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 Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion

Agenda Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion

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

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 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

Agenda Introduction Power Saving Mechanism in DCF New Power Saving Mechanism Simulation Conclusion Introduction Power Saving Mechanism in DCF New Power Saving Mechanism Simulation Conclusion

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 ) 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 )

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 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

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 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

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 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

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. 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.

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

Agenda Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion

Dynamic Power Saving Mechanism Dynamic power saving mechanism (DPSM) is similar to the IEEE MAC protocol, we first describe how IEEE works IEEE 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] Dynamic power saving mechanism (DPSM) is similar to the IEEE MAC protocol, we first describe how IEEE works IEEE 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]

IEEE MAC Protocol cw = CW min, 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 IEEE MAC Protocol cw = CW min, 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 Dynamic Power Saving Mechanism

IEEE 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 IEEE 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

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

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 Dynamic adjustment of ATIM window In the proposed DPSM scheme, each node independently chooses an ATIM window size based on observed network conditions

Dynamic Power Saving Mechanism Longer dozing time In PSM specified in IEEE , 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 Longer dozing time In PSM specified in IEEE , 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

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” 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”

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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

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 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

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 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

Agenda Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion

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

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 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

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) 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)

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 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

Simulation (fixed network load)

Simulation (dynamic network load)

Agenda Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion Introduction Power Saving Mechanism in DCF Dynamic Power Saving Mechanism Simulation Conclusion

The ATIM window size in PSM in IEEE 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 The ATIM window size in PSM in IEEE 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