T-110.5111 Computer Networks II Energy Efficiency 18.11.2012
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T-110.5111 Computer Networks II Energy efficiency 18.11.2012 Matti Siekkinen 23.9.2010 Outline • Motivation • Measuring energy • Modeling energy consumption • Optimizing energy consumption Green ICT Source: Google image What is Green ICT? • Green ICT – Reduce the energy consumption of ICT • What’s involved? – Networked Equipment • PCs, mobile phones, data centers, set-top boxes... – Network Equipment (infrastructure) • Routers, switches, wireless access points… Why we give a damn • ICT energy consumption – About 12% of global power consumption – 60billion KWh wasted by inefficient computing every year – Telecom data volume increases approximately by a factor of 10 every 5 years, which corresponds to an increase of the associated energy consumption of 16-20% every year • CO2 – At least 2% of global CO2 emission – As much as airplanes, and ¼ of cars • €€¥££ – Data center and network operators – Large part of operation costs Another viewpoint: Quality of experience • Energy constrained devices – Smartphones • Need to recharge more and more often – Sensors and sensor networks • Don’t want to or cannot change batteries often • Quality of experience and availability issue – Not directly about money – Not so much a ”greenness” issue either • Although scale is very large... • Mobile network infrastructure draws far more power than the phones • We focus today mostly on these issues Some questions worth asking • How much energy does it cost when you – make a phone call, – watch YouTube, or – send an email? • How much of that energy could be saved? • How to achieve these energy savings? 7 Outline • Motivation • Measuring energy • Modeling energy consumption • Optimizing energy consumption Power measurement • System-level power measurement Measure the total power consumption of the whole mobile phone. • Component-level power measurement 9 Measuring with software • Simplest way to measure • Nokia Energy Profiler – Easy to use – Sampling frequency is low (4Hz) – Get accurate information about, e.g. voltage(V), current(Am) • Measured error just a few % in active cases, more for low power cases (idle) – Only for Symbian L 10 Measuring with software (cont.) • PowerTutor – A power monitor for Android-based mobile platforms – Uses models to estimate power consumption • Accuracy may vary depending on usage (check their paper for details) http://ziyang.eecs.umich.edu/projects/powertutor/ 11 Glance at the power consumption consumption atthe power Glance Power(Watt) 0.000 0.500 1.000 1.500 2.000 2.500 3.000 Watching 1 (5,2.249) 5 10 (10,1.281) 15 20 25 YouTube 30 35 40 45 50 55 60 from 65 70 75 ? N95 80 85 90 95 WLAN (113,1.494) (113,1.494) 100 105 Time(second) Time(second) 110 115 120 125 130 12 135 140 WCDMA 145 150 155 160 ? 165 170 175 180 185 190 195 200 205 210 215 (211,2.516) 220 225 230 235 ? 240 245243 249 Using external instruments 13 Power measurement • System-level power measurement Measure the total power consumption of the whole mobile phone. • Component-level power measurement Example: Given a mobile phone, measure the power consumption of each CPU, network interface, and display separately. 14 Component-level power measurement • Requires information about power distribution network at the circuit level • Very few off-the-shelf devices can be measured on component-level – e.g. Openmoko Neo Freerunner – See: Aaron Carroll and Gernot Heiser. An analysis of power consumption in a smartphone. In Proceedings of the USENIXATC 2010. 15 Outline • Motivation • Measuring energy • Modeling energy consumption • Optimizing energy consumption Where does the energy go? • Hardware consumes the energy • Amount of energy consumed depends on – Hardware physical characteristics – Hardware operating mode – Workload generated by software running on top of hardware 17 Power modeling • Allows to estimate energy/power consumption even when direct measurement is impossible – Impractical: external instruments usable only in lab settings – Software not available • Why interesting? – Understand and improve energy consumption behavior of existing protocols and services • Also in setups which aren’t possible in a lab • Help redesign for better energy efficiency – Develop energy-aware protocols and applications • Run-time estimation of energy consumption • E.g., choose energy efficient paths, peers, servers • 18 Power modeling (cont.) • Power models describe – Transmission energy, computational energy, other energy expenditure – Power consumption of each hardware component or software component – Power consumption of a service • Methodology – Deterministic modeling – Statistical modeling Power measurement is needed for both methods. 19 Example: Transmission energy • Which one consumes less energy? – 3G, WLAN, or LTE? – Lumia 920 or iPhone 5? – P2P or C/S? – ... • No simple answers... 