A survey of Energy Efficient Network Protocols for Wireless Networks

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

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

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

4. Energy Efficiency Research in Protocol Stack

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

6. Sources of Power Consumption • Two types • Communication related • Computation related • Tradeoff between them

7. Communication related • Three modes: • Transmit • Receive • Standby • Example: • 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

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

9. General Guidelines and Mechanisms 1/5 • Reduce collisions in MAC • Retransmissions lead to power consumption and delays • Cannot be completely eliminated due to user mobility and varying set of mobiles • Change typical broadcast mechanism • 802.11: Receiver keeps track of channel status through constant monitoring

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

11. 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: • Avoid transmissions when channel conditions are poor • Combine ARQ and FEC mechanisms

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

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

14. MAC Sublayer • Three specific MAC protocols • IEEE 802.11 • EC-MAC • PAMAS

15. IEEE 802.11 Standard 1/2 • A mobile that wishes to conserve power may switch to sleep mode and inform the base station • The base station • Buffers packets that are destined for the sleeping mobile • 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

16. IEEE 802.11 Standard 2/2 • Conserves power but results in additional delays and may affect the QoS • Experimental measurements of per packet energy consumption • Same incremental costs for both unicast and broadcast traffic • Higher fixed costs for unicast transmission because of MAC coordination and CTS and ACK messages

17. 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

18. EC-MAC Protocol 2/7

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

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

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

22. 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

23. 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 • Fixed are desirable from energy efficient perspective since a mobile will know when to wake up to receive FSM • Variable are better for meeting the demands of bursty traffic

24. 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

25. 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

26. 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: • It has no packets to transmit and a neighbor begins transmitting a packet not destined for it • 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)

27. 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

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

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

30. 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

31. 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 • Decreasing the transmission power results in an increased number of transmission attempts but may be more efficient than attempting to maximize the throughput

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

33. Adaptive Power Control andCoding Scheme • Each transmitter operates at a power-code pair • Power level lies between a specified minimum and maximum • The error code is chosen from a finite set • At each iteration (timeframe): • Receiver checks the word error rate (WER) • 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

34. 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: • Frequent topology updates • Improved routing • Consumes bandwidth • Infrequent topology updates • Decreased update messages • Inefficient routing and occasional missed packets

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

36. 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

37. Network Layer: Unicast Traffic 1/6 • Five different metrics • Energy consumed per packet • 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

38. Network Layer: Unicast Traffic 2/6 • Variance in power level across mobiles • All mobiles are equal and remain powered-on together for as long as possible • Cost per packet • Routes should be created such that mobiles with depleted energy reserves do not lie on many routes • 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

39. 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

40. 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

41. 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

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

43. 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

44. Network Layer: Broadcast Traffic 2/4 • In traditional networks broadcast technique is a simple flooding algorithm • No global information topology gathered • Requires little control overhead • 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

45. 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

46. 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

47. Transport Layer • TCP was designed initially for wired networks • Physical links are fairly reliable • Packet loss is random in nature • Over a wireless link it degrades significantly • 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

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

49. Split connection protocols 1/2

50. 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