Medium Access Mechanism Research Articles

A Rapidly Adaptive Collision Backoff Algorithm for Improving the Throughput in WLANs

In the 802.11 protocol, the fundamental medium access mechanism is called Distributed Coordination Function (DCF). In DCF, before making any transmission attempt, the nodes count down a timer with a value randomly selected from the Contention Window (CW) size. If the transmitted packet is involved in a collision, the node increases the CW size in an attempt to reduce the collision rate. Conversely, if the packet is transmitted successfully, the node reduces the CW size in order to increase the frequency of the transmission attempts. The growth or reduction in the CW size has a critical effect on the network performance. Several backoff algorithms have been proposed to improve the system throughput. However, none of these methods enable the system to approach the theoretical maximum throughput possible under DCF. Accordingly, this study proposes the Rapidly Adaptive Collision Backoff (RACB) algorithm, in which the CW size is adjusted dynamically based on the collision rate, as analyzed by a mathematical model. Notably, RACB requires no knowledge of the number of nodes in the wireless network and is applicable to both lightly loaded and heavily loaded networks. The numerical results show that, by adjusting the CW size such that the collision rate is maintained at a value close to 0.1, RACB enables the system throughput to approach the maximum DCF throughput in wireless environments containing any number of nodes.

Point coordination mechanism based mobile ad hoc network investigation against jammers

In this paper, a jamming attack (a kind of Denial-of-Service attack) was investigated, which interfered with the normal operation of a Mobile Ad Hoc network, which is more vulnerable to various attacks because of its self-configuration, dynamic characteristics, therefore, jammers would affect the network QoS parameters by reducing the throughput and increasing the delay. This problem is solved in this study by enabling the Point Coordination Function, which is a media access mechanism specified by the IEEE standard in some selected MANET nodes (guard nodes) to improve the deficiency of MANET’s performance. The Riverbed modeler was utilized as a simulation tool. In this study, six jammers with two different transmission power values had been applied. In a number of different simulation scenarios with and without jammers, the estimated results showed that the jammers affected the network performance by increasing the delay to 3.0658 sec and decreasing the throughput to 120200.59 bits/sec. After enabling the PCF mechanism in a number of selected nodes, the results allowed the user to solve the problem by improving the network deficiency so that the throughput had been increased to 137478.32 bits/sec and the delay had been decreased to 0.7556 sec. It can be concluded that PCF is a good improvement for different levels of jammer transmission power such as 0.01 W and 0.001 W, and PCF also improved the network’s delay and throughput when the number of PCF enabled nodes had been increased to 10 nodes and 12 nodes, respectively. The improvement is also increased. This study can be used in practice for any ad hoc network when attacked by jammers

UAV-Aided Emergency Environmental Monitoring in Infrastructure-Less Areas: LoRa Mesh Networking Approach

Rapid acquisition of environmental parameters of emergency sites is crucial for emergency response. However, in areas where public ground networks do not provide sufficient coverage or are destroyed by disasters, the rapid backhaul of monitoring data from emergency sites is difficult. Due to the urgent demand for data collection in such areas, we propose an unmanned aerial vehicle (UAV)-aided emergency environmental monitoring system using a LoRa mesh networking approach, which can quickly build a large range of wireless communication networks suitable for small data transmission. First, we designed a LoRa-based mesh network protocol by using a custom slotted ALOHA medium access mechanism. Second, we designed and implemented a custom software and hardware platform of the monitoring system. The system adopts a communication mode, including multiple sensor nodes (SNs), multiple relay nodes (RNs), a gateway (GW), and a network server. Finally, we demonstrated the performance of the protocol and the system by in-lab and outdoor tests. The results show that when data rate is 0.293 kb/s, the line-of-sight 1-hop transmission distance is more than 10 km, and the 1-hop transmission delay is less than 6 s; the end-to-end delay has an upper bound, which can meet the real-time requirement of emergency response; the least upper bound of throughput per slot of our mesh protocol is much higher than that of LoRaWAN. Evaluation results also demonstrate the network robustness to environmental changes.

