Pulse or digital communications – Synchronizers – Network synchronizing more than two stations
Reexamination Certificate
2000-01-11
2003-03-11
Phu, Phuong (Department: 2631)
Pulse or digital communications
Synchronizers
Network synchronizing more than two stations
C375S362000
Reexamination Certificate
active
06532274
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method and to an arrangement for synchronising at least one local oscillator with a central time-generating unit. The local oscillator serves a so-called element included in a network, and the time-generating unit is included in a so-called main unit, which is also included in the network.
The frequency of the local oscillator can be controlled by periodic and automatic calibration.
DESCRIPTION OF THE BACKGROUND ART
It has long been known that nodes or elements belonging to a telecommunications network need to be synchronised with a common time reference. This is particularly important in respect of telecommunications networks that serve mobile telephones, where one and the same time reference must be used between different base stations in order for a user to be able to move without hindrance between the areas covered by said base stations during an ongoing communication.
It has become more and more usual in recent times to use in telecommunications networks transmission media and transport techniques that although novel with respect to telecommunications have been used earlier for the transmission of data, for instance.
The problem encountered when using new transport techniques is one of transferring synchronisation from the core of the network to its peripheral borders. The physical medium and the protocol used are not always suitable for the transference of synchronisation. A typical example in this respect is the use of Internet Protocol (IP) technique between base stations and Base Station Controllers (BSC) in mobile telecommunications networks.
Known solutions to these problems are found in the use of autonomous clocks or in clocks that are locked to an available navigational system, such as the Global Positioning System (GPS).
For reasons of cost, autonomous clocks are normally comprised of quartz oscillators. These clocks, however, require periodic manual calibration in order to be able to generate a time reference signal within set requirements.
It is also known that oscillators can be controlled automatically with respect to frequency without requiring manual work, by automatically assigning to the oscillator a correction value in accordance with a given periodicity. One example of oscillators that can be controlled automatically are voltage-controlled oscillators with which a voltage level determines the frequency of the oscillator. In the case of an oscillator of this kind, it is possible to calibrate and adjust the oscillator periodically and automatically, by controlling the voltage level in question. However, this requires a relevant correction value to be given, i.e. a voltage level that corresponds to the frequency to which the oscillator shall be set. Accordingly, the expression “the frequency can be controlled by periodic and automatic calibration” used in this description of the present invention refers to an oscillator that can be controlled frequency-wise by setting a signal, such as a voltage level, to a specific value in the absence of manual work.
It is not always possible to use the GPS, because it is not always possible to receive requisite signals, such as in the case of underground base station installations. The use of the GPS may also be impractical purely for cost reasons.
The problems of transferring a synchronising reference signal can be divided into two separate groups. Firstly, it may be necessary for an absolute time to be known, i.e. the time of day (ToD), and secondly a local oscillator may be required to oscillate at the same frequency as a specified reference frequency within a given error tolerance. The present invention relates mainly to this latter problem, in other words to the possibility of allowing the oscillators active in the network to oscillate at a common frequency.
Listed below are prior publications, which describe various solutions to problems that can occur within this particular technical field.
Publication EP-A1-0 838 916 teaches a method of synchronising a receiving node to the bit rate of incoming bits. A time stamp is allocated to a data packet arriving at a receiving buffer. When the data packet is read from the buffer, the time at which the data packet was read is compared with the time stamp and the time difference, i.e. the time between reading the data packet into the buffer and reading said data packet from said buffer provides a measurement of the number of data packets in the buffer.
The bit rate with respect to the receiving node is regulated in accordance with a desired number of data packets in the buffer. The bit rate is decreased when the number of data packets becomes too small and is increased when the number of data packets becomes too large. The local bit rate can be adjusted to be the same as the bit rate of the transmitting node in this way.
Publication U.S. Pat. No. 5,822,383 also describes a solution to the problem of synchronising a receiving node to the bit rate of a transmitting node.
Publication WO 98/13969 deals with the time of day problem. A good time reference is already found available through GPS signals to network nodes. Publications WO 98/13966 and SE-C2 508 460 also describe a solution in which all system nodes have access to a GPS signal.
In Proceedings: Twelfth Real-Time Systems Symposium, there is presented the article “Efficient Synchronization of Clocks in a Distributed System” by Sympath Rangarajan and Satish K. Tripathi, which describes the possibility of allocating different clocks in a system a common frequency that is based on a mean value of the different frequencies of the system clocks, by virtue of the different clocks in the system sending time stamps to one another and thereafter being synchronised to a common clock value calculated in accordance with a common statistical algorithm. The system clocks are synchronised in this way to a statistically calculated common frequency and not as slave clocks to a master clock.
A known method of synchronising a local oscillator with a central oscillator is to use time stamps, where a first time stamp is sent from the local oscillator to the central oscillator and a second time stamp is sent back in return. The information that is sent back includes information as to when the first time stamp was received and when the second time stamp was sent. The local oscillator can be synchronised with the central oscillator on the basis of the round-trip delay and on the basis of the information included in the second time stamp.
In the 1997 IEEE International Conference on Communications, there was presented an article “On Assessing Unidirectional Latencies in Packet-Switching Networks” by Andreas Fasbender and Ingo Rulands, which describe the possibility of synchronising a local oscillator with a centrally positioned oscillator on the basis of round-trip delay.
This article also describes the possibility of using a one-way delay in ascertaining whether or not the communications path between two units in a network is suitable for transmitting time information that is reliable from the aspect of synchronisation.
It should also be mentioned that in a proposal to an American standard designated ANSI T1.101-1994 “Draft American National Standard for Telecommunications—Synchronization Interface Standard”, there are described and specified types of digital network interfaces that can transfer references for synchronisation and synchronisation specifications relating to reference signals in network interfaces between DS1 and SONET.
Section 8 of this publication describes how a local oscillator can be controlled to determine whether it maintains a specific quality standard for oscillators after a reference signal to the oscillator has been lost.
Oscillators can be divided into categories designated “Stratum”, where Stratum levels, Stratum 1 to Stratum 4, denote the accuracy levels of the oscillators and where Stratum 1 denotes a highest level and Stratum 4 a lowest level. Each level can also be divided into sublevels in accordance with different alphabetical designations.
ANSI T1.101-1994
Phu Phuong
Telefonaktiebolaget LM Ericsson (publ)
LandOfFree
Synchronization method and arrangement does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Synchronization method and arrangement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Synchronization method and arrangement will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3044067