stump: exploiting position diversity in the staggered tdma underwater mac protocol kurtis kredo ii,...
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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