TDMAとDCFの組み合わせによるアドホックネットワーク上でのQoS通信の実現方式

　An ad hoc network does not rely on the fixed network infrastructure; it uses a distributed network management method. With the popularity of the smart devices, ad hoc network has received more and more attention, supporting QoS in ad hoc network has become inevitable. Many researches have been done for provision of QoS in ad hoc networks. These researches can be divided into three types.　The first type is contention-based approach which is the most widely used. IEEE 802.11e MAC (media access control) protocol belongs to this type which is an extension of IEEE 802.11 DCF(Distributed Coordination Function). It specifies a procedure to guarantee QoS by providing more transmission opportunities for high priority data. However, since IEEE 802.11eis designed based on the premise that access points are used, when the number of QoS flows increases, packet collisions could occur in multi-hop ad hoc network.　The second type is using TDMA-based approach. The TDMA approach can provide contention-free access for QoS traffics through the appropriate time slot reservation. The current TDMA approaches reserve time slots for both QoS traffics and best-effort traffics. However, it is difficult for TDMA as the only approach to allocating channel access time for best-effort traffics sincet he required bandwidth of the best-effort traffics changes frequently.　We propose a QoS scheme, which takes advantage of both contention-based approach and TDMA-based approach. In the proposed scheme, contention-based approach DCF provides easy and fair channel time for best-effort traffics, and TDMA approach serves the QoS traffics. A time frame structure is designed to manage the bandwidth allocation. A time frame is divided into two periods, specifically the TDMA periods and the DCF periods. The proportion of two periods is decided by QoS traffics. Therefore the QoS traffics are given absolutely higher priority than best-effort traffics. In order to guarantee the transmission of each QoS packet in TDMA period, a time slot assignment algorithm based on QoS data rate has been proposed. The proposed scheme also employs an admission control scheme, which rejects the new QoS user when the channel capacity is reached. In addition, we provide the configuration of the proposed scheme in the mobile environment. The procedures are designed for route changes and new-adding users. 　The proposed scheme is simulated in the QualNet simulator. In the static environment, the performance of the proposed scheme is evaluated in the case of a gradual increase in the number TCP flows and in the case of gradual increase in QoS data rate. Simulation results show that in the static environment the proposed scheme can not only provide effective QoS performance, but also can provide good support for best-effort flows. In the mobile environment, we simulated the performance of the proposed scheme at different moving speed (maximum is 5 Km/h) when the ARF (Auto Rate Fallback) is available. From the simulation results, in a specific mobile environment, the proposed scheme can support the QoS transmission well.電気通信大学201

Design and optimization of QoS-based medium access control protocols for next-generation wireless LANs

