Method for vehicle-mounted Ad Hoc network based on improved AODV (Ad Hoc Ondemand Distance Vector) protocol

[0030] The embodiments of the present invention are described in detail below with reference to the accompanying drawings: this embodiment is implemented on the premise of the method of the present invention, and detailed implementation modes and specific operation procedures are given, but the protection scope of the present invention is not limited to the following 的实施例。 Example.

[0031] The scene setting of this embodiment takes an on-board Ad Hoc network on a highway as an example.

[0032] Step 1: When the source node has data to send to the destination node, an RREQ message will be generated. The source node will add its own location, speed and direction, and route expiration time into the RREQ message and send it to neighboring nodes.

[0033] The source node adding information such as its own location, speed and direction, and route expiration time into the RREQ message refers to making certain modifications to the format of the RREQ message to meet the need for storing relevant information. The revised RREQ message adds five new information storage domains, which store the abscissa, ordinate, movement speed, the angle between the movement direction and the abscissa axis and the route expiration time of the node currently forwarding the RREQ message. The modified RREQ message is as figure 1 Shown.

[0034] When the source node generates the RREQ message, it first fills in the original information fields of the RREQ message, and the filled format and content are the same as the original AODV protocol. Then the source node calls the GPS module of the vehicle to obtain the position, speed and direction information of its own node and fill it in the new message field. The abscissa, ordinate, movement speed, angle between the movement direction and the abscissa axis of the current forwarding RREQ message node Area. The initial value of the route expiration time RET is set to 100s. After the source node generates the RREQ message in the above format, it broadcasts it to the neighboring node. The flow chart of the source node generating RREQ is as follows Figure 4 Shown.

[0035] Step 2: Each node that receives the RREQ will calculate the stability between the two nodes based on the node that sent the RREQ message and its own position, speed and direction information, and the node that does not meet the stability conditions will discard the RREQ message. Processing, the node meeting the stability condition will update the RREQ message and continue to forward it until the destination node is found.

[0036] The stated stability condition means that the distance between the neighboring node receiving the RREQ message and the node sending the RREQ is less than the threshold t×R or the distance is greater than the threshold t×R, but as the two nodes move, the distance becomes closer. R is the communication radius between nodes, and t is a preset parameter value. Preferably, in order to balance the number of nodes that directly forward RREQ messages and whether to calculate whether to forward RREQ messages, t=0.7. figure 2 Shown is the node communication area diagram. When the distance between neighboring nodes and node A is less than 0.7R, they can forward RREQ messages when they receive RREQ messages; when the distance between neighboring nodes and node A is greater than 0.7R, further steps are required Judge the change of the distance between two nodes. image 3 It is a schematic diagram of the movement of two nodes. With the movement of the two nodes, the condition for node B and node A to become closer is The coordinates are expressed as: Where v i And v j Is the horizontal and vertical components of the relative velocity of node B to node A, d i And d j Is the horizontal and vertical components of the relative position of node B to node A. So when the distance between two nodes is greater than 0.7R, it satisfies The neighboring node will forward the RREQ message, otherwise the RREQ message is discarded without processing.

[0037] When the intermediate node receives the RREQ message for the first time, it first determines whether the flag RREP_FLAG is 1. If RREP_FLAG is 1, it means that the node has received the reply message RREP, and then the node no longer processes the repeated RREQ message. If RREP_FLAG is not 1, add the information in the received RREQ message to the broadcast ID cache table, and search for whether there is a reverse route to the source node (reverse route is used for RREP messages to reach the source node), If it does not exist, the reverse route will be added to the local routing table. If the RREQ message is received repeatedly, the intermediate node will compare the RET in the RREQ message with the RET in the broadcast ID cache table old Value size, if RET> RET old , Indicating that the current reverse routing is more stable, the node updates the reverse routing table, and uses RET as the new RET old Otherwise, the repeated RREQ message is discarded without processing. Then the node judges whether it is the destination node. If it is the destination node, it will generate and reply to the RREP message; if it is not the destination node, it will forward the RREQ message to the neighboring node. After receiving the RREQ message, the neighboring node will perform the same operation until the destination node is found. The node's processing process for RREQ messages is as follows Figure 5 Shown.

[0038] Step 3: After the destination node receives the RREQ message for the first time, it caches the content of the RREQ message and starts a waiting timer. All RREQ messages received within the waiting timer time will be cached by the destination node.

[0039] The RREQ message received within the waiting timer time will be cached by the destination node means that the destination will not reply to the RREQ message immediately after receiving the RREQ message for the first time, but will start the waiting timer, and will wait The RREQ message received within the timer time is buffered for selecting a more stable route. The length of the waiting timer needs to be considered in a compromise between the number of received RREQ messages and the delay, and it is preferably set to 500 ms.

[0040] Step 4: When waiting for the timer to expire, the destination node will select a more stable route from multiple routes to reply to the RREP message, and finally the source node will receive this RREP message and use this more stable route for data transmission.

[0041] The relatively stable route refers to the route with the longest route expiration time.

[0042] The said route expiration time (Route Expiration Time, RET) refers to the time that the entire established route can exist stably and continuously. It is the minimum value of the expiration time of all links in the entire route, expressed as among them Two adjacent nodes n k-1 And n k The effective time maintained by the communication link is defined as the link period time (Link Expiration Time, LET). according to image 3 , Can be calculated

[0043] LET ab = - ( v i · d i + v j · d j ) + ( v i 2 + v j 2 ) · r 2 - ( v i · d j - d i · v j ) 2 v i 2 + v j 2 .

[0044] The larger the route expiration time value, the longer the stable existence time of the route and the more reliable the route. The destination node selects the route with the longest route expiration time to reply to the RREP message. After the destination node generates the RREP message, it sets the RREQ_FLAG in the broadcast ID cache table to 1, indicating that it has responded to the RREQ message, and then transmits the RREP to the next hop node according to the routing information in the reverse routing table, which generates the RREP message. The process is like Image 6 Shown.

[0045] Step 5: Each node that receives the RREP message updates the route to the destination node, and then unicasts the RREP message to the next hop node pointed to in the reverse routing table. The node processing RREP message is as follows Figure 7 Shown.

[0046] Step 6: The source node receives the RREP message and obtains the route to the destination node, and starts sending data.

[0047] In the route discovery process, through the first step of improvement, only nodes that meet the stability conditions can forward RREQ messages. On the one hand, nodes with relatively stable links are selected to prepare for the subsequent selection of stable routes; on the other hand, it is restricted The flooding of RREQ messages in the entire network reduces network overhead. In the routing selection process, the second step is taken to improve, providing a strategy for selecting a more stable route based on the route expiration time. The destination node replies to the RREP message according to the selected more stable route, and finally the source node uses it after receiving the RREP message. This more stable route performs data transmission. Figure 8 Shown is the overall flow chart of the improved protocol. Picture 9 The performance of the improved AODV protocol and the original AODV protocol is compared. The data packet delivery rate in the figure refers to the ratio of the number of data packets arriving at the destination node to the number of data packets sent from the source node. From Picture 9 It can be seen that the method based on the improved AODV protocol has a better data packet delivery rate, that is, it improves the data transmission success rate.