EDCA For Background Users Computer Science Essay

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In present, there is an increase in the ratio of applications based on heterogeneous multimedia traffic with new enhancements and conditions. The network traffic with different applications in accordance with the requirements in terms of delay, jitter, loss, throughput, interaction and reliability features.

In the past, wireless local area networks with IEEE 802.11 standard did not support QoS management especially in case of voice, voice and delay sensitive application but due to the increase of different multimedia traffic flow/services and in order to get rid of the service issue; it needed to raise a new standard to support these kind of multimedia services and applications i.e. 802.11e.This standard does support the quality of service (QoS) over WLANs. There are still further improvements going on to improve the performance of the multimedia traffic service over WLAN's so that the acceptable service with quality should be provided to the multimedia traffic in the best interest of the WLAN's.

Summary of Background Material:

During the last two decades we have been trying to maintain the fast growing heterogeneous multimedia traffic like video streaming, voice over IP, file transfer and web browsing. Some applications need to be transferred via internet or IP-based wireless local area networks. Basically the IP protocol was designed and made for sending packet of data from one side to another side without knowing all the specific requirements keeping in mind in terms of packet loss, delay, jitter and multimedia traffic. In order to solve this problem we need to have a look on the Quality of Services (QoS).

Quality of Services (QoS) Specifications

The quality of services management over WLAN's with reliable delivery of multimedia is strongly based on wireless channels and different requirement from different types of traffic applications. It is difficult to find the parameters and metrics which define multimedia flow degradation when crossing network like video streaming or VoIP calls.

QoS Parameters

There are the numbers of parameters from which we can analyze and monitor whether we have received a level of service offered is achieved or not. These are the following parameter [1] [2]:

Network availability

This plays very crucial role in the best interest of the quality services if network is not available for sometime then there could be adverse performance issue. It consists of many types of things from which combines to make a whole network for instance network card, routers connections, routing protocols, physical connection and etc.

Bandwidth

It also has a significant parameter that affects the QoS. In this parameter we have two types of bandwidth as available or guaranteed. If we talk about available bandwidth there is no guarantee at all its depends upon the situation if there is a good service you could even the same service agreed over your ADSL connection otherwise may be less than it. On the other hand if you have a got a guaranteed bandwidth service from the service provider than you will be prioritized according to the service level agreement (SLA) and provided by the guaranteed service either by logical connection or VLAN's.

Delay

This is the time take place of the packet from the leaving point to the entering point. It is very important issue regarding QoS and some application it is acceptable whereas in video and voice applications can cause timeout and fail.

Jitter

It is the delay variation between the repeated packets for a same traffic flow and cause QoS issue regarding delay sensitive real-time applications like video and voice. In the real-time we need to have a fixed delay in the packets in order to get the same quality as we are expecting.

Loss

Loss can comes with the errors occur during the physical transmission. It crop up mostly due to the geographic condition like fog, rain or physical interruptions like buildings, trees and etc. It also occurs when the network gets choke up and drop packets.

Multimedia Traffic Typologies

As we have already seen how the Quality of Services play a vital role in order to get the expected level of sensitive multimedia traffic delivery. As new applications and services continuously developing with different requirement and attributes so the best way is to have the classification of these various traffic flows.

Network Management Applications

These applications are used for the network control like routing protocols, quality of services. They require high priority in such case to get rid of delay, jitter and packet loss.

Interactive Applications(Real-time)

These applications are efficient so that simultaneous user can communicate properly in real-time and possibly there would be no high delay between the sender and receiver especially in voice and video application.

Reactive Applications(Near-real-time)

These are the applications which include low reaction time and they are consider to be a near-real-time having less severe requirements in terms of delay, jitter and jitter than the real time applications.

Reliable Applications(No-real-time)

There are no specific requirements in terms of jitter and delay because overall they need the reliable delivery of packets. For instance email, file transfer and they are based on TCP so it can be retransmission in case of error or packet lost.

Real Multimedia Scenario Applications

These are the important application regarding real-time multimedia communication:

Voice

Voice over IP applications are now a days in the market are very popular and to maintain the standard of Quality of Service in these application there should be no delay more than 1 percent and jitter should not be more than 30 ms. These applications are usually low-bandwidth in order to have a reliable delivery of packets.

