1. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. Wireless LANs Session 1544 15

2. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 2 1544 Wireless LANs Overview  Introduction  IEEE 802.11 Standard  Types of 802.11 WLANs  802.11 WLAN Mobility Types  WM Access Mechanisms  WLAN Problems  Quality of Service  Simulation Models  Simulation Results & Analysis  Conclusions

3. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 3 1544 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 802.11, specifies the technologies for WLANs. IEEE 802.11 WLAN architecture LLC PHY MAC HL Wireless Function Datalink Layer

4. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 4 1544 Wireless LANs The IEEE 802.11 Standard  Original 802.11 2.4 GHz operating frequency Data rates of 1 Mbps/2Mbps Frequency Hopping (802.11 FHSS) Direct Sequence (802.11 DSSS)  Supplements of 802.11 802.11a – Operation up to 54 Mbps using OFDM in the 5 GHz frequency range 802.11b – Extension of the initial DSSS 2.4 GHz band up to 11 Mbps 802.11e – MAC layer DCF and PCF enhancements for QoS assurance

5. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 5 1544 Wireless LANs Types of 802.11 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).

6. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 6 1544 Wireless LANs 802.11 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)

7. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 7 1544 Wireless LANs The 802.11 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

8. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 8 1544 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.

9. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 9 1544 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

10. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 10 1544 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 Bandwidth Network Availability Media Access Delay Throughput DCF and PCF are evaluated using Media Access Delay and Throughput parameters.

11. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 11 1544 Wireless LANs OPNET Simulation Model – DCF DCF ParametersValues Type of NetworkAd hoc Length of Simulation180 seconds Packet Size Range1400-2000 Bytes Interarrival time0.1-0.2 seconds RTS Threshold1700 Bytes Number of Stations10 Physical CharacteristicsDirect Sequence Data Rate1 Mbps PCF FunctionalityDisabled

12. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 12 1544 Wireless LANs OPNET Simulation Model – PCF PCF ParametersValues Type of NetworkInfrastructure Length of Simulation180 seconds Packet Size Range1400-2000 Bytes Interarrival time0.1-0.2 seconds RTS Threshold1700 Bytes Number of Stations8 Physical CharacteristicsDirect Sequence Data Rate1 Mbps PCF FunctionalityEnabled CFP Beacon Multiple1 CFP Offset0 CFP Interval0.01 Max Failed Polls2 Beacon Interval0.02 seconds

13. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 13 1544 Wireless LANs OPNET Simulation Results – DCF

14. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 14 1544 Wireless LANs OPNET Simulation Results – PCF

15. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 15 1544 Wireless LANs OPNET Simulation Results – Analysis  DCF Mode For this mode, the delay is low and the throughput high when the network uses RTS/CTS mechanism. This maybe as a result of fewer collisions caused by RTS/CTS.  PCF Mode For this mode, the delay and throughput for a PCF workstation are lower than that of a DCF workstation. The lower delay may be attributed to the fact that PCF operates under the CF mode and the lower throughput may be attributed to the overhead involved in the transmission of polling packets.

16. Copyright © 2003 OPNET Technologies, Inc. Confidential, not for distribution to third parties. 16 1544 Wireless LANs Conclusions  Summary This presentation described the current status of IEEE 802.11 Wireless LANs and evaluated the two basic wireless medium access mechanisms DCF and PCF using OPNET Modeler. The QoS offered by the two mechanisms was measured using Media Access Delay and Throughput metrics.  Future Work DCF and PCF do not provide prioritized access to the wireless medium. The IEEE 802.11e is currently working on enhancing the 802.11 MAC with mechanisms like Enhanced DCF (EDCF) and HCF. It will be interesting to evaluate these mechanisms using OPNET.