Saving Energy during Channel Contention in 802.11 WLANs

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).

The IEEE 802.11b standard access scheme

• 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)

The IEEE 802.11b standard access scheme

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

• SIFS

The IEEE 802.11b standard access scheme

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

o.

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

Numerical results

Numerical results

• The average energy consumption per successful packet.

• The throughput fairness.

• As fairness, we use Jain’s index

Numerical results

Numerical results

Numerical results

Numerical results

Numerical results

Numerical results

Numerical results

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.