Stefano Lucetti

Providing air-time usage fairness in IEEE 802.11 networks with the deficit transmission time (DTT) scheduler

Wireless systems based on the IEEE 802.11 standard are known to suffer a performance degradation when just a single station in the network experiences bad channel conditions toward the Access Point (AP). This phenomenon, known as the “performance anomaly”, is mainly due to the max-min throughput fairness of the CSMA/CA algorithm of the 802.11 MAC. The simple FIFO scheduling policy usually implemented in the AP also contributes to this problem. In order to overcome the performance anomaly, we propose the Deficit Transmission Time (DTT) scheduler. The aim of DTT is guaranteeing each station a fair medium usage in terms of transmission time. This feature, directly related to the proportional fairness concept, allows to ideally achieve exact isolation among the traffic flows addressed to different stations. DTT achieves this goal taking advantage of measurements of actual frame transmission times. Experiments carried out using a prototype implementation of DTT are compared with analogous tests performed with a classic FIFO queue of a commercial AP and a recently proposed traffic shaping scheme aimed at solving the same 802.11 performance anomaly.

The wireless hierarchical token bucket: a channel aware scheduler for 802.11 networks

The paper proposes an architecture for a scheduling algorithm to be integrated in IEEE 802.11 access points (AP), able to take into account, besides the transport service class required by the destination user, the channel quality experienced by the destination mobile station (STA). The relevance of this topic is due to the observation that when one or more STAs experience poor radio channel conditions, they increase the time spent to transmit a single packet, due to the retransmission of corrupted frames and the adoption of transmission techniques at a lower bit rate, leading to an inefficient use of the shared medium. These phenomena give, as a consequence, a worsening of the performance of all the STAs sharing the wireless medium, independent of their radio channel conditions. The adoption of a scheduling algorithm able to manage information on channel quality permits these effects to be reduced, not penalizing the STAs experiencing good channel conditions and, as a consequence, their experienced throughput. As a result, only the STAs in bad channel conditions experience a reduction of throughput. A prototype of an AP equipment implementing the proposed architecture is presented: it was obtained by modifying the hierarchical token bucket (HTB) and is referred to as wireless hierarchical token bucket (WHTB). The performance of the presented prototype is evaluated experimentally and compared with those obtained with standard scheduling algorithms, which do not take into account information on channel quality.

On synchronization techniques: performance and impact on time metrics monitoring

The paper presents an experimental comparison of device synchronization strategies to monitor quality of service (QoS) time metrics, such as the one-way delay and the delay variation. The compared strategies are based on global positioning system (GPS), public Internet network time protocol (NTP) servers and ad hoc GPS-based NTP servers with different treatments of the NTP traffic. The presented experimental results allow to quantitatively evaluate the level of performance achievable using each synchronization technique. Finally, considering a satellite test bed, we focus our analysis on two relevant aspects of time metrics monitoring: the impact of the synchronization errors on the metrics observation and the different values that can be measured when a metric is monitored at the application or data link level. Copyright

A Model-based Admission Control for IEEE 802.11e Networks

The recently approved IEEE 802.11e standard improves the support for real-time multimedia applications in Wireless LANs. Yet, to provide some level of guarantee in terms of QoS parameters, it is essential to use an admission control scheme. We have devised one such scheme, which bases the admissibility test on the time occupancy of the medium. The network state is assessed through an analytical model of the EDCA in non-saturation conditions. In the paper, beyond describing the proposed scheme, we also show its effectiveness in the case of voice over IP applications.

Admission region of multimedia services for EDCA in IEEE 802.11e access networks

This paper presents a simulation analysis for the evaluation of the admission region of a IEEE 802.11e network adopting the EDCA (Enhanced Distributed Channel Access) mechanism. In particular, this study gives an estimate of the number of QoS-aware applications, namely videoconference and Voice over IP (VoIP), that can be admitted to the transport service offered by the EDCA while satisfying their QoS requirements. The traffic sources adopted for the simulation are obtained from measurement campaigns led on the emulation of VoIP and videoconference services based respectively on the G.723.1 and the H.263 codecs. The results emphasize the bottleneck role played by the Access Point when services producing symmetrical traffic are conveyed over an 802.11e access network. Furthermore, the QoS parameters experienced in a mix of VoIP, videoconference and TCP traffic under EDCA are compared with those obtained when the DCF mechanism is adopted. This comparison clearly highlights the efficiency in traffic differentiation of the EDCA algorithm.

