Medium Access Algorithm Research Articles

Adaptive Spatial Scheduling for Event Traffic in LoRaWAN Networks.

Low-Power Wide-Area Networks constitute a leading, emerging Internet-of-Things technology, with important applications in environmental and industrial monitoring and disaster prevention and management. In such sensor networks, external detectable events can trigger synchronized alarm report transmissions. In LoRaWANs, and more generally in networks with a random access-based medium access algorithm, this can lead to a cascade of frame collisions, temporarily resulting in degraded performance and diminished system operational capacity, despite LoRaWANs' physical layer interference and collision reduction techniques. In this paper, a novel scheduling algorithm is proposed that can increase system reliability in the case of such events. The new adaptive spatial scheduling algorithm is based on learning automata, as well as previous developments in scheduling over LoRaWANs, and it leverages network feedback information and traffic spatial correlation to increase network performance while maintaining high reliability. The proposed algorithm is investigated via an extensive simulation under a variety of network conditions and compared with a previously proposed scheduler for event-triggered traffic. The results show a decrease of up to 30% in average frame delay compared to the previous approach and an order of magnitude lower delay compared to the baseline algorithm. These findings highlight the importance of using spatial information in adaptive schemes for improving network performance, especially in location-sensitive applications.

Knowledge Transfer-Based Multiagent Q-Learning for Medium Access in Dense Cellular Networks

Inter-cell interference in dense cellular networks causes severe network performance degradation because the base stations (BSs) and devices cannot fully utilize all the network status information in practice. Reinforcement learning enables the wireless network to find an effective transmission strategy with limited information and the network entities to operate in a distributed manner. We herein propose a medium access algorithm based on multi-agent reinforcement learning by exchanging knowledge of transmissions from other cells. Specifically, a state mapping function between the device and the BS and a transfer value iteration are proposed. The BS transfers reward processes derived from inter-cell interference so that the partially observed medium of devices is expanded to monitor entire networks. Extensive simulation results show that simple knowledge transfer can help performance improvement in terms of network stability and throughput.

Link Availability Based Routing Protocol for Mobile Ad-Hoc Networks based on Link Life Time Prediction

A self-configured MANET has a set of wireless devices. With no prior administration, the Mobile Nodes (MNs) communicate via the wireless links. The limited bandwidth, transmission errors, dynamic topology, energy constraints, and link stability fluctuations lead to node mobility. The links between nodes are unreliable and might break on account of such node mobility. Link breakage leads to the rerouting process at the sender node (where the link breakage occurs) or Source Node (SN). This work proposed Link availability based routing protocol (LBRP) for MANET to attain load balancing via the Multi-Path (MP) communication. The traffic data could completely pass through parallel multiple paths, which enhances the protocol efficiency. On weighing against the existing routing protocols (RPs), the LBRP chiefly considers frequent link failures and unpredictable change in topology. The simulation outcomes validate the proposed LBRP. The LBRP performs well when contrasted to the DSR, ZRP, and AODV in all the experiments. This work would serve as the groundwork for examining and optimizing certain network protocols, and could help in designing algorithms for transport control and medium access.

Stability conditions for a decentralised medium access algorithm: single- and multi-hop networks

We consider a decentralised multi-access algorithm, motivated primarily by the control of transmissions in a wireless network. For a finite single-hop network with arbitrary interference constraints, we prove stochastic stability under the natural conditions. For infinite and finite single-hop networks, we obtain broad rate-stability conditions. We also consider symmetric finite multi-hop networks and show that the natural condition is sufficient for stochastic stability.

