A survey of Energy Efficient Network Protocols for Wireless Networks Christine E. Jones Krishna M. Sivalingam Prathima Agrawal Jyh-Cheng Chen

Issue 1/2 Rapid expansion of wireless services, mobile data and wireless LANs Greatest limitation: finite power supplies

Issue 2/2 Typical example of power consumption from a mobile computer (Toshiba 410 CDT): o 36% Display o 21% CPU/memory o 18% Wireless interface o 18% Hard drive Goal o Incorporate energy conservation at all layers of protocol stack

Energy Efficiency Research in Protocol Stack

Physical Layer Two different perspectives Increase battery capacity o Increase capacity while restricting weight o However battery technology hasn’t experienced significant advancement in the past 30 years Decrease of energy consumed o Variable clock speed CPUs o Flash memory o Disk spindown

Sources of Power Consumption Two types o Communication related o Computation related Tradeoff between them

Communication related Three modes: o Transmit o Receive o Standby Example: o Proxim RangeLAN2 2.4 GHz 1.6 Mbps PCMCIA card 1.5W transmit, 0.75W receive, 0.01W standby Turnaround between transmit and receive typically takes 6 to 30 microseconds Optimize the transceiver usage

Computation related Usage of CPU, main memory and disk Data compression techniques for reduction of packet length increase power consumption

General Guidelines and Mechanisms 1/5 Reduce collisions in MAC o Retransmissions lead to power consumption and delays o Cannot be completely eliminated due to user mobility and varying set of mobiles Change typical broadcast mechanism o : Receiver keeps track of channel status through constant monitoring

General Guidelines and Mechanisms 2/5 Turnaround between transmit and receive mode spends time and power o Allocate contiguous slots for transmission or reception o Request multiple transmission slots with a single reservation packet Computation of transmission schedule should be relegated to base station

General Guidelines and Mechanisms 3/5 Scheduling algorithm may additionally consider battery power level Allow mobile to re-arrange allocated slots under low-power conditions At link layer: o Avoid transmissions when channel conditions are poor o Combine ARQ and FEC mechanisms

General Guidelines and Mechanisms 4/5 Energy efficient routing protocols o Ensure all nodes equally deplete their power level o Avoid routing through nodes with lower battery power Requires mechanism for dissemination of node battery power o Periodicity of routing updates can be reduced May result in inefficient routes

General Guidelines and Mechanisms 5/5 OS level o Suspend of specific sub-unit (disk, memory, display etc.) when detect prolonged inactivity

MAC Sublayer Three specific MAC protocols o IEEE o EC-MAC o PAMAS

IEEE Standard 1/2 A mobile that wishes to conserve power may switch to sleep mode and inform the base station The base station o Buffers packets that are destined for the sleeping mobile o Periodically transmits a beacon that contains information about such buffered packets When the mobile wakes up, it listens for this beacon, and responds to the base station which then forwards the packets

IEEE Standard 2/2 Conserves power but results in additional delays and may affect the QoS Experimental measurements of per packet energy consumption o Same incremental costs for both unicast and broadcast traffic o Higher fixed costs for unicast transmission because of MAC coordination and CTS and ACK messages

EC-MAC Protocol 1/7 Energy Conserving-Medium Access Control Developed with the issue of energy efficiency as a primary goal Defined for infrastructure network but can be extended to ad-hoc by allowing mobiles to elect a coordinator It is based on reservation and scheduling and supports QoS

EC-MAC Protocol 2/7

EC-MAC Protocol 3/7 FSM: o transmitted at the start of each frame by the base station o contains synchronization information and uplink transmission order for subsequent reservation phase Request/Update Phase: o Each registered mobile transmits new connection requests and status of established queues o Collisions avoided

EC-MAC Protocol 4/7 New User Phase (Aloha): o Registration of new users o Collisions occur o Provides time for BS to compute the data phase transmission schedule Schedule Message: o Broadcasted by the base station o Contains the slot permissions for the subsequent data phase

EC-MAC Protocol 5/7 Data phase (Downlink): o Transmission from base station to mobiles o Scheduled considering QoS requirements Data phase (Uplink): o Slots allocated using a suitable scheduling algorithm

EC-MAC Protocol 6/7 Collisions are avoided and this reduces the number of retransmissions Mobile receivers are not required to monitor the channel because of schedules Centralized scheduler can optimize schedule so that mobiles transmit and receive within contiguous slots

EC-MAC Protocol 7/7 Scheduling algorithms may consider also battery power level in addition to packet priority Frames may be fixed or variable length o Fixed are desirable from energy efficient perspective since a mobile will know when to wake up to receive FSM o Variable are better for meeting the demands of bursty traffic

PAMAS Protocol 1/3 Designed for ad hoc network, with energy efficiency as primary goal Provides separate channels for RTS/CTS control packets and data packets

