通信工程外文翻译文献

中英文对照外文翻译文献(文档含英文原文和中文翻译)
计算机网络冗余GPS时间同步电路板的设计与实现
摘要：如今，在计算机网络系统中准确和可靠的时间是一个基本要求。

为实现这一必要性，时间同步想法产生了。

同时在某些情况下，可靠的时间是如此的重要，以致于一个冗余的结构得以应用。

在本文中，时间同步系统的主要研究是设计和实施一个时间同步电路，该电路能够通过NTP协议与计算机网络同步时间。

在本设计中还嵌入了冗余方案以便提供更高的可靠性。

关键字：计算机网络GPS时间NTP 冗余时间同步时间同步协议时间服务器
1. 引言
我们通常会把电脑的时间和手表的误差设置在一两分钟内，但另一方面，准确和可靠的时间对于财务和法律事务、运输、分销系统，和许多其他涉及资源分布广泛的应用程序是必要的。

举一个例子说明，在一个分布式的机票预订系统，如果分布式计算机时间不同，座椅可以卖出两倍价格甚至更多，或者在网上股票交易完成之前会产生法律后果。

在这方面，世界协调时和时钟同步已开发出来。

基础的时间尺度已随着历史得到改进，以地球自转为基础的地球时和原子时也产生了。

一些重要的时间尺度还包括国际原子时（TAI）、通用协调时间尺度（UTC）、和标准时间或民用时间。

时钟同步协议的想法是，即使最初设置准确，但电脑的内部时钟也可能与世界时钟不同。

之后，由于时钟漂移，会有相当大的误差，所以总是有必要将这些漂移的时钟同步到参考时钟源。

时间同步源包括地球上的无线电同步技术（WWV, WWVH, WWVB, DCF77 and LORAN-C）、卫星时间同步技术（GOES, GPS, GLONASS, and Galileo）、互联网时间同步技术以及电话拨号时间同步技术。

