Multi-hop Ad Hoc Networks, Voice Communications Technology

Mutihop Ad Hoc network, which dose not rely on fixed basic facilities and has no central control node, is a kind of distributed wireless network with self-organized capability. So it is applied to military communications, emergency rescue and the environment which can't get the support of the cable or need temporary communications in order to handle emergencies. The expansion of voice communication, which combines image, voice and data would save cost and bandwidth, improve utilization of spectrum and be more suitable to practical application. However, voice transmission is very sensitive to delay and jitter. Especially Ad Hoc network has the characteristic of distributed network, channel sharing, self-organized, dynamic topology and so on, so the existing protocols can't provide good quality assurance for voice transmission. Therefore, how to guarantee the quality of service of voice transmission in Ad Hoc network becomes one of the key issues.This paper mainly studies MAC protocol of multihop Ad Hoc network which was adapted to voice transmission and proposed the merging algorithm and adaptive backoff algorithm which were based on IEEE 802.11 DCF protocol. The adaptive backoff algorithm achieved the transporting priority of different nodes.DCF adopts CSMA/CA protocol. Because of the fairness of CSMA/CA media access mechanism, even if the bottleneck node has more voice packets in sending buffer, it needs to contend the channel. So the congestion of the bottleneck nodes would increase loss rate and jitter of voice packets. Using the features of the broadcasting of the wireless channel and the short length of voice packets, the merging algorithm merged several voice frames together to transport. The neighbor nodes filtered the frame after receiving the whole frame. This algorithm let the bottleneck nodes send more voice packets in each opportunity of getting the channel. It decreased delay caused by channel contending and the spending of frame header. Through the implementation of piggybacking method, the adaptive backoff mechanism let a relay node which had stored more voice packets have higher priority to send frames. It can reduce the probability of frame collision and the problem of packet loss and delay caused by the overflowing of the sending buffer in relay nodes.First, this paper introduces the characteristic of Ad Hoc network which is different from the traditional network and is not fit to voice transporting, the effect of the congestion caused by bottleneck nodes to the performance of voice and the related work of resolving the network congestion. Then it analyzes the main factors which affects the performance of voice transmission and the current research developments of VoIP QoS. Finally, to address the effect of the bottleneck nodes, this paper presented merging algorithm and adaptive backoff algorithm and analyzed them with theory and simulation separately. The simulation results showed that the algorithms could not only decrease delay and jitter of voice packets, but also increase network throughput.