Analysis of Enhanced Collision Avoidance Scheme Proposed for IEEE 802.11e-Enhanced Distributed Channel Access Protocol

In enhanced distributed channel access (EDCA) protocol, small contention window (CW) sizes are used for frequent channel access by high-priority traffic (such as voice). But these small CW sizes, which may be suboptimal for a given network scenario, can introduce more packet collisions, and thereby, reduce overall throughput. This paper proposes enhanced collision avoidance (ECA) scheme for AC_VO access category queues present in EDCA protocol. The proposed ECA scheme alleviates intensive collisions between AC_VO queues to improve voice throughput under the same suboptimal yet necessary (small size) CW restrictions. The proposed ECA scheme is studied in detail using Markov chain numerical analysis and simulations carried out in NS-2 network simulator. The performance of ECA scheme is compared with original (legacy) EDCA protocol in both voice and multimedia scenarios. Also mixed scenarios containing legacy EDCA and ECA stations are presented to study their coexistence. Comparisons reveal that ECA scheme improves voice throughput performance without seriously degrading the throughput of other traffic types.     

Two-level protection and guarantee for multimedia traffic in IEEE 802.11e distributed WLANs

In order to support multimedia applications such as voice and video over the wireless medium, a contention-based channel access function, called Enhanced Distributed Channel Access (EDCA), has been developed in the emerging standard IEEE 802.11e. In the EDCA, differentiated channel access is provided for different traffic classes. In this paper, we propose a two-level protection and guarantee mechanism for voice and video traffic in the EDCA-based distributed wireless LANs. In the first-level protection, the existing voice and video flows are protected from the new and other existing voice and video flows via a distributed admission control with tried-and-known and early-protection enhancements. In the second-level protection, the voice and video flows are protected from the best-effort data traffic by adopting frame-based and limit-based data control mechanisms. Performance evaluations are conducted in terms of throughput, delay, transmission limit, number of collisions, and throughput square relative difference. Extensive simulation results demonstrate that the proposed two-level protection and guarantee mechanism is very effective in terms of the protection and guarantee of existing voice and video flows as well as the utilization of the channel capacity. An early version of this paper was presented at IEEE INFOCOM 2004.     

Performance of Integrated Packet Voice/Data Token-Passing Rings

This paper presents performance results that indicate that packetized voice service can be provided on a token-passing ring without adversely affecting the performance of data traffic. This is accomplished by introducing a relatively mild priority structure: stations are limited to a single packet transmission per medium access, and voice packets are given access priority over data packets at the same station. In addition, voice traffic is allowed longer packet lengths than data traffic. Several versions of this basic scheme are considered: 1) the number of active stations is constrained so that voice packets are guaranteed access within one packetization period, 2) no guarantee on access time is provided and voice packets are discarded when the waiting time exceeds one packetization period, and 3) no guarantee on access time is provided and voice packets are buffered until they can be transmitted.     

Supporting multimedia streaming and best‐effort data transmission over IEEE 802.11e EDCA

As demand for broadband multimedia wireless services increases, improving quality of service (QoS) of the widely deployed IEEE 802.11 wireless LAN (WLAN) has become crucial. To support the QoS required by a wide range of applications, the IEEE 802.11 working group has defined a new standard: IEEE 802.11e. In this paper, we propose a measurement‐based dynamic media time allocation (MBDMTA) scheme combined with a concatenating window scheme to support real‐time variable bit rate (rt‐VBR) video and best‐effort (BE) data transmission using IEEE 802.11e enhanced distributed channel access (EDCA). To provide the QoS guarantee for rt‐VBR video, the proposed MBDMTA scheme dynamically assigns channel time to the rt‐VBR video based on the estimate of the required network resources. On the other hand, the concatenating window scheme controls the contention window (CW) ranges of different priority flows such that real‐time services always have higher channel access probability, thus achieving the capability of preemptive priorities. In addition, the concatenating window scheme preserves fairness among flows of the same class and attains high channel utilization under different network conditions. Simulation results demonstrate that the throughput and delay performance improve significantly for the transmission of rt‐VBR video and BE traffic as compared to those for the 802.11e EDCA specification. It is also revealed that combining the two proposed schemes provides seamless integration and reliable transmission of digital video and data service within the 802.11e EDCA framework. Copyright © 2007 John Wiley & Sons, Ltd.     