Comparison of LRD and SRD traffic models for the performance evaluation of finite buffer systems

The paper presents a comparison of several traffic models in order to investigate their adequacy to approximate the behaviour of actual traffic in a single server queue under realistic working conditions. Hence, considering queueing systems with finite buffers, models parsimoniously fitting the long range dependence (LRD) property are compared to intrinsically short range dependent models and to flexible ones, capturing both LRD and short range dependence (SRD). The paper highlights an optimistic queueing behaviour evaluated by means of models capturing only LRD property, when realistic buffer sizes are considered and a strong SRD correlation component is present. The simulation results point out that the circulant modulated Poisson processes (CMPP) and fractional autoregressive integrated moving average (p,d,0) (f-ARIMA(p,d,0)) are suitable models to estimate the performance of the queueing system under heavy load conditions. On the contrary, when the system works under low-medium load conditions, the relevance of the marginal distribution to queueing performance and the inaccuracy of the CMPP model on fitting this statistic in the case of actual traffic data lead to a reduction of the accuracy of this model on system performance evaluation.

Extension and Application of the Network Utilization Characteristic Metric to IEEE 802.11e

In this paper we describe a time-based admission control scheme for multimedia over IP services in wireless LANs based on the IEEE 802.11e standard. We build on an existing work where it was defined a parameter, called network utilization characteristic (NUC), to measure the temporal occupancy of the wireless channel of an IEEE 802.11 network. The work we present is twofold. At first, we extend this parameter to 802.11e systems. A simple Markov chain is used for this purpose. Then we build an admission control scheme based on the so-extended NUC and verify its effectiveness in a practical case: a network with mixed voice and video traffic. Simulations, also with different link qualities, are used to validate the effectiveness of our scheme.

Admission region for multimedia services in IEEE 802.11e systems

The work presents the simulation analysis aimed at evaluating the admission region of a IEEE 802.11 e draft 4.0 standard, adopting the EDCF (enhanced distributed coordination function) access control mechanism. In particular, the study gives an estimate of the number of QoS aware services, namely videoconference and voice over IP (VoIP), that can be admitted to the transport service offered by the EDCF, while their QoS requirements are satisfied. In the study, the ideal radio channel model is assumed. Furthermore, the traffic sources are obtained from traffic data acquired by means of measurement campaigns made during emulation of VoIP and videoconference services, based respectively on G.723.1 and H.261 codec. Finally the QoS parameters experienced by a mix of VoIP, videoconference and TCP traffic transmitted according to the EDCF mechanism are compared with those obtained when the DCF mechanism is adopted. The result of this comparison clearly highlights the efficiency in the traffic differentiation of the EDCF mechanism.

A comparison of HTB based channel-aware schedulers for 802.11 systems

The 802.11 commercial devices adopt a simple FIFO scheduler, which does not allow to isolate flows addressed to different destinations. This characteristic leads to the well-known performance anomaly of 802.11: when one or more STAs experiment poor radio channel conditions, they increase the time spent to transmit a single packet leading to an inefficient use of the shared medium. These phenomena have as a consequence the worsening of the performance of all the STAs sharing the wireless medium independently of their radio channel conditions. In this scenario, the paper proposes an architecture for a scheduling algorithm to implement in the AP for the downlink traffic, based on Hierarchical Token Bucket (HTB). The peculiarity of the proposed architecture is its ability to take into account, besides the transport service class required by the destination user, the channel quality experimented by the destination mobile STAtion (STA). Starting from this architecture two scheduling schemes are then proposed: the wireless HTB (WHTB) and the time-based WHTB (TWHTB). Both schemes are implemented in a prototype of AP. Hence, the performance of the proposed scheduling algorithms are experimentally evaluated and compared with those obtained with standard scheduling algorithm, which do not take into account information on channel quality. Finally, the paper presents a comparison in terms of implementation issues highlighted by the two proposed schemes, during the developing phase and the experimental analysis.

Interference-Aware Channel Assignment in Wireless Mesh Networks

The emerging IEEE 802.11-based Wireless Mesh Networks (WMNs) suffer of poor performance in terms of throughput. The use of multi-hop paths to relay the traffic raises the problems of intra-flow and inter-flow interference in the wireless backbone. A possible solution to this problem is exploiting a multi-channel framework. In this context, we develop an innovative interfere-ence model that permits to take into account the actual signal power received from each interference source, the interference produced by sources external to the WMN, the link utilization factor and directive antennas. Starting from this new model, we propose a channel assignment algorithm for multi-radio WMNs which is based on graph theory. Its performance is evaluated through simulations, which show a great improvement over existing channel assignment algorithms.