Combining contention‐based access and dynamic service period allocation for performance improvement in IEEE 802.11ad mmWave WLAN

In IEEE 802.11ad millimeter wave wireless LANs, the directional multi‐giga bit stations (DMGSTAs) use contention‐based access periods (CBAPs) and scheduled service periods (SPs) for medium access. The STAs carrying non‐QoS traffic use CBAPs, while SPs are allocated for STAs carrying high QoS applications. During CBAP, the STAs use enhanced distributed channel access (EDCA) scheme. Further, 802.11ad advocates dynamic allocation of SP for guaranteed data transmission based on a centralized polling scheme. The standard advocates that the coverage area around the access point can be divided into several sectors, and the total CBAP shall be divided among the sectors on a time sharing basis. The STAs residing within a sector simultaneously contend during the CBAP fraction of that sector. However, such STAs have to defer their transmission attempts and wait for the designated CBAP fraction of the succeeding beacon interval (BI), if the residual time in the current CBAP fraction is not sufficient for a frame transmission. This leads to very high delay and reduced throughput. The objective of this paper is to propose an efficient hybrid medium access control scheme, where the deferred STAs during CBAP are scheduled again by utilizing the unallocated slots in the SP of the same BI. We describe an analytical model for the throughput and the average frame delay, under the proposed scheme, and compare the performance against the legacy scheme, where the STAs use CBAP alone for channel access. The analytical and simulation results establish that the proposed scheme significantly improves the throughput and reduces the average frame delay.     

Model-based admission control for IEEE 802.11e enhanced distributed channel access

IEEE 802.11e enhanced distributed channel access (EDCA) is a distributed medium access scheme based on carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this paper, a model-based admission control (MBAC) scheme that performs real-timely at medium access control (MAC) layer is proposed for the decision of accepting or rejecting requests for adding traffic streams to an IEEE 802.11e EDCA wireless local area network (WLAN). The admission control strategy is implemented in access point (AP), which employs collision probability and access delay measures from active flows to estimate throughput and packet delay of each traffic class by the proposed unsaturation analytical model. Simulation results prove accuracy of the proposed analytical model and effectiveness of MBAC scheme.     

Performance analysis of the IEEE 802.11 distributed coordinationfunction

The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS/CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol     

Queuing Scheme for Improved Downlink Throughput on WLANs

A transmission queuing scheme is described that increases downlink throughput on wireless local area networks (WLANs) while also increasing the total throughput. When the amount of uplink traffic increases on a WLAN, the carrier sense multiple access with collision avoidance (CSMA/CA) protocol, which is the prescribed scheme for IEEE 802.11 WLAN channel access, may substantially reduce the rate of downlink data frame transmission. This results in severe throughput degradation for mobile stations with downlink traffic. The proposed scheme comprises a transmission control function based on consecutive transmission, as described in the IEEE 802.11e standard, and a dynamic queue prioritization algorithm. Simulation results demonstrate that the proposed scheme increases the maximum total throughput for uplink and downlink traffic by 17% compared with the conventional distributed coordination function (DCF) scheme and that it reduces the difference between uplink and downlink throughput. In an environment where transmission errors occur, the difference in throughput is reduced by about 50% compared with the conventional schemes.     

IEEE 802.11n: enhancements for higher throughput in wireless LANs

This article introduces a new standardization effort, IEEE 802.11n, an amendment to IEEE 802.11 standards that is capable of much higher throughputs, with a maximum throughput of at least 100 Mb/s, as measured at the medium access control data services access point. The IEEE 802.11n will provide both physical layer and MAC enhancements. In this article we introduce some PHY proposals and study the fundamental issue of MAC inefficiency. We propose several MAC enhancements via various frame aggregation mechanisms that overcome the theoretical throughput limit and reach higher throughput. We classify frame aggregation mechanisms into many different and orthogonal aspects, such as distributed vs. centrally controlled, ad hoc vs. infrastructure, uplink vs. downlink, single-destination vs. multi-destination, PHY-level vs. MAC-level, single-rate vs. multirate, immediate ACK vs. delayed ACK, and no spacing vs. SIFS spacing.     