Stability of a standard decentralised medium access

We consider a stochastic queueing system modelling the behaviour of a wireless network with nodes employing a discrete-time version of the standard decentralised medium access algorithm. The system is unsaturated " each node receives an exogenous flow of packets at the rate ! packets per time slot. Each packet takes one slot to transmit, but neighbouring nodes cannot transmit simultaneously. The algorithm we study is standard in that: a node with empty queue does not compete for medium access; the access procedure by a node does not depend on its queue length, as long as it is non-zero. Two system topologies are considered, with nodes arranged in a circle and in a line. We prove that, for either topology, the system is stochastically stable under condition ! < 2/5. This result is intuitive for the circle topology as the throughput each node receives in a saturated system (with infinite queues) is equal to the so-called parking constant, which is larger than 2/5. (This fact, however, does not help to prove our result.) The result is not intuitive at all for the line topology as in a saturated system some nodes receive a throughput lower than 2/5.

Delay Scaling in Many-Sources Wireless Networks without Queue State Information

We examine a canonical scenario where several wireless data sources generate sporadic delay-sensitive messages that need to be transmitted to a common access point. The access point operates in a time-slotted fashion, and can instruct the various sources in each slot with what probability to transmit a message, if they have any. When several sources transmit simultaneously, the access point can detect a collision, but is unable to infer the identities of the sources involved. While the access point can use the channel activity observations to obtain estimates of the queue states at the various sources, it does not have any explicit queue length information otherwise. We explore the achievable delay performance in a regime where the number of sources n grows large while the relative load remains fixed. This scaling is particularly pertinent in Internet of Things (IoT) scenarios, where a key challenge is to achieve low delay when the overall traffic activity is dispersed across massive numbers of highly intermittent sources. We establish that, under any medium access algorithm without queue state information, the average delay must be at least of the order of n slots when the load exceeds some threshold λ* < 1$. This demonstrates that bounded delay can only be achieved if a positive fraction of the system capacity is sacrificed. Furthermore, we introduce a scalable Two-Phase algorithm for low-delay IoT applications which achieves a delay upper bounded uniformly in n when the load is below e -1 , and a delay of the order of n slots when the load is between e -1 and 1. Additionally, this algorithm provides robustness against correlated source activity, which is also prevalent in IoT scenarios.

Delay Scaling in Many-Sources Wireless Networks without Queue State Information

We examine a canonical scenario where several wireless data sources generate sporadic delay-sensitive messages that need to be transmitted to a common access point. The access point operates in a time-slotted fashion, and can instruct the various sources in each slot with what probability to transmit a message, if they have any. When several sources transmit simultaneously, the access point can detect a collision, but is unable to infer the identities of the sources involved. While the access point can use the channel activity observations to obtain estimates of the queue states at the various sources, it does not have any explicit queue length information otherwise. We explore the achievable delay performance in a regime where the number of sources n grows large while the relative load remains fixed. We establish that, under any medium access algorithm without queue state information, the average delay must be at least of the order of n slots when the load exceeds some threshold lambda* < 1. This demonstrates that bounded delay can only be achieved if a positive fraction of the system capacity is sacrificed. Furthermore, we introduce a scalable Two-Phase algorithm which achieves a delay upper bounded uniformly in n when the load is below e -1 , and a delay of the order of n slots when the load is between e -1 and 1. Additionally, this algorithm provides robustness against correlated source activity.

Stability conditions for a discrete-time decentralised medium access algorithm

We consider a stochastic queueing system modelling the behaviour of a wireless network with nodes employing a discrete-time version of the standard decentralised medium access algorithm. The system is unsaturated—each node receives an exogenous flow of packets at the rate of $\lambda$ packets per time slot. Each packet takes one slot to transmit, but neighbouring nodes cannot transmit simultaneously. The algorithm we study is standard in the following sense: a node with an empty queue does not compete for medium access; the access procedure by a node does not depend on its queue length as long as it is nonzero. Two system topologies are considered, with nodes arranged in a circle and in a line. We prove that, for either topology, the system is stochastically stable under the condition $\lambda<2/5$. This result is intuitive for the circle topology as the throughput each node receives in the saturated system (with infinite queues) is equal to the so-called parking constant, which is larger than $2/5$. (This fact, however, does not help us to prove the result.) The result is not intuitive for the line topology as in the saturated system some nodes receive a throughput lower than $2/5$.

