Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks

1. Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks Yu-chee tseng et. al National Chio Tung University, Taiwan INFOCOM ‘02 Presented by Joo, Jaikwan CS710 : Special issues in Computer Architecture

2. Contents • Introduction • General power saving • Power saving modes in IEEE 802.11 • Three asynchronous power saving protocols for MANET • Dominating-awake-interval • Periodically-fully-awake-interval • Quorum-based • Simulations • Conclusions CS710 : Special issues in Computer Architecture

3. Introduction • The critical issue of MANET(Mobile Ad hoc NETwork) : power saving • Battery technology is not likely progress as fast as computing and communication technologies. • The category of power-saving solution • Transmission power control • Topology control • Power aware routing • Base on mobile host power level • Low-power mode • IEEE 802.11 has power saving mode which is a radio only needs to be awake periodically. • Bluetooth : park, hold, sniff mode. CS710 : Special issues in Computer Architecture

4. Introduction • MANET • Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization • Two challenge of power saving • Clock synchronization • No central control, variable packet delay due to unpredictable mobility and radio interference. • Neighbor discovery • Because PS host will reduce its transmitting/receiving activity • Routing problem CS710 : Special issues in Computer Architecture

5. Introduction • Basic idea of protocol • Enforces PS hosts send more beacon packets than the original IEEE 802.11 standard • Arrange the wake-up and sleep patterns of PS hosts such that any two hosts are guaranteed to detect each other in finite time even under PS mode CS710 : Special issues in Computer Architecture

6. Introduction • Power saving modes in IEEE 802.11 • Two power modes : active and power saving(PS) • Under infrastructure(with AP) • AP monitors the mode of each mobile host. • A host in PS mode only awakes up periodically to check for possible incoming packet from AP. • A host always notifies its AP when changing mode. • Periodically AP transmit beacon frames. • In each beacon frame, a Traffic Indication Map(TIM) will be delivered, which contains ID’s of those PS host with buffered unicast packet in the AP. • A PS host, on hearing its ID, should stay awake remaining beacon interval. • On DCF, awake PS host issue PS-POLL • On PCF, awake PS wait for AP Poll • To send Buffered broadcast packet, AP send DTIM(Delivery TIM), after that, buffered broadcast packet will be sent. CS710 : Special issues in Computer Architecture

7. Introduction • Under an ad hoc network • PS hosts also wake up periodically • ATIM window : short interval that PS hosts wake up. • Assuming that hosts are fully connected and all synchronized. • In the beginning of each ATIM window, each mobile host will contend to send a beacon frame. • Successful beacon serve for synchronizing mobile host’s clock. • This beacon also inhibits other hosts from sending their beacon • To avoid collisions among beacons, use random back-off [0-2*CWmin –1] CS710 : Special issues in Computer Architecture

8. Introduction • After the beacon, host can send a direct • ATIM frame to each of its intended receivers • in PS mode. • After transmitted an ATIM frame, keep remaining • awake • On reception of the ATIM frame, reply with an ACK • and remain active for the remaining period • - Data is sent based on the normal DCF access. CS710 : Special issues in Computer Architecture

9. Introduction • Problem statement • PS mode of 802.11 is designed for single hop(fully connected) ad hoc network. • If applied for multi-hop • Clock synchronization • Communication delay and mobility are all unpredictable • Neighbor discovery • A host in PS mode is reduced its chance to transmit • Network partitioning • Inaccurate neighbor information may lead to long packet delay or even network partitioning problem. CS710 : Special issues in Computer Architecture

10. Three asynchronous power saving protocols for MANET • Guidelines in designing protocol • More beacon • To prevent the inaccurate-neighbor problem • A PS host should not inhibit its beacon in ATIM window even if it has heard other beacons. • Allow multiple beacon in a ATIM window • Overlapping Awake interval • Since protocol don’t count on clock synchronization • The wake-up pattern of two PS host must overlap with each other. • Wake-up prediction • To drive PS host’s wake-up pattern based on their time difference. CS710 : Special issues in Computer Architecture

11. Three asynchronous power saving protocols for MANET • Three power-saving protocols, each with a different wakeup pattern for PS host • Beacon interval • For each PS host, it divides its time axis into a number of fixed length interval • Active window • On state • Beacon window • PS hosts send its beacon • MTIM window • Other hosts send their MTIM frames to the PS host. • Excluding these three windows, PS host with no packet to send or receive may go to the sleep mode. CS710 : Special issues in Computer Architecture

12. Three asynchronous power saving protocols for MANET • Access procedure • Back-off delay • [0 ~ 2*CWmin –1 slot] • Notation used in this paper • BI : length of a beacon interval • AW : length of an active window • BW : length of a beacon window • MW : length of an MTIM window CS710 : Special issues in Computer Architecture

