Multicast Time Distribution

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Multicast Time Distribution

Master’s Thesis performed for

SP Swedish National Testing and Research Institute by

EROLD PERSSON

Information Networks Division Reg nr: LiTH-ISY-EX-3510-2004

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Multicast Time Distribution

Master’s Thesis performed for

SP Swedish National Testing and Research Institute by

EROLD PERSSON

Information Networks Division Department of Electrical Engineering

Link¨oping University Reg nr: LiTH-ISY-EX-3510-2004

Supervisor: Kenneth Jaldehag, SP

Carsten Rieck, SP

Examiner: Robert Forchheimer, ISY Link¨oping 19th March 2004.

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Avdelning, Institution Division, Department

Institutionen för systemteknik

581 83 LINKÖPING

Datum Date 2004-03-16 Språk

Language Rapporttyp Report category ISBN Svenska/Swedish

X Engelska/English Licentiatavhandling X Examensarbete ISRN LITH-ISY-EX-3510-2004

C-uppsats

D-uppsats Serietitel och serienummer _{Title of series, numbering} ISSN Övrig rapport

____

URL för elektronisk version

http://www.ep.liu.se/exjobb/isy/2004/3510/

Titel

Title Tidsdistribution i multicast-mod Multicast Time Distribution

Författare

Author Erold Persson

Sammanfattning

Abstract

The Swedish National Testing and Research Institute is maintaining the Swedish realization of the world time scale UTC, called UTC(SP). One area of research and development for The Swedish National Laboratory of Time and Frequency is time synchronization and how UTC(SP) can be distributed in Sweden. Dissemination of time information by SP is in Sweden mainly performed via Internet using the Network Time Protocol (NTP) as well as via a modem dial up service and a speaking clock (Fr¨oken Ur). In addition to these services, time information from the Global Positioning System (GPS) and from the long-wave transmitter DCF77 in Germany, is also available in Sweden.

This master’s thesis considers how different available commercial communication systems could be used for multicast time distribution. DECT, Bluetooth, Mobile Telecommunication and Radio Broadcasting are di_erent techniques that are investigated. One application of Radio Broadcasting, DARC, was found to be interesting for a more detailed study. A theoretical description of how DARC could be used for national time distribution is accomplished and a practical

implementation of a test system is developed to evaluate the possibilities to use DARC for multicast time distribution.

The tests of DARC and the radio broadcast system showed that these could be interesting

techniques to distribute time with an accuracy of a couple of milliseconds. This quality level is not obtained today but would be possible with some alterations of the system.

Nyckelord

Keyword

time distribution, multicast, DECT, UMTS, Bluetooth, RDS, DARC, DAB, UTC(SP), accuracy, stability

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Abstract

The Swedish National Testing and Research Institute is maintaining the Swedish realization of the world time scale UTC, called UTC(SP). One area of research and development for The Swedish National Laboratory of Time and Frequency is time synchronization and how UTC(SP) can be distributed in Sweden. Dissemina-tion of time informaDissemina-tion by SP is in Sweden mainly performed via Internet using the Network Time Protocol (NTP) as well as via a modem dial up service and a speaking clock (Fr¨oken Ur). In addition to these services, time information from the Global Positioning System (GPS) and from the long-wave transmitter DCF77 in Germany, is also available in Sweden.

This master’s thesis considers how different available commercial communica-tion systems could be used for multicast time distribucommunica-tion. DECT, Bluetooth, Mobile Telecommunication and Radio Broadcasting are different techniques that are investigated. One application of Radio Broadcasting, DARC, was found to be interesting for a more detailed study. A theoretical description of how DARC could be used for national time distribution is accomplished and a practical implemen-tation of a test system is developed to evaluate the possibilities to use DARC for multicast time distribution.

The tests of DARC and the radio broadcast system showed that these could be interesting techniques to distribute time with an accuracy of a couple of millisec-onds. This quality level is not obtained today but would be possible with some alterations of the system.

Keywords: time distribution, multicast, DECT, UMTS, Bluetooth, RDS, DARC, DAB, UTC(SP), accuracy, stability

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Preface

This report is written as the master’s thesis for the degree in Master of Science in Applied Physics and Electrical Engineering at Link¨oping University.

The work has been performed at the Swedish National Testing and Research Institute, SP, in Bor˚as.

There are a number of persons that have been valuable for this work. First, I want to express my gratitude to the supervisors at SP, Kenneth Jaldehag and Carsten Rieck, for their support of the work and their comments of the report. Second, the remainder staff at the section of Electricity and Time, deserves to be mentioned for the kind reception of me that made me feel welcome at SP. Finally, Charlotte who is very important to me, has given me great mental support and love as well as assistance with the proofreading of this report.

Link¨oping, March 2004 Erold Persson

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Abbreviations

BIMP International Bureau of Weights and Measures in France BIC Block Identification Code

BMCh Block Message Channel CAP CTM Access Profile CBS Cell Broadcast Channel CDMA Code Division Multiple Access CPICH Common Pilot Channel

CT Clock-Time and date information CTM Cordless Terminal Mobility DAB Digital Audio Broadcasting DARC Data Radio Channel

DECT Digital Enhanced Cordless Telecommunications ETSI European Telecommunications Standard Institute FDD Frequency Division Duplex

FDM Frequency Division Multiplex FDMA Frequency Division Multiple Access FIC Fast Information Channel

GAP Generic Access Profile GPRS General Packet Radio Service GPS Global Positioning System GPS CV GPS Common View

GSM Global System for Mobile Communications IPv4 Internet Protocol ver 4

IPv6 Internet Protocol ver 6 LMCh Long Message Channel

LMSK Level-controlled Minimum Shift Keying MCI Multiplex Configuration Information MJD Modified Julian Date

MSC Main Service Channel

NITZ Network Information Time Zone NTP Network Time Protocol

NWS NetWork Server

OFDM Orthogonal Frequency Division Multiplex PSCH Physical Synchronization Channel

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PTB Physikalisch-Technische Bundesanstalt RDS Radio Data System

SCH Synchronization Channel SeCh Service Channel

SI Service Information SMCh Short Message Channel SOS Swift Over Serial

SP Swedish National Testing and Research Institute TAF Time Accuracy Field

TAI International Atomic Time scale

TDPNT Time, Date, Position and Network name Table TDT Time and Date Table

TDD Time Division Duplex TDM Time Division Multiplex TDMA Time Division Multiple Access TM Transmission Mode

TSE Transmitting Station Equipment

USART Universal Synchronous Asynchronous Receiver Transmitter UT1 Universal Time

UTC Coordinated Universal Time VHF Very High Frequency

WCDMA Wideband Code Division Multiple Access

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Introduction

1.1 Background

Time plays in different ways an important role for everyone. For everyday use time is for instance needed to decide meetings between people. For scientists time can be of academic interest in the daily work. Depending of the different usage, various quality of time is needed and expected.

