Clock Synchronisation for
RTS
Dr. Hugh Melvin, Dept. of IT, NUI,G
1
Importance of RTS Clocks
• RealTime implies need for accurate timekeeping
• Precise Time & Timing
– Two distinct issues
– Both often required for RTS
• Time (of day)
– UTC (Universal Coordinated Time)
– Chronological event recording
– Crucial for fault diagnosis in distributed systems
• Timing
– Frequency is key issue
– Crucial in synchronising components
Dr. Hugh Melvin, Dept. of IT, NUI,G
2
Time & Timing
• Time
– UTC evolved from
Greenwich Mean Time
GMT
– Time Synchronisation
• How closely 2 clocks
agree on Time of Day
• Timing
• How closely 2 entities
operate at same frequency
Dr. Hugh Melvin, Dept. of IT, NUI,G
3
1
Greenwich
• Home of Time & the
Prime Meridian
• Longitude 0 & GMT
agreed in 1884
– GMT driven by
p
of railways
y
expansion
and need for
national/international
consistency
• “Timeball” rises and
falls at 13.00 hrs
– Then site of London
docks
– In service since 1833
Dr. Hugh Melvin, Dept. of IT, NUI,G
4
Greenwich
Dr. Hugh Melvin, Dept. of IT, NUI,G
5
18th Century GPS
• 18th century maritime navigation
– Latitude
• Astronomical
– Longitude dilemma
• Solutions
– Astronomical
– Time based
• Need for robust maritime clock
Dr. Hugh Melvin, Dept. of IT, NUI,G
6
2
18th Century GPS
• Time based location
– Set ship clock before setting sail from time
ball
– Compare with 'local time' to give one's
position east or west of the home port.
• 360° = 24 hr  15 ° = 1 hr
– Problem was getting clock to work well on
a ship
– Lincolnshire carpenter, John Harrison
• H4 won him the great Longitude Prize
of £20,000. .. in 1759
Dr. Hugh Melvin, Dept. of IT, NUI,G
7
Time & Timing Examples
• Time
• Distributed Control Systems : Moneypoint
– Facilitates fault diagnosis
•
•
•
•
•
Power Line Fault Detection
VoIP delay monitoring
SLA adherence monitoring
g systems
y
Billing
Database/File Integrity timestamps
• Timing
• Synchronising redundant devices 2v3,2v4 voters etc
• TDM within GSM/POTS
– Avoids bit errors
• MM applications : skew issues due to ‘timing’ mismatch
– Delay/Buffer problems
Dr. Hugh Melvin, Dept. of IT, NUI,G
8
Power System Control
• AS station
– Time: Token Bus Synchronisation via Master Clock
•
•
•
•
Critical for chronological data logging / fault diagnosis
Approx 1 msec level synch reqd
Provided via GPS
Generator Earth Fault / Overcurrent ..
– Which came first .. msec level data required
– Timing:
Ti i
S
Synchronising
h i i 2
2v3
3 voter
t systems
t
• Need to deliver verdicts simultaneously
• Power Line Fault Monitoring
• Noise burst travels in both directions
• Speed of light c = 3* 108 m/s  0.3 m / nanosec
» synch level of usec needed (300 m)
• Synchroscope
– Frequency alignment
Dr. Hugh Melvin, Dept. of IT, NUI,G
9
3
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
10
Power Line Fault Detection System
Dr. Hugh Melvin, Dept. of IT, NUI,G
11
Power Line Fault Detection System
Dr. Hugh Melvin, Dept. of IT, NUI,G
12
4
Timing : 2v3 Redundancy
Bus A
Bus B
CS275
CPU 1
..
U/I
U/I
N8
N8
CPU 2
CPU 3
..
..
..
..
..