Multiple preemptive EDCA for emergency medium access control in distributed WLANs

The increasingly use of wireless local area networks (WLANs) in public safety and emergency network services demands for a strict quality of service (QoS) guarantee especially a large number of users report an emergency for immediate channel access. Unfortunately, the traditional IEEE 802.11e-based enhanced distributed channel access (EDCA) does not support a strict QoS guarantee for life saving emergency traffic under high loads. Previous studies have attempted to enhance the performance of EDCA called the Channel Preemtive EDCA (CP-EDCA) which is a promising idea to support emergency traffic in WLANs. However, CP-EDCA supports a single emergency traffic only (i.e. no emergency service differentiation) with high delays for increased traffic loads. To overcome this problem, we propose a class of EDCA protocol called Multiple Preemption EDCA (MP-EDCA) as a candidate to support multiple emergency traffics under high loads. Each MP-EDCA node can support up to four emergency traffics (life, health, property and environment) with different priorities in addition to support background (non-emergency) traffic. The proposed protocol privileged the high priority life-saving emergency traffic to preempt the services of low priority ones without much starvation in the network to maintain a strict QoS guarantee. The paper evaluates the performance of MP-EDCA through an extensive analysis of simulation outcome. The results obtained show that MP-EDCA outperforms CP-EDCA in achieving lower medium access control and emergency node delays.     

Efficient Back-off Mechanism for Multimedia Support in 802.11e EDCA Wireless Ad-Hoc Networks

This paper presents an effective back-off mechanism to improve quality of service (QoS) for multimedia applications over the IEEE 802.11e enhanced distribution channel access (EDCA) standard. It can be done with proposed algorithm called dynamic fast adaptation for contention-based EDCA (DFA-EDCA) mechanism. The main concept of proposed DFA-EDCA mechanism is to use the exponential functions to adaptively tune the back-off parameters in IEEE 802.11e EDCA according to the changes on a network load within a short period. In this proposed mechanism an intra-access category (intra-AC) differentiation mechanism is provided to increase its back-off time randomly and achieve discrimination of same priority traffic on different stations. The performance evaluations have been conducted by using Network Simulator (NS-2). The simulation results show that the proposed DFA-EDCA mechanism has greatly outperformed the previous mechanisms such as non-linear dynamic adaptation scheme of the minimum contention window (CW_min-HA), dynamic adaptation algorithm of the maximum contention window (CW_max-adaptation), adaptive enhanced distributed coordination function and the conventional EDCA in terms of goodput, gain of goodput, packet delay, collision rate and channel utilization ratio (CUR). It has significantly reduced both packet delay and collision rate simultaneously together with an obviously increment in both goodput and CUR, which lead to the improvement in QoS for multimedia applications.     

基于预留信道和强占优先相结合的接入策略

报道了一种用于话音／数据综合的蜂窝移动通信系统中的基于预留切换信道和强占优先策略相结合的业务接入方案。在本方案中为具有越区切换请求的话音呼叫提供了预留切换信道和强占数据业务的优先权相结合的接入策略。同时为了提高系统总的承载话音的业务量,也为新近产生的话音呼叫提供了一定的强占数据业务的优先权。结果表明：我们的方案可以为移动用户提供更好的服务质量。     

Performance evaluation on IEEE 802.11e considering emergency calls in congested situation

To enhance the quality of service (QoS) support in IEE 802.11, IEEE 802.11e has been studied, which introduces the so-called hybrid coordination function (HCF). HCF includes two medium access mechanisms contention-based channel access (EDCA), and contention-free channel access (HCCA). Although IEEE 802.11e has provided differentiated channel access mechanism, when call demand rises for important festivals such as New Year's Day or large scale natural disasters such as earthquakes, the delay of voice will increase and the QoS of voice nodes will drop down rapidly. Through our simulation study, in order to guarantee the QoS of emergency voice calls in congested situation, a higher priority for these calls will be required.     

Reducing the Impact of Legacy Stations on Voice Traffic in 802.11e EDCA WLANs

The EDCA access mechanism of the 802.11e standard supports legacy DCF stations, but the performance of high priority traffic is substantially degraded in their presence. In a previous letter we proposed the ACKS technique to mitigate the impact of legacy stations and studied this technique under high priority data traffic. In this letter we analyze the suitability of the ACKS technique for protecting voice traffic. Results show that voice performance is significantly improved as a result of using this technique     

An integrated services token-controlled ring network

A ring protocol is proposed that allows voice and data traffic to coexist within the ring. This ring has the following distinct features: (1) it allows synchronous traffic such as voice and video to have a definite access to the channel within each packetization period (or frame); (2) it allows data messages to have a higher channel access priority provided that the synchronous traffic is not delayed by more than one frame; (3) it supports variable rate data circuits. Simulation results show that the data-message delay is much smaller than for other integrated services schemes. Urgent messages can be transmitted with a higher priority over voice. Since the voice packet delay is bounded within one packetization period, no time-stamping is needed and the voice loss can be completely avoided by reserving a sufficient number of slots. Continual speech reception is possible by synchronizing the speech regeneration process to the end of each frame. Since the ring is synchronized, gateway switching to external circuit-switched and packet-switched networks is very simple     

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.     

