Optimum WLAN Performance Under Saturated Traffic Loading
Chapter One
Objective of the Study
The objective of this thesis was to determine the optimum WLAN performance parameters under saturated traffic loading. This is intended to provide a congestion control measure for WLAN in a high traffic loading situation for network service providers.
The said objective is to be realized by pursuing the following sub-objectives:
- The development of WLAN network model.
- The conversion of the WLAN network model into a computer simulation model using MATLAB block oriented simulated package.
- The simulation of the computer model and collection of simulation results.
- The analysis of the simulation results to establish the set of optimum performance parameters.
CHAPTER TWO
LITERATURE REVIEW
Introduction
A local area network LAN is a data communication network which is geographically limited (typically to 1 km radius) and allows easy interconnection of data terminals, computers and computing systems [7]. A Wireless LAN is a shared – communication network that broadcast information over wireless link for all stations within the network footprint or within a geographical area approximately 1km radius [8]. A WLAN basically consists of one or more wireless devices connected to each other in a peer-to-peer manner or through APs, which in turn are connected to the backbone network providing wireless connectivity to the covered area [8]. WLAN is widely considered to play a major role in wireless multimedia communications that require high speed transmission. It links two or more devices using some wireless distribution method (typically spread spectrum and OFDM radio), and usually providing a connection through an access point to the wider internet [8].
In this chapter, wireless networks and its variations with emphasis on IEEE 802.11 standard is reviewed. The evolution of IEEE 802.11x series was presented. Significant points were reviewed as the standards evolved from the originally released standard in 1997 to the current state-of-the-art, with more emphasis on IEEE 802.11e (QoS) and IEEE 802.11b. Based on previous work, the IEEE 802.11 architecture, the physical layer, the MAC sub-layer protocols, and the concept of basic access mechanism which is the DCF protocol (CSMA/CA) of the proposed architecture, RTS/CTS mechanism and PCF were described. Furthermore, the frame structure and it’s addressing, general frame format, WLAN RF Allocation were also depicted in this chapter. Finally, the chapter ended with the review of related works.
Wireless Networks
One of the most promising and discussed technologies in the last decade is the wireless technology which allows users to utilize devices that enable the access to information at any time any place. These needs make wireless networks the best solution for interconnecting devices and people. Wireless networks are comprised of devices that communicate using radio and infra-red signals. Wireless networks are generally classified into two categories: infrastructure-based and ad-hoc wireless networks.
Infrastructure-based wireless network consists of base stations (access points) that are localized at convenient places and predefined infrastructure. They provide wireless connectivity to devices within their coverage area. Examples of this category are Wireless Local Area Networks (WLANs) and cellular networks. In other words, a WLAN is a flexible data communication system implemented as an extension to a wired LAN within a building or campus. [3]
On the other hand, ad hoc wireless networks do not have a pre-established infrastructure. Moreover, nodes connect to each other through automatic configuration when they are in transmission range and willing to communicate. In this way, an ad hoc wireless network is formed which is both flexible and powerful. Therefore, these capabilities make wireless ad hoc networks suitable for many applications where one central node may not be convenient, and where minimal configuration and quick deployment is required in emergency situations.
Wireless ad hoc networks can be further classified by their application in mobile ad hoc networks, wireless sensor networks, and wireless mesh networks [3]. Therefore, mobile ad-hoc network, wireless sensor network, and wireless mesh network make-up the categories of multi-hop wireless networks as illustrated below.
Multi-hop Wireless Networks
The Multi-hop Wireless Networks consist of wireless networks that primarily use multi-hop wireless relaying. The major categories in the multi-hop wireless networks are (1) Mobile ad-hoc networks, (2) wireless sensor networks, (3) wireless mesh networks, and (4) hybrid wireless networks [3], [9]. In the following subsections, are classifications of multi-hop wireless networks?
Mobile Ad hoc Networks (MANET)
In MANET, devices are mobile nodes which provide the functionality required to connect users allowing them to exchange information in an environment with no pre-established infrastructure. Therefore, MANET is an infrastructure-less network with highly dynamic topology [3]. Devices are free to move randomly and organize themselves arbitrarily; thus, the wireless network topology may change quickly and is unpredictable. Also, some devices may be connected to other resources such as the Internet, file servers, etc., allowing users to gain access to this resources. In the past, these networks have been used for tactical network related applications in battlefield communications and survivability [3], [10]. Military operations generally cannot rely on a fixed communication infrastructure, then MANET allows a suitable framework to overcome issues that radio signals and radio frequency present. In general, MANET also has problems due to wireless communication and wireless networks such as the limited transmission range, channel that is not protected from other signals and has time-varying and asymmetric properties and hidden and exposed terminal problems may occur.
CHAPTER THREE
WIRELESS LOCAL AREA NETWORK (WLAN) MODEL
Introduction
The performance of a given network can be analyzed using two approaches – analysis on a real life system and on a model [100]. Analysis on a real-world system is possible, but may have due consequences. In this case, carrying out WLAN performance analysis under saturated traffic loading on a practical network could result in the disruption of network operation. Therefore, such an approach may be costly. As a result of this, a modeling approach was adopted. A model will therefore be designed and implemented using MATLAB block oriented simulation package. Many types of modeling techniques exist; these include descriptive, physical, mathematical, flowchart, schematics, and computer program [100]. There are three basic classifications of models commonly used in communication engineering; they are analytical, graphical and computer program model. The network model for this thesis is computer program model and is based on the adopted WLAN architecture.
