Research On Medium Access Control Problems For Control-Oriented Networks

The rapid development of information technology has incurred profound transformation of automations, and gradually leads to the open and interconnected communication networks in control areas. More and more networks of various types are integrated into system's control loop, bringing great opportunities and unprecedented challenges for control system's analysis and design. Because of introducing special nonlinearities, an important issue arising simultaneously in the process of networking control system is that the negative influence cast by networks on control systems must be minimized. For traditional control theory, it means new methods must be developed to analysis and design corresponding networked systems. At the other side, from the point of communication networks, applying appropriate communication protocols is also a very important issue that is helpful to solve the new network problems in essential and reduce complexity of the theoretic research.Today, Ethernet has become the most prospective wired control network technology that also may compensate shortcomings of Fieldbus. Its development prospects in the future are even more expected conformably by the enterprises and the researchers. However, one of the main obstacles still constricting its applications in control areas is that due to the lack of real-time performances, Ethernet doesn't meet such industrial requirements as transmission determinism, order and priority. Even applying Ethernet switch or constructing real time protocols in high stack layers does not sufficiently qualify by introducing new sustitute problems of shifted nondeterminism, increased inter-layer delivery delay, and complexity. Wireless control networks have advantages of supporting cable-freedom, mobility and low cost and could be applied in many special areas working as the effective complement of wired networks, such as wireless sensor networks. Due to the lack of constant power supply, transmit power control in wireless control networks has become one significant issue that is crucial to quality of wireless signal, lifetime of battery or wireless node, and network topology. Real-time Ethernet and wireless transmit power control are two distinct problems confronted in control networks, both of which are close related with medium access control protocol. Companying with the most recent advancements, we studied corresponding medium access control problems for control networks in the dissertation and achieve the following major contributions.Considering providing real time performances by the high layers may lead to increased intermediate transfer layers and processing delay for a message, we analysis the possibility of defining priority mechanism at data link layer, and propose a physical signaling method based on carrier's variable length character. Variant lengths of standard frame are used to realize multi-channel state feedback with the help of the intrinsic carrier sensing function of CSMA networks. Then global information of transmission's collision, success and priority is easily obtained at data link layer. Beside, priority management may also be handled to high layer protocols and made application oriented, where intermediate layers are treated as transparent. So the new method is more general in practices. In fact, the proposed mechanism makes great efforts to utilize the intrinsic function and character of CSMA network itself, which also makes it easy to implement.Considering the non-determinism problem of Ethernet, we propose a real time medium access control protocol compatible with Ethernet standard. Obtaining multi-channel state information using the variant frame length signaling method, the proposed scheme determines channel access right by introducing persistent frame contending and channel blocking in the event of collisions. By this means, it realizes prioritized transmission and promises that for the highest priority class message, delivery delay is deterministic when it satisfying certain pseudo periodic condition. When there generates no collision, the best-effort CSMA/CD is chosen as the channel access rule to make the message transmit as fast as possible. Comparing with existent methods, new protocol tries to conserve the virtue and character of standard Ethernet meanwhile. It is also completely compatible with Ethernet. The new real time Ethernet bus can be connected directly to current gateway without any protocol transfer equipment. Ordered transmit schedule is very common in control networks. Considering the non-strictness problem of such schedule caused by random delayed retransmission strategy in the virtual time protocol, we propose a timely collision resolution algorithm, BOVTA, carrying also on virtual time axis. Not the same as its counterparts, BOVTA utilizes a principle consistent to the virtual time channel access rule to perform resolution synchronization and thus avoids introducing extra station dependent information. It makes the collision resolution more simple and easy to perform, meanwhile keeping the channel overhead relatively low. We then propose a multi-priority virtual time Ethernet protocol based on BOVTA algorithm, which promises strictness of the ordered transmit schedule.Considering power control problems for wireless control networks, we first point out an over-protect drawback of PCM which cause extra energy efficiency price, then propose an adaptive power control scheme named IP-PCM. Without violating the minimized power principle for data transmission, transmit power of busy tone is adaptively controlled according to an interference balancing mechanism. As the result, the carrier sensing range is optimized independent of the interference range. Comparing with PCM, IP-PCM apparently improved the spatial reuse and finally increased the energy efficiency.Utilizing the famous OPNET modeling and simulation tool for communication networks, we describe the modeling principle in detail for the studied MAC protocols based on discrete-event-driven finite state machine (FSM). It provides a basis for further theoretic analysis and design of these solutions.