What is wireless mesh network technology?

Wireless network technology

I. Definition

Wireless mesh network refers to a large number of terminals connected wirelessly into a mesh structure, each node exchanges data through routing, is a low-power multilevel hopping point system.

II.? Working Principle

1. Its core is to allow each node in the network to send and receive signals, ? So that ordinary wireless technology in the past has been the existence of low expandability and poor transmission reliability and other problems are solved. A large number of terminal devices in the network can be automatically connected into a mesh structure through wireless,? Each node in the network is equipped with automatic routing function,? Each node only communicates with neighboring nodes,? Therefore, it is a kind of self-organization,? self-managed intelligent network, without the need for a backbone network can be constructed flexible network. Traditional wireless communication networks must be designed and arranged in advance,? Its transmission path is fixed,? The transmission path of a mesh network is dynamic.

2.? Wireless?Mesh?Network is a special form of Mobile Ad?Hoc Networks, and all its early research originated from the research and development of Mobile Ad?Hoc Networks. It is a high-capacity, high-speed distributed network, different from traditional wireless networks, and can be regarded as a kind of fusion of WLAN and Ad Hoc network, and play the advantages of the two, as a new network structure that can solve the bottleneck problem of the "last kilometer" WMN is written into the IEEE802.16 (i.e., World?Informatization?Network), and is a special form of Mobile Ad Hoc Network. （WMN is included in the IEEE802.16 (World Interoperability for Microwave Access (WiMax)) Wireless Municipal Area Network (WMAN) standard.

3.? Each node in a wireless mesh network is capable of receiving/transmitting data, and also, like a router, passes data to its neighbors. Through relay processing, the data packets arrive at the specified destination with a reliable communication link through the intervening nodes.?　?　Similar to the Internet and other point-to-point routing networks, mesh-format networks have multiple redundant communication paths. If one path is interrupted for any reason, including RF interference, the mesh network automatically selects another path to maintain normal communications. In general, a mesh network automatically selects the shortest path, improving the quality of the connection.?　In practice, if the distance is reduced by a factor of two, the signal strength at the receiving end is increased by a factor of four, making the link more reliable without increasing the transmit power of the nodes. In a grid-format network, simply increasing the number of nodes can increase the reachable range or bring more reliability from the increase in redundant links.

4.? Today's grid-format wireless LANs primarily use 802.11a/b/g-based standards as well as 802.15.4?s Zigbee RF technology. Industry heavyweights, such as Cisco and Intel, confirm that mesh technology is the next logical extension of current wireless communications conformity. The use of meshes can help organizations to quickly set up new wireless networks or extend existing WLANs without the need for wireline base stations, as they can choose the optimal path for data transmission. In addition, industrial users can quickly set up networks of sensors and controllers with embedded wireless meshes for industrial and transportation management.

3. Features?

1. Reliability is greatly enhanced

The mesh topology used by the wireless mesh network avoids the point-to-multipoint star structure,? Such as ?8?0?2?... ?1?1?WL?A?N and cellular networks, etc. due to the centralized control method of service aggregation, ? center network congestion as well as interference,? single point of failure,? thereby greatly enhancing its reliability.

2. Conflict protection mechanism

Wireless mesh network can generate collision link identification at the same time optional links and its own links between the angle of obtuse angle, mitigating the interference between the links.

3. Simplify link design

Wireless grids usually require shorter wireless link lengths, which reduces the cost of antennas, and on the other hand, reduces the transmit power, which also reduces interference between RF signals of different systems and the system's self-interference, and ultimately simplifies the design of the wireless link.

4. Increased network coverage

End-users can access the network or connect to other nodes from anywhere. Compared with traditional networks, the range of access points is greatly enhanced, and the spectrum utilization is improved, increasing the capacity of the system.

5. Flexible networking, easy maintenance

Because of the wireless mesh network's own networking characteristics, as long as the need to add a small number of wireless devices, that is, with the existing facilities to form a wireless broadband access network. The routing characteristics of the wireless mesh network so that link interruptions or local expansion and upgrading does not affect the operation of the entire network, thus improving the flexibility and feasibility of the network, and the traditional network compared to the more powerful, more complete.

