Self Organization and Energy Efficient TDMA MAC Protocol by

Wake Up For Wireless Sensor Networks

Zhihui Chen; Ashfaq Khokhar

ECE/CS Dept., University of Illinois at Chicago

IEEE SECON 2004

Presented by Yung-Lin Yu

Outline• Introduction

• Motivation

• Channel and Traffic Assumption

• TDMA-W: Details– Self-Organization– TDMA-W Channel Access Protocol

• Simulation Results

• Conclusion

Introduction (1/2)• Sensor networks are different from other

wireless communication networks– In WSN, Traffic rate is very low

• Typical communication frequency is at minutes or hours level

– Sensor networks are battery powered and recharging is usually unavailable

• Energy is an extremely expensive resource

Introduction (2/2)– Sensor nodes are generally stationary after their

deployment

– Sensor nodes coordinate with each other to implement a certain function

• Traffic is not randomly generated as those in mobile ad hoc networks

• Due to these differences, existing wireless MAC protocol are not suitable for WSN

Motivation (1/2)• Design a MAC protocol for WSN has two

aspects– Contention-based– Schedule-based

• This paper proposed a schedule-based MAC protocol as TDMA-W – Due to traffic rate is relatively low, we can have fix

timings for door knocking for each node– It is a suitable MAC protocol for WSN for its

collision-free and maintenance simplicity

Motivation (2/2)• TDMA-W

– Each node is assigned two slots– Transmission/Send slot (s-slot)– Wakeup slot (w-slot)

5 6 7 8 1 2 3 4 5 6 7 8 1 2

1 ZZZ ZZZ ListenWakeup Nd3

Send ZZZ ZZZ ZZZ ZZZ ZZZ Listen ZZZ ZZZ ZZZ

2 ZZZ ZZZ ZZZWakeup Nd3 ZZZ Send ZZZ ZZZ ZZZ ZZZ ZZZ Listen ZZZ ZZZ

3 ZZZ ZZZ

Wakeup

Nd4Listen Listen Listen Send ZZZ ZZZ ZZZ ZZZ Listen ZZZ ZZZ

4 ZZZ ZZZ ListenWakeup Nd6 ZZZ ZZZ Listen Send ZZZ ZZZ Listen ZZZ ZZZ ZZZ

5 ZZZ ZZZ ListenWakeup Nd6 ZZZ ZZZ ZZZ ZZZ Send ZZZ Listen ZZZ ZZZ ZZZ

6 ZZZ ZZZ ZZZ Listen ZZZ ZZZ ZZZ Listen Listen ZZZ ZZZ Listen ZZZ ZZZ

Channel and Traffic Assumption

• A TDMA-W frame lasts for Tframe seconds

• Tframe is known to all nodes and is preset before deployment

• A TDMA-W frame is divided into slots

• Each node is assigned one slot for transmission and one slot for wakeup

• Networks are synchronized

Self-Organization (1/9)• Assign time slots to the sensors within each

TDMA-W frame

• Assume sensor networks has data rate of 1 Mbps

• Transmission of a 512 byte packet occupies the channel for about 3.9 ms

• Assume a TDMA-W frame of 1 second divided into 256 slots– Each slot is of 3.9 ms– Capable of communicating 512 bytes

Self-Organization (2/9)1. Each node randomly selects a slot with

uniform probability among all slots to be its s-slot

2. During its selected s-slot, each node broadcasts

– It’s Node ID– It’s S-slot number– One-hop neighbors’ IDs– One-hop neighbors’ S-slot assignments– Slot number of any s-slot during which this node

has identified a collision

Self-Organization (3/9)3. When a node is not transmitting, it turns on

its receiver circuit and listens to the traffic from neighbors

• The node should record all the information being broadcast by all its neighbors

• Their s-slot assignments and their node IDs

• The slot number of any slot being broadcast as a collision-prone slot

Self-Organization (4/9)4. If a node determines that

– it is involved in a collision – or finds out that one of its two-hop neighbors has

the same s-slot– It then randomly selects an unused slot and go to s

tep 2

Self-Organization (5/9)5. If

– no new nodes are joining in– or s-slot assignments are not changing– or no collisions are detected for a certain period– It implies all neighbor nodes are found and all the

s-slots are final

Self-Organization (6/9)6. Each node identifies an unused slot or any

s-slot being used by the nodes beyond its two-hop neighbors and declares it as its w-slot

• Note that w-slots need not be unique

7. Each node broadcasts its w-slot and the self-organization is complete

Self-Organization (7/9)• Some problems

Slot=5 Slot=5

Collision Collision

Select another

Select another

Self-Organization (8/9)• Some problems

– To solve this problem• Let one node go to the listening mode in its assigned s-

slot with a probability

Slot=5 Slot=5

Self-Organization (9/9)

– To listen during s-slot with a probability– To set a collision counter

• Collision in the same slot repeats, nodes can realize a deadlock has occurred

slot=1

slot=1

slot=1

slot=1

slot=2

slot=2

slot=2

slot=2

Report collide

TDMA-W Channel Access Protocol (1/4)

1. Each node maintains a pair of counters for every neighbors

– Outgoing counters – Incoming counters– These counters are preset to an initial value

2. If no outgoing data is sent to a node in a TDMA-W frame

– The node decrements the corresponding outgoing counter by one

– Otherwise it resets the counter to the initial value

TDMA-W Channel Access Protocol (2/4)

3. If no incoming data is received from a neighboring node in a TDMA-W frame

– The node decrements the corresponding incoming counter by one

– If the counter is less than or equal to zero, the node stop listening to that slot starting from next TDMA-W frame

TDMA-W Channel Access Protocol (3/4)

4. If a outgoing data transmission request arrives– The node first checks the outgoing counter– If the counter is greater than zero, then the link is

considered active and the packet can be sent out during the s-slot

– If the counter is less than or equal to zero, a wakeup packet is sent out during the w-slot of the destination node prior to the data transmission

TDMA-W Channel Access Protocol (4/4)

5. If a node receives a wakeup packet in its w-slot

– It turns itself on during the s-slot corresponding to the source node ID contained in the wakeup packet

Simulation Results (1/6)• Nodes are deployed randomly in a 500x500 sq.

ft. area

• Communication range is 100 feet for all nodes

• Assume an IEEE 802.11 basic rate of 1 Mbps as the physical layer transmission rate

• Slot length is set to be 4 ms– Long enough for transmitting a 512-byte packet

• Tframe is set to 1 second

– A TDMA-W frame has 250 slots

Simulation Results (2/6)• Self-organization protocol

Simulation Results (3/6)• Power consumption

– Transmission : Receiving/Listening : Sleeping

= 1.83: 1 : 0.001– The network is synchronized

• All the nodes become active at the same time

– All data packets are fixed to be 256 bytes in length– Control packets (RTS, CTS, ACK in S-MAC and

Wakeup packet in TDMA-W) are about 20 bytes in length

– Initial value for counters is set to 3

Simulation Results (4/6)• One-Hop Random Traffic

0.7%0.16%

10.1%

4.7%

Simulation Results (5/6)• Delay of Random One-Hop Traffic

Simulation Results (6/6)• Delay of All to One Reduction Operation Traffic

Conclusion• The proposed protocol only consumes 1.5% to

15% power of 10% S-MAC– It is about 6~67 times longer than 10% S-MAC

• The proposed scheme also show the event with a delay comparable to S-MAC for one-hop traffic

• The proposed protocol is collision free for data traffic so reliable transmission is guaranteed for all types of traffic