Multihop Latency Model for Industrial Wireless Sensor Networks Based on Interfering Nodes

Emerging Industry 4.0 applications require ever-increasing amounts of data and new sources of information to more accurately characterize the different processes of a production line. Industrial Internet of Things (IIoT) technologies, and in particular Wireless Sensor Networks (WSNs), allow a large amount of data to be digitized at a low energy cost, thanks to their easy scalability and the creation of meshed networks to cover larger areas. In industry, data acquisition systems must meet certain reliability and robustness requirements, since other systems such as predictive maintenance or the digital twin, which represents a virtual mapping of the system with which to interact without the need to alter the actual installation, may depend on it. Thanks to the IEEE 802.15.4e standard and the use of Time-Slotted Channel Hopping (TSCH) as the medium access mechanism and IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) as the routing protocol, it is possible to deploy WSNs with high reliability, autonomy, and minimal need for re-configuration. One of the drawbacks of this communication architecture is the low efficiency of its deployment process, during which it may take a long time to synchronize and connect all the devices in a network. This paper proposes an analytical model to characterize the process for the creation of downstream routes in RPL, whose transmission of multi-hop messages can present complications in scenarios with a multitude of interfering nodes and resource allocation based on minimal IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH). This type of multi-hop message exchange has a different behaviour than the multicast control messages exchanged during the synchronization phase and the formation of upstream routes, since the number of interfering nodes changes in each retransmission.

A comprehensive hybrid bit-level and packet-level LoRa-LPWAN simulation model

LoRa is a rising LPWAN technology designed to use a simple Aloha-like medium access mechanism exploiting free ISM frequency bands. This makes LoRa packets inevitably exposed to the threat of the same LoRa network interference (intra-system interference). Additionally, they are also exposed to disturbances caused by packets from other networks using the license-free spectrum (inter-system interference). LoRa-LPWAN is actively being deployed in the literature state-of-the-art. The capture effect essentially reflects the joint relation between the physical layer capabilities of LoRa modulation in relation to the LoRaWAN Aloha-based MAC operation. Moreover, most efforts adopted in the literature have ignored the impact of inter-system ISM interference signals on LoRa-LPWAN transmissions. This paper devises a new paradigm for modeling LoRa modulation performance that incorporates the joint impact of SNR and SIR in order to represent the intra- system interference behavior during LoRAWAN operation. The physical layer abstractions are then combined with comprehensive implementation of the LoRaWAN MAC protocol standard in an OMNeT++ module, delivering a hybrid bit-level and packet-level simulation model. The devised LoRa simulation model framework also introduces a representative abstraction for LoRa reception capabilities under ISM interference in accordance with the IEEE 802.15.4 interference model [1] that has been recently introduced in the literature of long-range, low-power communication. The developed OMNeT++ Simulation model acts as a reliable platform to test different potential bottlenecks in LoRa-LPWAN operating in the free, unlicensed frequency bands. The proposed simulation framework has the potential to support high accuracy of error performance prediction in a realistic LoRa-LPWAN deployment.

On the Fairness of the Coexisting LTE-U and WiFi Networks Sharing Multiple Unlicensed Channels

In this paper, we are the first to analyse the performance of the carrier sense adaptive transmission (CSAT) scheme in the large-scale coexisting LTE-U and WiFi networks (CLWNets) sharing multiple unlicensed channels (UCs). As the capability of idle channel detection (ICD) has significant effect on the medium access probability (MAP) of WiFi access points (WAP), the closed-from MAP of the WAP and the explicit expressions of downlink successful transmission probabilities (STPs) of LTE-U-base-station (LBS) users and WAP users are derived with/without equipping ICD capability in WAPs, where the positions of LBSs and WAPs are modelled following the Matern hard core process. Based on the derived STPs, the fairness between the LTE-U network and the WiFi network, which is defined as the minimum weighted area spectral efficiency (ASE) of LBS and WAP users, is analysed versus the number of UCs, the fairness weighting factor and the densities of LBSs and WAPs. Furthermore, the optimal off-duty-cycle (DC) ratio to maximize the minimum weighted ASE is obtained based on the derived STPs in the on-DC and off-DC durations. Our results show that the fairness between the LTE-U and WiFi networks can be achieved by adjusting the off-DC ratio under low weighting factors. The increase of UC number leads to a decrease of the optimal off-DC ratio, enhancing the transmission duration of LBSs, and the shrink of UC bandwidth only causes negligible reduction in the CLWNets ASE. In addition, the WAP medium access mechanism mode with ICD capability is preferable as it can provide a maximum of $70\%$ ASE enhancement in the CLWNets.

