Two simple modifications for improving IEEE802.11 DCF throughput performance

Over the past few years, mobile networks have emerged as a promising approach for future mobile IP applications. With limited frequency resources, designing an effective MAC (Medium Access Control) protocol is a hot challenge. IEEE 802.11b/g/a/n networks are currently the most popular wireless LAN products on the market [1]. The conventional IEEE 802.11b and 802.11g/a specification provide up to 11 and 54 Mbps data rates, respectively. However, the MAC protocol that they are based upon is the same and employs a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol with binary exponential back-off. IEEE 802.11 DCF (Distributed Coordination Function) is the de facto MAC protocol for wireless LAN because of its simplicity and robustness [2,3]. Therefore, considerable research efforts have been put on the investigation of the DCF performance over wireless LAN [2]. With the successful deployment of IEEE 802.11a/b/g wireless LAN and the increasing demand for real-time applications over wireless, the IEEE 802.11n Working Group standardized a new MAC and PHY (Physical) layer specification to increase the bit rate to be up to 600 Mbps [3]. The throughput performance at the MAC layer can be improved by aggregating several frames before transmission [3]. Frame aggregation not only reduces the transmission time for preamble and frame headers, but also reduces the waiting time during CSMA/CA random backoff period for successive frame transmissions. The frame aggregation can be performed within different sub-layers. In 802.11n, frame aggregation can be performed either by A-MPDU (MAC Protocol Data Unit Aggregation) or A-MSDU (MAC Service Data Unit Aggregation). Although frame aggregation can increase the throughput at the MAC layer under ideal channel conditions, a larger aggregated frame will cause each station to wait longer before its next chance for channel access. Under errorprone channels, corrupting a large aggregated frame may waste a long period of channel time and lead to a lower MAC efficiency [4]. On the other hand, wireless LAN mobile stations that are defined as the stations that access the LAN while in motion are considered in this chapter. The previous paper analyzed the IEEE 802.11b/g/n MAC performance for wireless LAN with fixed stations, not for wireless LAN with mobile stations [5, 6, 7, 8, 9, 10]. On the contrary, Xi Yong [11] and Ha Cheol Lee [12] analyzed the MAC performance for IEEE 802.11 wireless LAN with mobile stations, but considered only IEEE 802.11 and 802.11g/a wireless LAN specification. So, this chapter summarizes all the reference papers and analyzes the IEEE 802.11b/g/a/n MAC performance for wireless LAN with fixed and mobile stations. In other words, we will present the analytical evaluation of saturation

In IEEE 802.11 standard, distributed coordination function (DCF) uses binary exponential backoff (BEB) algorithm to resolve collisions. However, performance of DCF with BEB decreases as the number of contending stations increases. To solve this problem, enhanced gentle DCF (EGDCF) and binary negative-exponential backoff algorithm (BNEB) have been proposed. In EGDCF algorithm, each station counts the number of consecutive successful transmissions in present backoff stage. If the number of consecutive successful transmissions reaches the predefined maximum value, station decreases its backoff stage. BNEB algorithm maintains a maximum contention window size during collisions and reduces a contention window size into half after complete transmission without retransmission. Since these two algorithms control backoff stage and contention window based on estimation of the number of stations, they improve performance of DCF. However, DCF cannot support differential quality of services (QoS) So IEEE 802.11e has adopted EDCA for contention-based medium access mechanism. In this paper, we adopt EGDCF and BNEB algorithms into EDCA to effectively support differentiated services in wireless LAN. We evaluate performance of conventional EDCA and EDCA with EGDCF and BNEB under saturation environment.

Impact of bursty error rates on the performance of wireless local area network (WLAN)

In recent years, Wireless Local Area Networks (WLANs) have become extremely popular. IEEE 802.11 protocol is the dominating standards for WLANs employing the Distributed Coordination Function (DCF) as its Medium Access Control (MAC) mechanism. In the literature, several papers have stuided performance of IEEE 802.11 protocol and assume that a station always have a packet available for transmission. This paper presents a Markov chain model to compute IEEE 802.11 DCF performance taking into account the packet arrival rates. Throughput and packet delay analysis are carried out in order to study the performance of IEEE 802.11 DCF under various traffic conditions. We present the analytical results of throughput and packet delay through network size and arrival rates. Results indicate that throughput and packet delay depends on the number of stations and packet arrival rates.

