Napoli - 21 February 2004 – Simone Merlin

SLIDE 1

Analysis of the hidden terminal effect in multi-rate IEEE 802.11b

networksSimone Merlin

Department of Information Engineering

Università degli Studi di Padova

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

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 RxHN

Tx Rx

HN

Weak Interf

Napoli - 21 February 2004 – Simone Merlin

SLIDE 4Some 802.11b specifications

)/( ob NEfBER

MPDU PLCP HEAD PRE

1,2,5.5,11 Mbps 1 (2) 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

iNo PPGNoEb

rP/

SNIRSpread Gain

Napoli - 21 February 2004 – Simone Merlin

SLIDE 5NAV 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.

RTSCTS

Napoli - 21 February 2004 – Simone Merlin

SLIDE 6B.A. vs RTS/CTS access methods

PLCP H ≈ MPDU PLCP ≈ MPDU

TX RXRX TX

RTSCTS

Hidden NodeHidden Nodes FREE

area, due to CTS

For LOW bit-rate

Interfering nodes far from receiverPossible high interference

Basic access

RTS/CTS access

HN

HN

Napoli - 21 February 2004 – Simone Merlin

SLIDE 7B.A. vs RTS/CTS access methods

PLCP H PLCP H

TX RXRX TX

RTSCTS

Hidden Node

MPDU 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

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

Napoli - 21 February 2004 – Simone Merlin

SLIDE 9Goodput

For a fair comparison, we account for RTS/CTS overhead defining the GOODPUT for the two acces methods:

RCRC

ba

TT

DG

T

DG

0

0

D

T

T

RC

0 = Time for BA transmission of a packet

= Time for RTS/CTS transmission for a packet

= Packet data (with retransmission)

Napoli - 21 February 2004 – Simone Merlin

SLIDE 10Best 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

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

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

Napoli - 21 February 2004 – Simone Merlin

SLIDE 13Conclusion

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

Napoli - 21 February 2004 – Simone Merlin

SLIDE 14