Hybrid Coordination Function Research Articles

Deterministic Backoff: Toward Efficient Polling for IEEE 802.11e HCCA in Wireless Home Networks

The emergence of video streaming over wireless home networks creates renewed interests in design and analysis of new MAC protocols toward QoS provisioning for video applications. IEEE 802.11e Hybrid coordination function Controlled Channel Access (HCCA) exhibits good QoS provisioning for constant bit rate (CBR) video streams in a single collision domain. However, its performance degrades significantly for variable bit rate (VBR) video streams particularly in multicollision domains. In addition, HCCA has the disadvantage of high complexity. In this paper, we introduce a deterministic backoff (DEB) method into the HCCA mechanism, which achieves virtual polling via carrier sense on the wireless channel. DEB intentionally sets each station's backoff counter to a different value, thus stations can access the shared wireless channel at different time slots, which avoids network collisions. By proper controlling of each station's backoff counter, DEB achieves polling like HCCA, but in a more flexible and efficient way. It considerably mitigates inter-AP interference as well due to its carrier sense nature. Results show that, compared to HCCA, DEB always exhibits improvement in performance, particularly in multicollision domains where improvement is remarkable.

QoS in Wireless LAN: A comparison between feedback-based and earliest due-date approaches

The Hybrid Coordination Function is a very powerful mechanism to provide differentiated services in IEEE 802.11 WLANs. In recent years, its adoption in conjunction with centralized scheduling algorithms has lead to many interesting solutions to support services with very sharp delay constraints. Notably, two main approaches have been proposed to face real-time service provisioning, i.e., using the well known earliest due date scheme or feedback control theory. Unfortunately, it is still missing a deep comparison among them. To bridge this gap, we propose herein a novel feedback-based mechanism for infrastructure WLANs and compare it with respect to the most important delay-EDD algorithm, in complex scenarios with video, voice, and best effort flows. Simulation results, obtained using ns-2, demonstrate that (i) both algorithms are able to guarantee upper bounded packet delays, (ii) a more efficient bandwidth usage and a higher network throughput is achieved using the feedback based approach. In particular, it has been shown that the feedback based approach allows a throughput gain ranging from 10% to 20% with respect to delay-EDD.

Selectivity function scheduler for IEEE 802.11e HCCA access mode

SUMMARYIn this paper, we present a scheduling algorithm that enhances the performance of the standard IEEE 802.11e scheduler for the Hybrid Coordination Function Controlled Channel Access mode. The main contribution in designing the proposed scheduler is the ability to accommodate multiple streams with different levels of Quality of Service requirements concurrently running on the same station. This is achieved by dynamically calculating the Transmission Opportunities of each active traffic stream (TS) and the appropriate Service Interval of each active station. The proposed algorithm optimizes the utilization of the scarce bandwidth resources by only polling active stations. The algorithm incorporates a selectivity function to assign polling priorities to the active streams only based on their diverse requirements and their link‐attainable transmission rates. The performance of the proposed Selectivity Function Scheduler (SFS) scheme is evaluated against the standard scheduler. Simulation results show that the SFS outperforms the standard scheduler in terms of enhancing streams' throughput, reducing packet dropping ratio and maintaining high fairness amongst the admitted TS. Copyright © 2011 John Wiley & Sons, Ltd.

AWPP: A New Scheme for Wireless Access Control Proportional to Traffic Priority and Rate

Cutting-edge wireless networking approaches are required to efficiently differentiate traffic and handle it according to its special characteristics. The current Medium Access Control (MAC) scheme which is expected to be sufficiently supported by well-known networking vendors comes from the IEEE 802.11e workgroup. The standardized solution is the Hybrid Coordination Function (HCF), that includes the mandatory Enhanced Distributed Channel Access (EDCA) protocol and the optional Hybrid Control Channel Access (HCCA) protocol. These two protocols greatly differ in nature and they both have significant limitations. The objective of this work is the development of a high-performance MAC scheme for wireless networks, capable of providing predictable Quality of Service (QoS) via an efficient traffic differentiation algorithm in proportion to the traffic priority and generation rate. The proposed Adaptive Weighted and Prioritized Polling (AWPP) protocol is analyzed, and its superior deterministic operation is revealed.

