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Unformatted text preview: Performance Analysis of IEEE 802.11b Distributed Coordination Function Function WLAN INTRODUCTION WLAN A WLAN is a system of WLAN nodes that access a common wireless channel within the same frequency band. band. WLAN products first WLAN appeared in the late 1980s, marketed as substitutes for traditional wired LANs. In WLAN a basic service In set, consists of stations that executes the same MAC protocol and compete for access to the same shared wireless medium. medium. A basic service set may basic be isolated or connected to a backbone distribution system through an access point (AP). point Evolution of WLAN Standards The first wireless Ethernet standard, IEEE 802.11, provided for three The physical layer (PHY) specifications including infrared, 1-2 Mbps FHSS and DSSS in the 2.4 GHz ISM band. DSSS Two years later, 802.11b specification increased data rates well beyond Two the critical 10 Mbps mark, maintained compatibility with the original 802.11 DSSS standard. 802.11b incorporated two types of coding schemes, PBCC as an option for higher performance in the form of range at the 5.5 and 11 Mbps rates and CCK to attain a top-end data rate of 11 Mbps. and The new standard was which came into existence in July 2003 is named The as IEEE 802.11g and operates into 2.4 GHz ISM band and the data rate is up to 54 Mbps. up IEEE 802.11 MAC Architecture IEEE The IEEE-802.11 MAC sub layer is responsible for frame addressing The and formatting, error checking, channel allocation procedures, fragmentation and reassembly. Two channel allocation procedures are defined in the IEEE-802.11 Two MAC architecture: DCF and PCF. MAC DCF supports asynchronous data transfer in which, all the stations DCF contend for the channels for each packet. PCF uses a centralized decision maker, such as an AP, to regulate transmissions. decision DCF protocol DCF A station senses the medium to determine its state, and if the medium is free, station the transmission may proceed after ensuring that the medium is idle for a duration, defined as DCF Inter-frame Space (DIFS). If the medium is busy, the station will defer until the end of the current If transmission plus a DIFS delay. After the deferral, the station will apply a random back off procedure. random To begin the procedure, the station will select a random back off interval back To off slots in the range of 0-7. The station performing the back off procedure will use the carrier sense mechanism. use If the medium is busy at any time during a back off slot, the back off procedure If is suspended at the beginning of the back off slot and is resumed only if the medium is idle for a DIFS period again. If the medium is idle for the duration of a particular back off slot, the station If will decrement its back off interval by one slot. After the completion of back off, the station transmits its frame immediately. If two or more stations complete their back off procedure at the same time, a If collision will occur, and each station will have to select a new number collision DIFFERENT STEPS INVOLVED IN PROTOCOL PROTOCOL Generation of packets in terminal Generation stations. stations. Packet sensing block Channel sensing block DIFFERENT STEPS INVOLVED IN PROTOCOL PROTOCOL DIFS block Back off block Transmission block Transmission DIFFERENT STEPS INVOLVED IN PROTOCOL PROTOCOL Acknowledgement reception Acknowledgement Collision detection block Collision Retransmission Retransmission of collided packets. Flowchart of the WLAN simulation model Flowchart Simulation Results Simulation Assumptions
Mobile stations are 100. Packet size is 1024 bytes Network load is varied from 0.1 to 0.9 Data rate is 11mbps Mean inter arrival time of packets is 1micro second = 11 time units based on 11 mbps rate. 1micro DIFS = 770 time units or 70 micro seconds. CW min =32 and is varied for analysis. Back off slot time is 220 time units or 20 micro seconds. The header sizes The MAC layer header = 272 time units or 24.72 micro seconds MAC Physical overhead = 2112 time units or 192 micro seconds. Physical 2112 Different characteristics of the network Average Throughput of the network Average Delay per each packet transmitted Probability of collision Simulation results showing variations in throughput for changes in CWmin = 32, 64, 128 for Simulation results showing variations in Delay/packet for changes in CWmin = 32, 64, 128 Delay/packet Simulation results showing variations in CollisionProbability for changes in CWmin = 32, 64, 128 Probability Final Result Final The average throughput value for a network The architecture following 802.11b DCF is 0.420747 CONCLUSION CONCLUSION By increasing the Minimum contention window:Throughput increases Throughput Delay per packet increases Delay Collision probability decreases Hence by properly increasing the Minimum Hence Contention window size the maximum throughput and a decrease in probability of collision can be achieved. achieved. FUTURE SCOPE FUTURE This project can be extended to include the priority This based transmission of the VOIP (Voice Over Internet Protocol) data traffic. This project can be still extended to implement the This other WLAN IEEE protocols like 802.11a, 802.11d, 802.11g, 802.11h. REFERENCES REFERENCES  DeSimone, Antonio, Mooi Choo Chuah, and On-Ching Yue.  “Throughput Performance of Transport-Layer over Wireless LANs” IEEE Globecom: Global Telecommunications Conference, vol. 1, pages 542-549, Houston, TX, USA, 1993. Conference  Hodge, James R. A Framework for Simulating Wireless  Mobile Client-Server Computing. Master’s thesis, Department Mobile Master’s of Computer Science, University of Waterloo, July 1996. of  Hodge, James R. Virtual Network Simulation Package Hodge, User’s Manual. University Of Waterloo, October 1996. User’s  IEEE P802.11, Working Group for Wireless Local Area  Networks. “Wireless LANMedium Access Control (MAC) and Physical Layer (PHY) Specifications” IEEEApproved Draft Standard, P802.11D6.1, May 1997. Standard, Thanking you Thanking Queries? Queries? ...
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This note was uploaded on 02/22/2011 for the course CS 57 taught by Professor Dr.xiao during the Fall '08 term at The University of Akron.
- Fall '08