Saving Energy during Channel Contention in 802.11 WLANs

transcript

V. BAIAMONTE and C.-F. CHIASSERINISupported by the Italian Ministry of University

Mobile Networks and Applications 11, 287–296, 2006

Outline

• Introduction

• The IEEE 802.11b standard access scheme

• The EDA technique

• Numerical results

• Conclusions

Introduction

• Energy consumption is a critical issues.

• Various operational states• Transmitting/receiving,

• Overhearing/idling, and dozing.

• Other operational states

Introduction

• Distributed Coordination Function (DCF), is based on the CSMA/CA mechanism.

• A amount of time in idling/overhearing.

Introduction

• PSM： switch to dozing when does not have any data to transmit/receive.

• Drawback： a delay in traffic service leads to a severe Quality of Service (QoS) degradation.

Introduction

• Aims at while a WLAN card is involved in channel contention.

• At MAC layer, called the Energy-efficient Distributed Access (EDA) scheme.

The IEEE 802.11b standard access scheme

• The DCF exploits both a physical and a virtual channel sensing.

• Overhearing a transmission can set their NAV (Network Allocation Vector).

• If physical or virtual carrier sense detects the channel as busy, then waits to become idle.– DIFS (or EIFS if the previous frame was receiv

ed in error)

• By using the NAV, operations are not interrupted.– RTS/CTS or data transmission

• SIFS

The EDA technique

• First： convert the time in overhearing into a low-power state, namely the idling state.

• Second： degradation traffic delivery delay

The EDA technique

• Slightly modifying two aspects of the DCF protocol: – (i) the virtual sensing mechanism and – (ii) the backoff procedure.

• All other mechanisms are unchanged

The EDA technique

• Case1： NAV=0 and channel as idle.– behaves as in the standard DCF

• Case2： NAV=0 but channel is busy.– Sets its NAV accordingly and enters the low-po

wer state.

• Case3： NAV>0– Senses the channel for DIFS and draws a backo

ff value.

The EDA technique

• While backoff > 0, stay in low-power mode regardless of the channel state.– cannot overhear– cannot set its NAV. – The backoff counter continues till it reaches zer

The EDA technique

• While backoff = 0, the WSTA does not immediately transmit. It listens to the channel for PIFS.– If the channel is idle, it transmits and ends the c

urrent channel contention;

– Otherwise, the WSTA doubles its CW size and draws a new backoff value.

The EDA technique

Numerical results

• The average energy consumption per successful packet.

• The throughput fairness.

• As fairness, we use Jain’s index

Numerical results

Conclusions

• Enable to be in low-power mode while activity and contends for the channel.

• Allow to save energy without degradation of the QoS performance.

• Energy saving as large as 80% and 28% under UDP and TCP traffic.

Saving Energy during Channel Contention in 802.11 WLANs

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