The Swedish National Testing and Research Institute, SP, is responsible for the national time scale UTC(SP). One of the commissions for the Swedish National Laboratory for Time and Frequency at SP is to offer and develop services for time synchronization traceable to UTC(SP).

Time traceable to UTC(SP) is today distributed by SP via modems connected to the telecommunication network, via Internet and via a speaking clock that can be called up. Besides these synchronization sources maintained by SP, international time sources like GPS and DCF77 are used to meet the need of time synchronization in Sweden.

There are in principle two ways to distribute information in general, point-to-point or multicast. A point-to-point-to-point-to-point connection is established between one trans-mitter and one single receiver. The information flow is in most cases bidirectional. Multicast communication involves one transmitter and multiple receivers listening to the same information channel.

The purpose of this master’s thesis is to investigate weather different available commercial communication methods can be used for time distribution in multicast.

1.2 Project specification

As a first part of the work different commercial communication methods are in-vestigated. The systems are described from the perspective of time distribution. Advantages and drawbacks of the different systems are described.

One of the systems studied is chosen for a more detailed investigation of the 1

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2 Introduction

possibilities to use it for time distribution. A detailed description is given and a practical implementation is performed to evaluate how well the theoretical assump-tions correspond to the practical reality.

1.3 Thesis outline

In Chapter 2 different properties of time in general are discussed. Expressions like accuracy, stability and time scales are explained. Chapter 3 presents general information distribution methods and examples of time distribution systems used today are mentioned. In Chapter 4 different possible communication methods for multicast time distribution are investigated. An overview is given and advantages respectively drawbacks with each system are presented. One of the systems de-scribed in Chapter 4 is selected and studied in more detail in Chapter 5. Practical implementations are developed and the achieved results of the test measurements are presented. In Chapter 6 some conclusions sum up the work and recommenda-tions for future work is given.

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Chapter 2

Time

2.1 What is time?

Ever since the beginning of everything time is proceeding unchangeably. The hu-man has always tried to understand or explain time and scientists like Newton and Einstein have spent years of studying this phenomenon. They, as well as scientists today, often ended up in a philosophical thinking where it is hard to discern right and wrong.

Despite these difficulties of the subject, time plays an important role in the practical life for all of us. It is from this point of view time is considered in this report.

2.2 Epoch and time interval

When considering the concept of time there are mainly two different ways to see it depending on the field of applications. When an event is decided to happen at a special point of time we are talking about the epoch or date of this special event, when something is going to take place. Another way is to consider time only as a time interval which does not care about when, but for how long something takes place.

Often, when time is used, it is not only used by one isolated part independent of everything else. Instead time is often used as a tool to time things together. In this case synchronization between the different parts is very important regardless of if we are talking about epochs or time intervals.

2.3 Sources of time

There are a large number of cycles and rhythms going on all around us, even inside us, having the regularity that they can be used as sources of or references for time. The most natural of these, and even maybe the most used historically,

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4 Time

is the rotation of the earth, both relative to its own axis and relative to the sun. Both the variations between day and night and between different times of the year originate from these rotations and historically this is the main source for the human consciousness of time. Soon one realized that it was possible to count different hallmarks of these variations to know the current time. The sundial was developed.

In order to know the time also when the sun does not shine or during the night different mechanized devices to interpolate time between checks with the sun have been developed. These devices could be flowing water or sand adjusted to the cycles of the sun. Later, counting swings of pendulums became a useful and important measurement of time. Hereby the early resonator was developed and resonators in various forms are the main sources of time in our days.

2.3.1 Resonators

Many different resonators are used for time keeping purposes. All of them swing with their own resonant frequency and with various stability and accuracy. The cheapest and most used resonator, for example in wrist-watches and electronic devices, is the quartz crystal. It is based on the piezoelectric effect. A small piece of the crystal vibrates when an alternating electric voltage is applied to it. Once it is made to vibrate the crystal will continue to generate an oscillatory voltage with its own resonate frequency. The applied voltage together with the crystal constitute a feedback system that is regulated by the resonant frequency of the crystal.

Another group of resonators are the atomic resonators. The atomic resonators use the fact that a certain atom has so called resonant frequencies. A resonant frequency is derived from movements of the electron between the different possi-ble energy levels an electron can occupy. When an atom is exposed to radiation of a particular frequency, of which the energy corresponds to the energy gap be-tween two energy levels of the atom, the electrons change energy level by absorbing the radiated energy. For every atom there is at least one frequency of radiation which gives a peak of electrons changing their energy level and this is the resonant frequency for that particular atom.

A few atoms are better than others to use for time keeping purposes. Today clocks derived from rubidium, cesium and hydrogen are the most used. The rubid-ium oscillator is the cheapest one of these while the hydrogen maser is the most advanced and thereby the most expensive. The cesium clock is aligned in between and is used by national and international time keeping laboratories because of its high accuracy and long term stability. The definition of the SI-unit of time, one second, is since 1967 based on the cesium atom as well. The unperturbed resonant frequency of a Cesium-133 atom is defined as 9 192 631 770 Hz. The duration of one second is stated as the duration of the defined number of cycles of an elec-tromagnetic signal, of which the energy corresponds to the special energy gap of cesium-133 [14].

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2.4 Time scales and traceability 5

2.3.2 Q, Accuracy and Stability

When an ideal resonator gets an initial push, it will run forever. Of course this is not the case in reality because of friction. The Q-factor is a measure of how many swings a resonator makes until its energy from the initial push is consumed. A mechanical clock have a Q of about 100 whereas an atomic clock have millions in Q-value.

Two very important quality measurements in the field of time, besides Q, are accuracy and stability. The difference between these two expressions will be ex-plained. The stability expresses the precision of something momently compared in two different points of time, independent of each other. An ideally stable resonator will give us exactly the same resonant frequency now as in any later moment cho-sen randomly[14]. The accuracy means how the average frequency of the resonator agree with the stated frequency[14]. For an accurate clock, the length of single seconds may vary during a period of time but the average length of a second and thereby the total length of the time period agrees with the definition of time. A res-onator can be stable without being accurate if there is a constant but non-changing error of the frequency.

