IEEE 1588 Implementation Precision Time Protocol

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IEEE 1588 ImplementationPrecision Time Protocol
Integrated System Health Management (ISHM)
David Rauth
Special Topics in ECE ISHM 0909-504-04 Dr. John
Schmalzel Rowan University
Thursday April 21, 2005
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ISHM Overview

• ISHM adds an intelligent component to all levels
  of data acquisition and control systems giving
  them the ability to self-assess their condition
  and make real-time decisions based on that
  knowledge.
• Increased Safety
• Increased Performance
• Increased Reliability
• Decreased Cost

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The Time Synchronization Problem

• Current Solution
• DAQ server clock synchronized to UTC via IRIG
• Samples time-stamped at DAQ server
• Centralized DAQ
• All your eggs are in one basket
• High computation load on server
• One time source
• Time-stamp accuracy

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The Time Synchronization Problem

• Major Network-based Solutions
• Network Time Protocol (NTP)
• Most widely used protocol
• Targets large distributed computing systems
• Physical layer Ethernet
• Millisecond accuracy typical
• Time Triggered Protocol (TTP)
• Targets safety-critical systems (aircraft, etc.)
• Protocol addresses many other issues beyond time
  sync.
• Physical layer Primarily CAN RS-485
• Microsecond accuracy typical (depending on no. of
  nodes)

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The Time Synchronization Problem

• Major Network-based Solutions (continued)
• Serial Real-Time Communications System (SERCOS)
• Targets motion-control systems (robots, etc.)
• Protocol addresses many other issues beyond time
  sync.
• Physical layer SERCOS (deterministic bus)
• Sub-microsecond accuracy typical
• Precision Time Protocol (PTP)
• Targets measurement and control systems
• Sole-purpose is low network load time
  synchronization
• Physical layer Any Multicast Network
• Sub-microsecond accuracy typical

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The Time Synchronization Problem

• Our PTP Solution
• Time synchronization via Ethernet infrastructure
• Increased accuracy
• Samples time-stamped at sensor level
• Distributed DAQ time-base
• High reliability
• Less infrastructure
• Easy maintenance
• Low cost

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PTP Clock Synchronization Model

• Overview (ordinary clocks)
• Each node contains a PTP clock (henceforth clock)
• Each clock has two ports to the communication
  path
• Event port
• General port
• One clock on the comm. path will be selected
  master
• Selection made by examining Sync messages
• Sync messages are sent by any clock claiming to
  be master
• Master clock status determined using best master
  clock alg.
• One clock in the sub-domain will be grandmaster
• Best accuracy of all the masters (ultimate time
  source)
• Each node synchronizes its clock to the master
  via Sync messages

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PTP Clock Synchronization Model

• Assumptions
• Network must support multicast communication
• It must be possible to prevent multicast messages
  from propagating beyond a subnet
• Each clock shall meet performance requirements
• A clocks stated properties must accurately
  describe the clock

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PTP Clock Synchronization Model

• Clock types
• Ordinary clocks
• Communication over a single comm. path
• Contained within subnet
• Boundary clocks
• Communication via multiple comm. paths
• Connect multiple subnets

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PTP Clock Synchronization Model

• PTP Communication Protocol
• All non-management messages are sent as multicast
  communications
• All non-management PTP messages are subnet
  specific
• Increased efficiency
• Scales easily as number of clocks increase

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PTP Clock Synchronization Model

• Major Clock Properties
• UUID for each port
• Clock type (ordinary or boundary)
• Clock stratum identifier
• ATOM - 1 (UTC calibrated clock accuracy lt 25ns)
• GPS - 1 (time derived from GPS receiver accuracy
  lt 100ns)
• ATOM - 2 (accuracy at last sync lt 100ns)
• GPS - 2 (accuracy at last sync lt 100ns)
• NTP - 2 (accuracy at last sync lt 50ms)
• HAND - 2 (set by operator accuracy lt 10s)
• INIT - 2 (arbitrary time set unspecified
  accuracy)
• DFLT - 3 (none of the above apply)