Time Coordinated V2I Communications and Handover for WAVE Networks

A wide variety of information technology (IT) services will become available for vehicles through the IEEE 802.11p, IEEE 1609, and wireless access in vehicular environment (WAVE) standards, and naturally the need for service handover will become a necessity. For WAVE networks, a time coordinated medium access control (MAC) protocol named WAVE point coordination function (WPCF) is proposed in this paper. The mathematical derivations and simulation analysis demonstrate that the proposed WPCF protocol can significantly enhance the utilization efficiency and can support more users based on an equal-length access period when compared to point coordination function (PCF), enhanced distributed channel access (EDCA), or hybrid coordination function (HCF) controlled channel access (HCCA) for vehicle to infrastructure (V2I) communications. The paper also demonstrates the effectiveness of the proposed WAVE handover controller (WHC) and WPCF in minimizing service disconnection time when used for V2I handover. In addition, technical enhancements and messages are added to enable seamless communication services over soft-handover (SHO), and enable minimum handover delay in hard-handover (HHO) situations. The mathematical performance evaluation and the simulation results show that the proposed scheme can significantly improve the handover latency compared to the performance that can be delivered by the current protocols.

Enhancing IEEE 802.11p/WAVE to provide infotainment applications in VANETs

IEEE 802.11p/WAVE (Wireless Access for Vehicular Environment) is the emerging standard to enable wireless access in the vehicular environment. Most of the research contributions in this area has focused on safety-related applications, while comfort and information/entertainment applications (such as on board Internet access, point-of-interest notification, e-map download) have been considered only recently. Notwithstanding, the user interest in this kind of applications is expected to become a big market driver in a near future. In this paper, an extension to IEEE 802.11p is proposed that is compliant with the multi-channel operation of the WAVE architecture and targets at the support of non-safety applications, while preserving the delivery of safety services. The proposed W-HCF (WAVE-based Hybrid Coordination Function) protocol leverages controlled access capabilities on top of the basic contention-based access of the IEEE 802.11p; it exploits vehicles’ position information and coordination among WAVE providers in order to improve performances of delay-constrained and loss-sensitive non-safety applications.

Enhanced HCCA mechanism for multimedia traffics with QoS support in IEEE 802.11e networks

In broadband wireless networks, providing Quality of Service (QoS) support for multimedia traffics is no doubt crucial to many application requirements such as real-time audio and video streaming communications. IEEE 802.11e Medium Access Control (MAC) standard proposes Hybrid Coordination Function (HCF), which is composed of HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA) mechanisms for QoS support. HCCA performs a polling procedure with parameterized channel. However, a station may not always have pending data to transmit. That is, polling all the stations may waste time and therefore deteriorate the transmission performance. In this paper, we propose a modified HCCA mechanism using the beacon frame broadcasted by the AP to determine which station can transmit its pending data so as to improve the transmission performance. The simulation results show that the performance improvement of our proposed mechanism is significant for QoS support particularly under light load condition.

Study on bandwidth control for multiple transmission rates in IEEE 802.11e EDCAF

AbstractIEEE 802.11e supports the guaranteed quality of service (QoS) by providing different transmission priorities. IEEE 802.11e improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). Signal quality may affect the available bandwidth and transmission rate, because the characteristic of communication channel in wireless environment is in random time‐variation manner. Generally a station using a lower transmission rate will occupy communication channel for a longer time and degrade system performance, which causes unfairness and cannot provide the guaranteed QoS for the stations with higher transmission rates. We propose a bandwidth control scheme (BCS) by combining the IEEE 802.11e enhanced distributed channel access function (EDCAF) protocol to overcome the guaranteed bandwidth issue in multirate environments. A multirate discrete Markov chain model is analyzed for the multirate transmission system in this paper. According to the obtained results, BCS improves performance especially in throughput and makes the different QoS requirements be processed efficiently and flexibly. Copyright © 2009 John Wiley & Sons, Ltd.

A time slots coordination mechanism forIEEE 802.11 WLANs

A software-based time slots coordination mechanism (TSCM) for IEEE 802.11 WLANs is proposed in this letter to guarantee the quality of service (QoS) for voice traffic and avoid starvation for data traffic. With the complexity analysis, relative simplicity of TSCM is shown. Via simulations, we demonstrate that TSCM outperforms the enhanced distributed channel access (EDCA), hybrid coordination function (HCF) controlled channel access (HCCA), and Blackburst in terms of number of supported voice sessions, normalized throughput, and fairness index.