在这些时钟源中，全球定位系统（GPS）提供了一些特殊的优点，如时间精度、抗噪声干扰、在世界各地都可用、并不断引用国际标准。

如今，相比其他时钟资源，全球定位系统时钟的使用更为广泛。

图1显示了一个典型的时间同步结构，其中时间服务器从GPS接收的数据作为时间同步源。

接下来，时间服务器为需要时间的设备发送满足同步协议的准确时间消息。

这些设备通过收到的消息同步它们的内部时钟。

当今各种各样的同步协议为时间同步提供了不同的手段，但他们都遵循两种常用的一般模式。

无论是客户端向服务器发出请求，并且服务器以当前时间响应客户端，或者是服务器向组内或所有的客户发送消息。

一些重要的标准计算机网络时间同步协议如下：时间协议：时间协议规定在RFC868，返回一个32位并行的二进制数表示时间，参考自1900年1月1日协调世界时秒。

服务器监听端口37上的时间请求，并响应TCP/IP 和UDP/IP 格式的请求。

白天协议：白天协议规定在RFC867，发送时间使用ASCII字符。

服务器监听端口13，并且响应TCP/IP和UDP/IP格式的请求。

网络时间协议（NTP）：协议规定在RFC-5905，对于通过网络同步电脑始终来说，是最
古老的（并且仍在使用）和最先进的时间同步协议。

这个NTP服务器监听123端口，并通过发送一个满足NTP协议格式的UDP数据包来响应请求。

简单网络时间协议（SNTP）：SNTP是NTP协议的一个不太复杂的实现版本。

当完整的NTP最终表现不需要的时候可以使用SNTP。

精密时间协议（PTP）：PTP正式出现在IEEE 1588-2008，是一个旨在提高相对于传统的基于以太网协议比如NTP的时间精确度，但是在实际应用当中更昂贵。

对于开源，对于典型的应用来说NTP具有足够的精度，并且有能力工作在大型网络中，NTP是公共互联网中使用最广泛的，并且已经为许多私人网络服务了超过三十年。

可靠性要求对于时间同步来说可能太严格了，以致于一个单一的时间服务器都可能不可信。

因此，在一个冗余结构中需要使用多个时间服务器。

在本文中，我们将探讨时间同步系统的设计与实现，通过从一个低成本的GPS接收器获取时间的数据，然后使用NTP协议同步计算机网络。

在本设计中，以使用两个时间同步板以提供更高的可靠性的方式嵌入了冗余管理。

这份文件的组织如下：第二部分给出了关于NTP协议的一个简短解释，并且使用它同步时间。

第三部分给出关于冗余的更多解释。

第四部分给出了建立一个网络时间同步器的结构，包括硬件设计和软件算法。

第五部分给出最后的测试和结果。

2. 什么是NTP？
NTP主要包含三个部分：NTP软件程序，在UNIX系统中称为守护进程，在WINDOWS 系统中成为服务；用于在服务器和客户机之间交换时间值的协议；和一套处理时间值提前或延缓系统时钟的算法。

