With the continuous development of wireless communication technology, a new type of wireless network, Mobile Ad Hoc Network (Mobile Ad Hoc Network, MANET) has become a research hotspot. Mobile Ad Hoc network is a multi-hop, temporary autonomous system formed by a group of mobile nodes. Due to the distribution, dynamics, autonomy, ease of construction and mobility of Ad Hoc networks, wireless mobile Ad Hoc networks can be widely used in the military field, natural disaster emergency treatment, scientific investigation, exploration, emergency communications and other fields. However, Ad Hoc networks have their special limitations, such as limited bandwidth, highly dynamic network topology, link interference, limited link range, and broadcast characteristics. This makes the establishment of reliable, fast and efficient routing protocols one of the hotspots in current research.
1 MANET routing protocol
The routing protocol of MANET is roughly divided into ReacTIve routing protocol and ProacTIve routing protocol.
1.1 Reactive routing protocol
Reactive routing protocol, also known as on-demand routing protocol, is a routing algorithm that searches for routes only when data needs to be sent. In this routing protocol, the node does not need to maintain timely and accurate routing information. When sending a message to the destination node, the source node initiates the route search process in the network to find the corresponding route.
The currently widely used reactive routing protocols are DSR (Dynamic Source RouTIng) and AODV (Ad Hoc On Demand Distance Vector). The DSR protocol uses source routing, which mainly includes route discovery and route maintenance. When a node sends data, it first checks whether there is an unexpired route to the destination node in the cache, and it can be used directly, otherwise it uses flooding to realize the route discovery process.
AODV is an algorithm based on distance vectors, and differs from DSR in that AODV only maintains the required routes. It uses the destination sequence number to avoid invalid paths and does not use the periodic update method. When a node has routing requirements, the node generates a REEQ and broadcasts it to the temporary node until the destination node receives it, and then the destination node sends back the RREP signal until the source node. In the maintenance process, periodically broadcast the HELLOW signal to indicate the existence of a node.
In addition to this, there are many reactive routing protocols, such as: Temporary Sequential Routing Protocol (TORA), Sequential Routing Protocol (SSR), etc.
1.2 A priori routing protocol
A priori routing protocols are also called table-driven routing protocols. In these protocols, each node maintains a routing table that contains routing information to other nodes. When a network topology change is detected, the node sends updates in the network Message, the node receiving the update message updates its routing table. Once the source node wants to send a message, it can establish an immediate route to the destination node. The current mainstream table-driven routing protocols are DSDV (DesTImation Sequenced Distance Vector) and WRP (Wireless Routing Protocol). DSDV is an improvement to the Bellman Ford routing algorithm, adding a routing loop avoidance mechanism, each node saves a routing table, the routing table contains all nodes and their distance information, through broadcast to maintain network node connectivity, while Use the destination node serial number to distinguish the old and new routes. The WRP routing algorithm is also an improvement of the path discovery algorithm PFA. It uses the path length to the node and the information of the corresponding path to the penultimate node to accelerate the convergence rate of the routing protocol, thereby achieving improved routing loop problems.
2 OPNET simulation platform
At present, among the numerous dedicated network simulation software, there are commercial software developed by software companies, as well as scientific research software developed by universities and research institutes.
OPNET is an excellent network simulation and modeling tool that supports object-oriented modeling and provides a graphical editing interface for users to use. Its powerful functions and comprehensiveness can simulate almost any network device and support various network technologies. In addition to being able to simulate fixed communication models, OPNET's wireless modeler can also be used to establish packet wireless network and satellite communication network models. In addition, a well-functioning result analyzer provides an effective and intuitive tool for network performance analysis. OPNET's Molder is a software specifically designed for visual prototyping. Its use not only facilitates the establishment of network models, but also reduces the workload of programming. Molder provides a variety of editors to help users complete network modeling and simulation operations. It includes a Network Editor, a Node Editor, and a Process Editor.
3 Network simulation
Here, model, simulate and analyze the classic routing protocols AODV and DSR in the MANET network.
3.1 Network model
3.1.1 Protocol performance evaluation indicators
(1) Routing Find Time (Routing Find Time): The convergence speed of routing changes is a key factor to measure conventional routing protocols, but for Ad Hoe networks, routing protocols do not converge, so route discovery time is used as a performance test indicator .
(2) End-to-end average delay (Delay): This parameter refers to the time between the source node sending data to the destination node, including route discovery, queue queuing, data sending, and propagation. It reflects whether the network is unobstructed, the smaller the delay, the more unobstructed the network, and its unit is s.
(3) Packet Delivery Fraction (Packet Delivery Fraction): This parameter counts the ratio of packets delivered to the destination node and CBR packets generated by the source node in%.
3.1.2 Network model building process
The mobile Ad Hoc network model consists of 10 mobile nodes, in order 0, 1, 2, ..., 10, randomly distributed in the campus network environment within the area of ​​1 000 m × 1 000 m, and the physical wireless communication OPNET passes through the pipeline (Pipeline) to simulate, it includes 14 pipeline stages:
(1) Receiver group;
(2) Transmission delay;
(3) Link blocking;
(4) Channel matching;
(5) Transmitter antenna gain;
(6) Propagation delay;
(7) Receiver antenna gain;
(8) Receiver power;
(9) Background noise;
(10) Interference noise;
(11) Signal to noise ratio;
(12) Bit error rate;
(13) Error distribution;
(14) Error correction.
The movement of the node is defined by the trajectory item. Here, each movement time is selected to be 5 m / s, which belongs to medium-speed movement. Among them: source node service flow is CBR (Continuous Bit-Rate); packet interval is 4 packet / s; packet size is 512 B; simulation start time is 5 s; simulation time is 500 s; node transmission power is 0.005 W; channel The bandwidth is 2 000 kHz.
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