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

An Energy Efficient MAC Protocol for Wireless LANs, E.-S. Jung and N.H. Vaidya,

INFOCOM 2002, June 2002

An Energy Efficient MAC Protocol for Wireless LANs, E.-S. Jung and N.H. Vaidya,

INFOCOM 2002, June 2002 吳豐州吳豐州

AgendaAgenda

IntroductionPower Saving Mechanism in DCFDynamic Power Saving MechanismSimulationConclusion


AgendaAgenda



IntroductionIntroduction

Battery power is one of the critical resources in WLAN Power Limited!!

Battery managementPower controlEnergy-efficiency protocol

Battery power is one of the critical resources in WLAN Power Limited!!

Battery managementPower controlEnergy-efficiency protocol

IntroductionIntroduction

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

AgendaAgenda

IntroductionPower Saving Mechanism in DCFNew Power Saving MechanismSimulationConclusion

IntroductionPower Saving Mechanism in DCFNew Power Saving MechanismSimulationConclusion

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 )



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



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



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



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.



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

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

AgendaAgenda



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 ProtocolWhen 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 802.11 MAC protocol, we first describe how IEEE 802.11 works

IEEE 802.11 MAC ProtocolWhen 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 802.11 MAC Protocolcw = CWmin, at the beginning and also

after each successful transmissionS 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 802.11 MAC Protocolcw = CWmin, at the beginning and also

after each successful transmissionS 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 802.11 MAC ProtocolAbsence 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 802.11 MAC ProtocolAbsence 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



Key Features of DSPMDynamic adjustment of ATIM windowLonger dozing time (more energy

saving)

Key Features of DSPMDynamic adjustment of ATIM windowLonger dozing time (more energy

saving)



Dynamic adjustment of ATIM windowIn the proposed DPSM scheme, each

node independently chooses an ATIM window size based on observed network conditions

Dynamic adjustment of ATIM windowIn the proposed DPSM scheme, each

node independently chooses an ATIM window size based on observed network conditions



Longer dozing timeIn 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

Longer dozing timeIn 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



Longer dozing timethere 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 timethere 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”



In DPSM Operation, following modifications are madeAnnounce one ATIM frame per

destinationIncreasing and decreasing ATIM

window sizeBackoff algorithm for ATIM framePacket markingPiggybacking of ATIM window size

In DPSM Operation, following modifications are madeAnnounce one ATIM frame per

destinationIncreasing and decreasing ATIM

window sizeBackoff algorithm for ATIM framePacket markingPiggybacking of ATIM window size



Announce one ATIM frame per destinationWhen 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 destinationWhen 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 destinationIf 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 destinationIf 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



Increasing and decreasing ATIM window sizeWe 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 sizeWe 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



Backoff algorithm for ATIM framewhile 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 framewhile 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



Packet markingIf 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 markingIf 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





Piggybacking of ATIM window sizeEach 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 sizeEach 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 sizefor 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 sizefor 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



Rules for Dynamic ATIM Window AdjustmentInitially, each node begins with ATIM

window size equal to ATIMmin

Rules for Dynamic ATIM Window AdjustmentInitially, each node begins with ATIM

window size equal to ATIMmin



Rules for increasing the ATIM window sizeBased on the number of pending

packets that could not be announced during the ATIM window

Based on overheard informationReceiving a marked packetReceiving an ATIM frame after ATIM

window

Rules for increasing the ATIM window sizeBased on the number of pending

packets that could not be announced during the ATIM window

Based on overheard informationReceiving a marked packetReceiving an ATIM frame after ATIM

window





Rules for decreasing the ATIM window sizeDuring 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 sizeDuring 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

AgendaAgenda



SimulationSimulation

Two metrics are used for evaluationAggregate throughput over all flows

in the networkAggregate throughput per unit of

energy consumption

Two metrics are used for evaluationAggregate throughput over all flows

in the networkAggregate throughput per unit of

energy consumption


Simulation modelDuration 25 secSource 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 modelDuration 25 secSource 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 modelWireless 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 modelWireless 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)


Wireless LAN scenarioNetwork 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 scenarioNetwork 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 (fixed network load)



Simulation (dynamic network load)Simulation (dynamic network load)



AgendaAgenda



ConclusionConclusion

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

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

Documents

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