2 out of 3
Non-Redundant
Control Circuits
Double Redundant
Safety Circuits
Dr. Hugh Melvin, Dept. of IT, NUI,G
Triple Redundant Safety Circuits
13
Timing: Synchroscope
• Frequency alignment
between generator
and transmission grid
• No room for
significant error
Dr. Hugh Melvin, Dept. of IT, NUI,G
14
Synchroscope
Dr. Hugh Melvin, Dept. of IT, NUI,G
15
5
Telecommunications
• Precise timing synch fundamental to current
systems
– POTS
• SDH/SONET
– Cellular Networks
• GSM/CDMA
– Loss of synch
• Bit errors (slip)  data loss
• System failure
• IP-based NGN
– Circuit  Packet switched network
• Loss of core synch key challenge
• http://forum.telecom-sync.com/
Dr. Hugh Melvin, Dept. of IT, NUI,G
16
Dr. Hugh Melvin, Dept. of IT, NUI,G
17
Dr. Hugh Melvin, Dept. of IT, NUI,G
18
6
Dr. Hugh Melvin, Dept. of IT, NUI,G
19
Soft RTS
• POTS operation based on TDM
• PCME1E2..E4 SDH/SONET
• Precise timing synchronisation reqd throughout
the network for correct system operation
• GSM : FDM + TDM
• Each FDM channel divided out to 8 users via
TDM
Dr. Hugh Melvin, Dept. of IT, NUI,G
20
Soft RTS
• IP Multimedia Applications
– Time
• Delay / Jitter measurement imp in packet (IP) networks
• More advanced QoS possible through synchronised time
– Recall G
G.1010
1010
– Basis of SLA  delay/jitter measurement important
– Timing
• Skew Issues between various clocks
– Time & Timing
• Lip Synch challenge
• MMOG
Dr. Hugh Melvin, Dept. of IT, NUI,G
21
7
Dr. Hugh Melvin, Dept. of IT, NUI,G
22
Dr. Hugh Melvin, Dept. of IT, NUI,G
23
Audio-System Clock Skew
Dr. Hugh Melvin, Dept. of IT, NUI,G
24
8
Timing ‘Skew’ for IP Multimedia
Dr. Hugh Melvin, Dept. of IT, NUI,G
25
Lip Synch ETSI STQ
C
Undetectability plateau
C'
Diffgrade)
Subjective evaluation results (D
0
B
B'
Detectability threshold
-0,5
-1
-1,5
Acceptability threshold
A
A'
Sound delay wrt
vision
Sound advanced wrt
vision
-2
-200 -180
-160
-140
-120
-100
-80
-60
-40
-20
Delay time (ms)
0
20
40
60
80
Dr. Hugh Melvin, Dept. of IT, NUI,G
100
26
Synch Time for MMOG
Dr. Hugh Melvin, Dept. of IT, NUI,G
27
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
C
t value
l ttranslated
l t d to
t time
ti
standard
t d d
– 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
28
Computer Clocks
• Accuracy relates to how close the crystal freq
is to its rated value
– Determined largely by manufacturing process
• Get what you pay for!
• Stability relates to how frequency varies
– Influenced
f
by parameters such as:
• Temperature .. Eg. 2ppm /C
• Ageing
– Eg. Cesium Beam: 3 x 10-12 / year
• Noise
• Note: Frequency i.e. timing errors lead to time
(of day) errors
Dr. Hugh Melvin, Dept. of IT, NUI,G
29
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)
.. or Radio Receiver DCF77, MSF .. LFR signal .. less accurate
GPS most popular approach to providing accurate/stable time
– Option C : Cheaper Approach
• Software based approach to discipline cheap crystal clocks
• Crude but useful for certain applications
Dr. Hugh Melvin, Dept. of IT, NUI,G
30
10
Dr. Hugh Melvin, Dept. of IT, NUI,G
31
Dr. Hugh Melvin, Dept. of IT, NUI,G
32
Clock Options
• TCXO
– 105 µsec / day = 1.15 10-6 freq instability
• 1.15 ppm
• OXCO
– 10 µsec / day = 1.15 10-10 freq instability
• 1.15 10-4 ppm
• Cesium
– 10-3 µsec / day = 1.15 10-14 freq instability
• 1.15 10-8 ppm
Dr. Hugh Melvin, Dept. of IT, NUI,G
33
11
Clock Terminology
• Some 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
34
Dr. Hugh Melvin, Dept. of IT, NUI,G
35
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 significant except over long timescales
– Note linear relationship in previous slide
• No drift evident
Dr. Hugh Melvin, Dept. of IT, NUI,G
36
12
Cone of Acceptability
Slope = 1 +

Slope = 1 = True Clock
Clock
Time
Sl
Slope
=1-

Real Time
Q: How are accuracy and stability related to cone of acceptability?