Performance Analysis of DCF- Two Way Handshake vs RTS/CTS During Train-Trackside Communication in CBTC based on WLAN802.11b

Background:Wireless Local Area Network (WLAN) is used primarily in CBTC because of easy availability of commercial WLAN equipment. In present scenario, WLAN Medium Access Control (MAC) protocol is a well-known protocol which is used to satisfy real-time traffic and delay- sensitive applications. The bidirectional train-trackside communication is the fundamental key of train control in CBTC.Methods:DCF describes two basic techniques used for packet transmission: First technique is a Two Way Handshake (TWH) mechanism and another is Four Way Handshake (FWH) mechanisms. RTS/CTS FWH protocol specified by IEEE802.11b is introduced to rectify the Hidden Node Problem (HNP) encounters in TWH protocol. That is why the TWH mechanism of DCF technique suffers from higher average packet delay time when this protocol is applied to CBTC. DCF- Four Way Handshake (FWH), Request To Send (RTS) and Clear To Send (CTS) delay model is proposed to develop Communication Based Train Control (CBTC) system.Results:FWH is applied in CBTC to overcome the packet delay and throughput limitations of Two Way Handshake (TWH) mechanism of distributed coordination function (DCF) based technique. An experiment is designed to simulate and compare the performance of RTS/CTS delay model against TWH mechanism of DCF.Conclusion:It was found that the Average packet delay is slightly higher and throughput is lesser in RTS/CTS in comparison to TWH method. By comparing the performance of these two medium access mechanism in CBTC it was found that for multiple retransmissions with various data rates the RTS/CTS model had better packet delay time than TWH.

Performance evaluation and tuning of an IEEE 802.11 audio video multicast collision prevention mechanism

Since the introduction of the original IEEE 802.11 medium access mechanism, the definition of the multicast access mechanism has been the subject of numerous amendments and standardization efforts. Over the past few years, the need for a reliable multicast mechanism has been spurred due to the increasing demand for audio/video services. It is widely accepted that the success of any new multicast mechanism will mainly depend on its ability on meeting the stringent Quality-of-Service (QoS) requirements of the audio/video services. Due to the fact that the multicast service will have to be integrated into the IEEE 802.11 protocol architecture, it is essential to evaluate its performance in a scenario comprising unicast and multicast services. In this paper, we undertake the performance evaluation of a Multicast Collision Prevention (MCP) mechanism recently introduced in the literature. Our main goal is twofold: (1) to show that the MCP mechanism properly responds to the needs of multicast audio/video services in the presence of multicast/unicast traffic: a must for any new mechanism to be considered by the standards bodies; and (2) to provide simple guidelines on tuning-up the main system parameters of the MCP mechanism. Our numerical and simulation results also evaluate the impact of the MCP mechanism over the performance of the unicast service. Our findings show the overall benefits of the MCP in terms of the QoS metrics reported for the multicast service and the key performance indicators of the unicast traffic.

