Clock Synchronisation for RTS

transcript

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

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

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

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

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

AS EHF

125N16R30

141NAT-24Synogate

U/IA U/IB

127 N-BK Bus 1

MasterClock

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

Audio-System Clock Skew

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

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

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

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)

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

Cone of Acceptability

Real Time

Clock Time

Slope = 1 = True Clock

Slope = 1 -

Slope = 1 +

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

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)

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

VCOComparatorInput Signal

VCO = Voltage Controlled Oscillator

Freq controlled by applied input voltage

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)

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

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

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

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

P/F-Lock Loop

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

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

Clock Discipline

• Recall– No time reversal!– Avoid step changes

• Hybrid phase/frequency-lock (PLL/FLL) feedback loop

• PLL/FLL Mode: Depends on polling interval

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

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

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

Refid:

DCF: 77.5 KHz Radio Signal

PTB: German time signal

Time difference

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

NTP Robustness Issues

• Redundancy

• Path Diversity

• Symmetric Networks

• Proximity to Primary Reference Sources– See results

• OS & Network Load– Platform Dependencies

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

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?

Server Offsets: Problem?

Clock Synchronisation for RTS

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