The development of protocol standards and the low cost of wireless equipment increased the number of wireless network installations in public (airports, universities campuses) and private (enterprises, homes) areas all over the world. This research effort examines the IEEE 802.11 protocol that dominates wireless local area networks (WLANs). Specifically, it investigates the performance of the Distributed Coordination Function (DCF) of ΙEEE 802.11 Medium Access Control (MAC) protocol. The objective of this thesis is to contribute in the research for the development of a simple and effective MAC sub-layer for WLANs that utilize the IEEE 802.11 protocol. Mathematical models are developed to calculate the following metrics in order to examine the performance of DCF that use Binary Exponential Backoff (BEB) algorithm: a) average packet delay per backoff stage and probability per backoff stage b) average packet delay, c) packet jitter, d) packet drop probability, e) average packet drop time, f) packet delay distribution, and g) cumulative packet delay distribution. An admission control algorithm is also presented. The proposed mathematical models are a) simple, as expressed with relatively simple mathematical relations, b) accurate, since they are validated by comparing analytical against simulation results and show that the proposed models are fairly accurate, and c) provide results in short period of time. Different scenarios of WLANs are examined to investigate the effect of DCF parameters in performance and to define the system parameters values that improve the performance of the IEEE 802.11 protocol. The analysis of metrics and the application of their outcomes to WLANs contribute in the design and development of more efficient access methods to wireless medium. The main contribution of this thesis is that packet delay is investigated thoroughly as a measure of WLAN performance.%%%%Η ανάπτυξη προτύπων για τα ασύρματα τοπικά δίκτυα σe συνδυασμό μe το χαμηλό κόστος της ασύρματης τeχνολογίας eπέτρeψe την ραγδαία eξαπλωση των ασυρμάτων δικτύων σe κοινόχρηστους και σe ιδιωτικούς χώρους όπως πανeπιστήμια, πλατeίeς, αeροδρόμια, γραφeία eπιχeιρήσeων, οικίeς, κλπ. Η παρούσα διατριβή μeλeτά το πρωτόκολλο ΙEEE 802.11 το οποίο κυριαρχeί στα ασύρματα τοπικά δίκτυα. Eιδικότeρα eρeυνά την απόδοση της κατανeμημένης διαδικασίας πρόσβασης DCF του υποeπιπέδου MAC του ΙEEE 802.11. Σκοπός της παρούσας eρeυνητικής προσπάθeιας eίναι η συμβολή στην έρeυνα για την ανάπτυξη eνός απλού και αποτeλeσματικού υποeπιπέδου πρόσβασης μέσου MAC στα ασύρματα δίκτυα που βασίζονται στο πρωτόκολλο ΙEEE 802.11. Αναπτύσσονται μαθηματικά μοντέλα για τον υπολογισμό των ακόλουθων μeτρικών για την απόδοση της διαδικασίας DCF που χρησιμοποιeί τον αλγόριθμο eκθeτικής οπισθοχώρησης ΒEΒ: α) την μέση καθυστέρηση των πακέτων ανά βαθμίδα οπισθοχώρησης και την πιθανότητα ανά βαθμίδα οπισθοχώρησης, β) την μέση καθυστέρηση των πακέτων, γ) την κατανομή των καθυστeρήσeων των πακέτων, δ) την αθροιστική κατανομή των καθυστeρήσeων, e) την διακύμανση των καθυστeρήσeων των…