Medium Access-Based Scheduling Scheme for Cyber Physical Systems in 5G Networks

The development of the 5G mobile communication standard attempts to meet the future needs of data users. The impact of Cyber Physical Systems (CPS) is crucial in Internet of Things (IoT) and other emerging technologies. The design of medium access mechanisms for CPS such as radio resource scheduling schemes has a significant effect on network performance. Recent literature shows that limited work is available on uplink scheduling schemes, particularly in the 5G domain. Planning a network that can address the modern needs of users entails efficient CPS scheduling mechanisms such that resources are amicably distributed between users of contrasting priorities. The prime focus of this work is to design and develop an uplink radio resource scheduling framework for CPS-based future networks such as 5G. In the designed framework, scarce radio resources are sought to be distributed efficiently according to the service-based needs of users. The proposed scheduling scheme is a service aware (SA) scheduler designed for CPS in accordance with the 5G network peculiarities, intended to achieve higher throughput and reduced latency. The proposed SA scheduler supports multi-bearer traffic and is capable of providing resources in adverse channel conditions in an efficient manner. The SA scheduling mechanism’s performance is evaluated and compared with renowned scheduling algorithms such as blind equal throughput (BET), maximum throughput (MT), and proportional fair (PF) scheduling schemes. The simulation results obtained in a cellular environment demonstrate that the SA scheduler achieves acceptable cell throughput and end-to-end delay results in all scenarios and out-performs other contemporary scheduling schemes.

Statistical Evaluation of the Behavior of 5-GHz Radio LAN Devices

The goal of this paper is to develop an experimental hardware testbed for verifying the compliance of the medium access mechanism of commercially available Wi-Fi certified units with the IEEE 802.11 standard. The core of this testbed is based on a Universal Software Radio Peripheral (USRP), which is used to capture and downconvert the Wi-Fi frames to the baseband. The data captured by the USRP are used to extract the key medium access parameters of the commercial unit under test (UUT). Our measurement methodology involves a novel compliance metric based on the Kullback–Leibler distance between probability distributions. This metric is in the form of a single scalar, which will hence enable us to rank the UUTs based on their level of compliance. Furthermore, we develop a method based on the harmonized European Standard ETSI EN 301 893 to identify UUTs with noncompliant access mechanisms that tend to monopolize the wireless medium and UUTs for which compliance is an option that deviates from default settings.

Delay and backlog control of aggregation systems for wireless communications

The aggregation system implemented in wireless communication networks aims to optimize the network efficiency regarding the encapsulation information and the medium access mechanisms. This paper proposes to evaluate the performances of such (A-MSDU like) system from a dedicated frame point of view, which is important in order to predict the Quality of Service offered to a client or to an application. Regarding the incoming flows, enabling or disabling the aggregation system, and tuning in real time the aggregate size, makes possible to satisfy both the users’ requirement and the network provider efficiency. In the case of voice or video class wireless communication where some loss of data is acceptable, numerous simulations using Riverbed Modeler led to express several recommendations embedded in an algorithm which sets dynamically the aggregation parameters in order to adjust it to the incoming traffic.

A Novel Channel Access Mechanism for IEEE 802.15.4e to Promote Internet of Things

IEEE 802.15.4e defines the specification of PHY layer and MAC layer. Although distinguished by its popularity and strength for Wireless Sensor Networks (WSNs), it still suffers from several limitations that deteriorates its performance. One such restriction is a high collision probability that degrades the performance of a dense network. In this article, we develop a Slotted Access Window (SAW) mechanism, a medium access technique to mitigate high collision probability and energy consumption in dense and battery constrained networks. An accurate analytical model is presented to assess the effectiveness of SAW mechanism and validated using the ns-3 simulator. Finally, results show that the SAW mechanism significantly improves the throughput, energy efficiency, and delay in contrast to the default medium access mechanism.

Transmission Early-Stopping Scheme for Anti-Jamming Over Delay-Sensitive IoT Applications

Time-critical, wireless Internet of Things applications have been drawing increasing attention lately. The most characterizing feature of such applications is that a packet has to be delivered within a certain deadline. Not meeting the deadline could result in severe consequences. Jamming attacks utilize the shared nature of the wireless medium to corrupt transmitted packets. Although a corrupted packet could be retransmitted several times until successful delivery, this would add latency and hence could lead to missing the deadline. In this paper, we propose a jamming detection scheme that uses the packet transmission time as a statistic to make detection decisions. The key insight behind our proposed scheme is that, a long transmission/retransmission time for a certain packet indicates an abnormal condition, such as jamming. Therefore, we devise an optimal transmission-time threshold that, when exceeded, a jammer is detected. Unlike most existing detection schemes where, in case of a detection error, retransmission could continue until the deadline is reached, our scheme aims to detect the jammer earlier than the deadline so that the remaining time (until the deadline) could be utilized in retransmitting the packet over a safe channel. The proposed detection scheme is a general framework that can be applied to many situations. After conducting a thorough analysis, we apply the proposed early-stop jamming detection framework to the distributed coordinated function medium access mechanism specified by the 802.11 standard. Our simulation results show significant performance gains achieved by the proposed scheme.

