A scheduling algorithm for wireless networks with large propagation delays

Underwater acoustic (UWA) networks are a key form of communications for human exploration and activities in the oceanographic space of the earth. A fundamental issue of UWA communications is large propagation delays due to water medium, which has posed a grand challenge in UWA network protocol design. Conventional wisdom of addressing this issue is to live with this disadvantage by inserting a guard interval to introduce immunity to propagation delays. Recent advances in interference alignment (IA) open up a new direction to address this issue and promise a great potential to improve network throughput by exploiting large propagation delays. In this paper, we investigate propagation delay-based IA (PD-IA) in multi-hop UWA networks. We first develop a set of simple constraints to characterize PD-IA feasible region at the physical layer. Based on the set of PD-IA constraints, we develop a distributed PD-IA scheduling algorithm to greedily maximize interference overlapping possibilities in a multi-hop UWA network. Simulation results show that the proposed PD-IA algorithm yields higher throughput than an idealized benchmark algorithm without propagation delays, indicating that large propagation delays are not adversarial but beneficial for network throughput performance.

The potential of exploiting large propagation delays in underwater acoustic (UWA) networks to maximize the network throughput is established in the recent past. Transmission scheduling strategies have been proposed to take advantage of large propagation delay. Super-TDMA is one among such Medium Access Control (MAC) strategies proposed. It is a form of Time Division Multiple Access (TDMA) protocol in which multiple transmissions are allowed in the same time slot and hence concurrently propagate in the medium. We present the implementation challenges of Super-TDMA on underwater acoustic modems and the experimental results demonstrating interference alignment, the crossing of simultaneously transmitted packets in water, and the time synchronization among the deployed nodes in the network.

The extreme conditions under which multi-hop underwater acoustic sensor networks (UASNs) operate constrain the performance of medium access control (MAC) protocols. The MAC protocol employed significantly impacts the operation of the network supported, and such impacts must be carefully considered when developing protocols for networks constrained by both bandwidth and propagation delay. Time-based coordination, such as TDMA, have limited applicability due to the dynamic nature of the water channel used to propagate the sound signals, as well as the significant effect of relatively small changes in propagation distance on the propagation time. These effects cause inaccurate time synchronization and therefore make time-based access protocols less viable. The large propagation delays also diminish the effectiveness of carrier sense protocols as they do not predict with any certainty the status of the intended recipients at the point when the traffic would arrive. Thus, CSMA protocols do not perform well in UASNs, either. Reservation-based protocols have seldom been successful in commercial products over the past 50 years due to many drawbacks, such as limited scalability, relatively low robustness, etc. In particular, the impact of propagation delays in UASNs and other such constrained networks obfuscate the operation of the reservation protocols and diminish, if not completely negate, the benefit of reservations. The efficacy of the well-known RTS-CTS scheme, as a reservation-based enhancement to the CSMA protocol, is also adversely impacted by long propagation delays. An alternative to these MAC protocols is the much less complex ALOHA protocol, or one of its variants. However, the performance of such protocols within the context of multi-hop networks is not well studied. In this paper we identify the challenges of modeling contention-based MAC protocols and present models for analyzing ALOHA and p-persistent ALOHA variants for a simple string topology. As expected, an application of the model suggests that ALOHA variants are very sensitive to traffic loads. Indeed, when the traffic load is small, utilization becomes insensible to p values. A key finding, though, is the significance of the network size on the protocols' performance, in terms of successful delivery of traffic from outlying nodes, indicating that such protocols are only appropriate for very small networks, as measured by hop count.

The use of wireless access networks, particularly IEEE 802.11 wireless access networks, is an economical method for realizing a communication network between a core information network of a smart grid and a rural area network. This paper presents a novel analysis of the throughput performance of the IEEE 802.11 carrier sense multiple access with collision avoidance (CSMA/CA) protocol with a large propagation delay. The throughput analysis is performed by using a real model. A Markov chain model is used to analyze the throughput performance of the IEEE 802.11 CSMA/CA binary exponential backoff algorithm. The results of analysis and computer simulations show that the theoretical throughput is in good agreement with the throughput obtained from computer simulations under the propagation delay less than 20 µs. When the propagation delay is more than 20 µs, the throughput obtained from the simulation results becomes larger than the theoretical throughput because of short-term unfairness. The throughput degradation for a transmission rate of 6 Mb/s and a propagation delay of 40 µs is 16.8 %; this propagation delay corresponds to a transmission distance of 12 km.

