基于IEEE802.15.4退避算法的改进机制

IEEE802.15.4是供低速率、低功耗和低成本设备使用的短距离无线通信的协议,定义了低速无线个域网(LR-WPANs)的MAC层和物理层规范.由于在例如无线体域网(WBAN)应用中,能量消耗是备受关注的问题,因此提出了一种基于时槽机制的CSMA/CA过程中改进的退避算法,在每次CSMA/CA开始时根据过去的传输状况动态地调整竞争窗口的最小值.通过NS-2进行仿真,实验结果表明这样的改进方案在业务负载比较高或者包的大小比较小的时候可以减少数据包碰撞概率和重传概率,从而可以减少功耗并且提高吞吐量性能.     

An Improvement of Slotted CSMA/CA Algorithm in IEEE 802.15.4 Medium Access Layer

The standard of IEEE 802.15.4 is able to achieve low-power transmissions in low-rate and short-distance wireless personal area networks. It employs the slotted carrier sense multiple access with collision avoidance (CSMA/CA) for the contention mechanism. The blind backoff process in the slotted CSMA/CA will cause lower channel utilization. Sensor node performs backoff process immediately when the clear channel assessment (CCA) detecting busy channel. It may neglect the implicit information of CCA failed detection and further cause the redundant senses. This paper proposes an additional carrier sensing (ACS) algorithm based on IEEE 802.15.4 to enhance the carrier sensing mechanisms in original slotted CSMA/CA. An analytic model is developed to evaluate the performance of ACS algorithm. The analytical and simulation results demonstrate that the proposed scheme significantly improves the throughput, average medium access control delay and power consumption of CCA detecting.     

An Efficient Backoff Algorithm for IEEE 802.15.4 Wireless Sensor Networks

IEEE 802.15.4 is one of the most prominent MAC protocol standard designed to achieve low-power, low-cost, and low-rate wireless personal area networks. The contention access period of IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA) algorithm. A long random backoff time causes longer average delay, while a small one gives a high collision rate. In this paper, we propose an efficient backoff algorithm, called EBA-15.4MAC that enhances the performance of slotted CSMA/CA algorithm. EBA-15.4MAC is designed based on two new techniques; firstly, it updates the contention window size based on the probability of collision parameter. Secondly, EBA-15.4MAC resolves the problem of access collision via the deployment of a novel Temporary Backoff (TB) and Next Temporary Backoff (NTB). In this case, the nodes not choose backoff exponent randomly as mentioned in the standard but they select TB and NTB values which can be 10–50 % of the actual backoff delay selected by the node randomly. By using these two new methods, EBA-15.4MAC minimizes the level of collision since the probability of two nodes selecting the same backoff period will be low. To evaluate the performance of EBA-15.4MAC mechanism, the network simulator has been conducted. Simulation results demonstrate that the proposed scheme significantly improves the throughput, delivery ratio, power consumption and average delay.     

Short Clear Channel Assessment in Slotted IEEE 802.15.4 Networks

One of the major concerns in wireless sensor networks is energy saving due to lack of power sources. IEEE 802.15.4 provides CSMA/CA to reduce the possibility of a collision by checking whether the medium is in use. This checking operation is CCA (Clear Channel Assessment). Currently, IEEE 802.15.4 defines CCA as two consecutive channel assessment for slotted CSMA/CA. Obviously, always performing the channel assessment twice is wasteful and inefficient, since it requires more delay and energy consumption than when the channel assessment is performed once. In this paper, we propose a SCCA (Short Clear Channel Assessment) scheme which performs the channel assessment once, not twice, with minimal effort. Simulation results show that the SCCA operation provides reduced CCA processing time and energy consumption than when IEEE 802.15.4 CCA is used.     

