Masami Kihara

Two-way time transfer through 2.4 Gb/s optical SDH system

Abstract : An experiment to transfer time and frequency over 2.488 Gbits SDH (Synchronous Digital Hierarchy) systems using 175-km commercial optical fibers has been set up by CRL and NTT. We confirm that the frequency stability of the time comparison data is 10(12)/square root of tau at averaging times above 10 s. This equals that of the Cs frequency standard (H5071A) used in this experiment. The time comparison resolution is of the order of 10(-11) s (square root of time variance). The long-term stability of this system is expected to be better than 1 ns. The time comparison results of this experiment agree well the GPS common-view results.

Accurate time/frequency transfer method using bidirectional WDM transmission

An accurate time transfer method is proposed using bidirectional wavelength division multiplexing (WDM) signal transmission along a single optical fiber. This method will be used in digital telecommunication networks to yield a time synchronization accuracy of better than 1 ns for long transmission lines over several tens of kilometers. The method can accurately measure the difference in delay between two wavelength signals caused by the chromatic dispersion in the fiber for conventional simple bidirectional dual-wavelength frequency transfer methods. We describe the characteristics of this difference in delay and show that the accuracy of the delay measurement can be obtained below 0.1 ns by transmitting 156-Mb/s time reference signals of 1.31 and 1.55 /spl mu/m along a 50-km fiber using the proposed method. Subnanosecond delay measurement using the simple bidirectional dual-wavelength transmission along a 100-km fiber with a wavelength spacing of 1 nm in the 1.55-/spl mu/m range is also shown.

Long-term propagation delay characteristics of telecommunication lines

The variations in outgoing path delay and incoming delay of a paired transmission path have been measured simultaneously and independently for over one year. The characteristics will allow the performance of delay compensation schemes for precise time synchronization of digital telecommunication networks to be evaluated. The annual delay variation is caused by temperature changes, and its magnitude is about 3 mu s for the 2400-km line investigated. It is shown that the variation of annual delay can be estimated by measuring air temperature above the cable. The delay variations, which vary slowly with a period of over 10/sup 3/ s, such as annual and daily variations, are clearly correlated for the outgoing and incoming paths. Asymmetry of slow delay variation is below 1 ns (indicated noise deviation). The cause of the delay variation that has a period of 10/sup 0/ >

Time signal distribution in communication networks based on synchronous digital hierarchy

A new method that uses round-trip paths to accurately measure transmission delay for submicrosecond time synchronization in synchronous digital hierarchy (SDH) networks is proposed. Since the proposed system uses virtual container-3 (VC-3) signal as the time reference signals, existing SDH equipment can be used without any changes. The feature of this method is that it separately measures the initial round-trip path delay and subsequent variations in round-trip path delay. The initial uncertain delay due to the administrative unit-3 (AU-3) pointer operations in SDH equipment is determined by controlling the reference clock of the equipment. The characteristics of the equipment delay caused by the pointer operations are also described. In an experiment with actual SDH equipment, the error in initial delay determination was suppressed to 30 ns. The delay variation measurement of the round-trip path is not affected by the pointer operation in the proposed method. The precision of delay variation measurement can be better than 10 ns.

Prototype of an optically pumped cesium beam frequency standard

The prototype of an optically pumped cesium beam frequency standard is composed of three modules: a cesium beam tube, a microwave module, and a laser diode module. In the cesium beam tube, background lights from the windows are reduced. In the microwave module, an automatic level controller is used in order to reduce long-term frequency instabilities caused by the microwave level fluctuation. In the laser diode module, three laser diodes are connected to the cesium beam tube by polarization maintaining fibers and fiber couplers. The short-term frequency stability (two-sample standard deviation), which is estimated from the preliminary experimental results gained from the prototype, is about sigma /sub y/( tau )=1*10/sup 12// square root tau .<<ETX>>

Short-Term Frequency Stability Estimation of an Optically Pumped Cesium Beam Frequency Standard

A theory is proposed for the calculation of the short-term frequency stabiliry of any optically pumped Cs beam frequency standard. This theory takes into account the influence of fluorescence photon statistics and laser linewidths. The accuracy of the theory is confirmed by measuring the signal to noise ratio of an optically pumped Cs beam resonator and then estimating the short-term frequency stability of the standard based on the resonator. The theory predicts the observed performance well.

Transmission time-based authentication scheme using 3G mobile device for DRM system

The recent widespread deployment of 3G technology has opened the possibility for high value digital content to be delivered to wireless-driven output terminals that may or may not be mobile. This calls for a robust DRM (Digital Rights Management) system that can protect copyrighted content from being illegally accessed. The conventional method of authenticating a registered user or a prospective content purchaser is by means of the username and password combination. However, this method raises many vulnerabilities, especially now that password attacks and phishing techniques are rampant. Enhanced user authentication schemes are thus needed to ensure that the content is delivered to the legitimate purchaser, and to combat piracy in the Internet in general. Our assumed DRM system utilizes 3G mobile devices, such as HSDPA (High Speed Downlink Packet Access) enabled mobile devices, to access the content. An essential part of the system is an access area restriction scheme; it is based on the physical location of the users mobile device as estimated from the packet transmission time between the mobile device and the server. Together with personal information, the access location must be pre-registered to realize location-specific enforcement policies. This system is extremely flexible and supports a wide range of security policies; for example, the content provider can stipulate that service is terminated if the registered user moves away from the registered terminal through which he is accessing the content.

An experimental analysis of packet train probe for a stable RTT measurement

One of the metrics used to ensure an efficient management of network services is the packet delay. However, packet delay is highly variable due to queuing mechanism in many network devices, including the host systems. In this paper, the protocol stack packet delay on Linux system is presented. Also, a new RTT probe model based on packet train is proposed in order to achieve a stable RTT measurement without making any modification, optimization and tuning to the current Linux kernel. Experiment results show that using packet train the minimum peak RTT could be determined from the “tail” packets and that its variation is within few microseconds.

An Atomic Beam Collimator for Cs Beam Frequency Standards

It is necessary to predict the directivity of atomic beam collimators when designing Cs beam frequency standards. This paper presents an improved beam collimator theory. Collision processes are not neglected in this theory. This theory can analyze the temperature characteristics of a Cs atomic beam collimator of various lengths. Systematic experiments using three atomic beam collimators (lengths of 17, 34, and 50 mm) are performed. Beam profiles and peak beam intensities are compared with calculated values. Beam profiles calculated with this theory agree well with the measured profiles.

Anatomy of Computer Time and its Application to a DRM Scheme

This paper presents the computer timing mechanism and its relationship with the packet transmission delay at the protocol stack. The experiment platform uses two general purpose personal computers connected directly via a network interface. This setup allows us to ignore the influence of end-to-end network devices between the hosts which can cause delay variation. In trials we measured the round-trip time (RTT) between the two hosts in various parameters, such as packet size, packet interval, burst length and burst interval. We show the various RTT characteristics and recommend several parameters that can render precise and stable measurement. Furthermore, we present a proposal for a location-based identification system in the digital rights management (DRM) scheme for a secure digital content distribution via the Internet. This scheme allows the user storage system to be identified by its network location, among other personal identification systems, for its strict content management and prevention of illegitimate activities on the Internet.