In a wireless network with large propagation delay S-ALOHA(Slotted ALOHA) requires large guard time which causes lower normalized throughput. To reduce the large guard time in the ISA-ALOHA, a time alignment mechanism was proposed under the assumption of propagation delay estimation. In this letter, we propose a framed structure which is able to estimate propagation delay by employing coordinator beaconing. The framed structure consists of a time period for beaconing and a group of multiple time slots for random access. The proposed Beacon-based S-ALOHA(BS-ALOHA) can make packets generated during the time of beaconing evenly distributed over the random access period. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time and show that BS-ALOHA provides higher normalized throughput than both ALOHA and S-ALOHA employing the large guard time.

The large propagation delay of underwater acoustic signals significantly affects the throughput performance of underwater communication networks. While past research focused on mitigating the impact of large propagation delays, recent work has suggested exploiting large delays. In this paper, we consider an underwater linear unicast network which employs a time- division based scheduling strategy to exploit large propagation delays to improve network throughput. We assume the protocol model in a network with partially overlapping collision domains, where the transmission range is normalized as 1 and the interference range is an integer k. We systematically discuss the throughput of the linear networks with single traffic flow, showing that the average throughput of an N-node linear network with single traffic flow cannot exceed (N-1)/k. We then propose a general transmission scheduling strategy that can achieve the throughput upper bound and also give some examples of the optimal schedules.

Downlink data transmission scheduling algorithms in wireless networks

Transmission algorithms are introduced for use in a single-hop packet switching system with nonuniform traffic and with propagation delay that Is large relative to the packet transmission time. The traffic model allows arbitrary traffic streams subject only to a constraint on the number of data packets which can arrive at any individual source in the system or for any individual destination in the system over time periods of specified length. The algorithms are based primarily on sending transmission schedules to the receivers immediately before transmitting each data packet multiple times so that the receiver can maximize the number of packets it captures. An algorithm based on matchings in a random graph is shown to provide mean total delay divided by mean propagation delay arbitrarily close to one, as the propagation delay tends to infinity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Any medium access control (MAC) protocol designed for underwater acoustic networks (UANs) must take some unique characteristics of underwater channels into consideration for apt functioning, such as long propagation delay, high bit-error rate, limited bandwidth and so forth. Already, many MAC protocols are designed for UANs, which have fully or partially taken these characteristics into account and could be classified as handshake-free and handshake-based protocols. In this paper, these two classes are investigated through a comprehensive simulation campaign by employing their prominent representative protocols, namely ALOHA, carrier sense multiple access (CSMA) and multiple access with collision avoidance based adaptive packet train (MACA-APT) protocol. The contribution of this paper lies as follows: (i) unearthing the optimum values of various parameters on which the performance of a protocol relies; (ii) investigating the performance of all three protocols through a realistic channel model simulator by varying a couple of network parameters, and iii) discovering their effectiveness on various applications.

Multiple access with collision avoidance (MACA) is a popular medium-access control (MAC) protocol for terrestrial networks (e.g., 802.11). Underwater acoustic networks (UANs) differ from terrestrial networks as they are characterized by long propagation delays and higher data loss. Most of the published analysis for MACA in terrestrial networks ignores these factors, and, therefore, cannot be directly applied to UANs. As a result of the high data loss in UANs, it is common to implement reliability at a link level, rather than end-to-end acknowledgements. Keeping this in mind, we present a Markov chain analysis for a reliable variant of the MACA protocol for ad hoc UANs and derive closed-form expressions for mean service time and throughput. We show that the network performance is vastly improved with a few changes to the protocol, and propose a novel MACA-based protocol for use in UANs. For best performance, the protocol parameters such as batch size and backoff window have to be optimally chosen. We show that an optimum batch size minimizes the total waiting time. Finally, we compare our analysis results with experimental results obtained from a deployment of a small UAN.

