Due to such features as low cost, ease of deployment, increased coverage, and enhanced capacity, multihop wireless networks such as ad hoc networks, mesh networks, and sensor networks that form the network in a self-organized manner without relying on fixed infrastructure is touted as the new frontier of wireless networking. Providing efficient quality of service (QoS) support is essential for such networks, as they need to deliver real-time services like video, audio, and voice over IP besides the traditional data service. Various solutions have been proposed to provide soft QoS over multihop wireless networks from different layers in the network protocol stack. However, the layered concept was primarily created for wired networks, and multihop wireless networks oppose strict layered design because of their dynamic nature, infrastructureless architecture, and time-varying unstable links and topology. The concept of cross-layer design is based on architecture where different layers can exchange information in order to improve the overall network performance. Promising results achieved by cross-layer optimizations initiated significant research activity in this area. This paper aims to review the present study on the cross-layer paradigm for QoS support in multihop wireless networks. Several examples of evolutionary and revolutionary cross-layer approaches are presented in detail. Realizing the new trends for wireless networking, such as cooperative communication and networking, opportunistic transmission, real system performance evaluation, etc., several open issues related to cross-layer design for QoS support over multihop wireless networks are also discussed in the paper.

http://repository.ust.hk/ir/bitstream/1783.1-3342/1/043999781.pdf

Cross-Layer Design for QoS Support in Multihop Wireless Networks

IEEE 802.11 is the most important standard for wireless local area networks (WLANs). In IEEE 802.11, the fundamental medium access control (MAC) scheme is the distributed coordination function (DCF). To understand the performance of WLANs, it is important to analyze IEEE 802.11 DCF. Recently, several analytical models have been proposed to evaluate the performance of DCF under different incoming traffic conditions. However, to the best of the authors' knowledge, there is no accurate model that takes into account both the incoming traffic loads and the effect of imperfect wireless channels, in which unsuccessful packet delivery may occur due to bit transmission errors. In this paper, the authors address this issue and provide an analytical model to evaluate the performance of DCF in imperfect wireless channels. The authors consider the impact of different factors together, including the binary exponential backoff mechanism in DCF, various incoming traffic loads, distribution of incoming packet size, queueing system at the MAC layer, and the imperfect wireless channels, which has never been done before. Extensive simulation and analysis results show that the proposed analytical model can accurately predict the delay and throughput performance of IEEE 802.11 DCF under different channel and traffic conditions

/pdf/performance-analysis-of-ieee-802-11-dcf-in-imperfect-1ucwt7h9qf.pdf

Performance Analysis of IEEE 802.11 DCF in Imperfect Channels

We consider the vector fixed point equations arising out of the analysis of the saturation throughput of a single cell IEEE 802.11e wireless local area network with nodes that have different back-off parameters, including different Arbitration InterFrame Space (AIFS) values. We consider balanced and unbalanced solutions of the fixed point equations arising in homogeneous and nonhomogeneous networks. We are concerned, in particular, with (i) whether the fixed point is balanced within a class, and (ii) whether the fixed point is unique. Our simulations show that when multiple unbalanced fixed points exist in a homogeneous system then the time behaviour of the system demonstrates severe short term unfairness (or multistability). Implications for the use of the fixed point formulation for performance analysis are also discussed. We provide a condition for the fixed point solution to be balanced within a class, and also a condition for uniqueness. We then provide an extension of our general fixed point analysis to capture AIFS based differentiation; again a condition for uniqueness is established. An asymptotic analysis of the fixed point is provided for the case in which packets are never abandoned, and the number of nodes goes to ∞. Finally the fixed point equations are used to obtain insights into the throughput differentiation provided by different initial back-offs, persistence factors, and AIFS, for finite number of nodes, and for differentiation parameter values similar to those in the standard.