PAMAS Protocol 2/3 A mobile with a packet to transmit sends a RTS over the control channel, and awaits the CTS If no CTS arrives the mobile enters a backoff state However, if CTS is received, then the mobile transmits the packet over the data channel The receiving mobile transmits a “busy tone” over the control channel for the others to determine that the data channel is busy

PAMAS Protocol 3/3 The use of control channel allows mobiles to determine when and for how long to power off A mobile can power off when: o It has no packets to transmit and a neighbor begins transmitting a packet not destined for it o It does have packets to transmit but at least one neighbor-pair is communicating The length of power off time is determined through the use of a probe protocol (Singh and Raghavendra, 1998)

LLC Sublayer Is responsible for the error control The two most common techniques for the error control are Automatic Repeat Request (ARQ) and Forward Error Correction (FEC) Both waste network bandwidth and power resources due to retransmissions and greater overhead

LLC Sublayer Recent research has addressed low- power error control and several energy efficient link layer protocols have been proposed: o Adaptive Error Control with ARQ o Adaptive Error Control with ARQ/FEC Combination o Adaptive Power Control and Coding Scheme

Adaptive Error Control with ARQ 1/3 Three guidelines: o Avoid persistence in retransmitting data o Trade off number of retransmission attempts for probability of successful transmission o Inhibit transmission when channel conditions are poor

Adaptive Error Control with ARQ 2/3 Works as normal until the transmitter detects an error due to the lack of a received ACK. Then the protocol enters a probing mode in which a probing packet is transmitted every t slots. Probe packet contains only header This mode continues until an ACK is received. Then the protocol returns to normal mode and continues transmission from where it was interrupted

Adaptive Error Control with ARQ 3/3 Analysis results show that under slow fading channel conditions it is superior to standard ARQ in terms of energy efficiency There is an optimal transmission power in respect to energy efficiency o Decreasing the transmission power results in an increased number of transmission attempts but may be more efficient than attempting to maximize the throughput

Adaptive Error Control with ARQ/FEC Combination Each packet stream o is associated with service quality parameters (packet size, QoS requirements) o maintains its own time-adaptive customized error control scheme Error control scheme o is a combination of an ARQ scheme (Go-Back-N, CACK, SACK, etc.) and a FEC scheme o modifies as channel conditions change over time

Adaptive Power Control and Coding Scheme Each transmitter operates at a power-code pair o Power level lies between a specified minimum and maximum o The error code is chosen from a finite set At each iteration (timeframe): o Receiver checks the word error rate (WER) o If the WER lies within an acceptable range, power-code is retained, otherwise a new power- code pair is computed by the transmitter Variations of algorithm include average WER

Network Layer Energy efficient routing algorithms for ad hoc networks Does not apply to infrastructure networks because all traffic is routed through BS Two different approaches: o Frequent topology updates Improved routing Consumes bandwidth o Infrequent topology updates Decreased update messages Inefficient routing and occasional missed packets

Network Layer Typical metrics for ad hoc routing protocols o Shortest-hop o Shortest-delay o Locality-stability However they may result in the overuse of energy resources of a small set of mobiles decreasing mobile and network life

Network Layer example Using shortest-hop routing, traffic from A to D will always be routed through E E’s energy reserves will be drained faster and then F will be disconnected from network A to D traffic should also use the B-C path extending networks life

Network Layer: Unicast Traffic 1/6 Five different metrics o Energy consumed per packet o Time to network partition Given a network topology, a minimal set of mobiles exist such that their removal will cause the network to partition The traffic in that mobiles should be divided in such a way that they drain their power at equal rates

Network Layer: Unicast Traffic 2/6 o Variance in power level across mobiles All mobiles are equal and remain powered-on together for as long as possible o Cost per packet Routes should be created such that mobiles with depleted energy reserves do not lie on many routes o Maximum mobile cost By minimizing the cost experienced by a mobile when routing a packet through it significant reductions in the maximum mobile cost result

Network Layer: Unicast Traffic 3/6 The goal is to minimize all the metrics except for the second which should be maximized Shortest-cost routing protocol is more appropriate instead of shortest-hop So although packets may be routed through longer paths, the paths contain mobiles that have greater amounts of energy reserves Also routing traffic through lightly loaded mobiles conserves energy because it minimizes contention and retransmission

Network Layer: Unicast Traffic 4/6 Simulation results showed no extra delay over the traditional shortest-hop metric This is true because congested paths are often avoided However this approach requires that every mobile have knowledge of every other mobile and the links between them This creates significant communication overhead and increased delay

Network Layer: Unicast Traffic 5/6 Stojmenovic and Lin proposed localized routing algorithms These algorithms depend only on information about the source location, the location of neighbors and location of the destination This information is collected through GPS receivers which are included in every mobile

