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Overview

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. HL. MAC. PHY. Introduction.

lindamcdonald
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Overview

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  1. 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

  2. HL MAC PHY 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 Datalink Layer Wireless Function

  3. 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

  4. 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).

  5. 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)

  6. 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

  7. 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.

  8. 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

  9. 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.

  10. OPNET Simulation Model – DCF

  11. OPNET Simulation Model – PCF

  12. OPNET Simulation Results – DCF

  13. OPNET Simulation Results – PCF

  14. OPNET Simulation Results – Analysis • DCF Mode Forthis 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 ModeFor 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.

  15. 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 mechanismswas 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.

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