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A Rate-Adaptive MAC Protocol for Wireless Networks

A Rate-Adaptive MAC Protocol for Wireless Networks. By Gavin Holland, Nitin Vaidya and Paramvir Bahl. Contents. Introduction MAC protocol issues RBAR description Simulation Summary. Introduction. IEEE 802.11 is an established MAC layer protocol for Wireless Local Area Networks

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A Rate-Adaptive MAC Protocol for Wireless Networks

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  1. A Rate-Adaptive MAC Protocol for Wireless Networks By Gavin Holland, Nitin Vaidya and Paramvir Bahl

  2. Contents • Introduction • MAC protocol issues • RBAR description • Simulation • Summary

  3. Introduction • IEEE 802.11 is an established MAC layer protocol for Wireless Local Area Networks • Supports DSSS, FHSS, and IRDA at the physical layer. • Transmits a data rate up to 11 Mbps. • Uses CSMA/CA (RTS/CTS hand-shake) to combat the Hidden Terminal problem.

  4. The Need for Rate Adaptation

  5. Issues Involved • Standard supports Reservation Access Control based on duration field T. T = TCTS + TDATA + TACK + 3 * SIFS • Standard supports Rate Adaptation using a (PLCP) header. • Rate selection mechanism intentionally unspecified.

  6. Overview of the Receiver-Based Autorate (RBAR) Protocol • S sends a RTS to R with rate r1 and packet-size n. • A hears the RTS and tentatively calculates the reservation duration. • R selects r2 using channel estimation and sends a CTS. • B hears the CTS and calculates the reservation using r2 and n • S puts r2 in the data packet header and transmits at the rate r2. • A hears the data packet and recalculates the reservation. A S R B r1, n r1, n r2, n r2, n r2, n r2, n ACK

  7. Modifications to 802.11 • Change the 16-bit duration field in RTS and CTS packets to a 4-bit rate sub-field and 12-bit packet length sub-field. • A CRC is added to the MAC header for data packets. • Encoding of the PLCP signal field is divided into two 4-bit rate fields corresponding to the rate for the MAC header and the data.

  8. Network Simulation • Modified the NS-2 Simulator to include a model for Rayleigh fading. • Used threshold based rate-selection from an Instantaneous SINR sampled at the end of a packet reception. • Compared results with the Lucent AutoRate Fallback (ARF) protocol.

  9. The Lucent ARF protocol • Rate is reduced when two consecutive ACKs are lost. • A timer is started after rate reduction. • After the timer expires the rate is increased for the first packet (a probe). • If ACK is lost then rate is immediately reduced and the timer restarted.

  10. Simulation Results

  11. Results Cont.

  12. Results Cont.

  13. Results Discussion • Impact of Node Speed on CBR • An increase in mean-node speed resulted in a decrease in relative performance. • Impact of Bursty Traffic • Showed the most performance gain when traffic is lightest. • TCP versus UDP • Performance over TCP sources is notably better when compared with performance over UDP sources.

  14. Research Contribution • Receiver-Based Rate Control • RTS serves as a probe for estimating channel quality. • Channel quality feedback is not required. • Implementation based on existing IEEE802.11 Standard. • RBAR outperforms the Lucent WaveLAN Auto-Rate Fallback protocol .

  15. Possible Flaws • Requires separate processing of MAC header and data payload. • Channel Estimation Technique • Computationally complex for thin-client receivers. • Constrained by channel coherence time. • Lack of QoS enforcements.

  16. Conclusion • Showed that a modified version of 802.11 could improve network throughput. • Proposed a hybrid approach of the RBAR and ARF protocol for future studies.

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