Fixed point analysis of single cell IEEE 802.11e WLANs: uniqueness, multistability and throughput differentiation

We propose an analytical model for the performance analysis of the enhanced distributed channel access (EDCA) function of the IEEE 802.11e standard. The proposed discrete-time Markov chain (DTMC) model incorporates the main quality-of-service (QoS) features of 802.11e, namely, contention window (CW), arbitration interframe space (AIFS), and transmit opportunity (TXOP) differentiation. Due to its specific design, the proposed DTMC model jointly considers the state of the medium access control (MAC) layer buffer and the MAC differentiation mechanisms which facilitates a novel performance analysis framework at an arbitrary traffic load. Analytical and simulation results are compared to demonstrate the accuracy of the proposed approach for varying traffic load, EDCA parameters, and MAC layer buffer space.

Analysis of the 802.11e enhanced distributed channel access function

In this paper we use stochastic geometry to propose two models for Aloha-based linear VANETs. The first one uses Signal over Interference plus Noise Ratio (SINR) capture condition to qualify a successful transmission, while the second one expresses the transmission throughput as a function of SINR using Shannon's law. Assuming Poisson distribution of vehicles, power-law mean path-loss and Rayleigh fading, in these models we derive explicit formulas for the mean throughput and the probability of a successful reception at a given distance. Furthermore, we optimize two quantities directly linked to the achievable network throughput: the mean density of packet progress and the mean density of information transport. This is realized by tuning the communication range and the probability of channel access. We also present numerical examples and study the impact of external noise on an optimal tuning of network parameters.

https://hal.inria.fr/inria-00439793/document

Maximizing throughput of linear vehicular Ad-hoc NETworks (VANETs) — a stochastic approach

In the IEEE 80211 MAC layer protocol, the basic access method is the DCF which is based on the CSMA/CA The IEEE 80211 adopts exponential backoff We use the fixed contention window instead of the exponentially increased contention window We employ the "linear feedback model" to compute the throughput and the delay for CSMA/CA Based on our analytical equations, we can get the optimal contention window size The results show that our optimal contention window scheme can greatly improve the performance compared to IEEE 80211 CSMA/CA MAC protocol This paper has also given the analytical results for the effect of RTS/CTS It is shown that although RTS/CTS does add the overhead to the system by using additional bandwidth, it becomes essential when the hidden terminal problem is dominant, traffic is heavy, or the packet length is very large

http://ieeexplore.ieee.org/iel5/8474/26699/01191520.pdf

Performance analysis and enhancement for IEEE 802.11 MAC protocol

The IEEE 802.11e working group aims to enhance the current 802.11 medium access control (MAC) to support integrated data and voice (or video) communications. Till now, a draft of the IEEE 802.11e enhanced distributed coordination function (EDCF) has been proposed. In the literature, there is very limited analysis about EDCF since it is a new protocol, and most related work are only confined to simulation or saturation state. In this paper, we evaluate the performance of EDCF by dividing the traffic into two groups, namely real-time packets and non-real-time packets, and use an analytical model to quantify the performance of both IFS priority and contention window (CW) priority in the EDCF. In our analysis, we assume that the traffic arrival is a Poisson process. We provide a queue model for EDCF. Since there are multiple stations contending for one channel in the system, the queue model for EDCF cannot be a regular M/G/1 model. We redefine the service time and the waiting time so that we can still use M/G/1 model in the analysis. To validate the accuracy of our analytical results, we have done extensive simulations and we observed that EDCF does provide service differentiation between different traffic categories. But due to the inherent contention characteristic of carrier sense multiple access with collision avoidance (CSMA/CA) MAC protocol, the delay cannot be guaranteed, even for the highest priority traffic. A lot more work needs to be done for the MAC protocol to effectively support service differentiation in the future. Copyright © 2004 John Wiley & Sons, Ltd.

Performance evaluation for IEEE 802.11e enhanced distributed coordination function

The IEEE 80211e working group aims to enhance the current 80211 MAC to support the integrated data and voice (or video) communications Till now a draft of the IEEE 80211e EDCF has been proposed In this paper, we evaluate the performance of the EDCF by dividing the traffic into two categories, namely real-time packets and non real-time packets, and use an analytical model to quantify the performance of both IFS priority and CW priority in the EDCF We also propose a new priority scheme, which allows the user to continuously send real-time packets We define the maximum number of real-time packets that a user can continuously send to get a good balance between the fairness and the priority Our proposed scheme is shown to provide much better results than IEEE 80211e EDCF