There are differences between different resonators in terms of stability and ac-curacy. The cesium clock is very accurate in the long run and the definition of the atomic second is based on the cesium atom. Besides this the cesium clock is not that stable in comparison with for example a hydrogen maser in the short term. The maser has a natural vibration of 1 420 405 752 Hz and is not very accurate with the definition of one second, but its frequency is very stable for short time periods of less than a day[14].

2.4 Time scales and traceability

There are a lot of different time scales around the world maintained by national time laboratories like SP in Sweden. The users of these different time scales want to be sure of the fact that the time they are using is accurate and reliable. To guarantee this accuracy the national time is made traceable to one international standard which is the same worldwide. The national time scale of Sweden UTC(SP) is traceable to UTC and thereby an accurate time source [24].

2.4.1 International time keeping

The Coordinated Universal Time, UTC, is the international time scale for world distribution of time and is a combination of the International Atomic Time scale, TAI, and Universal Time, UT1.

UT1 is a dynamic time scale derived from the rotation of the Earth and is pro-portional to the angle of the rotation of the Earth. The scale is dynamic depending on different variations of the earth’s rotation and that implies that the scale follows the rise and the set of the sun. UT1 is maintained by IERS (International Earth Rotation Service)

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6 Time

TAI is an atomic time scale. It is calculated by BIPM (International Bureau of Weights and Measures), located in France outside Paris, as a weighted average of data from 230 atomic clocks located in about 60 national laboratories worldwide. By definition UTC has the same metrological properties as TAI, but to follow the rotation of the earth UTC is corrected with so called leap seconds. The correction assures that the difference between UTC and UT1 is always less than 0.9 seconds and the difference between UTC and TAI is always a integer number of seconds.

UTC and TAI are virtual time scale on world basis and they are realized on a national basis, for example through UTC(SP) in Sweden maintained by SP.

2.4.2 National time keeping

The Swedish standard time is derived from UTC and realized as the national time scale UTC(SP) by SP. The base for UTC(SP) is made up by atomic clocks, cesium clocks and hydrogen masers, continuously compared with the international time scale UTC and TAI. The comparison is established via BIPM and is obtained by measurements against satellites in the GPS-system (GPS Common-View see 3.2.2) and the Two-Way Satellite Time and Frequency Transfer (TWSTFT). The Swedish time scale UTC(SP) is also one of the national time scales all over the world that are the base for UTC.

2.5 Why accurate time?

The need for accurate time is increasing in conjunction with the general technical development today. The different techniques available for fast communication and precise navigation need high timing precision and they push the development for-ward by their demands at the same time as available accurate timing possibilities push the technical development. A result of this development is that people in general expect that clocks around them give accurate time as well. The need for synchronized and accurate public clocks introduce demands also on cheaper time distribution methods with less accuracy than the expensive and technical advanced systems with very high accuracy.

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Chapter 3

Time Distribution

3.1 Distribution techniques

There are different techniques to distribute information in general. One major dif-ference is how the communication is established between the transmitter and the receiver. Expressions that are used in these contexts are unicast, anycast, many-cast, multimany-cast, broadcast and point-to-point. The meaning of these expressions will be explained in the next sections. Unicast, anycast and multicast are defined distribution methods in the Internet Protocols (IPv4 and IPv6) while manycast, broadcast and point-to-point are expressions used in different other communication situations and their meaning may vary as well.

3.1.1 Unicast

Most of the connections in traditional communication networks are unicast or point-to-point connections. A unicast connection is established between one transmitter and one receiver. The information flow may be only in one direction but in most cases it is bidirectional. Example of unicast communication is a connection between two computers on the Internet or the connection between the base station and the mobile terminal during a call in a mobile system. An unicast system may contain several point-to-point connections between one transmitter and a number of receivers where the information flowing in every connection is identical. This is still unicast communication due to the fact that the information is sent on multiple channels.

3.1.2 Anycast and manycast

Anycast defines communication with a receiver that is the nearest in a group of sev-eral possible receivers offering the same service. In IPv6 this is used as a technique for chain-updating a group of routers with new routing information. A variant of anycast is manycast. In manycast, the one to communicate within a group of

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8 Time Distribution

devices offering the same service, is chosen by a set of optional criteria. The chosen part does not have to be the one that is the nearest in a topological point of view.

3.1.3 Multicast and broadcast

In multicast and broadcast communication identical data is distributed to sev-eral receivers without sending the data more than once. The difference between broadcast and multicast is often explained differently depending on the application. One difference is how the receiver group is specified. A multicast group is defined by a given number of receivers where each receiver is addressed in the group. A broadcast group may instead include all the members in a whole network.

Broadcast is also an expression for the transmissions of public radio and tele-vision. Here, the meaning of broadcast is that everyone with the appropriate equipment can receive the broadcasted information.

The information flow in broadcast and multicast communication is single di-rected, from transmitter to receiver. For time distribution the broadcast and mul-ticast techniques are interesting because of the possibilities to reach many receivers with the same information with relatively simple methods. When the information is transmitted the transmitter does not have to care about who is receiving. A sig-nificant drawback of this is that the possibilities to correct for varying transmission delays in the channel are very small due to the single direction communication. This increases the demands on the stability of the communication channel significantly.

3.2 Multicast methods available today

The methods for time distribution are under constant development. The multicast distribution technique is generally very interesting for time distribution. It gives possibilities to transmit time information to a large group of users with limited resources. There are no restrictions of how many users that are allowed to receive a multicast transmission. A number of existing multicast systems offer possibilities for time synchronization.

Measurements on the synchronization in the television signal was earlier a fre-quently used time source. The measurements were performed first and then cor-rections were made manually to get the time comparison.

The radio navigation system Loran C can as well be used for time synchroniza-tion purposes. The Loran C transmitters are located along the coasts. No regular time code is transmitted but the frequency information in the system are used by many receivers to output synchronized second pulses.

GPS and DCF77 are the most used systems in Europe for time synchronization nowadays. Both of these systems can be regarded as broadcast systems. For time synchronizing via Internet the Network Time Protocol (NTP) is most used. This is primary a unicast system but has modes for manycast and multicast as well. NTP is not further treated in this report.

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3.2 Multicast methods available today 9

3.2.1 DCF77

DCF77 is a time distribution system with a transmitter located in Mainflingen, 25 km south-east of Frankfurt am Main, in Germany. It transmits time and frequency information on a carrier frequency of 77.5 kHz.

The carrier signal is amplitude modulated with second marks. At the beginning of each second the carrier amplitude is reduced to 25 percent for a duration of either 0.1 or 0.2 seconds. These different pulses correspond to binary high or low and are used to transmit a time code. The values for minute, hour, day, weekday, month and year are BCD-coded and the whole time code is sent during a period of one minute. The time code is based on the German national time scale UTC(PTB) provided by Physikalisch-Technische Bundesanstalt (PTB) [13].