20 Transmission energy metrics Two metrics • How many Joules are needed for transmitting or receiving one Bit (energy utility)? – Not constant – Depends on how you send or receive the data – Depends on how you control the operating mode of the network interface – Contextual dependency • E.g. poor signal strength requires more transmit power • How many Bits do you need to transmit or receive? – In bad network conditions, you also need to consider the power consumed by data retransmission (TCP) 21 Transmission: WNI states and transitions • Managing wireless network interface happens through different states – Set of states are technology specific • WNI transitions from state to another according to some rules – Usually timer specified transitions • What states? – E.g. receive, idle, and sleep in WiFi – CELL_DCH, CELL_PCH etc. for 3g • Correspond to different kind of resource allocation (i.e. channel type) • States have different power characteristics – Part of circuitry can be powered off at run time (sleep) CHAPTER 2. LTE 13 For real-time streaming applications such as voice calls, the data sent during every transmission and the bandwidth utilized are very small. For Wi-Fi, 3G, LTE: different powerapplications transmitting states small amount of data for many times semi persis- tent scheduling is more adaptable in which the UE does not need to request for a Grant each time for transmissionTraffic of data. Instead,RRC the Inactivity UE is provided with a transmission pattern which it follows and transmits the data during that particular time slot. For example,burst during a voicetimer call running the UE sends an initial SR and gets the transmission pattern from the scheduler after which the data is sent on the allocated time. This reduces the complexities and overhead of the scheduler and the UE. There are many di↵erent types of scheduling algorithms which are de- signed based on the need of vendors. Commonly used algorithms include Round Robin [42], Proportional Fair [47], Maximum Channel Quality Index (CQI) [43], Channel aware scheduling algorithmRRC_IDLE [42]. 2.3 LTE UE and RRC States In LTE, a UE can shift between three states namely, RRC DISCONNECTED, RRC CONNECTED and RRC IDLE. SLEEP RECEIVE PSM TRANSMIT Timeout IDLE Figure 2.4: States of LTE UE Initially, when the UE is in the OFF state, it does not hold any ac- tive connection with the eNodeB. The state at which the UE is turned on and is searching for a possible base station for registration or the phase when the UE is in Airplane mode could be termed as RRC DETACHED Transmission: Tail energy • All wireless network interfaces exhibit tail energy – Energy spent being idle with radio on à wasted energy • Due to inactivity timers – Mandate how long radio remains in active state (rx on) before state transition to inactive state (rx (partly) off) • Timers help with interactive apps and limit signaling – Sporadic communication – Switching radio on and off causes some delay – State transitions require signaling between phone and and base station • Timer values vary between technology – Wifi≈100-200ms – 3G≈11s (varies between ISPs) – LTE≈10s (varies between ISPs) How to minimize tail energy • Wi-Fi tail is already short • 3G has Fast Dormancy – Phone requests the network to transition to CELL_PCH – Network maintains control and allows or denies (e.g. too frequent requests) – Cuts tail duration down to 3-5s • LTE has DRX/DTX (discontinuous reception/ transmission) – 3G has also CPC (continuous packet connectivity) which is similar (not (yet) fully supported by deployed networks) 2 RRC IDLE. DRX being activated, the transmission and reception of data happens in the RRC CONNECTED mode. When the UE does not receive any data for a certain time, it enters the DRX cycle phase and activates the Short DRX cycle with Short DRX timer (Ts). In the Short DRX cycle state, the UE monitors the How to get rid of tail energyPDCCH (cont.) during the DRX OnTimer period. • DRX works in LTE’s Fig.connected 1. States of LTE UE state – Hence, also called cDRX (connected Initially, when the UE is in the OFF state, it does not hold mode DRX)any active connection with the eNodeB. The state at which the UE is turned on and is searching for a possible base station • DRX operatesfor registration in cycles or the phase when the UE is in Airplane mode could be termed as RRC DISCONNECTED state. Once the – Check periodicallyUE finds base if station new coverage, data it registers is waiting to the Mobility Management Entity (MME) through the LTE Attach proce- – Very similardure. to AfterPSM the registration in 802.11 is successful, the UE moves to the RRC CONNECTEDRRCstate. In inactivity the RRC CONNECTED timerstate (T the ) arriving data UE has a Radio Resource Control (RRC) connection with theidle eNodeB and maintains an active connection through various Fig. 2. RRC State transition of LTE UE CONNECTED STATE IDLE STATE Rx on signaling messages. Once the UE sleeps for a longer duration without any data transmission, it moves to the RRC IDLE state after the expiry of inactivity timer (Inactivity). When The Short DRX timer is active for a time period called as the Short DRX timer period (TS)after which the UE activates Rx off the UE is out of coverage area, or during the process of area update it de-registers with the eNodeB and moves to the Long DRX timer (Tl).