In this chapter, the adopted typical WLAN architecture is presented in figure 3.1. The DCF access mechanism is implemented on MATLAB Simevent Environment as shown on figure 3.2 and figure 4.9.
Adopted Typical WLAN Architecture
The WLAN architecture intended for infrastructure mode of operation as recommended by IEEE 802.11 standard [2] is adopted for this project as illustrated in figure 3.1. The architecture is comprised of WLAN Access Point (WLAN AP), Distribution System (DS), Server, Very Small Aperture Terminal (VSAT) and the attached Workstations (Wkstn).
In the infrastructure mode, a station needs to join a BSS to communicate. It obtains synchronization information from periodic beacons from the BS. It can either obtain this information by requesting it from the BS (active probing), or it can wait for the periodic beacon from the BS. Before being able to send and receive data, the station has to go through an authentication and association process [3]. The distribution system connects BSS to the Server and the VSAT through the access point and then to the internet.
All components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped with wireless network interface cards. Access points (APs), normally routers, are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with. A distribution system (DS) connects access points in an extended service set or connect an access point in a basic service set. The concept of a DS can be used to increase network coverage through roaming between cells. DS can be wired or wireless. VSAT provides access to the internet.
CHAPTER FOUR
SIMULATION RESULTS AND RESULTS ANALYSIS
Introduction
The model presented in chapter three (figure 3.2) is intended to be simulated under varying and different traffic conditions. The traffic considered includes Lognormal, Binomial, and Bernoulli traffic distributions. The implication is that all users always have packets available for transmission and enough to flood the network. Throughput under saturated traffic situation is the upper limit of the throughput achieved by the system, and it represents the maximum load the system can carry in the stable condition.
In this chapter, the model, figure 3.2 was converted into MATLAB Computer Simulation Model as presented in figure 4.9 on page 80. The model was simulated in order to determine the relationships between varying traffic intensity, delay, loss and throughput.
CHAPTER FIVE
Conclusion and Recommendation.
The objective of this research is to carry out a performance analysis of IEEE 802.11b medium access control distributed co-ordination function traffic loading under network saturation. This is achieved by investigating the effect of traffic loading and traffic distribution patterns on the quality of service (QoS) parameter on WLAN at different data rate, in other to establish the sort for optimum WLAN Performance. In order to evaluate the mean packet delay, loss rate and network throughput performance, this research has presented a traffic loading for MAC DCF which supports WLAN’s QoS Parameters. Using the proposed model, we have evaluated the mean packet delay, loss rate and network throughput performance of IEEE 802.11 DCF for basic access method under network saturation condition using MATLAB. Based on this physical model, a computer simulation model was developed over a MATLAB Simulink Simevent Environment. it is characterized by a MAC DCF protocol that enhances station‘s transmission probability and fair access to the channel, which is more realistic, such as non ideal channel conditions. In other words, The basic access mechanism is a key feature of the IEEE 802.11 standard that uses virtual carrier sensing and random back off procedure to avoid the probability of two or more station from accessing the channel at the same time, thus, causing collision. To realize this feature, the WLAN MAC architecture is organized, modeled, and implemented in the Simevent within the MATLAB Simulink framework, in an organized manner following the software engineering best practices.
This thesis considered three different traffic distribution patterns namely Lognormal, Binomial and Bernoulli traffic distribution for analysis. The results showed that lognormal traffic distribution has considerable (better) packet loss rate, throughput, and delay performance at different data rate when compared with the other two traffic distributions whereas Bernoulli has the least performance in terms of high loss rate, high delay and low throughput also at different data rate. This means that lognormal traffic distribution will yield optimum performance at network saturation traffic loading within the data rate limit of IEEE 802.11b. The effect of the traffic distribution patterns on the network slightly vary at low and partly medium traffic loading and also vary at different data rate.
In this thesis, results show that the IEEE 802.11b does not perform well in terms of high throughput, low mean delay and low loss rate at high traffic load conditions. If the number of workstations increases, mean packet delay, loss rate and throughput performance of the IEEE 802.11b protocol degrades significantly. Moreover, as the number of workstations increases, improvement increases. Similarly, it was shown that the mean packet delay of arrived packets decreases as the number of workstations decreases with different network load levels. At saturation, it was also shown that throughput decreases, mean packet delay increases, packet loss rate increases with continuous increase in the number of workstation. The IEEE 802.11b optimum parameters under high traffic loadings, at 11Mbps data rate are 4.6 Mbps throughput, 300 ms delay and 0.00017 loss rate. Clearly, the existing IEEE 802.11b WLANs cannot be used for high bandwidth real – time applications serving large number of users. Therefore, to achieve an optimum network performance, it was observed that the IEEE 802.11b WLAN requires an improvement on its window back-off algorithm and fairness. Although various enhancements to the original IEEE 802.11 protocol have been proposed in the past, the problem of efficient channel utilization, higher throughput and good fairness has not been fully solved yet. Therefore, a planned extension of the present study is recommended, as the joint MAC-physical layer design approach for performance improvement of the IEEE 802.11b.
Lastly, figure 4.3, 4.5, 4.7 and 4.8 showed that WLAN performance reaches its optimum when 40 workstations are used in a wireless LAN hotspot scenario. In order words, it means that a WLAN Performance is best when a total number of workstations equal to 40 are connected to an access point at network saturation, and it represents the maximum load the system can carry in the stable condition. When each workstation generates 1,763 Erlang traffic intensity.
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