1. CS710 : Special issues in Computer Architecture 1/22 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

2. CS710 : Special issues in Computer Architecture 2/22 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

3. CS710 : Special issues in Computer Architecture 3/22 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.

4. CS710 : Special issues in Computer Architecture 4/22 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

5. CS710 : Special issues in Computer Architecture 5/22 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

6. CS710 : Special issues in Computer Architecture 6/22 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.

7. CS710 : Special issues in Computer Architecture 7/22 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*CW min –1]

8. CS710 : Special issues in Computer Architecture 8/22 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.

9. CS710 : Special issues in Computer Architecture 9/22 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.

10. CS710 : Special issues in Computer Architecture 10/22 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.

11. CS710 : Special issues in Computer Architecture 11/22 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.

12. CS710 : Special issues in Computer Architecture 12/22 Three asynchronous power saving protocols for MANET  Access procedure  Back-off delay  [0 ~ 2*CW min –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

13. CS710 : Special issues in Computer Architecture 13/22  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

14. CS710 : Special issues in Computer Architecture 14/22  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

15. CS710 : Special issues in Computer Architecture 15/22 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

16. CS710 : Special issues in Computer Architecture 16/22 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.

17. CS710 : Special issues in Computer Architecture 17/22  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.

18. CS710 : Special issues in Computer Architecture 18/22 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

19. CS710 : Special issues in Computer Architecture 19/22  Impact of beacon interval length [Traffic load=10pkts/sec, “ON” probability=80%][For unicast Traffic load=10pkts/sec, “ON” probability=80%] [For broadcast Traffic load=10pkts/sec, “ON” probability=80%] P(4) is good for both

20. CS710 : Special issues in Computer Architecture 20/22  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.

21. CS710 : Special issues in Computer Architecture 21/22  Impact of traffic load For unicast(“on”=80%, Beacon interval=300ms) Higher traffic load incurs higher power consumption since hosts have less chance to sleep. Higher load makes transmitting a packet less costly because multiple packets may be transmitted in one beacon interval.

22. CS710 : Special issues in Computer Architecture 22/22 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.