The invention relates to the mutual synchronizing of the network elements of a data transmission network. The object of the invention is a method and system for transmitting time information between the network elements of a data transmission network.
In many data transmission networks, there is a need to synchronize the clock machineries provided in the network elements, so that the time data maintained by the clock machineries of different network elements are mutually equal as accurately as possible. In other words, the clock machineries in different network elements should as accurately as possible show a common time for all network elements to be mutually synchronized. Said common time is generally called the universal wall clock time. Said network elements can be for example routers or base stations of a mobile phone network. For instance, in new generation mobile phone networks, it is a requisite for a reliable operation of the data transmission between a mobile phone that moves from the coverage of a base station to the coverage of another base station and the base station network that the base stations follow a common notion of time at a sufficient accuracy.
In an arrangement according to the prior art, the clock machineries of network elements to be mutually synchronized are synchronized by means of a GPS (Global Positioning System) signal received from a satellite. Naturally this kind of solution requires that each network element to be synchronized is provided with a GPS receiver. A GPS receiver together with its antenna systems increases the manufacturing costs of the network element. In addition, when placing a network element in its working environment, care must be taken that the GPS signal is received at a sufficiently high power level. Network elements are often placed in underground bunkers. In that case there must be built antenna and signal path systems, by means of which the GPS signal is brought to the network element located in the underground bunker.
In another arrangement according to the prior art, mutually synchronized network elements transmit to each other time stamp messages, on the basis of which each network element adjusts the operation frequency and/or operation stage of its own clock machinery. The operation frequency represents the growth rate of the time data given by the clock machinery, and the operation stage represents the value of the time data given by the clock machinery at a certain moment of time. Thus the operation frequency is the derivative of the operation stage with respect to time. On the basis of the information contained by the time stamp messages, the network elements to be mutually synchronized tend to adjust their clock machineries, so that the operation frequencies of the clock machineries of different network elements are as close as possible, and respectively their operation stages are as close as possible. A time stamp message contains that time data value, given by the clock machinery of the network element transmitting the time stamp, that corresponds to the transmission moment of said time stamp message.
In order to illustrate the synchronizing operation based on time stamps, let us now observe two network elements A and B. Let us assume that the network element A transmits to the network element B a time stamp message V1 at a point of time when the time value given by the clock machinery of the network element A is t1. In other words, said time stamp message V1 contains the data t1. The network element B receives said time stamp message V1 at a point of time when the time value given by the clock machinery of the network element B is t2. The difference t2−t1 contains two components, which are the difference Ds1 between the operation stages of the clock machineries of the network elements A and B at the moment of receiving the time stamp message V1, and the transmission delay S1 of the time stamp message V1 from the network element A to the network element B. In other words, t2−t1=Ds1+S1. Let us assume that the network element B transmits to the network element A a time stamp message V2 at a point of time when the time value given by the clock machinery of the network element B is t3. In other words, said time stamp message V2 contains the data t3. The network element A receives said time stamp message V2 at a point of time when the time value given by the clock machinery of the network element A is t4. The difference t4−t3 contains two components, which are the difference Ds2 of the operation stages of the clock machineries of the network elements B and A at the moment of receiving the time stamp V2, and the transmission delay S2 of the time stamp from the message network element B to the network element A. In other words, t4−t3=Ds2+S2. In case the transmission delays S1 and S2 are mutually equal, and the difference of the operation stages of the clock machineries of the network elements A and B is not changed during the time between the moments of reception of the time stamp messages V1 and V2, the difference of the operation stages can be calculated as follows:
where Ds=Ds1=−Ds2. In case the time data t2 is transmitted from the network element B to the network element A, the network element A can, according to the equation (1), calculate how much the operation stage of the clock machinery of the network element A deviates from the operation stage of the clock machinery of the network element B.
In a prior art synchronizing arrangement based on time stamp messages and used in packet, frame or cell switched data transmission networks, the time stamp messages are transmitted between different network elements as data packets, frames or cells. As was already described above, the calculation of the differences of the operation stages by means of the equation (1) is based on assumptions that the difference of the operation steps of the clock machineries of the separate network elements is not changed during the time between the moments of reception of the time stamp messages, and that the transmission delays in different directions are mutually equal. The quality of modern clock machineries is generally so high that this assumption as regards the difference between the operation stages usually holds true. On the other hand, often the assumption regarding the transmission delay in packet, frame and cell-switched data transmission networks does not hold true at a sufficient accuracy, because the transmission delay contains a remarkable random-type share. The reason for said random-type share is, among others, in the queuing delays experienced by the data packets, frames or cells in the transmission buffers and/or reception buffers of the network elements.