The DCF77 is used for synchronization of real time clocks as well as frequency reference with relatively high demands of stability. The system covers the southern part of Sweden and can here be used as time reference with an accuracy of a couple of microseconds with a high quality time receiver. A major drawback for Sweden is the long distance between the transmitter and the receiver. The signal might be very weak and it is often difficult to receive it, particularly indoors.

3.2.2 GPS

The Global Positioning System (GPS) is today the globally most used for time and frequency dissemination. The system is primary a navigation and positioning system but it also offers time and frequency information with high accuracy and stability. It consists of more than 24 satellites in orbits around the earth. Every satellite is equipped with atomic clocks that are accurately monitored against UTC and the GPS time is an average time calculated of all satellite clocks and a number of additional clocks located on the earth. The GPS-time is realized through the GPS atomic clocks and related to UTC. UTC(GPS), received with a GPS receiver, reaches an accuracy of less than 100 ns relative to UTC depending on the type of the receiver[24].

There are GPS receivers that are relatively cheap but the requirements of the antenna are still high. To get a high time accuracy, the position of the antenna has to be known and the GPS always requires free sight to the satellites as well.

As an alternative to use the GPS signal directly for time synchronization, GPS Common-View (GPS CV) can be used to compare two clocks at different locations. With this method observations of the GPS satellites are performed at the different locations during the same instant of time. The observations are then compared and the time difference between the clocks is obtained[24].

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Chapter 4

Possible multicast systems

This chapter presents a couple of existing commercial communication systems that can possibly be used for multicast time distribution. A brief description of every system is made and advantages as well as disadvantages are presented.

4.1 DECT

A DECT network, used for voice communication, covers the whole of the office area at SP. The network, together with the switchboard, is supplied by Ericsson. What are the possibilities to use the existing DECT network at SP for time distribution?

4.1.1 The DECT System

DECT, Digital Enhanced Cordless Telecommunications, is a standard specified by the European Telecommunications Standards Institute, ETSI, for cordless tele-phones. The DECT system is mostly used for indoor telecommunications as an wireless alternative to the traditional wired telephone but with the mobile advan-tages from the cellular network. The network comprises base stations and mobile terminals and the coverage varies from one private house with one base station to large office complexes with thousands of users. Every single network is usually connected to the Public Phone station.

The Physical Layer of DECT is based on both TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access). One channel consists of a time slot and a frequency slot which are dynamically selected by the base stations. This gives a very flexible system for interference free communication. There are 10 carrier frequencies between 1880 and 1900 MHz available. One frame of 10 ms contains 24 time slots and full duplex (bidirectional communication) provided by Time Division Duplex results in 12 parallel channels on each frequency. The total data rate is given to 1152 kbit/s at each frequency[20].

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12 Possible multicast systems

4.1.2 Time distribution via DECT

There are roughly two different ways to use a DECT system for time distribution purposes. One possible method could be to use some type of already implemented time information in the system. Another way would be to use DECT as a data channel and transmit a time code via some data interface supported by the system. The DECT-standard released by ETSI [6, 7] does not mention anything about system time or clock time that can be used for this purpose, however different manufactures have made time information available via DECT for their own mobile terminals. Ericsson distributes the current system time of the switchboard to the terminal every time the terminal is powered on and connected to the system. The time information is not broadcasted by the base station and to get access to this time information one must in principle connect with a device that is subscribed in the network. This implies that there is no possibility to snap up the current time by only listening to the DECT network.

The other possibility is to distribute time as a special payload with some kind of modem solution designed for DECT systems. The Base Stations from Ericsson comply with two different profiles for communication in DECT, GAP (Generic Ac-cess Profile) and CAP (CTM (Cordless Terminal Mobility) AcAc-cess Profile), both specified in the standard by ETSI. These profiles do not support any kind of plain data communication. There are different data profiles specified in the DECT stan-dard but they can not be used without being supported by the system network.

4.2 Mobile telecommunication

Mobile telecommunication systems offer possibilities of data and voice communi-cation with high national coverage through different types of networks. The early digital systems, sometimes called the second generation systems, are today in prin-ciple finalized. The third generation systems are under construction and within these the first operational network in Sweden started offering service during 2003. What are the possibilities to use these systems for distribution of time in multicast and what are the limitations?

There are many different technical network solutions for cellular systems in different parts of the world. For the digital second generation systems, GSM is dominating in Europe and is partly spread in the rest of the world too. IS-95 CDMA is another second generation system. It is used in North America and is interesting in connection with time distribution. For the third generation systems there are two competing standards, WCDMA mostly used in Europe and CDMA2000 which is a further development of IS-95 CDMA.

CDMA, Code Division Multiple Access, is an access method that uses different spreading codes to distinguish the different channels from each other. It is used instead of FDMA and TDMA discussed in 4.1.1.

This overview concentrates on WCDMA, which is the future dominating system in Europe, and one application of IS-95 CDMA is mentioned. The first issue to con-sider is at what level in the system the time information might be distributed. Is it

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4.2 Mobile telecommunication 13

possible to send time information as a special payload on any available multicast or broadcast data channel or is it possible to use the synchronization message broad-casted by the base station for time distribution purposes? This will be discussed in the following two sections.

4.2.1 Multicast data services available

From a general point of view, there are problems with the prerequisites for time transmissions at higher levels. The delay of the channel may be difficult to estimate and a possible instability of this delay affects the accuracy. The most known mod-ern service for data transfer is the General Radio Packet Service, GPRS. GPRS is available in GSM as well as WCDMA and offers various solutions for data commu-nication. GPRS is not a multicast service though and the communication is based on point to point connections. The quality of the service does not comply with the requirements for time transmission either because the capacity available for GPRS in the network may change dynamically. With this capacity change the delay of the channel may change as well.

There are different multicast/broadcast services defined for multimedia pur-poses in the specification of WCDMA but the real time properties are often not specified. In cases where they are specified, they do not fulfill the requirements for the time transmission purposes. The reason is often that multimedia does not require these type of high qualities of the transmission channel.

There is also a low data rate broadcast channel, Cell Broadcast Channel (CBS), specified. This is aimed for SMS-like messages that can be broadcasted over a ge-ographical limited area. There is no information given about real time or delay properties of this channel either but the parallel with SMS makes it not very in-teresting for real time applications.