6. Low investment cost

Wireless grid network initial construction cost is low. Wireless mesh network has the advantages of scalability, easy expansion, automatic configuration and wide range of applications

Wireless mesh network hybrid networking

Fourth, the key technology of WMN

1. Orthogonal Division Multiple Access (QDMA) technology

QDMA technology is designed specifically for the optimization of wide-area communications and mobile mesh network systems. It originated in the military to provide a reliable means of communication in special environments or emergency situations.QDMA technology uses Direct Sequence Spread Spectrum (DSSS) modulation and operates in the ISM band at 2.4 GHz. Because it uses a multi-channel approach (three data channels and one control channel) at the MAC sublayer, it is more suitable for high-density WMN terminal equipment than a single channel.QDMA technology provides a high-performance RF front-end that incorporates functionality similar to that of a multi-tap Rake receiver (typically used in cellular networks) and a fairness algorithm to overcome the rapid changes in the RF environment.

QDMA provides strong error correction over a wide range of mobile communications, while enhanced interference immunity and signal sensitivity enable QDMA-based networks to provide mobile speeds of up to 250mph, compared to 20mph for IEEE802.11 protocols in real-world multiple-access environments, and QDMA data transmission is currently available over a range of up to 1600m, compared to 802.11 protocols. Currently, QDMA has a data transmission range of 1600m, while 802.11b only has a range of 20-50m. In addition to the range and speed of communication, QDMA is unique in that it has built-in localization technology that can accurately locate communication devices without relying on the Global Positioning System (GPS), with an error of no more than 10m?

2. Hidden Terminal Problem Handling Technology

Since WMN adopts wireless transmission medium, it inevitably exists hidden terminal and exposed terminal problems just like other wireless transmission networks. Due to the specificity of the wireless medium, both hidden terminal problems may occur, and both will lead to signal collision. Currently, it can be avoided by the RTS/CTS protocol (Request to Send/Clearance to Send Protocol) in IEEE802.11, but it can't completely solve the hidden terminal and exposed terminal problems. Although the probability and time of conflicts in the hidden terminal problem can be reduced by the handshake mechanism, there are still conflicts of control messages between nodes and the exposed terminal problem cannot be solved. In fact, WMN can be regarded as a simplified Ad Hoc network, so the hidden terminal and exposed terminal problems can be solved according to some existing mature schemes in Ad Hoc networks .

3. Routing Technology

The multi-hop wireless network of WMN is characterized by dynamic topology, so there are a lot of requirements for its routing protocols.

The routing protocols of WMN can refer to the existing routing protocols of Ad Hoc networks, which can be broadly classified into Proactive, Reactive and Reactive routing protocols. Reactive) routing protocols and hybrid routing protocols. Several typical routing algorithms are DSDV (Destination Sequential Distance Vector Routing Protocol), DSR (Dynamic Source Routing Protocol), TORA (Temporary Ordered Routing Algorithm) and AODV (Ad Hoc On-Demand Distance Vector Routing Protocol). Recently, Microsoft has proposed a routing protocol MR-LQSR for multi-radio transceiver, multi-hop wireless networks, the main idea is to use the maximum throughput criterion based on the DSR protocol, which has begun to take into account the characteristics of WMNs .

4. Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing (OFDM) can be used in the physical layer of a WMN.OFDM is a technique in which high-speed data streams are assigned to a number of orthogonal sub-channels with relatively low transmission rates through serial/parallel transformations, and narrow-band modulation and transmission are carried out on each of the sub-channels, which reduces the interactions of the sub-channels. interference between the sub-channels is reduced. The signal bandwidth on each sub-channel is less than the correlation bandwidth of the channel, so that the frequency selective fading on each sub-channel is flat, greatly eliminating inter-symbol interference. The digital information modulation used is Time Differential Phase Shift Keying (TDPSK) and Frequency Differential Phase Shift Keying (FDPSK), and the digital information modulation and demodulation functions are implemented with Fast Fourier Transform (IFFT and FFT) algorithms. Due to the frequency selectivity of the wireless channel, all the sub-channels are not in deep fading at the same time, so the system performance can be enhanced by utilizing sub-channels with high signal-to-noise ratio by means of dynamic bit allocation as well as dynamic sub-channel allocation. Since narrowband interference can only affect a small fraction of the subcarriers, OFDM systems are somehow resistant to this interference.OFDM combines diversity, space-time coding, interference and inter-channel interference suppression, and smart antenna techniques to maximize the system performance so that the WMN performance can be further optimized.