Contention Window Value Research Articles

Performance Analysis of Different Backoff Algorithms for WBAN-Based Emerging Sensor Networks

The Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) procedure of IEEE 802.15.6 Medium Access Control (MAC) protocols for the Wireless Body Area Network (WBAN) use an Alternative Binary Exponential Backoff (ABEB) procedure. The backoff algorithm plays an important role to avoid collision in wireless networks. The Binary Exponential Backoff (BEB) algorithm used in different standards does not obtain the optimum performance due to enormous Contention Window (CW) gaps induced from packet collisions. Therefore, The IEEE 802.15.6 CSMA/CA has developed the ABEB procedure to avoid the large CW gaps upon each collision. However, the ABEB algorithm may lead to a high collision rate (as the CW size is incremented on every alternative collision) and poor utilization of the channel due to the gap between the subsequent CW. To minimize the gap between subsequent CW sizes, we adopted the Prioritized Fibonacci Backoff (PFB) procedure. This procedure leads to a smooth and gradual increase in the CW size, after each collision, which eventually decreases the waiting time, and the contending node can access the channel promptly with little delay; while ABEB leads to irregular and fluctuated CW values, which eventually increase collision and waiting time before a re-transmission attempt. We analytically approach this problem by employing a Markov chain to design the PFB scheme for the CSMA/CA procedure of the IEEE 80.15.6 standard. The performance of the PFB algorithm is compared against the ABEB function of WBAN CSMA/CA. The results show that the PFB procedure adopted for IEEE 802.15.6 CSMA/CA outperforms the ABEB procedure.

Fair Downlink Traffic Management for Hybrid LAA-LTE/Wi-Fi Networks

Due to the scarcity of the licensed spectrum allocated for mobile communication systems, licensed-assisted access long-term evolution (LAA-LTE) network is recently proposed to deploy in unlicensed spectrum, which is currently occupied by different Wi-Fi systems. It is a very challenging problem to ensure fair coexistence between LAA-LTE and Wi-Fi networks, in terms of spectrum sharing and traffic management. To solve this problem, a fair downlink traffic management (FDTM) scheme is proposed in this paper for hybrid LAA-LTE/Wi-Fi networks. FDTM aims to tune the minimum contention window ( $CW_{min}$ ) values and assigns feasible weights for the LAA eNBs with different traffic loads, thus to achieve; 1) fair spectrum sharing with the coexisting Wi-Fi networks in unlicensed spectrum and 2) fair service differentiation for downlink LAA-LTE traffic. Numerical results show our FDTM scheme can guarantee the throughput performance of Wi-Fi networks in shared unlicensed spectrum while supporting proportional fairness for the LAA eNBs with different traffic loads.

MAC Performance Analysis for Drive-Thru Internet Networks With Rayleigh Capture

In practical radio transmissions, channel capture is a dominating factor that affects wireless network performance. The capture effect can occur in wireless network when packets arrive with different powers. Packets with high power can effectively swamp low power packets, such that they are received successfully, when otherwise a collision would have occurred. We present a vehicular network performance-prediction model for a Rayleigh capture channel in Drive-thru Internet scenario. The model incorporates the capture effect into a 2-D Markov chain modeling the high-node mobility and distributed coordination function broadcast scheme. The performance-prediction model unveils the impacts of mobility velocity and number of vehicles on the throughput in a Rayleigh capture channel. We use a vehicular traffic flow model to predict vehicular movement on road by aggregating all vehicles into a flow. Simulation results confirm that our performance-prediction model accurately predicts the performance of traveling vehicles with Rayleigh capture channel in the Drive-thru Internet scenario. We demonstrate that using our performance-prediction model, we can obtain optimal contention window value, by which the best system throughput can be reached without wasting contention time. This is also proved by Anastasi et al.

