[Popular technology] ad hoc point-to-point communication mode-Mobile Self-Organizing Network

I. Concept

The ad-hoc mode is similar to the previous direct connection twisted pair. It is a P2P connection, so it cannot communicate with other networks. Generally, wireless terminal devices such as PMP, PSP, and PMA use the ad-hoc mode. In the establishment of a home wireless LAN, I think everyone knows that the simplest thing is to implement wireless interconnection between two computers with wireless NICs. One computer can share bandwidth when connected to the Internet.

Supplementary concepts:

1) PMP (Portable Media player, simplified media producer) is a type of multimedia computer file format that uses mpeg-4. its sub-file name is .mp4, which stores digital audio and video. Later, the video and audio can be played in MP4 format.

2) PSP (PlayStation portable, portable TV game machine-handheld game machine) is the head of Sony Corporation in Japan and has many functions such as games, music and videos.

Ad-hoc network is a mobile network with temporary fast and automatic networking.

Ii. Features of Ad Hoc Networks
Ad hoc network is a special wireless mobile network. Network in progressEqual status of all nodesYou do not need to set any central control nodes. Nodes in the network not only have the functions required by ordinary mobile terminals, but also have the packet forwarding capability. Compared with ordinary mobile networks and fixed networks, it has the following features:
1) No center
Ad hoc networks do not have a strict control center. All nodes have equal status, that is, a peer-to-peer network. Nodes can join or leave the network at any time. Faults at any node do not affect the operation of the entire network and have a strong anti-destruction capability.
2) Self-Organizing
Network deployment or expansion does not depend on any preset network facilities. Nodes use hierarchical protocols and distributed algorithms to coordinate their behavior. After the node is started, it can quickly and automatically form an independent network.
3) multi-hop routing
When a node needs to communicate with a node out of its coverage, it needs to jump to the middle node. Unlike multiple hops in a fixed network, multi-hop routes in an ad hoc network are completed by common network nodes rather than dedicated routing devices (such as routers.
4) dynamic topology
An ad hoc network is a dynamic network. The network nodes can be moved everywhere, and can be started or shut down at any time, which will change the network topology at any time. These features make the Ad Hoc network architecture, network organization, protocol design, and other aspects have a significant difference with the ordinary cellular mobile communication network and fixed communication network.

Iii. application fields of Ad Hoc Networks
Due to the particularity of Ad Hoc Networks, its application fields are significantly different from common communication networks. It is suitable for scenarios where network facilities cannot or are inconvenient to be laid in advance, and where fast automatic networking is required. Research on ad hoc networks is initiated by military applications. Therefore, military applications are still the main application fields of ad hoc networks. However, ad hoc networks are also widely used in civil applications.
Its application scenarios mainly include the following:
1) military applications
Military applications are the main application fields of Ad Hoc network technology. Because of its unique features such as no need to set up network facilities, fast expansion, and strong anti-destruction ability, it is the preferred technology for digital man battlefield communication. Ad hoc network technology has become the core technology of the tactical Internet of the US military. Recently, major communication equipment such as digital radio stations and wireless Internet controllers of the US military have adopted ad hoc network technology.

2) Sensor Networks
Sensor Networks are another major application area of Ad Hoc network technology. For many applications, sensor networks only use wireless communication technology. Considering factors such as volume and energy saving, the transmit power of the sensor cannot be very large. Using ad hoc networks to implement multi-hop communication is a very practical solution. Sensors scattered everywhere form an ad hoc network, which enables communication between sensors and the control center. This has broad application prospects in fields such as explosive residue detection.

3) Emergency and temporary occasions
In the event of earthquakes, floods, strong tropical storms, or other disasters, fixed communication network facilities (such as wired communication networks, base stations of cellular mobile communication networks, satellite communications earth stations, and microwave relay stations) may be completely destroyed or cannot work normally, for disaster recovery, ad hoc networks, such as self-organizing network technology, can be quickly deployed without any fixed network facilities. Similarly, in remote or remote field areas, it is impossible to rely on fixed or preset network facilities for communication. The independent Networking capability and self-organization Characteristics of Ad Hoc network technology are the best choice for communications in these scenarios.

4) Personal Communication
Personal Area Network (PAN) is another application area of Ad Hoc network technology. It can be used not only for communication between personal electronic communication devices such as PDA, mobile phone, and laptop, but also for multi-hop communication between personal LAN. Scatternet in Bluetooth is a typical example.

5) integration with Mobile Communication Systems
Ad hoc networks can also be combined with cellular mobile communication systems, the multi-hop transmission capability of the mobile station is used to expand the coverage of the mobile communication system, balance the services of adjacent cells, and increase the data rate at the edge of the cell. In practical applications, in addition to local communication, an ad hoc network can also act as an end subnet to access other fixed or mobile communication networks through access points, communicates with hosts other than the Ad Hoc network. Therefore, ad hoc networks can also be used as one of the wireless access methods for various communication networks.

