Rami Melhem Sameh Gobriel & Daniel Mosse Modeling an Energy-Efficient MAC Layer Protocol

Rami Melhem

Sameh Gobriel & Daniel Mosse

Modeling an Energy-Efficient MAC Modeling an Energy-Efficient MAC Layer ProtocolLayer Protocol

2Modeling An Energy-Efficient MAC Layer Protocol December, 2004

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Wireless Adhoc NetworksIntroduction

A collection of mobile nodes forming a temporary network.

Each host is an independent router.

Significant impact on military and civil applications:

Conferences & meetings. Combat field surveillance. Target tracking. Sensor networks. Search & rescue operations.


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Energy Consumption Challenge Nodes are low power, low cost devices.

Very limited supply energy.

It may be hard (or undesirable) to retrieve the nodes to change or recharge the batteries.

Considerable challenge on the “Energy Consumption”.

In our recent previous work [Infocom 04]: Wasted energy in collisions & collision resolution

is a significant portion of the energy consumption.

BLAM goal is to minimize the energy wasted in collisions.

Motivation


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802.11 MAC Protocol & Collisions

Collision can only occur during transmission of control frames.

When a collision occurs the station defer for a random time uniformly distributed [0..CW].

The Value of the CW depends on the number of failed transmissions.

CWmin

2nd R

CWmax

1st T 1st T

1st R 1st R

802.11 MAC Protocol

A B CRTS

CTS

Data

ACK

RTS

RTS

RTS / CTS

RTS


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Why Collision Energy is Significant ??

In adhoc network a message is forwarded in more than one hop collision is faced at each hop.

A Power-aware adhoc network decrease energy by increasing number of intermediate hops. (tendency for collision at each relay node)

rCPTx

Collision Energy

Reason 1: Number of Hops

P2 < P1


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C DA B

Data Ack

RTS CTS

CA B DCx We can’t use low power for

both data and control frames

Previous work, [e.g. Vaidya, 02] proposed sending RTS-CTS with max power and Data-ACK with lower power

However, control frames are the ones that face collision and retransmission

Collision Energy

Reason 2: Control Frames Tx Power


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Energy-poor nodes are the most critical ones because: They have a lot of data to send. They are located in the confluence of many

routes. Leaving these nodes to die may cause a

“network partition”.

In 802.11, all colliding nodes are equally treated all nodes try retransmission at subsequent times.

Energy-poor nodes can drain their energy colliding with high-energy nodes.

Collision Energy

1

2

4

3 6

75

Reason 3: Critical Nodes

XCollision


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BLAM: An Energy Efficient MACBLAM

BLAM conserves the channel bandwidth and energy consumption by decreasing the total number of collisions.

New partitioning philosophy nodes are split into virtual groups based on their residual energy (no contention between low-energy and high-energy nodes).

In IEEE 802.11: When fresh data packet arrives at a node, it senses the channel. If the channel is

idle the node sends with probability 1. When a collision is detected, the node defers transmission for a random period of

time chosen uniformly in the interval [0..CW].

In BLAM: when a fresh data packet arrives at a node or when a collision is detected the

node waits a random period of time before trying to transmit. The random period of time is normally distributed with mean and variance depend

on the residual energy.


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CW Size Distribution

0 100 2000

0.001

0.002

0.003

g w( )

w

0 100 2000.001

0.002g w( )

w

0 100 2000.001

0.002g w( )

w

0 100 2000.001

0.002g w( )

w

0 100 2000

0.001

0.002

0.003

g w( )

w

Capacity = 1 Capacity = 0.75 Capacity = 0.5

Capacity = 0.25 Capacity = 0

Full capacity Mean =0 most probable to pick smaller window. As capacity decreases Mean moves to left picking larger window (i.e.

waiting longer). Priority to high-energy nodes for channel access over low-energy nodes.

BLAM

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Modeling An Energy-Efficient MAC Layer Protocol December, 2004

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Transmission Priorities BLAM distributes the channel access based on the node energy.

The network nodes are divided among a continuous set of transmission priorities based on the remaining energies.

Probability of contention among members of the different priorities will be low energy-poor nodes will not waste their energy colliding with high-energy nodes.

The deferring time distribution is more selective (variance low) at the two extremes (C = 0 & C = 1) while less selective at C = 0.5 (close to uniform) separate as much as possible between high and low energy nodes while not be very selective when C=0.5 (node majority).

However: Probability of contention among members of the same priority will be high (pick comparable value for CW).

BLAM

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BLAM Effectiveness BLAM modifications are only based on the local host

information: No communication with central node is needed and no “Request-

Status” messages are transmitted to neighbors.

No additional fields in the frame and no changes in the frame handling technique.

BLAM can be implemented as an open-loop control circuit the node energy (input) and generates a random number (output) used to determine the transmission probability and random deferring time.

BLAM is transparent to the upper layers no specific support nor changes are required.

BLAM is backward compatible can be deployed in a network that uses IEEE 802.11.