Priority-Based Data Gathering Framework in UAV-Assisted Wireless Sensor Networks

This paper proposes a novel data acquisition framework in sensor networks using an unmanned aerial vehicle (UAV) with the goal of increasing the efficiency of the data gathering efforts. To maximize the system throughput, we introduce a priority-based frame selection scheme to suppress the number of redundant data transmissions between sensor nodes and the UAV. Toward this goal, we classify the nodes inside the UAV's coverage area into different frames according to their locations. Taking advantage of the mobility of the UAV, we assign different transmission priorities to nodes in different frames. To do that, we introduce an adjustment to the contention window value used in IEEE 802.11 MAC, thereby defining a lower contention window range to the frame with higher priority (urgent area) and a higher contention window range to the frame with lower priority (less important area). The proposed framework leads to a reduction in packet collisions and, at the same time, minimizes the packet loss originated from nodes in the rear-side of the UAV when the UAV moves in the forward direction. To optimize the networks' energy consumption, we present a novel routing protocol based on the aforementioned framework. By leveraging the proposed framework and routing algorithm, we aim to reduce the transmission distances between senders and receivers. A shorter distance leads to better channel quality and energy savings, as is verified by our simulation studies and results.

An adaptive traffic MAC protocol based on correlation of nodes

EA-MAC protocol is proposed on the basis of SMAC protocol to remedy the shortcomings of SMAC. In the EA-MAC, node correlation analysis algorithm and traffic adaptive duty cycle mechanism are added. Meanwhile, the residual energy is introduced into the existing traffic adaptive back-off mechanism to measure the saving efficiency. In the node correlation algorithm, all network nodes are divided into several areas by computing node correlation according to the collected information. The clustering mechanism is applied for choosing representative node in each area for transmitting data. This method can effectively reduce redundant nodes transmitting duplicate data. In traffic adaptive duty cycle mechanism, the duty cycle is regulated dynamically to decrease idle listening by comparing the threshold set with the flow value obtained from the predict flow model. In back-off mechanisms, by adjusting the value of contention window and back-off time, data collisions can be effectively avoided when network traffic is heavy. In addition, nodes with more remaining energy have priority to access the channel and have shorter back-off time, which can keep the balance of the whole network energy consumption and lengthen network lifetime. Simulation results show that the EA-MAC protocol has better energy saving, throughput, shorter delay performance, and low packet loss rate than that of SMAC protocol in dynamic traffic sensor networks.

Appraisement of IEEE 802.11s based Mesh Networks with Mean Backoff Algorithm

Wireless mesh networks, consisting of mesh routers and mesh clients are very robust, reliable and easily maintained networks. The current work is based on IEEE 802.11 standard amendment IEEE 802.11s specific for mesh topology based networks to improve the connectivity and coverage. It aims to control the shared medium access among the stations using Distributed Coordination Function (DCF) MAC protocol for reducing collisions and delays, thereby increasing throughput of the network.. The actual Binary Exponential Backoff (BEB) algorithm as implemented in IEEE 802.11 resets the contention window to its least value after an acknowledged transmission. From the previous works it has been observed that this sudden reset to minimum value of contention window does not ensure a reduction in collisions. It may lead to more contention in the network thereby increasing delays and affecting its throughput. The proposed work presents a Mean Backoff Algorithm in order to solve this flaw of BEB algorithm. The proposed algorithm aims to bring the contention window to some appropriate value in order to cope up with the unfairness caused due to its minimum value on successful transmission.

Improving aggregate utility in IEEE 802.11p based vehicle-to-infrastructure networks

IEEE802.11p, also known as wireless access in vehicular environment defines amendments to IEEE 802.11 to support intelligent transportation systems applications, by enabling both vehicle-to-vehicle and vehicle-to-infrastructure (V2I) communications. The medium access control layer in IEEE 802.11p is based on IEEE 802.11e enhanced distributed channel access, while the physical layer is based on IEEE 802.11a standard. This paper investigates the problem of improving the aggregate utility in IEEE 802.11p based V2I networks, while ensuring fairness among the competing vehicles. Firstly, we consider a V2I network in drive-thru Internet scenario, formed by vehicles moving on a multi-lane highway with different mean velocities in different lanes, in which all the competing vehicles use the same data rate. For error-prone channels, we derive analytical expressions for the class specific optimal minimum contention window ($$CW_{\min }$$CWmin) values that simultaneously maximize the aggregate data transferred and provide fairness among vehicles belonging to distinct mean velocity classes in the sense of equal chance of communicating with the road side units. We also obtain an analytical expression for the maximum aggregate data transferred in the presence of channel error. In the second part, we extend the analytical model to compute the amount of successfully transferred data in a multi-rate multi-lane V2I network. In addition to the unfairness problem caused by the distinct velocities, vehicles in such networks suffer from a performance anomaly problem as well, due to the use of distinct data rates. We determine analytical expressions for the $$CW_{\min }$$CWmin values required to simultaneously resolve both the problems. Results show that, with proper tuning of $$CW_{\min }$$CWmin the aggregate data transferred in the network improves significantly. The analytical results are corroborated using extensive simulation studies.

