Significant TCP unfairness in ad hoc wireless networks has been reported during the past several years. This unfairness results from the nature of the shared wireless medium and location dependency. If we view a node and its interfering nodes to form a “neighborhood”, the aggregate of local queues at these nodes represents the distributed queue for this neighborhood. However, this queue is not a FIFO queue. Flows sharing the queue have different, dynamically changing priorities determined by the topology and traffic patterns. Thus, they get different feedback in terms of packet loss rate and packet delay when congestion occurs. In wired networks, the Randomly Early Detection (RED) scheme was found to improve TCP fairness. In this paper, we show that the RED scheme does not work when running on individual queues in wireless nodes. We then propose a Neighborhood RED (NRED) scheme, which extends the RED concept to the distributed neighborhood queue. Simulation studies confirm that the NRED scheme can improve TCP unfairness substantially in ad hoc networks. Moreover, the NRED scheme acts at the network level, without MAC protocol modifications. This considerably simplifies its deployment.

Ad hoc networks are completely wireless networks of mobile hosts, in which the topology rapidly changes due to the movement of mobile hosts. This frequent topology change may lead to sudden packet losses and delays. Transport protocols like TCP, which have been designed for reliable fixed networks, misinterpret this packet loss as congestion and invoke congestion control, leading to unnecessary retransmissions and loss of throughput. To overcome this problem, a feedback scheme is proposed so that the source can distinguish between a route failure and network congestion. When a route is disrupted, the source is sent a Route Failure Notification packet, allowing it to invalidate its timers and stop sending packets. When the route is reestablished, the source is informed through a Route Reestablishment Notification packet, upon which it resumes packet transmissions. Simulation experiments show that in the event of route failures, as the route reestablishment time increases, the use of feedback provides significant improvements in performance.

Routing in Ad hoc networks is a challenging problem because nodes are mobile and links are continuously being created and broken. Existing on-demand Ad hoc routing algorithms initiate route discovery only after a path breaks, incurring a significant cost in detecting the disconnection and establishing a new route. In this work, we investigate adding proactive route selection and maintenance to on-demand Ad hoc routing algorithms. More specifically, when a path is likely to be broken, a warning is sent to the source indicating the likelihood of a disconnection. The source can then initiate path discovery early, potentially avoiding the disconnection altogether. A path is considered likely to break when the received packet power becomes close to the minimum detectable power (other approaches are possible). Care must be taken to avoid initiating false route warnings due to fluctuations in received power caused by fading, multipath effects and similar random transient phenomena. Experiments demonstrate that adding proactive route selection and maintenance to DSR and AODV (on-demand ad hoc routing protocols) significantly reduces the number of broken paths, with a small increase in protocol overhead. Packet latency and jitter also goes down in most cases. Because preemptive routing reduces the number of broken paths, it also has a secondary effect on TCP performance – unnecessary congestion handling measures are avoided. This is observed for TCP traffic under different traffic patterns (telnet, ftp and

Abstract — We present a model for the joint design of congestion control and media access control (MAC) for ad hoc wireless networks. Using contention graph and contention matrix, we formulate resource allocation in the network as a utility maximization problem with constraints that arise from contention for channel access. We present two algorithms that are not only distributed spatially, but more interestingly, they decompose vertically into two protocol layers where TCP and MAC jointly solve the system problem. The first is a primal algorithm where the MAC layer at the links generates congestion (contention) prices based on local aggregate source rates, and TCP sources adjust their rates based on the aggregate prices in their paths. The second is a dual subgradient algorithm where the MAC sub-algorithm is implemented through scheduling linklayer flows according to the congestion prices of the links. Global convergence properties of these algorithms are proved. This is a preliminary step towards a systematic approach to jointly design TCP congestion control algorithms and MAC algorithms, not only to improve performance, but more importantly, to make their interaction more transparent.

