a mesh node. This method is transparent for WMN and WSN. This method’s main disadvantage is complexity caused by translation processes.
E. Stack virtualization To translate protocols or metrics, a complex translation process is needed. As used in Akari Project [9], putting a virtual WSN stack which includes all WSN network protocols all, to mesh nodes reduces the complexity and assuring the backwards compatibility. When a sensor packet is sent between two mesh node, a mesh link is used and the packet is encapsulated. Before and after encapsulation operation, virtual sensor stack manipulate the packet. To communicate between WMN and WSN, the gateway uses the sensor stack which is implemented on its own sensor interface. The key issue of this technique is that it is impossible to put such a stack on an existing mesh network. F. Identical stack As we mentioned, WMNs and WSNs are not different basically. An approach suggested for WSNs can be adapted for WMNs. Taking advantages of both technologies and forming an heterogeneous node like in [10] bring with a lot of benefit. In the light of these information, for interconnecting of WMNs and WSNs using of identical routing and addressing schemes can be a solution. In this method, identical upper layer stack usage might cause poor employment of different node types. For avoiding this situation, node needs, operation characteristics should be considered.
IV.
CONCLUSION
For providing the most appropriate efficient, ubiquitous data access to modern day users, taking advantages of existing technologies might be needed. In the future, wireless sensor might be mobile and interconnecting of this sensor and mesh networks which have high data rate, power efficiency implies lots of gain. The important thing of this process is the strategy which is followed on interconnection of sensor and mesh networks that are the similar basically. As mentioned in this work, there exists several strategy presented but stil there is not a method which can reduce the complexitiy of interconnection. By suggesting of new methods to solve these problems, differences between mesh and sensor networks might be disappear and WMSNs can be a feasible solution with their easy deployment, flexibilty, scalability and cost-effectivity features.
IV. REFERENCES
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