Implementation and performance evaluation of IEEE 802.15.4 unslotted CSMA/CA protocol on Contiki OS

In the wireless sensor networks domain, one of the most used standards is IEEE 802.15.4. This standard has been made available on many low power operating systems such as TinyOS and Contiki OS. It is crucial for the implementation to be compliant with the specifications of the standard. In the case of Contiki OS, the provided version of the main medium access algorithm, unslotted Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA), presents many flaws. In this paper, we discuss the implementation of the provided CSMA/CA algorithm and we point out to the parts that do not respect the standard specifications. We also propose and implement a compliant version of this algorithm and show through simulation the correctness of the implementation.

Overhead caused by WiFi on ZigBee Networks using Slotted CSMA/CA

This paper presents an in-depth analysis on the effect of the coexistence in the 2.4 GHz band between IEEE 802.15.4 and IEEE 802.11 on the medium access control. We focus on the slotted CSMA/CA medium access algorithm that is used by IEEE 802.15.4 and analyse the delay that frames undergo when suffering from interference from a WiFi activity. We measure the overhead caused by the additional delay spent in the MAC sublayer frame queue before accessing the medium due to the presence of WiFi interference. We experimented different scenarios with overlapping channels and non-overlapping channels. We show that the two wireless protocols can coexist if we take into considerations the relative positions of the nodes to avoid very high interference and if we avoid monopolizing the channel with a very high rate WiFi traffic even under overlapping channels.

QoS-Guaranted Slot Allocation Algorithm for Efficient Medium Access in HR-WPAN

It is very important to provide a parameterized quality of service (QoS) using traffic specification (TSPEC), such as mean data rate, maximum burst size, and peak data rate, when packets from the application layer need to be transmitted with guaranteed services in a high-rate wireless personal area network (HR-WPAN). As medium resources are limited, the optimal medium time required for each device needs to be estimated to share the resources efficiently among devices. This paper proposes a variable-service interval-based resource allocation algorithm to efficiently make a reservation of medium resources based on a parameterized QoS. In other words, the proposed algorithm calculates the number of medium access slots (MASs) based on TSPEC, local resources, and local conditions and determines suitable locations for the MASs within a superframe to accommodate more devices. The simulation results show that the proposed algorithm can accommodate more devices and has greater than 10% resource allocation efficiency in an HR-WPAN compared to existing schemes.

CADMA: collision-avoidance directional medium access for vehicular ad hoc networks

Quick and accurate message transmission is an important research topic for vehicular ad hoc networks (VANET). Most studies assume that the periodic broadcast of beacon frames between vehicles increases the safety of the driver. In particular, there has been a lot of research into broadcasting based on carrier sense multiple access with collision avoidance (CSMA/CA) algorithms for medium access. However, the CSMA/CA algorithm is not an optimum technique for the VANET system, due to the transfer delay that occurs in inducing frequent collisions on transmission signals. In this paper, we propose a collision-avoidance directional medium access (CADMA) protocol and infrastructure-utilized clustering method for VANET to support reliable data transfer. In the proposed scheme, the CADMA protocol uses non-competitive transmission methods and cluster heads (CHs) to manage access by allocating the nodes resources. In addition, the roadside unit (RSU) helps with the clustering process. The simulation results indicate that the CADMA can reduce transmission delays and the collision rate of the broadcasting signal, and have shown that the CADMA can be effectively utilized for the VANET systems.

