I-DTMC: An Integrated-Discrete Time Markov Chain Model for Performance Analysis in Future WLANs

The prediction of numbers of newborns is an important issue in hospital management. Relying on the inherent non-aftereffect property, discrete-time Markov chain (DTMC) is a candidate for solving the problem. But the classical DTMC is unable to handle the uncertainty of states, especially when the state space is not discrete, which would lead to instable predicted results. In order to overcome the limitation of the existing DTMC model, a belief Markov chain (BMC) model is proposed by synthesizing the classical DTMC and Dempster-Shafer theory effectively. Depending on the advantages of Dempster-Shafer theory in expressing uncertainty, the proposed BMC model is capable of dealing with various uncertainties, which improves and perfects the classical DTMC model. An illustrative example demonstrates the effectiveness of the proposed model. Moreover, a comparison between the proposed BMC model and the classical and fuzzy states modified DTMC models is given to show the superiority of the proposed model against the other two. Finally, the stability of the proposed model has been proven.

Wireless Local Area Networks (WLAN)s are today common for access to Internet; well-known and used is IEEE 802.11 standardized technology. Some drawbacks of the standardized distributed medium access scheme, Distributed Coordination Function (DCF) has resulted in considerations made to extend the DCF from single-hop communications to multi-hop. Multihopping has the potential to reduce energy consumption and increase throughput and coverage. The drawback is lack of Quality-of-Service (QoS) support, which already with single-hop communication was bad, but extending a communication between two peers to multi-hop significantly reduces the possibility to offer reliable transmissions. The contention-based medium access performed at each hop with the DCF results in large uncertainties as to when a packet arrives to its end-destination. There are two general approaches for extending the DCF with QoS functionality: 1. Allocate resources for a flow and 2. Assign a flow higher priority than other flows. My hypothesis is that for strict QoS guarantees, a reservation-based approach is the better one. This has been my motivation to design a new medium access protocol; based on the DCF, periodic time slots for QoS-demanding real-time applications are reserved. The Distributed Allocation of time slots for Real-time traffic (DARE) is unique as it totally distributed sets up an end-to-end reservation before the transmission of data begins, repairs broken reservations, support many periods and time slot sizes and distribute piggy-backed information even to a two-hop radius from a receiver. This thesis describes the DARE protocol and also presents results from comparisons of DARE, DCF and the IEEE 802.11 E standardized priority-based QoS medium access protocol Enhanced Distributed Channel Access (EDCA). Using simulations, this thesis shows that DARE offers constant end-to-end packet delay for a flow, very low average packet delay in the whole system and constant throughput. EDCA and DCF depend strongly on the total network load; average packet delay for a real-time flow increases rapidly with the surrounding load. DARE outperforms both DCF and EDCA when many flows are present, even in networks with frequent topology changes where reservations must be repaired. Thus, the reservation-based DARE protocol is the most suitable approach for extending DCF with QoS.

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.

Over the past few years, wireless local area networks (WLANs) have gained an increased attention and a large number of WLANs are being deployed in universities, companies, airports etc. Majority of the IEEE 802.11 based WLANs employ distributed coordination function (DCF) in wireless access points (AP) to arbitrate the wireless channel among Wireless Stations (STAs). However, DCF poses serious unfairness problem between uplink and downlink flows. To overcome this unfairness problem, we propose a simple enhancement to the IEEE 802.11 DCF which provides priority to the AP and thus enables it to acquire a larger share of the channel when required. We have demonstrated the unfairness problem through systematic measurements in an experimental test bed of WLAN using the legacy 802.11 DCF. We also developed analytical models to calculate the throughput of AP and the STAs and verify these results through thorough simulations in ns-2. We observe that our simulation results find in good agreement with our analytical models. Results show that our proposed enhancement achieves a fair distribution of bandwidth and improves the throughput (by nearly 300%) for the downlink flows as compared to the DCF, without severely affecting the performance of uplink flows

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 high diversity node situation, single-channel MAC protocols suffer from many collisions. To solve this problem, the research of multichannel MAC protocol has become a hotspot. And the cyclic quorum-based multichannel (CQM) MAC protocol outperformed others owing to its high frequency utilization. In addition, it can avoid the bottleneck that others suffered from and can be easily realized with only one transceiver. To obtain the accurate performance of CQM MAC protocol, a Markov chain model, which combines the channel hopping strategy of CQM protocol and IEEE 802.11 distributed coordination function (DCF), is proposed. The metrics (throughput and average packet transmission delay) are calculated in performance analysis, with respect to node number, packet rate, channel slot length and channel number. The results of numerical analysis show that the optimal performance of CQM protocol can be obtained in saturation bound situation. And then we obtain the saturation bound of CQM system by bird swarm algorithm (BSA). Finally, the Markov chain model and saturation bound are verified by Qualnet platform. And the simulation results show that the analytic and simulation results match very well.