Using Incompletely Cooperative Game Theory in Wireless Mesh Networks

Recently, game theory has become a useful and powerful tool in research on wireless mesh networks. In this article the authors present a novel concept of incompletely cooperative game theory and use it to improve the performance of MAC protocols in WMNs. In this game, first, each node estimates the current game state (e.g., the number of competing nodes). Second, the node adjusts its equilibrium strategy by tuning its local contention parameters (e.g., the minimum contention window) to the estimated game state. Finally, the game is repeated several times to get the optimal performance. To use the game effectively in WMNs, the authors present a hybrid CSMA/CA protocol by integrating a proposed virtual CSMA/CA and the standard CSMA/CA protocol. When a node has no packet to send, it contends for the channel in virtual CSMA/CA mode. In this way the node can estimate the game state and obtain the optimal strategy. When a node has packets to send, it contends for the channel in standard CSMA/CA mode with the optimal strategy obtained in virtual CSMA/CA mode, switching smoothly from virtual to standard CSMA/CA mode. At the same time, the node keeps adjusting its strategy to the variable game state. In addition, the authors propose a simplified game-theoretic MAC protocol (G-CSMA/CA) by designing an auto degressive backoff mechanism based on the incompletely cooperative game. G-CSMA/CA can easily be implemented in mesh nodes. Finally, simulation results show that the incompletely cooperative game can increase system throughput, decrease delay, jitter, and packet loss rate, and support the game effectively.     

A Dynamic Adaptation Mechanism for Traffic Conditions in Wireless Sensor Network

As overall network traffic pursue to expand, a lot of low-power medium access control protocols have been proposed to deal with burst traffic in wireless sensor network. Although most of them provide low throughput but do not well optimize the energy consumption. In this paper, we propose a new hybrid carrier sense multiple access with collision avoidance (CSMA/CA) and time division multiple access (TDMA) protocol that arranges nodes into two categories of priority according to their traffic rate and data transmission delay. Nodes that have continuous data should send its data during the contention free period, those one will be classified as low priority and its data will be scheduling using TDMA. Others nodes who have a random data should transmit it immediately during the contention access period (CAP) using a fuzzy logic algorithm, based on their queue length and implemented in the CSMA/CA algorithm. Therefore, the proposed scheme dynamically changes the CAP length to ensure that nodes can complete its transaction during the same super-frame. Simulation results are done using the network simulator tools (NS-2) and have improved good efficiency regarding the IEEE 802.15.4 standard. The mechanism has improved the energy consumption, minimised the packet loss probability, increased the throughput variation in the network and also minimised the average end to end delay.     

An adaptive CSMA/TDMA hybrid MAC for energy and throughput improvement of wireless sensor networks

IEEE 802.15.4 as a standard for low rate wireless personal area networks (LR-WPAN) is an applicative choice for implementation of wireless sensor networks. Due to the advantages of this standard and its capabilities for more specification to wireless sensor networks, we were persuaded to resolve some of its proven weaknesses in such environments. The slotted CSMA/CA method utilized in beacon-enabled mode of 802.15.4 causes unacceptable level of energy consumption and throughput in conditions like high loads. To overcome these issues, we proposed an adaptable CSMA/TDMA hybrid channel access method by applying some modifications to the 802.15.4 standard. The energy and throughput improvement is achieved by dedicating a part of the contention access period to a time division medium access protocol (TDMA). To evaluate our proposed method in comparison with 802.15.4, we developed a simulation in OMNeT++. Analysis of the simulation results indicates general improvement of energy consumption and throughput. As a sensor network grows more populated or the load increases, the proposed method shows a better performance in comparison with IEEE 802.15.4 standard.     