A Scalable CSMA and Self-Organizing TDMA MAC for IEEE 802.11 p/1609.x in VANETs

vehicular ad hoc networks (VANETs) have been a key topic for research community and industry alike. The wireless access in vehicular environment standard employs the IEEE 802.11p/1609.4 for the Medium Access Control (MAC) layer implementation for VANETs. However, the carrier sense multiple access (CSMA) based mechanism cannot provide reliable broadcast services, and the multi-channel operation defined in IEEE 1609.4 divides the available access time into fixed alternating control channel intervals (CCH) and service channel (SCH) intervals, which may lead to the low utilization of the scarce resources. In this paper, a novel multichannel MAC protocol called CS-TDMA considering the channel access scheduling and channel switching concurrently is proposed. The protocol combines CSMA with the time division multiple access (TDMA) to improve the broadcast performance in VANETs. Meanwhile, the dwelling ratio between CCH and SCH changes dynamically according to the traffic density, resulting in the improvement of resource utilization efficiency. Simulation results are presented to verify the effectiveness of our mechanism and comparisons are made with three existing MAC protocols, IEEE MAC, SOFT MAC and VeMAC. The simulation results demonstrate the superiority of CS-TDMA in the reduction of transmission delay and packet collision rate and improvement of network throughput.     

Performance Analysis for a New Medium Access Control Protocol in Wireless LANs

One fundamental issue in high-speed wireless local area networks (LANs) is to develop efficient medium access control (MAC) protocols. In this paper, we focus on the performance improvement in both MAC layer and transport layer by using a novel medium access control protocol for high-speed wireless LANs deploying carrier sense multiple access/collision avoidance (CSMA/CA). We first present a recently proposed distributed contention-based MAC protocol utilizing a Fast Collision Resolution (FCR) algorithm and show that the proposed FCR algorithm provides high throughput and low latency while improving the fairness performance. The performance of the FCR algorithm is compared with that of the IEEE 802.11 MAC algorithm via extensive simulation studies on both MAC layer and transport layer. The results show that the FCR algorithm achieves a significantly higher efficiency than the IEEE 802.11 MAC and can significantly improve transport layer performance.     

基于CC2500的Ad Hoc开发平台MAC协议接口设计

随着Ad Hoc网络的发展和应用,MAC协议成为研究的热点之一。文中参考广泛应用的IEEE02、11MAC（媒体访问控制）协议,以CC2500射频收发芯片作为物理层模块,在自主设计的Ad Hoc开发平台上实现了CSMA／CA（带避撞的载波侦听多路接入）。由于物理层模块的特性对MAC层设计影响,设计了MAC层与物理层的接口,还设计了MAC帧格式和帧间隔。CSMA／CA经实验证明高效可靠。这一设计具有广阔的应用前景,同时,通过在真实的物理环境下对MAC协议性能进行测试,有利于加深对MAC协议的理解,并且对协议的研究和改进具有重要的意义。     

Saturation throughput analysis of a carrier sensing based MU-MIMO MAC protocol in a WLAN under fading and shadowing

In wireless local area networks (WLANs), the traditional carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol cannot use the full benefits from multiuser multiple-input multiple-output (MU-MIMO) technique due to random medium access of the users. In this paper, we propose a carrier sensing based MAC protocol for a MU-MIMO based WLAN with full utilization of MU-MIMO technique. By modeling the WLAN system under the proposed MAC protocol as a discrete time Markov chain, we develop an analytical model for computing the saturation throughput in presence of path loss, Rayleigh fading and log-normal shadowing. The analytical model is then validated via simulation. By means of numerical and simulation results, we demonstrate that the proposed MAC protocol significantly improves throughput performance than the traditional CSMA/CA MAC protocol. Further, we compare the performance of the proposed MAC protocol with a MU-MIMO MAC protocol called Uni-MUMAC protocol and find that the proposed MAC protocol performs better than the Uni-MUMAC protocol. We also explore the effect of some of the network and wireless channel parameters on the performance of the proposed MAC protocol.

     

CLACK: Cross-layer ACK-Aided Opportunistic Transmission in Wireless Networks

Wi-Fi has gained tremendous attention from the research community, yielding successful technological advancements. However, the data throughput efficiency (the ratio of application throughput to the maximum achievable physical data rate) degrades rapidly as the PHY data rate increases when using the current 802.11 medium access control (MAC) protocol. To address this MAC inefficiency, many protocols have been introduced and standardized. This paper describes and examines these state-of-the-art enhancements to MAC efficiency for the 802.11 standard, and proposes a CLACK (Cross-Layer ACK) method that tackles this issue in totally different manner to those previous schemes. The main idea is simple: When a receiver sends an ACK, it transmits the data using the ACK transmission opportunity, and avoids channel contention necessary for data transmissions. The receiver’s short signature is piggybacked in the PHY instead of the MAC to acknowledge the packet reception. We have implemented CLACK using USRP toolkits and GNU Software Define Radio. Our implementation demonstrates the feasibility of our key techniques for both PHY and MAC design. Further, we use detailed simulation to evaluate CLACK in general wireless environments under different traffic loads and varying channel conditions. Our results show that CLACK gains up to 52 % in terms of throughput, when compared to the basic 802.11 scheme, and up to 18 % when compared to existing advanced 802.11e/n schemes.     

