| Because of the development of sensor, embed and wireless telecommunication, the wireless sensor network is more and more suitable to the practical use. And the application perspective of wireless sensor network is much brighter, such as daily work, industry manufacture, scientific research and education. In 2003, in the top ten technologies of influencing future human life most which are listed by Technology Review, wireless sensor network was in the first place. As the technology is more and more mature, the network formed by sensors gains more attention from people.But because of the features of wireless sensor network, it still has some flaws in the security consideration. So this paper designs and implements a wireless sensor network protocol which has the ability to change communication channel dynamic. It has some advantages: it is easy to implement, it doesn't need more devices to improve the security and it can also ease the communication pressure.To implement this goal, in this paper, my work includes designing and implementing a TDMA protocol, researching common random arithmetic and finding a two suitable to wireless sensor network and eventually designing and implementing wireless sensor network protocol which has the ability to change communication channel dynamic.1. The wireless sensor network includes three types of devices: the Base Station, routers and terminal. All types of devices have similar working states. There are three working states: off-line, on-line and transition. When the nodes'power is on, the state should be transition, and the state will change into off-line. The enter-network steps start. If that works, the state should be on-line. The main steps in enter-network are broadcasting, selecting parent node, sending route message, time synchronization, network layer registration, network command configuration and state transition. And the on-line work flow is: probe parent node, time synchronization, network command configuration, network management and upload freeze data.2. This paper has researched three methods to generate random number in protocol. Linearity fitting is an easy way to implement and has little requirement in hardware. And the wireless sensor network platform support AES encryption by hardware. So it also can be implemented easily. And BBS generator has many things to compute, but the arithmetic is not complicated. So it can be implemented on the hardware of wireless sensor network. And this paper finally chooses two methods. One is linearity fitting, because when the parameter is set properly, the sequence generated is similar to the true random sequence. But when the seed X 0is decided, the following numbers are not random anymore. Persons can use the already know numbers to calculate the following number. So when the application does not need very high level security. We can use this method to implement. Another is BBS generator. BBS generator has the highest security level ever known. And the sequence that generated can be used in applications need more security. To hack BBS generator means to do the big integer factor resolve. And if the integer is big enough, it is an impossible mission. To improve the security, we also have to pay the cost of doing more computing. So there are some limitation of using BBS, it can only used for encryption, because the running speed of the arithmetic is very slow.3. Then this paper designs a proposal which has the ability to change communication channel dynamic based on the linear fitting and BBS generator. And the working steps are as follow: splitting the data needed to send into several pieces. And create a data frame for every data segment. Then send the first data segment in the default channel. If the other node gives response, change the channel according to the scheduled channel choosing plan. And send the next frame. If it does not receive the response for a long time, then it will change to the default channel and send the message again. If the node receives the other node's sending data frame, it will send a response message back. And change the channel according to the scheduled channel choosing plan. And if it does receive response for the node for a long time, it will change back to default channel and wait. We can use the linear fitting as an example. The working flow is as follow: the node A and B all turn on the power. And the node A sends time request message to the node B. If the node B receives the time request message, then it send back time synchronization message 1 and 2. And both nodes use time synchronization to generate seed of random arithmetic. After node B getting seed, it will send frequency hopping message to node A in default channel. If node A receives node B's frequency hopping message, it will also send frequency hopping message to node B. After that, node A and B use linear fitting method to generate channel used in next time. Node B will send frequency hopping message to node A after changing communication channel. And it will repeat these steps until the communication stop.This paper improves the security of wireless sensor network in some areas. And it also relieves the congestion of the network. But the proposal in this paper still has many disadvantages. It has to be improved in the future work. So it can have more practical value. |
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