Saving Energy during Channel Contention in 802.11 WLANs V. BAIAMONTE and C.-F. CHIASSERINI Supported 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 received in error)
The IEEE 802.11b standard access scheme • By using the NAV, operations are not interrupted. • RTS/CTS or data transmission • SIFS
The EDA technique • First：convertthe 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-power state. • Case3：NAV>0 • Senses the channel for DIFS and draws a backoff 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 zero.
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 current channel contention; – Otherwise, the WSTA doubles its CW size and draws a new backoff value.
Numerical results • The average energy consumption per successful packet. • The throughput fairness. • As fairness, we use Jain’s index
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.