STUMP: Exploiting Position Diversity in the Staggered TDMA Underwater MAC Protocol

Kurtis Kredo II, Petar Djukic, Prasant Mohapatra

IEEE INFOCOM 2009

Outline Introduction Staggered TDMA Underwater MAC Protocol Network Model Conflict-Free Scheduling Scheduling Algorithms Numerical Results Conclusions

Introduction Acoustic communication forces protocol

designers for underwater networks Avoiding or reducing collisions becomes

vitally important Previous work has focused on overcoming the

challenges of the acoustic channel STUMP schedule overlapping transmissions

without collisions

Staggered TDMA Protocol To communications without collisions and

increase channel utilization STUMP nodes develop schedule constraints share propagation delay estimates share time slot requirements among two-hop

neighbors

Network Model Scheduled protocols require nodes maintain

time synchronization define σ as the maximum synchronization error at

any node from a global time any two nodes differs by at most 2σ

Define π as the maximum error experienced in estimating the one way propagation delay

Four Possible Conflicts

Conflict-Free Scheduling A set of time slots assigned to each node for transmi

ssion, S = {si}, prevents all conflicts Define constraints, C, ensure node transmission time

s are sufficiently separated Binary ordering variables, O = {oij}

oij = 1, node i transmits before receiving node j’s packet

Finding sets S and O that satisfy the schedule constraints C and node demands Δ

STUMP Schedule Constraints Node i transmitting to neighbor j transmits for

Δij time slots starting in slot sij

Define pij as the propagation delay from node i to node j

pij may not equal pji due to different propagation paths between the two nodes

RX-RX Conflicts (1) Node k finishes receiving node i’s packet

This yields the inequality

RX-RX Conflicts (2) Node j’s transmission does not cause a collisi

on with node i in the next frame

Node i : 2 slot packet

Node j : 3 slot packet

K

m

Frame size

RX-RX Conflicts (3)

TX-RX-TX Conflicts Ensure an interference packet does not arrive

at a node while it is receiving a valid packet nearly identical to the RX-RX conflict

TX-TX Conflicts Neighbor 1 is farther than neighbor 0,

neighbor 2 is farther than neighbor 1

Node k is the next farther neighbor than node j from node i

TX-RX Conflicts TX-RX conflicts ensure nodes do not transmit

while receiving a packet

TDMA Schedule Constraints Define Gi as the guard slots required after the

transmission of node i using TDMA

Ensure nodes cause interference to each other are assigned non-overlapping time slots

Centralized Scheduling Algorithms An appropriate objective function, minimum

frame size or minimum uplink delay yields an integer linear programming problem enough computational resources require significant overhead to collect the

network information

Distributed Scheduling Algorithms Determine the ordering variables, oij, by priori

tizing nodes node i has a higher priority than node j, then oij =

1, otherwise oij = 0

With fixed ordering, scheduling constraints become a set of difference equations Bellman-Ford algorithm

Determine Ordering Variables A simple way to find node priorities is to

select them at random does not guarantee any level of performance

Leaf nodes have the highest priority (an RX-RX conflict) may get the same priority may resulting in non-optimal uplink delay.

Numerical Results Compare STUMP with the TDMA and Aloha

protocols evaluating average throughput and delay

performance over 100 random topologies Nodes small movements caused by ocean

currents the synchronization and propagation delay

estimate error parameters σ and π

Normalized Throughput as σ Varies

Average Maximum Uplink Delay

Average Maximum Delay as σ Varies

Traffic Load Varies

Conclusion The Staggered TDMA Underwater MAC

Protocol increases the performance by using propagation delay estimates to schedule

overlapping transmissions Provide users to develop scheduling

algorithms suited to their application