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| Introduction |
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| Measurement of
accurate Time and synchronization of computers and systems
in a networked environment is of paramount importance
in various applications. In the recent years GPS based
timing products are extensively being used for time displays
and time synchronization applications. |
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| Following are
the example areas of Application of GPS Based Timing Products |
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- Air Traffic Control and Position Reporting
and other Transportation Systems.
- Radio and TV Broadcasting.
- Data Streaming.
- Enterprise Network Monitoring, Control,
Intruder Detection, Location and Time Stamping.
- E-Commerce Transactions with Time
Stamps
- Secure document transaction.(with
encryption)
- Electrical Power Systems.
- Telecommunication systems.(wired and
wireless)
- Event Synchronization on Distributed
networks.
- Industrial Automation.
- Networked Time Displays in Airports,
Railway Stations, Stadia and Public Centers.
- Defence base operations.
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| Existing GPS Based
Timing Products |
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GPS Time Synchronization
Products are relatively new and have indeed evolved since
the last decade. Among them, the Network Time Protocol
(NTP) servers based on UNIX workstations dominate the
market. The more recent products however provide higher
precision and reliability, use embedded software and Linux
based systems.
These products broadly fall under the following categories |
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- Network Time Displays (Referenced
Digital Clocks),
- Network Time Synchronization Servers,
- Frequency and Pulse Synchronization
(for Control Applications) and
- Stand Alone Computer Time Synchronization
(with GPS on PCI Bus).
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| Typical Network
Time Synchronisation System |
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| Among the GPS
time synchronization products, the Network based systems
are popular. This is represented by the following block
diagram. |
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| Time Synchronization
System |
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| Precision timing
and synchronization requirements of networks is addressed
by acquiring accurate time as a primary source from satellites
using Global Positioning System Receiver, tracking at
least one satellite intermittently with an update rate
of 1 sample per second. A Reference Oscillator serves
as a secondary time source in the absence of the GPS signal.
The other components of the system are, Microprocessor
Control System with Ethernet Network Interface, time code
outputs that include IRIG (Inter Range Instrumentation
Group) time code and protocols that include Network Time
Protocol (NTP) Stack implemented on Ethernet for synchronizing
the clients in the network with the Primary or Secondary
Time Sources. |
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| Network Time Protocol |
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NTP is the defacto
standard for network time synchronization. The synchronization
protocol determines the time offset of the server clock
relative to the client clock and sends the appropriate
corrections.
NTP protocols work in one or more association modes, viz
Client /Server, Broadcast/ Multicast etc. The Broadcast
mode allows many clients to synchronize to one or few
servers, thereby reducing network traffic when large numbers
of clients are involved. Multicast is used when the subnet
spans across multiple networks.
NTP Client side software is installed on the client's
system for synchronizing the clients with server.
By implementing appropriate clock disciplining algorithms
in conjunction with NTP, reliable Network Time Synchronization
can be achieved with accuracies of 1 to 100 milliseconds. |
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| Other Software
Protocols Required for the System |
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| The other protocols
that complement NTP to synchronize networks and maintain
security and implement other network management functions
are Simple Network Time Protocol (SNTP), TIME, Day Time,
MD5, Simple Network Management Protocol (SNMP), MIB II,
DHCP, FTP and TELNET. |
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| Price Information |
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| Network Time Servers
with GPS, Rubidium Reference Oscillator, NTP and other
Protocols cost in the range US $5000 -8,000. The cost
depends on the timing accuracies required for the application
with respect to UTC (Universal Coordinate Time), number
of clients to be serviced, application software features,
user Interface to the system and the level of ruggedization
required. |
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| Typical Product
Specifications of GPS Based Network Time Server |
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| Sl No |
Parameter |
Specifications |
| 1 |
GPS Receiver Characteristics |
- 12 channel parallel receiver
- Minimum satellites for time
: 1 intermittently
- Time Update Rate : 1 sample
per second
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| 2 |
Antenna Characteristics |
- Window mount
- Gain requirement: Minimum 16
dB gain
- Noise Figure: Less than 2 dB
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| 3 |
Input/Output connections for NTP |
- Network : 10/100 Base-T Ethernet
auto sensing (Two redundant NTP time outputs)
- Serial : RS-232 (optional)
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| 4 |
1 PPS Characteristics |
- Pulse width : 1 to 5 ms
- Level : Differential TTL
- Accuracy : ± 100 ns
- Rise time : 10 ns
- Synchronisation : Rising edge
synchronized to GPS time within ± 100 ns accuracy
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| 5 |
Hold over accuracy |
- In the absence of GPS signal,
hold over accuracy < 5 µsec per hour
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| 6 |
Clock Reference |
- Rubidium oscillator 10 MHz Frequency
output
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| 7 |
Network protocols |
- NTP, NTP broadcast mode, SNTP,
TIME (RFC 868), DAYTIME (RFC 867), MD5 (RFC
1321), SNMP (RFC 1157), MIB II (RFC 1213), DHCP
(RFC 2132), FTP (RFC 959) & Telnet (RFC 854)
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| 8 |
User Interface (Outputs) |
- 1 or 2 NTP time outputs on Ethernet
- PPS output
- IRIG B/E AM/TTL Output
- Display : 32 character backlit
LCD
- LED Indicators1
- Power ON
- GPS Fix
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| 9 |
Stratum |
- Stratum 1 Operation via GPS
satellites
- Stratum 2 Operation via NTP
servers
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| 10 |
Client Software |
- Software for synchronizing client-side
with network server for Windows and Linux
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| 11 |
Timing Accuracy |
- Network : 1-100 millisecond
typical client accuracy
- GPS: < 1 u sec to UTC
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| 12 |
Environmental |
- Mounting : EIA 1U 19'' Rack
Mount
- Dimensions (480 mm W x 44 mm
H x 305 mm D)
- Operating Temperature: 0 to
50 Deg C.
- Storage Temperature: -40 to
+ 80 Deg C.
- Relative Humidity: 0 to 95 %
non condensing
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| 13 |
Power Supply Requirements |
- 90 - 230V AC and/or 24 V DC
input
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| For abbreviations
and references, please go to: Appendix |
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More
>> |
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