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吳豐州

An Energy Efficient MAC Protocol for Wireless LANs, E.-S. Jung and N.H. Vaidya, INFOCOM 2002, June 2002. 吳豐州. Agenda. Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion. Agenda. Introduction Power Saving Mechanism in DCF

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吳豐州

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  1. An Energy Efficient MAC Protocol for Wireless LANs, E.-S. Jung and N.H. Vaidya, INFOCOM 2002, June 2002 吳豐州

  2. Agenda • Introduction • Power Saving Mechanism in DCF • Dynamic Power Saving Mechanism • Simulation • Conclusion

  3. Agenda • Introduction • Power Saving Mechanism in DCF • Dynamic Power Saving Mechanism • Simulation • Conclusion

  4. Introduction • Battery power is one of the critical resources in WLAN Power Limited!! • Battery management • Power control • Energy-efficiency protocol

  5. Introduction • Wireless interface consumes significant power, and can be in either the awake or doze state • In awake state, there are three different modes, transmit, receive, idle, and each consumes 1.65W, 1.4W, 1.15W respectively. As a contrast, in doze state consumes 0.045W • thus power saving mechanism (PSM) is often putting wireless interface into a doze state

  6. Agenda • Introduction • Power Saving Mechanism in DCF • New Power Saving Mechanism • Simulation • Conclusion

  7. Power Saving Mechanism in DCF

  8. Power Saving Mechanism in DCF • Time is divided into beacon intervals • At the beginning of beacon interval, there exists a specific time interval, called ATIM window (Ad-hoc Traffic Indication Message Window )

  9. Power Saving Mechanism in DCF • ATIM window is utilized to announce any packets pending transmission to nodes in doze state and every node is awake during ATIM window • When a node wants to transmit, it sends ATIM frame in ATIM window first, and then a destination node ready to receive, it replies an ATIM-ACK

  10. Power Saving Mechanism in DCF • After the ATIM handshake, both source and destination node will be stay awake for the remaining beacon interval to perform the data transmission • A node that ha not transmitted or received an ATIM frame may enter the doze state for saving energy after finishing its ATIM window

  11. Power Saving Mechanism in DCF • During ATIM window, only ATIM and ATIM-ACK can be transmitted, real data transmission can only occur after the ATIM window • Overhead in energy consumption is incurred for transmitting or receiving ATIM and ATIM-ACK, and there is overhead in time due to the ATIM window

  12. Power Saving Mechanism in DCF • All nodes use the same (fixed) ATIM window size critically affects throughput and energy consumption, and a fixed ATIM window does not perform well in all situations • If the ATIM window is chosen to be too small, there may not be enough time available to announce buffered packets, potentially degrading throughput.

  13. Power Saving Mechanism in DCF • If the ATIM window is too large, there would be less time for the actual data transmission, since data is transmitted after the end of the ATIM window, again degrading throughput at high loads

  14. Agenda • Introduction • Power Saving Mechanism in DCF • Dynamic Power Saving Mechanism • Simulation • Conclusion

  15. Dynamic Power Saving Mechanism • Dynamic power saving mechanism (DPSM) is similar to the IEEE 802.11 MAC protocol, we first describe how IEEE 802.11 works • IEEE 802.11 MAC Protocol • When a node S wants to transmit a packet to a node D it choose a “backoff” counter uniformly distributed in the interval [0,cw]

  16. Dynamic Power Saving Mechanism • IEEE 802.11 MAC Protocol • cw = CWmin, at the beginning and also after each successful transmission • S waits until medium is idle, and then the backoff counter is decremented by 1 after each “slot time” • When counter reaches 0, S transmit an RTS. After D receiving RTS, D replies a CTS to S if D can communicate with S at the present time

  17. Dynamic Power Saving Mechanism • IEEE 802.11 MAC Protocol • Absence of the CTS is taken as an indication of congestion, and S doubles its cw, picks a new backoff counter uniformly distributed over [0,cw], and repeats the above procedure • After RTS-CTS, S sends DATA to D and after D receiving DATA successfully, D sends an ACK to S

  18. Dynamic Power Saving Mechanism • Key Features of DSPM • Dynamic adjustment of ATIM window • Longer dozing time (more energy saving)

  19. Dynamic Power Saving Mechanism • Dynamic adjustment of ATIM window • In the proposed DPSM scheme, each node independently chooses an ATIM window size based on observed network conditions

  20. Dynamic Power Saving Mechanism • Longer dozing time • In PSM specified in IEEE 802.11, when a node transmits or receives an ATIM frame during an ATIM window, it must stay awake during the entire beacon interval • we allow a node to enter the doze state after completing any transmissions that are explicitly announced in the ATIM window

  21. Dynamic Power Saving Mechanism • Longer dozing time • there is a finite delay associated with the doze-to-awake transition, in addition to a higher energy consumption. Therefore, in our scheme, a node will not enter the doze state after completing packet transmissions if the remaining duration in the current beacon interval is “too small”