Dr. Hugh Melvin, Dept. of IT, NUI,G
37
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)
• Distributed clocks over non-deterministic network
– More difficult scenario
– Eg. Internet Clock Synchronisation via NTP
Dr. Hugh Melvin, Dept. of IT, NUI,G
38
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
• Resolve timing errors  accurate time
• Similar approach used in widely distributed scenario
– GPS / POTS / GSM all use variants of this approach
• But only for timing synchronisation .. Not really concerned with time
Dr. Hugh Melvin, Dept. of IT, NUI,G
39
13
PLL
Input
Signal
Comparator
VCO
VCO = Voltage Controlled Oscillator
Freq controlled by applied input voltage
Dr. Hugh Melvin, Dept. of IT, NUI,G
40
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) (www.ntp.org)
– RFC 1305 (www.ietf.org)
• Unix-based NTP daemon now ported to most OS
Dr. Hugh Melvin, Dept. of IT, NUI,G
41
NTP Clock Synchronisation
• More concerned with time rather than timing
• 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
42
14
NTP
• Network Time Protocol (NTP) synchronises
clocks of hosts and routers in the Internet
– Determines timing errors and compensates to
produce accurate time
• Increasingly deployed in the Internet
– Increased need for time synchronisation
– Facilitated via always-on Internet connection
• Can provide nominal accuracies
– low milliseconds on WANs
– submilliseconds on LANs
– submicroseconds on workstations
• using a precision time source cesium oscillator/GPS
Dr. Hugh Melvin, Dept. of IT, NUI,G
43
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
44
NTP System
GPS/Radio Clock
Timin
g S
igna
l
Timing Signal
GPS Satellite
Timing Signals
NTP
NT
P
P
NT NTP Primaryy 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
45
15
NTP Operation
Peer 1
Filter 1
Peer 2
Filter 2
Peer 3
Intersection
and
Clustering
Algorithms
Combining
Algorithm
Loop Filter
P/F-Lock Loop
Filter 3
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
46
NTP Operation
• NTP Algorithms act upon a set of variables
– Offset / Delay / Dispersion
– Dispersion
n 1
 j    ij wi 1
i 0
 ij   i   j
• w = 0.75
• These are relative to both peer and root
Offset
Delay
Dispersion
θ
δ
ε
Θ
Δ
Ε
Dr. Hugh Melvin, Dept. of IT, NUI,G
47
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 p
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
48
16
NTP Operation
B 3.59.020
C 3.59.022
15 ms
15 ms
A 3.59.000
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
49
Filtering Algorithm
•
Filtering algorithm looks at last 8 samples
•
Reduces offset errors by a factor of about ten
•
Effective at removing spikes
•Chooses
sample with min RTD
Dr. Hugh Melvin, Dept. of IT, NUI,G
50
Intersection Algorithm
1
2
4
3
Clocks 1, 2 ,3 are truechimers
4 is a falseticker
X1
X2
•
•
•
•
Selects a subset of peers
Based on intersection of confidence intervals
Identifies truechimers & falsetickers
eg. From 1,2,3,4 above
Dr. Hugh Melvin, Dept. of IT, NUI,G
51
17
Intersection Algorithm
•
Estimated offset to each clock is mid pt
But: Any point in each confidence interval may
represent actual time as seen by that peer
•
•
If clocks 14 are correct, there must exist a
common intersection
 Clock
Cl k 4 mostt lik
likely
l iincorrect..
t di
disregard
d
•
Interval X1 = smallest intersection containing
points from 1,2,3
•
But also include the max no of interval midpoints
–
Select X2 interval
–
Could select mid pt of X2 .. or refine further
Dr. Hugh Melvin, Dept. of IT, NUI,G
52
Clustering (Clock Selection)
• Sort surviving clocks by stratum and incr synch
distance (RTD/2 + disp), S1 S2 S3
• Remove outliers that have significant dispersion
relative to other survisors
– Compute Select Dispersion of each clock
• Weighted sum of differences to other clocks
– Compute Sample Dispersion of each clock
• Weighted sum of diff relative to past samples of same
clocks
– If Max SelDisp > Min SamDisp
• Remove this survivor and repeat
• Favours candidates at start of sorted list
 Favours lowest stratum / delay
Dr. Hugh Melvin, Dept. of IT, NUI,G
53
Clustering algorithm
Sort survivors of intersection algorithm by increasing
synchronization distance(RTD/2 + dispersion). Let n = no of
survivors and nmin a lower limit (eg.3).
For each survivor si, compute the select dispersion (weighted sum
of clock difference) between si and all others.
