“2017 National First Military Simulation Competition” Status Report
May 15 – Registration Deadline Various sub-regions are gradually carrying out preliminary work
1 Introduction
Wireless distributed networks have gained increasing attention as a rapidly developing technology in the field of communications in recent years. The predecessor of wireless distributed networks is the ad hoc network, which originated from the packet radio network (PRNET) proposed by the U.S. DAPRA in 1972. It is a multi-hop, temporary self-organizing system composed of a group of mobile nodes with wireless transceivers. With continuous research and the expansion of ad hoc network applications, wireless sensor networks and wireless mesh networks have emerged. They also adopt distributed and self-organizing networking concepts but have characteristics different from ad hoc networks in specific application environments. Therefore, as technology continues to develop, wireless distributed networks have become a very broad concept, mainly consisting of three types: ad hoc networks, wireless sensor networks, and wireless mesh networks. This article focuses on analyzing, studying, and comparing the basic concepts, technical characteristics, and application scenarios of these three types of wireless distributed networks.
2 Ad Hoc Networks
2.1 Basic Concepts
“Ad hoc” comes from Latin, meaning “for this purpose”. It consists of a series of movable nodes, where network nodes are dynamically and arbitrarily distributed, and nodes are interconnected wirelessly. Each network node has dual functions as both a terminal and a router. Figure 1 shows the structure of a typical ad hoc network. Due to its self-organizing characteristics, the topology, channel environment, and service patterns of ad hoc networks change dynamically with the movement of nodes. The initial purpose of ad hoc network research was to meet military needs for survival on the battlefield. In harsh battlefield environments, communication cannot rely on pre-installed communication infrastructure because, on the one hand, these facilities may not exist at all, and on the other hand, these facilities may be destroyed at any time. Due to its rapid, flexible, and convenient networking capabilities, ad hoc networks have gained extensive attention in academia and industry, increasingly becoming an important direction in the development of mobile communications.
Figure 1 Ad Hoc Network Structure
2.2 Technical Characteristics and Application Scenarios
Due to the self-organizing characteristics of ad hoc networks and their provision of a more flexible networking method, they possess many characteristics not found in traditional wired or wireless networks:
(1) No Central Control and Self-Organization
This is the greatest feature of ad hoc networks. There is no absolute control center in the network; all nodes are equal in status, and nodes coordinate their behaviors through distributed algorithms without manual intervention or any other pre-installed network facilities, allowing for rapid deployment and automatic networking anywhere at any time. Due to the distributed nature of the network, redundancy of nodes, and the absence of a single point of failure, ad hoc networks exhibit excellent robustness and resilience.
(2) Dynamically Changing Network Topology
In ad hoc networks, mobile terminals can move within the network at any speed and in any manner, and can turn off their radios at any time. Combined with the various types of antennas used for wireless transmission, variations in transmission power, mutual interference among wireless channels, terrain, and weather, the network topology formed by mobile terminals through wireless channels can change at any time, and the manner and speed of these changes are difficult to predict.
(3) Limited and Time-Varying Wireless Transmission Bandwidth
Ad hoc networks use wireless transmission technology as the underlying communication means. Due to the physical characteristics of wireless channels, the bandwidth they can provide is much lower than that of wired channels. Additionally, considering various factors such as conflicts arising from competing shared wireless channels, signal attenuation, noise, and interference between channels, the actual bandwidth available to mobile terminals is far less than the theoretical maximum bandwidth. Furthermore, unlike wired networks, due to dynamically changing topologies, the amount of traffic forwarded by each node that is not destined for itself varies over time, resulting in time-varying link capacity.
(4) Multi-Hop Routing
Due to limited transmission power of nodes and the limited coverage range, when a node needs to communicate with another node outside its coverage area, it requires intermediate nodes to relay. Moreover, multi-hop routing in self-organizing networks is accomplished by ordinary nodes cooperating, rather than by dedicated routing devices (such as routers).
(5) Energy Constraints
Due to the mobility characteristics of network nodes, most nodes are powered by batteries. Therefore, energy efficiency becomes a very important metric during system design.
(6) Poor Security
Ad hoc networks are a special type of wireless mobile network, and due to the use of wireless channels, limited power sources, and distributed control, they are more susceptible to network attacks such as passive eavesdropping, active intrusion, and denial of service. If security measures are not added at the network or data link layer, ad hoc networks can easily fall victim to attacks such as monitoring network transmissions, retransmissions, manipulating packet headers, and redirecting routing information. Therefore, channel encryption, interference resistance, user authentication, and other security measures need to be specially considered.