  22. Dynamic Power Saving Mechanism • In DPSM Operation, following modifications are made • Announce one ATIM frame per destination • Increasing and decreasing ATIM window size • Backoff algorithm for ATIM frame • Packet marking • Piggybacking of ATIM window size

  23. Dynamic Power Saving Mechanism • Announce one ATIM frame per destination • When a node, say node A, successfully transmits an ATIM frame to another node, say node B, node A will not transmit another ATIM frame to the same destination in the same beacon interval

  24. Dynamic Power Saving Mechanism • Announce one ATIM frame per destination • If node A could not deliver all pending packets that were previously announced to node B, and the current beacon interval expires, nodes A and B both stay up in the next beacon interval, with B anticipating the remaining packets from node A, without node A having to send an ATIM frame to node B

  25. Dynamic Power Saving Mechanism • Increasing and decreasing ATIM window size • We specify a finite set of ATIM window sizes that may be used by each node, with the smallest ATIM window size being denoted as ATIMmin. Each allowed window is called a level

  26. Dynamic Power Saving Mechanism • Backoff algorithm for ATIM frame • while the backoff interval is being decremented, say, at node A, the ATIM window of node A might end. In this event, the node will attempt to send an ATIM frame for the corresponding destination again in the next beacon interval

  27. Dynamic Power Saving Mechanism • Packet marking • If ATIM-ACK has not been received after three transmissions, the transmitted packet is “marked” and re-buffered for another try (also up to 3 times) in the next beacon interval • after three attempts in a beacon interval, the ATIM frame for a given destination is only transmitted again in the next beacon interval

  28. Dynamic Power Saving Mechanism

  29. Dynamic Power Saving Mechanism • Piggybacking of ATIM window size • Each node piggybacks its own ATIM window size on all transmitted packets • The packets pending to be transmitted are sorted by the size of the ATIM window at their destinations

  30. Dynamic Power Saving Mechanism • Piggybacking of ATIM window size • for implementing the above scheme consists of several queues, one queue corresponding to each allowed level of the ATIM window, the smallest value of the ATIM window being ATIMmin • the packet is re-buffered in the queue corresponding to ATIM window size ATIMmin, to give a higher transmission priority to such packets

  31. Dynamic Power Saving Mechanism • Rules for Dynamic ATIM Window Adjustment • Initially, each node begins with ATIM window size equal to ATIMmin

  32. Dynamic Power Saving Mechanism • Rules for increasing the ATIM window size • Based on the number of pending packets that could not be announced during the ATIM window • Based on overheard information • Receiving a marked packet • Receiving an ATIM frame after ATIM window

  33. Dynamic Power Saving Mechanism

  34. Dynamic Power Saving Mechanism • Rules for decreasing the ATIM window size • During an ATIM window, if a node has successfully announced one ATIM frame to all destinations that have pending packets and no window increasing rule defined above is satisfied, it means that the current ATIM window size was big enough

  35. Agenda • Introduction • Power Saving Mechanism in DCF • Dynamic Power Saving Mechanism • Simulation • Conclusion

  36. Simulation • Two metrics are used for evaluation • Aggregate throughput over all flows in the network • Aggregate throughput per unit of energy consumption

  37. Simulation • Simulation model • Duration 25 sec • Source node generates CBR traffic, Packet size of each flow is 512 bytes • The initial energy for each nodes is 1000 joules so nodes do not run out of energy during the simulations • The beacon interval 100 ms both PSM and NPSM

  38. Simulation • Simulation model • Wireless interface consumes 1.65W, 1.4W, 1.15W, and 0.045W in the transmit, receive, and idle modes and the doze state, respectively • 800 μs as the doze-to-awake transition time and a node will consume twice as much power as the idle mode (i.e., 2.3W)

  39. Simulation • Wireless LAN scenario • Network sizes are 8, 16, 32, 64 and half the nodes are source and the other half are destination • Simulated network loads are 5%, 10%, 20%, 30%, 40%, and 50%, measured as a fraction of the channel bit rate of 2 Mbps • With a total load of 10%, and 4 traffic sources, each traffic has a rate of 0.05 Mbps

  40. Simulation (fixed network load)

  41. Simulation (fixed network load)

  42. Simulation (fixed network load)

  43. Simulation (dynamic network load)

  44. Simulation (dynamic network load)

  45. Simulation (dynamic network load)

  46. Agenda • Introduction • Power Saving Mechanism in DCF • Dynamic Power Saving Mechanism • Simulation • Conclusion

  47. Conclusion • The ATIM window size in PSM in IEEE 802.11 significantly affects the throughput and the amount of energy saving • In PSM, if the ATIM window is too small, the throughput degrades as the network load becomes heavier. If the ATIM is too large, the energy gain from power saving mode become small, since each node must stay awake during the ATIM window • In DPSM, a node also can power off its wireless network interface whenever it finishes packet transmission for the announced packets. Simulation results show that the proposed scheme can improve energy consumption without degrading throughput

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