Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. Quality of Service(QoS) in IEEE Wireless LANs: Evaluation of Distributed Coordination Function (DCF) & Point Coordination Function (PCF) Session 1544 Praveen Durbha, Matthew Sherman

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Overview  Introduction  IEEE Standard  Types of WLANs  WLAN Mobility Types  WM Access Mechanisms  WLAN Problems  Quality of Service  Simulation Models  Simulation Results & Analysis  Conclusions

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Introduction A wireless LAN is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection. A standard, IEEE , specifies the technologies for WLANs. IEEE WLAN architecture LLC PHY MAC HL Wireless Function Datalink Layer

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs The IEEE Standard  Original GHz operating frequency Data rates of 1 Mbps/2Mbps Frequency Hopping ( FHSS) Direct Sequence ( DSSS)  Supplements of a – Operation up to 54 Mbps using OFDM in the 5 GHz frequency range b – Extension of the initial DSSS 2.4 GHz band up to 11 Mbps e – MAC layer DCF and PCF enhancements for QoS assurance

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Types of WLANs  Ad hoc Wireless Network This type of wireless network does not have any backbone infrastructure and has at least two wireless stations. It is also referred to as Independent Base Service Set (IBSS).  Infrastructure Wireless Network This type of wireless network consists of multiple cells interconnected by Access Points (APs) and a Distribution System (DS) such as Ethernet. It is also called as Extended Service Set (ESS).

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Mobility Types  No transition Refers to stations that do not move and are moving within a BSS. (Supported)  BSS transition Refers to stations that move from one BSS to another BSS within the same ESS. (Supported)  ESS transition Refers to stations that move from a BSS in one ESS to another BSS in a different ESS. (Not supported)

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs The MAC  Distributed Coordination Function (DCF) Mandatory implementation Uses CSMA/CA protocol No service differentiation Works for both IBSS & ESS Operates during the Contention Period (CP) Waits a period of DIFS interval before transmission  Point Coordination Function (PCF) Optional Implementation Uses a Point Coordinator (PC) which resides in the AP Works only for ESS Operates during the Contention Free Period (CFP) Waits a period of PIFS interval before transmission

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs DCF Algorithm Wireless station senses the medium If medium free for DIFS, transmit frame else wait till the end of current transmission Receiver sends ACK to sender after SIFS (Success) If collision, wait for EIFS and then back-off (Failure) Back-off = Random_number () * Slot_time Random_number range [0,CW] Contention window (CW) doubled for every repeated collision Back-off timer decremented Station with least back-off has access to the medium Stations wanting to transmit additional packets also back-off

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs PCF Algorithm Point Coordinator senses medium Idle for PIFS, send out polling packet Polled stations send packet to AP after SIFS AP sends ACK to polled stations after SIFS Medium busy, defer access till the end of current transmission No back-off involved Process repeats till all packets are forwarded through AP.

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Hidden Terminal Problem Statement Every station in a wireless network has limited radio transmitting range. This may lead to two stations communicating with the same receiving station which results in a collision. The station causing the collision is termed as “hidden” with reference to the receiving station. Solution The RequestToSend(RTS) & ClearToSend(CTS) mechanism is used to resolve the hidden terminal problem.

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs RTS/CTS Mechanism Algorithm Sender transmits RTS frame Receiver acknowledges RTS with CTS frame Sender receives the CTS frame and the channel is reserved Sender sends the DAT frame Receiver sends the ACK frame to the sender to end the 4-way handshake

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Quality of Service Definition QoS is a broad term used to describe the overall experience the end-user or application will receive over a wireless network. Standard parameters used for measuring QoS are Media Access Delay Throughput Packets Dropped (Packet Loss Rate - PLR) Network Availability DCF and PCF are evaluated using Media Access Delay and Throughput parameters.

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Model – DCF DCF Station ParametersValues Type of NetworkInfrastructure Length of Simulation60 seconds Packet Size Range Bytes Packet Size Distributionuniform_int Interarrival Time (Low Load) seconds Interarrival Time (High Load) seconds Interarrival Time (Overload) seconds Interarrival Time DistributionUniform RTS Threshold256/1024 Bytes Number of Stations10 Station Traffic DestinationNode_0 (AP) AP Traffic DestinationNode_1 Physical CharacteristicsDirect Sequence Data Rate11 Mbps PCF FunctionalityDisabled Offered Load (Low)1.7 Mbps Offered Load(High)3.4 Mbps Offered Load(Overload)6.8 Mbps Center Node (Node_0)Access Point

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Model – PCF PCF Station ParametersValues Type of NetworkInfrastructure Length of Simulation60 seconds Packet Size Range Bytes Packet Size Distributionuniform_int Interarrival Time (Low Load) seconds Interarrival Time (High Load) seconds Interarrival Time (Overload) Interarrival Distributionuniform Number of Stations10 Station Traffic DestinationNode_0 (AP) AP Traffic DestinationNode_1 Physical CharacteristicsDirect Sequence Data Rate11 Mbps PCF FunctionalityEnabled CFP Beacon Multiple1 CFP Offset0 CFP Interval0.01 Max Failed Polls2 Beacon Interval0.02 seconds Offered Load (Low)1.7 Mbps Offered Load (High)3.4 Mbps Offered Load (Overload)6.8 Mbps Center Node (Node_0)Access Point

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Low Load Delay least for DCF no RTS/CTS scenario PCF records higher delay Good QoS as delay < 1 sec

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Low Load Throughput same for all scenarios as no data packets dropped (see next slide) Full Load carried Load=Throughput + Data Dropped

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Low Load No data dropped (PLR) in any of the scenarios Good QoS

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – High Load PCF remains stable. No effect on DCF with no RTS/CTS. DCF with RTS/CTS approaching criticality.

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – High Load Offered Load doubled Load=Throughput for PCF,DCF no RTS & DCF with 1024 RTS Load≠Throughput for DCF with 256 RTS Some packet drops (see next slide)

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – High Load PLR = 0 for PCF, DCF no RTS & DCF with 1024 RTS PLR = 325 Kbps for DCF with 256 RTS

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Overload All Scenarios in overload Packets being dropped PCF best delay performance

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Overload Maximum throughput for PCF PCF offers highest capacity

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Simulation Results – Overload Maximum PLR for DCF with 256 RTS Minimum PLR for PCF

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs Conclusions  Summary This presentation evaluated the two basic wireless medium access mechanisms DCF and PCF under low and high load conditions. DCF was observed to outperform PCF under low load and PCF outperform DCF under high load conditions. RTS/CTS offered no advantage, but there were no hidden terminals. The observations were done using Media Access Delay, Throughput & Data Dropped (PLR) metrics.  Future Work DCF and PCF do not provide prioritized access to the wireless medium. IEEE e is currently working on enhancing the MAC with mechanisms like Enhanced DCF (EDCF) and HCF. It will be interesting to evaluate these mechanisms using OPNET once the e draft has stabilized.

Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties Wireless LANs References [1 ] IEEE Working Group, IEEE standard for wireless LANs, 1999, Reaffirmed [2] Jim Geier,”Wireless LANs: Implementing High Performance IEEE Networks”, Second Edition, SAMS publishing 2002 [3] Qiang Ni, Lamia Romdhani, Thierry Turletti, Imad Aad, “QoS Issues and Enhancements for IEEE Wireless LAN, INRIA, Sophia Antipolis, Cedex, France, November [4] Dongyan Chen, Sachin Garg, Martin Kappes, Kishore Trivedi, “Supporting VBR VoIP Traffic in IEEE Wireless LAN in PCF Mode OPNETWORK'02, Washington DC, August 2002.