Effective Backoff Scheme to ease the Network Congestion on IEEE 802.11 DCF

Since all transmitting stations in the range share the wireless channel, the IEEE 802.11 MAC uses the Binary Exponential Backoff (BEB) algorithm to minimize collision. The BEB technique resets the Contention Window values after each successful frame transmission regardless of the network state, and a sudden CW decrease incurs performance degradation as the number of competing stations increases. To resolve this problem, we propose a new algorithm that adjusts CW values using (m,k)-firm scheduling technique. The proposed algorithm changes CW using the violating distance instead of resetting the CW to minimum value after every successful transmission. The performance results are shown to identify the improvement of our scheme in terms of waiting time comparing to the BEB.

무선 메쉬 네트워크에서 적응적 경쟁 윈도우를 이용한 TCP Fairness 개선 방안

Wireless Mesh Networks(WMNs) is generally composed of radio nodes in the mesh topology. WMNs consists of mesh client, mesh router and gateway connected to a wired network. Each client and router relay messages to the gateway for communication. WMNs is widely used recently in many areas can provide extended coverage based on multi-hop communication and ubiquitous communication at any time and any location. However the competition and collision between each node to transmit data is inevitable when the same channel is used for transmission. The transmission opportunities and the throughput of nodes located far from gateway decrease more if the communication channel is accessed based on competitive CSMA/CA scheme using DCF(Distributed Coordination Function) provided by IEEE 802.11 MAC. In this paper, we improve the performance of the TCP fairness and throughput of the nodes with more than 2 hops by applying various algorithms for controlling contention window values. Also, we evaluate the performance using ns-2 simulator, According to the results, proposed scheme can enhance the fairness characteristic of each node irrespective of data to the gateway.

A Cross-Layer Approach to Reduce Channel Access Delay Jitter in IEEE 802.11 WLANs

In this paper, we propose a consistent random backoff (CRB) scheme to reduce the channel access delay jitter in voice over wireless local area networks. In the CRB scheme, a contention window (CW) size at each backoff stage is determined by hashing the session identifier and the talk spurt index. Therefore, all packets in the same talk spurt of a session have the same CW sizes if they are transmitted at the same backoff stage. Since a modulo-division operation with the identical maximum CW value is applied, fairness with the legacy backoff scheme (i.e., binary exponential backoff) is also provided. Extensive simulation results demonstrate that the CRB scheme can reduce the channel access delay jitter by 54 %.

MAC Protocol for Ad Hoc Networks Using Smart Antennas for Mitigating Hidden and Deafness Problems

We herein propose a MAC protocol for the smart antenna network, which applies pulse/tone exchange prior to the RTS/CTS handshake process. RTS frame collisions are drastically reduced with little additional overhead due to pulse/tone exchange in the proposed protocol. In addition, the number of exposed nodes is reduced by using smart antennas. Furthermore, since the occurrence of the deafness problem can be identified by pulse/tone exchange failure, retransmission is conducted using a fixed contention window value. Therefore, the wastage of wireless resources due to the deafness problem is reduced. As a result, the network throughput can be effectively improved compared with that for previous protocols. Simulation results demonstrate the validity and effectiveness of the proposed protocol. key words: smart antennas, ad hoc networks, DMAC, pulse/tone

Arbitration Interframe Space-controlled Medium Access Control: a medium access control protocol guaranteeing absolute priority in wireless local area networks

SUMMARY The demand for multimedia services, such as voice over Internet Protocol, video on demand, information dissemination, and file sharing, is increasing explosively in wireless local area networks. These multimedia services require a certain level of QoS. Thus, it is important to provide QoS for multimedia applications. IEEE 802.11e tries to meet the QoS requirement of multimedia services by using Enhanced Distributed Channel Access. This gives more weights to high-priority traffic than low-priority traffic in accessing the wireless channel. However, Enhanced Distributed Channel Access suffers from many problems such as low aggregate throughput, high collision rates, and ineffective QoS differentiation among priority classes. In this paper, we propose a new medium access scheme, the Arbitration Interframe Space-controlled Medium Access Control (AC-MAC), that guarantees absolute priority in 802.11 wireless networks. In AC-MAC, the AIFS and contention window values are controlled, so that a higher-priority traffic can preferentially access and effectively utilize the channel. Extensive simulations show that AC-MAC can perfectly provide absolute priority and good throughput performance regardless of the number of contending nodes. In the simulation of voice over Internet Protocol service, AC-MAC provides effective QoS differentiation among services and also meets the high level of QoS requirements. AC-MAC also adapts quickly in a dynamic environment and provides good fairness among the nodes belonging to the same priority class. Copyright © 2011 John Wiley & Sons, Ltd.

