Enhanced Slotted Aloha Protocols for Underwater Sensor Networks with Large Propagation Delay

Kai Chen† Jianhua He§

Yi Zhou‡ Haibing Guan‡

†School of Information Security and Engineering,

Shanghai Jiaotong University, China.

‡School of Electronic, Information and Electrical Engineering,

Shanghai Jiaotong University, China.

§School of Engineering, Swansea University, UK.

IEEE VTC 2011

Outline

• Introduction• System model• SA-ALOHA• ISA-ALOHA• Simulation• Conclusion

Introduction

• Underwater sensor networks(UWSNs) have attracted large research interests from both academy and industry

• Due to the importance of MAC protocols for UWSNs– Design and analysis of MAC protocols

Introduction

• Among the existing MAC protocols in shared wireless medium– ALOHA protocols

Pure-ALOHA

Time

A

B

C

DATA

DATA

DATA

DATA

DATA

Slotted-ALOHA

Time

A

B

C

DATA

DATA

DATA

DATA

DATA

Introduction

• This frame can be received successfully– There are no other frames arrival at node in

conflict free period (Pcf)

],[ ,,,, fAikifAikicf TTtTTtP

ti,k: node i send the kth frame to node ATi,A: actual propagation delayTf: frame transmission time

Introduction

• Conflict free for two different frame transmission

fijij TTt

∆tij: the difference of start for two frame transmission∆Tij: the difference between propagation delay Ni to NA and that Nj to NA

Tf: frame transmission time

Introduction

• RF network scenarios

fij Tt

Pure-ALOHA Slotted-ALOHA

protocol

∆tij Any value Multiple of Tf

throughput 0.18 0.36

Time Time

A

B

C

A

B

C

DATA

DATA

DATA

DATA

DATA

DATA

DATA

DATA

DATA

DATA

TIME

Sender

Receiver

Radio wavePropagation speed : 3*108 m/s

Pure-ALOHA Slotted-ALOHA

protocol

∆tij random Multiple of Tf

∆Tij random random

throughput 0.18 0.18

Introduction

• UWSN scenarios

fijij TTt Pure-ALOHA Slotted-ALOHA

protocol

∆tij random Multiple of Tf

∆Tij random random

Time Time

A

B

C

A

B

C

DATA DATA

DATADATA

DATA

collisionDATA

DATA

DATA

collision

TIMEPropagation delay

UnderwaterSender

Receiver

Acoustic wavePropagation speed : 1500 m/s

Introduction-motivation

• The performance of Aloha in UWSNs can drop quickly with increasing propagation delay

Goal

• Reuse the idea of synchronous transmission in S-Aloha protocol– SA-ALOHA– ISA-ALOHA• Improvement for the SA-ALOHA

System model & Assumption

• Single hop star topology with a central control node A

• Fixed N ordinary nodes• Position information• Time synchronized• Each ordinary nodes may occasionally send

data frames to the central control node

SA-ALOHA

• Example of frame transmission and reception

SA-protocol

• Calculation of Tpre

slotAiipre TTT mod,,

Ti,A: propagation delat of node i to node ATslot: slot size

SA-protocol

• Problem– Propagation delay is not accurate

AiAierrAi TTT ,,,

~

Terr: Estimation error of Ti,j

T~i,A: estimated value of Ti,j

Tslot,isa: Tslot for ISA protocol

ISA-protocol

• Solution

]),1[(),max( , NiT errAi

max* ef TkT

maxeT

isaslotT , incslot TT

Tinc: the increment of time from Tslot for Tslot,isa

ISA-protocol

• Solution

Sender , N1

Sender , N2

Receiver , NA

Tslot,isa

=Tf+k*Temax

Frame1 Tx

Frame2 Tx

Frame1 Rec

Frame2 Rec

Tolerant of the error of estimation

Simulation

parameter value

Ordinary node 50

Transmission range 1000

RF UWSN

Propagation speed 3*108 m/s 1500 m/s

Frame length 512 bytes 50 bytes

Data rate 2 Mbps 2 kbps

Tf 2.048 ms 200 ms

Tslot = 1.01*Tf 2.06848 ms 202 ms

1000 mCenter node

Simulation result

Simulation result

Simulation result

Simulation results

Simulation result

Temax = 33 ms

Conclusion

• In this paper, the authors investigated the performances of Aloha protocols in (UWSN)– Long propagation delay• SA-protocol• ISA-protocol

• Simulation results demonstrate the effectiveness of our proposed protocols

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