On Multiple Traffic Type Integration over Wireless TDMA Channels with Adjustable Request Bandwidth

A new medium access control (MAC) protocol for mobile wireless communications is presented and investigated. We explore, via an extensive simulation study, the performance of the protocol when integrating voice, video and data packet traffic over a wireless channel of high capacity (referring to an indoor microcellular environment). Depending on the number of video users admitted into the system, our protocol varies: a) the request bandwidth dedicated to resolving the voice users contention, and b) the probability with which the base station grants information slots to voice users, in order to preserve full priority for video traffic. We evaluate the maximum voice capacity and mean access delay, as well as the aggregate channel throughput, for various voice and video load conditions, and the maximum voice capacity, aggregate channel throughput and average data message delays, for various video, voice and data load conditions. As proven by the comparison with a recently introduced efficient MAC scheme (DPRMA), when integrating voice and video traffic our scheme obtains higher voice capacity and aggregate channel throughput. When integrating all three traffic types, our scheme achieves high aggregate channel throughput in all cases of traffic load.     

Adaptive per hop differentiation for end-to-end delay assurance in multihop wireless networks

As the rapid growth of smart hand-held devices, multihop wireless access networks have a lot of potential applications in a variety of fields in civilian and military environments. Many of these applications, such as realtime audio/video streaming, will require some form of end-to-end QoS assurance. In this paper, we present an adaptive per hop differentiation (APHD) scheme towards achieving end-to-end delay assurance in multihop wireless networks. Our scheme is based on EDCA technique which is proposed in 802.11e draft. In EDCA, data packets of different priorities will use different MAC contention parameter set, which translate into different delays. Our APHD scheme extends the capability of EDCA into multihop environment by taking end-to-end delay requirement into consideration at each intermediate hop. Following a cross-layer design approach, APHD is aimed to be a distributed and localized technique. Individual nodes keep track of the channel state independently without any intercommunication overhead. Data packets carry end-to-end delay requirement along with other important information in the packet header. At an intermediate node, based on data packet’s end-to-end requirement, its accumulative delay so far, and the current node’s channel status, APHD smartly adjusts data packet’s priority level in order to satisfy its end-to-end delay requirement. Simulation results show that APHD scheme can provide excellent end-to-end delay assurance while achieving much higher network utilization, compared to a pure EDCA scheme.     

A Medium Access Control Protocol for Voice/Data Integrated Wireless CDMA Systems

In this paper, a medium access control protocol is proposed for integrated voice and data services in wireless local networks. Uplink channels for the proposed protocol are composed of time slots with multiple spreading codes per slot based on slotted code division multiple access (CDMA) systems. The proposed protocol uses spreading code sensing and reservation schemes. This protocol gives higher access priority to delay‐sensitive voice traffic than to data traffic. The voice terminal reserves an available spreading code to transmit multiple voice packets during a talkspurt. On the other hand, the data terminal transmits a packet without making a reservation over one of the available spreading codes that are not used by voice terminals. In this protocol, voice packets do not come into collision with data packets. The numerical results show that this protocol can increase the system capacity for voice service by applying the reservation scheme. The performance for data traffic will decrease in the case of high voice traffic load because of its low access priority. But it shows that the data traffic performance can be increased in proportion to the number of spreading codes.     

话音/数据综合的多时隙预约多址协议及其性能分析

本文提出了一种综合话音和数据的多时隙预约多址协议.该协议在保证话音终端的优先权的情况下,允许数据终端在报文的传输期间在连续多个帧中预约多个信息时隙.文中对协议进行了理论分析,并推导出了协议的重要性能指标(如话音分组丢失率、数据报文平均接入时延、系统平均吞吐率等)的解析表达式.研究表明,该协议可以支持比IPRMA、NC-IPRMA更高的等效数据终端速率,而且系统平均吞吐率在很大的负载范围内接近最大值.     

