Subodh Pudasaini

COMIC: Intelligent Contention Window Control for Distributed Medium Access

In this letter, a scheme for Collision Mitigation with Intelligent Contention Window Control (COMIC) is proposed for backoff based collision resolution algorithm. COMIC intelligently mitigates collisions by probabilistically maximizing the selection likelihood of relatively less collision-probable contention slots over the backoffed contention window. A unified Markovian model for the Distribution Coordination Function (DCF) that incorporates COMIC, DCFcomic, is formulated for the performance analysis. The performance results show that DCFcomic outperforms the conventional DCF in both throughput and average packet delay due to the significant reduction in packet collisions.

Carrier sense multiple access with improvised collision avoidance and short-term fairness

In this paper, we present a simple method to simultaneously enhance collision avoidance efficiency and short-term fairness of a most popular contention based medium access control protocol, carrier sense multiple access with collision avoidance. The key idea here is to adaptively tune the shape of contention slot selection distribution over the temporal contention window during ongoing collision resolution process which, in the legacy scheme, used to be flat throughout. The tuning mechanism is such designed that it not only maximizes the selection likelihood of relatively less collision prone contention slots over the contention window but also compensates the idle delay that the contending stations have suffered in their recent access attempt. Through rigorous numerical and simulation based analysis, the proposed scheme is shown to enhance the performance of a IEEE 802.11 based distributed wireless network in terms of network throughput efficiency and packet transmission delay while allowing individual stations to share the channel fairly even in short time scale.

Cross-Layer Performance Analysis of CSMA/iCA Based Wireless Local Area Network

Carrier sense multiple access with improvised collision avoidance (CSMA/iCA) has recently been proposed as an enhancement to CSMA/CA. It has been reported to be superior than the legacy counterpart in terms of throughput efficiency, packet transmission delay and quantitative fairness index. Nevertheless, the superiority has been shown assuming ideal network conditions: error-free physical layer (L1) and saturated (always non empty) queue at medium access control layer (L2). These strict assumptions, however, do not accurately hold in the real-world Wireless Local Area Networks since the wireless medium is generally error-prone and the arrival of the packets at L2 queue is generally bursty resulting in non-saturated queue occupancy. Thus, the reported performance, especially throughput, in such typical L1/L2 settings is not complete to understand the performance benefit that CSMA/iCA offers under the realistic network settings. In this paper, we relax the aforesaid ideal assumptions and present a cross-layer (L1/L2) performance analysis. Our cross-layer analytical model considers the effect of Rayleigh fading induced bit errors in L1 and non-saturated queue occupancy due to Poisson packet arrival at L2 queue. By virtue of the validated numerical results, we show that the CSMA/iCA consistently retains its throughput gain over CSMA/CA for the non-ideal wireless settings as well.

A New Emergency-Handling Mechanism based on IEEE 802.15.4 for Health-Monitoring Applications

The recent advances in wireless communication systems and semiconductor technologies are paving the way for new applications over wireless sensor networks. Health-monitoring application (HMA) is one such emerging technology that is focused on sensing and reporting human vital signs through the communication network comprising sensor devices in the vicinity of the human body. The sensed vital signs can be divided into two categories based on the importance and the frequency of occurrence: occasional emergency signs and regular normal signs. The occasional emergency signs are critical, so they have to be delivered by the specified deadlines, whereas the regular normal signs are non-critical and are only required to be delivered with best effort. Handling the occasional emergency sign is one of the most important attributes in HMA because a human life may depend on correct handling of the situation. That is why the underlying network protocol suite for HMA should ensure that the emergency signs will be reported in a timelymanner. However, HMA based on IEEE 802.15.4 might not be able to do so owing to the lack of an appropriate emergency-handling mechanism. Hence, in this paper, we propose a new emergency-handling mechanism to reduce the emergency reporting delay in IEEE 802.15.4 through the modified superframe structure. A fraction of an inactive period is modified into three new periods called the emergency reporting period, emergency beacon period, and emergency transmission period, which are used opportunistically only for immediate emergency reporting and reliable data transmission. Extensive simulation is performed to evaluate the performance of the proposed scheme. The results reveal that the proposed scheme achieves improved latency and higher emergency packets delivery ratio compared with the conventional IEEE 802.15.4 MAC.

Throughput and reliability analysis of a scalable broadcast MAC for distributed wireless networks

In carrier-sense-multiple-access-based distributed wireless networks, the legacy mechanism of arbitrating the access attempts of multiple broadcast users (using a fixed-size temporal contention window consisting of multiple contention slots having uniform selection probability) has been well understood to be inefficient in avoiding collisions, especially when the network is large. Recently, it has been found that this inefficiency can be mitigated by exploiting an additional degree of freedom (DoF) over the contention window, referred to as contention slot selection distribution (CSSD). In this article, the authors examine how the additional DoF contributes to enhancing network performance and quantify the performance gains that the network can achieve in terms of easily understandable performance indicators like network throughput efficiency and broadcast reliability. A discrete-time Markov chain is formulated to characterize the access arbitration process involving a reverse exponential CSSD and its steady-state solution is fed to a renewal-reward process-based network model to estimate the performances of interest. Validated numerical results are presented which confirm that this new channel access arbitration mechanism can significantly enhance the scalability of distributed broadcast networks.

