Hikari Telephone JJY

Hikari Telephone JJY is a timing distribution system that provides Japan Standard Time over modern digital telephone lines. Testing starred on May 26, 2016, and the system It has been in full-scale operation from February 1, 2019. Since early 2022, it is used more frequently than the previous Telephone JJY service that it was designed to replace because direct analog telephone connections are no longer available.

The Hikari Telephone JJY service is used by users that cannot or do not want to connect to the internet, but still require a reliable time source without factors of uncertainty such as the reception of GPS signals or Standard Radio Signals. Often, such users are broadcasters. The Hikari Telephone JJY system provides them with a traceable link to Japan Standard Time with an uncertainty of less than one millisecond.

If you have any questions about the service that are not answered on this page, please check our answers to frequently asked questions.

If you are interested in the background and development of the service, please visit [this page]. We have also collected answers to [frequently asked questions]. ((It would be great to add another page based on the development history that was in the Journal of NICT article!))

How to use Hikari Telephone JJY

Using the Hikari Telephone JJY service requires the Hikari Denwa digital telephone line that is offered by NTT in combination with the FLET’S Hikari internet connection. The connection to NICT’s systems requires a router that is compatible with the Data Connect service. The time synchronization itself either uses specialized hardware, or a suitably configured computer system.

Registration

The Hikari Telephone JJY service requires a free-of-charge registration. Additionally, the caller-ID information has to be enabled for the telephone line so that the Hikari Telephone JJY service can identify the incoming connection.

Please complete the user application form (Japanese) and send the result to jst-service with the subject line “光テレホンJJY利用登録申請書” (user application file). Processing the application usually takes about 5 business days, and you will receive an email when everything is ready.

Connecting to the service

To access the Hikari Telephone JJY service, the router needs to be configured to create a tunnel from the local network to the systems at NICT. Please see this configuration manual for details.

Starting the Data Connect service then creates an IP-UDP tunnel. Data Connect’s guaranteed bandwidth and QoS (Quality of Service) features ensure a consistently low latency. The time synchronization then uses NTP-compatible packets that are exchanged over this connection.

Since the Data Connect bandwidth is severely limited, and excess traffic quickly degrades the quality of the time synchronization, it is important that network traffic does not accidentally get routed over the tunnel connection. This is often enforced by placing only the time synchronization equipment on the LAN connected to the router. If high-speed access is performed, the time synchronization error increases because the delay time changes due to the bandwidth limitation. In addition, please pay attention to the access interval.

Usage example

Specialized time server systems are available from commercial suppliers, but since the Hikari Telephone JJY Service uses extended NTP packets for synchronization, it can also be used with just a Linux- or Unix-based PC running the ntpd service. A typical application would be an NTP server providing time information in a network that is isolated from the internet.

By configuring ntp.conf as

the ntpd server will run as a stratum 1 server based on the computer’s own clock. To stop the clock error from accumulating, the system is then set to periodically access the Hikari Telephone JJY service and synchronize the time by a command such as

Here, IP 192.168.200.1 is the Hikari Telephone JJY host address (NICT Headquarters) when connected via Data Connect as described in the configuration manual.

The option ‘-u’ sends the outgoing packets from an unprivileged port, which allows the use of ntpdate even while the ntpd service is active. Option ‘-B’ performs gradually performs the time synchronization in slew mode. Use option ‘-b’ if you want to synchronize instantly, but this may result in forwards or backwards time jumps.

The option ‘-p8’ causes a total of 8 packets to be sent. Sending multiple packets ensures that time synchronization can still be performed even though the initial packets are lost during the time it takes to establish the telephone connection.

Automated synchronization every 6 hours can be enabled by creating a /root/htelsync.sh file

and then executing it periodically by editing the crontab file: to include:

This will initiate a time synchronization every six hours, at 01:05, 07:05, 13:05 and 19:05. In a test at NICT, this was sufficient to keep the NTP server clock accurate to within ± 0.1 s. Further improvement or an extended hold-over time between synchronizations may be possible by adjusting the ntpd drift file. Without synchronization, the clock of the server used in the experiment lags by about 0.5 s per day.

Please note that this simple method does not support leap seconds, which require manual configuration. The communications cost for this configuration –as of April 2017– is ¥1 (without tax) per synchronization, for a total of about ¥120 per month.

Precautions

Please be careful not to send NTP packets continuously.

If NTP packets are sent too rapidly, the connection will be throttled due to the limited bandwidth. This will increase the transmission delay and consequently the error in the time synchronization.

Please do not access the system more than once per hour and keep the connection time to 3 minutes or less.

This helps avoid excessive burden on the Hikari Telephone JJY system and keeps the system available to connections by all users.

Please do not use the Hikari Telephone JJY service as a simple top-level NTP server.

If you are looking for an NTP server, please use NICT's publicly accessible stratum 1 NTP server. The Hikari Telephone JJY is different from a standard NTP server, and is a complex system with limited capacity that should only be used where its specific features are required.

Data format

The Hikari Telephone JJY system performs time synchronization by NTP-compatible packets exchanged through an IP UDP tunnel established over the Data Connect service. NTP packets contain time information in terms of Coordinated Universal Time, without information on local time. To make the time information directly traceable to Japan Standard Time (JST), the provided by Hikari Telephone JJY service makes use of the extension fields that were introduced in version 4 of the NTP standard.

The extended data is only included in the reply when the initiating user packet also contained a 20-byte extension field. In the returned packet, the extended data is replaced by the following information:

control is a 1 byte value that is unspecified so far and currently returns 0.
server ID is a 1 byte value that contains the hosts server ID as described in the text.
JST offset is a signed 1 byte value that describes the offset of JST from UTC. This should always be +9.
DST offset is a signed 1 byte value that describes the offset of Japanese Time from UTC during daylight saving time. As Japan presently does not implement daylight saving time, a hypothetical value of +10 is currently returned.
start of DST is a 4 byte value corresponding to the integer part of an NTP timestamp. It indicates when the local time switches from JST offset to DST offset.
end of DST is a 4 byte value that indicates when local time returns to JST offset.
As long as there is no switch to daylight saving time in Japan, both of these fields will remain set to 0.
option is a currently unused 8-byte field for future extension.