Transcript Slide 1

Clock Synchronisation for
RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
1
Importance of RTS Clocks
• RealTime implies need for accurate
timekeeping
• Examples
– Hard RTS
• Distributed Control Systems
• Power System / Fly-by-wire
– Soft/Firm RTS
• TDM within GSM/POTS
– POTS : SONET/SDH
» Synchronous Opt. Network /Synch. Digital Hierarchy
• MM applications
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Power System Control
• AS station
– Token Bus Synchronisation via Master Clock
• Critical for chronological data logging / fault diagnosis
– Timeslicing for token management
– Synchronising 2v3 voter systems
• Need to deliver verdicts simultaneously
• Fault Diagnosis
– Impossible without Chronological Data
• Generator Earth Fault / Overcurrent ..
– Which came first .. msec level data required
• Power Line Fault Monitoring
– Noise burst travels in both directions .. usec level synch
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Token Bus : Master Clock
U/IA
U/IB
U/IB
U/IB
U/IA
125
N16
R30
U/IA
U/IB
103
N8
AS220E
102
N8
AS220E
101
N8
AS220E
U/IA
U/IA
U/IB
123
N-UHR
M-Clock
U/IA
141
NAT-24
Synogate
U/IA
Master
Clock
127
N-BK
Bus 1
126
N-BK
Bus 0
U/IB
U/IA
U/IB
U/IA
U/IB
104
N8
AS220E
105
N8
AS220E
U/IB
U/IA
121
N16
OS254
U/IA
U/IB
160
NS5NAT
PG750
U/IB
106
N8
AS220E
U/IB
112
N8
AS220E
U/IA
U/IA
U/IB
133
N8
AS EHF
Dr. Hugh Melvin, Dept. of IT, NUI,G
U/IA
U/IB
107
N8
AS220E
U/IB
111
N8
AS220E
U/IA
U/IA
U/IB
132
N8
AS EHF
U/IA
U/IB
108
N8
AS220E
U/IB
110
N8
AS220E
U/IA
U/IA
U/IB
131
N8
AS EHF
U/IA
U/IB
109
N8
AS220E
U/IA
U/IB
128
N8
AS231
4
Soft-Firm RTS
• POTS operation based on TDM
– PCME1E2..E4 SDH/SONET
– Precise synchronisation reqd throughout the network
for correct system operation
• GSM : FDM + TDM
– Each FDM channel divided out to 8 users via TDM
• Multimedia Applications
– Delay / Jitter Measurement increasingly imp in
packet (IP) networks
– More advanced QoS through synchronised time
• Recall G.1010
– Basis of SLA  measurement important
– Skew Issues between various system/media clocks
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Dr. Hugh Melvin, Dept. of IT, NUI,G
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Dr. Hugh Melvin, Dept. of IT, NUI,G
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Audio-System Clock Skew
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Computer Clocks
• Most commonly consist of quartz crystal and
a counter
• Crystal oscillates at defined rate (Hz) which
generates a consistent tick and increments a
software counter
• Counter value translated to time standard
– UTC (Univ. Coord. Time) .. Based on GMT
• Primary Source: Atomic Clocks TAI (International Atomic
Time)
– But requires leap seconds every few years!
– UTC = TAI + Leap_Seconds
• Crystal Quality described by Accuracy &
Stability
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Computer Clocks
• Accuracy relates to how close the
crystal freq is to rated value
– Determined by manufacturing process
• Get what you pay for!
• Stability relates to how frequency varies
– Influenced by parameters such as:
• Temperature .. Eg. 2ppm /C
• Ageing
– Eg. Cesium Beam: 3 x 10-12 / year
• Noise
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Computer Clocks
• Improved Quality Timekeeping ?
– Option A: Stick with crystals
• Precision manufacturing  costly
• Temperature Compensated Crystal Osc.(TCXO)
• Oven Controlled Crystal Osc.(OCXO)
– Option B
:
• Buy an Atomic Clock
– .. or GPS Receiver (based on atomic clock)
• Most popular approach to providing accurate/stable time
– Option C : Cheaper Approach
• Software based approach to discipline cheap crystal
clocks
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Terminology
• Confusion with terms in literature
– Paxson/Mills terminology used here
– Offset
• Difference between time reported by clock C, C(t) and
true clock (UTC) at true time t.
