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Time Synchronization in Packet Networks and Influence of Network Devices on Synchronization Accuracy

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Time Synchronization in Packet Networks and Influence of Network Devices on Synchronization Accuracy VOL. 1, NO. 3, SEPTEMBER 2010 Time Synchronization in Packet Networks and Influence of Network Devices on Synchronization Accuracy Michal Pravda 1, Pavel Lafata 1, Jiří Vodrážka 1 1Department of Telecommunication Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague Email: {pravdmic,lafatpav,vodrazka}@fel.cvut.cz Abstract – Time synchronization and its distribution still one of the most popular protocols used for computer time represent a serious problem in modern data networks, synchronization across Internet. SNTP [2] is a simplified especially in packet networks such as the Ethernet. This NTP with worse accuracy. Today, the standard IEEE 1588 article deals with a relatively new synchronization known as PTP (Precision Time Protocol) is increasingly protocol - the IEEE 1588 Precision Clock Synchronization wide-spreading. The first standard, which describes PTP, Protocol (PTP). This protocol reaches the typical accuracy was issued in 2002, but in 2008 a new revision of this better than 100 nanoseconds and uses hierarchical document was published [3]. The main difference between synchronization network infrastructure, which enables to NTP and PTP protocol is the method of their synchronize all devices connected into this network. The implementation. While NTP allows only software first part of the article describes the Precise Time implementation, PTP protocol allows also hardware Protocol, while the second part deals with synchronization implementation. Hardware implementation enables more accuracy measurements for various network topologies accurate determination of arrival and departure time of with different network devices. This paper contains the PTP packet, which is a significant improvement of the presentation of a laboratory synchronization network as synchronization accuracy [4]. well as it includes practical results and comments The NTP is very popular and widely used protocol concerning its functionality and performance. Several mostly in computer networks. The precision of NTP is in measurements were performed and the comparisons of the several milliseconds. It is possible to reach better accuracy accuracy of precision time distribution for different by the statistical methods and the filtering algorithms, but network topologies are presented in this article for various it is suitable only for LANs (Local Area Network) [5]. This network conditions and devices. The conclusion contains a method uses the filtering algorithm with IIR filter and summary of measurements, and it shows the resulting reaches the accuracy approximately dozens of accuracy of PTP synchronization. microseconds. The determination of precise time is provided by exchanging timestamps between the client and a synchronization server. The PTP uses the same principle 1 Introduction with several specific improvements. The PTP is the best accurate protocol today for packet- Time synchronization and time distribution are very based networks. Its accuracy is usually better than hundred important parts of the present telecommunication of nanoseconds. The main reason of the better accuracy is networks. The precise time synchronization is necessary in a hardware implementation and precise determination of for correct transmissions of high-speed digital signals, the timestamps on network interface. The main reducing their jitter, wander, slips and the bit error rate disadvantage comparing to the NTP is the necessity of during the transmission. More and more complex devices special network card, which determines the precise time of need to know the very precise time for their good work. packet arrival and which process timestamps. The PTP and Special synchronization networks and protocols are used to its accuracy is the main topic of this article. We decided to synchronize in backbone networks, e.g. SDH and OTH, build the laboratory workplace with demonstrational PTP but today it is also necessary to deliver the synchronization synchronization network in order to research the synchronization possibilities and accuracy of PTP protocol. into other type of networks, such as the Ethernet. The We also performed several measurements of the precision synchronization is very important for real-time synchronization accuracy and its dependency on the services, such as Voice over IP [9]. network topology and conditions. The results are presented In general, packet networks such as Ethernet are the in this article with several conclusions. fastest growing networks today. We encounter them everywhere and therefore should be used for time 2 IEEE 1588 - Precision Time Protocol synchronization. This paper focuses on clock synchronization and time distribution across packet network. Network Time Protocol (NTP), Simple Network The Precision Time Protocol (PTP) is a name of the Time Protocol (SNTP) and relatively new IEEE-1588 synchronization protocol which is described in the standard Precision Time Protocol (PTP) belong to the most popular IEEE 1588. This standard was published in 2002, but today synchronization protocols. Network Time Protocol [1] is there is the second version with small changes and improvements. 