ACKNOWLEDGEMENT A BIG THANKS TO ALL WHO HAVE MADE THIS PROJECT POSSIBLE ESPECIALLY TO THE HEAD OF OUR DEPARTMENT FOR GIVING US THE PERMISSION TO USE THE M.TECH LAB FOR OUR PROJRCT WORK. A VERY SPECIAL THANKS TO OUR PROJECT GUIDE MRS. SANDHYA PATTANAYAK.WITHOUT HER THIS PROJECT WOULD HAVE BEEN IMPOSSIBLE.
CONTENTS AIM OF PROJECT OVERVIEW OF 802.11 MAC PROTOCOLS DCF PROTOCOL EDCF PROTOCOL NETWORK SIMULATION & RESULTS CONCLUSION
AIM OF PROJECTThe main objectives of the Project are: The study of basic concepts and issues of Wireless/Cellular network that can improve the QoS of a cellular WLAN. Mainly focusing on Medium Access Control layer of Open Systems Interconnection (OSI) model. Study various existing Medium Access Control protocols of cellular WLAN. Implementation and comparison of efficient mechanisms that can improve the QoS of WLAN by using OPNET Modeler’s wireless module. Study the results obtained, and recommend the best possible protocol that can provide high QoS under respective network traffic conditions.
WHAT IS WLAN? A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier. The last link with the users is wireless, to give a network connection to all users in a building or campus. The backbone network usually uses cables. Wireless LANs operate in almost the same way as wired LANs, using the same networking protocols and supporting the most of the same applications.
AN OVERVIEW OF IEEE 802.11 IEEE 802.11 advantages： • Broadband bandwidth capability • Low deployment cost • Internet services access anytime, anywhere • Mobility and connectivity 7
IEEE 802.11 disadvantages： Best effort services No build in QoS modification of existing standards required Shared medium 8
Interframe Spacing Varying interframe spacings create different priority levels for different types of traffic. The high-priority traffic doesnt have to wait as long after the medium has become idle. To assist with interoperability between different data rates, the interframe space is a fixed amount of time, independent of the transmission speed. 9
Short interframe space (SIFS) The SIFS is used for the highest-priority transmissions, such as RTS/CTS frames and positive acknowledgments. PCF interframe space (PIFS) The PIFS is used by the PCF during contention-free operation. Stations with data to transmit in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic. 10
DCF interframe space (DIFS) The DIFS is the minimum medium idle time for contention-based services. Stations may have immediate access to the medium if it has been free for a period longer than the DIFS. Extended interframe space (EIFS) It is not a fixed interval. It is used only when there is an error in frame transmission. 11
Distributed Coordination Function(DCF) The DCF is the basis of the standard CSMA/CA access mechanism. Like Ethernet, it first checks to see that the radio link is clear before transmitting. To avoid collisions, stations use a random backoff after each frame, with the first transmitter seizing the channel. 13
In some circumstances, the DCF may use the CTS/RTS clearing technique to further reduce the possibility of collisions. Most traffic uses the DCF, which provides a standard Ethernet-like contention-based service. The DCF allows multiple independent stations to interact without central control, and thus may be used in either IBSS networks or in infrastructure networks. 14
Backoff with the DCF A period called the contention window or backoff window follows the DIFS. This window is divided into slots. Stations pick a random slot and wait for that slot before attempting to access the medium; all slots are equally likely selections. When several stations are attempting to transmit, the station that picks the first slot (the station with the lowest random number) wins. 16
Figure 7. Diagrammatic representation of DCF access method 17
EDCF ( Enhanced DCF ) EDCF is a contention-based channel access scheme. EDCF provides differentiated service, distributed access to the wireless medium for 8 delivery priorities. EDCF access channel on each ESTA uses at most 8 prioritized output queues, one for each delivery priority, called Traffic Categories (TCs). 18
The CWmin and CWmax parameters can be set differently for different traffic categories, such as, a high priority TC with small values of CWmin and CWmax. Instead of using a DIFS, as a minimum specified idle duration time as defined in DCF, a new kind of interframe space called Arbitration Interframe Space (AIFS) is used. 19
Figure : Diagrammatic representation of EDCF access method 20
A single station may implement up to eight transmission queues realized as virtual stations inside a station, with QoS parameters that determine their priorities. If the counters of two or more parallel TCs in a single station reach zero at the same time, a scheduler inside the station avoids the virtual collision. 21
Figure : Virtual backoff of eight traffic categories 22
MAC DELAY THE MAC DELAY VARIES WITH THE PRIORITY LEVELS OF THE APPLICATION. HIGHER THE PRIORITY LOWER THE MAC DELAY.
COMPARISON OF DCF & EDCF THE THROUGHPUT OF BOTH PROTOCOLS SEEMS TO BE IDENTICAL. THROUGHPUT
COMPARISON-MAC DELAY THE MAC DELAY OF EDCF IS GREATER THAN THAT OF DCF. DCF PERFORMS BETTER IN THIS REGARD MAC DELAY
COMPARISON-RETRANSMISSIONATTEMPTS THE NO. OF RETRANSMISSION ATTEMPTS FOR EDCF IS MORE THAN DCF BECAUSE OF A LARGE NO COLLISSIONS. DCF HAS COLLISSION AVOIDANCE MECHANISM THUS HAS LOW NO. OF RETRANSMISSION. RETRANSMISSION ATTEMPTS
CONCLUSIONThe results obtained from simulation shows that EnhancedDistribution Coordination Function provides efficientmechanism for service differentiation and hence provides qualityof service to the Wireless LAN. However, this improvementcomes at a cost of a decrease in quality of the lower prioritytraffic up to the point of starvation. The acquisition of the radiochannel by the higher priority traffic is much more aggressivethan for the lower priority. Higher priority traffic benefited,while lower priority traffic suffered. In terms of overallperformance (under the used simulation conditions in thisparticular study of QoS of Wireless LAN), DCF performsmarginally well than EDCF. This happens due to reason that inEDCF mechanism, each AC function acts like a virtual station formedium access, so more collision will be expected for EDCFscenario.