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INET2001 Paper: Advanced NTP Synchronization Device

The Wayback Machine - https://web.archive.org/web/20050210104920/http://www.isoc.org:80/inet2001/C…

Advanced NTP Synchronization Device
for Internet Monitoring Tools

Haruo Okazawa <okazawa@crl.go.jp>
Communications Research Laboratory,
JAPAN
Akihiko Machizawa <machi@crl.go.jp>
Communications Research Laboratory,JAPAN
Shin-ichi Nakagawa <snakagaw@crl.go.jp>
Communications Research Laboratory,JAPAN
Yoshiaki Kitaguchi <kita@genesis.tao.go.jp>
Telecommunications Advancement Organization
GENESIS Project,JAPANTohru Asami <asami@kddlabs.co.jp>KDDI R&D Laboratories, Inc.
JAPANAtsushi Ito <at-ito@gsec.keio.ac.jp>
Keio University,JAPANAbstract
The latency is one of the important parameters for the Quality of Service(QoS) and it is measured at both ends of the network. This analysis is subject
to the machine's clock and depends heavily on its precision. For the timesynchronization among nodes, NTP (Network Time Protocol) [1] is usedthrough the Internet, and the time accuracy is also very important at each
NTP server.
In this research, we aimed to improve the time stability of the top NTP server(stratum1), and then we developed a server with higher stability, using thehigh precision frequency signal from the outside, instead of using the crystal
oscillator on the motherboard of PC (Personal Computer). To input externally,
we used the cesium atomic time (10-12/day) and the high precision crystal
oscillator (10-9/day), but we found that the time stability of PC was at 1
microsecond order each, for the Linux time resolution.
Table of Contents
Introduction
NTP Server
Advanced NTP Synchronization Device
Cesium Atomic ClockHigh Accuracy Crystal Oscillator
Conclusion
Reference
1. Introduction

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INET2001 Paper: Advanced NTP Synchronization Device

Quality of Service (QoS) is one of the next-generation Internet technologies. And thelatency is one of the important parameters of QoS. When the measuring the latency, all
the measuring equipments must be synchronized for a regular number of days (forexample, one month), for high quality of services. And the general method for its
synchronization on the network is NTP (Network Time Protocol). Recently, for theelectronic commerce, it has become more important to know an exact packet arrival time
to a group of commerce servers to identify the first comer to serve and the timesynchronization technology with high precision has also be requested. Currently, the time
accuracy of stratum 1, based on GPS (Global Positioning System) is more than 10
microseconds. [2] (See the table 1, when the polling interval to GPS is 32 seconds atdefault). However, on a high-speed network such as OC-12, the transmission time of apacket with a typical size of 500 byte, is about 6 microseconds, and as a result, we mustsay that the time resolution of NTP server is not enough accurate now.
In this research, we aimed to improve the time accuracy of Linux OS of AT compatible
machines for general purpose and used a high precision signal from the exterior toachieve it.
Table 1: Trimble Palisade receiver operating system compatibility

2. NTP Server
NTP, which is widely used as a network time synchronization technology, constitutes thetime synchronization system with a hierarchical structure. And this NTP acquires the UTC1
(Coordinated Universal Time) at the top NTP server (stratum 1) from the external timesources. Recently, GPS has been used widely an external time source because of lower
change of GPS receivers than before.
The smaller the polling interval to the external time source is, the better the time
accuracy of Stratum 1 obtain. And when that interval is only 1 second, as precise as about
1 microsecond can be kept. (Refer to the table 1). However, when modifying the time,sometimes the time would be jumped or skipped. Therefore, we need to think anothermethod and we suggest having better crystal oscillator inside the PC.

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Figure 1: Accuracy vs. polling interval
3. Advanced NTP Synchronization Device
To replace the standard signal of 14.318MHz inside the PC, we developed an externalsignal inputting device which is PC-mountable. This device can convert the external
10MHz signal into 14.318MHz one, and as a time source, if only 10MHz signal can begenerated, any sort of oscillators can be available.
Using this device, we measured the time stability when each signal of cesium atomic clock
and the high precision crystal oscillator is used. The NTP server system constitution isshown in table 2. For the comparison of the time source, GPS is used with Palisade NTP ofTrimble.
Table 2: The specification of our test system

4. Cesium Atomic Clock
The cesium atomic clock controls and makes use of the cesium atom and it defines TAI2
(International Atomic Time). This time, the clock is the one of HP5070A typed cesium
atomic clock by Hewlett-Packard. This keeps the high precision with an accuracy of only
less than 20 nanoseconds frequency stability (2.0 x 10-13/day).
We measured the time difference between the NTP server and the one gained by the GPSreference. Loopstat, being attached to ntp-4.099, was used for this measurement. Table
2-2 shows its result, and as it shows, the standard deviation a day was found to be 0.55
microseconds. The time resolution of Linux, 1 microsecond, defines the NTP server time
accuracy.
5. High Accuracy Crystal Oscillator
Next, we tried out a measurement, using a crystal oscillator that has lower precision butcosts less than a cesium atomic clock. This oscillator, which is the universal time interval
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counter by Standard Research System, keeps the accuracy of 10-9/day.
As table 2-3 shows, we gained the result of 0.57 microseconds for the standard deviation
less than 1 microsecond just as the secession 4. And then we conclude that there is no
specific difference by the current Linux OS.

Figure 2: Time comparison between NTP server and GPS time
6. Conclusion
We developed a device to improve the precision of the standard frequency of 14.318 MHz
inside the PC, using the external high stable signal. When the signal of the cesium atomic
clock is input, the NTP server precision was found to be 0.55 microseconds as thestandard deviation. Even when using the high stable crystal oscillator, we still gained 0.57
microseconds, so there is no any specific by the current Linux time resolution.
We need to improve the Linux time resolution for the NTP server of nanosecond precision
for the future and this device will be necessary for the further network measurement.

1 UTC: Since 1972, UTC is the atomic time made to approximate by "leaptime adjustment" at the time of astronomy. The length for 1 second is thesame as that of TAI.
2 TAI: Defines the time, in which the duration of 9,192,632,770 periods of thecesium 133 radiations as 1 second. This time has been measured successively
since 1955.

Reference
1D. L. Mills, "Network Time Protocol (Version 3)", RFC 1305, March 1992.
2Trimble Palisade Receiver, URL http://www.trimble.com/oem/ntp/driver29.htm
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