1. DESYNC: Self-Organizing Desynchronization and TDMA on Wireless Sensor Networks (IPSN’07)
Julius Degesys, Ian Rose, Ankit Patel, Radhika Nagpal
Division of Engineering And Applied Sciences, Harvard University

2. Introduction Synchronization is biologically–inspired:
cardiac cells beating together, fireflies
It is an important primitive in WSN:
decentralized algorithms for node synchr
so that the network remains stable despite
individual faults / topology changes

3. Desynchronization Is the logical opposite of synchronization
Nodes performs their tasks as far away as possible from all other nodes
The goal is to interleave periodic events to occur in a round-robin schedule:
imagine not fireflies flashing in unison
but in a uniformly distributed fashion

4. Desynchronization Setting: Motes performing periodic tasks
Synchronization: Sometimes desirable to perform tasks at the same time

5. Desynchronization Setting: Motes performing periodic tasks
Synchronization: Sometimes desirable to perform tasks at the same time
Perfect Desynchronization: Other times, want even distribution
No two are happening at the same time
Perfect round-robin schedule
Wireless spectrum not “2.4GHz”

6. DESYNCH
A biologically inspired, self-mantaining algorithm to achieve desynchronization in a single-hop network.
Possible applications:
equally distribute monitoring tasks
organize sleep cycles
improve MAC protocols such as TDMA

7. Background - Synch modelling
Many synchronized systems are modeled as networks of oscillators :
each node pulses at a fixed frequency
existence of an algorithm converging to synchrony
(Mirollo-Strogatz, Synchronization of pulse-coupled biological oscillators, SIAM 1990)
Desynch can be considered as a patterned synch where all oscillators pulse at evenly spaced intervals:
algorithm converging to desynchrony (DESYNC) in O( n^2), for n < 500

8. Background 2 – Channel sharing in WSN
In WSN nodes needs to share the channel
MAC protocols:
Contention-based (CSMA/CA):
nodes checks the channel before transmitting
if busy they randomly back off and try later
Schedule-based (TDMA):
time is divided into slots, each node picks one of them to transmit collision-free

9. CSMA/CA Pro: simple and adaptive Draws:
used when the expected contention is low /
in case of bursty traffic
Draws:
unable to detect collision while transmitting
hidden terminal problem
large backoffs and message loss under high load low bandwith utilization

10. TDMA Pro: Two main drawbacks:
low latency & good bandwith (fixed frequency)
good for predictable traffic (fixed bit rate)
Two main drawbacks:
overhead: tyme synch among nodes & slot schedule negotiation
wasted slots: slot goes unused when node does not need to transmit

11. Key Observation
TDMA only requires nodes to desynchronize their transmissions
There is no need to agree on global time
If nodes could self mantain desynchronization, we can solve the above drawbacks simultaneously
DESYNC applied to TDMA

12. The Framework Single-hop network
Each node performs a task with period T
Фi(t)= phase of node i at time t, 0 ≤ i ≤ n-1
Nodes moving clockwise on a ring
Upon reaching Фi(t)= 1 node i “fires”, indicating the end of its cycle to others

13. All nodes observe the firing, and:
use this information to jump forward or backward in phase
Being Δi(t)= Фi(t) – Фi-1(t):
GOAL: Δi(t)= 1/n , 0 ≤ i ≤ n-1 Desync
Nodes oblivious of the current state,
aware only of the firing events

14. Framework ? How do we get from a random start to desynchronization?
Single node periodically “fires” by broadcasting a message
A typical starting configuration (random)
A system in desynchronization
How do we get from a random start to desynchronization? ?
Don’t use the word “Sample,” broadcast a message to let other nodes know that it is performing its task
This represents the progression of the node’s internal timer.

15. DESYNC Algorithm Local view of three nodes, let’s focus on B
FIRE A A B B C
Local view of three nodes, let’s focus on B
Node C has already fired, as well as B.
B has tracked C’s firing time.
Node A is about to fire.
When back neighbor fires, B jumps towards the
midpoint of the nodes preceiding and following it
A node pay attention to the timing of the firings
before and after its own (phase neighbors)

16. DESYNC Algorithm(2)
Δi+1, Δi = firing time of previous and next nodes relative of node i’s firing
Node i can compute the phase of its previous neighbor:
Фi+1(t)= Фi(t) + Δi+1 (mod 1)
and the one of its next neighbor
Фi-1(t)= Фi(t) - Δi (mod 1)
note Δi+1 it’s in node’s memory (stale)!