Wireless LAN has become increasingly popular as a medium to connect portable devices to the network. Most of the existing wireless LAN protocols based on IEEE standard 802.11 assume that the wireless channel is time invariant and thus assign radio resources accordingly. In this paper, we devise a modified MAC level channel allocation method by carefully scheduling radio resource according to temporal channel characteristics, assuming the influence of channel fading. Based on this method we propose new protocols for the distributed coordination function (DCF) and the point coordination function (PCF), the two modes of communication defined in the IEEE standard for WLAN. The proposed protocols introduce minimum computational overhead. Our simulations show that the proposed PCF protocol improves the channel capacity up to 14% and the DCF protocol improves the channel capacity up to 90%, depending on the loss rate and temporal characteristics of the wireless channel. We also show that the proposed DCF protocol can lower SNR requirements thus potentially extending the battery life of portable devices that use WLAN.

IEEE 802.11 DCF mode is a distributed channel access method for standard Wireless Local Area Networks (WLAN) with a goal of providing long-term fair channel access for the nodes. Due to the characteristics of radio signals, such as propagation attenuation and capture effect, WLAN users may experience unfair medium allocations depending on their locations. Our analysis shows that a user near the access point (AP) can access the medium several times more than a user located far away. Advanced Distributed Coordination Function (ADCF), a variant of Distributed Coordination Function (DCF) mode of IEEE 802.11, was proposed to achieve distributed weighted fair queuing. In this paper, we show that the ADCF alleviates this location dependent unfairness. Simulation results show that the ADCF mode improves the overall performance of a WLAN considerably.

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

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

The Distributed Coordination Function (DCF) is the fundamental access method defined in the IEEE 802.11 Standard for Wireless Local Area Networks (WLANs). With this standard, the Access Point (AP) and the mobile stations consume a significant amount of energy to contend for access to the shared wireless channel. In order to improve the energy efficiency of WLANs, we investigate in this paper a simple and backwards compatible mechanism, called Bi-Directional DCF (BD-DCF), that enables bidirectional communications between wireless stations with a single channel access invocation. The key idea is to let the AP or any mobile station transmit a data packet together with the acknowledgement upon the successful reception of a data packet. This approach reduces the communication overhead, the channel contention, and better balances uplink and downlink transmission opportunities. We evaluate the performance of the proposed BD-DCF by means of computer-based simulation considering different traffic loads, degrees of traffic symmetry, data packet sizes, and data rates. The results presented in this paper show that the BD-DCF protocol can improve the energy efficiency of DCF up to 50%.

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.

A multi-objective approach for energy-efficient and reliable dynamic VM consolidation in cloud data centers

Cognitive Radio (CR) is expected to bring about a more flexible wireless communication environment by the efficient utilization of spectrum resources. In this paper, a CR coexisting with IEEE 802.11 Wireless Local Area Networks (WLANs) is proposed. In the Distributed Coordination Function (DCF) access scheme in IEEE 802.11 WLAN, a station (STA) transmits a data frame by executing a random backoff procedure after Distributed Inter Frame Space (DIFS) period, and the destination STA of the data frame responds with Ack frame to the source STA after Short Inter Frame Space (SIFS) period. After the Ack frame is transmitted, the same procedures are repeated. The proposed CR terminal recognizes the DIFS period and the SIFS period, and then it transmits CR signals during these periods with the transmission power that does not affect the IEEE 802.11 WLAN protocol. Thus, the proposed CR terminals recognize the periods during which IEEE 802.11 STAs do not transmit any frames and they use the periods to transmit CR signals. In this paper, IEEE 802.11 WLAN STA that has the capability for the proposed CR technique in addition to the normal 802.11 WLAN capability is considered and the improved average throughputs by the CR communications are evaluated in the computer simulation, and then the effectiveness of the proposed method is clarified.

IEEE 802.11 MAC protocol in distributed coordination function (DCF) mode is not scalable as the number of contending users increases. In this paper, we propose two new contention resolution schemes to reduce the number of collisions due to a large number of contending users. The first scheme, hybrid DCF (H-DCF), splits the monolithic contention resolution into two phases to enhance scalability. The other scheme, enhanced hybrid DCF (EH-DCF), provides fairness with the current IEEE 802.11 MAC and yet improves the efficiency of contention resolution. The major contribution of our work is to provide inter-operability with the IEEE 802.11 DCF as well as an efficient channel access, unlike the previous works. We conduct comprehensive simulation experiments by using an ns-2 simulator to demonstrate the advantages of these two schemes. The results show that they improve network performance by up to 10~30%, compared to the original IEEE 802.11 DCF. Therefore, H-DCF and EH-DCF can be good candidates for MAC protocols for hotspots where numerous users access wireless LANs intensively.