IEEE 802.11 무선랜의 업링크와 다운링크간 공평성 향상을 위한 Contention Window 조절 알고리즘

본 논문은 IEEE 802.11 무선랜에서의 업링크와 디운링크 플로우간의 공평성 문제를 다룬다. 기존 연구에서는 AP의 최소 Contention Window (CWmin) 값을 단말의 CWmin 값보다 작게 설정하여 무선랜에서의 업링크와 다운링크 트래픽 간의 공평한 채널 사용을 보장하고자 하였다. 기존 논문의 해결책과 달리 본 논문에서 제안하는 알고리즘은 공평성 개선뿐만 아니라 동시에 시스템의 전체 throughput을 높일 수 있는 CWmin 값을 찾는 것을 목적으로 한다. 이를 위해 제안 알고리즘에서는 AP가 다운링크 플로우를 가지고 있는 단말의 수와 업링크 플로우을 가지고 있는 단말의 수를 실시간으로 확인하고 최적의 CWmin 값을 계산하여 단말에게 알린다. 실험 결과를 통해 제안 알고리즘이 단말간의 공평한 채널 사용을 보장함과 동시에 시스템의 throughput을 향상시키는 것을 확인할 수 있었다. This paper addresses the fairness issue between uplink and downlink traffic in IEEE 802.11 WLANs. Some solutions in existing work try to solve this issue by giving smaller minimum contention window (CWmin) value to an AP compared to stations. In contrast to the existing solutions, a proposed algorithm in this paper aims at finding CWmin values that not only provides fairness between uplink and downlink traffic among stations but also achieves high throughput. For this, in the proposed algorithm, an AP checks the number of stations that have uplink and downlink traffic, respectively. Based on this information, the AP calculates optimal CWmin values and announces it to stations. Our simulation results show that the proposed algorithm outperforms existing algorithms in terms of fairness and throughput.

A utility-optimal backoff algorithm for wireless sensor networks

A novel backoff algorithm in CSMA/CA-based medium access control (MAC) protocols for clustered sensor networks was proposed. The algorithm requires that all sensor nodes have the same value of contention window (CW) in a cluster, which is revealed by formulating resource allocation as a network utility maximization problem. Then, by maximizing the total network utility with constrains of minimizing collision probability, the optimal value of CW (Wopt) can be computed according to the number of sensor nodes. The new backoff algorithm uses the common optimal value Wopt and leads to fewer collisions than binary exponential backoff algorithm. The simulation results show that the proposed algorithm outperforms standard 802.11 DCF and S-MAC in average collision times, packet delay, total energy consumption, and system throughput.

A simple recursive scheme for adjusting the contention window size in IEEE 802.11e wireless ad hoc networks

The IEEE 802.11e standard has been introduced recently for providing Quality of Service (QoS) capabilities in the emerging wireless local area networks. This standard introduces a contention window based Enhanced Distributed Channel Access (EDCA) technique that provides a prioritized traffic to guarantee the minimum bandwidth needed for time critical applications. However, the EDCA technique resets the Contention Window (CW) of the mobile station statically after each successful transmission. This static behavior does not adapt to the network state since it reduces the network usage and results in bad performance and poor link utilization whenever the demand for link utilization increases. This paper proposes a new adaptive differentiation technique for IEEE 802.11e Wireless Local Area Networks that takes into account the network state before resetting the contention window. In the new technique, the congestion level of the network is sensed by using previous CW values. Three other enhancement techniques that focus on network adaptation are also discussed. Their main limitations are the high complexity of the implemented algorithms and their slow adaptation to the network state when the channel experiences bursty traffic. The proposed technique is compared to the original differentiation techniques of IEEE 802.11a and IEEE 802.11e standards, as well as to the enhancement schemes. Results show that the proposed adaptive technique outperforms IEEE 802.11e and is comparable to the other enhancement schemes while maintaining relatively low complexity requirements.