两种异构CSMA/CA机制OSTS/BSTS无线传感网络公平性、实时性分析比较

IEEE802.15.4作为一种专为低速率无线个人区域网络(WPAN)而设计的低成本、低功耗、低速率的短距离无线通信新标准,为无线传感器网络提供了一种很好的解决方案。本文针对异构、非饱和无线网络,提出了两种新的CS-MA/CA机制:OSTS/BSTS机制;异构节点数据到达率不同,其各自获取的吞吐量不同,由此分析获得异构网络节点的公平性。OSTS/BSTS机制最大的特点是异构节点被赋予了公平的机会来访问信道,不存在优先权等级的问题。这两种机制采用两个半马尔可夫链模型来分别表达两组节点的访问过程,一个宏观马尔可夫链模型来表达信道状态转换过程,结合队列理论模型来分析异构节点的延时量、吞吐量、传输概率等特性,以获取网络实时性、公平性理论模型,并采用NS-2仿真工具对分析结果进行了仿真。     

A dynamic ID management protocol for CSMA/IC in ad hoc networks

Contention based MAC protocols are widely used in ad hoc networks because they are suitable, where no central control node exists. However, contention based MAC protocols waste much time because of frequent collisions and long contention times. Moreover, it is hard for them to fairly distribute medium access opportunities. As a result, the problem of unfair medium access may arise under normal network conditions. Recently, another contention based MAC protocol, named the Carrier Sense Multiple Access/ID Countdown (CSMA/IC) was proposed. CSMA/IC resolves medium access contention by comparing the IDs of contending nodes with a simple signaling process. Therefore, medium access collisions never happen as long as each node possesses a unique ID, and the time cost for contention may be smaller than any other contention based MAC protocols if the number of IDs is managed so as to be as small as possible. Furthermore, CSMA/IC may support fair medium access by manipulating the ID of each node properly. In this paper, we propose a novel dynamic ID management protocol which enables a node to acquire a unique ID without any message exchanges and fairly distributed the number of medium access opportunities to all contending nodes. The proposed protocol also makes the contention process of CSMA/IC efficient by dynamically managing the length of the ID field according to the network traffic. The simulation results show that the proposed ID management protocol significantly improves the aforementioned aspects of CSMA/IC MAC protocol compared to previous ID management schemes.     

A successive transmission medium access scheme with dynamic contention window for VLC system with saturated traffic

The visible light communication (VLC) network is usually relatively small scale and can provide high-data-rate information transmission, where multiple users get access to the network according to the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism specified by IEEE 802.15.7 standard. In this paper, we propose a novel dynamic contention window with successive transmission (DCW-ST) scheme to improve the performance of this channel access mechanism and to achieve better network throughput without delay performance degradation. Specifically, we propose to adjust the contention window dynamically to adapt to the time-changing network size. Further, we derive the contention window size to achieve trade-off of throughput and delay, and the minimum contention window size required for the throughput enhancement. In addition, in order to further improve the delay performance, we present a successive transmission scheme that allows the nodes which have completed one transmission successfully to get the chance of transmitting information successively according to the network condition. Simulations are performed for the VLC system in saturated traffic and compared with the theoretical performances, which demonstrate that our proposed scheme outperforms the legacy CSMA/CA of IEEE 802.15.7.     

无线传感器网络突发感知的拥塞控制协议(英文)

In monitoring Wireless Sensor Networks(WSNs),the traffic usually has bursty characteristics when an event occurs.Transient congestion would increase delay and packet loss rate severely,which greatly reduces network performance.To solve this problem,we propose a Burstiness-aware Congestion Control Protocol(BCCP) for wireless sensor networks.In BCCP,the backoff delay is adopted as a congestion indication.Normally,sensor nodes work on contention-based MAC protocol(such as CSMA/CA).However,when congestion occur...     

Performance of Carrier Sense Multiple Access with Collision Avoidance Protocols in Wireless LANs

The performance of Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocols, which is adopted as a draft standard in IEEE 802.11, is analyzed in the view of throughput and packet delay. We consider three kinds of CSMA/CA protocols, which include Basic, Stop-and-Wait and 4-Way Handshake CSMA/CA, and introduce a theoretical analysis for them. First, we consider that a network consists of a finite population and then expand to an infinite population model. We model the CSMA/CA protocol as a hybrid protocol of a 1-persistent CSMA and a p-persistent CSMA protocol. We calculate the throughput and packet delay for three kinds of CSMA/CA protocols and verify analytical results by computer simulation. We have found that 4-Way Handshake CSMA/CA shows better performance than those of other two type CSMA/CA in high traffic load and analytical results are very close to simulation ones.     