17. Using that info, node i computes the desired midpoint as:
Фmid(t)= 1/2 [Фi+1(t) + Фi-1(t)]
Now he can jump towards it:
Ф’(t)= (1-α) Фi(t) + α Фmid(t)
where α Є (0,1).
It turned out that α = 1 does not always allow convergence ( oscillation due to memory time delay)

18. Simple Implementation: constant memory regardless of the network size
DESYNC Algorithm(3)
Convergence to Desynchrony: regardless the usage of stale informations.
Simple Implementation: constant memory regardless of the network size
Self-Adapting: if number of nodes changes, those closest to the disturbance adjust their phases, leading the system back
to desynchronization.

19. DESYNC-TDMA Algorithm
Nodes use earlier firings to compute the TDMA slots near the time of their next firing
Node i’s TDMA slot begins at previously computed midpoint between i and its previous phase neighbor
It ends at the midpoint between i and its next phase neighbor

21. A node will never fire outside its slot: If node B doesn’t jump:
ФA < ФB < ФC ФA+ФB < 2ФB < ФC+ФB
mid(AB) < ФB < mid(BC)
Being B’ = mid(AC) the target jump point of B, this point is always within B’s time slot:
ФB < ФC ФA < ФB
ФA+ФB < ФA+ФC ФA+ФC < ФB+ФC
ФA+ФB < ФA+ФC < ФB+ФC
mid(AB) < mid(AC) < mid(BC)

22. DESYNC-TDMA Key Features
1)The algorithm fully utilizes the channel
defining a set of non-overlapping slots covering T
Collision free transmission, fully utilized bandwidth

23. DESYNC-TDMA Key Features(2)
2) TDMA schedule adapts to nodes entering or leaving.
When a node leaves, its neighbors readjust their slot boundaries
if node does not need to transmit:
leave the protocol, sleep & re-enter
When a node enters he must interrupt:
cost of one time period latency /
one data slot interrupted

24. DESYNC-TDMA Key Features(3)
3) The algorithm is self-contained:
No need to know the network size
No need to discover neightbors IDs
No need to agree on a global time
No need for a time synchronization protocol
The round-robin schedule is a result of the order in which nodes enter the process

25. DESYNC-TDMA – Implementation
Implemented on Telos WSN with TinyOS
Motes use a 250Kbps wireless transceiver.
Default CSMA radio interface to transmit, with initial backoff reduced to 1.2 ms
Motes use their local clock to track firings
MAC-level TS used to insert a delay into msg

26. Evaluation – Experimental Setup
Single-hop network, 20 Telos motes around a Tmote Sky acting as a base station
The BS logs all messages

27. Two experiments:
Fixed-Size: a fraction of total motes (n = 4,10, 20) to transmit data in the entire slot, to test TDMA-like performance under peak load.
Node Removal-Addiction: in order to evaluate the effect of motes entering leaving.
Initially n = 8, then a mote removed
at t = 135. At t = 180 three motes enter the system.

28. Evaluation Metrics Average Desync error: is the average deviation
from desired slot-size of T/n for a given round
Normalized throughput: best possible data throughput is 62.8 Kbps (single mote to BS).
Ratio between measured data msg throughput and the above value, meaured at each round
Fairness: average, min & max throughput per node
Message Loss: BS detect missed msg by SN
Ratio between missed msg and expected msg.

29. Experimental Results
Motes arrival and departure experiment: 8 motes started
At t= 135 one mote left and three motes woke up at t=180
Time of each mote’s firing relative to those of a single mote
( fixed-size experiment, n= 10)
Desynchronization error over time for different n
Desync error and throughput changes over time
( fixed-size experiment, n= 10)

30. Comparison to other MAC Protocols
Ideal TDMA: upper bound, collision-free slots of T/n size, fully utilized bandwidth.
Fixed TDMA: lower bound, slot size of T/N
Hybrid TDMA: (Z-MAC). TDMA is modified so that unowned slots can be used by other motes.
CSMA: simple and adaptive, works well for small networks and variable traffic.
Large backoffsand message loss under high load.

31. Comparison to other MAC protocols
Throughput and message loss across different protocols
Normalized throughput over n, for different protocols where
each node tries to send as much as possible (high data rate)
Total throughput on a 10 nodes network varying data rates

32. Summary and Conclusion
Desync-TDMA is a new way of thinking about TDMA scheduling.
Draws:
- 1 round latency before transmitting
- Smaller slot for several round until re achieving desynchronization
- Fairness could lead to wasted bandwidth
- Effect of lost firing messages on desynchronization accuracy?
Pro:
- No explicit scheduling / time synchronization
- Excellent total throughput and collision- free transmission under high loads
- Fairness and predictable message latencies
- Self-adaptive to topology changes

33. Future Work and Open Issues
Applying DESYNC to coordinate sleep schedules
Extend DESYNC-TDMA to multi-hop networks.
This is much more complex due to:
Intersecting neighborhoods
Overlapping broadcast regions:
hidden terminal

34. Any question?

35. Thanks for your attention