4.2.2 Broadcast synchronization channels

The communication between the base station and the mobile terminal on lower level is divided in different physical channels for downlink and uplink. Among the downlink physical channels especially one group of channels are of interest, the synchronization channels. They are used when the terminal synchronizes with the base station. These channels are broadcasted over the entire cell from the base station.

There are two different modes of WCDMA characterized by the duplex method, Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In FDD the uplink and downlink transmissions are separated in different frequency bands and in TDD the uplink and downlink transmissions are multiplexed on the same carrier. The different duplex methods involve a different structure of the synchronization solution. WCDMA FDD constitutes today the generally so called third generation cellular network in Sweden and Europe.

In FDD mode the base station works asynchronous and there is no need for base stations to be synchronized between themselves. For synchronization between base

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station and terminal two channels are used, the Synchronization Channel (SCH) and the Common Pilot Channel (CPICH). There are a primary and a secondary SCH and they are used for slot and frame synchronization. The terminal must be able to synchronize to the cell before knowing the downlink scrambling code and therefore the SCH is not under cell specific scrambling. After the synchronization the CPICH is used to determine the exact primary scrambling code for the cell. CPICH is also phase reference to other downlink channels and aid the channel estimation at the terminal for the downlink channels[15].

For TDD-CDMA, base station synchronization is desirable because of intercell and interoperator interference. The synchronization accuracy must be only on symbol level, not chip level. It can be achieved with GPS receivers at the base station or a common clock signal with extra cabling. The synchronization between terminal and base station as well as the code group of a cell for TDD-CDMA, are achieved via the Physical Synchronization Channel (PSCH)

Independent of whether the base stations are synchronized between themselves or not, none of WCDMA-FDD or WCDMA-TDD offers any time specific informa-tion in broadcasted synchronizainforma-tion channels besides the chip-rate and frequency of the channel. There are no information in the base station broadcast messages that can be used for time distribution without relating to something else.

IS-95 CDMA, adopted as a second generation digital cellular standard in North America, uses GPS for synchronization and can be used for UTC time synchro-nization as well with GPS as a base. This is briefly described below.

Indirect GPS via IS-95 CDMA

In IS-95 CDMA the base stations are separated by spread-spectrum modulation and every station uses the same Pseudo-Noise code but each one with a delay of 64 chips of the code, equate to 52.0833... microseconds. With this delay in the code a maximum timing error of 10 microsecond is allowed in order to maintain an adequate separation. To obtain these timing requirements in the system the IS-95 standard defines the system time to the same time as the GPS time, which requires the base stations to be synchronized to GPS time within 10 microseconds. To maintain this synchronization every base station has to be equipped with a GPS receiver.

The GPS time information is contained in the synchronization channel broad-casted from the base station to the terminal. By receiving and demodulating this synchronization message and the pilot message broadcasted by the base station, UTC(GPS) can be received with an accuracy 10 microseconds[16]. This possibil-ity also applies to the third generation system, CDMA 2000, which uses the same technique for synchronization.

4.2.3 NITZ

Network Information Time Zone, NITZ, is a service message that is optional in both GSM and WCDMA networks[9, 8]. The main purpose of the service is to

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4.3 Radio Broadcasting 15

provide mobile terminals with information of current time zone during roaming between different networks. Some manufactures, e.g. Nokia Mobile Phones, use this service to continuously synchronize the real time clock in the mobile terminal. Nokia also offers GSM-modules with this feature embedded. In the specification of NITZ the accuracy of the service is given to a couple of minutes. The NITZ message is not broadcasted but available when the terminal is connected to the base station.

A contact was taken with Telia to discuss NITZ and its accuracy. Telia has the service implemented in their network but they do not give any guarantees for the time accuracy at the time of this report. They were very interested in why and how we want to use the time and they did not sound unfamiliar to specify it for a special usage.

4.3 Radio Broadcasting

Today there are possibilities to send digital data via the existing infrastructure for public analog radio broadcasting. The information is sent via a subcarrier which is added to the original stereo multiplex signal. There are two different channels, Radio Data System (RDS) and Data Radio Channel (DARC). Both of these could be possible transmission channels for time distribution.

In the near future Digital Audio Broadcasting, DAB, is probably going to break through for public radio. This digital radio system will give us highly increased data rates for broadcasted radio and DAB also offers transmission of other digital data than audio.

4.3.1 RDS

RDS, specified in[12], is a low data rate channel used as an information channel by transmitters and radio stations where different information such as program name and program type are transmitted. The information gives the listener increased functional service such that mobile receivers automatically can hold a particular radio station and change to the frequency with the strongest signal.

In Sweden nearly every radio station transmitted on the FM network supports RDS. However the functions supported by every station can be different. Teracom AB, who is transmitting the Swedish radio stations with national coverage, P1 to P4, has also been responsible for the development of the Swedish RDS network.

The bit rate of the channel is relatively low, 1187.5 bit/s. The data stream is based on data blocks of 26 bits where 16 bits are information bits and the remaining 10 bits are for error correction. Four data blocks constitute one data group which is the largest element of the structure. Every data group contains 104 bits and has a duration of 87.6 ms. There are different types of data groups, each type containing different information. Every type of the group has a unique identity.

One service transmitted in RDS is

”Clock-Time and date information”and the message type is called CT. This message could be used for time distribution. A

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CT-group is transmitted every minute and is inserted so that the end of the group will occur as near the minute edge as possible depending on the group resolution. An uncertainty of half a block length, about 43 ms, is introduced here because of the fact that the group edge does not synchronize with the minute edge. The RDS standard states the total accuracy to± 100 ms. There are different possible sources of uncertainty besides the group and minute synchronize. The path delay between transmitter and receiver varies depending on the geographical location, see the discussion in 4.3.4. The source of time also has a great impact and there can be delays in transmitter and receiver that have an effect on the accuracy. These delays are difficult to estimate without practical tests because they may differ depending on the type of the transmitter and the receiver.

Experience shows that the discipline of the operators to transmit the correct time varies and there are examples of local radio stations whose time reference differ several minutes to UTC. The most reliable time is transmitted by Teracom who is responsible for the transmission and time reference of P1, P2, P3 and P4. Their time reference has until now been a rubidium clock synchronized to UTC via DCF77. During this autumn however, Teracom is going to change reference and start using NTP and their local network time instead.

RDS is already used as a time reference for synchronization of radio receivers. The time received via RDS is also used in other situations where the requirements of the accuracy are less important. By this time it is not recommended as a reliable and quality assured time source by SP.