A distributed MAC scheme to achieve QoS in ad hoc networks

Real-time applications introduce new requirements on wireless networks and impose quality thresholds on parameters like delay, jitter, throughput, and packet loss in order to run smoothly. This paper addresses this issue by presenting a MAC scheme that offers real-time applications the opportunity to reserve transmission time based on their QoS requirements for contention-free medium access. Our scheme, which is called EDCA with Resource Reservation (EDCA/RR), operates in a fully distributed manner, is compatible with IEEE 802.11, and provides both prioritized and parameterized QoS. In this study, we have extended EDCA/RR to handle reservation collisions and, through extensive simulations, we show that our proposal can handle multiple reservations as well as uninformed stations that lie outside the transmission range of both the transmitter and the receiver while providing QoS guarantees. We compare EDCA/RR with EDCA and our results show that, as the traffic in the network increases, EDCA/RR succeeds providing the required service to QoS-demanding applications whereas EDCA fails in this task. In addition, when the medium is lossy we show that, not only does EDCA/RR give better service to real-time traffic, but also to contending non-real-time traffic.     

On the unfairness in IEEE 802.11e networks: Desynchronization of its medium access mechanism

Since the advent of the first IEEE 802.11 standard, several papers have proposed means of providing QoS to IEEE 802.11 networks and evaluate various traffic-prioritization mechanisms. Nevertheless, studies on the assignment of AIFS times defined in IEEE 802.11e reveal that the various priority levels work in a synchronized manner. The studies show that, under large loads of high-priority traffic, EDCA starves low-priority frames, which is undesirable. We argue that QoS traffic needs to be prioritized, but users sending best-effort frames should also obtain the expected service. High-priority traffic can also suffer performance degradation when using EDCA because of heavy loads of low-priority frames. Thus, we have proposed a mechanism based on desynchronizing the IEEE 802.11e working procedure. It prevents stations that belong to different priority classes from attempting simultaneous transmission, prioritizes independent collision groups and achieves better short-term and long-term channel access fairness. We have evaluated the proposal based on extensive analytical and simulation results. It prevents the strangulation of low-priority traffic, and, moreover, reduces the degradation of high-priority traffic with the increased presence of low-priority frames.     

Effects of base doping and carrier lifetime on differential current gain and temperature coefficient of 4H-SiC power bipolar junction transistors

Considering the circumstance of heterogeneous voice flows, first, by applying Markov chain, this paper proposes an unsaturated analytical model for the IEEE 802.11e EDCA protocol, which considers the condition of non-ideal transmission channel and the character of the occurrence of backoff countdown at the beginning of time slot in EDCA protocol. Furthermore, according to the proposed model, the media access delay and throughput of a flow are analysed, and the flow-oriented call admission control (CAC) scheme is proposed. Finally, the simulation results are shown to confirm that the proposed CAC scheme can guarantee the requirements of throughput and delay of voice flows, and can admit more voice flows to improve the utilisation efficiency of network resources by choosing the appropriate values of the minimum contention window or the appropriate varieties of voice flows.     

A Distributed End-to-End Reservation Protocol for IEEE 802.11-Based Wireless Mesh Networks

This paper presents an end-to-end reservation protocol for quality-of-service (QoS) support in the medium access control layer of wireless multihop mesh networks. It reserves periodically repeating time slots for QoS-demanding applications, while retaining the distributed coordination function (DCF) for best effort applications. The key features of the new protocol, called "distributed end-to-end allocation of time slots for real-time traffic (DARE), are distributed setup, interference protection, and scheduling of real-time data packets, as well as the repair of broken reservations and the release of unused reservations. A simulation-based performance study compares the delay and throughput of DARE with those of DCF and the priority-based enhanced distributed channel access (EDCA) used in IEEE 802.11e. In contrast to DCF and EDCA, DARE has a low, nonvarying delay and a constant throughput for each reserved flow     

Throughput and delay analysis for the IEEE 802.11e enhanced distributed channel access

In this letter, we propose a three-dimensional Markov chain model for the 802.11e enhanced distributed channel access (EDCA) mode. This model can be used to compute the maximum sustainable throughput and service delay distribution for each priority class when under saturation load. The new framework models the performance impact of major quality-of-service (QoS)-specific features (e.g., CWMin, CWMax, AIFS, internal collision resolution) of the 802.11e EDCA mode, and hence can provide an analytical approach to pick the parameter values associated with EDCA to meet the QoS requirements of each priority.