This paper investigates differentiated services in wireless packet networks using a fully distributed approach that supports service differentiation, radio monitoring, and admission control. While our proposal is generally applicable to distributed wireless access schemes, we design, implement, and evaluate our framework within the context of existing wireless technology. Service differentiation is based on the IEEE 802.11 Distributed Coordination Function (DCF) originally designed to support best-effort data services. We analyze the delay experienced by a mobile host implementing the IEEE 802.11 DCF and derive a closed-form formula.We then extend the DCF to provide service differentiation for delay-sensitive and best-effort traffic based on the results from the analysis. Two distributed estimation algorithms are proposed. These algorithms are evaluated using simulation, analysis, and experimentation. A Virtual MAC (VMAC) algorithm passively monitors the radio channel and estimates locally achievable service levels. The VMAC estimates key MAC level statistics related to service quality such as delay, delay variation, packet collision, and packet loss. We show the efficiency of the VMAC algorithm through simulation and consider significantly overlapping cells and highly bursty traffic mixes. In addition, we implement and evaluate the VMAC in an experimental differentiated services wireless testbed. A Virtual Source (VS) algorithm utilizes the VMAC to estimate application -level service quality. The VS allows application parameters to be tuned in response to dynamic channel conditions based on "virtual delay curves." We demonstrate through simulation that when these distributed virtual algorithms are applied to the admission control of the radio channel then a globally st...

This paper evaluates four mechanisms for providing service differentiation in IEEE 802.11 wireless LANs. The evaluated schemes are the Point Coordinator Function (PCF) of IEEE 802.11, the Enhanced Distributed Coordinator Function (EDCF) of the proposed IEEE 802.11e extension to IEEE 802.11, Distributed Fair Scheduling (DFS), and Blackburst. The evaluation was done using the ns-2 simulator. Furthermore, the impact of some parameter settings on performance has also been investigated. The metrics used in the evaluation are throughput, medium utilization, collision rate, average access delay, and delay distribution for a variable load of real time and background traffic. The simulations show that the best performance is achieved by Blackburst. PCF and EDCF are also able to provide pretty good service differentiation. DFS can give a relative differentiation and consequently avoids starvation of low priority traffic.

Abstract—Voice over Internet Protocol (VoIP) over a wireless local area network (WLAN) is poised to become an important Internet application. However, two major technical problems that stand in the way are: 1) low VoIP capacity in WLAN and 2) unacceptable VoIP performance in the presence of coexisting traffic from other applications. With each VoIP stream typically requiring less than 10 kb/s, an 802.11b WLAN operated at 11 Mb/s could in principle support more than 500 VoIP sessions. In actuality, no more than a few sessions can be supported due to various protocol overheads (for GSM 6.10, it is about 12). This paper proposes and investigates a scheme that can improve the VoIP capacity by close to 100 % without changing the standard 802.11 CSMA/CA protocol. In addition, we show that VoIP delay and loss performance in WLAN can be compromised severely in the presence of coexisting transmission-control protocol (TCP) traffic, even when the number of VoIP sessions is limited to half its potential capacity. A touted advantage of VoIP over traditional telephony is that it enables the creation of novel applications that integrate voice with data. The inability of VoIP and TCP traffic to coexist harmoniously over the WLAN poses a severe challenge to this vision. Fortunately, the problem can be largely solved by simple solutions that require only changes to the medium-access control (MAC) protocol at the access point. Specifically, in our proposed solutions, the MAC protocol at the wireless end stations does not need to be modified, making the solutions more readily deployable over the existing network infrastructure. Index Terms—Capacity, IEEE 802.11, quality of service (QoS), voice over Internet Protocol (VoIP), wireless local area network

Member, IEEE With the success of wireless technologies in consumer electronics, standard wireless technologies are envisioned for the deployment in industrial environments as well. Industrial applications involving mobile subsystems or just the desire to save cabling make wireless technologies attractive. Nevertheless, these applications often have stringent requirements on reliability and timing. In wired environments, timing and reliability are well catered for by fieldbus systems (which are a mature technology designed to enable communication between digital controllers and the sensors and actuators interfacing to a physical process). When wireless links are included, reliability and timing requirements are significantly more difficult to meet, due to the adverse properties of the radio channels. In this paper we thus discuss some key issues coming up in wireless fieldbus and wireless industrial communication systems: i) fundamental problems like achieving timely and reliable transmission despite channel errors; ii) the usage of existing wireless technologies for this specific field of applications, and iii) the creation of hybrid systems in which wireless stations are included into existing wired systems.

IEEE 802.11 LANs feature two basic modes of operation: Distributed Coordination Function (DCF) and Point Coordination Function (PCF). DCF allows for independent and distributed channel access, PCF requires a dedicated station (access point, AP) which provides a centralized channel access arbitration. In this paper we study at which point it makes sense for best-effort, real-time, and mixed traffic scenarios to switch from DCF to PCF mode. We will show that it is particularly useful to use the DCF mode for low trac and less number of mobile scenarios, also in the case of real-time traffic. Furthermore, we show that the PCF function should be used for high load scenarios and to reduce contention in WLAN cells containing a large number of mobiles. In addition, we will show, that the PCF enhances the network capacity. We also present a novel approach based on an implicit signaling to optimize the access point polling structure and to reduce the number of unsuccessful polling attempts resulting in...