BLAM

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Collision ModelCollision Model

The wireless channel around node X can be: Idle Transmit: when node X correctly transmits a data frame to R RTS-Col: when two or more nodes in the coverage area of X transmits

an RTS CTS-Col: When a hidden node from X transmits an RTS to collide with

the CTS sent by R.

a CTS

Hiddenareafrom

sender

a RTS

x R

a data

CoverageArea of x

IDLE RTS-col

Transmit

CTS-col

1

1

1

Pit

Pir

Pic

Pii

We compare analytically BLAM and IEEE 802.11 MAC protocol. We developed a collision model for the wireless network through which we

can derive the total network throughput and the steady state probability of collision.

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Probability of Transmission1. Assume that the probability of transmission for a node in a given time

slot is p.

2. Using p we compute the probabilities of transition from each state to the other assuming a uniform distributed network.

3. Using the equilibrium equations of the state transition diagram we get the steady state collision probability and also the percentage of total time the wireless channel around node x is in each state.

Collision Model

The difference between BLAM and IEEE 802.11 is in p the probability of transmission per time slot.

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Probability of Transmission1. As an approximation, if we assume that the CW value is held constant (at mid

range) then: In IEEE 802.11 the probability of transmission per time slot [As proved in Bianchi,00] :

In BLAM the probability of node X to transmit in slot “i” is a function of the node’s energy level Rx.

2. For any neighborhood with a given distribution of energies among M nodes the probability of transmission in a given time slot i is given by:

1

2)(11.802

CWip

Collision Model

i

i

xblamxblam dtRtpRip1

),(),(

M

jjblamblam Rip

Mip

1

),(1

)(

BLAM

802.11

i

Pdf

of T

x

0 CW

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Comparing BLAM & 802.11

Using the steady state equations of the collision model we compare BLAM and IEEE 802.11: Average collision probability Total network throughput

The comparison is done in two cases: BLAM Worst case nodes in the neighborhood are having equal residual energy. BLAM Best case nodes in the neighborhood are having a uniform distribution of

remaining energy.

Analytical Results

Parameter ValueRTS Duration 13 slot time

CTS Duration 12 slot time

Data Packet Duration 287 slot time

ACK Duration 12 slot time

CW size 256 slot time

Nodes per neighborhood 16 nodes

Probability of Tx P802.11(i) & Pblam(i)

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Comparing BLAM & 802.11 To verify the correctness of the collision model we simulated a single hop network using NS2.

Two sets of scenarios are simulated: All the nodes have full energy The nodes have uniform distribution of the remaining battery energy.

The energy distribution is forced to be fixed from the start to the end of the simulation.

Analytical Results

Worst case: BLAM only increased the probability of collision by 13% Best case: BLAM decreased the probability of collision by almost 4 folds.

Normalized number of Collision

0

0.2

0.4

0.6

0.8

1

1.2

Worst Case Best Case

Model Simulation

1.13

1.293

0.2910.405

Normalized Throughput

0

0.2

0.4

0.6

0.8

1

Worst Case Best Case

Model Simulation

0.971 0.958 1.012 1.02

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Simulation EnvironmentSimulation

Parameter Value

Number of simulation runs 10

Network size 375 X 375 m2

Node range 250 m

Number of nodes 32

Number of connections 60

Flow Type CBR

Packet Size 512 bytes

Transmission rate per source 6 pkts/sec

The previous simulation analysis is presented to verify the correctness of the model (single-hop network, fixed-energy, fully-saturated & uniform-distributed nodes).

Another simulation analysis is presented for a real network scenario.

In our simulations, we compared 3 MAC layer protocols: Basic 802.11 Modified 802.11 BLAM

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Total Number of Collisions

BLAM decreased Collisions by 40% over basic protocol.

Initially the collision should be higher.

Once a node starts transmission, it moves towards another priority (no contention).

Simulation Results

0

1000

2000

3000

4000

Basic 802.11 Modified 802.11 BLAM

Total Number of Collisions

3939

2760

2038

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Network LifetimeSimulation Results

BLAM saved the energy wasted in Collisions, resolution & retransmissions.

BLAM conserved the energy of the critical nodes by avoiding colliding with high-energy nodes.

BLAM increased the network lifetime by 15%.

100

150

200

250


Network Lifetime (FND)

225 230242

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Total Number of Rx PacketsSimulation Results

Energy savings can be reached trivially by making nodes send less frequently (decrease throughput).

BLAM was able to deliver more data packets to its final destination.

BLAM increased the number of data packets received by 39% over the basic protocol.

0

2500

5000

7500

10000


Total Number of Rx Pkts

8740

9952

11147

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ConclusionsConclusion

We showed that the IEEE 802.11 protocol is not optimal for adhoc networks.

We introduce BLAM, a new energy-efficient MAC layer enhancement.

We used a collision model to analytically compare the behavior of BLAM and the IEEE 802.11 DCF protocols.

We showed that the worst case probability of collision in BLAM is 13% higher than that of the IEEE 802.11, while in the best case a 4 folds improvement in the collision probability is achievable.

We validated the correctness of the proposed model through simulation analysis for a single hop adhoc network.

We also simulated a real network scenario and showed that BLAM decreased the number of collisions & increased the network lifetime This indicates that the worst case of BLAM is not frequent.

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Documents

Rami Melhem Sameh Gobriel & Daniel Mosse Modeling an Energy-Efficient MAC Layer Protocol