因为NTP软件往往是和操作系统捆绑在一起的（如大多数的WINDOWS和UNIX系统），是计算机网络时间同步中最常用的协议。

例如，我们不打算涵盖所有三部分，但我们打算描述设计一个网络时间服务器中所涉及的协议。

更详细的细节可在正式规范中浏览。

NTP协议是基于互联网协议（IP）和用户数据报协议（UDP）建立的，在IEEE 802.3以太网帧中。

该NTP协议报头，在可选的扩展域和一个可选的消息认证码（MAC）后有12个字。

NTP工作模式，包括C/S模式和广播模式。

在客户机/服务器模式中，客户端向服务器发出请求，服务器以当前时间响应请求。

然而在广播模式，时间服务器周期性地向客户端发送NTP数据包，客户端使用接收到的数据包调整自己的时间。

在NTP数据包中最重要的领域是时间戳字段。

一个NTP的时间戳是一个64位无符号定点数的整数部分，在前32位显示以1900年1月为参考的时间和小数部分在最后32位。

这种表示的精度是约2 - 32秒（233皮秒）。

时间同步的精度取决于网络环境。

今天，在互联网的大多数地方，提供准确的时间为10 - 100毫秒，而如果在一个良好的条件，没有太多的路由器的局域网上，正常同步在几毫秒。

3. 什么是冗余？
冗余是一种通常用于提高系统可靠性的技术。

它有多种形式和类型。

冗余最一般的形式
冗余是Mout-of-N（MooN）冗余，其中MooN至少必须是有效的系统函数。

这方面的例子如图2所示。

M-out-of-N冗余广泛应用于不同的领域，例如，在安全系统、编程等。

同时，尊重切换时间，冗余可分为并行和备用冗余。

在并行模式下，冗余的部分是连续运行，在待机模式下，当一个正常运行的操作组件失败时，他们才切换至一个操作模式。

两者之间的本质区别如图3所示。

只要时间同步系统需要更多的可靠性，冗余技术就可以用。

图4显示了一个在时间同步系统中的1oo3冗余结构。

根据这个数据，三个时间服务器正在从是时间同步源（GPS）那里获取时间，然后作为时间服务器，反馈给和一个共同网络连接的群体的客户端以准确和可靠的时间。

值得一提的是，不同的冗余度也可以增加时间同步的可靠性。

这是通过让时间从一种以上的时间参考实现的。

例如一个时间服务器从GPS获取时间，而另一个时间服务器从DC F77广播时间源获取时间。

4. 时间同步电路设计
A. 硬件设计
在这里，我们提出一个方案，一个时间服务器从GPS获取时间数据，并且通过NTP协议为客户端提供准确的时间。

图5给出了时间服务器主板的内部结构。

它主要由五个部分组成：低成本的GPS接收器、单片机、以太网接口、用户接口和电源。

在提出了时间服务器中，单片机中起着重要的作用。

它应该被适当地编程来和其他部分交流，让他们作为一个时间服务器一起工作。

GPS接收器通过串行接口连接到单片机。

它为单片机提供NMEA协议格式的时间数据和与UTC同步的1PPS信号（秒脉冲）。

以太网控制器接口计算机网络建立一个连接并通过它传输以及接收数据。

用户接口包括一个液晶显示器和触摸屏。

它从用户得到所需的配置数据从而显示时间同步的结果。

B. 软件设计
关于时间服务器中单片机的算法设计包括三个子算法，如图7所示：主回路、GPS 1PPS 中断服务程序（ISR），和GPS串行接口（ISR）。

主回路首先初始化以太网控制器、GPS接收器、和用户接口，然后进入一个死循环，等待GPS时间脉冲和串行接口的中断。

这1PPS ISR被设置为上升沿触发。

本ISR具有最高优先级，并且能够让单片机在数万纳秒精度的情况下获得世界协调时。

在这个程序中计数器设置为每10秒发送一次NTP数据包。

NTP的工作模式是广播模式，让所有连接到网络的客户端可以接收数据包。

GPS的串行接口ISR每次运行都会接收来自GPS接收机的一个串行数据。

如果收到的数据是一个时间数据，就会进行解码并且存储在“Time”变量中。

同时NTP数据包会在下一个1PPS被发送出去，其时间戳提前1秒。

冗余也嵌入算法当中。

如图6所示，为此板“优先级”和“优先权”的参数。

主机发送时间信息到以太网。

此外，主机发送一个“活消息”给另一个板的地址。

监测器接收“活消息”，如果超过3秒还没有收到，从站承担主机的责任向以太网发送时间消息和“活消息”。

5. 结果
A. 硬件实现
在测试中实现网络时间显示，选择NEO-5Q GPS接收器，atmeaga128单片机，ENC28j60以太网控制器，45以太网插座，128 *64液晶显示器和触摸屏，和一个电源（周围建立的电压调节器）。

图8显示了根据结构图5建立的一个原型板。

B. 软件实现
为实现时间服务器的软件使用了“CodeVisionA VR”。

根据上述算法编写了C++程序，本程序编译并下载到单片机中。

图9显示了设计的由菜单显示的同步结果和由用户配置的数据。

C. 同步结果
为了测试系统板，通过一个HUB与安装了“Sun Virtual Box”虚拟机软件的计算机相连接。

其中一个是作为主机，另一个作为从机。

下一步，建立两个虚拟机，并且都装有Windows XP，所有三个互联。

通过操纵注册表键的综合时间同步服务（W32服务）让他们NTP时间同步。

所有计算机都成功同步到了时间服务器。

图10显示Wireshark软件的截屏（免费的网络协议分析仪），捕捉NTP数据包在这些电脑之间的传送，服务器的客户端请求和服务器答复溯源。

测试冗余功能，主机与网络断开，从机在3秒内成功地承担了主机的责任。

因为客户端每10秒接收NTP数据包，他们不觉得时间服务器更改了。

6. 结论
如今，准确的时间对于以计算机为基础的系统来说必不可少，在这方面，通常用一个时间同步系统从一个时间参考来获取的时间，同时通过时间同步协议为需要时间的系统提供精确的时间。

考虑到GPS作为一个参考时间源的巨大优势，也因为其具有高度的准确性和对NTP协议广泛的适用性，它常用在时间同步系统中。

在本文中探讨了一个典型的时间同步系统的设计和实现方法，时间服务器从一个低成本的GPS接收器上获取时间数据然后使用NTP协议给计算机网络同步时间。

同时提供更可靠的时间同步服务，建造了两个时间服务器主板，设计了一个冗余方案，并且在一个1oo2冗余结构中实现了方案。

参考文献
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分布式计算，6卷，pp.211- 219，1991
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[2] K. Behrendt 和K. Fodero。

”完美的时间：时间同步技术测试”。

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[3] Postel，J .时间协议。

美国网络工作组的报告RFC-868，南加州大学信息科学研究所，1983。

[4] Postel，J .白天协议。

美国网络工作组的报告RFC-867，南加州大学信息科学研究所，1983
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美国网络工作组的报告RFC-5905，
德拉瓦大学，2010年6月。

[6] Mills, D.L简单网络时间协议（种）4版本为IP v6和IP v4，开放系统互连，RFC 4430。

德拉瓦大学，2006 年1月。

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[8] D . L .Mills，计算机网络时间协议：在地球和空间运用的NTP。

第二版，出版社，2011。

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2011年10月6日访问。

1. GPS得到广泛应用，但仍存在不足。

GPS已成为全球性的高新技术产业，成为一种信息基础设施，正逐步进入人们的日常生活。

存在的不足是十分明显的，它是个军方控制的系统，不可能在任何时候任何地点保证民用；在闹市、密林，或有遮挡和环境恶劣的情况下，其可用性受到限制；单系统本身就存在着局限性，不可能确保任何应用。