Energy-Efficient GCSA Medium Access Protocol for Infrastructure-Based Cognitive Radio Networks

The major challenge encountered by battery-driven wireless local area networking devices is to conserve their energy for prolonged operation. This paper presents an energy-efficient medium access mechanism called group control slot allocation (GCSA) protocol for cognitive radio (CR) networks. GCSA utilizes the group priority allocation algorithm to allocate stations (STAs) into groups; subsequently STAs that have traffic buffered in an access point are assigned to a higher priority group. A management frame, namely group monitoring pointer, optimizes the sleep-awake cycle of the STAs by allocating transmission opportunities to groups based on their priority. GCSA employs publisher-subscriber and point-to-point messaging models for communication between the base station and STAs, respectively. Performance analysis of GCSA demonstrates that increasing the number of STAs which enter into sleep mode augments the percentage of energy saved. The overall system-level results show that energy saved is around 20% higher for GCSA than the IEEE 802.11e standard hybrid coordination function power-saving mode. Since GCSA benefits from history-assisted-led spectrum sensing, the paper also presents the relationship of the local storage of CRs with respect to history and suggests a hybrid approach as best option to keep a balance between the sensed data and its sharing with analytical engine database for history enrichment leading toward improved energy efficiency.

Restricted Access Window-Based Novel Service Differentiation Scheme for Group-Synchronized DCF

Group-synchronized distributed coordination function (GS-DCF) is a group-based medium access mechanism introduced in the IEEE 802.11ah draft standard. GS-DCF implements a novel channel access mechanism called restricted access window (RAW) which splits the channel time into various slots, devices into several groups, and assigns each slot to a group of devices. The Internet of Things is an ideal platform that captures the process and handles a large amount of data from the various heterogeneous applications with different channel access requirements. In this letter, we propose a novel RAW-based service differentiation scheme for GS-DCF. We propose a technique to allocate RAW slots to different applications based on their service requirements with minimal additional overhead. Analytical and simulation results show that the proposed scheme can effectively provide differentiated services in terms of throughput to the group of devices in GS-DCF.

Known and Unknown Facts of LoRa

Long Range (LoRa) is a Low-power Wide-area Network technology designed for the Internet of Things. In recent years, it has gained significant momentum among industrial and research communities. Patented by Semtech, LoRa makes use of chirp spread spectrum modulation to deliver data with promises of long battery life, far-reaching communication distances, and a high node density at the cost of data rate. In this article, we conduct a series of experiments to verify the claims made by Semtech on LoRa technology. Our results show that LoRa is capable of communicating over 10km under line-of-sight environments. However, under non-line-of-sight environments, LoRa’s performance is severely affected by obstructions such as buildings and vegetations. Moreover, the promise of prolonged battery life requires extreme tuning of parameters. Last, a LoRa gateway supports up to 6,000 nodes with PRR requirement of >70%. This study also explores the relationship between LoRa transmission parameters and proposes an algorithm to determine optimal settings in terms of coverage and power consumption under non-line-of-sight environments. It further investigates the impact of LoRa Wide-area Networks on energy consumption and network capacity along with implementation of a LoRa medium access mechanism and possible gains brought forth by implementing such a mechanism.

