Cyclone Time Technology Deriving Consistent Time Base Using Local Clock Information

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Cyclone Time TechnologyDeriving Consistent Time
Base Using Local Clock Information

• Ashok Agrawala
• Moustafa Youssef
• Bao Trinh
• University of Maryland
• College Park, MD 20742

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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

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Two Nodes
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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, ?AB, 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

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Outline

• Introduction
• Drift correction scheme (Cyclone)
• Results
• Virtual Clock Scheme
• Conclusions

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Scheme 1Drift 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

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Two Nodes
pA(2)

pA(0)
pA(1)
Node A
Node B
pB(2)

pB(0)
pB(1)
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Algorithm

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

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Algorithm

• 3. For subsequent beats Node B calculates
• 4. It sends the (n1)st beat at

Kb
px2B
pxkbB
px1B
B
pBB
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Analysis
Similarly
Therefore
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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.
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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.

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Outline

• Introduction
• Drift correction scheme (Cyclone)
• Results
• Virtual Clock Scheme
• Conclusions

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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

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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.

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Convergence Time for Different Network Topologies
Nodes Convergence time in Seconds Convergence time in Seconds Convergence time in Seconds Convergence time in Seconds
Nodes Star Chain Bidirectional Random
20 2 5 5 4
50 15 62.5 62.5 31.125
100 15 62.5 62.5 31.125
200 N/A N/A N/A 31.125
500 N/A N/A N/A 31.125
1000 N/A N/A N/A 31.125
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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.

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Transit Delay and Convergence
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Latency Perturbations
Node ID 0.01 0.10 1
Node ID CTJ CTJ CTJ
1 0.000 0.000 0.000
2 0.001 0.008 0.055
3 0.001 0.007 0.064
4 0.001 0.008 0.074
5 0.001 0.008 0.072
6 0.001 0.007 0.083
7 0.001 0.008 0.073
8 0.001 0.007 0.072
9 0.001 0.008 0.083
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Drift Rate Perturbation
Node ID 100 PPM 10 PPM 1 PPM
Node ID CTJ CTJ CTJ
1 0.000 0.000 0.000
2 0.014 0.001 0.000
3 0.004 0.001 0.000
4 0.008 0.001 0.000
5 0.007 0.002 0.000
6 0.011 0.001 0.000
7 0.012 0.002 0.000
8 0.015 0.001 0.000
9 0.010 0.001 0.000
10 0.009 0.001 0.000
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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 ??

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Outline

• Introduction
• Drift correction scheme (Cyclone)
• Results
• Virtual Clock Scheme
• Conclusions

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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

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Assumptions

• For the discussion right now we add two
  additional assumptions
• All connections are bidirectional
• Transit time in two directions are the same

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Approach

• Carry out the first scheme and reach convergence
• At convergence we note that

• The time when node A sends its nath beat

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Two Node Case

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

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

• When this phase starts
• For each of its incoming links node A calculates

It initializes
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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.

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On Convergence

• Node A has values

It can convert its local clock values to Virtual
global clock as
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Current Status

• Simulation Results confirm the claims
• Working on prototype implementations using
  standard NICs

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