13. Dominating-awake-interval • PS host stay awake sufficiently long so as to ensure that neighboring host can know each other. • Dominating awake property • AW >= BI/2 + BW • This guarantees any PS host’s beacon window to overlap with any neighboring PS host’s active window. • In every two beacon interval, PS host can receive all its neighbor’s beacon  short response timesuitable for highly mobile • The sequence of beacon intervals are alternatively labeled as odd and even interval CS710 : Special issues in Computer Architecture

14. Periodically–fully-awake-interval • Two types of beacon interval • Low power intervals • AW is reduced to the minimum • PS host send out its beacon to inform others its existence • AW = BW + MW, in the rest of the time , the host can go to the sleep mode. • Fully awake intervals • AW is extended to the maximum • Arrives periodically every T intervals • AW = BI, rest of the time must remain awake • PS hosts discover who are in its neighborhood. • By collecting other host’s beacons, hosts predict when its neighboring host will wake up. • a lot of power, so they only appear periodically and are interleaved by low power intervals. • Response time to get aware of a newly appearing host • T beacon interval • Suitable for slowly mobile environments CS710 : Special issues in Computer Architecture

15. BI • Quorum-based • PS host only needs to send beacon O(1/n) of the all beacon intervals. • Design PS host’s wakeup pattern so as to guarantee a PS host’s beacons can always be heard by other’s active windows. • Quorum interval • Beacon + MTIM, AW = BI • Non quorum intervals • Start with an MTIM window, after that, host may go to sleep mode, AW=MW • As long as n=>4, this amount of awaking time is less than 50% • Suitable for expensive transmission cost CS710 : Special issues in Computer Architecture

16. Communication protocols for power-saving hosts • Since the PS host is not always active, the sending host has to predict when the PS host will wake up. • Beacon packet has to carry the clock value of the sending host so as for other hosts to calculate their time differences. • S predict the receiving side’s MTIM window, S contends to MTIM packets to notify the receiver, after which the buffered data packet can be send. • Unicast • During the receiver’s MTIM window, sender contends to send its MTIM packet to the receiver. • Receivers will reply an ACK after SIFS, stay awake in the remaining of the beacon interval. • After the MTIM window, sender will contend to send the buffered packet to the receiver based on the DCF procedure. CS710 : Special issues in Computer Architecture

17. Broadcast • To reduce the number of transmissions, divide these asynchronous neighbors into group and notify them separately in multiple runs. • When S intends broadcast a packet, it first check the arrival time of the MTIM windows of all neighbors. • S picks the host, whose first MTIM window arrives earliest, S picks those MTIM window have overlapping with Y’s first MTIM. • After the these notification, S repeats the same process. • A neighbor, on receiving a MTIM carrying a broadcast indication, should remain awake until broadcast received or time value expires. • Broadcast packet should send based on DCF procedure. CS710 : Special issues in Computer Architecture

18. Simulation experiments • Environment • Implemented by C • Transmission radius : 250m • Rate : 2Mbps • Traffic load : Poisson distribution 5~30pkts/sec • “On-off model” to simulate mobility(in every 5 sec) • On probability : uniform distribution 50%~100% • Beacon interval : 100ms~500ms • Simulation time : 100sec • Assume all hosts are in the PS mode • Metric • Power consumption • The average power consumption per mobile host thru one simulation run • Power efficiency • average power consumption for each successful packet transmission • Neighbor discovery time • Average time to discover a newly approaching neighbor CS710 : Special issues in Computer Architecture

19. Impact of beacon interval length [Traffic load=10pkts/sec, “ON” probability=80%] [For unicast Traffic load=10pkts/sec, “ON” probability=80%] P(4) is good for both [For broadcast Traffic load=10pkts/sec, “ON” probability=80%] CS710 : Special issues in Computer Architecture

20. Impact of mobility For unicast(traffic load=10pkts/sec, Beacon interval=300ms) In case of broadcast Broadcast packet is counted as successful as long as some neighbors are there to receive the packet. CS710 : Special issues in Computer Architecture

21. Impact of traffic load For unicast(“on”=80%, Beacon interval=300ms) For unicast(“on”=80%, Beacon interval=300ms) Higher load makes transmitting a packet less costly because multiple packets may be transmitted in one beacon interval. Higher traffic load incurs higher power consumption since hosts have less chance to sleep. CS710 : Special issues in Computer Architecture

22. Conclusions • Three power saving protocol based on IEEE802.11, multi-hop, asynchronous MANETs. • Dominating awake interval • Most power consumption, the lowest neighbor discovery time. • Quorum based • The most power saving, the longest neighbor discovery time. • Periodically-fully-awake interval • Balance both power consumption and neighbor discovery time. CS710 : Special issues in Computer Architecture