4.3.2 DARC

The Data Radio Channel, specified in[5], is another channel for transmitting digital data on the analog radio network. It is, like RDS, transmitted on a subcarrier added to the original FM multiplex signal. The raw bit rate is 16 kbit/s and the channel can be used for various data applications. The main usage today is dissemination of DGPS-corrections for GPS navigation and news transmissions for hand-hold computers.

There are three different DARC channels with national coverage transmitted via the national radio stations P1, P2 and P3. P4 has DARC enabled in some local areas. P2 is exclusively used by the Swedish national defense for information concerning air force information. The DARC networks is managed by Teracom, who is responsible for the infrastructure as well as for the daily operation.

The data on the DARC channel is arranged in blocks were one block consists of 288 bits which corresponds to a block duration of 18 ms. 272 blocks form one frame, which is the largest element in the data structure.

Besides the general pay load data transmitted on a DARC channel there are various service information messages for the channel transmitted as well. One of these messages contains time and date information. The message is called Time and Date Table, TDT, and contains information about current date and clock time in Modified Julian Day, hours, minutes and seconds. Apart from this, a Time Accuracy Field, TAF, can be present and states in that case the number of blocks

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4.3 Radio Broadcasting 17

since the beginning of the current second. With TAF enabled, the TDT can offer a maximum accuracy of± 9 ms according to how the time field is specified. The standard of DARC does not specify any accuracy of the time information for the final user but there is a bit in the TAF telling weather the time is accurate within one second or one block.

In order to reach a higher accuracy, time information with a resolution of mil-liseconds could be transmitted with a change of the current time format or as a special payload.

4.3.3 DAB

During the last ten years DAB, specified in[11], has been developed and today only economical restrictions limit the changeover to digital radio for the public service. Today there are working test networks covering metropolitan areas in Sweden and a decision of an extension is close at hand. DAB is giving a total data rate of 2.432 Mbits/s.

The DAB data channel is divided into two main channels, the Main Service Channel (MSC) and the Fast Information Channel (FIC), which for time distri-bution have very different properties. The MSC is time interleaved to obtain an effective error correction but at the price of a very long delay in the channel, 360 ms. The FIC, on the other hand, uses high protection and frequent repetition of the data and is available without the delay caused by the error correction.

The FIC is primary used to transmit the multiplex configuration of the DAB transmission but there are also possibilities to send Service Information (SI) via this channel. One of these SI messages is Date and Time which allow an operator to send time information at a millisecond level. The service information is optional for the operator but the data format is specified in the DAB standard.

Data modulation and structure

DAB uses a multi-carrier modulation method called OFDM (Orthogonal Frequency Division Multiplex). The idea behind multi-carrier modulation is to split up the high rate data stream into a number of (K) parallel data streams of low data rate and modulate each of them separately on its own carrier. This leads to an increased symbol time with preserved data rate and makes the system less sensitive to intersymbol interference. There are four different Transmission Modes (TM), each with different OFDM parameters to suit various physical situations.

Mode K T_s Frame length FIBs per Frame TM I 1536 1246µs 96 ms 12

TM II 384 312 µs 24 ms 4

TM III 192 156 µs 24 ms 3

TM IV 768 623 µs 48 ms 6

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The data in DAB is structured in frames with a length of 24 ms, 48 ms or 96 ms depending on which TM it is operating in. Every frame consists of two synchronization symbols and a number FIC and MSC symbols. The number of symbols and the symbol time, T_s, depends on the operating mode. For TM II there are three FIC symbols and 72 MSC symbols. The gross bit rate for MSC is always 2.304 Mbits/s whereas the gross bit rate for the FIC is 96 kbits/s for every mode except TM III which has 128 kbits/s.

The FIC consists of a number of Fast Information Blocks (FIB) which carry the information. For the FIC a error coding rate of 1/3 is used and by that a net bit rate of 32 kbit/s is obtained for TM I,II,IV and for TM III the net bit rate is increased by a factor 4/3. Every FIB comprises 32 bytes of data, of which 2 bytes are used for error detection (CRC) and the remaining bytes are used for Multiplex Configuration Information (MCI) and SI. The number of FIBs varies depending on the Transmission Mode and is adjusted to get the appropriate data rate.

SI - Time and Date

The Service Information, which is optional to transmit, may contain Time and Date information and this could with advantage be used for time distribution purposes. The information includes Modified Julian Day and the time in UTC. The time can be given in two formats, a short-form with only hours and minutes and a long-form format with seconds and milliseconds as well. There is also a confidence indicator, a bit that tells weather the time is within the agreed tolerance.

There is no information about the accuracy of the time in the standard besides the fact that the time can be made available in millisecond resolution. The fact that the data rate of DAB is high and the symbol time is relatively short would give opportunities to distribute time at millisecond level. Considerations may be necessary when choosing the time source and designing the receiver, and maybe even the transmitter, to reach this accuracy.

4.3.4 Time distribution in broadcast radio networks

Most broadcast radio transmitters are active in the VHF-band (30 - 300 MHz) and have limited coverage areas. To obtain a national coverage several transmitters have to be used. This may be a problem for the purposes of time distribution. For high timing capabilities it is be desirable that all transmitters are synchronized. The optimal situation would be to have a cesium clock as time source, located at every transmitting station. This is however a very expensive solution and practically impossible to carry out.

A more practicable solution would be to have one accurate time source located at a central position such as Kakn¨astornet. It is located in Stockholm and from here the transmissions of the national radio stations are distributed to the local trans-mitters. How would the delays be affected depending of the different geographical locations of the local transmitters according to Kakn¨astornet? The maximum dis-tance between the source and the transmitter would be about 1400 km. If the

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4.4 Bluetooth 19

speed of light is regarded as the signal propagation velocity the maximum delay would be about 5 ms. In fact, the delay would be higher because the communica-tion lines to the transmitting stacommunica-tions does not support light speed communicacommunica-tion. The real delays are difficult to predict at a theoretical level because of the lack of available written system information from Teracom. Presumably these delays are not even known by Teracom because they do not have these real time demands in the network today. A very rough assumption would be that the delays are in the region of 20 ms.

In DARC, the position of the transmitter may be included in the time message. If there is a considerable delay, proportional to the distance between Kakn¨astornet and the transmitter, it could be corrected by using this position.

The transmitted signal in the VHF-band is not reflected in the ionosphere. Only the ground wave will reach the receiver and thereby the limited coverage area of one transmitter. The advantage of having no ionospheric reflection is that the reception is never disturbed by the sky wave and the propagation time does not vary from day to day depending on different ionospheric conditions.

