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In recent years, the traditional wired networks are unable to meet the requirements of the present world. the reach of wireless network have increased as it have been deployed in various places like buildings, parks, streets and many other places. The annual industry revenues have already exceeded US$4 billion . Wireless networks are getting popular and has become a matter of interest for developers and the researchers. More and more people are now interested in having wireless networks because of the ease of use, performance, mobility and flexibility. Among many wireless technologies, the use of IEEE 802.11 Wireless Local Area Networks (WLAN) is becoming very popular due to its ease of installation, the free of charge availability in different circumstances and its high performance . The reach of IEEE 802.11 wireless LAN technology has widened as they are now used in areas like wireless mesh networks and WSN (wireless sensor networks). IEEE 802.11 is now consider as a prominent contender to play an important role in 4G (4th Generation) telecommunication network . During recent years, real time applications like Voice over IP (VoIP) and video over wireless networks have increased, so efforts are made to improve and enhance Quality of Service (QoS) for real time traffic.
Motivation for this topic came from the fact that IEEE 802.11 wireless LAN network is considered as one of the integral part in the future technologies like 4G (4th generation telecommunication network, and it is expected that there will be an exponential growth in the use of video and voice over wireless network. The success and popularity of IEEE 802.11 wireless LAN has encourage researchers and developers to work on improving the shortcomings and performance of existing technology. There certain shortcomings in this technology: Its performance highly depends on selection of interim hops. An ideal interim hop should have good radio channels with its preceding hop as well as proceeding hop . But such perfect interim hops cannot be always guaranteed in practical wireless network . IEEE 802.11 MAC layer access mechanism is Distributed Coordination Function (DCF). The channel access procedure of DCF is based on CSMA/CA (Carrier Sense multiple access with collision avoidance) backoff mechanism, in which every station share channel equally and quality of service (QoS) is not supported.
Wireless LAN use RF (Radio Frequency) technology to receive and transmit data. IEEE established IEEE 802.11 as standard for wireless LAN. Because of its low price and interesting features like mobility and ease of use it has now became the most popular and accredit standard for wireless LANs. The main difference between WLANs and LANs is the limited bandwidth and the varying topology because of the mobility of the nodes . Wireless LAN consist of 2 parts nodes (computers, printers) and access point (transceivers) which provide connection between nodes or between wireless LAN and other networks.
IEEE 802.11 standard comes under the scope of data link layer and physical layer (layer 1&2) of OSI reference model. In case of 802.11 data link layer is divided in to 2 sub layers, MAC (Medium Access Control) & LLC (Logic Link Control). The MAC defines the rules to send data and access medium while reception and transmission is done by PHY (physical layer)
IEEE 802.11 Components:
There are 4 physical major components of 802.11 wireless networks and those are:
This component of 802.11 network is a logical component i-e it is use to connect different access point to make a big area. Its main purpose is to receive frame from source and forward it to destination (access point).
Wireless Access Point
An Access point is a device that connects wireless communication devices to each other. The access points mostly connect with some kind of wired network and it relays data b/w wired devices and wireless devices.ÂÂ
The most common standard medium for wireless communication are Radio frequencies and infra red.
Stations are devices between which wireless communication occurs. Stations are usually computers or printers with wireless interface card.
Wireless Network Types:
Basic service set is the basic block of IEEE 802.11 wireless network and it has two types:
IBSS (Independent Network)
In this type of network the devices (stations) communicate with each other directly without intervention of any other device. For the existence of this network the station should be in range. It is also known as ad hoc network.
In this type of network the stations does not communicate directly with each other instead they are connected through access point, so it will take 2 hops for communication, 1st hop is from source to access point while the 2nd hop is from access point to destination device.
ESS (Extended Service Set)
This service set is for large service area, in this type of service set stations from one BSS can communicate to the station present in the other BSS if these BSSs are a part of same Extended Service Set (ESS).
IEEE 802.11 MAC Layer:
The MAC layer is responsible to manage and maintain access of communication between different stations. Its functionality includes security, power management and most importantly frame formatting.
There are 2 access co-ordination functions defined in the IEEE 802.11 standards, one is contention free period while the other is contention based period. Contention based period works on the principle of listen before talk also known as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). DCF (Distributed co-ordination Function) provides contention based period. While in case of contention free period times slots are given to stations to send data to avoid contention. PCF (Point Co-ordination Function).
DCF (Distributed Co-ordination Function):
The distributed co-ordination functions on the mechanism of collision avoidance which reduces the possibility of collision in wireless network. Before starting any transmission station follows a backoff procedure, in this procedure the medium is sensed by every station for a particular random time also known as backoff time. When it is sensed that the medium is idle for a particular time DIFS (DCF Inter-Frame Space). The DCF has a drawback that it only supports best effort traffic so it canââ‚¬â„¢t provide different services to frame having different priorities. So there is no QoS for real time traffic in DCF.