Medium access control in ad hoc networks with MIMO links: optimization considerations and algorithms

we present a medium access control (MAC) protocol for ad hoc networks with multiple input multiple output (MIMO) links. MIMO links provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links coupled with several key optimization considerations, necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm called stream-controlled medium access (SCMA) that has the key optimization considerations incorporated in its design. Finally, we present a distributed SCMA protocol that approximates the centralized algorithm and compare its performance against that of baseline protocols that are CSMA/CA variants.     

A load awareness medium access control protocol for single‐hop wireless ad hoc networks

A contention‐based wireless ad hoc medium access control (MAC) protocol, such as carrier sense multiple access with collision avoidance (CSMA/CA), has excellent efficiency when the system is light loaded. The main drawback of such protocols is their inefficiency and unbounded delay when the system load is heavy. On the other hand, a contention‐free MAC protocol, such as token passing, has a better and fair throughput when the system is heavy loaded. The main drawback of such protocols is their inefficiency when only a small amount of users want to transmit. In this paper, we propose a new load awareness single‐hop wireless ad hoc MAC protocol (which is called the LA protocol) that exploits the benefits of both contention‐based and contention‐free protocols. A contention‐based MAC protocol is used when the system is light loaded and a contention‐free one is used otherwise. Our LA protocol, which operates in a distributed fashion and is fully compatible with the IEEE 802.11 wireless local area network (WLAN) standard, can switch smoothly between the contention‐based protocol and the contention‐free one. Simulation results show that our protocol indeed extracts the better part of two kinds of protocols. Copyright © 2006 John Wiley & Sons, Ltd.     

A Beacon-Based Collision-Free Channel Access Scheme for IEEE 802.11 WLANs

In IEEE 802.11 based WLAN standard, distributed coordination function is the fundamental medium access control (MAC) technique. It employs a CSMA/CA with random binary exponential backoff algorithm and provides contention-based distributed channel access for stations to share the wireless medium. However, performance of this mechanism drops dramatically due to random structure of the backoff process, high collision probability and frame errors. That is why development of an efficient MAC protocol, providing both high throughput for data traffic and quality of service (QoS) support for real-time applications, has become a major focus in WLAN research. In this paper, we propose an adaptive beacon-based collision-free MAC adaptation. The proposed scheme makes use of beacon frames sent periodically by access point, lets stations enter the collision-free state and reduces the number of idle slots regardless of the number of stations and their traffic load (saturated or unsaturated) on the medium. Simulation results indicate that the proposed scheme dramatically enhances the overall throughput and supports QoS by reducing the delay, delay variation and dropping probability of frames.     

100+ VoIP Calls on 802.11b: The Power of Combining Voice Frame Aggregation and Uplink-Downlink Bandwidth Control in Wireless LANs

The bandwidth efficiency of voice over IP (VoIP) traffic on the IEEE 802.11 WLAN is notoriously low. VoIP over 802.11 incurs high bandwidth cost for voice frame packetization and MAC/PHY framing, which is aggravated by channel access overhead. For instance, 10 calls with the G.729 codec can barely be supported on 802.11b with acceptable QoS - less than 2% efficiency. As WLANs and VoIP services become increasingly widespread, this inefficiency must be overcome. This paper proposes a solution that boosts the efficiency high enough to support a significantly larger number of calls than existing schemes, with fair call quality. The solution comes in two parts: adaptive frame aggregation and uplink/downlink bandwidth equalization. The former reduces the absolute number of MAC frames according to the link congestion level, and the latter balances the bandwidth usage between the access point (AP) and wireless stations. When used in combination, they yield superior performance, for instance, supporting more than 100 VoIP calls over an IEEE 802.11b link. The authors demonstrate the performance of the proposed approach through extensive simulation, and validate the simulation through analysis.     

POWMAC: a single-channel power-control protocol for throughput enhancement in wireless ad hoc networks

Transmission power control (TPC) has great potential to increase the throughput of a mobile ad hoc network (MANET). Existing TPC schemes achieve this goal by using additional hardware (e.g., multiple transceivers), by compromising the collision avoidance property of the channel access scheme, by making impractical assumptions on the operation of the medium access control (MAC) protocol, or by overlooking the protection of link-layer acknowledgment packets. In this paper, we present a novel power controlled MAC protocol called POWMAC, which enjoys the same single-channel, single-transceiver design of the IEEE 802.11 ad hoc MAC protocol but which achieves a significant throughput improvement over the 802.11 protocol. Instead of alternating between the transmission of control (RTS/CTS) and data packets, as done in the 802.11 scheme, POWMAC uses an access window (AW) to allow for a series of request-to-send/clear-to-send (RTS/CTS) exchanges to take place before several concurrent data packet transmissions can commence. The length of the AW is dynamically adjusted based on localized information to allow for multiple interference-limited concurrent transmissions to take place in the same vicinity of a receiving terminal. Collision avoidance information is inserted into the CTS packet and is used to bound/ the transmission power of potentially interfering terminals in the vicinity of the receiver, rather than silencing such terminals. Simulation results are used to demonstrate the significant throughput and energy gains that can be obtained under the POWMAC protocol.     

