Clock Synchronisation for RTS
description
Transcript of Clock Synchronisation for RTS
![Page 1: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/1.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 1
Clock Synchronisation for RTS
![Page 2: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/2.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 2
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
![Page 3: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/3.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 3
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
![Page 4: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/4.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 4
Token Bus : Master Clock
U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB U/IA U/IBU/IA U/IB
101N8
AS220E
102N8
AS220E
103N8
AS220E
104N8
AS220E
105N8
AS220E
106N8
AS220E
107N8
AS220E
108N8
AS220E
U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB
126N-BKBus 0
123N-UHRM-Clock
121N16
OS254
112N8
AS220E
111N8
AS220E
110N8
AS220E
109N8
AS220E
U/IA U/IB U/IA U/IB U/IA U/IB U/IA U/IBU/IA U/IB U/IA U/IB
160NS5NAT
PG750
133N8
AS EHF
132N8
AS EHF
131N8
AS EHF
128N8
AS231
125N16R30
141NAT-24Synogate
U/IA U/IB
127 N-BK Bus 1
MasterClock
![Page 5: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/5.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 5
Soft-Firm RTS• POTS operation based on TDM
– PCME1E2..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
![Page 6: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/6.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 6
![Page 7: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/7.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 7
![Page 8: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/8.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 8
Audio-System Clock Skew
![Page 9: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/9.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 9
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
![Page 10: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/10.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 10
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
![Page 11: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/11.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 11
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
![Page 12: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/12.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 12
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)
![Page 13: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/13.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 13
![Page 14: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/14.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 14
Clock Terminology• Skew
– A large skew rate rapidly increasing offset frequent resynchronisation
– If specify max abs skew rate for clock C of
– 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
))(1()()())(1(
121212tttCtCtt
![Page 15: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/15.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 15
Cone of Acceptability
Real Time
Clock Time
Slope = 1 = True Clock
Slope = 1 -
Slope = 1 +
![Page 16: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/16.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 16
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
![Page 17: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/17.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 17
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)
![Page 18: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/18.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 18
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
![Page 19: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/19.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 19
PLL
VCOComparatorInput Signal
VCO = Voltage Controlled Oscillator
Freq controlled by applied input voltage
![Page 20: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/20.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 20
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)
![Page 21: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/21.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 21
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
![Page 22: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/22.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 22
NTPThe 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
![Page 23: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/23.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 23
Timing Signal
Tim in g S ig n als
G PS Satell ite
G PS/Rad io C lock
N T P S e c o nd ary S e rve rS tratum 3
N T P S e c o nd ary S e rve r(S tra tum 3 )
N T P S e c o nd ary S e rve rS tratum 3
N T P S e c . S e rve rS trat. 2
N T P S e c .S e rve r S tra t. 2
N T P P rim ary S e rve rS tratum 1
NTP System
N T P
![Page 24: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/24.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 24
NTP Operation
• Complex Software comprising various algorithms• Filtering Alg.• Clustering and Intersection Alg.• Combining Alg.• Clock Discipline
NTP Messages
Peer 1
Peer 2
Filter 1
Peer 3
Filter 2
Filter 3
Intersectionand
ClusteringAlgorithms
CombiningAlgorithm
Loop Filter
VFO
P/F-Lock Loop
![Page 25: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/25.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 26
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
![Page 26: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/26.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 27
NTP Operation
C 3.59.022
D 3.59.032
B 3.59.020
A 3.59.000
15 ms 15 ms
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
![Page 27: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/27.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 35
Clock Discipline
• Recall– No time reversal!– Avoid step changes
• Hybrid phase/frequency-lock (PLL/FLL) feedback loop
• PLL/FLL Mode: Depends on polling interval
![Page 28: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/28.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 37
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
![Page 29: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/29.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 38
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
![Page 30: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/30.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 39
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
![Page 31: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/31.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 40
Refid:
DCF: 77.5 KHz Radio Signal
PTB: German time signal
![Page 32: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/32.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 41
![Page 33: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/33.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 42
Time difference
![Page 34: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/34.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 43
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
![Page 35: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/35.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 44
![Page 36: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/36.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 45
NTP Robustness Issues
• Redundancy
• Path Diversity
• Symmetric Networks
• Proximity to Primary Reference Sources– See results
• OS & Network Load– Platform Dependencies
![Page 37: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/37.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 46
NTP Operation : Asymmetry
C 3.59.017
D 3.59.032
B 3.59.015
A 3.59.000
10 ms 20 ms
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
![Page 38: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/38.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 47
NTP Operation : Asymmetry
C 3.59.017
D 3.59.032
B 3.59.015
A 3.59.000
15 ms 15 ms
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?
![Page 39: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/39.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 48
![Page 40: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/40.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 49
![Page 41: Clock Synchronisation for RTS](https://reader036.fdocuments.in/reader036/viewer/2022062314/56814595550346895db28451/html5/thumbnails/41.jpg)
Dr. Hugh Melvin, Dept. of IT, NUI,G 50
Server Offsets: Problem?