Adaptive TXOP Allocation Based on Channel Conditions and Traffic Requirements in IEEE 802.11e Networks

IEEE 802.11e has established a new access mechanism called the hybrid coordination function (HCF) as a step toward provisioning quality-of service (QoS) support. In this mechanism, a simple HCF scheduler is introduced to take the QoS requirements of admitted flows into account and allocate transmission opportunities (TXOPs), which is the time under which a station (STA) can send its burst of data packets, to STAs. Although many approaches have been taken to address the mechanism's inherent problems, unsubstantial effort has been afforded in tackling a channel-aware allocation mechanism. An STA can be in either the good or bad channel state in any feasible data rate, which can be modeled as a Markov chain of two states for each data rate. In view of this, we propose a mechanism of adaptive TXOP allocation that is applicable to existing scheduling algorithms. Our method works in accordance with channel and traffic conditions and complies with the link adaptation mechanism. Extensive simulation results verify that our method shows improved performance while ensuring long-term fairness among STAs and being adaptive to the channel conditions and underlying physical transmission rates.

A High Throughput Medium Access Control Implementation Based on IEEE 802.11e Standard

With the growing demand for high-performance multimedia applications over wireless channels, we need to develop a Medium Access Control (MAC) system that supports high throughput and quality of service enhancements. This paper presents the standard analysis, design architecture and design issues leading to the implementation of an IEEE 802.11e based MAC system that supports MAC throughput of over 100Mbps. In order to meet the MAC layer timing constraints, a hardware/software co-design approach is adopted. The proposed MAC architecture is implemented on the Xilinx Virtex-II Pro Field-Programmable Gate Array (FPGA) (XC2VP70-5FF1704C) prototype, and connected to a host computer through an external Universal Serial Bus (USB) interface. The total FPGA resource utilization is 11, 508 out of 33, 088 (34%) available slices. The measured MAC throughput is 100.7Mbps and 109.2Mbps for voice and video access categories, transmitted at a data rate of 260Mbps based on IEEE 802.11n Physical Layer (PHY), using the contention-based hybrid coordination function channel access mechanism.

A Priority Queuing Model for HCF Controlled Channel Access (HCCA) in Wireless LANs

Recently, there has been a rapid growing interest in new applications requiring quality of service (QoS) guarantees through wireless local area networks (WLAN). These demands have led to the introduction of new 802.11 standard series to enhance access medium supporting QoS for multimedia applications. However, some applications such as variable bit rate (VBR) traffic address some challenges in the hybrid coordination function (HCF) nominated to provide QoS. This paper presents a novel priority queuing model to analyze a medium access in the HCF controlled channel access (HCCA) mode. This model makes use of a MAP (Markovian Arrival Process)/PH (Phase Type)/1 queue with two types of jobs which are suitable to support VBR traffic. Using a MAP for traffic arrival process and PH distribution for service process, the inclusion of vacation period makes our analysis very general and comprehensive to support various types of practical traffic streams. The proposed priority queuing model is very useful to evaluate and enhance the performance of the scheduler and the admission controller in the HCCA mechanism.

An 802.11e HCCA scheduler with an end-to-end quality aware territory method

In this paper we present a solution for the IEEE 802.11e HCCA ( Hybrid coordination function Controlled Channel Access) mechanism which aims both at supporting strict real-time traffic requirements and, simultaneously, at handling TCP applications efficiently. Our proposal combines a packet scheduler and a dynamic resource allocation algorithm. The scheduling discipline is based on the Monolithic Shaper-Scheduler, which is a modification of a General Processor Sharing (GPS) related scheduler. It supports minimum-bandwidth and delay guarantees and, at the same time, it achieves the optimal latency of all the GPS-related schedulers. In addition, our innovative resource allocation procedure, called the territory method, aims at prioritizing real time services and at improving the performance of TCP applications. For this purpose, it splits the wireless channel capacity (in terms of transmission opportunities) into different territories for the different types of traffic, taking into account the end-to-end network dynamics. In order to give support to the desired applications, we consider the following traffic classes: conversational, streaming, interactive and best-effort. The so called territories shrink or expand depending on the current quality experienced by the corresponding traffic class. We evaluated the performance of our solution through extensive simulations in a heterogeneous wired-cum-wireless scenario under different traffic conditions. Additionally, we compare our proposal to other HCCA scheduling algorithms, the HCCA reference scheduler and Fair Hybrid Coordination Function (FHCF). The results show that the combination of the MSS and the territory method obtains higher system capacity for VoIP traffic (up to 32 users) in the simulated scenario, compared to FHCF and the HCCA reference scheduler (13 users). In addition, the MSS with the territory method also improves the throughput of TCP sources (one FTP application achieves between 6.1 Mbps without VoIP traffic and 2.1 Mbps with 20 VoIP users) compared to the reference scheduler (at most 388 kbps) and FHCF (with a maximum FTP throughput of 4.8 Mbps).

