The wireless mesh network is a multi-point to multi-point wireless network developed by the Ad Hoc network. At present, the routing protocols of the wireless mesh network refer to the routing protocol of the Ad Hocl network to a large extent, among which the classic type The routing protocol directly applies the Ad Hoc routing protocol to the wireless mesh network environment. Introduce the classic wireless mesh routing protocol, and analyze the representative protocols among them.
1. Routing Protocol of Wireless Mesh Network
Traditional routing protocols are designed specifically for wired networks and are not suitable for wireless mesh networks. Because traditional routing protocols cannot handle the rapid changes in topology and link quality that are common in wireless mesh networks. Wireless mesh networks have some significant characteristics, such as: high dynamics, intelligence, end-to-end optimal path selection, multi-hop, usually limited bandwidth and insufficient computing power. There are two reasons for the high dynamic nature of wireless mesh networks. First, the router itself may move and cause rapid changes in the network topology. Second, even if the router itself does not move, the quality of the radio link may still change rapidly due to interference, geography, and environmental factors.
From the above characteristics, we can know that a complete wireless mesh routing protocol must have the following characteristics: ①distributed operation; ② fast convergence (suitable for faster movement); ③ scalability: ④ suitable for a large number of small devices; ⑤ Active operation with limited bandwidth and computing power (reduce initial delay): ⑥ Consider the quality and capacity of the radio link when selecting a route; ⑦ Avoid loops: ⑧ Security.
Because the wireless mesh network is a wireless network developed from the Ad Hoc network. There is a certain similarity between Ad Hoc network and wireless mesh network, so the existing mainstream wireless mesh network routing protocol is also developed from the AdHoc network routing protocol, mainly including three types of routing protocols : One is a priori routing protocol: one is a reactive routing protocol; the other is a mixture of the two, called a hybrid routing protocol.
Second, a priori routing protocol
(1) Introduction
The priori routing protocol is a form-based routing protocol. In this protocol, each node maintains one or more tables that contain routing information to all other nodes in the network. When a change in the network topology is detected, the node sends routing update information in the network. The node that receives the update information updates its own table to maintain consistent, timely, and accurate routing information. The difference between different a priori routing protocols is the way topological update information is transmitted in the network and the type of tables that need to be stored. The a priori routing protocol continuously detects changes in network topology and link quality, and updates the routing table according to the changes, so the routing table can accurately reflect the network topology. Once the source node needs to send a message, it can immediately get the route to the destination node.
(2) Typical a priori routing protocol DSDV protocol
The basic principle of DSDV is that each node maintains a routing table to other nodes, and the content of the table is the "next hop" node of the route. The innovation of DSDV is to set a sequence number for each route. The route with the larger sequence number is the preferred route. When the sequence number is the same, the route with fewer hops is the preferred route. Under normal circumstances, the sequence number broadcast by the node is an even number that increases monotonically. When Node B finds that the route to Node D (the route sequence number is s) is interrupted, Node B broadcasts a routing message to inform the sequence number s + l, and set the hop count to infinity, so that the routing table of any node A that sends information through B includes an infinite distance. This process until A receives a valid route to D (routing sequence No. is s + 1-1).
In this solution, all mobile terminals in the network establish a routing table, including the number of hops (or path matrix identifying the distance vector) of all destination nodes to reach each destination node. Each routing record has a serial number set by the target node. The serial number enables the mobile terminal to distinguish between the currently valid routing path and the outdated routing path. The routing table periodically updates the entire network to maintain the communication effectiveness of the entire network. Generally, in order to reduce the transmission of a large amount of routing information due to routing table updates and reduce network routing overhead, two routing update methods can be used. The first is the full clearing method, that is, routing update information is transmitted in the entire network through multiple network protocol data units. If the terminals in the network move, the new routing packet information is sent to all terminals in the network from time to time. The second is the partial update method, or incremental update method, that is, after the last full clear transmission, only the routing information related to the change is transmitted for transmission, which is usually placed in a standard NPDU, thus Reduce the amount of routing information transmitted. In the incremental update mode, the mobile terminal can add another additional table to store routing update information.
The broadcast information of the new routing information includes the address of the target node, the number of hops to each target node, the serial number of the received information, and the unique broadcast serial number. The new routing information applies the latest serial number. If two updates have the same sequence number, the route with the smaller distance vector array has priority. Because it represents the shortest path (or the least number of hops). Under normal circumstances, there may be multiple paths from the source node to the destination node. In the process of determining the optimal routing path, the mobile terminal tracks the time of different routing paths, and the optimal routing path is the path with the shortest time. Before finding the best path, the time fluctuates convergently. Once the path is determined, this information is stored in the routing table of each terminal until the node receives new routing information. 3. Reactive protocol
The reactive routing protocol is a routing method that searches for a route only when a path from the source node to the destination node is needed for data transmission. Nodes do not save timely and accurate routing information for the entire network. When the source node wants to send a message to the destination node, the source node initiates a route lookup process in the network and finds the corresponding route before sending the message. In order to improve efficiency, the node can save the found route in the cache for subsequent transmission. The characteristics of reactive routing protocol on-demand routing can better adapt to the wireless network environment where nodes move more frequently. After the node moves, it only needs to update the routing information of the relevant path that needs to send data.
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