PCF (Point Co-ordination Function):
As DCF is unable to insure good quality real time traffic because it does not support QoS. So to support real time applications like voice and video, IEEE 802.11 includes another type of coordination function i-e point coordination function. PCF is only limited WLAN working in infrastructure mode. Because of its complicated hardware implementation, PCF is an optional function. When PCF is enabled in WLAN, it divides the channel access time into periodic intervals also know as beacon intervals. The beacon interval consists of contention based period and contention free period. At the time of CFP (Contention Free Period), access points maintain a list of stations that have PCF enabled & polls them.
But during heavy traffic performance of both PCF and DCF decreases.
Problem Description & Issues in the technology:
In recent years, the need of voice and video traffic is increasing exponentially, but current WLANs are unable to meet the QoS requirements for real time applications. The 2 most important QoS issues in real time applications are failure of priority and congestion management in the network. There is a suggested or recommended table for QoS requirement for real time applications by ITU-2001 .
Voice issues over Wireless LAN:
There are some issues that are still remain in the quality of voice over wireless LANs, when the number of calls increases the quality of calls decreases and sometimes it is not even acceptable for communication. Like when there are many access points (APs) involve in a voice communication, the calls have to shift from one AP to other and this causes some delay which can be noticed by our ears.
Noise because of the background traffic also irritates the listeners . Sometimes because of the overwhelming of the radio channels, listening becomes difficult for the communicating parties.
Power supply is also a factor of concern is VoIP calls, because in case of mobiles batteries are not good enough to handle VoIP calls, as compare to wired networks wireless networks do not have good sources to rely on. Bandwidth is also one of the major concerns as more often the delay and jitter is caused due to lack of bandwidth.
Video issues over Wireless LAN:
Many efforts are done to meet the requirements of QoS of real time application like video and voice over wireless medium. Efficient allocation of bandwidth is the most important concept as the mobile network carries mixed heterogeneous traffic.
In video streaming network topology have vital role, as in mesh network there are multiple paths for the packets to reach destination as all nodes in the network acts like a router .
The variable nature of low latency videos makes them difficult to handle in wireless environment . Service time in access points (APs) also causes delay, jitter and packet loss in video transmission .
Video conferencing issues over Wireless LAN:
In recent years, video conferencing is getting more and more popular. In case of cell phones in which transmission of voice and video occur simultaneously the issues becomes much bigger. When voice and video run simultaneously video quality goes down and pixel start breaking while some calls drop. So QoS is necessary to have reliable multimedia traffic.
Current status and Developments of Research or Technology:
To overcome Quality of Service issues in WLAN many researchers and developers came up different ideas some of them are discussed below.
Literature Survey and Analysis:
QoS IEEE 802.11:
As distributed coordination function DCF and point coordination functions are unable to provide QoS, IEEE came up with a new standard for QoS known as 802.11e. This standard offers that high priority data should have higher possibility of being sent rather than low priority data or traffic.
The IEEE 802.11e proposed a new access mechanism HCF (Hybrid Coordination Function) which supports 2 mechanisms for channel access to create enhancements in 802.11 MAC to have QoS for multimedia applications in wireless LANs . HCF is the combination of both the previous coordination functions (DCF and PCF). HCP propose 2 channel access techniques, EDCA (Enhanced Distributed Channel Access) for contention based access while the other is HCCA (HCF Controlled Channel Access) for contention free channel access.
Enhanced Distributed Channel Access (EDCA):
EDCA allows the priority to be assigned on the packets before entering the MAC layer. With the help of Access Categories (AC), it is possible differentiate data and assign them different priorities. There are 4 access categories Voice, Video, Best effort and Background where voice have the highest priority while background have the lowest priority. Every station enabled with EDCA should implement these 4 categories. The priorities assigned to different access categories depends on 3 parameters Arbitrary Inter Frame Space number, Contention Window, Transmission Opportunity limit. AIFS is the time interval b/w the medium sensed by the STA and transmission, TXOP is time duration for transmission by station after acquiring the channel.
HCF Controlled Channel Access (HCCA):
HCCA is efficient for real time applications like video and voice. The HCF controlled channel Access allows a reservation of transmission opportunity with HCCA enabled AP. The STA request for TXOP according to their requirements and HC make the decision of accepting or rejecting it on the basis of admission control policy. Upon the acceptance of the request, TXOP is scheduled by HC for station and AP. In case of transmission from station, the station is polled by HC based on the same parameters that were by the station at the time of request. While in case of transmission from AP, AP get TXOP from HC which is within AP.
22.214.171.124 RELATED WORKS:
The research carried out in the field of industrial wireless networks with provisioning QoS is very confined. In , Grilo et al and in  Ramos et al measured the performance of the HCCA but their findings are based on the home or office applications such as Voice over IP (VOIP) and video streaming. Similarly [5, 6, 7] also measured the performance of HCCA and suggested some solutions but were also based on multimedia applications.
In , Krommenacker and Lecuire presented results on wireless automation system in which 802.11g with focus on the PCF was considered. According to , the HCCA mechanism was found to be inefficient for the real-time applications, such as voice over IP and video streaming, even though this kind of traffic does not allow an assessment of the same mechanism in an environment with different traffic characteristics like industrial communication systems . It has been shown in  that the characteristics of voice and video traffic differ significantly from those of industrial real-time communication,