4.4 Bluetooth

Bluetooth is a wireless communication method developed for cheap, simple and secure communication with relatively short range, for example flexible connections between mobile phones and accessories such as headsets and laptops. It operates at 2.4 GHz, which is a globally available license-free frequency band reserved for industrial, scientific and medical applications which is called the ISM-band. The modulation is very modern and uses both Time Division Multiplex (TDM) and Frequency Division Multiplex (FDM). During transmission a frequency hopping technique is used and in principle the channel frequency can change for each time slot, every 625 microseconds. The maximum bit rate for Bluetooth is 723.2 kbit/s in downlink and 57.6 kbit/s in uplink or 433.9 kbit/s symmetric[4].

There are two types of network topologies for a Bluetooth network. One master and up to seven slaves can form a piconet and several piconets can be connected to each other to form a scatternet. In the scatternet every master has the capability to act as both master and slave in different time slots and by that connect the two piconets. The scatternet forms a so called ad hoc network. There are different classes of output power and these give a master slave connection a range of between 10 and 100 meters.

Bluetooth is primarily developed for point-to-point communication although a network may include several nodes. However there is a broadcast function where one can send one packet to every active slave in one moment. This is a one way communication channel without possibility to check whether the slaves received the transmitted package or not. It is possible to use the multiplexing service supported by Bluetooth to send different data types on the same channel.

There is no time information embedded in Bluetooth as system. The possibility to use Bluetooth as time distribution system is to transfer a special payload with

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time information. This distribution system would however cover a very local area. Another issue might be that there are several layers between the physical channel and the common development interface, providing many functions that are not very useful for time distribution. The risk is that these functions will create transmission delays that are difficult to estimate. The data rate of Bluetooth is also unnecessarily high. For time distribution only, a simpler communication solution might fulfill the requirements.

4.5 Summary of described systems

4.5.1 DECT

DECT is today mostly used for voice communication and is not an interesting alternative for a new establishment of a time distribution network. One possibility would be to use an existing DECT network and receive the time stated by the switchboard which already may be available in the mobile terminals. It is difficult to evaluate the quality of the switchboard as time source though and to make it reliable would presumably require a large work.

4.5.2 Mobile telecommunication

The mobile telecommunication systems dominating in Sweden are today GSM and in the future WCDMA. There is no obvious way to use GSM or UMTS for multicast time distribution with high accuracy. The possible multicast channels available are designed for multimedia and do not comply with the demands of a real time channel for time distribution. There is no time information in the broadcasted synchronization message that can be used either. The complexity of these systems is very high. Changes in order to allow accurate time distribution would be a very large project and has to be done in close co-operation with mobile system developers.

4.5.3 Radio Broadcasting

The opportunities to send digital data via the public broadcasting network are very interesting for multicast time distribution. The three systems RDS, DARC and DAB are reviewed. All of the three systems contain time information that can be used for multicast time distribution.

RDS

RDS offers time with an accuracy of 100 ms according to the RDS-standard. The relatively low data rate and long symbol time does not make this system interesting for a further development to obtain a higher accuracy.

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4.5 Summary of described systems 21

DARC

DARC has a higher data rate than RDS. With a receiver designed for time reception and a reliable time source, the accuracy could be at least 18 ms. The limitation is that the time is stated with a resolution of one block and the block length is 18 ms. With changes of the time format so that the time is stated with millisecond resolution the accuracy could possibly reach a couple of milliseconds.

DAB

DAB is the latest broadcasting technology and is completely digital. The time format available may contain a resolution of milliseconds. If the receiver and the transmitter (encoder) are designed properly and an accurate source is used, the time accuracy would probably be at least a couple milliseconds thanks to the high data rate and short symbol time.

4.5.4 Bluetooth

For short-range communication of up to 10 and maybe 100 meters Bluetooth should be a cheap and robust wireless technology. For time distribution Bluetooth may be interesting for short distances. One possible drawback is the protocol structure that is rather complicated and it includes many functions not needed for time distribution. This may result in delays difficult to estimate.

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Chapter 5

A deeper review of DARC

The are a number of conditions that make the public broadcasting networks inter-esting for time distribution with national coverage.

• The transmitter network is already established.

• The limited complexity of the technical solution makes it suitable for real time communication.

• There are often time information already available in the systems that can be used. Modifications may however be necessary to increase the accuracy. • The systems are constructed for multicast/broadcast communication. In order to investigate possibilities and restrictions of time transmission via the public broadcasting networks DARC was selected for a detailed study. In the DARC transmission, time information is included and this information is used for the investigation. Two experimental receiver systems were constructed with a commercial receiver connected to a PC respectively a microprocessor. Different measurements were then performed to evaluate the accuracy and stability of the transmitted time.

5.1 The DARC network

The DARC signal is transmitted as a subcarrier on the FM multiplex signal used for public broadcasted analogous radio. The advantage of transmitting via the FM signal is that the existing infrastructure is used for the transmission. The transmitters are already available with an almost complete national coverage and the communication out to the transmitters is established as well.

In Sweden there are three parallel networks corresponding to the three national radio stations P1, P2 and P3. There is also a fourth network covering metropolitan

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24 A deeper review of DARC

areas via the P4 station. All networks are operated by Teracom but there are different service providers for each of them.

The DARC transmitted via P1 is used by Sectra AB which primarily sends financial information for their CitySurfer, a hand-hold DARC receiver with dis-play. Via P2 the Swedish Military is using DARC for LuLIS, a tactical broadcast information system that provides air situational awareness to all military units in real time. Finally the DARC via P3 is used by Teracom AB where smaller service providers are allowed to rent more or less capacity. The local P4 network is used for transmitting Differential GPS-corrections (DGPS) to final users.

The two main components that are specific for DARC in the network infras-tructure are the NetWork Server (NWS) and the Transmitting Station Equipment (TSE). There is one NWS for every network and for the Swedish networks operated by Teracom AB the NWS is located at Kakn¨astornet in Stockholm. The NWS is responsible for controlling the transmission, the direction and the quantity of the information which is sent to the TSE. The TSE is located at the FM transmitter site and consequently there is a TSE, one for each network, at every transmitter. The data stream is received by the TSE from the NWS and the TSE is the DARC encoder that generates the DARC compliant 76 kHz subcarrier.

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5.2 DARC communication layers 25

5.2 DARC communication layers

The communication between two DARC devices, a transmitter and a receiver, is structured according to the Open System Interconnection reference model (OSI ref-erence model). The OSI refref-erence model is nowadays the most used for structuring communication between different devices.

