Research Of MAC Protocols In Wireless Ad Hoc Networks

In recent years, with the in-depth research of related technologies and the improvement in hardware, there are more and more deployed wireless ad hoc networks in practice. A wireless ad hoc network is a network temporarily and spontaneously established by wireless communication nodes through distributed protocols, with the characteristics of infrastructureless, multihop, distributed and mobility. As one important part of the wireless ad hoc network protocol suite, the media access control (MAC) protocol determines how one wireless communication node transmits and receives packets through wireless channel. Thus, the MAC protocol largely determines the network performance which can be measured in terms of throughput, transmission delay and energy consumption, etc.Focus on how to improve the wireless ad hoc MAC protocol, this thesis first deeply analyses the essential factors which determine the performance of a MAC protocol. Then, this dissertation proposes and evaluates new solutions for concurrent transmission, intra-flow contention and inter-flow contention.The major contributions of this thesis include:1. Research on concurrent transmission mechanism. In a given area of the wireless ad hoc network, more scheduled transmissions means higher throughout. So, without violating each other, it is a promising approach to improve the throughput of a wireless ad hoc network MAC protocol by scheduling as more as possible transmissions in a given space. In wireless communication, a packet can be received successfully even if there exist other overlapping or interfering packets. This phenomenon is the so called capture effect. Based on the capture effect, this dissertation proposes a concurrent transmission MAC protocol (CTMAC), which can work on existing IEEE 802.11 hardware.CTMAC inserts additional control gap between the transmission of control packets (RTS/CTS/ATS) and DATA packet. After the successful exchange of control packets, a node waits until the finish of current additional control gap. Additionally, the control packets of CTMAC will not always silence the neighboring nodes. Thus, the additional control gap allows a series of RTS/CTS exchanges to take place between the nodes in the vicinity of the transmitting or receiving node to schedule possible multiple, concurrent data transmissions. To safeguard the concurrent data transmissions, collision avoidance information is included in the control packets and used by the neighboring nodes to determine whether they should begin their transmissions. Also, to isolate the possible interference between DATA packets and ACK packets, a novel ACK sequence mechanism is proposed. Because the size of additional control gap has a significant impact on the performance of the CTMAC, this thesis proposes a packet-level adaptive mechanism to adjust the length of additional control gap according to the number of concurrent transmissions in the vicinity. Simulation results show that a significant gain in throughput can be obtained by the CTMAC protocol compared with the existing work including the IEEE 802.11 MAC protocol.2. Research of intra-flow contention problem under multihop scenarios. Due to the characteristics of infrastructureless and multihop, most of the traffics in a wireless ad hoc network are multihop. To fulfill the forwarding of the received packets, the neighboring nodes on a multihop route will contend with each other inevitably. This is the so called intra-flow contention problem, which limits the performance of the MAC protocol. By analyzing the essential characteristics of a multihop traffic, this dissertation proposed a novel voluntary waiting MAC protocol (VWMAC) to solve this problem. Through voluntary waiting by wireless hosts according to the length of data packet transmitted and the distance between neighboring nodes on the multihop path, VWMAC uses a surprisingly simple strategy to achieve significant performance enhancement. This thesis gains the equation to compute the length of voluntary waiting time and validate its correctness through simulation. In addition, voluntary waiting of participating nodes present opportunity for these nodes to transit to sleep state to conserve scarce energy. Our simulation results show that VWMAC outperforms IEEE 802.11 and existing approaches in terms of throughput, transmission delay and energy efficiency.3. Research of mechanisms for dealing with the MAC layer contentions. At the MAC layer, a wireless communication node faces various kinds of contentions which include intra-flow contention and inter-flow contention. The MAC layer contentions and the resulting collisions have significant impact on the performance of a MAC protocol. Dealing with these problems quickly and actively is necessary for a desirable wireless ad hoc MAC protocol. In this thesis, a novel MAC protocol, dynamic waiting MAC (DWMAC), is proposed to manage the contending actions of a wireless node dynamically according to the contention level.In DWMAC, a node waits voluntarily after the successful transmission of a DATA packet. The duration of waiting is determined jointly by the contention level in the vicinity and the length of the contended transmissions. In the state of dynamic waiting, a node loses the right of transmitting, even if it has packet waiting in it's transmit queue. However, it can receive passively from neighboring nodes when RTS packet is received.The cross node which is on the routes of multiple multihop traffics needs more consideration, because it requires more chances to forward the received packets. In this dissertation, a marking mechanism is proposed to assign the cross node higher priority during the contention for the shared channel. Because the flows in the wireless ad hoc network have some intrinsic relationship, DWMAC uses an ARMA Filter method to utilize the history information and improve the performance further. Compared with existing solutions, DWMAC deals with the MAC layer contentions more effectively. The simulation results show that DWMAC protocol is able to adjust the action of a wireless communication node timely and actively, resulting significant improvements in throughput and packet delay.