In some data transmission systems, an error of the size of the transmission delay between the operation stages of the clock machineries of different network elements can be allowed. If in a situation like this, one network element serves as the master device and the other network elements serve as slave devices that tend to be synchronized with the master device, it suffices that said master device transmits time stamp messages to said slave devices. In this case we are talking about unidirectional transmission of time stamp messages. Also in this kind of working environment, the fluctuation of the transmission delay makes it difficult to adjust the operation frequency and stage of the clock machineries, because the fluctuation in the transmission delays may give a false impression of the change in the difference of the operation stages. The effect of the fluctuation of the transmission delay can be attenuated by low-pass filtering, but as is well known, low-pass filtering slows down the adjusting process.
The invention relates to a system for transmitting time stamp information between network elements, so that the limitations and drawbacks connected to the prior art can be eliminated or alleviated. The invention also relates to a method for transmitting time stamp information between network elements, so that the limitations and drawbacks connected to the prior art can be eliminated or alleviated. The invention also relates to a network element for transmitting time stamp information, so that the limitations and drawbacks connected to the prior art can be eliminated or alleviated.
In the present invention, it has surprisingly been discovered that in a data network transmitting packet, frame or cell switched data traffic, time stamp information can be transmitted from a network element to another network element by using that part of the bit stream passing between the network elements, which part in the network element transmitting said bit stream is connected to said bit stream in a location that is in said transmitting network element placed after the transmission buffer buffering data packets, frames or cells, in the flowing direction of said bit stream.
By means of the invention, there are achieved remarkable advantages:
In the present document, both data packets, frames and cells are called by the general term Protocol Data Units, PDU.
An arrangement according to the invention for transmitting time stamp information from a first network element to a second network element, said arrangement including:
The method according to the invention for transmitting time stamp information from a first network element to a second network element, in which method:
The network element according to the invention for transmitting time stamp information to a data transmission link, said network element including:
The various embodiments of the invention are characterized by what is set forth in the dependent claims.
The invention is explained in more detail below, with reference to the preferred embodiments described by way of example, and to the appended drawings, where
In a system according to an alternative embodiment of the invention the writing unit 106 is arranged to place the time stamp information AL in those bits of the combined bit stream 111a emitted from the multiplexer 104 that represent the bit stream 113a or part of the bit stream 113a.
In a system according to an embodiment of the invention, the reading unit 110 is arranged to read the time stamp information AL from those bits of the combined bit stream 111c that represent the bit stream 113c or part of the bit stream 113c. Now the demultiplexer 109 is not needed for transmitting the time stamp information AL from the network element 101 to the network element 102.
The drawing references 111a, 111b and 111c represent the above mentioned combined bit stream, so that the drawing reference 111a refers to the bits transmitted and/or stored in the signal processing elements of the network element 101 and representing said combined bit stream, the drawing reference 111b refers to the bits transmitted by the data transmission link and representing said combined bit stream, and the drawing reference 111c refers to the bits transmitted and/or stored in the signal processing elements of the network element 102 and representing said combined bit stream. Respectively, the drawing reference 112a represents in the network element 101 the same bit stream as the drawing reference 112c in the network element 102, and the drawing reference 113a represents in the network element 101 the same bit stream as the drawing reference 113c in the network element 102.
In a system according to an embodiment of the invention, the protocol data units are Ethernet protocol MAC layer (Media Access Control) data frames, and the bit stream 113a, 113c is included in the interframe gaps that are transmitted over the physical layer of the Ethernet protocol (Ethernet Phy) in between chronologically successive MAC layer data frames. In other words, the time stamp information AL is transmitted in one or several interframe gaps.
In a system according to an embodiment of the invention, the transmitter 105 and the receiver 108 are realized by commercially available physical layer Ethernet adapter circuits (Ethernet-Phy circuits). The data transmission interface between the transmission buffer 103 and the multiplexer 104 is advantageously MII (Media Independent Interface) or, in case of a 1 Gbit/s transmission rate Ethernet, GMII (Gigabit Media Independent Interface). The multiplexer 104 and the writing unit 106 can be realized for example by one or several programmable processors, an ASIC and/or FPGA circuit (Application Specific Integrated Circuit, Field Programmable Gate Array). Respectively, the demultiplexer 109 and the reading unit 110 can be realized for example by one or several programmable processors, an ASIC and/or FPGA circuit.