Due to the uniqueness of ad hoc networks, their application domains are significantly different from those of ordinary communication networks. They are suitable for situations where it is not possible or convenient to lay network facilities in advance and where rapid automatic networking is required. Military applications remain the primary domain for ad hoc networks, but there are also very broad prospects for civil applications, including military applications, emergency incidents, temporary situations, personal communication, and integration with mobile communication systems.
3 Wireless Sensor Networks
3.1 Basic Concepts
Wireless sensor networks are considered one of the most important technologies of the 21st century, which will have a profound impact on human lifestyles in the future. In recent years, with the rapid development of wireless communications, integrated circuits, sensors, and micro-electromechanical systems (MEMS), large-scale production of low-cost, low-power, multifunctional micro wireless sensors has become possible. These micro wireless sensors have capabilities such as wireless communication, data collection and processing, and collaborative operation. Wireless sensor networks (referred to as sensor networks) are organized by many such micro wireless sensor nodes. The nodes of the sensor network can be deployed randomly or specifically in the target environment, and they can self-organize through specific protocols to gather information about the surrounding environment and collaborate to complete specific tasks. Figure 2 shows the structure of a wireless sensor network.
Figure 2 Structure of Wireless Sensor Network
The sensor network originally stemmed from a research project by the U.S. DARPA. Due to the limitations of technology at that time, the application of sensor networks could only be confined to some military projects, making it difficult to promote and develop. In recent years, with the development of wireless communications, microprocessors, MEMS, and other technologies, the ideal blueprint for sensor networks has been realized, and its application prospects are becoming increasingly broad, with foreign research institutions actively researching it.
3.2 Technical Characteristics and Application Scenarios
Sensor networks have some unique characteristics compared to traditional networks, and it is precisely these characteristics that present many new problems and challenges. The main characteristics of sensor networks include:
(1) Limited and Non-rechargeable Battery Energy of Sensor Network Nodes
Due to the miniaturization of sensor nodes, the battery energy of nodes is limited, and due to physical environmental constraints, it is difficult to replace the batteries of nodes, making the limited battery energy of sensor nodes one of the most critical constraints in the design of the entire sensor network, directly determining the operational lifespan of the network.
(2) Large Number and High Density of Sensor Network Nodes
Due to the miniaturization of sensor network nodes, the communication and sensing radius of each node is very limited, generally within a few dozen meters. Moreover, to save energy, sensor nodes are mostly in sleep mode, so a large number of sensor nodes are deployed to ensure network quality. The number and density of nodes in sensor networks can be several orders of magnitude higher than in ad hoc networks, potentially reaching hundreds of nodes per square meter, making it even impossible to assign a unified physical address to individual nodes. This brings a series of problems, such as signal conflicts, the selection of effective transmission paths, and how to coordinate the operation of a large number of nodes.
(3) Data-Centric
Data-centric means that nodes aggregate data from multiple routes to eliminate redundancy, minimizing the transmitted data to achieve energy savings. Sensor networks are high-density networks, and many nodes may detect the same physical phenomenon, leading to redundancy in the data sent by sensor nodes. Wireless sensor networks consider routing and data aggregation within the network together to achieve maximum energy savings.
(4) Dynamic Changes in Topology Structure
Sensor nodes switch between working and sleeping states, and sensor nodes may fail for various reasons or new sensor nodes may be added to improve network quality. These characteristics cause the topology of sensor networks to change rapidly, posing challenges to the effectiveness of various algorithms (such as routing algorithms and link quality control protocols). Additionally, if nodes have mobility, it can also lead to changes in network topology.
Wireless sensor networks, with their self-organizing, miniaturized, low-cost, and flexible characteristics, have very broad application prospects in military, environmental science, healthcare, space exploration, business applications, and other fields.
4 Wireless Mesh Networks
4.1 Basic Concepts
Wireless mesh networks are a key technology in next-generation wireless networks, which have gained widespread attention and rapid development in recent years. It is a dynamically self-organizing network where nodes form networks in an ad hoc manner and maintain a mesh structure. It is often seen as a simplified version of ad hoc networks, but there are certain differences between the two. Access points in wireless mesh networks can serve as both peer data forwarding entities for ad hoc networks to complete data routing functions and as bridge connectors to connect to other wired networks. Wireless mesh networks are high-capacity, high-rate multipoint-to-multipoint networks, proposed to solve the “last mile” problem in wireless distributed networks.