In-band simultaneous transmission between an access point (AP) and a node was enabled by recent full-duplex radio communication research. While this event was expected to increase throughput within the feasibility of full-duplex radio communication, the conventional Medium Access Control (MAC) Dist ributed Coordination Function (DCF) protocol in wireless local area networks (WLAN) limits this performance enhancement. Each node competes for transmission opportunities because DCF is based on half-duplex communication principles. The competition among nodes and an AP creates excessive overhead and many collisions. In this paper, we propose a new MAC protocol called Reservation-Based Medium-Access Control (RMAC) based on node reservation in WLAN with full-duplex radio. RMAC is compatible with DCF. RMAC decreases collisions by reducing the number of competing nodes and increases throughput by reducing competition overhead. Our RMAC assures nodes a transmission opportunity without collision if a node has several packets to send. We show that RMAC achieves at least 86.3% more throughput than full-duplex radio based on DCF. The RMAC protocol also maintains high throughput even in cases of dense node deployment.

The carrier-sensing multiple access with collision avoidance (CSMA/CA) protocol is the most well-known medium access control (MAC) protocol for wireless networks. Both the distributed coordination function (DCF) defined in IEEE 802.11 and the MAC layer defined in IEEE 802.15.4 are based on the CSMA/CA protocol. Nevertheless, these two standards have quite different carrier-sensing mechanisms. Different to continuous carrier sensing in DCF, an IEEE 802.15.4 node only senses the channel once just after a backoff. Sensing-once mechanism can reduce the computation loading on the CPU. However, it significantly increases the probability of failure transmission because a node is not fully aware of channel activity. This paper first proposes a software architecture integrating proper hardware features for designing a DCF-based MAC protocol and then successfully implements it on a low-power transceiver. In addition, this paper conducts experiments in a star topology network to compare the performance of the above DCF-MAC protocol with the IEEE 802.15.4 MAC protocol. Experimental results show that, without continuous sensing, the IEEE 802.15.4 network suffers a high transmission failure probability as the network size increases. Consequently, the proposed DCF-based MAC protocol outperforms the IEEE 802.15.4 MAC protocol in terms of packet loss probability and throughput.

Chapter 4 - Wireless Local Area Networks

In mobile ad hoc networks (MANETs), the estimation of packet end-to-end delay depends on that of one-hop packet delay. In this paper, we conduct an analysis of the one-hop packet delay in MANETs, where the medium access control (MAC) layer uses the IEEE 802.11 distributed coordination function (DCF) to share the medium. In the MANET, each node runs the IEEE 802.11 DCF and a routing protocol. It is assumed that all nodes are one-hop neighbors, and that any pair of nodes can send data over the wireless channel with a fixed data rate. The light traffic condition is used, i.e., each node generates packets at the network layer according to a Poisson process. By modeling each wireless node as an M/M/1 queueing system, we derive the mean one-hop packet delay analytically under the light traffic condition. Simulation analysis is carried out to verify the derived results. Results show that the mean one-hop packet delay increases with either the network size or the packet generation rate in networks subject to the light traffic condition. The mean one-hop packet delay derived in this paper is analytical and exact for networks under the light traffic condition. Results that can be found in the literature are usually based on the heavy traffic condition, and they tend to overestimate by a large amount the mean one-hop delay for networks with light traffic.

Vehicle-to-vehicle (V2V) communications network is a subclass of mobile ad hoc networks (MANET). With the new dedicated short range communications (DSRC) at 5.9 GHz, vehicles equipped with DSRC devices will communicate and collaborate to broadcast their safety-critical information to each other. DSRC is to be used in a wide range of advanced vehicle safety applications such as intersection collision avoidance system. The DSRC specification is based on the physical (PHY) and medium access control (MAC) layers of the IEEE 802.11a. The MAC mechanism for the IEEE 802.11a is the distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). Mobile nodes contend for the channel using a DCF random backoff timer. A random backoff algorithm chooses a Contention Window (CW) value between aCWmin and aCWmax. The random number, CW, is the number of time slots the mobile node has to sense the channel idle before it may transmit. In this paper, we developed an intersection traffic simulator to evaluate the settings of IEEE 802.11a DCF aCWmax on the available bandwidth per vehicle and on the required communication range. Our simulation results show that it is sufficient to set the aCWmax to 200 and the communication range to 200m for the intersection collision avoidance system enabled by DSRC.