Network Layer: Unicast Traffic 6/6 They proposed a new power-cost metric o Incorporates both a mobile’s lifetime and distance based power metrics Three power-aware localized routing algorithms were developed o Power Minimize total amount of power utilized when transmitting a packet o Cost Avoid mobiles with low battery reserves o Power-cost Combination of the other two

Network Layer: Broadcast Traffic 1/4 Each mobile needs to receive a packet only once Intermediate mobiles are required to retransmit the packet Key idea: allow each mobile’s radio to turn off after receiving a packet if its neighbors have already received a copy of the packet

Network Layer: Broadcast Traffic 2/4 In traditional networks broadcast technique is a simple flooding algorithm o No global information topology gathered o Requires little control overhead o Completes with minimum number of hops Not suitable for wireless networks because many intermediate nodes must retransmit packets needlessly It is more beneficial to spend some energy in gathering topology information in order to determine the most efficient broadcast tree

Network Layer: Broadcast Traffic 3/4 A broadcast approach is presented in (Singh et al., 1999) The tree is constructed starting from the source and expanding to the neighbor that has the lowest cost per outgoing degree Mobile costs continuously change so broadcast transmissions may traverse different trees Simulations showed very little difference in delay but 20% or better in energy consumption

Network Layer: Broadcast Traffic 4/4 In (Wieselthier et al., 2000) is presented an algorithm for determining the minimum- energy tree There exists an optimal point in the trade-off between reaching greater number of mobiles in a single hop by using higher transmission power versus reaching fewer mobiles but using lower power levels

Transport Layer TCP was designed initially for wired networks o Physical links are fairly reliable o Packet loss is random in nature Over a wireless link it degrades significantly o It resorts to a larger number of retransmissions and frequently invoke congestion control measures because it confuses link errors and loss as channel congestion The increased retransmissions consume battery energy and bandwidth

Transport Layer Various schemes have been proposed o Split connection protocols o Link-layer protocols o End-to-end protocols

Split connection protocols 1/2

Split connection protocols 2/2 Completely hide the wireless link from the wired network by splitting each TCP connection into two separate connections at the BS The second one may use modified versions of TCP that enhance performance over the wireless channel

Link-layer protocols 1/2

Link-layer protocols 2/2 Hides link related losses from the TCP source Uses a combination of local retransmissions and FEC over the wireless link Local retransmissions use techniques that are tuned to the characteristics of the wireless channel

End-to-end protocols Include modified versions of TCP that are more sensitive to the wireless environment Uses mechanisms such as o SACK allow the TCP source to recover from multiple packet losses o ELN Aid the TCP source to distinguish between congestion and other forms of loss

Energy Consumption Analysis of TCP 1/4 The energy consumption of Tahoe, Reno and New Reno is analyzed in (Zorzi and Rao, 2000) Efficiency is defined as the average number of successful transmissions per energy unit Results demonstrate that o error correlation affects the energy performance o congestion control algorithms of TCP allow for greater energy savings by backing off and wait during error bursts o energy efficiency is sensitive to the version of TCP

Energy Consumption Analysis of TCP 2/4 The same versions of TCP were studied in (Tsaoussidis et al., 2000a) in terms of energy/throughput tradeoffs Results showed that o no single version is most appropriate within wired/wireless heterogeneous networks o the key to balancing energy and throughput is through the error control mechanism They proposed a modified version of TCP, referred to as TCP-Probing in (Tsaoussidis and Badr, 2000)

Energy Consumption Analysis of TCP 3/4 In TCP-Probing when a segment is delayed or lost, instead of invoking congestion control, transmission is suspended and a probe cycle is initiated Probe cycle: o exchange of probe segments (TCP header with no payload) between sender and receiver o terminates when two consecutive RTT are successfully measured

Energy Consumption Analysis of TCP 4/4 The sender invokes standard TCP congestion control if persistent error conditions are detected However, if conditions indicate transient random error, then the sender resume transmissions according to available network bandwidth

OS/Middleware The main functions of an operating system is to manage access to physical resources like CPU, memory and disk space CPUs can be operated at lower speeds by scaling down the supply voltage (quadratic relationship between power and supply voltage) Predictive shutdown Different page placement algorithms exploit the new power management features of memory technology

Application Layer 1/2 Load Partitioning o Applications may be selectively partitioned between the mobile and base station o Most of the power intensive computations of an application are executed at the BS Proxies o Middleware that automatically adapt the applications to changes in battery power and bandwidth o Either on the mobile or BS side of wireless link

Application Layer 2/2 Databases o Minimize power consumed per transaction through embedded indexing Video Processing o Reduce effective bit rate of video o Carefully discard video frames