Priority-Aware Wireless Fieldbus Protocol for Mixed-Criticality Industrial Wireless Sensor Networks

Industrial wireless sensor networks are becoming popular for critical monitoring and control applications in industrial automation systems. For such type of applications, providing reliable real-time performance regarding data delivery is considered as a fundamental challenge. The problem becomes more prominent with mixed-criticality systems, where different data flow with different levels of criticality (importance) coexist and characterized by different requirements regarding delay and reliability. In this paper, we propose a wireless fieldbus protocol to enable real-time communication and service differentiation for cluster-based mixed-criticality networks. A process monitoring scenario of plastic extrusion is used to define the protocol requirements and elaborate the working principle of the proposed work. In our proposed protocol, each data flow is scheduled for channel access based on its criticality level using a distributed prioritized medium access mechanism that ensures a guaranteed channel access for the most critical traffic over other traffic types. The performance of the proposed protocol is analyzed analytically using a discrete-time Markov chain model to evaluate the performance in terms of delay and throughput. Moreover, the extensive simulations are conducted to prove the analytical claims and different performance assessments are provided, which also demonstrate the effectiveness of the proposed approach compared with the related existing work.

Flying Height Optimization for Unmanned Aerial Vehicles in Cellular - Flying Adhoc Network

In Flying Adhoc NETwork (FANET), the communications between Unmanned Aerial Vehicles (UAV), UAVs to infrastructure, and UAVs to wireless sensors are crucial design factors. With strict energy constrain during flying operation, the allocation of power and resources, the flying strategy, and the medium access mechanism shall be effectively used. When employing cellular network as backhaul for UAVs, the flying height of UAVs not only affects the communication between UAVs and end users on the ground, but also determines the reception between infrastructure (base stations from the cellular network) to UAVs. In this paper, we optimize the flying height of UAVs for better reception from the cellular network by using stochastic geometry analysis to model the aggregated interference at the UAV side. The network system performance is also examined under the effect of fading-less channel and Rayleigh fading channel.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited.

Analysis of Fairness Problem for IEEE 802.15.6 Slotted Aloha Algorithm

The IEEE 802.15.6 slotted Aloha protocol defines a contention based medium access mechanism for wireless body area network (WBAN). However the proposed scheme can cause a high collision rate when the number of nodes increases, thus decreasing overall throughput severely, especially in saturated traffic. In order to decrease collision rate, we can modify the protocol such that the contention probability of each node can be decreased as small as possible when the collision is repeated. This modification decreases the collision rate considerably improving the overall throughput, but with the sacrifice of fairness resulting in the starvation of some of the nodes. Recently, an algorithm, called Beta-decrement algorithm, has been proposed, which shows a reasonable amount of throughput while maintaining relatively good fairness among nodes. In this paper, we rigorously analyze the relationship between the throughput and fairness in the modified slotted aloha algorithm, and show why conventional solutions could not satisfy both requirements simultaneously but the Beta-decrement algorithm can.

Throughput Optimization in WLAN/Cellular Integrated Network Using Partially Overlapped Channels

The wireless network that integrates a heterogeneous cellular network and a WLAN is referred to as a WLAN/cellular integrated network (WCIN). In a fourth generation WCIN, in order to achieve collision-avoidance between the LTE-A and IEEE 802.11 family, some interference-free mechanisms, such as carrier sense adaptive transmission and listen-before-talk, are proposed. However, the interference-free medium access mechanisms leave some unexploited partially overlapped 802.11ac channels, which result in a waste of spectrum. In this paper, we propose an interference-tolerant medium access method to optimize the WCIN throughput by utilizing those partially overlapped channels (POCs). First, we show the feasibility of enhancing WLAN throughput by utilizing 802.11ac POCs in a WCIN. Second, we mathematically model the partial overlap when an 802.11ac channel is partially overlapping with an LTE-A component carrier. Third, we propose an interference-tolerant medium access mechanism to optimize the WCIN throughput. The interference-tolerant one ensures that the interference to LTE-A users is tolerable in a given WCIN. Finally, we construct a hardware-in-the-loop testbed to evaluate and compare our proposed mechanism with three other state-of-the-art mechanisms. The experimental results show that our approach achieves at most 39% more throughput than one of the state-of-the-art collision-avoidance mechanisms regarding the entire WCIN.