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PTP Clock Synchronization Model

• Clock Capabilities
• Follow-up (optional)
• Clock provides a Follow_Up message for each Sync
  message issued
• Variance
• Clock maintains an estimate of its inherent
  stability
• Latency
• Each port time-stamps inbound and outbound
  messages for use in delay correction

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PTP Clock Synchronization Model

• Clock Capabilities (continued)
• Latency (continued)

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PTP Clock Synchronization Model

• Port Categories
• Event Port
• Sync Info. for time synchronization
• Delay_Req Management message for requesting TX
  delay
• General Port
• Follow_Up More accurate time-stamp value
• Delay_Resp Management message containing TX
  delay

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PTP Clock Synchronization Model

• Synchronization Interval
• Seconds between Sync messages
• Shall be 1, 2, 8, 16, or 64 seconds
• All clocks must perform using any of the
  intervals

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PTP Specification

• Time structure
• PTP Epoch started at 0hrs 1 Jan. 1970
• Epochs occur ever 8,925,512 years

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PTP Specification

• Protocol engine state machine
• PTP_INITIALIZING
• Initialize data sets, hardware, and comm.
  properties
• PTP_FAULTY
• Clock shall not participate in synchronization
• Implementation-specific measures taken to clear
  fault
• PTP_DISABLED
• Disabled and not responsive to messages
• PTP_LISTENING
• Waiting for Sync message or Sync message timeout
• PTP_MASTER
• Clock is behaving as a master

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PTP Specification

• Protocol engine state machine (continued)
• PTP_PRE_MASTER
• Clock is behaving as a master except it does not
  send any non-management messages
• PTP_PASSIVE
• Not transmitting periodic Sync messages unless
  requested
• PTP_UNCALIBRATED
• More than one master detected in subnet
• Transient state to allow a new master clock to
  come online
• PTP_SLAVE
• Clock is synchronizing to selected master

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PTP Specification

• Protocol engine state machine (continued)

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PTP Specification

• Best master clock (BMC) algorithm
• Determines which of all the clocks it can see
  (including itself) is best
• Algorithm runs independently on each clock
• Algorithm avoids a configuration with two
  masters, no masters, or oscillation

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PTP Specification

• BMC algorithm definitions
• C0 Typical clock
• D0 Default data set
• r Port of C0 (one r for ordinary clock)
• Erbest Best message of those considered
• Ebest Best of all messages

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PTP Specification

• BMC state decision algorithm

Data set comparison algorithm
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PTP Specification

• BMC data set comparison algorithm goals
• Find which clock derives its time from better
  grandmaster
• If grandmasters are equivalent, choose clock
  whose path length is lesser
• Choose clock via tie-breaking techniques

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PTP Specification

• BMC data set comparison algorithm
• Part 1 of 4

Variance Comparison
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PTP Specification

• PTP Variance
• Represents performance of clock
• Based on Allan deviation
• Gives statistic on variation of local clock
  frequency
• xk, xk1, xk2 are residual time
  measurements ? is the sample period between
  measurements
• N is the number of measurements

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PTP Specification

• PTP Variance (continued)
• PTP variance based on statistics of time
  differences measured against a reference clock
• xk, xk1, xk2 are time difference
  measurements N is the number of measurements

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PTP Specification

• Local clock synchronization via PTP

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PTP Implementation

• What other network protocols can support PTP?
• (Remember multicast requirement)
• Besides ISHM where else can PTP be applied?

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PTP Implementation

• Ethernet implementation
• PTP messages sent via UDP
• PTP interface at PHY/MAC boundary
• Xilinx FPGA-based solution
• Implementation testing with OnTime T200 Switch

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Questions?

• Key References
• IEEE 1588-2002 PTP Standard
• IEEE 1588 NIST Website (http//ieee1588.nist.gov)
• Links to NTP, TTP, SERCOS informational sites
• Implementation information
• OnTime Networks (http//www.ontimenet.com)