2. GLONASS备受关注，却令人失望。

GLONASS一度给人们以很乐观的情景，现实又如此无情，经济和技术的双重原因，使它只能残缺不全地工作，今仅有不足十颗星工作，且很不稳定。

3. Galileo先声夺人，与GPS组合应用为总体思路。

Galileo计划高举的纯民用旗帜，给人一个放心和安全感。

而且它公开宣布与GPS兼容互动，更增强了其吸引力和竞争力，以及服务保证承诺。

丰富多彩的服务可满足不同档次和应用领域的需求。

4. 多系统组合并非多多益善，应用设备做到适可而止。

多年后，可能有多个卫星导航系统在天空运行，是不是多多益善，一是没有必要，二是应用接收机不能做得太复杂，不然会大幅增加成本，无法大批量推广，三是接收到的卫星数达到一定数目后，卫星信号再多也不会带来明显好处。

一般有双系统足矣。

5. 增强系统层出不穷，最终出路是多系统的组合。

现在各种各样的卫星导航增强系统层出不穷，仅以广域增强系统而言，就有美国的W AAS，欧洲的EGNOS，中国的CW AAS，日本的QZSS（还有一种为MSAS）。

一旦有两个完整的全球导航系统（GNSS）工作，在轨卫星达50-60个，就没有必要去建设那些广域增强系统了。

更多地是要走既省钱，又办事的国际合作之路。

Redundant GPS Time Synchronization Boards
for Computer Networks
Abstract--Nowadays, accurate and reliable time is an essential requirement in computer network based systems. To fulfill this necessity, time synchronization idea has been developed. Meanwhile in some cases, reliable time is so vital, therefore a redundant structure can be used. In this paper, a time synchronization system is investigated by designing and implementing a time synchronization board capable of synchronizing time of computer networks using NTP(Network Time Protocol). A redundant scheme is also embedded in this work to provide more reliability.
Keywords--Computer Network, GPS Time, NTP, Redundancy, Time Synchronization, Time Synchronization Protocols, Time Server.
I. INTRODUCTION
We may usually set our computer’s time by our wristwatch to within a minute or two, but on the other side, accurate and reliable time is necessary for financial and legal transactions, transportation, distribution systems, and many other applications involving widely distributed resources. To make sense, as an example, in a distributed airline reservation system, a seat can be sold twice or not at all if the distributed computers vary in time or there may be legal consequences when an online stock trade is completed, before it is bid [1]. In this regard, coordination to an international time scale and clock synchronization have been developed. The basis for the time scales has been refined throughout the history and sidereal time, earth rotation based time, and atomic time have been developed. Some important time scales include International Atomic Time scale (TAI), Coordinated Universal Time scale (UTC), and Standard Time or civil time.
Clock synchronization deals with the idea that internal clocks of computers may differ even if initially set accurately. After a while due to clock drift, there will be considerable clock errors, so there is always need for keeping these drifty clock synchrone to a time synchronization source as a reference clock.
Time synchronization sources include earth-based radio transmission (WWV, WWVH, WWVB, DCF77 and LORAN-C), satellite-based signal transmission (GOES, GPS, GLONASS, and Galileo), and time-setting messages via communications networks and telephone modem services [2]. Among these sources, Global Positioning System (GPS) offers some special advantages such as great time accuracy, noise immunity, worldwide availability for free, and continuously referenced to an international standard. Nowadays GPS based clocks are used very often as the time synchronization sources over the other clock recourses.
Fig. 1 shows a typical time synchronization structure, where a time server receives time data form GPS as a time synchronization sources. The time server then provides the time needing
devices with accurate time by sending them a massage in form of a synchronization protocol. The devices synchronize their internal clocks using the received massage.
The various synchronization protocols in use today provide different means to time synchronization, but they all follow two general models. Whether the client sends a request to the server and the server responds with its current time, or the server sends messages to a group or all of the clients. Some important standard computer network time synchronization protocols are as follows:
Time Protocol: Time protocol specified in RFC868 [3], returns a 32-bit unformatted binary number that represents the time in UTC seconds since January 1st, 1900. The server listens to time requests on port 37, and responds in either TCP/IP or UDP/IP formats.
Daytime Protocol: Daytime Protocol specified in RFC867 [4], sends time using ASCII characters. The server listens on port 13, and responds to requests in either TCP/IP or UDP/IP formats.
Network Time Protocol (NTP): NTP specified in RFC-5905 [5], is the oldest (and still in use) and most sophisticated time protocols for synchronizing computer clocks across a network. The NTP servers listen for a NTP request on port 123, and respond by sending a UDP/IP data packet in the NTP format.
Simple Network Time Protocol (SNTP): SNTP is a less complex implementation version of NTP protocol [6]. SNTP can be used when the ultimate performance of the full NTP implementation is not needed.