Video

Video applications involve the one way interaction within two or more users and there should no more than 1 percent packet loss with a maximum delay of 400 ms. As comparative to voice these applications need more bandwidth to get rid of delays, losses and bit rate fluctuations in video.

Video Streaming

In these applications the latency and loss should not be increased more than 4 to 5 percent. It needs more bandwidth than the simple interactive video applications.

Data

In this case there are no particular requirements in terms of delay, jitter and packet loss where as the main focus is based on the reliable delivery of packets on receiver side.

IEEE 802.11

It is important to have a background on IEEE-802.11 standard. It considers the physical and data link layer of ISO-OSI but it does not supports the Quality of Services (QoS) management and frames of data (Wireless station) are queued in a separate queue and waiting to be sent into the channel especially in case of heterogeneous flows having different QoS requirements. In this standard an algorithm named DCF (Distributed Coordination Function) is used by the channel access with interframe space duration and Contention window dimensions (CW min and CW max).

As observed in the 802.11 DCF administer the traffic so that they could mange on equally basis but unfortunately it is not appropriate for real-time and interactive applications. So we can say that we need to add the prioritization function for the traffic flow in to the DCF and work on the man and min values of contention windows and interframe space duration in the best interest of the quality services.

IEEE 802.11e with QoS

As we know it delineates the enhancement of QoS with new function called Hybrid Coordination Function (HCF) with two types of access modes.

EDCA ( Enhanced Distributed Channel Access)

HCCA ( HCF Controlled Channel Access)

This standard designed on for the priority management so it can manage and control QoS on 802.11 wireless networks as well. In EDCA there are four access categories (AC) which are implemented at each QoS enabled station (QSTA) being it a wireless host. For the implementation of priorities EDCA observes differentiated channel access probabilities to frames challenging for channel resources. It assigns the priority according to the QoS requirements by using the set of EDCA parameters which are broadcasted by the AP though beacon frame.

EDCA Parameters

Arbitrary Inter Frame Space (AIFS)

It has the same role as DIFS but here in 802.11e having different values of each Access category. In AIFS the station must detect an idle channel for at least AIFS (AC) interval before starting its back-off timer.

AIFS [AC] = SIFS + AIFSN [AC] x Slot time

CWmin and CWmax ( Contention Windows Parameters)

In 802.11 DCF as have already seen them as they control minimum and maximum dimension of back-off window size. Now they are called CWmin [AC] and CWmax [AC] because of the widow size is doubled.

Transmission opportunity (TXOP)

It delimits by a time and maximum duration in which a wireless station transmit data frames without any interruption and entering a new contention phase. If TXOP value is null that mean station can send only one frame and this parameter is consist of maximum value called TXOP limit.

HCCA

The HCCA provides the parametric quality of services support during controlled channel access period where hybrid coordinator broadcasts periodically beacon frames for network communication, management and control to the wireless stations. It can possibly starts CAP when medium access controlled by QAP that allocates TXOPs to given QSTA (QoS enables station). To have a access of channel QSTA runs QoS reservation for all traffic which need QoS support and have a strong interaction with HC.

Block ACK

The 802.11e introduces the improvement in the system efficiency new operation with transmission burst and aggregated acknowledgements (Block Ack).This method allows a block of frames to be transmitted with SIFS period and acknowledge by a final Ack frame called Block Ack.

Improvement in EDCA Performance

There had some improvements regarding QoS when traffic flow increased. There was a focus on the uplink and downlink fairness. Some methods used to enhance the EDCA performance by trimming the standards parameters.

Uplink/Downlink Fairness

The IEEE 802.11e wireless network is related to fairness between the uplink and downlink channel allocation in WLAN controlled by QAP.

Uplink Flow

The uplink flow refers with a packet flow produced by a QSTA towards the QAP. When the QATS have contention for channel access then the uplink packet is transmitted.

Downlink Flow

The downlink flow instead refers to a packet flow in opposite direction. The packets of downlink can be produced either by wired or wireless QSTAs of same BSS and QAP must compete for channel access with other QSTAs before sending packet.