An Intelligent Contention Window Control Scheme for Distributed Medium Access

This paper proposes a novel contfntion window control scheme for backoff based CRA (Collision Resolution Algorithm). The proposed scheme demonstrates a distributed way to control random slot selection over the contention window in each contention stage with reference to nomlalized backoff value in the previous contention stage. Such control results an intelligent backoff time distribution over the contention window for each contention stage. Backoff time chosen for every access from such distribution potentially mitigates much collisions along with reduced channel ideal time. Analysis results show that, compared to the performance of legacy scheme incorporating DSSS (Direct Sequence Spread Spectrum) 802.11b system parameters, there is at least 13% normalized throughput enhancement with heavily reduced packet delay in an IBSS (Independent Basic Service Set) with more than 30 stations with saturated traffic.

QoS provisioning in CSMA/iCA based medium access control protocol for WLAN

The fundamental basis of QoS differentiation in IEEE 802.11e EDCA is to differentiate durations of several channel access parameters for initiating and pursuing contention over the shared channel. More precisely, EDCA specifies the durations of those parameters in such a way that they remain inverse-proportional to the corresponding priorities of the contending packets. As such, the higher priority packets have shorter channel access parameters which privilege them to possibly win contention even in the presence of lower priority packets. As the number of lower priority packets increases, however, frequent inevitable priority inversions and inter-priority class packet collisions degrade the perceived QoS grade of the higher priority packets. We identify that occurrence probability of such problematic events would increase if the conventional CSMA/CA in EDCA is replaced by its newly proposed enhanced version known as CSMA/iCA. We find that the problem is rooted to the CSMA/iCAs approach of tuning Contention Slot Selection Distribution (CSSD) over the contention window. In this paper, we present a simple scheme, named Push CSSD Mean Right, that slightly modifies the original CSSD tuning mechanism in CSMA/iCA in order to reduce the occurrence probability of such QoS-degrading problematic events. Via rigorous simulations in ns-2, we demonstrate the suitability of the proposed scheme in CSMA/iCA based EDCA for provisioning QoS.

Cross-layer analysis of CSMA/iCA based Wireless Local Area Network

Carrier Sense Multiple Access with intelligent Collision Avoidance (CSMA/iCA) has recently been proposed as an enhancement to CSMA/CA. It has been reported to be superior than the legacy counterpart in terms of throughput efficiency, packet transmission delay and quantitative fairness index. Nevertheless, the superiority has been shown assuming ideal network conditions: error-free physical layer (L1) and saturated (always non empty) queue at medium access control layer (L2). These strict assumptions, however, do not accurately hold in the real-world Wireless Local Area Networks (WLANs) since the wireless medium is generally error-prone and the arrival of the packets at L2 queue is generally bursty resulting in non-saturated queue occupancy. Thus, the reported performance, especially throughput, in such typical L1/L2 settings is not complete to understand the performance benefit that CSMA/iCA offers under the realistic network settings. In this paper, we relax the aforesaid ideal assumptions and present a cross-layer (L1/L2) performance analysis. Our cross-layer analytical model considers the effect of Rayleigh fading induced bit errors in L1 and non-saturated queue occupancy due to Poisson packet arrival at L2 queue. By virtue of the rigorous analysis, we have shown that CSMA/iCA consistently retains its throughput gain over CSMA/CA for the real world wireless settings as well.

Performance characterization of CSMA/CA adapted multi-user MIMO aware MAC in WLANs

To realize the multi-user multiple input multiple output (MIMO) advantage over WLANs, it requires significant changes in the MAC protocol. Either the dominant MAC protocol carrier sense multiple access/collision avoidance (CSMA/CA) needs to be replaced by a novel multi-user MIMO aware MAC protocol or it should be upgraded into multi-user MIMO aware CSMA/CA. Nevertheless, the simplest approach would be upgrading the CSMA/CA. Simple modifications in the control packets format and/or the channel access mechanism can upgrade CSMA/CA into simple, yet practicable, multi-user MIMO aware MAC protocol. By utilizing convenient changes, several modification approaches can be provisioned for this purpose. Hence, it is important to understand their performance benefits and trade-offs. In this article, we discuss some of such modification approaches that best represent the possible modifications. We provide their detail performance analysis based on analytical modeling and derived expressions in terms of throughput and delay. We also derive expressions for achievable performance and present their performance limits too.

A Reactive Cross Collision Exclusionary Backoff Algorithm in IEEE 802.11 Network

An inseparable challenge associated with every random access network is the design of an efficient Collision Resolution Algorithm (CRA), since collisions cannot be completely avoided in such network. To maximize the collision resolution efficiency of a popular CRA, namely Binary Exponential Backoff (BEB), we propose a reactive backoff algorithm. The proposed backoff algorithm is reactive in the sense that it updates the contention window based on the previously selected backoff value in the failed contention stage to avoid a typical type of collision, referred as cross-collision. Cross-collision would occur if the contention slot pointed by the currently selected backoff value appeared to be present in the overlapped portion of the adjacent (the previous and the current) windows. The proposed reactive algorithm contributes to significant performance improvements in the network since it offers a supplementary feature of Cross Collision Exclusion (XCE) and also retains the legacy collision mitigation features. We formulate a Markovian model to emulate the characteristics of the proposed algorithm. Based on the solution of the model, we then estimate the throughput and delay performances of WLAN following the signaling mechanisms of the Distributed Coordination Function (DCF) considering IEEE 802.11b system parameters. We validate the accuracy of the analytical performance estimation framework by comparing the analytically obtained results with the results that we obtain from the simulation experiments performed in ns-2. Through the rigorous analysis, based on the validated model, we show that the proposed reactive cross collision exclusionary backoff algorithm significantly enhances the throughput and reduces the average packet delay in the network.