/pdf/performance-analysis-of-ieee-802-11e-enhanced-distributed-tejywfypp9.pdf

Performance analysis of IEEE 802.11e enhanced distributed coordination function

In ad hoc networks, CSMA/CA is a widely used MAC layer protocol. There has been considerable work on the performance evaluation of this protocol. But most work is confined to saturation performance of single-hop ad hoc networks. In this paper, we employ a linear feedback model to evaluate the performance for CSMA/CA according to the Poisson distributed traffic in both single-hop and multi-hop ad hoc networks. To the best of our knowledge, this is the first attempt to analytically evaluate the performance for CSMA/CA protocol under a general assumption about the traffic. This paper also gives analytical results, showing the impact of RTS/CTS. Although RTS/CTS do add the overhead to the system, they become essential when either the hidden terminal problem is dominant, or the traffic is heavy, or the packet length is very large. This paper also shows that the performance degrades dramatically in multi-hop ad hoc networks when the number of competing nodes increases, which implies that the scalability is still a major problem in ad hoc networks. To validate our analytical results, we have done extensive simulations, and it is observed that the simulation results match the analytical results very well. Copyright © 2006 John Wiley & Sons, Ltd.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/wcm.425

Analytical modeling of MAC protocol in ad hoc networks

An ad hoc network is a basic WLAN. In the IEEE 802.11 WLANs, CSMA/CA is a widely used MAC layer protocol and considerable work has been done on the performance evaluation of this protocol. But most research is confined to saturation performance of single-hop ad hoc networks. In this work, we employ a linear feedback model to evaluate the performance for CSMA/CA according to the Poisson distributed traffic in both single-hop and multi-hop ad hoc networks. To the best of our knowledge, this is the first attempt to evaluate the performance for CSMA/CA protocol analytically under a general assumption about the traffic. This work also gives quantitative results, showing the impact of RTS/CTS. Although RTS/CTS do add the overhead to the system, they become essential when either the hidden terminal problem is dominant, or the traffic is heavy, or the packet length is very large. They have also shown that the performance degrades dramatically in multi-hop ad hoc networks when the number of competing nodes increases, which implies that the scalability is still a major problem in ad hoc networks. The IEEE 802.11e working group aims to enhance the current 802.11 MAC to support integrated data and voice (or video) communications. Till now a draft of the IEEE 802.11e EDCF has been proposed. In the literature, there is very limited analysis about EDCF since it is a new protocol, and most related work are only confined to the simulation or the saturation state. In this work, we evaluate the performance of EDCF by dividing the traffic into two groups, namely real-time packets and non real-time packets, and use an analytical model to quantify the performance of both IFS priority and CW priority in the EDCF. In our analysis, we assume that traffic arrival is a Poisson process. Since there are multiple stations contending for one channel in the system, the queue model for EDCF cannot be a regular M/G/1 model. We redefine the service time and the waiting time so that we can still use M/G/1 model in the analysis. To validate the accuracy of our analytical results, we have done extensive simulations and we observe that EDCF does provide service differentiation between different traffic categories. But due to the inherent contention characteristic of CSMA/CA MAC protocol, the delay cannot be guaranteed, even for the highest priority traffic. A lot more work need to be done for the MAC protocol to effectively support service differentiation in the future. To enhance the performance of 802.11, we proposed optimal contention window scheme. The results show that the optimal contention window scheme greatly outperforms the 802.11 exponential contention window scheme. We also proposed UEDCF, a priority scheme to provide further priority to real-time packets. It has been shown that UEDCF can improve the performance of real-time packets without degrading the overall performance. In multi-hop ad hoc networks, hidden terminal problem might still be present, even after employing RTS/CTS. We proposed a new MAC protocol called CAA, which mainly focuses on the correct transmission of RTS/CTS messages. It is observed that this protocol can greatly improve the performance.

Yunli Chen

Papers

Performance analysis and enhancement for IEEE 802.11 MAC protocol

Performance evaluation for IEEE 802.11e enhanced distributed coordination function

Performance analysis of IEEE 802.11e enhanced distributed coordination function

Analytical modeling of MAC protocol in ad hoc networks

Performance analysis and enhancement of mac protocol for ieee 802.11 wlans