Wireless technologies in the LAN environment are becoming increasingly important. The IEEE 802.11 standard is the most mature technology for wireless local area networks (WLANs). The performance of the medium access control (MAC) layer, which consists of distributed coordination function (DCF) and point coordination function (PCF), has been examined over the past years. We present an analytical model to compute the saturated throughput of 802.11 protocol in the absence of hidden stations and transmission errors. A throughput analysis is carried out in order to study the performance of 802.11 DCF. Using the analytical model, we develop a frame delay analysis under traffic conditions that correspond to the maximum load that the network can support in stable conditions. The behaviour of the exponential backoff algorithm used in 802.11 is also examined.

With the onset of multimedia applications, Quality of Service (QoS) in 802.11-based wireless ad hoc networks has been an essential area to look into. Standard wireless protocols such as IEEE 802.11b and IEEE 802.11e have been deemed unsuitable for multimedia traffic transmission in an ad hoc (multihop) network environment due to the unique network characteristics. As an ad hoc network is infrastructureless and distributed in nature, it gives rise to several challenges in QoS provisioning. In this thesis, we focus on the research of three key areas on QoS in ad hoc networks namely, buffer management in the medium access control (MAC) layer, cross-layer design and flow admission control. Firstly, we study MAC layer buffer management in the IEEE 802.11e protocol. The IEEE 802.11e MAC protocol provides QoS for multimedia traffic by probabilistic priority queueing. Ad hoc networks tend to generate a large volume of network control packets for route maintenance and these control packets are typically queued in the highest priority queue. As a result, higher priority control traffic is always being served, consuming most of the bandwidth and results in poor network performance for data traffic. We examine this problem and propose a dynamic buffer management scheme to mitigate it. By setting different threshold levels in different priority queues, we allow lower priority data traffic to be served when the higher priority control traffic is deemed to be excessive. Simulation of our proposed scheme has shown that our approach yielded better network performance compared to the standard protocol in terms of packet delivery ratio and average delay. Secondly, we study the cross-layer interaction between the MAC and Network layers to identify its impact on QoS. We select the IEEE 802.11e MAC protocol and Ad-hoc On Demand Distance Vector (AODV) routing protocol in our study since they are standardized protocols for wireless networks. In IEEE 802.11e MAC protocol, lower priority traffic has a higher probability of being blocked. Hence, the MAC layer will inform the Network layer of a broken link through the link notification function. In this case, AODV will re-route the paths that traverse this link. As a result, higher priority traffic which shares the same link as the lower priority traffic will undergo the same re-routing procedures. We termed this phenomenon path correlation effect. Under high traffic load, the path correlation effect will be more intense and hence degrade the QoS of higher priority traffic. To break the path correlation effect, we propose a cross-layer design called the Bundled Virtual Circuit (BVC). Our proposed BVC scheme not only addresses the path correlation issues, but also provides priority treatment from the Network layer to the MAC layer. Through our simulations, our proposed cross-layer design approach is proven to provide better network performance than the 802.11e/AODV protocols in terms of packet delivery ratio and delay. Finally, we examine flow admission control at the routing layer in ad hoc networks. Standard routing protocols such as AODV performs routing on a shortest path basis. In the presence of multiple routes, hotspots are likely to form among the intermediate nodes and thus lead to network congestion. In addition, existing routes are not well-protected from the introduction of new routes. We propose a flow admission control algorithm to be used with AODV. Our algorithm allows routes to be created through available bandwidth and congestion estimation. During the route discovery phase, each node will be gauged on its estimated available bandwidth and its backlog of packets in the MAC buffer. If a node does not have sufficient bandwidth, or has too many backlogged packets compared to the requested flow rate, it will not participate in the route discovery process. From simulations, our new scheme is able to provide better network performance than the standard AODV protocol in terms of delay. Our work in this thesis focuses on QoS provisioning in ad hoc networks from the MAC and network layer perspectives. By studying the network characteristics and protocols’ behavior, we investigate the various problems of providing QoS in an ad hoc network environment and propose three different schemes to improve the QoS provisioning at the MAC and network layers. We believe that our work is able to provide some insights to the research area on QoS in ad hoc networks.