Precision Time Protocol (PTP): PTP formalized by IEEE 1588-2008 [7], is a protocol designed to increase timing accuracy over traditional Ethernet based protocols like NTP but it is more expensive in implementation.
For being open source, having sufficient accuracy for typical applications and the ability to work on large networks, NTP is the one widely in use on the public Internet and numerous private networks for over almost three decades.
The reliability requirements for time synchronization may be so strict that a single time server cannot always be trusted. Therefore, more time servers can be used in a redundant structure.
In this paper, we will investigate time synchronization system by design and implementation of a time synchronization board which gets time data from a low-cost GPS receiver and then synchronize time of computer networks using NTP protocol. In this design, redundancy is also embedded in a way that two time synchronization boards can be used together to provide more reliability.
The remainder of this document is organized as follows. Section II presents a brief explanation of NTP protocol and time synchronization using it. Redundancy gets more interpreted in section III. A structural scheme for building a network time synchronizer board, hardware design and software algorithms are proposed in section IV. Finally the implemented board and the results are presented in sections V.
II. WHAT IS NTP?
NTP has three major parts: the NTP software program, called a Daemon in UNIX and a Service in Windows; a protocol that exchanges time values between servers and clients; and a suite of algorithms that processes the time values to advance or retard the system clock [8]. Because NTP software is often bundled with the operating systems (such as most flavors of Windows and UNIX) it is the most common used protocol for computer network time synchronizations. For instant, we are not going to cover all the three parts but we are intending to describe the Protocol which is in need for designing a NTP time server. Further details can be found in the formal specification in [5].
The NTP is built on the Internet Protocol (IP) and User Datagram Protocol (UDP) in an IEEE 802.3 Ethernet frame demonstrated in Table 1. The NTP packet header, shown in Table 2, has 12 words followed by optional extension fields and an optional Message Authentication Code (MAC).
NTP working modes include client/server and broadcast mode. In client/server mode, the client sends a request to the server and the server responds with its current time. In broadcast mode however, time server periodically sends NTP packets to the clients and they use the packet to adjust their time.
The most important field in the NTP packet is the timestamp field. An NTP timestamp is a 64-bit unsigned fixed-point number, with the integer part in the first 32 bits showing the past seconds from 0h 1 January 1900 and the fraction part in the last 32 bits. The precision of this representation is about 2-32 second (233 picoseconds).
Accuracy of NTP synchronization depends on the network environment. Today in most places of the Internet, NTP provides time accurate to the order of 10-100 mSec while under good conditions on a LAN without too many routers synchronization to within a few milliseconds is normal [8].
III. WHAT IS REDUNDANCY?
Redundancy is a technique usually used in order to increase the reliability of a system. It comes in many forms and types. The most general form of redundancy is the Mout-of-N (MooN) redundancy, where at least M of N components must be functional in order the system functions. Some examples of this mode are shown in Fig. 2. Applications of M-out-of-N redundancy can be found in various applied areas, for example, in safety systems, N-version programming, etc [9].
Meanwhile, respecting the switch over time, redundancy can be classified as Parallel and Standby redundancy [10]. In parallel mode, redundant components are continuously operating and in standby mode they are only switched into an operating mode when a normally operating operation component fails. The essential difference between these two modes is illustrated in Fig.
3.
Whenever time synchronization in a system needs to be done with more reliability, redundancy technique can be used. Fig. 4 shows a 1oo3 redundancy structure in a time synchronization system. According to this figure, three time servers are getting time from time synchronization source (GPS) themselves and then act as time servers to feed a much larger group of clients connected
with a common network with an accurate and reliable time.
It worth mentioning that diverse redundancy can also increase time synchronization reliability. This is done by getting time from more than one type of time references. For example a time server gets time from GPS satellites and the other one gets it from DCF77 radio time source. IV. TIME SYNCHRONIZATION BOARD DESIGN