With the ever-increasing number of multicast data applications recently, considerable efforts have been focused on the design of flow control schemes for multicast services. The main difficulties in designing a flow controller for multicast service are caused by heterogeneous multicast receivers, especially those with large propagation delays, since the feedback arriving at the source is somewhat outdated, and can be harmful to the control operations. To attack the above problem, the present paper describes a novel multicast flow control scheme, the so-called proportional, integrative, derivative plus neural network (PIDNN) predictive technique, which consists of two components: the proportional integrative plus derivative (PID) controller and the back propagation BP neural network (BPNN). This network-assisted property is different from the existing control schemes, in that the PIDNN controller can release the irresponsiveness of a multicast flow caused by those long propagation delays from the receivers. By using BPNN for the receivers with longer propagation delay, this active scheme makes the control more responsive to network status. Thus the rate adaptation can be performed in a timely manner, for the sender to respond to network congestion quickly. We analyze the theoretical aspects of the proposed algorithm, show how the control mechanism can be used to design a controller to support multi-rate multicast transmission based on feedback of explicit rates, and verify this matching using simulations. Simulation results demonstrate the efficiency of our scheme in terms of high link utilization, quick response, scalability, high unitary throughput, intra-session fairness and inter-session fairness.

The large-scale mobile Underwater Wireless Sensor Network (UWSN) is a useful networking paradigm to explore underwater environments. The characteristics of mobile UWSNs, such as low communication bandwidth, large propagation delay, and floating node mobility are significantly different from wireless sensor networks. In this paper, we propose a new efficient routing protocol, called multi-path routing (MPR) protocol, for UWSNs to improve the transmission delay. Multi-path is utilized during the path construction from the source node to the destination node, which is composed of a series of multi-subpaths. Each multi-subpath is a sub-path from a sending node to its two-hop neighboring node, called receiving node, by one more relay nodes, where these relay nodes simultaneously are neighboring nodes of sending and receiving nodes. Due to the different propagation delay of UWSN, the packet arrival time along the different sub-path is different; thus it may avoid the data collision in the receiving node when receiving different packets from different relay nodes. It surely sends the different packet along different sub-path to significantly improve delay time and packet delivery ratio. Finally, simulation results illustrate the performance achievements of the MPR protocol in improving delay time, packet delivery ratio, throughput and reducing overhead ratio.

Performance analysis of broadcast star network with collision-avoidance switch

Nanonetworks are expected to expand the capabilities of individual nanomachines by allowing them to cooperate and share information by molecular communication. The information molecules are released by the transmitter nanomachine and diffuse across the aqueous channel as a Brownian motion holding the feature of a strong random movement with a large propagation delay. In order to ensure an effective real-time cooperation, it is necessary to keep the clock synchronized among the nanomachines in the nanonetwork. This paper proposes a model on a two-way message exchange mechanism with the molecular propagation delay based on the inverse Gaussian distribution. The clock offset and clock skew are estimated by the maximum-likelihood estimation. Simulation results by MATLAB show that the mean square errors of the estimated clock offsets and clock skews can be reduced and converge with a number of rounds of message exchanges. The comparison of the proposed scheme with a clock synchronization method based on symmetrical propagation delay demonstrates that our proposed scheme can achieve better performance in terms of accuracy.

The limited channel capacity and the varying propagation conditions of radio signals have motivated research for boosting the achievable throughput in wireless networks. Among the effective optimization strategies is to dynamically adjust the packet size either to better suit the channel conditions or to minimize the number of overhead bits in the individual packets. However, multi‐levels of security requirements impose constraints on the data mix in the packet payload and may diminish the gains achievable by contemporary packet‐size optimization schemes. This paper presents a novel bandwidth optimization algorithm for wireless data acquisition networks where strict confidentiality requirements and access restriction policies have to be observed. The algorithm exploits the classification of data in minimizing the number of packet transmissions as well as the overhead within the individual packets. The idea is to combine the transmission of packets based on the time sensitivity and security attributes of the data in the payload. The performance of the proposed algorithm is validated through mathematical analysis and through simulation. The simulation results confirm the effectiveness of the algorithm in boosting data throughput in the network. Copyright © 2010 John Wiley &amp; Sons, Ltd.