• Also relative offset between clocks C1and C2
– C1(t) - C2(t)
– Skew
• Difference in frequency between clock C and a true clock
(UTC) , C’(t)
• Defined in ppm (usec per sec)
• +/-12 ppm approx = +/- 1 sec/day
• Also relative skew between clocks C1and C2
– C1’(t) - C2’(t)
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Terminology
• Skew
– A large skew rate  rapidly increasing offset 
frequent resynchronisation
– If specify max abs skew rate for clock C of

(1 )(t2  t1)  C(t2 )  C(t1)  (1  )(t2  t1)
– Clock should operate within cone of acceptability
• Drift
– Rate of change of frequency C’’(t)
• Eg. Ageing influence or change in temperature
– Not usually that significant except over long
timescales
– Note linear relationship in previous slide
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Cone of Acceptability
Slope = 1 +

Slope = 1 = True Clock
Clock
Time
Slope = 1 -

Real Time
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Synchronisation
• Perfect clocks do not exist
• Eg. PC System Clock  NTP Server  GPS Receiver 
GPS Atomic Clock  GPS Master Atomic Clock ??
• Examine two separate scenarios
• Localised Cluster of Clocks
– Eg. Power System Control / Fly-by-wire Systems
– Also widely distributed clocks over deterministic network
» Propagation time known (can be compensated for)
» Eg. POTS
• Widely distributed clocks over non-deterministic network
– More difficult scenario
– Eg. Internet Synchronisation
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Synchronisation
• Some General Principles
– Fault Tolerance critical
• Identify and isolate faulty clocks
• Note: A faulty clock is one that does not operate within
cone of acceptability
– Cf Clock Quality: May be stable but inaccurate
– Avoid setting clocks backward
– Event processing nightmare
– OS problems eg. Timers / timeslicing
– Avoid large step changes
• Amortize the required change (+/-) over a series of short
intervals (eg. over multiple ticks)
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Localised Cluster of Clocks
• Hardware-based Phase Locked Loops (PLL)
– Oscillator output is aligned to the input signal.
– Input signal can come from a
• Master Clock
• Combination of outputs from all other clocks
– Input signal used to drive its PLL
– Can also compensate for Propagation Delay
variations
– Expensive but precise approach
• Similar approach used in widely distributed
scenario
– GPS / POTS / GSM all use variants of this
approach
Dr. Hugh Melvin, Dept. of IT, NUI,G
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PLL
Input
Signal
Comparator
VCO
VCO = Voltage Controlled Oscillator
Freq controlled by applied input voltage
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Widely Distributed Clocks
• More difficult environment if underlying
network non deterministic
• Expense of hardware based approach
cannot be justified for many Soft-Firm
RTS
• Cheap software based approach
– Network Time Protocol (NTP)
– RFC 1305 (www.ietf.org)
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Synchronisation : NTP
• Network Time Protocol (NTP) synchronises
clocks of hosts and routers in the Internet
• Increasingly deployed in the Internet
– Increased need for time synchronisation
– Facilitated via always-on Internet connection
• Provides nominal accuracies of low
milliseconds on WANs, submilliseconds on
LANs, and submicroseconds on workstations
using a precision time source such as a
cesium oscillator or GPS receiver
• Unix-based NTP daemon now ported to most
OS
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP
The NTP architecture, protocol and algorithms have
evolved over the last twenty years to the latest NTP
Version 4
• Internet standard protocol for time synchronisation
and coordinated time distribution using UTC
• Fault tolerant protocol – automatically selects the
best of several available time sources to synchronise
with
• Highly scalable – nodes form a hierarchical structure
with reference clock(s) at the top
– Stratum 0: Time Reference Source
• GPS / GOES (GeoSat) / LORC (LoranC) / ATOM / DTS
– Stratum 1: Primary Time Server
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP System
GPS/Radio Clock
Timin
g S
ignal
Timing Signal
GPS Satellite
Timing Signals
NTP
NT
P
P
NT NTP Primary Server
Stratum 1
NTP Secondary Server
Stratum 3
NTP Sec. Server
Strat. 2
P
NT
NTP Sec. NT
Server Strat. 2 P
NTP Secondary Server
(Stratum 3)
Dr. Hugh Melvin, Dept. of IT, NUI,G
NTP Secondary Server
Stratum 3
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NTP Operation
Peer 1
Filter 1
Peer 2
Filter 2
Peer 3
Filter 3
Intersection
and
Clustering
Algorithms
Combining
Algorithm
Loop Filter
P/F-Lock Loop
VFO
NTP Messages
• Complex Software comprising various algorithms
• Filtering Alg.