Emerging technologies such as Bluetooth are expected to become a ubiquitous solution for providing short range, low power, low cost, pico-cellular wireless connectivity. Bluetooth is a Master driven Time Division Duplex (TDD) system that supports an asynchronous channel for data traffic as well as synchronous channels for voice traffic. Data applications running over Bluetooth such as http, ftp and real audio will need transport layer protocols such as TCP and UDP to send packets over the wireless links. In this paper we study several schemes designed to improve the performance of asynchronous data traffic over a Bluetooth piconet that supports multiple active slaves. We propose and compare a number of SAR policies and MAC scheduling algorithms with a view towards enhancing the performance of transport layer sessions. We investigate the effect of different FEC and ARQ schemes at the baseband level, using a two-state Markov channel model for the Bluetooth RF link. We also study how the presence of circuit-switched voice impacts the performance of data traffic. Keywords--- Medium Access Control (MAC), Scheduling, Time Division Duplex (TDD), Segmentation and Reassembly (SAR), Forward Error Correction (FEC), Automatic Repeat Request (ARQ), TCP, UDP. I.

Improving TCP performance has long been the focus of many research efforts in mobile ad hoc networks (MANET). In this paper, we address one aspect of this endeavor: how to properly set TCP's congestion window limit (CWL) to achieve optimal performance. Past research has shown that using a small CWL improves TCP performance in certain scenarios [1], [2], however, no comprehensive study has been given.

A central challenge in ad hoc networks is the design of routing protocols that can adapt their behavior to frequent and rapid changes in the network. The performance of proactive and reactive routing protocols varies with network characteristics, and one protocol may outperform the other in different network conditions. The optimal routing strategy depends on the underlying network topology, rate of change, and traffic pattern, and varies dynamically. This paper introduces the Sharp Hybrid Adaptive Routing Protocol (SHARP), which automatically finds the balance point between proactive and reactive routing by adjusting the degree to which route information is propagated proactively versus the degree to which it needs to be discovered reactively. SHARP enables each node to use a different application-specific performance metric to control the adaptation of the routing layer. This paper describes application-specific protocols built on top of SHARP for minimizing packet overhead, bounding loss rate, and controlling jitter. Simulation studies show that the resulting protocols outperform the purely proactive and purely reactive protocols across a wide range of network characteristics. 1

On-demand routing protocols for mobile ad hoc networks utilize route caching in different forms in order to reduce the routing overheads as well as to improve the route discovery latency. For route caches to be effective, they need to adapt to frequent topology changes. Using an ondemand protocol called "Dynamic Source Routing" (DSR), we study the problem of keeping the caches up-to-date in dynamic ad hoc networks. Previous studies have shown that cache staleness in DSR can significantly degrade performance. We present and evaluate three techniques to improve cache correctness in DSR namely wider error notification, route expiry mechanism with adaptive timeout selection and the use of negative caches. Simulation results show that the combination of the proposed techniques not only result in substantial improvement of both application and cache performance but also reduce the overheads. 1

Sensor networks can be considered distributed computing platforms with many severe constraints including limited CPU speed, memory size, power, and bandwidth. Individual nodes in sensor networks are typically unreliable and the network topology dynamically changes, possibly frequently. Sensor networks can also be considered a form of ad hoc network. However, here also many constraints in sensor networks are different or more severe. Sensor networks also differ because of their tight interaction with the physical environment via sensors and actuators. Due to all of these differences many solutions developed for general distributed computing platforms and for ad hoc networks cannot be applied to sensor networks. Many new and exciting research challenges exist. This paper discusses the state of the art and presents the key research challenges to be solved, some with initial solutions or approaches.

The widespread use of mobile and handheld devices is likely to popularize ad hoc networks, which do not require any wired infrastructure for intercommunication. The nodes of mobile ad hoc networks (MANETs) operate as end hosts as well as routers. They intercommunicate through single-hop and multi-hop paths in a peer-to-peer fashion. With the expanding range of applications of MANETs, the need for supporting Quality of Service (QoS) in these networks is becoming essential. This paper provides a survey of issues in supporting QoS in MANETs. We have considered a layered view of QoS provisioning in MANETs. In addition to the basic issues in QoS, the report describes the efforts on QoS support at each of the layers, starting from physical and going up to the application layer. A few proposals on the inter-layer approaches for QoS provisioning have also been addressed. The paper concludes with a discussion on the future directions and challenges in the areas of QoS support in MANETs.