A Time Fairness-Based MAC Algorithm for Throughput Maximization in 802.11 Networks

This paper focuses on designing a distributed medium access control algorithm for fairly sharing network resources among contending stations in an 802.11 wireless network. Because the notion of fairness is not universal and there lacks a rigorous analysis on the relationships among the four types of most popular fairness criteria, we first mathematically prove that there exist certain connections between these types of fairness criteria. We then propose an efficient medium access algorithm that aims at achieving time fairness and throughput enhancement in a fully distributed manner. The core idea of our proposed algorithm lies in that each station needs to select an appropriate contention window size so as to fairly share the channel occupancy time and maximize the throughput under the time fairness constraint. The derivation of the proper contention window size is addressed rigorously. We evaluate the performance of our proposed algorithm through an extensive simulation study, and the evaluation results demonstrate that our proposed algorithm leads to nearly perfect time fairness, high throughput, and low collision overhead.

Tug-of-war Model for Competitive Multi-armed Bandit Problem: Amoeba-inspired Algorithm for Cognitive Medium Access

Tug-of-war Model for Competitive Multi-armed Bandit Problem: Amoeba-inspired Algorithm for Cognitive Medium Access

Analyzing and Evaluating Contention Access Period of Slotted CSMA/CA for IEEE802.15.4

IEEE standard 802.15.4 is widely studied due to its huge applicability. This vary study is based upon analyzing techniques and methodology discussed in IEEE 802.15.4 standard in context of contention access period. According to standard specifications, there are two minor differences in CSMA/CA algorithm used in CAP along with different frequency ranges. These frequency ranges are accepted in different geographical regions of this world. Considering medium access algorithm used, one flavor supports ACK frame after successful transmission and other do not. ACK mode of CSMA/CA is widely analyzed analytically and simulated in earlier conducted researches,however, there is no study discussing behaviour of non-ACK mode. This non-ACK mode is prescribed as a different flavor in IEEE 802.15.4 for applications that do not focus on taking ACK packet after every transmission. In this work, we modify a markov chain model for non-ACK mode and compare both modes analytically followed by extensive simulations and discussions. For health care applications ACK mode shows its worth however, in streaming data or playing games, non-ACK mode can be preferred due to its lower delay, higher throughput and lower control load.

Amoeba-inspired algorithm for cognitive medium access

tug-of-war (TOW) is a unique parallel search algorithm for solving the multi-armed bandit problem (BP), which was inspired by the photoavoidance behavior of a single-celled amoeboid organism, the true slime mold Physarum polycephalum (1-4). The cognitive medium access (CMA) problem, which refers to multiuser channel allocations of the cognitive radio, can be interpreted as a multi-armed bandit problem (CBP) (5, 6). Unlike the normal BP, the CBP considers a competitive situation in which more than one user selects a channel whose reward probability (probability of which channel is free) varies depending on the number and combination of the selecting users as indicated in a payoff matrix. Depending on the payoff matrix, the CBP provides a hard problem instance in which the users should not be attracted to the Nash equilibrium to achieve the maximum, which is the most desirable state to obtain the maximum total score (throughput) for all the users. In this study, we propose two variants of the TOW model (solid type and liquid type) for the CBP toward developing a CMA protocol using a distributed control in uncertain environments. Using the minimum CBP cases where both the users choose a channel from the two considered channels, we show that the performance of our solid-type TOW model is better than that of the well-known upper confidence bound 1 (UCB1)-tuned algorithm, particularly for the hard problem instances. aim of this study is to explore how the users can achieve the social maximum in a decentralized manner. We also show that our liquid-type TOW model, which introduces direct interactions among the users for avoiding mutual collisions, makes it possible to achieve the social maximum for general CBP instances.

Overview on Multi-Channel Communications in Wireless Sensor Networks

In this paper, we focus on multi-channel MAC protocols specifically designed for Wireless Sensor Networks (WSNs). The use of multiple channels helps overcome interferences and thus enhance the overall network performance. In order to establish channel and slot allocation to enable simultaneous transmissions, many protocols have been proposed in the literature. Some of them are based on a TDMA approach which offers a more deterministic behavior but requires a strict synchronization, while others are based on CSMA/CA which is an asynchronous medium access algorithm that suffers from collisions due to its probabilistic behavior. Different classifications have been proposed in previous survey papers depending on the periodicity of channel switching, on whether they are centralized or distributed, synchronized or asynchronous, etc. In this paper, we present a survey of the different techniques used in WSNs and that are applicable on the IEEE 802.15.4 standard. We detail some of the most popular protocols and discuss their advantages and weaknesses. We conclude the paper with an open discussion concerning future research directions.

