1. University of Maryland
Cyclone Time Technology Deriving Consistent Time Base Using Local Clock Information
Ashok Agrawala
Moustafa Youssef
Bao Trinh
University of Maryland
College Park, MD 20742

2. Some Common Characteristics
Peer-to-Peer Architecture
The scheme relies only on local information or what they can obtain from their immediate neighbors
No central/master clock
Fast convergence

3. Clock Model Each node has a local clock which ticks at a constant rate
The clock is stable in that its drift rate does not change rapidly
Local time can be recorded at any instant by reading the clock which is an integer register incremented every tick time
Local time at any node A is represented as
Where
is the current local time at node A at time instant
is the drift rate of the clock at node A, and
is the offset

4. Two Nodes

5. Assumptions
All nodes are connected in that there is a path from any node to every other node.
The transit time for a message from Node A to Node B, ΔAB, is fixed ( relaxed later).
Each node is capable of timestamping an incoming message with its local clock time to within a clock tick.
Each node is capable of sending a message at a defined local time to within a clock tick
If there is a variability in timestamping operation, this gets lumped into the variability in the transit time

6. Outline Introduction Drift correction scheme (Cyclone) Results
Virtual Clock Scheme
Conclusions

7. Scheme 1 Drift Correction (Cyclone)
Goal : Correct drift at all nodes
Each node sends a beat message at times it determines from its local information
This message is only a time marker with no other information bits
Each node uses a common constant number whose value is fixed at design time

8. Two Nodes
pA(0)
pA(1)
pA(2) …
Node A
Node B
pB(0)
pB(1)
pB(2) …

9. Algorithm Initially each node sends the 0th beat at
2. Each node sends the 1st beat at

10. Algorithm 3. For subsequent beats Node B calculates
4. It sends the (n+1)st beat at Kb
πx2B
πx1B
πxkbB B
πBB

11. Analysis
Similarly
Therefore

12. Analysis Matrix Notation
Thus at each step we carry out a distributed calculation of
As A is a stochastic matrix, this iteration converges with all items in the
vector taking the same value.
Convergence rate is determined by the second dominant eigen value.

13. Practical Considerations
Transit delay
We assume it to be a constant. If it has some variability, it will have to be treated as a random variable. In that case the degree of clock synchronization depends on the jitter in the transit delay.
When transit time is much larger than the cycle time, special steps are required in the beginning of the operations
Finite precision
The development above assumed an infinite precision arithmetic and infinite resolution clocks.
These are small perturbations to the calculations but have to make sure that their affects do not accumulate.
Require phase locking.

14. Outline Introduction Drift correction scheme (Cyclone) Results
Virtual Clock Scheme
Conclusions

15. Simulation Parameters
Clock Tick Time – 100 ps (10 GHz)
Cycle Time – 125 ms (8000/sec)
Latencies – Random 0 and 80 cycles
Topologies
Chain
Star
Random
Drift rate - ±100 ppm

16. Simulation Results Convergence achieved every time
On convergence, jitter 1-2 clock ticks
Long Term Stability
Similar jitter and no drift after 12 hours of simulation time.

17. Convergence Time for Different Network Topologies
# Nodes
Convergence time in Seconds
Star
Chain
Bidirectional
Random 20 2 5 4 50 15
62.5
31.125
100
200
N/A
500
1000
Convergence numbers are in term of cycles. Assuming 8000 cycles/sec (for 10 GHz clock), worse case is 100-chain at 500K cycles, or 62 secs random converges in ½ the time, probably due to a smaller diameter.

18. Effects of Perturbations
Transit time
Varied by 10%
No more than the transit time change
Stabilizes very fast after that
Clock Drift
Varied again by 10%
Again, a step change results in immediate jitter determined by the change in clock drift
Stabilizes very fast.

19. Transit Delay and Convergence

20. Latency Perturbations
Node ID
0.01%
0.10% 1%
CTJ 1
0.000% 2
0.001%
0.008%
0.055% 3
0.007%
0.064% 4
0.074% 5
0.072% 6
0.083% 7
0.073% 8 9

21. Drift Rate Perturbation
Node ID
100 PPM
10 PPM
1 PPM
CTJ 1
0.000% 2
0.014%
0.001% 3
0.004% 4
0.008% 5
0.007%
0.002% 6
0.011% 7
0.012% 8
0.015% 9
0.010% 10
0.009%

22. Implications
This scheme achieves clock synchrony in that all nodes achieve a common cycle value of p*
The local value pA is different at each node
The beat instants are not synchronized
They do not drift
How to achieve a common clock reading ??

23. Outline Introduction Drift correction scheme (Cyclone) Results
Virtual Clock Scheme
Conclusions

24. Virtual Global Clock A clock with parameters b* and a*
We define a scheme such that based only on local information any node can convert its local time to the time at this Virtual Global Clock.
Key idea is to use a distributed consensus technique

25. Assumptions
For the discussion right now we add two additional assumptions:
All connections are bidirectional
Transit time in two directions are the same

26. Approach Carry out the first scheme and reach convergence
At convergence we note that
The time when node A sends its nath beat

27. Two Node Case As a part of the beat message node A also sends
Its converged cycle length
Current cycle number
Time
A value
Node B sends similar values

28. Calculations Similar calculations are done by node B
Node A can convert its local time to the time at Node B as

29. Multinode Operations When this phase starts It initializes
For each of its incoming links node A calculates
It initializes

30. Multinode Operations For each subsequent cycle
It calculates the new values of A and B as averages of incoming values of A and B adjusted to the local scale.

31. On Convergence Node A has values
It can convert its local clock values to Virtual global clock as

32. Current Status Simulation Results confirm the claims
Working on prototype implementations using standard NICs

33. Protocol specifies security
Comparisons
CTT
IEEE-1588
NTP
GPS
TTP
SERCOS
Spatial extent
General
A few subnets
Wide area
Local bus
Communications
Network
Internet
Satellite
Bus or star
Bus
Target accuracy
Sub-microsecond
Few milliseconds
Style
Distributed
Master/slave
Peer ensemble
Client/server
Master/Slave
Resources
Small network message and computation footprint
Moderate network and computation footprint
Moderate computation footprint
Moderate
Latency correction
Yes
Configured No
Drift Correction
Protocol specifies security
No (V2 may include security)
Administration
Self organizing
N/A
Hardware?
For highest accuracy
RF receiver and processor
Update interval
~2 seconds
Varies, nominally seconds
~1 second
Every TDMA cycle, ~ms

34. Concluding Remarks
Use of consensus approach simplifies the clock synchronization
As the scheme only depends on local information it is highly scalable
Primary results to date
Analytic
Simulation
Implementations ?
Appropriate estimators/filters
Practical considerations
Node Failure
Node Joining …