Buffer Sizing for TCP Flows in 802.11e WLANs

We consider the task of sizing buffers for TCP flows in 802.11e WLANs. A number of fundamental new issues arise compared to wired networks. These include that the mean service rate is dependent on the level of channel contention and packet inter-service times vary stochastically due to the random nature of CSMA/CA operation. We find that these considerations lead naturally to a requirement for adaptation of buffer sizes in response to changing network conditions.     

Sticky CSMA/CA: Implicit synchronization and real-time QoS in mesh networks

We propose a novel approach to QoS for real-time traffic over wireless mesh networks, in which application layer characteristics are exploited or shaped in the design of medium access control. Specifically, we consider the problem of efficiently supporting a mix of Voice over IP (VoIP) and delay-insensitive traffic, assuming a narrowband physical layer with CSMA/CA capabilities. The VoIP call carrying capacity of wireless mesh networks based on classical CSMA/CA (e.g., the IEEE 802.11 standard) is low compared to the raw available bandwidth, due to lack of bandwidth and delay guarantees. Time Division Multiplexing (TDM) could potentially provide such guarantees, but it requires fine-grained network-wide synchronization and scheduling, which are difficult to implement. In this paper, we introduce Sticky CSMA/CA, a new medium access mechanism that provides TDM-like performance to real-time flows without requiring explicit synchronization. We exploit the natural periodicity of VoIP flows to obtain implicit synchronization and multiplexing gains. Nodes monitor the medium using the standard CSMA/CA mechanism, except that they remember the recent history of activity in the medium. A newly arriving VoIP flow uses this information to grab the medium at the first available opportunity, and then sticks to a periodic schedule, providing delay and bandwidth guarantees. Delay-insensitive traffic fills the gaps left by the real-time flows using novel contention mechanisms to ensure efficient use of the leftover bandwidth. Large gains over IEEE 802.11 networks are demonstrated in terms of increased voice call carrying capacity (more than 100% in some cases). We briefly discuss extensions of these ideas to a broader class of real-time applications, in which artificially imposing periodicity (or some other form of regularity) at the application layer can lead to significant enhancements of QoS due to improved medium access.     

Analyzing split channel medium access control schemes

In this work, we analyze and evaluate the maximum achievable throughput of split-channel MAC schemes that are based on the RTS/CTS (ready-to-send/clear-to-send) dialogue and that rely on pure ALOHA or on p-persistent carrier sensing multiple access (CSMA) contention resolution techniques. Our results show that, when radio propagation delays are negligible and when the pure ALOHA mechanism is used, then for a network with relatively large number of nodes, the maximum achievable throughput of the split-channel MAC schemes is lower than that of the corresponding single-channel MAC schemes. When the split-channel MAC schemes employ the p-persistent CSMA mechanism, then they out-perform the corresponding single-channel schemes when the maximum end-to-end propagation delays are at least 25% of the transmission time of the control packets on the single shared channel.     

A Game-Theoretic Study of CSMA/CA Under a Backoff Attack

CSMA/CA, the contention mechanism of the IEEE 802.11 DCF medium access protocol, has recently been found vulnerable to selfish backoff attacks consisting in nonstandard configuration of the constituent backoff scheme. Such attacks can greatly increase a selfish station's bandwidth share at the expense of honest stations applying a standard configuration. The paper investigates the distribution of bandwidth among anonymous network stations, some of which are selfish. A station's obtained bandwidth share is regarded as a payoff in a noncooperative CSMA/CA game. Regardless of the IEEE 802.11 parameter setting, the payoff function is found similar to a multiplayer Prisoners' Dilemma; moreover, the number (though not the identities) of selfish stations can be inferred by observation of successful transmission attempts. Further, a repeated CSMA/CA game is defined, where a station can toggle between standard and nonstandard backoff configurations with a view of maximizing a long-term utility. It is argued that a desirable station strategy should yield a fair, Pareto efficient, and subgame perfect Nash equilibrium. One such strategy, called CRISP, is described and evaluated     