The writing unit 106 is preferably arranged to place the time stamp information AL in those bits of the interframe gaps, the values of which are independent of the operation of the Ethernet protocol in the data transmission carried out between the network elements 101 and 102. Now the transmission of the time stamp information AL from the network element 101 to the network element 102 does not disturb other data transmission between the network elements 101 and 102. Among the interframe gap bits, the values of which are independent of the operation of the Ethernet protocol in the data transmission carried out between the network elements 101 and 102, are those interframe gap bits, the values of which are not defined in the technical standards dealing with the Ethernet data transmission, as well as those interframe gap bits, for which there is in said standards defined a usage that is neither needed nor used in the data transmission carried out between the network elements 101 and 102.
In a system according to an embodiment of the invention, the writing unit 106 is arranged to place the time stamp information in one interframe gap, so that said time stamp information corresponds to the transmission moment of said interframe gap. In a system according to an alternative embodiment of the invention, the writing unit 106 is arranged to divide the time stamp information in at least two parts and to place said parts in at least two interframe gaps, so that said time stamp information corresponds to the transmission moment of the first transmitted interframe gap of said at least two interframe gaps.
In a system according to an embodiment of the invention, the bit stream 113a, 113c in
In a system according to an embodiment of the invention, the multiplexer 104 in
In a system according to an embodiment of the invention, the demultiplexer 109 in
The writing unit 106 is advantageously arranged to place the time stamp information AL in those control data bits of the SDH frames, the values of which are independent of the operation of the SDH protocol in the data transmission carried out between the network elements 101 and 102. Now the transmission of the time stamp information AL from the network element 101 to the network element 102 does not disturb other data transmission between the network elements 101 and 102. The control data bits of the SDH frame, the values of which are independent of the operation of the SDH protocol in the data transmission carried out between the network elements 101 and 102, are control data bits, the value of which is not defined in technical standards dealing with SDH data transmission, and control data bits, for which said standards have defined a usage that is neither needed nor used in the data transmission carried out between the network elements 101 and 102.
In a system according to an embodiment of the invention, the bit stream 113a, 113c in
In a system according to an embodiment of the invention, the multiplexer 104 in
In a system according to an embodiment of the invention, the demultiplexer 109 in
The writing unit 106 is preferably arranged to place the time stamp information AL in those control data bits of the Sonet frames, the values of which are independent of the operation of the Sonet protocol in the data transmission carried out between the network elements 101 and 102. Now the transmission of the time stamp information AL from the network element 101 to the network element 102 does not disturb other data transmission between the network elements 101 and 102. The Sonet frame control data bits, the values of which are independent of the operation of the Sonet protocol in the data transmission carried out between the network elements 101 and 102, are control data bits, the value of which is not defined in the technical standards dealing with Sonet data transmission, and control data bits, for which said standards have defined a usage that is neither needed nor used in the data transmission carried out between the network elements 101 and 102.
In a system according to an embodiment of the invention, the writing unit 106 in
In a system according to an embodiment of the invention, the bit stream 113a, 113c in
In a system according to an embodiment of the invention, the multiplexer 104 in
In a system according to an embodiment of the invention, the demultiplexer 109 in
The writing unit 106 in
In a method according to an alternative embodiment of the invention, said time stamp information AL is placed, in between steps 603 and 604, in the bits of the combined bit stream B3 that represent the bit stream B2 or part of the bit stream B2.
In a method according to an embodiment of the invention, said time stamp information AL is read in the network element B from those bits of the combined bit stream B3 that represent the bit stream B2 or part of the bit stream B2. Now the separation step 606 of the bit streams B1 and B2 is not needed.
In a method according to an embodiment of the invention, the protocol data units are Ethernet protocol MAC (Media Access Control) layer data frames, and the bit stream B2 is included in the interframe gaps that are transmitted over the physical layer of the Ethernet protocol (Ethernet Phy) in between chronologically successive MAC layer data frames. Consequently, the time stamp information AL is transmitted in one or several interframe gaps.
In a method according to an embodiment of the invention, the time stamp information AL is placed in step 602 in those bits of the interframe gaps, the values of which are independent of the operation of the Ethernet protocol in the data transmission carried out between the network elements A and B.
In a method according to an embodiment of the invention, the time stamp information AL is placed in step 602 in one interframe gap, so that said time stamp information corresponds to the transmission moment of said interframe gap. In a method according to an alternative embodiment of the invention, the time stamp information AL is divided into at least two parts, and said parts are placed in at least two interframe gaps, so that said time stamp information corresponds to the transmission moment of the first transmitted interframe gap of said at least two interframe gaps.
In a method according to an embodiment of the invention, the bit stream B2 represents the control data of SDH (Synchronous Digital Hierarchy) frames. Consequently, the time stamp information AL is transmitted in frame fields reserved for the control data of SDH frames. The protocol data units can be for example IP (Internet Protocol) data packets.
