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Department of Information Engineering University of Padova, ITALY Mathematical Analysis of IEEE Energy Efficiency {andrea.zanella, Andrea Zanella, Francesco De Pellegrini WPMC 2004, September 2004 Special Interest Group on NEtworking & Telecommunications

WPMC'04 Abano Terme, Padova (Italy) September 2004 Motivations  Wireless ad-hoc networks are becoming more and more popular  Self-organization  Mobility  Portability  IEEE offers native support for ad-hoc networking  Single cell managed by means of Distributed Coordination Function (DCF)‏  Terminals are battery-powered: energy consumption is a primary issue!  Energy consumption in transmission and reception is of the same order of magnitude [Feeney 01]  The carrier-sense mechanism (CSMA/CA) reduces collision probability but draws energy [Stemm 97]  Cost of sensing is exacerbated by transmissions occurring during the backoff  Also collisions and alien traffic involve an energetic cost

WPMC'04 Abano Terme, Padova (Italy) September 2004 Aim of the study  Goal  Providing a complete statistical description of the energy spent  Characterize the impact of RTS/CTS on energy consumption  Provide a mathematical tool for the design of energy-aware algorithms Case study  Reference scenario [Bianchi2000]  Ad hoc network with n saturated IEEE terminals Single-hop network No hidden or exposed node problem Heavy traffic conditions (saturation)‏ All terminals have always a packet ready for transmission

WPMC'04 Abano Terme, Padova (Italy) September 2004 Energy Model  Linear energetic model  Energy is drawn proportionally to the time spent in each mode [Feeney]  Each operating mode is associated to a different energetic coefficient Transmitting (  )‏ Receiving (  R )‏ Sensing (  S )‏ Virtual Sensing (  0 )‏ RTS CTS DATA ACK T RTS T SIFS T CTS T DATA T ACK T DIFS RTS CTS DATA ACK T SIFS RTS NAV T NAV A)‏ B)‏ C)‏ A B C  Energy spent during SIFS periods is neglected

WPMC'04 Abano Terme, Padova (Italy) September 2004 Detailing the Energy Consumption Overall energy spent for successful packet delivery Energy spent in non-colliding transmission Energy spent in colliding transmissions Energy spent during backoff Number of collisions before success Energy spent in each collision  Hypothesis  are i.i.d. and independent of E T  Probability of collision p independent of the system state [Bianchi01]

WPMC'04 Abano Terme, Padova (Italy) September 2004 Detailing E T & E Tc,j  E T : Energy required for transmitting a packet with success Basic Access RTS/CTS  E Tc,j : Energy spent during packet collision Basic Access RTS/CTS T EIFS

WPMC'04 Abano Terme, Padova (Italy) September 2004 Detailing E B  E B : Energy spent in backoff  W r : total number of tick periods spent in backoff  Tick Period time between two successive decrements (tick) in the backoff countdown process Idle channel: countdown 1 per time slot Busy channel: freeze until the channel returns idle for a DIFS, then resume countdown   j : energy spent in each tick period

WPMC'04 Abano Terme, Padova (Italy) September 2004 Detailing  j  During a tick period a node can be  sensing the radio channel  receiving a valid packet intended for that node  discarding a valid packet for other destinations  listening collided transmission on the channel Idle Channel Busy Channel sense receive listen discard

WPMC'04 Abano Terme, Padova (Italy) September 2004 Putting al pieces together...  Moment generating function for the energy spent by each node in the network

WPMC'04 Abano Terme, Padova (Italy) September 2004 Case Study  Lucent WaveLAN 11 Mbps [Feeney2001]  Transmitting  = 1 (normalized)‏  Receiving  R = 2/3  Sensing  S = 0.82  R  Possible power saving policy  Case 1 Energy spent during NAV phase is negligible (  0 =0)  Case 2 Energy spent during NAV phase is not negligible (  0 =0.5  S )  Case 3 Regular sensing is performed during NAV phase (  0 =  S )