As a main principle of structuring, the model subdivides the functionality in 7 functional layers. Layer 1 - 4 include functions needed for transferring data between devices and Layer 5 - 7 include functions needed to facilitate the interaction between users at application level. For our requirements concerning time distribution the main interests are Layer 1 - 4. The other layers offer a large number of additional functions but these are not needed or even not necessary for the time distribution purpose. DARC is specified in [5].

5.2.1 Physical layer, Layer 1

The transmitter side is responsible for modulating the subcarrier with the data received from Layer 2 and for adding the modulated subcarrier to the FM multiplex signal. The subcarrier frequency is 76 kHz ± 7.6 Hz and it is modulated with Level-controlled Minimum Shift Keying (LMSK). LMSK is a type of MSK where the amplitude of the modulated subcarrier is controlled by the level of L-R (left minus right) signal.

The receiver side is responsible for extracting the subcarrier from the FM mul-tiplex signal and demodulation of the subcarrier. The data from the demodulated subcarrier is sent to Layer 2.

The channel has a continuous gross bit rate of 16 kbit/s± 1.6 bit/s.

5.2.2 Data link layer, Layer 2

The Layer 2 includes logical functions related to the data transmission such as block and frame synchronization, data formatting, error protection and scrambling for energy dispersal.

The largest element in the data structure is the frame which consists of 78 336 bits divided in 272 blocks with 288 bits each. There are two types of blocks, information blocks and parity blocks. Every block is beginning with a Block Iden-tification Code (BIC) of 16 bits. The information block comprises 176 information bits and 96 bits for error correction besides the BIC and the parity block is exclu-sively used for error correction.

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The error correction is performed both horizontal for one block and vertical over the entire frame.

There are three different frame types specified, each one with different allocation and layout of the parity bits. The frame A comprises 190 information blocks and 82 parity blocks in one sequence. In frame B the parity blocks are spread almost uniformly over the frame in order to obtain an information transmission as uniform as possible. Frame C only consists of information blocks and no delay is caused by parity blocks. There is also a variant of frame A, frame A1, with real time blocks inserted between the parity blocks to suspend the delay caused by the parity blocks. The total number of blocks in that case is 272 + 12.

Figure 5.3. A DARC Layer 2 frame of type A

The BIC that starts every block is used to distinguish parity blocks from in-formation blocks and to retrieve frame and block synchronization. There are four types of the 2 bytes long BIC and they are designed to have poor cross-correlation with each other, while their auto-correlation make them suitable for synchroniza-tion. Layer 2 also scrambles the data to avoid restrictions on incoming data and to spread the modulation spectrum.

The transmitter side is responsible for sending Layer 2 frames once it is filled with data by Layer 3. The receiver side is responsible for interpretation of the incoming continuous bit stream as well as delivering received data blocks to Layer 3 with a quality check parameter included.

5.2.3 Network Layer, Layer 3

Layer 3 is responsible for providing data ready for Layer 2. The data is organized in Layer 3 blocks of 22 bytes or 176 bits each, corresponding to the length of the information part in the information blocks of Layer 2. One Layer 3 block consists of a Layer 3 header and a remaining data part. In Layer 3 there are four logical

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5.2 DARC communication layers 27

channels defined, each one with different functionality depending on the type of the transmitted data. The channels are presented below.

• The Service Channel (SeCh), handling Layer 4 service messages, is used for transmitting network and service information to the receivers.

• The Short Message Channel (SMCh), handling Layer 4 short messages, is used for transmitting data with real time capabilities.

• The Long Message Channel (LMCh), handling Layer 4 long messages, is used for transmitting data files.

• The Block Message Channel (BMCh), is used for transmitting blocks of data. The different channels are identified in the first four bits of the Layer 3 header and the rest of the Layer 3 header is then different depending on the channel type. The SeCh is of most interest for time application because there are special service messages containing time information in this channel.

Figure 5.4. Service massage divided in Layer 3 blocks

SeCh Layer 3 header

The Layer 3 header of the Service Channel is 3 bytes long and contains information about the service message in general, the type of the message, the length of it and the first bits indicate the service channel.

There are eight types of different service messages where two are time informa-tion messages, Time, Date, Posiinforma-tion and Network name Table (TDPNT) and Time and Date Table (TDT). The other service messages contain frequency information of nearby transmitting stations and multiplex information concerning services on the other message channels.

5.2.4 Layer 4, Service Channel

In the next layer, Layer 4, the different service messages as well as the messages of the other logical channels are implemented. The service messages comprise a

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Layer 4 header and the special service message. The header occupies three bytes and the message can occupy up to 301 bytes. The maximum total length of a service message is 304 bytes and in that case it will occupy 16 Layer 3 blocks.

Two of the service message types are TDPNT and TDT. The reason why there are two types of service messages for time information is that TDT replaces the earlier TDPNT. Both TDT and TDPNT contain current Modified Julian Day and UTC time in hours, minutes and seconds. The main difference is that it is possible to obtain higher accuracy with TDT. There is a Time Accuracy Field (TAF) in TDT that states the number of blocks since the beginning of the current second. With the block length of 18 ms it is possible to receive time information with a maximum accuracy of ± 9 ms without changing the time format. Today TDT should be used instead of the old TDPNT.

In both these message types network name and position of the transmitter can be included as well. This is optional for TDT but shall be included in TDPNT.

Figure 5.5. The format of the TDT Time and Date Table

Multicast Time Distribution

Multicast Time Distribution

Multicast Time Distribution

Institutionen för systemteknik

581 83 LINKÖPING

Abstract

Preface

Abbreviations

Contents

Chapter 1

Introduction

1.1

Background

1.2

Project specification

1.3

Thesis outline

Chapter 2

Time

2.1

What is time?

2.2

Epoch and time interval

2.3

Sources of time

2.3.1

Resonators

2.3.2

Q, Accuracy and Stability

2.4

Time scales and traceability

2.4.1

International time keeping

2.4.2

National time keeping

2.5

Why accurate time?

Chapter 3

Time Distribution

3.1

Distribution techniques

3.1.1

Unicast

3.1.2

Anycast and manycast

3.1.3

Multicast and broadcast

3.2

Multicast methods available today

3.2.1

DCF77

3.2.2

GPS

Chapter 4

Possible multicast systems

4.1

DECT

4.1.1

The DECT System

4.1.2

Time distribution via DECT

4.2

Mobile telecommunication

4.2.1

Multicast data services available

4.2.2

Broadcast synchronization channels

4.2.3

NITZ

4.3

Radio Broadcasting

4.3.1

RDS

4.3.2

DARC

4.3.3

DAB

4.3.4

Time distribution in broadcast radio networks

4.4