An Energy Efficient MAC Protocol for Wireless LANs

An Energy Efficient MAC Protocol

for Wireless LANs

2/19

Contents

Introduction

Power Saving Mechanism (PSM) for DCF in IEEE

802.11

Related Work

Proposed DPSM (Dynamic PSM) Scheme

Key Features of DPSM

DPSM Operation

Rules for Dynamic ATIM window adjustment

Performance Evaluation

Conclusion

3/19

Introduction

Energy conserving mechanisms at various layers Routing layer MAC layer Transport layer

Energy efficient MAC protocol For wireless LAN By putting the wireless interface in a “doze” state

Measured power consumption awake : transmit (1.65 W), receive (1.4 W), idle (1.15

W) doze (0.045 W)

4/19

PSM for DCF in IEEE 802.11 Two components in IEEE 802.11

PCF (Point Coordination Function)

DCF (Distributed Coordination Function)

Power Saving Mechanism for DCF Time is divided into Beacon Interval

All nodes are in awake state during an ATIM window

All nodes use the same ATIM window size

5/19

Related Work Adjust Beacon Interval and ATIM window [Woesner,

1998] Simulation results for the PSM

Enforce nodes to enter doze state [Cano, 2001] Use RTS/CTS for traffic indication message (per packet

basis)

Costs of doze-to-active transition

SPAN : Elects a group of coordinators [Chen, 2001] Stay awake and forward traffic for active connections

Use advertised traffic window following an ATIM window

PAMAS : use two separate channels [Singh, 1998] Separated transmission of control packet / data packet

Nodes determine when to power off and the duration

6/19

PSM with fixed ATIM window size

Affects throughput & energy consumption

Small window size

Not enough time available to announce traffic

Degrading throughput (potentially)

Large window size

Less time for actual data transmission

Higher energy consumption

DPSM : dynamically adjust the size of ATIM window

Dynamic Power Saving Mechanism

7/19

Key Features of DPSM

Dynamic adjustment of ATIM window

Each node uses a different ATIM window size

Longer dozing time (more energy saving)

Enter the doze state after announced packet delivery

Remained duration in the beacon interval is longer

than 1600 μs

8/19

DPSM Operation Announcing one ATIM frame per destination

Sender Informs the number of packets pending for

Receiver

If the announced packets are not delivered in a beacon

interval Stay up in the next beacon interval

Sender delivers remained packets without ATIM frame

Enter the doze state after successful packet transmission

Increasing and decreasing ATIM window size Finite set of ATIM window sizes

The smallest ATIM window size : ATIMmin

Each allowed window : level

Different nodes using different ATIM window size

9/19

Backoff algorithm for ATIM frame ATIM frame transmitted using CSMA/CA mechanism

Initial cw value is picked in the range [0, cwmin]

If an ATIM-ACK is not received Doubles the value of cw and selects a new backoff interval

If the ATIM window ends Use doubled cw value in the next beacon interval

i.e., cw will not be reset to cwmin

To decrease the probability of collision

DPSM Operation (cont.)

10/19

Packet marking Set retry limit for ATIM frame in a beacon interval as

3

If ATIM-ACK has not been received after 3

transmission Transmitted packet is “marked” and re-buffered for

another try

The node is free to send ATIM frame to another node

Re-buffered packet can stay in buffer for at most 2

beacon interval

Marking => dynamic increase of ATIM window size

DPSM Operation (cont.)

11/19

DPSM Operation (cont.) Piggybacking of ATIM window size

Each node announces its own ATIM window size

Nodes may be aware of some or all of other ATIM

window sizes

Packets pending to be transmitted are sorted the size of ATIM window at their destination

Destination node with small size of ATIM window gets

preference

If unknown, it is assumed to be equal to ATIMmin

ATIM frames are transmitted in the sorted order

Queues for each level of ATIM window

Re-buffered packet has a higher transmission priority

12/19

Rules for Dynamic ATIM Window Adjustment

Increasing rules The number of pending packets that could not be

announced during the ATIM window If the number of pending packets is more than 10

Overheard information If neighbor’s window size is at least two levels larger

Receiving an ATIM frame after ATIM window

Receiving a marked packet

13/19

Decreasing rules When the current ATIM window is big enough

No window increasing rule is satisfied

If a node has successfully announced one ATIM frame to

all destinations that have pending packets

Rules for Dynamic ATIM Window Adjustment

14/19

Performance Evaluation Performance metrics

Aggregate throughput over all flows

Aggregate throughput per unit of energy consumption

Simulation model Simulator : ns-2 with the CMU wireless extensions

Number of nodes : 8, 16, 32, or 64

Simulated flows : half of nodes

Network environment : LAN (one-hop network)

Traffic : CBR, 512 bytes packet in 2Mbps channel

Beacon Interval : 100 ms

ATIM window size : 2 ms ~ 50 ms

15/19

Simulation Results Aggregate Throughput (Fixed network load)

16/19

Simulation Results Aggregate Throughput per joule (Fixed network load)

17/19

Simulation Results Network load vs. ATIM window size

The number of pending packets is the main factor for a node

to increase its ATIM window

18/19

Simulation Results Dynamic network load

19/19

Conclusion

The ATIM window size in PSM in IEEE 802.11

Affects the throughput and the amount of energy saving

The network load is directly related to ATIM window size

Fixed ATIM window size can not achieve optimal performance

Dynamic PSM can

Adapt its ATIM window size according to observed network

conditions

Improve energy consumption without degrading throughput