8 VOL. 1, NO. 3, SEPTEMBER 2010 The PTP protocol is designed to synchronize the clock o = t − t + d (1) in all devices, which are connected in the same network. 2 1 MS The PTP protocol is based on the system of transmission of PTP messages between the synchronized clocks. For the From the Delay_Req and the Delay_Resp messages are determined the values of t and t . We can calculate the proper operation of the system, it is important to determine 3 4 clock offset from t and t . the boundary and ordinary clock and to establish a master- 3 4 slave synchronization hierarchy. PTP version 2 defines o = t4 − t3 + d SM (2) transparent clock in addition [6]. An example of a simple network system is shown in the Fig. 1 and it contains the Considering the assumption that the delay of packets two boundary and two ordinary clocks. One master clock will be the same in both directions ( dMS = d SM ), we can is always chosen for a single network segment, depending calculate them using the equation (3) [7]. on the accuracy of each device on each network segment. (t2 − t1 )+ (t4 − t3 ) This algorithm is called the Best Master Clock (BMC). d = (3) Detailed description of the topology design is given in the 2 chapter 3.3 [7]. The clock offset is then equal: (t − t )− (t − t ) o = 2 1 4 3 (4) 2 It is clear that the accuracy of timestamps has fundamental impact on the synchronization accuracy. Therefore, network devices allow the accurate determination of the packet arrival time. The specific network topology is presented in the chapter 3. Fig. 1. The network topology of a simple system. 3 Laboratory Network with IEEE 1588 protocol The accurate time is determined by calculating the delay of the transmission through the packet network. In this schematic process, the time server starts the 3.1 The Description of Equipments communication by sending the multicast packets (Sync and The IEEE 1588 synchronization protocol usually Follow up). These packets are received by the network operates above a hierarchically organized network. This card and are used for its synchronization. The means that there is typically one grand master-clock synchronization process is described in the Fig. 2. synchronization server in the network and all other network devices work in the synchronous-locked mode as Master Clock Time Slave Clock Time the slave clocks. As it was described in the previous section, the mechanism for the selection of the master- t1 Sync message clock device is already implemented in the IEEE 1588 t2m t2 protocol. For that reason, our solution of the Follow_Up message demonstrational laboratory network contains one synchronization server with the GPS receiver, one Ethernet switch and one computer network card with the IEEE 1588 protocol’s support and several standard Ethernet devices t3m t3 (routers, switches, hubs and network cards) that do not Delay_Req message support this synchronization protocol. t4 The LANTIME (Local Area Network Timeserver) Delay_Resp message M600/GPS server from the Meinberg - Radio Clocks co. was selected as the primary synchronization server for our laboratory network. This server provides a high precision timing signal via Ethernet (TCP/IP) network. The server is of a modular conception and thus can be equipped with the Master time Slave time various modules and interfaces. Typical configuration, which we also used in our network, contains the internal Fig. 2: The PTP synchronization process [7]. oscillator of OCXO LQ type, the 6 channels GPS C/A- code receiver, a single-board computer SBC LX800 500 The Sync message is received at the time t2. The MHz with integrated network card, and a power supply Follow_Up message contains the value of t1. When the unit. The server in our configuration also consists of four d packet delay will be known, it is possible to calculate the standard 100Base-T Ethernet interfaces, which support the clock offset o (1) of the Slave clock. 9 VOL. 1, NO. 3, SEPTEMBER 2010 NTP and SNTP protocols, one 100Base-T Ethernet 1.10 -11 . All these characteristics are temperature-dependent interface with the IEEE 1588 protocol’s support, several and their more detailed descriptions can be found in the BNC connectors with various frequency outputs, RS232 documentation [8]. console input/output, GPS antenna input and power input. The laboratory network further consists of a modular The server can be configured by the console commands switch from the Hirschmann co., which supports the IEEE using the RS232 port, by the Telnet protocol and by the 1588 protocol. The switch is of a modular industrial simple WWW environment using standard web browser Ethernet type and the configuration used in our network and the HTTP protocol. The front panel also contains consists of one module with the 4 Fast-Ethernet ports and several buttons and a small graphic display (vacuum the USB and V.24 management interfaces. The switch can be configured by the HTTP protocol and a web-browser or fluorescent graphic display - VFD) for the diagnostic by the SNMP protocol and it supports the synchronization purpose.
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