1

Understanding Spatio-TemporalUncertainty in Medium Access with ALOHA Protocols

Affan Syed, Wei Ye, Bhaskar Krishnamachari, and John Heidemann

ACM Proceedings of the second workshop on Underwater networks (WuWNet 2007)

2

Outline

Introduction Spatio-Temporal Uncertainty Understanding With ALOHA Handling Space Uncertainty Future Work Conclusion

3

Introduction

Underwater sensor network become an important area of research

Space-time Uncertainty, defined as the two dimensional uncertainty packet transmit time propagation latency

4

Space-Time Uncertainty

Same Transmission Time Different Transmission Time

no collision

5

Evaluation Methodology

ALOHA protocols are the simplest and earliest wireless MAC protocols

It consists of a single receiver that does not transmit nodes randomly deployed in a circular

region

6

ALOHA with Time Uncertainty

DataT

2T

vulnerability interval

DataT

T

vulnerability interval

Pure ALOHA Slotted ALOHA

7

Classical Throughput Analysis

8

ALOHA with Space Uncertainty

Simple ALOHA-constant delay

max propagation delay / transmit time

9

ALOHA with Space Uncertainty

Slotted ALOHA – any propagation latency

10

ALOHA with Space Uncertainty

Slotted ALOHA – any propagation latency

11

Slotted ALOHA with Guard Bands

Data

A’s Data

A

R

βtmax

β= 1

farthest

12

Choosing β

β= 1, no overlap at the receiver unless packets send at same time increase packet transmission latency

and bandwidth overhead β< 1, some node pair remains

overlap lowers band width overhead, increases

collision probability tradeoff

13

Parameters β (1)

14

Parameters β (2)

15

Analysis with Guard Bands

Throughput

Assume a > 0 and βa ≦1 expected number of collision-free packets

per slot is independent of a

each transmits a packet in a given slot

16

Varying Guard Bands Length

17

Varying Maximum Delay

18

Simple ALOHA with Guard Bands

19

Future Work

Plan to rigorously analyze the throughput capacity removing the assumption regarding

single and equidistant receiver Provide a framework to analyze

other wireless protocols CSMA increase understanding of the

characteristics of MAC in UWSN.

20

Conclusion

While designing MAC protocols under high latency environment it is necessary to consider both space

and time uncertainty Smaller ratio of propagation delay

to packet length result get better short-range communication