Performance analysis of MAC layer protocols in the IEEE 802.11 wireless LAN

In recent years, WLANs (Wireless Local Area Networks) based on the IEEE 802.11 standard have been taken a growing interest and developed widely all over the world. CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocols are the most popular MAC (Medium Access Control) protocols for WLANs. The performance of CSMA/CA protocols over wireless channels has been investigated over the past years. In this paper, we obtain the probability distribution function of the MAC layer packet service time, and we present the comprehensive performance analysis of IEEE 802.11 MAC protocol by investigating the queue dynamics of a wireless station based on the MAC layer packet service time. We adopt an MMPP(Markov Modulated Poisson Process) as the input traffic model that describes well the bursty nature of Internet traffic. The analysis on the throughput and the delay performance has been carried out by using the MMPP/G/1/K queueing model. We have some numerical results that represent the system throughput and the queue dynamics including the mean packet waiting time and packet blocking probability.     

Reactive routing evaluation using modified 802.11a with realistic vehicular mobility

Realistic mobility dynamics and underlying PHY/MAC layer implementation affect real deployment of routing protocols in vehicular ad hoc network (VANET). Currently, dedicated short range communication devices are using wireless access in vehicular environment (WAVE) mode of operation, but now IEEE is standardizing 802.11p WAVE. This work presents an in-depth simulation-based analysis of two reactive routing protocols, i.e., dynamic source routing (DSR) and ad hoc on-demand distance vector (AODV) with modified IEEE 802.11a PHY/MAC layers (comparable to 802.11p) in modified VANET mobility models (freeway, stop sign, and traffic sign) in terms of load, throughput, delay, number of hops, and retransmission attempts. Results obtained using OPNET simulator show that in urban/highway mobility scenarios, AODV??s performance with forthcoming 802.11p at high bit rate would be better than DSR in terms of high throughput, less delay, and retransmission attempts. Moreover, this comprehensive evaluation will assist to address challenges associated with future deployment of routing protocols integrated upon devices with upcoming IEEE 802.11p, concerning specific macro-/micro-mobility scenarios.     

EBA: an enhancement of the IEEE 802.11 DCF via distributed reservation

The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of the 802.11 WLAN is significantly compromised as the number of stations increases. In this paper, we propose a novel distributed reservation-based MAC protocol, called early backoff announcement (EBA), which is backward compatible with the legacy DCF. Under EBA, a station announces its future backoff information in terms of the number of backoff slots via the MAC header of its frame being transmitted. All the stations receiving the information avoid collisions by excluding the same backoff duration when selecting their future backoff value. Through extensive simulations, EBA is found to achieve a significant increase in the throughput performance as well as a higher degree of fairness compared to the 802.11 DCF.     

A hybrid multi-channel MAC protocol for wireless ad hoc networks

In a regular wireless ad hoc network, the Medium Access Control (MAC) protocol coordinates channel access among nodes, and the throughput of the network is limited by the bandwidth of a single channel. The multi-channel MAC protocols can exploit multiple channels to achieve high network throughput by enabling more concurrent transmissions. In this paper, we propose a hybrid and adaptive protocol, called H-MMAC, which utilizes multi-channel resources more efficiently than other multi-channel MAC protocols. The main idea is to adopt the IEEE 802.11 Power Saving Mechanism and to allow nodes to transmit data packets while other nodes try to negotiate the data channel during the Ad hoc Traffic Indication Message window based on the network traffic load. The analytical and simulation results show that the proposed H-MMAC protocol improves the network performance significantly in terms of the aggregate throughput, average delay, fairness and energy efficiency.     

IEEE 802.11-saturation throughput analysis

To satisfy the emerging need of wireless data communications, the IEEE is currently standardizing the 802.11 protocol for wireless local area networks. This standard adopts a CSMA/CA medium access control protocol with exponential backoff. We present a simple analytical model to compute the saturation throughput performance in the presence of a finite number of terminals and in the assumption of ideal channel conditions. The model applies to both basic and request-to-send/clear-to-send (RTS/CTS) access mechanisms. Comparison with simulation results shows that the model is extremely accurate in predicting the system throughput