18-08-2012, 11:22 AM
Quality of Service in Wireless LAN Using OPNET MODELER
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Introduction
As the technology is improving so are the demands of end users and their applications increasing. A wide variety of new applications are being invented daily. These applications have different demands from the underlying network protocol suite. High bandwidth internet connectivity has become a basic requirement to the success of almost all of these areas.
Wireless Local Area Networks (WLANs) has become one of the most promising and successful technology in recent years. WLANs provide free wireless connectivity to end users, offering an easy and viable access to the network and its services. Wireless networks are superior to wired networks with regard to aspects such as ease of installation and flexibility. They do, however, suffer from lower bandwidth, higher delays, higher bit-error rates, and higher costs than wired networks. With the advent of Wireless Local Area Networks (WLANs), bandwidth has increased and prices have decreased on Wireless networking solutions. These factors have made WLANs a very popular Wireless networking solution.
Wireless Network
Wireless refers to the transmission of voice and data over radio waves. It allows its users to communicate with each other without requiring a physical connection to the network. Wireless devices include anything that uses a wireless network to either send or receive data.Wireless communication has become the most promising way to connect people.
Cellular systems have experienced exponential growth over the last decade and there are currently around two billion users worldwide. The first digital network based on packet radio, ALOHANET, was developed at the University of Hawaii in 1971. The Defense Advanced Research Projects Agency (DARPA) invested significant resources to develop it. In 1990, the first digital communication based cellular system was introduced. Since then, Radio technology advanced rapidly to enable transmissions over larger distances with better quality and less power. It enabled mobile communications and wireless networking
Wireless Local Area Network (WLAN)
WLANs have revolutionized the way people are using their computers to communicate. As WLANs eliminate the need of wires for connecting end users, they provide a very easy, viable access to the network and its services. A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers without using wires. WLAN utilizes spread-spectrum modulation technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network. Wireless has become popular due to ease of installation and mobility. To transport the data on a wireless network radio frequency, microwave and infrared
are used as a transportation media [11].
Radio frequency (RF)
RF refers to frequencies of radio waves. RF is part of electromagnetic spectrum that ranges from 3 Hz - 300 GHz. Radio wave is radiated by an antenna and produced by alternating currents fed to the antenna. RF was long been used for radio and TV broadcasting, wireless local loop, mobile communications, and radio.
Microwave
Microwave is the upper part of RF spectrum having frequencies above 1 GHz.Because of the availability of larger bandwidth in microwave spectrum,microwave is used in many applications such as wireless Personal Area Network (Bluetooth), Wi-Fi, broadband wireless access (BWA) or wireless Metropolitan Area Network (WiMAX), wireless WAN (2G/3G cellular networks),satellite communications and radar. But it became very famous inhouses because of its use in microwave oven.
Infrared
Infrared light is part of electromagnetic spectrum that is shorter than radio waves but longer than visible light. Its frequency range is between 300 GHz and 400 THz that corresponds to wavelength from 1mm to 750 nm. Night vision equipment and TV remote control is using infrared from a long time.Infrared is also one of the physical media in the original wireless LAN standard, that's IEEE 802.11. Infra red use in communication and networking,defined by the IrDA (Infrared Data Association). Using IrDA specifications,infrared can be used in a wide range of applications, e.g. file transfer,synchronization, dial-up networking, and payment. However, IrDA is limitedin range (up to about 1 meter). It also requires the communicating devices tobe in LOS (Line of Sight) and within its 30-degree beam-cone [11].
Working of WLAN
Wireless networks perform functions similar to their wired Ethernet. Networks perform the following functions to enable the transfer of information from source to destination:
1. The medium provides a bit pipe (a path for data to flow) for the transmission
of data.
2. Medium access techniques facilitate the sharing of a common medium.
3. Synchronization and error control mechanisms ensure that each link transfers the data intact.
4. Routing mechanisms move the data from the originating source to the intended destination.
5. Connectivity software interfaces an appliance, such as a pen-based computer or bar code scanner, to application software hosted on a server.
Advantages of Wireless LAN
Flexibility: within radio coverage, nodes can communicate without further restriction.Radio waves can penetrate walls.
Planning: wireless ad hoc networks allow for communication without planning.Wired networks need wiring plans.
Robustness: wireless networks can survive disasters; if the wireless devices survive people can still communicate
Disadvantages of Wireless LAN
Connectivity: There are no wires to connect to the Wi-Fi network but then the area of
the hotspot is very limited and if the node gets out of the area it will be disconnected.
This is perhaps the greatest disadvantages you have to be within 100-150 ft of the
base station (indoors) and about 100-300 meters (outdoors) to get connected.
QoS (Quality of Service): WLANs offer typically lower QoS. Lower bandwidth due to
limitations in radio transmission and higher error rates due to interference.
Safety and security: using radio waves for data transmission might interfere with
other high-tech equipment. The greatest challenge faced by Wi-Fi providers today is
how to prevent outsiders from accessing the data.
Logical Architecture of WLAN
WLAN works in the lower two layers of OSI model. First one is the physical layer
which takes care of transmission of bits through a communication channel by defining
electrical, mechanical, and procedural specifications. Second one is the data link layer
which is sub-divided into two layers: logical link layer (LLC) and Medium Access
Control layer (MAC) . Only MAC layer is considered as the part of wireless LAN
Functions.