Wireless mesh networks consist of two types of nodes: mesh routers and mesh clients. Unlike traditional network bridges or gateways, mesh routers have other special functions to support mesh networks. Through multi-hop routing, mesh routers can cover the same area with lower power. To further enhance the flexibility of mesh networks, mesh routers are equipped with various wireless interfaces to support multiple wireless access technologies. Although there are many differences, mesh routers are fundamentally similar to traditional wireless network routers in hardware platforms.
Mesh routers typically do not have mobility; they form the backbone of the mesh network and provide wireless access services to mesh clients. Although mesh clients can temporarily act as mesh routers in certain situations, they are simplified in terms of hardware and software compared to mesh routers. For instance, in terms of communication protocols, mesh clients are lightweight and do not have the functions of gateways or bridges, only possessing a simple wireless interface. The structure of wireless mesh networks can be categorized into three types:
(1) Backbone Wireless Mesh Networks
Mesh networks form a backbone structure to ensure client access to the internet, as shown in Figure 3. The backbone structure can adopt various RF technologies, with IEEE802.11 technology being the most common. All mesh routers form a self-healing self-organizing network, and routers act as gateways to access the internet. This structure allows the mesh network to connect to the internet through the gateway and bridge functions of routers. For traditional clients, the same RF technology can be used to connect to the routers.
Figure 3 Structure of Backbone Wireless Mesh Network
(2) Client Wireless Mesh Networks
Client wireless mesh networks provide an end-to-end network structure, as shown in Figure 4. Clients form the actual network and provide services to other clients, so there is no need for mesh routers in this type of network. Client mesh networks typically use only one RF technology, making them somewhat similar to traditional ad hoc networks. However, in terms of network terminals, client mesh networks are significantly more robust compared to backbone wireless mesh networks.
Figure 4 Client Wireless Mesh Network
(3) Hybrid Mesh Networks
These combine the structural mesh network and client wireless mesh networks, where mesh network clients connect to the network via routers and can also form a mesh network with other mesh clients.
4.2 Technical Characteristics and Application Scenarios
As a broadband wireless distributed network, wireless mesh networks inherit some points from ad hoc networks while also differing in several aspects:
· Wireless mesh networks support ad hoc networks and possess self-forming, self-recovering, and self-organizing characteristics.
· Wireless mesh networks are multi-hop networks.
· Mesh routers lack mobility and can perform complex routing and configuration, greatly reducing the load on mesh terminals and clients.
· Mesh routers integrate hybrid networks, including wired and wireless networks. Therefore, multiple networks can coexist within wireless mesh networks.
· The power consumption limits of mesh routers and mesh clients differ.
· Wireless mesh networks are not isolated networks and must be compatible with other networks.
In addition to the above characteristics, wireless mesh networks have several advantages over traditional point-to-point network structures, mainly reflected in improved reliability, reduced collisions, simplified wireless link design, and ease of maintenance.
The primary application of wireless mesh networks is backbone mesh networks. The backbone of a wireless mesh network typically refers to the high-speed links that constitute the main data transmission lines in the network, specifically the wireless links between mesh routers. Traditionally, backbone networks use optical fibers to connect various edge networks, while in wireless mesh networks, only one or a few mesh routers connect to wired networks. The backbone of wireless mesh networks can be deployed indoors or outdoors, with outdoor devices typically mounted on streetlights or the outer surfaces of buildings where power is available.
5 Conclusion
In summary, ad hoc networks, wireless sensor networks, and wireless mesh networks, as the three most representative types of wireless distributed networks, all adopt distributed and self-organizing ideas to form networks, where each node possesses routing functions, providing routing and relay services for data transmission among other nodes at any time. Ad hoc networks primarily focus on applications in mobile environments, ensuring reliable communication between any two nodes in the network, with data flows in the network potentially including voice, data, and multimedia information; wireless sensor networks are a special form of ad hoc networks, focusing on monitoring physical phenomena in a certain area using distributed and self-organizing ideas, with the data flow transmitted in the network being of low rates; wireless mesh networks are wireless broadband access networks that utilize distributed ideas to construct networks, allowing users to access the internet wirelessly at high speeds anytime and anywhere.
Wireless distributed networks have rapidly developed in recent years, and people have high hopes for their future. However, due to their characteristics differing from traditional networks, there are many challenges in research and application, such as energy issues, efficient and reliable media access control protocols, and reliable multi-hop routing algorithms. These challenges will become obstacles to the smooth development of wireless distributed networks and require further research.
Source: Unmanned Aerial Vehicles
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