Abstract: A data communication network comprises a local clock (22) within a node (2) of the network which may be synchronized and syntonized by any node in the network. Each node contains a time packet detector (6) that detects and recognizes timing data packets and produces a recognition signal. Each node has a time server (10) that includes the local clock (22). The time server records the time of the recognition signal. The recorded time is used for correcting the local clocks of the various nodes (2) in the network. A transfer device such as a gateway, a bridge or a router may include a time server and a time packet detector to correct for the transit time of a time packet through such transfer device. The time packet detector (6) is connected at the point of final encoding for transmission or recovery of the clock and data.

Abstract: We address the problem of how to integrate fault-tolerant internal and external clock synchronization. We propose a new algorithm which provides both external and internal clock synchronization for as long as no more than F reference time servers out of a total of 2F+1 are faulty. When the number of faulty reference time servers exceeds F, the algorithm degrades to a fault-tolerant internal clock synchronization algorithm. We prove that at least 2F+1 reference time servers are necessary for achieving external clock synchronization when up to F reference time servers can suffer arbitrary failures, thus our algorithm provides maximum fault-tolerance. The algorithm is also optimal in another sense: we show that the maximum deviation between reference time and the clocks of nonreference time servers is minimal.

Abstract: Time and frequency data are often transmitted over public packet-switched networks, and the use of this mode of distribution is likely to increase in the near future as high-speed logical circuits transmitted via networks replace point-to-point physical circuits. ALthough these networks have many technical advantages, they are susceptible to evesdropping, spoofing, and the alteration of messages enroute using techniques that are relatively simple to implement and quite difficult to detect. I will discuss a number of solutions to these problems, including the authentication mechanism used in the Network Time Protocol (NTP) and the more general technique of signing time-stamps using public key cryptography. This public key method can also be used to implement the digital analog of a Notary Public, and I will discuss how such a system could be realized on a public network such as the Internet.

Abstract: PURPOSE:To correct the time of a timer server to proper time even if the time server becomes abnormal by re-setting the time of the time server to the time of all terminal equipments which are connected to a network. CONSTITUTION:The time server 2 requests all the terminal equipment 3a-3n which are connected to the network 1 to send the time of the internal timer of the terminal equipments 3a-3n back at certain intervals of time. The terminal equipments 3a-3n once receiving the time sending-back request from the time server 2 send the time of their internal timers back. The time server 2 statistically process the time of the respective terminal equipments 3a-3n which are sent back within a certain time to detect and substitute proper time for the time of the internal timer of the time server 2.

Abstract: Responding to an increased demand for reliable, accurate time on the Internet and Milnet, the U.S. Naval Observatory Time Service has established the network time servers, tick.usno.navy.mil and tock.usno.navy.mil. The system clocks of these HP9000/747i industrial work stations are synchronized to within a few tens of microseconds of USNO Master Clock 2 using VMEbus IRIG-B interfaces. Redundant time code is available from a VMEbus GPS receiver. UTC(USNO) is provided over the network via a number of protocols, including the Network Time Protocol (NTP) (DARPA Network Working Group Report RFC-1305), the Daytime Protocol (RFC-867), and the Time protocol (RFC-868). Access to USNO network time services is presently open and unrestricted. An overview of USNO time services and results of LAN and WAN time synchronization tests will be presented.

Abstract: The Network Time Protocol (NTP) is widely deployed in the Internet to synchronize computer clocks to each other and to international standards via telephone modem, radio and satellite. The protocols and algorithms have evolved over more than a decade to produce the present NTP Version 3 specification and implementations. Most of the estimated deployment of 100000 NTP servers and clients enjoy synchronization to within a few tens of milliseconds in the Internet of today. This paper describes specific improvements developed for NTP Version 3 which have resulted in increased accuracy, stability and reliability in both local-area and wide-area networks. These include engineered refinements of several algorithms used to measure time differences between a local clock and a number of peer clocks in the network, as well as to select the best subset from among an ensemble of peer clocks and combine their differences to produce a local dock accuracy better than any in the ensemble. This paper also describes engineered refinements of the algorithms used to adjust the time and frequency of the local clock, which functions as a disciplined oscillator. The refinements provide automatic adjustment of algorithm parameters in response to prevailing network conditions, in order to minimize network traffic between clients and busy servers while maintaining the best accuracy. Finally, this paper describes certain enhancements to the Unix operating system kernel software in order to realize submillisecond accuracies with fast workstations and networks. >