In a method according to an embodiment of the invention, the time stamp information AL is placed in step 602 in those bits of the control data of the SDH frames, the values of which are independent of the operation of the SDH protocol in the data transmission carried out between the network elements A and B.
In a method according to an embodiment of the invention, the bit stream B2 represents the control data of Sonet (Synchronous Optical Network) frames. Consequently, the time stamp information AL is transmitted in frame fields reserved for the control data of Sonet frames. The protocol data units can be for example POS (Packet Over Sonet) data packets.
In a method according to an embodiment of the invention, the time stamp information AL is placed in step 602 in those bits of the control data of the Sonet frames, the values of which are independent of the operation of the Sonet protocol in the data transmission carried out between the network elements A and B.
In a method according to an embodiment of the invention, the time stamp information AL is placed in step 602 in the control data of one SDH or Sonet frame, so that said time stamp information corresponds to the transmission moment of said SDH or Sonet frame. In a method according to an alternative embodiment of the invention, the time stamp information AL is divided into at least two parts, and said parts are is placed in the control data of at least two SDH or Sonet frames, so that said time stamp information corresponds to the transmission moment of the first transmitted SDH or Sonet frame of said at least two SDH or Sonet frames. In another alternative embodiment of the invention, the time stamp information AL is divided into at least two parts, and said parts are placed in the control data of at least two SDH or Sonet frames, so that said time stamp information corresponds to the transmission moment of the last transmitted SDH or Sonet frame of said at least two SDH or Sonet frames. The transmission moment of the last transmitted SDH or Sonet frame can be forecast when transmitting the first SDH or Sonet frame, if the chronologically successive SDH or Sonet frames are of a standard size, and the data transmission rate of the data transmission link is constant.
In a method according to an embodiment of the invention, the bit streams B1 and B2 are multiplexed in step 603 into a bit stream B3 by using an SDH framing circuit that is arranged to place the protocol data units in the payload frame fields of the SDH frames.
In a method according to an embodiment of the invention, the bit streams B1 and B2 are multiplexed in step 603 into a bit stream B3 by using a Sonet framing circuit that is arranged to place the protocol data units in the payload frame fields of the Sonet frames.
In a method according to an embodiment of the invention, the bit stream B2 represents the control data of PCM (Pulse Coded Modulation) frames. The time stamp information AL is transmitted in predetermined time slots of the PCM frames. Advantageously the time stamp information is transmitted in the time slot TS 0 of the PCM frames. In the present document, all PCM frame data except the payload data is considered to belong to the control data of the PCM frames. The protocol data units can be for example LAPF (Link Access Procedure for Frame mode services) data frames.
In a method according to an embodiment of the invention, the time stamp information AL is placed in those bits of the predetermined time slots of chronologically successive PCM frames, the values of which are independent of the operation of the PCM protocol in the data transmission carried out between the network elements A and B. Advantageously said predetermined time slots are the TS 0 of each PCM frame.
In a method according to an embodiment of the invention, the bit streams B1 and B2 are multiplexed in step 603 into a bit stream B3 by using a PCM framing circuit that is arranged to place the protocol data units in the payload frame fields of PCM frames.
In a network element according to an alternative embodiment of the invention, said writing unit 806 is arranged to place the time stamp information AL1 in the bits of the combined bit stream 811a emitted from the multiplexer 804, which represent the bit stream 813a or part of the bit stream 813a.
A network element according to an embodiment of the invention is provided with a receiver 808 that is arranged to receive the bit stream 811c through the receiving port 851, a demultiplexer 809 that is arranged to separate the bit stream 813c and the bit stream 812c representing the protocol data units from the bit stream 811c.
The network element is provided with a reading unit 810 that is arranged to read the time stamp information AL2 from the bit stream 813c. The dotted line arrow 822 represents an operation where the values of those bits that in a time slot between two points of time represent the bit stream 813c or part of the bit stream 813c are read.
In a network element according to an embodiment of the invention, the reading unit 810 is arranged to read the time stamp information AL2 from those bits of the bit stream 811c flowing to the demultiplexer 809 that represent the bit stream 813c or part of the bit stream 813c.
A network element according to an embodiment of the invention is an IP (Internet Protocol) router.
A network element according to an embodiment of the invention is an Ethernet switch.
A network element according to an embodiment of the invention is a base station of a mobile phone network.
A network element according to an embodiment of the invention is an MPLS (Multi Protocol Label Switching) switch.
As is obvious for a man skilled in the art, the invention and its various embodiments are not restricted to the above described examples only, but the invention and its embodiments can be modified within the scope of the independent claim.
Number | Date | Country | Kind |
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20060875 | Oct 2006 | FI | national |