WPMC'04 Abano Terme, Padova (Italy) September Normalized Lifetime Number of stations Basic Access RTS/CTS  0 = 0  0 =  S  0 = 1/2  S Results: node lifetime  Normalized Lifetime  Minimum theoretical energy per pck over Average energy per pck  RTS/CTS outperforms Basic Access mode   0 =0 leads to large gain in nodes lifetime  Gain rapidly fades for  0  1/2  S

WPMC'04 Abano Terme, Padova (Italy) September Payload [bits] Number of stations Energy-based threshold Throughput-base threshold [1]  0 = 0  0 =1/2  S 0 =S0 =S Basic Access-RTS/CTS threshold  Energy vs Throughput perspective  With  0  1/2  S payload threshold is lower than in Throughput-base case  Threshold shows less sensitivity to the number of nodes in the network With more than 20 nodes, the threshold remains almost const  Threshold increases as  0 gets close to  S Payload threshold after which RTS/CTS outperforms Basic Access [1] Bianchi2000

WPMC'04 Abano Terme, Padova (Italy) September 2004 Conclusions  Complete statistical description for energy consumption  Ad-hoc network with saturated IEEE nodes  Model allows for some interesting insights  Channel sensing during backoff has a relevant energetic cost  Switching to low-power mode during NAV can potentially save energy, but only for  0 <<  S  Payload length after which RTS/CTS outperforms Basic Access is lower for Energy-base than for Throughput-base perspective  Energy-based Threshold is less sensitive to the number of nodes in the network than Throughput-based Threshold

WPMC'04 Abano Terme, Padova (Italy) September 2004 Department of Information Engineering University of Padova, ITALY Mathematical Analysis of IEEE Energy Efficiency {andrea.zanella, Andrea Zanella, Francesco De Pellegrini WPMC 2004, September 2004 Questions?

WPMC'04 Abano Terme, Padova (Italy) September 2004 Extra Slides… Spare Slides

WPMC'04 Abano Terme, Padova (Italy) September 2004 Medium Access Control (MAC)‏  CSMA: Carrier Sensing Multiple Access  (Exponential) Backoff stage Choose a random number in the backoff window If the channel is sensed idle, then countdown by 1 for each slot If the channel is busy then freeze the countdown until the channel becomes idle again for at least a DIFS  When the countdown is over transmit the packet If no ACK is returned within a SIFS, a collision has occurred Double backoff window and re-enter the backoff stage Otherwise the transmission was successfull Reset the backoff window and enter the backoff stage for the next packet

WPMC'04 Abano Terme, Padova (Italy) September 2004 Collision Avoidance  Basic Access  Transmit data packet  RTS/CTS access  Try to reserve the channel before transmission Send a very short Request To Send (RTS) packet Receiver replies with a very short Clear To Send (CTS) packet Stations that get RTS or CTS packets avoid transmissions in the successive time interval (setting the NAV)‏

WPMC'04 Abano Terme, Padova (Italy) September 2004 Detailing E B : backoff strategy  E B : Energy spent in backoff  Backoff strategy  S(i) : backoff stage after i successive collisions S(i) = min(i,m)‏  CW i : i-th backoff window CW i =CW 0 2 S(i) -1  x i : i-th backoff counter x i =random{0,1,...,CW i }  Countdown x i tick periods then retransmit the packet

WPMC'04 Abano Terme, Padova (Italy) September 2004 Tick period (1/2)‏  Tick Period  time between two successive decrements (tick) of the backoff countdown process Idle channel countdown 1 per time slot Busy channel (valid or collided packet on the air)‏ freeze until the channel returns idle for a DIFS, then resume countdown  during a tick period a node can wait (idle channel)‏ receive valid packet intended for that node discard valid packet for other destinations listen collided transmission on the channel Idle Channel Busy Channel

WPMC'04 Abano Terme, Padova (Italy) September P [ E > e ] Normalized energy: e=E / min{E} Basic Access  0 =  S Results: complementary cdf of E  Energy actually spent for a packet transmission is many times the theoretical minimum  Jointly using RTS/CTS and smart sensing strategy drastically reduces energy costs Basic Access  0 = 0 RTS/CTS  0 =  S RTS/CTS  0 = 0