Ken Tang

MAC reliable broadcast in ad hoc networks

Traditional wireless ad hoc medium access control (MAC) protocols often utilize control frames such as request-to-send (RTS), clear-to-send (CTS) and acknowledgement (ACK) to reliably deliver unicast data. However, little effort has been given to improve the reliable delivery of broadcast data. Often, broadcast data are transmitted blindly without any consideration of hidden terminals. In this paper, we proposed a novel MAC protocol, broadcast medium window (BMW) that reliably delivers broadcast data.

TCP performance in wireless multi-hop networks

We investigate the interaction between TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between TCP data packets and TCP ACKs, which causes performance to degrade for window sizes greater than 1 packet. The second is the interaction between TCP and MAC layer backoff timers which causes severe unfairness and capture conditions. We show that these effects are particularly pronounced in two families of MAC protocols that have been extensively used in ad-hoc network simulation and implementations, namely CSMA and FAMA (a descendent of MACA). We then demonstrate that these problems are in part overcome by using MACAW, a MAC layer protocol which extends MACA by adding link level ACKs and a less aggressive backoff policy. We argue that link level protection, backoff policy and selective queue scheduling are critical elements for efficient and fair operation of ad-hoc networks under TCP. These conclusions are supported by extensive simulation and measurement experiments.

MAC layer broadcast support in 802.11 wireless networks

In this paper we introduce a simple extension to IEEE 802.11 that supports broadcasting in ad hoc networks. Ad hoc random access MAC protocols often treat unicast and broadcast packets differently. Unicast packets are preceded with MAC layer control frames, such as RTS, CTS and ACK, to ensure that the destination receives the unicast packets. Broadcast packets, on the other hand, are sent blindly without any control frames to assure the availability of the destinations. Therefore, the performance of the network degrades as contention increases. Our proposed extension to the broadcast mechanism of 802.11 helps alleviate such degradation when contention exists.

Random access MAC for efficient broadcast support in ad hoc networks

Wireless communications are becoming an important part of our everyday lifestyle. One major area that will have an enormous impact on the performance of wireless ad hoc networks is the medium access control (MAC) layer. Current random access MAC protocols for ad hoc networks support reliable unicast but not reliable broadcast. We propose a random access MAC protocol, broadcast support multiple access (BSMA), which improves the broadcast reliability in ad hoc networks.

Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks

In this study we investigate the performance of TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insights into the interactions between TCP and various MAC layer protocols, including CSMA, FAMA and 802.11. These MAC protocols were chosen because they provide an evolution of wireless medium access schemes, starting with carrie sing (CSMA), then evolving to the utilization of RTS/CTS control frames (FAMA) and finally progressing to collision avoidance and acknowledgements (802.11). We examine these interactions in various network topologies and in a mobile environment where node movements are unpredictable. In particular, we address the issue of fair sharing of MAC with multiple TCP flows.

Adaptive bandwidth management and QoS provisioning in large scale ad hoc networks

Quality of service provisioning in wireless ad hoc networks plays an integral part in determining the success of network-centric warfare as envisioned in future military operations. It requires good scalability of the QoS architecture since ad hoc networks in the battlefield tend to be large. Previous work attacking QoS in ad hoc networks seldom considers the scalability issues. In this paper, we propose a scalable QoS architecture for such networks. Our scheme draws upon the positive aspects of both IntServ and DiffServ, and extends upon the scalable LANMAR routing protocol to support QoS. The scheme is also capable of incorporating mobile backbone networks (MBNs) to further improve the scalability. Simulation results show that our proposed QoS architecture can achieve good scalability in terms of large network size and mobility.

Combining source- and localized recovery to achieve Reliable multicast in multi-hop ad hoc networks

This paper proposes a novel reliable multicast transport protocol for multi-hop, wireless ad hoc networks (or MANETs). To recover from the different types of losses that may occur in MANETs, our Reliable Adaptive Congestion-controlled Transport protocol, or Re-ACT, combines source-based congestion- and error control with receiver-initiated localized recovery. While the latter attempts to recover localized losses (e.g., caused by transmission errors), the former is invoked only for losses and congestion that could not be recovered locally (e.g., caused by global congestion). Loss differentiation is an important component of ReACT and uses medium access control (MAC) layer information to distinguish between different types of losses. Through extensive simulations, we evaluate ReACT’s performance under a variety of MANET scenarios, including different offered load and mobility conditions, and compare it against a strictly end-to-end (i.e., no localized recovery) scheme. Our results show that ReACT is the best performer in terms of reliability. Our results also showcase the effect of ReACT’s local recovery mechanism which quickly corrects error- and path breakage induced losses and thus manages to prevent the source from reducing its rate unnecessarily , thus achieving significant throughput improvement with lower overhead when compared to the strictly end-to-end protocol.

Efficient wireless network simulations with detailed propagation models

Accurate simulation of wireless networks requires realistic models of the channel propagation medium. The widely used free space model is computationally efficient but ignores many attenuation components which affect wireless signal propagation. This paper describes the impact of the accuracy of the wireless channel model on the accuracy of the results and on the execution time of large-scale network models. It then introduces means to reduce the runtime execution when deploying such detailed propagation models.

Effects of ad hoc layer medium access mechanisms under TCP

Mobile computing is the way of the future, as evident by such initiatives as Bluetooth, Iceberg and HomeRF. However, for mobile computing to be successful, an obvious layer, the MAC layer, must be efficient in channel access and reservation. Therefore, in-depth understanding is needed of the wireless MAC layer if wireless computing is to takeoff. Many random access wireless MAC protocols have been proposed and even standardized. However, there has yet been an attempt to understand why certain designs are used and what makes certain protocols better than others. In this paper, we survey several popular, contemporary, wireless, random access MAC protocols and determine the effects behind the design choices of these protocols.

Congestion control multicast in wireless ad hoc networks

In this paper, the interaction of the Medium Access Control (MAC) and routing layer is used to address the congestion control multicast routing problem in wireless ad hoc networks. We first introduce the Broadcast Medium Window (BMW) MAC protocol, which provides robust delivery to broadcast packets at the MAC layer. In doing so, we show that although BMW is able to provide high reliability under low to medium network load, reliability dramatically degrades under high network load. We then extend the wireless On-Demand Multicast Routing Protocol (ODMRP) to facilitate congestion control in ad hoc networks using BMW to combat the poor performance under highly congested network conditions. Through simulation, we show that ODMRP with congestion control adapts well to multicast sources that are aggressive in data transmissions.