Let smax be the survivor with max select dispersion (relative to all
other survivors)) and smin the survivor with min sample
p dispersion
p
(clock differences relative to past samples of the same survivor).
smax  smin or n nmin
yes
no
Delete the survivor smax; reduce n by one
The resulting survivors are processed by the combining algorithm
to produce a weighted average used as the final offset adjustment
Dr. Hugh Melvin, Dept. of IT, NUI,G
54
18
Combining Algorithm
• Combine result from survivors of
selection algorithm
• Weighted offset determined based on
– Offset of survivors Θ
– Synchronisation distance Λ
– Eg. 2 survivors (S1,S2) with parameters
(1 , 1 ), ( 2 ,  2 )
– Final Offset =
(1. 2   2 .1 ) /(1   2 )
Dr. Hugh Melvin, Dept. of IT, NUI,G
55
Combining Algorithm Example
(1. 2   2 .1 ) /( 1   2 )
• S1,S2 where S1 = (2 ms, 30) and S2 = (3
ms, 10)
• Final
Fi l Adjustment
Adj t
t=
– (2(10) + 3(30)) / (30 + 10)
= 110 / 40 = 2.75 msec
• Implemented via the Clock Discipline
Dr. Hugh Melvin, Dept. of IT, NUI,G
56
Clock Discipline
• Recall
– No time reversal!
– Avoid step changes
• Hybrid phase/frequency
phase/frequency-lock
lock (PLL/FLL)
feedback loop
• PLL/FLL Mode: Depends on polling interval
Dr. Hugh Melvin, Dept. of IT, NUI,G
57
19
PLL and FLL weight factors
•
Weight factors (not to scale)
• PLL predict (red) most important at shorter poll intervals to 24 s
• FLL predict (blue) most important at longer poll intervals to 217 s
Dr. Hugh Melvin, Dept. of IT, NUI,G
58
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
59
Simplified 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 maintains accurate time by resolving and
compensating for timing errors
Dr. Hugh Melvin, Dept. of IT, NUI,G
60
20
NTP Implementation
• Install NTP
• Set up ntp.conf file
– List of servers that you wish to connect to
– Need to consider
• Redundancy,
y, Path Diversity,
y, Low RTD
• Start up NTP daemon ntpd
• File ntp.drift records clock skew
– Unfortunate file name .. Adds to confusion !
• Other utilities
– ntpq, ntpdate
– See www.ntp.org
Dr. Hugh Melvin, Dept. of IT, NUI,G
61
Refid:
DCF: 77.5 KHz Radio Signal
PTB: German time signal
Dr. Hugh Melvin, Dept. of IT, NUI,G
62
Dr. Hugh Melvin, Dept. of IT, NUI,G
63
21
Time difference
Dr. Hugh Melvin, Dept. of IT, NUI,G
64
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
65
Dr. Hugh Melvin, Dept. of IT, NUI,G
66
22
NTP Robustness Issues
•
•
•
•
Redundancy
Path Diversity
y
Networks
Symmetric
Proximity to Primary Reference Sources
– See results
• OS & Network Load
– Platform Dependencies
Dr. Hugh Melvin, Dept. of IT, NUI,G
67
NTP Operation : Asymmetry
B 3.59.015
C 3.59.017
10 ms
20 ms
A 3.59.000
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
68
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
69
23
Dr. Hugh Melvin, Dept. of IT, NUI,G
70
Dr. Hugh Melvin, Dept. of IT, NUI,G
71
Server Offsets: Problem?
Dr. Hugh Melvin, Dept. of IT, NUI,G
72
24
NTP at NUI Galway
•
Public NTP servers
– ntp-galway.hea.net since 2002
– 2009
• 25,000 different clients
• 62,000 requests per hour
• Symmetricom S300 Server
– GPS
– DCF 77
– Oven Crystal
– Secondary server ntp2.it.nuigalway.ie
• Trimble GPS
• Linux based PC
Dr. Hugh Melvin, Dept. of IT, NUI,G
73
Dr. Hugh Melvin, Dept. of IT, NUI,G
74
Dr. Hugh Melvin, Dept. of IT, NUI,G
75
25
ntp-galway.hea.net
• GPS receiver
• DCF 77 Radio
• Also peers with
– Computer Services
– ntp2.it.nuigalway.ie
– Dublin, UK,
mainland european
servers
Dr. Hugh Melvin, Dept. of IT, NUI,G
76
26