Tuning EDCA Parameters

The correct tuning of EDCA parameters allows the reduction of waiting time due to channel contention and balance the resource allocation for QAP/QSTAs and also number of collision between frames and different ACs.

AIFSN

AIFSN needs to be enhanced due to different aspects. The reduction of AIFSN for AC at the QAP improves the uplink/Downlink fairness with QAP will have an advantage in channel contention and gets more channel time than the single QSTAs. Furthermore, reduction of AIFSN for QAP and QSTAs leads to a decrease of waiting times and increasing the WLAN throughput.

CWmin and CW max

The fairness of the uplink/downlink can have better improvement with the help of tuning the parameter with reduction and increase in waiting time and throughput we have another option is that if we augment the CW size of the AC when QAP get to know the number of active traffic sources related to the AC is a risky but due to this strategy we tried to save collisions and later performance issues. This strategy has a link with the TXOPlimit adjustment which I am going to discuss in the next portion.

TXOPlimit

The default values of TXOPlimit can be helpful in order to improve network performance of time bounded traffics with the break out transmission of several frames. About the uplink/downlink fairness we can assume when each one of the stations has to transmit frames and the AP has to transmit M * N frames back to back. Finally a reasonable method for TXOPlimit dynamic management give the opportunity to achieve a good uplink and downlink fairness if we have heavy traffic and also have many stations. On the other hand when an AC gets the channel the transmission time of will have an impact on delay of other ACs and will overcome the TXOPlimit value of transmitting AC. The value could lead to frames starvation with long TXOP transmission and channel with average numbers will be reduced and all other station will have less opportunity to transmit. So we can say it is proved that it could be helpful in terms of improving uplink and downlink fairness.

Objectives: The new IEEE standard 802.11 e adds QoS support to the Mac layer of WLAN networks. In order to further develop efficient QoS management schemes for the IEEE 802.11e networks, evaluate the throughput, delay and performance of mechanism and experience the background user's performance and quality issues.

First study on QoS in wireless local area networks and reinforce QoS guarantees for delay sensitive applications then based on the standard recommended, real-time simulations will test configuration are properly supported by 802.11 e and ensure QoS manages delay-sensitive multimedia and voice application traffic and all users receives acceptable quality of service, greater bandwidth, and less delay and jitter. The project work includes the design of configuration algorithms for the mechanisms studied and their evaluation in a simulator. Implementation work for programming the wireless local area networks and the tools to obtain the measurements will also be part of the project.

Requirement Analysis and Specification Plan

The aim of this project is to work and provide a simulation and ensure that acceptable Quality of Services in different applications and background user experience should be taking into account. Most of the work is based on the simulation rather than the hardware.

Firstly getting familiarity with the Simulators e.g. NS-2, OPNET and etc.

Test analysis with different application and checking QoS quality.

Checking the user Background experience is acceptable.

Research, analysis and testing procedure to enhance QoS.

Gant Chart

Gantt Chart

Tasks

Start Date

Duration(days)

Research

15-Mar-10

17

Requirements

02-Apr-10

15

Design

18-Apr-10

15

Development

04-May-10

75

Test / Evaluation

19-Jul-10

15

Conclusion

I have considered all the background study and requirements how there have been enhancement in order to achieve the good Quality of Service it started from the standard 802.11 to IEEE 802.11e and added with the new features that supports QoS. Furthermore, in terms of avoiding delay, jitter, packet loss and acceptable quality of services for every user experiencing the WLAN, none of the user should be disappointed from the quality service. I have stated a summary of background in which the changes made for QoS purposes and in the best interest of the services issues. I have studies all this information and will start working on real-time simulation to do the analysis first and see the result time by time.

Minimum Requirements

Understanding the project and all previous work done in related field.

Simulation analysis to check the Quality of services with different application and user experiences in terms of delay, jitter and packet loss avoidance.

Analyze and test state of the art heterogeneous traffic scenario and acceptable service for users.

Maximum Requirements

Quality of Service (QoS) enhancement with making the algorithms efficient and robust.

Also research and provide the smoothest way to achieve the required goal.

I will experience all the things in the best interest of completing my dissertation, there may be troubles in the way but with the help of guidance from my supervisor, i will discuss and try to sort out all the related problems regarding my project.

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