Iv. Architecture of Ad Hoc Networks

1) node Structure

Nodes in an ad hoc network must not only have the functions of common mobile terminals, but also have the packet forwarding capability, that is, they must have the functions of routers. Therefore, nodes can be divided into three parts: Host, router, and platform. The host provides the functions of common mobile terminals, including man-machine interfaces, data processing, and other application software. The vro module is mainly responsible for maintaining the network topology and routing information, and forwarding packets. The radio station provides wireless channel support for information transmission. The physical structure can be divided into the following categories: single host, single station, single host, multi-station, multi-host, single-station, and multi-host, multi-station. Hosts generally use a simple single-host, single-station structure. As a complex vehicle carrier, one node may include multiple hosts in the communication vehicle. Multiple radio stations can be used not only to build overlapping networks, but also as gateway nodes to interconnect multiple ad
Hoc network.

2) Network Structure

Ad hoc networks generally have two structures: A plane structure and a hierarchical structure. In a plane structure, all nodes are equal, so they can be called a peering structure. In a hierarchical structure, the network is divided into clusters. Each cluster consists of one cluster header and Multiple Cluster members. These clusters form a high-level network. In a higher-level network, you can cluster again to form a higher-level network until the highest level. In a hierarchical structure, the cluster header node forwards data between clusters. The cluster header can be pre-specified or automatically elected by nodes using algorithms. A hierarchical network can be divided into two types: single-frequency classification and multi-frequency classification. In a single-frequency hierarchical network, all nodes use the same frequency for communication. To realize the communication between cluster headers, Gateway nodes (nodes belonging to both clusters at the same time) must be supported. In a multi-frequency group network, different levels use different communication frequencies. The communication range of low-level nodes is small, while that of high-level nodes is large. Advanced nodes are in multiple levels at the same time and have multiple frequencies. different frequencies are used for different levels of communication. In a two-level network, the cluster head node has two frequencies. Frequency 1 is used for the communication between the cluster header and the cluster members. Frequency 2 is used for the communication between cluster headers. Each node of a hierarchical network can be a cluster header. Therefore, an appropriate cluster header Election Algorithm is required. The algorithm must be able to re-cluster based on network topology changes. The flat structure of the network is relatively simple, and all nodes in the network are completely equal. In principle, there is no bottleneck, so it is more robust. Its disadvantage is poor scalability: each node needs to know the route to all other nodes. Maintaining these dynamically changing routing information requires a large amount of control messages. In a hierarchical network, cluster members have simple functions and do not need to maintain complex routing information. This greatly reduces the number of route control information in the network, so it has good scalability. Because cluster head nodes can be elected at any time, the hierarchical structure is also highly resistant to destruction. The disadvantage of the hierarchical structure is that to maintain the hierarchical structure, the nodes need to execute the cluster head election algorithm, and the cluster head node may become the bottleneck of the network.
Therefore, when the network size is small, a simple flat structure can be used. When the network size increases, the application hierarchical structure is used. In its tactical Internet, the US military uses a dual-band hierarchical structure when using the recent digital radio station (ntdr, near term digital radio) networking.

V. Key Technologies in Ad Hoc Networks

1) Channel Access Technology

The wireless channels in Ad Hoc Networks are multi-hop shared multi-point channels, which are different from the shared broadcast channels in normal networks, point-to-point wireless channels, and the wireless channels controlled by base stations in cellular mobile communication systems. This technology controls how nodes can access wireless channels. The channel access technology is mainly used to solve the hidden terminal and exposed terminal problems. The two channels that have a great impact include the maca protocol, the control channel and the data channel split, and the MAC protocol based on the targeted antenna, and some Improved MAC protocols.

2) Network Architecture

The network is mainly designed for data services without too much consideration for the architecture. However, when an ad hoc network needs to provide multiple services and support certain QoS, we should consider selecting the most appropriate architecture and re-designing the original protocol stack.

3) Routing Protocol
The main challenge for ad hoc routing is how the traditional distributed routing database stored in nodes can adapt to the dynamic changes of the network topology. In Ad Hoc Networks, multi-hop routing is completed by common nodes, rather than by dedicated routing devices. Therefore, a dedicated and Efficient wireless multi-hop routing protocol must be designed. At present, the representative achievements that are generally recognized include DSDV, wrp, Oscar, DSR, Tora and zrp. So far, the research on routing protocols is still the most concentrated part of ad hoc networks.

4) QoS Assurance
Ad hoc networks are mainly used to transmit a small amount of data at the early stage. As applications continue to expand, multimedia information needs to be transmitted in the Ad hoc network. Multimedia Information imposes high requirements on latency and jitter, that is, it must provide QoS Assurance. QoS Assurance in Ad Hoc networks is a systemic problem. Different layers must provide corresponding mechanisms.

 

5) multicast/multicast protocol
Due to the special nature of Ad Hoc networks, the broadcast and multicast problems become very complex, and they need support at the link layer and network layer. At present, the research on this problem has made some progress.

6) security issues
Ad hoc networks have poor security and are vulnerable to eavesdropping and attacks. Therefore, we need to study the security architecture and security technologies suitable for ad hoc networks.

 

7) Network Management
Ad hoc network management involves a wide range of areas, including mobility management, address management and service management. corresponding mechanisms are required to solve node locating and automatic address configuration.

8) Energy Saving Control
The automatic power control mechanism can be used to adjust the power of the mobile node to compromise the transmission range and interference. The Intelligent sleep mechanism can also be used to adjust the power of the mobile node, use power Aware Routing and hardware with low power consumption to reduce node energy consumption.