Two methods to solve the broken chain problem in AODV protocol
2.1 Alternate routing method Since the conventional routing protocol maintains a complete routing table and can know the topology in the network, it is easy to find the alternate route when the routing table entry fails. However, due to the limitation of network characteristics, it is unrealistic to maintain the complete routing table in the ad hoc network. Each node in the existing AODV needs to maintain only one route to the destination address. A compromise solution to the problem of routing broken links is to maintain multiple routes to the destination node, typically AODV-BR [5], which uses RREP information sent to other nodes by neighboring nodes that are heard by the broadcast channel in wireless communication. Establish a backup route. When it detects a broken link in the original route, it sends an application request to establish a backup route, which effectively uses the bandwidth of the wireless channel, so that the broadcast RREP can be used to establish a backup route, and the backup route is near the original route. , There will not be great changes. Establishing multiple routes to be used as a backup route in the event of a broken link can reduce the operations after a broken link, reduce delay, and reduce packet loss, but it can have a great negative impact on the performance of the network at the cost of increasing the overhead of each node. . Taking AODV-BR as an example, the simulation results of the researchers show that the network throughput and end-to-end delay have been improved to a certain extent, but AODV-BR consumes a lot of resources on the node itself. Every node in the network, regardless of whether it is the destination node or not, must intentionally listen to the RREP in the network, which will consume a lot of processing power. Moreover, not only the nodes around the routing link run this kind of listening, all the nodes running the protocol in the network will continue to listen, so this kind of protocol is not suitable for many occasions because it consumes too much for the mobile terminal itself. Therefore, the establishment of multiple backup links is not necessarily suitable for ad hoc networks. First of all, this method has been similar to the active routing protocol, borrowing some of its mechanisms to maintain network connections. Although it has advantages, such as rapid recovery of broken links, etc., it has a great negative impact on the ad hoc network because the self-organization network Most of the mobile terminals have limited processing power. Secondly, ad hoc network interruption chains often occur due to the rapid topology changes, and the rapidly dynamic topology changes can easily cause backup routes to fail. In fact, many of the backup links that are set up at a great cost are actually wasted. As with on-demand routing, it is more effective and practical to only find available routes in real time when a link break occurs and restore the connection. Therefore, the current solution to the cost of the protocol, including network cost and terminal cost, is the most important issue in the design of an ad hoc network routing protocol.
2.2 Local repair method The traditional AODV uses the method of source node recovery when a route is found to be broken, that is, when RERR is transmitted back to the source node to inform that the route is broken, the source node re-discovers the route. Although this method is reliable but has a large delay, the latest AODV mentions the idea of ​​local repair. Because the node that caused the disconnection may still be nearby, the upstream node at the disconnection can use RREQ with a relatively small time to live (TTL) Broadcast to fix the route. However, the efficiency of local repair is limited, especially the mobility of the nodes in the network has a significant impact. The results of simulation using OPNET software are shown in Figures 1 and 2. In the simulation, it is assumed that 20 nodes move within the range of 2 500 m × 600 m, the node movement speed is set between 0 ~ 20 m / s, and the node movement direction is 0 Randomly select ~ 360 °, re-select the movement direction randomly when reaching the boundary or after each movement for a certain time, the simulation time is 10 min.
The simulation results in Figure 1 show that the data delay after using local repair is smaller than the source re-initiation method when the node mobility is small, but it increases as the mobility increases and gradually becomes worse than the latter. Because when the source is re-initiated, the data packet is discarded after the disconnection; after the local repair is used, the data packet will be stored in the node's buffer when the disconnection occurs, and the transmission will continue after the link is repaired. After comparison, using local repair can obtain a lower data packet loss rate.
The above results can also be further explained by the simulation results for transmission efficiency in FIG. 2. When the link is broken when the node mobility is not high, using local repair can reduce the delay and improve the efficiency of data transmission. If the source re-initiation method is used, the node will broadcast RERR, discard the datagram, and request the source to re-establish the route, reducing the transmission efficiency. When the mobility of the nodes in the network increases to a very high level, the rapid and large-scale changes in the network topology will cause the local repair to fail, but will reduce the transmission efficiency of the data packet. Using the source re-initiation method, the transmission efficiency is basically keep it steady.
Figure 1 Mobility and average data transmission delay Figure 2 Mobility and transmission efficiency
Improvement of AODV Routing Protocol Break Chain Repair in Ad Hoc Network
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