1. Napoli - 21 February 2004 – Simone Merlin SLIDE 1 Analysis of the hidden terminal effect in multi-rate IEEE 802.11b networks Simone Merlin Department of Information Engineering Università degli Studi di Padova

2. Napoli - 21 February 2004 – Simone Merlin SLIDE 2 Outline  Introduction:  the Hidden Terminal Problem in DCF (distributed Coordination Function) access method.  802.11b:  Multirate standard specifications  Basic access VS RTS/CTS mechanism  Numerical Results:  Distance Ratio for a target BER  Average Nominal Goodput  Conclusion

3. Napoli - 21 February 2004 – Simone Merlin SLIDE 3  The problem arises if HN starts transmitting when Rx is still receiving a valid packet from Tx  RTS/CTS packets solve in part the problem:  Reducing the probability of collision (RTS/CTS are small packets)  Reserving channel to transmit the useful data (Virtual carrier sensing)  RTS/CTS don’t affect the weak interferers that lie in the border of the sensing region  Only interference produced by hidden nodes and weak interferers are taken into account. We do not consider the effect of collisions. The Hidden Terminal Problem Tx Rx HN Tx Rx HN Weak Interf

4. Napoli - 21 February 2004 – Simone Merlin SLIDE 4 Some 802.11b specifications MPDU PLCP HEAD PRE 1,2,5.5,11 Mbps 1 (2) Mbps PPDU TX RX PLCP HEAD reception radius MPDU reception radius Given a target BER, the reception radius depends upon the rate: PLCP rec. range ≥ MPDU rec. range SNIR Spread Gain

5. Napoli - 21 February 2004 – Simone Merlin SLIDE 5 NAV setting ranges NAV is set as far as PLCP reception range PLCP HEADER contains field for NAV setting RTS, CTS packets usually sent at 1 Mbps NAV is set as far as 1 Mbps reception range B.A. RTS CTS

6. Napoli - 21 February 2004 – Simone Merlin SLIDE 6 B.A. vs RTS/CTS access methods PLCP H ≈ MPDU PLCP ≈ MPDU TX RX TX RTS CTS Hidden Node Hidden Nodes FREE area, due to CTS For LOW bit-rate Interfering nodes far from receiver Possible high interference Basic access RTS/CTS access HN

7. Napoli - 21 February 2004 – Simone Merlin SLIDE 7 B.A. vs RTS/CTS access methods PLCP H TXRX TX RTS CTS Hidden Node MPDU Hidden Nodes FREE area, due to CTS: SMALL For HIGH bit-rate RTS/CTS access method is less effective Basic access RTS/CTS access HN

8. Napoli - 21 February 2004 – Simone Merlin SLIDE 8 Numerical Results: Distance Ratio for a target BER  Definition: Distance ratio for a target BER (< 10 -6 ) Maximum distance at a given BER for Basic Access Maximum distance at a given BER for RTS/CTS All hidden node are active except for the closest to Rx With all the hidden nodes BA becomes sensibly worsen than RTS/CTS, but at high data rate BA is 92% of DCS/DTS

9. Napoli - 21 February 2004 – Simone Merlin SLIDE 9 Goodput  For a fair comparison, we account for RTS/CTS overhead defining the GOODPUT for the two acces methods: = Time for BA transmission of a packet = Time for RTS/CTS transmission for a packet = Packet data (with retransmission)

10. Napoli - 21 February 2004 – Simone Merlin SLIDE 10 Best Average Nominal Goodput  For each distance the optimal rate is chosen. Best case Worst case RTS/CTS is effective in reducing the hidden node interference only for low rates L = 1250 bytes = half the maximum payload

11. Napoli - 21 February 2004 – Simone Merlin SLIDE 11 Distance Ratio for a target Nominal Goodput  Definition: distance ratio for a target Goodput (40% of the nominal rate) BA outperforms RTS/CTS

12. Napoli - 21 February 2004 – Simone Merlin SLIDE 12 Nominal Goodput for a simulated scenario  Results obtained in the simulated scenario lie between the best and worst case

13. Napoli - 21 February 2004 – Simone Merlin SLIDE 13 Conclusion  Under the assumption of no collision and retransmission we have found:  RTS /CTS results to be effective against hidden node interference only for low transmission rate  At high data rate both techniques attain similar performance ..however for highly loaded networks  RTS/CTS could be convenient also at high transmission rate.  Future Work..  Evaluate in a ‘real’ scenario the effectiveness of RTS/CTS method against traffic load  Extend study to 802.11a

14. Napoli - 21 February 2004 – Simone Merlin SLIDE 14