We hope that it is more important to know where you are going than to get there quickly. SNU INC Lab. A Survey of Energy Efficient Network Protocols for Wireless Networks Hayoung, Oh 20 Nov 2006

SNU INC Lab. Contents Introduction Background Power consumption and conservation mechanisms MAC sublayer LLC sublayer Network layer Transport layer OS/middleware and application layers Summary

SNU INC Lab. Introduction Power management is one of the most challenging problem in wireless communication This paper addresses – Incorporation of energy conservation at all layers of the protocol stack for wireless networks

SNU INC Lab. Background Wireless network architecture Protocol layers

SNU INC Lab. Power consumption and conservation mechanisms(1) Sources of power consumption – Communication Transmit(1.5W)>receive(0.75W)>standby(0.01w)  Goal is to optimize the transceiver usage – Computation Protocol processing aspects – Ex) usage of CPU and main memory, data compression.. Tradeoff btw communication costs and computation – communication costs computation needs  Goal is to balance btw communication and computation

SNU INC Lab. Power consumption and conservation mechanisms(2) General conservation guidelines and mechanisms – Network layer Balance the amount of traffic carried by each node – MAC layer : collisions should be eliminated Using a small packet size for registration and bandwidth request – Link layer Transmissions may be avoided when channel are poor Error control schemes (ARQ + FEC) – In a typical broadcast environment Broadcast a schedule that contains data transmission starting times for each mobile Turn off the transceiver whenever the node determines that it will not be receiving data for a period of time – In switching from transmit to receive modes, and vice versa Continuous allocation Aggregate packet requests

SNU INC Lab. MAC sublayer(1) IEEE standard – CSMA/CA (Collision Sense Multiple Access/Collision Avoidance) – For power conservation (QoS problem) A mobile – Active mode/Power Saving mode Base station – buffering for PS mode mobile – Per-packet energy consumption Energy cost = fixed cost (MAC operation) + incremental cost (packet size) Fixed cost of Unicast > Fixed cost of broadcast ∵ receiver’s CTS/AcK messages Incremental cost of Unicast = Incremental cost of broadcast

SNU INC Lab. MAC sublayer(2) EC-MAC protocol (Energy Conserving) – Using centralized scheduler ∴ optimize the transmission schedule FSM-Frame synchronization message (BS->mobile)

SNU INC Lab. MAC sublayer(3) PAMSA protocol (Power Aware Multi-Access) – For Ad hoc network – Separate channels to determine when and how long to power off RTS/CTS control packets Data packets 1. RTS Data A B Control channel Data channel 2. CTS X 2’. backoff 1. RTS Data A B 2. CTS 3. Data 4. transmits a “busy tone” over the control channel CC Do not overhear Turn off

SNU INC Lab. LLC sublayer(1) Adaptive error control with ARQ ≠ ARQ (Automatic Repeat Request) – The new metric for energy efficiency total amount of data delivered,total energy consumed( 일정 ) – To maximize the energy efficiency of the protocol Avoid persistence in retransmission data Trade off # of retransmission attempts for probability of successful transmission Inhibit transmission when channel conditions are poor ARQ works as normal until the transmitter detects an error (lack of AcK) ARQ enters a probing mode (a probing packet is transmitted every t slots) until a properly received AcK is encountered

SNU INC Lab. LLC sublayer(2) Adaptive error control with ARQ+FEC (Forward Error Correction) – To keep energy consumption at a minimum The error control scheme associated with each stream may need to be modified as channel conditions change – Certain setup parameters and a channel model Packetsize, QoS requirements (used by MAC sublayer) and packet scheduler are associated with each data stream

SNU INC Lab. Network layer(1) Typical routing algorithms for Ad hoc – Frequent topology updates Improved routing but consume precious bandwidth – Infrequent topology updates Decrease update messages but inefficient routing Typical metrics – Shortest-hop, shortest-delay, and locality stability In wireless additional metrics – Energy resources, network life time

SNU INC Lab. Network layer(2) Unicast traffic – A shortest-hop routing (x), A shortest-cost routing (o) 1. Energy consumed per packet – Total energy consumed is also minimized 2. Variance in power levels across mobiles – All mobiles are equal 3. Cost per packet – Metrics other than energy consumed per packet – Mobiles with depleted energy do not lie on many routes 4. Maximum mobile cost – By minimizing the cost per mobile

SNU INC Lab. Network layer(3) Broadcast traffic – Each mobile’s radio turns off after receiving a packet if its neighbors have already received a copy of the packet – Power-aware broadcast tree approach The tree is constructed starting from a source and expanding to the neighbors – have consumed lower amounts of power – have not already received the data transmission

SNU INC Lab. Transport layer(1) TCP degrades significantly over a wireless link (handoff) – A larger # of retransmissions – Frequently invoke congestion control measures  throughput, delay, energy consumption Three schemes for reducing retransmission – Split connection protocols – Link layer approach (ex: ARQ+FEC) – End-to-End protocol

SNU INC Lab. Transport layer(2) Energy consumption analysis of TCP – The average # of successes per transmission attempt – Error correlation – Congestion control of TCP By backing off and waiting during error bursts – TCP Probing A probe cycle is initiated during error instead of congestion control – If persistent error conditions are detected  Sender invokes standard TCP congestion control – If transient random error  Sender resumes transmission using available network bandwidth

SNU INC Lab. OS/middleware and application layers(1) OS/middleware for energy efficiency – Mobility impacts the design of OS, middleware – Mobile computers’ processing is expensive (battery power) – Architecture techniques pipelining and parallelism – Predictive shutdown during periods of inactivity – Separation both computation activity (External events) and inactivity

SNU INC Lab. OS/middleware and application layers(2) Application layer for energy efficiency – Load partitioning Selectively partitioned btw the mobile and base station – Mobile host : display, acquire multimedia data – Base station : Intensive computations – Proxies Middleware automatically adapt the applications to changes in battery power and bandwidth – Databases Access time (probe wait, bcast wait) minimization – Video processing Under severe bandwidth constraints or low-power situations – Carefully discarding selected packets – Decreasing the # of transmitted bits

SNU INC Lab. Conclusions The key to energy conservation in wireless communications – Network layer Balance the amount of traffic carried by each node – Link layer Transmissions may be avoided when channel are poor Error control schemes (ARQ + FEC) – MAC layer : collisions should be eliminated Using a small packet size for registration and bandwidth request

SNU INC Lab. References CE. Jones, et al, "A Survey of Energy Efficient Network Protocols for Wireless Networks”, Wireless Networks, 2001.