Design Of MAC Protocal For Terminal Node In Opportunistic Network

With the popularization of short-range wireless communication device in recent years, wireless ad hoc network has developed rapidly and has showed a great prospect. However, due to the influence of the mobility of node, the sparsity of node and other external environmental factors, it is possible that no complete connected path exists between the source and destination node, as a result, the opportunistic network was proposed since the traditional wireless ad hoc network cannot work effectively in this disconnected scenario. Opportunistic network, which is a self-organizing, multi-hop, delay tolerance and disconnected wireless network, makes use of the encounter opportunity due to the mobility of node and transmits data in store-carry-forward mode. This kind of network has great value when applied in military battle field, public transport, underground construction and other environments where there is no infrastructure or the infrastructure is highly vulnerable.As an important part of the opportunistic network, opportunistic terminal node is a key factor related to the effectiveness of upper layer applications, depending on whether it can utilize the wireless channel resource properly or not. As a result, this thesis focuses on the design of an access mechanism in the Media Access Control (MAC) layer, which could be suitable for terminal nodes in the wireless opportunistic networks, so as to exploit the full advantage of the opportunistic network, as well as effectively applied to the frequent broadcast signal and improve network performance and spatial multiplexing gain. According to IEEE 802.11 access standards, this thesis proposes an opportunistic network MAC layer access mechanism based on layered hybrid coordination and the corresponding improved strategy, which are aimed for underground construction, and both theoretical and simulation results are provided to verify the practicability and the feasibility of the proposed MAC protocol.The main research work is as follows:Firstly, we investigate on the access mechanism of the terminal nodes in opportunity networks. Based on IEEE 802.11 access standards, we discuss key issues for MAC protocol design in the opportunistic network, such as the own characteristics and structural characteristics of the network, which sets clear objectives for us and also provides a theoretical basis for this thesis.Based on comprehensive consideration on the advantages and disadvantages of the three existing MAC protocols, namely, based on competition, based on scheduling and based on QoS, we propose a layered hybrid coordination access (LHCA) protocol for opportunistic network in underground scenarios. Layered architecture meets the scalability requirement of the opportunistic network, hybrid coordination mechanism could distinguish from polling cycle and competition cycle effectively, which ensures the two-stage position of the nodes, it also allows the burst traffic or high priority service to access to the competition cycle with control and ensures the QoS requirements. In addition, we set up a mathematical model for the hierarchical polling mechanism of LHCA using the system probability generating function and Markov chain. Theoretical calculations indicate that the hierarchical polling has better performance than general polling mechanisms, which guarantees the reasonability of our strategy from mathematical perspective.We further make improvements on LHCA in terms of QoS as well as competitive backoff mechanism and propose an enhanced LHCA protocol-LHCA/ECA. In the aspect of QoS, the backoff window size in the LHCA/ECA can be changed dynamically according to the traffic volume of the node, which reduces the waste of channel resource. In the aspect of backoff mechanism, LHCA/ECA combines both fixed and random backoff mechanism, therefore, reducing the collision probability and avoid the "avalanche" phenomenon.Finally, we build the system simulation model of opportunistic network based on the hierarchical hybrid coordination access mechanism on OPNET simulation platform. This model is implemented from three aspects, namely, network domain, node domain and process domain. We compare the system performance under CSMA/CA, LHCA and LHCA/ECA protocols to verify the feasibility of the proposed scheme.