移动Ad HOc网络中的公平按需多址接入协议

基于快速、有效竞争预约接入、无冲突轮询传输的思想和带冲突预防的冲突分解策略,本文提出了适于移动Ad Hoc网络的公平按需多址接入(FODA)协议.该协议在分群结构的基础上,利用公平冲突预防算法预约信道资源获得轮询服务,从而完全消除了载波侦听方式下多跳无线网络业务传输中的隐藏终端和暴露终端问题.另外,公平冲突预防算法解决了节点竞争接入时的冲突问题和不公平现象.最后,仿真结果表明,与带冲突避免的载波侦听多址接入(CSMA/CA)和轮询协议相比,FODA协议可以提供较高的信道吞吐量、较低的平均消息丢弃率和平均消息时延.     

Collision avoidance in TV white spaces: a cross-layer design approach for cognitive radio networks

One of the most promising applications of cognitive radio networks (CRNs) is the efficient exploitation of TV white spaces (TVWSs) for enhancing the performance of wireless networks. In this paper, we propose a cross-layer design (CLD) of carrier sense multiple access with collision avoidance (CSMA/CA) mechanism at the medium access control (MAC) layer with spectrum sensing (SpSe) at the physical layer, for identifying the occupancy status of TV bands. The proposed CLD relies on a Markov chain model with a state pair containing both the SpSe and the CSMA/CA from which we derive the collision probability and the achievable throughput. Analytical and simulation results are obtained for different collision avoidance and SpSe implementation scenarios by varying the contention window, back off stage and probability of detection. The obtained results depict the achievable throughput under different collision avoidance and SpSe implementation scenarios indicating thereby the performance of collision avoidance in TVWSs-based CRNs.     

Variable Beam Width Selection for Improving the Throughput of IEEE 802.11ad Wireless LAN in the Contention Based Access Period

The hybrid MAC protocol specified by IEEE 802.11ad for millimeter wave wireless LANs consist of carrier sense multiple access/collision avoidance (CSMA/CA) during the contention based access periods (CBAPs) and TDMA during the service periods. To provide channel access during CBAP, the coverage area around the access point (AP) can be divided into several quasi omni (QO) beam levels. When uplink channel access during CBAP is considered, every directional multigigabit station residing within a QO level uses CSMA/CA protocol for getting the transmission opportunity. With equal beam width receive QO levels at the AP, we present an analytical model to compute the uplink throughput of the network in the CBAP, by closely following the 802.11ad MAC protocol specifications. We demonstrate that PHY layer MCS (modulation and coding scheme) dependent adaptive selection of QO levels can improve the throughput performance. In the second part of the paper, we consider that PCP/AP can have at most three radios, each tuned to operate in non-overlapping frequency bands as specified by 802.11ad PHY. We establish that such an arrangement can lead to concurrent transmissions in the network and improve the uplink throughput performance.     

基于ZigBee信标模式的无线语音通信研究

ZigBee技术以其低功耗,高网络容量和灵活的自组网能力适合于很多无线通信场景。对ZigBee网络信标模式下,CSMA/CA和GTS两种传输机制分别进行仿真建模,研究在不同语音编码速率时,星型网络能容纳的语音节点数。仿真结果表明:在CSMA/CA方式下,语音编码速率依次为5.15,16和32 kb/s时,ZigBee网络能容纳的节点数分别为8,4和3;在GTS方式下,端节点数超过4时,网络丢包率和吞吐量均优于CSMA/CA,能容纳至多7个端节点同时通信。