• Clustering and Intersection Alg.
• Combining Alg.
• Clock Discipline
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Client Server Mode
• UDP/IP packets for data transfer
– Several packet exchanges between client/server
– Client
• originate timestamp A within packet being sent.
– Server receives such a packet:
• receive timestamp B
• transmit timestamp C
– Client
• Processes A,B,C as well as final packet arrival D
• Determine offset and Round Trip Delay (RTD)
• Note: RTD != RTT
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Operation
B 3.59.020
C 3.59.022
15 ms
A 3.59.000
15 ms
D 3.59.032
Symmetric Network : 15 ms each way (actual delay)
RTD = (D - A) – (C – B) = 32 – 2 = 30 msec (RTT =?)
Offset = ½[(B-A) - (D-C)] = (20 – 10)/2 = 5 ms
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Discipline
• Recall
– No time reversal!
– Avoid step changes
• Hybrid phase/frequency-lock (PLL/FLL)
feedback loop
• PLL/FLL Mode: Depends on polling interval
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Clock Models
• Unix Clock Model
• settimeofday( ), adjtime( )
• Kernel variables tick , tickadj
• adjtime adjusts clock every tick
– Can amortise reqd change gradually by making
adjustment every tick eg. every 10 msec
– Note: Newer Unix/Linux kernels 1000Hz  1msec
• 3 clock rates
– Normal rate .. Add 10 msec every tick (100 Hz)
– Normal Rate +/- tickadj
– Eg. If tickadj = 5us  Normal Rate +/- 500 ppm
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Operation
• NTP adjusts every sec via adjtime
– Eg. If clock skew is +100 ppm & tickadj=5us
• NTP will operate to keep clock effectively running at
correct rate
– Normal Rate - 500 ppm over 0.2 sec
– Normal Rate for 0.8 sec
–  Effective skew = 0 ppm
– Results in sawtooth – pattern
• Newer Unix Kernels have advanced NTP
features
– ntp_adjtime( ), ntp_gettime()
– Eliminates the sawtooth pattern
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Implementation
• Install NTP
• Set up ntp.conf file
– List of servers that you wish to connect to
– Redundancy & Path Diversity & Low RTD
• Start up NTP daemon ntpd
• File ntp.drift records clock skew
• Other utilities
– ntpq, ntpdate
– See www.ntp.org
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Refid:
DCF: 77.5 KHz Radio Signal
PTB: German time signal
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Time difference
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Server Details
• when: no of sec since last response
• poll : interval between queries
• reach : Reachability in octal
– 11111111 = 3778 = max
– 11101110 = 3568  last + 5th probe lost
• Symbol to LHS of server
– * : Synch Source – survivor with smallest dispersion
– +
:other candidates included in final combination alg
– -
: Discarded by clustering alg
– x
: Falseticker acc to intersection alg
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Robustness Issues
•
•
•
•
Redundancy
Path Diversity
Symmetric Networks
Proximity to Primary Reference Sources
– See results
• OS & Network Load
– Platform Dependencies
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Operation : Asymmetry
B 3.59.015
C 3.59.017
10 ms
A 3.59.000
20 ms
D 3.59.032
Offset still 5 ms but Asymmetric Network
RTD = (D - A) – (C – B) = 32 – 2 = 30 msec
Offset = ½[(B-A) - (D-C)] = (15 – 15)/2 = 0 ms .. Error
Dr. Hugh Melvin, Dept. of IT, NUI,G
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NTP Operation : Asymmetry
B 3.59.015
C 3.59.017
15 ms
A 3.59.000
15 ms
D 3.59.032
NTP’s Symmetric view of Asymmetric Network
RTD = (D - A) – (C – B) = 32 – 2 = 30 msec
Offset = ½[(B-A) - (D-C)] = (15 – 15)/2 = 0 ms !
Exercise: What is the maximum error in this calculation?
Dr. Hugh Melvin, Dept. of IT, NUI,G
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Server Offsets: Problem?
Dr. Hugh Melvin, Dept. of IT, NUI,G
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