An Efficient Scheduling Scheme for Assigning Transmission Opportunity in QoS-Guaranteed Wireless LAN

The IEEE 802.11e standard defines several important techniques that guarantee the quality of service (QoS) of multimedia communications in wireless LAN applications. A key technique introduced in the standard is an efficient media access control based on the Hybrid Coordination Function (HCF), which assigns an appropriate transmission opportunity (TXOP) to each flow for satisfying several requirements on the flow, such as delay bound and communication bandwidth. Although the simple scheduler designed in HCF could provide an efficient tool to guarantee the QoS in wireless LAN applications, it has a serious drawback, wherein a useless bandwidth assignment frequently occurs. Such assignments occur primarily due to a limitation of the scheme, wherein all the flows share the same service interval (SI). In this paper, we propose a new scheduling scheme that associates each flow with its own SI according to the hardness of the required SI. The effect of this approach is experimentally evaluated by simulation, the results of which indicate that the number of admitted flows and network resource utilization increase due to the proposed scheme.

Intelligent MAC model for traffic scheduling in IEEE 802.11e wireless LANs

IEEE 802.11e hybrid coordination function (HCF) medium access control (MAC) protocol aims to provide quality of service (QoS) in wireless local area networks (WLANs), which includes a new access mechanism, called enhanced distributed channel access (EDCA), and a polling-based scheme, called HCF controlled channel access (HCCA). In this paper, focusing on the problem of improving QoS in a mixed EDCA and HCCA scenario, an intelligent MAC model for traffic scheduling located at the QoS-enhanced access point (QAP) is proposed. The proposed MAC model includes two parts: two status estimators and an adaptive scheduler. The model not only takes into account the requirements of traffic streams (TSs) such as throughput, delay and priority, but also takes into account current system status. In order to evaluate current system status, decide call admission control and reserve resource accurately, the MAC model employs an unsaturation EDCA model in EDCA mode and a dynamic HCCA scheduling algorithm in HCCA mode. With the requirements information sent by TSs and the evaluated system status, the scheduler adaptively schedules TSs to transmit frames in the appropriate access mode. Simulation results prove the effectiveness of our approach and demonstrate that the proposed MAC model is effective and significantly improves QoS in terms of throughput and delay metrics.

Controlled Channel Access Scheduling for Guaranteed QoS in 802.11e-Based WLANs

The IEEE 802.11e standard developed by IEEE 802.11TGe intends to provide the quality of service (QoS) support in IEEE 802.11-based wireless local area networks (WLANs) through the introduction of hybrid coordination function (HCF). The HCF controlled channel access (HCCA) designed as a part of HCF is the medium access mechanism for parameterized QoS and is suitable for multimedia applications requiring hard QoS guarantees. The standard also defines a reference scheduler to complement HCCA in meeting these guarantees. In this paper, we investigate the performance of the reference scheduler described in the standard by using a novel queueing analytic framework. The analysis reveals the performance deficiencies of the reference scheduler. Afterwards, to overcome these deficiencies, we propose a new scheduling scheme, namely, the prediction and optimization-based HCCA (PRO-HCCA), based on the insights gained from the queueing analysis. Simulation experiments show that the proposed scheme overcomes the problems of the reference scheduler and successfully enables the HCCA to fulfill the QoS guarantees for multimedia applications.