Efficient Distributed Algorithms for Dynamic Access to Shared Multiuser Channels in SINR-Constrained Wireless Networks

In wireless networks, simultaneously active transmitters typically operate in separate communication channels to avoid mutual interference. This study focuses on the challenge of increasing the capacity of a wireless network by enabling multiple transmissions in each available channel. Active transmitters are assumed to maintain the receiver signal-to-noise-and-interference ratio (SINR) at a predetermined target value via power control to promote the quality of wireless connections. To this end, we propose distributed medium access algorithms that allow every transmitter-receiver pair to determine whether a target SINR is physically achievable through iterative power control in a given shared channel. The proposed algorithms are shown by theoretical analysis to be fast, accurate, and energy efficient. Numerical simulations demonstrate their ability to outperform related medium access schemes based on random access, carrier sensing, controlled power up, or invariant channel probing. Our major contribution consists of solving the open problem of accurate real-time computation of the spectral radius of an unknown network information matrix. This makes our framework applicable not only to testing target SINR achievability, but also to other aspects of wireless engineering such as energy efficiency, power control stability, and handover prioritization, in which knowledge of the spectral radius plays a key role.

Congestion control meets medium access

This paper looks at the problem of designing medium access algorithm for wireless networks with the objective of providing high throughput and low delay performance to the users, while requiring only a modest computational effort at the transmitters and receivers. Additive inter-user interference at the receivers is an important physical layer characteristic of wireless networks. Today's Wi-Fi networks are based upon the abstraction of physical layer where inter-user interference is considered as noise leading to the 'collision' model in which users are required to co-ordinate their transmissions through Carrier Sensing Multiple Access (CSMA)-based schemes to avoid interference. This, in turn, leads to an inherent performance trade-off [1]: it is impossible to obtain high throughput and low delay by means of low complexity medium access algorithm (unless P=NP). As the main result, we establish that this trade-off is primarily due to treating interference as noise in the current wireless architecture. Concretely, we develop a simple medium access algorithm that allows for simultaneous transmissions of users to the same receiver by performing joint decoding at receivers, over time. For a receiver to be able to decode multiple transmissions quickly enough, we develop appropriate congestion control where each transmitter maintains a "window" of undecoded transmitted data that is adjusted based upon the "feedback" from the receiver. In summary, this provides an efficient, low complexity "online" code operating at varying rate, and the system as a whole experiences only small amount of delay (including decoding time) while operating at high throughput.

Distributed Algorithms for Learning and Cognitive Medium Access with Logarithmic Regret

The problem of distributed learning and channel access is considered in a cognitive network with multiple secondary users. The availability statistics of the channels are initially unknown to the secondary users and are estimated using sensing decisions. There is no explicit information exchange or prior agreement among the secondary users and sensing and access decisions are undertaken by them in a completely distributed manner. We propose policies for distributed learning and access which achieve order-optimal cognitive system throughput (number of successful secondary transmissions) under self play, i.e., when implemented at all the secondary users. Equivalently, our policies minimize the sum regret in distributed learning and access, which is the loss in secondary throughput due to learning and distributed access. For the scenario when the number of secondary users is known to the policy, we prove that the total regret is logarithmic in the number of transmission slots. This policy achieves order-optimal regret based on a logarithmic lower bound for regret under any uniformly-good learning and access policy. We then consider the case when the number of secondary users is fixed but unknown, and is estimated at each user through feedback. We propose a policy whose sum regret grows only slightly faster than logarithmic in the number of transmission slots.