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.In recent years, there have been tremendous advances in wireless & mobile communications, including wireless radio techniques, networking protocols, and mobile devices. It is expected that different broadband wireless access technologies, e.g., WiFi (IEEE 802.11) and WiMAX (IEEE 802.16) will coexist in the future. In the meantime, multimedia applications have experienced an explosive growth with increasing user demands. Nowadays, people expect to receive high-speed video, audio, voice and web services even when being mobile. The key question that needs to be answered, then, is how do we ensure that users always have the "best" network performance with the "lowest" costs in such complicated situations? The latest IEEE 802.11n standards attains rates of more than 100 Mbps by introducing innovative enhancements at the PHY and MAC layer, e.g. MIMO and Frame Aggregation, respectively. However, in this thesis we demonstrate that frame aggregation's performance adheres due to the EDCA scheduler's priority mechanism and consequently resulting in the network's poor overall performance. Short waiting times for high priority flows into the aggregation queue resolves to poor channel utilization. A Delayed Channel Access algorithm was designed to intentionally postpone the channel access procedure so that the number of packets in a formed frame can be increased and so will the network's overall performance. However, in some cases, the DCA algorithm has a negative impact on the applications that utilize the TCP protocol, especially the when small TCP window sizes are engaged. So, the TCP process starts to refrain from sending data due to delayed acknowledgements and the overall throughput drops. In this thesis, we address the above issues by firstly demonstrating the potential performance benefits of frame aggregation over the next generation wireless networks. The efficiency and behaviour of frame aggregation within a single queue, are mathematically analysed with the aid of a M=G[a;b]=1=K model. Results show that a trade-off choice has to be taken into account over minimizing the waiting time or maximizing utilization. We also point out that there isn't an optimum batch collection rule which can be assumed as generally valid but individual cases have to be considered separately. Secondly, we demonstrate through extensive simulations that by introducing a method, the DCA algorithm, which dynamically determines and adapts batch collections based upon the traffic's characteristics, QoS requirements and server's maximum capacity, also improves e ciency. Thirdly, it is important to understand the behaviour of the TCP ows over the WLAN and the influence that DCA has over the degrading performance of the TCP protocol. We investigate the cause of the problem and provide the foundations of designing and implementing possible solutions. Fourthly, we introduce two innovative proposals, one amendment and one extension to the original DCA algorithm, called Adaptive DCA and Selective DCA, respectively. Both solutions have been implemented in OPNET and extensive simulation runs over a wide set of scenarios show their effectiveness over the network's overall performance, each in its own way.This study was supported by the Engineering and Physical Sciences Research Council (EPSRC)

VoIP Call Admission Control in WLANs in Presence of Elastic Traffic

VoIP service over WLAN networks is a promising alternative to provide mobile voice communications. However, several performance problems appear due to i) heavy protocol overheads, ii) unfairness and asymmetry between the uplink and downlink flows, and iii) the coexistence with other traffic flows. This paper addresses the performance of VoIP communications with simultaneous presence of bidirectional TCP traffic, and shows how the presence of elastic flows drastically reduces the capacity of the system. To solve this limitation a simple solution is proposed using an adaptive Admission and Rate Control algorithm which tunes the BEB (Binary Exponential Backoff) parameters. Analytical results are obtained by using an IEEE 802.11e user centric queuing model based on a bulk service M=G[1;B]=1=K queue, which is able to capture the main dynamics of the EDCA-based traffic differentiation parameters (AIFS, BEB and TXOP). The results show that the improvement achieved by our scheme on the overall VoIP performance is significant

Adaptive delayed channel access for IEEE 802.11n WLANs

Abstract— In this paper we investigate potential benefits that an adaptive delayed channel access algorithm can attain for the next-generation wireless LANs, the IEEE 802.11n. We show that the performance of frame aggregation introduced by the 802.11n adheres due to the priority mechanism of the legacy 802.11e EDCA scheduler, resulting in a poor overall performance. Because high priority flows have low channel utilization, the low priority flows throughputs can be amerced further. By introducing an additional delay at the MAC layer, before the channel access scheduling, it will retain aggregate sizes at higher numbers and consequently a better channel utilization. Also, in order to support both UDP and TCP transport layer protocols, the algorithm’s operational conditions are kept adaptive. The simulation results demonstrate that our proposed adaptive delayed channel access outperforms significantly the current 802.11n specification and non-adaptive delayed channel access

IEEE 802.11n MAC frame aggregation mechanisms for next-generation high-throughput WLANs [Medium access control protocols for wireless LANs]

IEEE 802.11n is an ongoing next-generation wireless LAN standard that supports a very highspeed connection with more than 100 Mb/s data throughput measured at the medium access control layer. This article investigates the key MAC enhancements that help 802.11n achieve high throughput and high efficiency. A detailed description is given for various frame aggregation mechanisms proposed in the latest 802.11n draft standard. Our simulation results confirm that A-MSDU, A-MPDU, and a combination of these methods improve extensively the channel efficiency and data throughput. We analyze the performance of each frame aggregation scheme in distinct scenarios, and we conclude that overall, the two-level aggregation is the most efficacious