Data rate estimation algorithm for IEEE 802.11e HCCA scheduler

PurposeAs the real‐time applications used in today's wireless network grow, some schemes are required to provide better quality of services. From the evaluation of referenced scheduling scheme provided in IEEE 802.11e standard document, we know that it does not perform well on traffic which is not strictly constant‐bit‐rate (CBR). Therefore, it is necessary to design a more flexible scheme to dynamically adjust the estimation of transmission opportunity allocated to QoS‐enhanced stations with different characteristics of applications. This paper aims to present a data rate estimation algorithm for the scheduler of the IEEE 802.11e hybrid coordination function controlled channel access.Design/methodology/approachThe performance of the algorithm is evaluated through Network Simulator ‐2.FindingsWith the proposed algorithm, the QoS access point can provide guaranteed parameters such as delay and throughput for both the real‐time variable‐bit‐rate and CBR traffics.Originality/valveThe paper presents a data rate estimation algorithm for IEEE 802‐11e HCCA scheduler.

Effective transmission opportunity allocation scheme for real-time variable bit rate traffic flows with different delay bounds

The medium access control of IEEE 802.11e defines a novel coordination function, namely, hybrid coordination function (HCF), which allocates transmission opportunity (TXOP) to stations taking their quality of service (QoS) requirements into account. However, the reference TXOP allocation scheme of HCF controlled channel access, a contention-free channel access function of HCF, is only suitable for constant bit rate traffic. For variable bit rate traffic, packet loss may occur seriously. The authors propose a TXOP allocation scheme to efficiently allocate bandwidth and meet the QoS requirements in terms of both delay bound and packet loss probability. To achieve high bandwidth efficiency, the authors take advantage of not only intra-flow multiplexing gain of traffic flows with large delay bounds, but also inter-flow multiplexing gain of multiple traffic flows with different delay bounds. According to numerical results obtained by computer simulations, the proposed TXOP allocation scheme results in much higher bandwidth efficiency than previous algorithms under the same constraints of delay bounds and packet loss probability.

A delay-based admission control mechanism for multimedia support in IEEE 802.11e wireless LANs

Multimedia over IEEE 802.11 wireless local area networks (WLANs) has recently been the focus of many researchers due to its rapidly increasing popularity. Unlike their best-effort counterparts, multimedia applications have quality of service (QoS) needs typically expressed in terms of the maximum allowed delay and/or the minimum required throughput. Therefore, prior to accepting a multimedia application, the network must assure the satisfaction of its QoS requirements. In this paper, we develop a mechanism that can be used to control the admissibility of multimedia applications into WLANs. To develop the proposed mechanism, we first derive an analytical approximation of the delay experienced by packets when travelled through these networks. The analytical approximation of the delay is then used to propose an admission control mechanism for the enhanced distributed channel access (EDCA) method used by the hybrid coordination function (HCF) of IEEE 802.11e. The proposed delay-based admission control mechanism is validated via simulations of voice traffic.

Service Interval Optimization with Delay Bound Guarantee for HCCA in IEEE 802.11e WLANs

IEEE 802.11e Medium Access Control (MAC) is a supplement to the IEEE 802.11 Wireless Network (WLAN) standard to support Quality of Service (QoS). The 802.11e MAC defines a new coordination function, namely Hybrid Coordination Function (HCF), which takes the QoS requirements of flows into account and allocates Transmission Opportunity (TXOP) to stations. On the basis of mean sending rate, delay of Variable Bit Rate (VBR) traffic cannot be bounded with the reference HCF scheduling algorithm proposed in this supplement. In this paper, we propose a new Connection Admission Control (CAC) and a scheduling algorithm that utilize the token bucket and a modified Latency-Rate (LR) scheduling algorithm to guarantee a bounded delay for HCF Controlled Channel Access (HCCA). The new Service Interval (SI) is calculated to optimize the number of stations accommodated and takes into account delay bound and token bucket parameters. We show that it is possible to obtain worst-case performance guarantees on delay. First, we analyze the behavior of the new scheduler with a loss free wireless channel model and after this, with a burst loss model and we explain how it is possible to extend this scheduler for a multi-rate scheme. Properties of the proposal are investigated both theoretically and using ns-2 simulations. We present a set of simulations with both Constant Bit Rate (CBR) and VBR flows and performance comparisons with HCF scheduling algorithm. The results show that the delay upper bound can be achieved for a large range of networks load with bandwidth optimization.