Wireless LAN administrators are often called upon to deal with the problem of sporadic user congestion at certain popular spaces ("hot-spots") within the network. To address this problem, we describe and evaluate two new approaches, explicit channel switching and networkdirected roaming for providing hot-spot congestion relief while maintaining pre-negotiated user bandwidth agreements with the network. The goals of these algorithms are: (i) to accommodate more users by dynamically providing capacity where it is needed, when it is needed; (ii) to improve overall network utilization by making more efficient use of deployed resources; and (iii) to guarantee at least a minimum amount of bandwidth to users. We propose that both the network and its users should explicitly and cooperatively adapt themselves to changing load conditions depending on their geographic location within the network. We describe how these algorithms enable the network to transparently adapt to user demands and balance load across its access points (APs). We evaluate the effectiveness of these algorithms on improving user service rates and network utilization using simulations. Our algorithms improve the degree of load balance in the system by over 30%, and user bandwidth allocation by up to 52% in comparison to existing schemes that offer little or no load balancing. 1.

This paper studies an important problem in the IEEE 802.11 distributed coordination function (DCF)-based wireless local area network (WLAN): how well can the network support quality of service (QoS). Specifically, this paper analyzes the network's performance in terms of maximum protocol capacity or throughput, delay, and packet loss rate. Although the performance of the 802.11 protocol, such as throughput or delay, has been extensively studied in the saturated case, it is demonstrated that maximum protocol capacity can only be achieved in the nonsaturated case and is almost independent of the number of active nodes. By analyzing packet delay, consisting of medium access control (MAC) service time and waiting time, accurate estimates were derived for delay and delay variation when the throughput increases from zero to the maximum value. Packet loss rate is also given for the nonsaturated case. Furthermore, it is shown that the channel busyness ratio provides precise and robust information about the current network status, which can be utilized to facilitate QoS provisioning. The authors have conducted a comprehensive simulation study to verify their analytical results and to tune the 802.11 to work at the optimal point with maximum throughput and low delay and packet loss rate. The simulation results show that by controlling the total traffic rate, the original 802.11 protocol can support strict QoS requirements, such as those required by voice over Internet protocol (VoIP) or streaming video, and at the same time achieve high channel utilization.

IEEE 802.11 contains a mechanism for transmission of data with realtime constraints known as Point Coordination Function. This supplementary medium access protocol resides on top of the basic medium access mechanism Distributed Coordination Function and uses a centralized polling approach. Due to the complexity of a PCF implementation and the predicted ineciency of the PCF several proposals have been presented for providing QoS support without the need of a centralized scheduler. Those solutions suer from the fact that they are shifting implementation complexity from the access point to the mobile nodes. In this paper we compare the suitability of the basic DCF and PCF protocols for the transmission of audio data in an interactive scenario. We show that a simple priority mechanism used on the mobiles as well as the access point is suitable for providing improved QoS in terms of bandwidth and without the need of an extended DCF protocol. In combination with the PCF an adequate delay characteristic for audio ows is achievable as well. To overcome the limitations in channel capacity caused by the PCF we suggest an implicit signaling scheme for improving the channel capacity by avoiding unsuccessful PCF polling attempts. Keywords IEEE 802.11, WLAN, real{time, best{eort, voice transmission, scheduling, PCF, DCF 1.

The recently proposed IEEE 802.11 MAC protocol provides shared access to a wireless channel. This paper uses an analytic model to study the channel capacity --- i.e. maximum throughput --- when using the basic access (two-way handshaking) method in this protocol. It provides closed-form approximations for (1) the probability of collision p, (2) the maximum throughput S and (3) the limit on the number of stations in a wireless cell. The analysis also shows that (4) p does not depend on the packet length, the latency in crossing the MAC and physical layers, the acknowledgement timeout, the interframe spaces and the slot size; (5) p and S (and other performance measures) depend on the minimum window size W and the number of stations n only through a variable g = W=(n+ 1) --- consequently, halving W is like doubling n; (6) the maximum contention window size has minimal effect on p and S; (7) the choice of W that maximizes S is proportional to the square root of the packet length; (8) S ...

In this paper, we present an analytic model for evaluating the queueing delays at nodes in an IEEE 802.11 MAC based wireless network. The model can account for arbitrary arrival patterns, packet size distributions and number of nodes. Our model gives closed form expressions for obtaining the delay and queue length characteristics. We model each node as a discrete time G/G/1 queue and derive the service time distribution while accounting for a number of factors including the channel access delay due to the shared medium, impact of packet collisions, the resulting backoffs as well as the packet size distribution. The model is also extended for ongoing proposals under consideration for 802.11e wherein a number of packets may be transmitted in a burst once the channel is accessed. Our analytical results are verified through extensive simulations. The results of our model can also be used for providing probabilistic quality of service guarantees and determining the number of nodes that can be accommodated while satisfying a given delay constraint.