With the rapid growth of the Internet and other Internet-protocol related applications, many improvements have been made at the data link layer in wireless systems to support a wider variety of data services. However, further improvements in packet data admission, scheduling and policing between nodes in the network are necessary to maximize throughput and user satisfaction. In the above three, scheduling has the greatest impact on system throughput. Also, proper scheduling can provide user satisfaction. In this paper, a combination of an Inter Frame Space (IFS) based Distributed Fair Scheduling (IDFS) with the backbone of IEEE 802.11 DCF is designed to achieve better performance in terms of throughput and fairness. Such an attempt has not been made so far in the literature up to our knowledge. By implementing this mechanism, it is proved that improved throughput is achieved compared to traditional DCF and Inter Frame Space (IFS) based Distributed Fair Queuing (IDFQ) separately. It involves a Cross-Layer Approach by considering the physical characteristics of wireless channel and designing a mapping function at Medium Access control (MAC) layer. The inclusion of weighted fair service discipline in the system remedies the performance anomaly problem of IEEE 802.11 Medium Access Control. The proposed methodology avoids collisions of packets to the greater extent and thus helps to improve the throughput in the simulated environment. It is also shown that better performance measure is achieved when validated with NS2 simulations.

The IEEE 802.11e standard has recently been ratified. This standard provides quality-of-service (QoS) guarantees in the medium access control (MAC) layer of 802.11 wireless local area networks (WLANs). In other words, the 802.11e standard updates the MAC layer of the former 802.11 standard for QoS. In particular, the enhanced distributed channel access (EDCA) function of 802.11e is an enhancement of the distributed coordination function (DCF) of 802.11. In this paper, we present an analytical model for EDCA in saturation mode. Our model incorporates an accurate contention window (CW) and arbitration interframe space (AIFS) differentiation. In addition, it provides virtual collision treatment for an access category (AC). Validating the theoretical results via simulations, we show that i) the proposed model accurately captures EDCA saturation throughput, and ii) this model is substantially more accurate than the models previously proposed in the literature.

Opportunistic medium access (i.e., multiuser diversity) and MIMO techniques (i.e., multiple-antenna techniques) are two effective ways to achieve a substantial throughput gain in a multiuser wireless system. In this paper, we propose a medium access control (MAC) protocol with opportunistic medium access and multiuser MIMO techniques (MAC-OMA/MM) in Multi-channel Multi-radioWireless Local Area Networks (WLANs) to explore the utility of the joint design of these two techniques for the challenging MAC design. Specially, in addition to utilizing multiple channels simultaneously and multiple radio transceivers dynamically, multiuser spatial multiplexing and multiuser diversity are employed in each frequency channel to improve system performance. The key ideas of MAC-OMA/MM can be summarized as follows. By utilizing ATIM (Ad-hoc Traffic Indication Message) windows as in IEEE 802.11 power saving mechanism (PSM) under the distributed coordinate function (DCF) mode, user selection and channel negotiation are conducted between the AP and users via ATIM messages on a common channel. Multiuser diversity are employed to opportunistically schedule among multiple candidate users to optimize data transmission. During data exchange, on each frequency channel, the AP can transmit data to two distinct users simultaneously in the downlink with the help of multiuser spatial multiplexing, and two users can concurrently send data to the AP by uplinkdownlink duality in the uplink, which creates an extra dimension in spatial domain to further leverage the effect of multiuser diversity and multi-channel gains. Another contribution of this paper is to provide an analytical model to characterize the impact of our protocol on the system throughput and energy efficiency performance. Extensive simulations have been conducted and the results demonstrate that our protocol outperforms existing multi-channel MAC protocols with only minimal additional overhead and minor enhancements to the IEEE 802.11 PSM.