A. Hardware design
Here we present a scheme of a time server board which gets timing data from GPS and provides accurate time for the clients via NTP protocol. Fig. 5 presents the internal structure of the proposed time server board. It consists of five main parts; low-cost GPS receiver, Ethernet interface, Microcontroller, user interface and power supply.
In this structure the Microcontroller plays an important role in the proposed time server. It should be programmed properly to communicate with the other parts and make them work together as a time server. GPS receiver connects to the microcontroller via a serial interface. It provides microcontroller with timing data in form of NMEA protocol [11] and a 1PPS (one Pulse Per Second) signal which is synchronized to UTC. The Ethernet controller interface establishes a connection to the computer network and transmit/receive packets to/from it. The user interface consists of a LCD display and a touchscreen. It gets the needed configuration data from user and displays time synchronization results.
B. Software design
The designed algorithm for microcontroller in the time server consists of three sub algorithms, shown in Fig. 7; main loop, GPS 1PPS Interrupt Service Routine (ISR), and GPS serial interface ISR.
The main loop first initializes Ethernet controller, GPS receiver, and the user interface, then it enters an endless loop, waiting for interrupts from GPS time pulse and serial interface.
The 1PPS ISR is set to be called at the rising edge of time pulse. This ISR has the highest priority and lets the microcontroller knows the happening of UTC seconds with tens of nanosecond accuracy. In this routine a counter is set to send NTP packets every 10 seconds. The NTP working mode is Broadcast mode, so all of the clients connected to the network can receive the packets.
The GPS serial interface ISR runs every time a serial data is received from GPS reviver. If received data be a time data, it is decoded and stored in “Time” variable. As the NTP packet will be sent at the next 1PPS, its timestamps should point to 1 second in advance.
Redundancy is also embedded in the algorithm. As Fig. 6 illustrates, for this purpose one board is parameterized as “Priority Master” and the other as “Priority Slave”. The Master transmits the time message to the Ethernet. In addition, the Master transmits an “Alive message” to the address of the other board. The Slave monitors the reception of the “Alive message” and if it is not received for more than 3 seconds, the Slave assumes the duty of the Master and transmits both the time message and the “Alive message” to the Ethernet.
V. RESULTS
A. Hardware Implementation
For implementing the Network Time Display in practice, NEO-5Q GPS receiver, ATmeaga128 Microcontroller, ENC28j60 Ethernet Controller, RJ-45 Ethernet Socket, 128*64 graphical LCD with touch-screen, and a power supply (build around V oltage Regulators) were selected. Fig. 8 shows a prototype board built according to the structure in Fig. 5.
B. Software Implementation
In order to implement the software for time server, “CodeVisionA VR” was used. Some C++ codes were written according to the mentioned algorithm in Fig. 7 to build the whole program. This program is then compiled and transferred to the Microcontroller. Fig . 9 shows some of the designed menus for showing the synchronization results and configuring the board by user.
C. Synchronization Results
In order to test the boards, they were connected via a HUB to a computer with “Sun Virtual Box” [12] virtual machine software installed in it. Then one of them was set as priority master and the other as priority slave. Next, two virtual machines were made in Sun Virtual Box, Windows XP was installed in each and all three were networked together. By manipulating registry keys [13] of the integrated windows time synchronization service (w32time service) they we made to be synchronized from the NTP time. All computers successfully synchronized to the time server.
Fig. 10 shows screenshot of Wireshark software (a free network protocol analyzer [14]) capturing the NTP packets transmitted between three computers and the time server in which client requests and time server replies are traceable.
To test the redundancy feature, Priority Master Board was disconnected from the network, Priority Slave board successfully assumed the duty of the Master board within 3 seconds. Because the clients receive NTP packets every 10 seconds, they did not felt the time server change.
VI. CONCLUSION
