Clock Synchronisation for RTS Dr Hugh Melvin Dept

Clock Synchronisation for RTS Dr. Hugh Melvin, Dept. of IT, NUI, G 1

Importance of RTS Clocks • Real. Time 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 2

Power System Control • AS station – Token Bus Synchronisation via Master Clock • Critical for chronological data logging / fault diagnosis – Timeslicing for token management – Synchronising 2 v 3 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 3

Token Bus : Master Clock U/IA U/IB U/IA 125 N 16 R 30 U/IA U/IB 103 N 8 AS 220 E 102 N 8 AS 220 E 101 N 8 AS 220 E 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 104 N 8 AS 220 E 105 N 8 AS 220 E U/IB U/IA 121 N 16 OS 254 U/IA U/IB 160 NS 5 NAT PG 750 U/IB 106 N 8 AS 220 E U/IB 112 N 8 AS 220 E U/IA U/IB 133 N 8 AS EHF Dr. Hugh Melvin, Dept. of IT, NUI, G U/IA U/IB 107 N 8 AS 220 E U/IB 111 N 8 AS 220 E U/IA U/IB 132 N 8 AS EHF U/IA U/IB 108 N 8 AS 220 E U/IB 110 N 8 AS 220 E U/IA U/IB 131 N 8 AS EHF U/IA U/IB 109 N 8 AS 220 E U/IA U/IB 128 N 8 AS 231 4

Soft-Firm RTS • POTS operation based on TDM – PCM E 1 E 2. . E 4 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 Qo. S 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 5

Dr. Hugh Melvin, Dept. of IT, NUI, G 6

Dr. Hugh Melvin, Dept. of IT, NUI, G 7

Audio-System Clock Skew Dr. Hugh Melvin, Dept. of IT, NUI, G 8

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 9

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. 2 ppm /C • Ageing – Eg. Cesium Beam: 3 x 10 -12 / year • Noise Dr. Hugh Melvin, Dept. of IT, NUI, G 10

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 11

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 C 1 and C 2 – C 1(t) - C 2(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 C 1 and C 2 – C 1’(t) - C 2’(t) Dr. Hugh Melvin, Dept. of IT, NUI, G 12

Dr. Hugh Melvin, Dept. of IT, NUI, G 13

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 Dr. Hugh Melvin, Dept. of IT, NUI, G 14

Cone of Acceptability Slope = 1 + Slope = 1 = True Clock Time Slope = 1 - Real Time Dr. Hugh Melvin, Dept. of IT, NUI, G 15

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 16

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 17

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 18

PLL Input Signal Comparator VCO = Voltage Controlled Oscillator Freq controlled by applied input voltage Dr. Hugh Melvin, Dept. of IT, NUI, G 19

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 20

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 21

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 (Geo. Sat) / LORC (Loran. C) / ATOM / DTS – Stratum 1: Primary Time Server Dr. Hugh Melvin, Dept. of IT, NUI, G 22

Dr. Hugh Melvin, Dept. of IT, NUI, G 23

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 24

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 26

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 27

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 35

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 1000 Hz 1 msec • 3 clock rates – Normal rate. . Add 10 msec every tick (100 Hz) – Normal Rate +/- tickadj – Eg. If tickadj = 5 us Normal Rate +/- 500 ppm Dr. Hugh Melvin, Dept. of IT, NUI, G 37

NTP Operation • NTP adjusts every sec via adjtime – Eg. If clock skew is +100 ppm & tickadj=5 us • 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 38

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 39

Refid: DCF: 77. 5 KHz Radio Signal PTB: German time signal Dr. Hugh Melvin, Dept. of IT, NUI, G 40

Dr. Hugh Melvin, Dept. of IT, NUI, G 41

Time difference Dr. Hugh Melvin, Dept. of IT, NUI, G 42

Server Details • when: no of sec since last response • poll : interval between queries • reach : Reachability in octal – 1111 = 3778 = max – 1110 = 3568 last + 5 th 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 43

Dr. Hugh Melvin, Dept. of IT, NUI, G 44

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 45

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 46

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 47

Dr. Hugh Melvin, Dept. of IT, NUI, G 48

Dr. Hugh Melvin, Dept. of IT, NUI, G 49

Server Offsets: Problem? Dr. Hugh Melvin, Dept. of IT, NUI, G 50