Alessandro Andreadis

QoS Enhancement with Dynamic TXOP Allocation in IEEE 802.11e

The transmission opportunity (TXOP) mechanism defined in the IEEE 802.11 e Hybrid Coordination Function (HCF) is not optimized to meet the QoS requirements of heterogeneous applications, since it is usually allocated by default according to the different Access Categories (AC). In this paper we propose a new algorithm, named DTXOP, for the dynamic assignment of the TXOP maximum duration at the Access Point (AP) of an IEEE 802.lie WLAN. DTXOP is periodically updated according to the current traffic conditions of each specific AC. Simulation experiments show that DTXOP allows to enhance delay and throughput performance and to maintain fairness between upstream and downstream channel access times.

A performance evaluation approach for GSM-based information services

The ever increasing diffusion of mobile communications will lead to massive mobile access to the Internet. The paper focuses on the wireless application protocol (WAP) for providing mobile information services, as envisaged by the personalized access to local information and services for tourists (PALIO) project within the fifth research framework of the European Commission. In particular, we consider a global system for mobile communications (GSM) network where WAP traffic is transported by the short message service (SMS) on specific GSM signaling channels. We develop suitable traffic models for both the WAP downlink traffic and the signaling traffic. An analytical approach is described that allows evaluation of the mean transmission delay of a WAP page. Finally, a downlink traffic scheduling policy is proposed in order to reduce the delay variance so that users experience a more regular traffic behavior (service usability requirement). The theoretical derivations for the GSM-SMS scenario are general and can also be tailored for other mobile communication systems. The study allows dimensioning of both the service and the downlink signaling traffic capacity in order to fulfill quality-of-service requirements for users.

FEC coding for H.263 compatible video transmission

A FEC coding strategy compatible with the H.263 standard is proposed, which uses unequal error protection to improve image quality for low bit rate video transmission over noisy channels. The H.263 bit stream is partitioned into classes of different sensitivity, without introducing any modification into the standard stream syntax. The experimental results show that the proposed scheme can be used either when video is transmitted over channels having high round-trip delay and limited bandwidth, or as a FEC segment of an integrated FEC-ARQ strategy.

An admission control algorithm for QoS provisioning in IEEE 802.11e EDCA

The IEEE 802.11e standard introduces QoS features for delivering multimedia applications in a WLAN environment. However, under heavy traffic loads, network saturation is easily achieved and consequently QoS performance are inevitably degraded. In this paper an admission control algorithm for resource sharing in a wireless LAN environment is proposed, with the aim at enhancing QoS support for real-time applications (e.g., VoIP and video) that are particularly sensitive to delay and jitter. The algorithm avoids saturation and protects the admitted traffics from being degraded, through a continuous monitoring of the available resources. A further optimization of the algorithm is finally implemented through the adoption of dynamic bursty transmissions times (i.e., TXOP). Performance evaluation is carried out through computer simulations, showing the benefits gained by the 802.11e EDCA scheme in terms of goodput and delay.

Qos enhancement for multimedia traffics with dynamic txoplimit in ieee 802.11e

The transmission opportunity (TXOP) mechanism defined in the IEEE 802.11e Hybrid Coordination Function (HCF) allows a wireless station to transmit a burst of frames back-to-back without re-entering the channel contention phase. Unfortunately, the adoption of a unique value for its maximum duration (named TXOPlimit) for all stations, included the AP, results in an unfair behavior, that provokes an unbalance between uplink and downlink throughputs and consequent QoS degradation. To solve this problem we propose an algorithm capable of tuning the TXOPlimit value at the AP, in order to dynamically change its channel resources according to the current traffic conditions. Extensive simulations show that the proposed algorithm allows to enhance significantly QoS performance of multimedia traffics and to increase the global WLAN throughput.

Improving QoS Performance in IEEE 802.11e Under Heavy Traffic Loads

IEEE 802.11e standard is a concrete attempt to QoS challenge, but when the volume of traffic flows increases, this approach is not sufficient. Several techniques have been developed to improve WLAN QoS performance, mainly introducing a trade-off between performance and standard compatibility. This work describes the techniques used to enhance EDCA efficiency by suitably regulating standard’s parameters and it introduces an innovative algorithm, named Dynamic TXOP (DTXOP), capable of enhancing fairness between upstream and downstream resource allocation in Wi-Fi networks. Finally, after a brief review of admission control algorithms for QoS support under heavy traffic loads, a suitable admission control scheme is integrated with DTXOP as a possible solution for facing QoS degradation of active sources due to an excessive network load. The benefits obtained by integrating DTXOP and the proposed admission control policy are shown in terms of QoS enhancement and efficiency in resource allocation.

Performance analysis of the WAP protocol over GSM-SMS

This paper, based on the PALIO project, deals with an information tourist service accessible through mobile phones by means of the Wireless Application Protocol (WAP). We have considered a Global System for Mobile communications (GSM) network where the WAP traffic is transported by the Short Message Service (SMS) on a logical channel that also conveys signaling messages. Suitable models have been proposed for both WAP and signaling traffic. An analytical approach (validated by simulations) has been developed to evaluate the mean delay to browse a WAP page.

Evaluation of QoS Support for Multimedia Traffics in IEEE 802.11e

The efficient delivery of multimedia over wireless LANs is strongly dependent on the requirements imposed by different traffics contending the channel resources. Mechanisms for supporting QoS are needed, especially when voice and video applications come into play with stringent constraints in terms of delay and throughput. In this paper we evaluate, through simulations, the benefits derived from the adoption of the IEEE 802.11e standard in delivering multimedia over WLANs. The adoption of fine-tuned parameters regulating the EDCA MAC scheme permits to achieve high performance in terms of goodput and delay, allowing multimedia traffics to fully satisfy their QoS requirements. Realistic assumptions have been made in our simulations, by taking into account the shadowing effects, typical of closed environments, and by adopting suitable parameters to simulate 802.11e channels as exactly as possible

High-capacity resource sharing schemes for broadband wireless networks

The unique capabilities of new cellular systems are expected to provide users with integrated multimedia services. Since the air interface still represents the system bottleneck, this paper proposes novel scheduling techniques to integrate efficiently the support of real-time traffic (i.e. voice and video) and data bursty traffic under quality of service (QoS) guarantees. Prioritization among traffic classes is adopted and a polling service discipline is employed within a class, where the permission rights of each traffic source are determined on the basis of token bucket regulators. Two polling-based approaches are compared to serve the sources of a class: (i) when a source is enabled to transmit, a burst of packets is sent at once; (ii) within the time interval destined to a traffic class, a cyclic service of the sources is allowed on a packet basis. With realistic assumptions on both radio channel conditions and protocol signaling overhead, this paper compares these two different approaches and the dynamic slot assignment++ (DSA++) scheme appeared in the literature. The obtained results highlight that our second scheme (case ii) allows increasing the number of supported video traffic sources of many units with respect to DSA++. Finally, an analytical approach has been proposed for our second polling scheme. Copyright

Proposal of a packet access scheme to integrate isochronous and data bursty traffic in low Earth orbit-mobile satellite systems

A novel medium access control (MAC) protocol, named dynamic resource assignment multiple access enhanced version (DRAMA+), is proposed for low Earth orbit-mobile satellite systems (LEO-MSSs). It is based on a time division multiple access air interface and utilizes access slots distinct from information slots. The DRAMA+ scheme realizes an efficient access phase and manages transmission requests with an on board scheduler. On the basis of suitable models for both isochronous voice sources with activity detection and data bursty sources, we have shown that DRAMA+ outperforms other protocols in the literature.