Nowadays accurate time is an essential need in computer based systems.In this regard, usually a time synchronization system is used to get time from a time reference and then provide the time needing systems with precise time via a time synchronization protocol.
Regarding the significant GPS advantages as a reference time source and also because of great accuracy and worldwide popularity of NTP protocol they are commonly used in time synchronization systems.
In this paper a typical time synchronization system was investigated by design and implementation of a time server board which gets time data from a low-cost GPS receiver and then synchronizes time of computer networks using NTP. Meanwhile to provide more reliability in time synchronization service, two time server boards were built and a redundant scheme was devised and implemented to use them in a 1oo2 redundant structure.
REFERENCES
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[2] K. Behrendt and K. Fodero. “The perfect time: An exami nation of time synchronization
techniques”. Proceedings of the 33rd Annual Western Protective Relay Conference, Spokane, W A, October 2006.
[3] Postel, J. Time protocol. DARPA Network Working Group Report RFC-868, USC Information Sciences Institute, May 1983.
[4] Postel, J. Daytime protocol. DARPA Network Working Group Report RFC-867, USC Information Sciences Institute, May. 1983.
[5] Mills, D.L. Network Time Protocol (Version 4) -Protocol and Algorithms Specification. DARPA Network Working Group Report RFC-5905, University of Delaware, June 2010.
[6] Mills, D.L. Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI, RFC 4430. University of Delaware, Jan. 2006.
[7] IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, IEEE Standard 1588-2008, July. 2008.
[8] D. L. Mills, Computer Network Time Synchronization: the Network Time Protocol on Earth and in Space. Second Edition, CRC Press, 2011.
[9] R. Billinton and R. N. Allan, Reliability Evaluation of Engineering Systems. New York: Plenum, 1992.
[10] NMEA 0183 standard for interfacing marine electronics devices, National Marine Electronics Association, 1983.
[11] , VirtualBox, Available: /, accessed on Sept. 16.
[12] Microsoft Support, Registry entries for the W32Time service, Available: /kb/223184, accessed on Oct. 6, 2011.
[13] The world’s foremost network protocol analyzer, Wireshark-Go deep, Available: /, accessed on Oct. 6, 2011.
1. GPS obtains the wide-spread application, but it still had the insufficiency. GPS has become
a global high and new technology industry, which is one kind of information infrastructure and is entering people's daily life gradually. The existence insufficiency is very obvious, it is the system which controls by military, and it is impossible at any time any place guarantee civil; In the bustling streets, the jungle, or in the mask and the environment bad situation, its usability is restricted; The single system itself has the limitation, and it is impossible to guarantee any application.
2. GLONASS receives the attention, but it is actually disappointing. GLONASS once gave people the very optimistic scene, the reality is heartless because of the economical and the technical dual reasons, which enable it only works incompletely, now only has the insufficiency ten star work, and is very unstable.
3. Galileo forestalls opponent by a show of strength, with GPS combination application for overall mentality. The Galileo plan lifts up high the pure civil flag, feels relieved for the human with the security sense. Moreover its announce publicly and the GPS compatible interaction,
strengthened its attraction and the competitive power, as well as service guarantee pledge. The richly colorful service may satisfy the different scale and the application domain demand.
4. The multi-system combination is better and better, using the equipment is limd. Many years later, possibly has many satellite navigational system in the sky movement, is the better and better, one is not unnecessary; second, cannot do too using the receiver complex, will otherwise increase the cost largely, the being unable mass promotion; third, will receive after the satellite number will achieve certain number, the satellite signal many (will not bring the obvious advantage again. Generally has the pair of system foot.
5. The enhancement system emerges one after another incessantly, the final outlet is the multi-system's combination. Now various satellite navigation enhancement system emerges one after another incessantly, only says by the macrozonality enhancement system, has US's WAAS, Europe's EGNOS, China's CW AAS, Japan's QZSS (also has one kind is MSAS). Once has two complete whole world guidance system (GNSS) work, in-orbit satellite reaches 50-60, was not unnecessary to construct these macrozonalities to strengthen is systematic. More is must walk both economizes, and handles matters road of the international cooperation.。