Abstract:
The invention relates to transmitting time information between elements of a data network. In the present invention, it has surprisingly been discovered that in a network transmitting packet, frame or cell switched data traffic, time stamp information can be transmitted from a network element ( 101 ) to another network element ( 102 ) by using that part ( 113   a,    113   c ) of the bit stream ( 111   b ) to be transmitted which is connected, in the transmitting network element ( 101 ), to the transmitted bit stream in a location that is placed, in the flowing direction of the transmitted bit stream, after a transmission buffer ( 103 ) buffering data packets, frames or cells. In an arrangement according to the invention, the random-type share of the transmission delay experienced by the time stamp information is slight, because the time stamp information does not have to queue in the transmission buffer ( 103 ) buffering data packets, frames or cells.

Description:
FIELD OF THE INVENTION 
       [0001]    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. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    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. 
         [0004]    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. 
         [0005]    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 V 1  at a point of time when the time value given by the clock machinery of the network element A is t 1 . In other words, said time stamp message V 1  contains the data t 1 . The network element B receives said time stamp message V 1  at a point of time when the time value given by the clock machinery of the network element B is t 2 . The difference t 2 −t 1  contains two components, which are the difference Ds 1  between the operation stages of the clock machineries of the network elements A and B at the moment of receiving the time stamp message V 1 , and the transmission delay S 1  of the time stamp message V 1  from the network element A to the network element B. In other words, t 2 −t 1 =Ds 1 +S 1 . Let us assume that the network element B transmits to the network element A a time stamp message V 2  at a point of time when the time value given by the clock machinery of the network element B is t 3 . In other words, said time stamp message V 2  contains the data t 3 . The network element A receives said time stamp message V 2  at a point of time when the time value given by the clock machinery of the network element A is t 4 . The difference t 4 −t 3  contains two components, which are the difference Ds 2  of the operation stages of the clock machineries of the network elements B and A at the moment of receiving the time stamp V 2 , and the transmission delay S 2  of the time stamp from the message network element B to the network element A. In other words, t 4 −t 3 =Ds 2 +S 2 . In case the transmission delays S 1  and S 2  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 V 1  and V 2 , the difference of the operation stages can be calculated as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     Ds 
                     = 
                     
                       
                         
                           ( 
                           
                             
                               t 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             
                               t 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               t 
                                
                               
                                   
                               
                                
                               4 
                             
                             - 
                             
                               t 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                       
                       2 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Ds=Ds 1 =−Ds 2 . In case the time data t 2  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. 
         [0006]    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. 
         [0007]    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. 
       SUMMARY OF THE INVENTION 
       [0008]    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. 
         [0009]    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. 
         [0010]    By means of the invention, there are achieved remarkable advantages:
       the random-type share of the transmission delay experienced by the time stamp information is smaller than in the above described prior art solution based on time stamp messages, because the time stamp information need not queue in a transmission buffer that buffers data packets, frames or cells,   the network elements obtain more accurate information of the difference of the operation stages between the clock machineries of the network elements than in the above described prior art solution based on time stamp messages, because the random-type share of the transmission delay experienced by the time stamp information is smaller, and   the network elements can adjust their clock machineries to more accurately indicate wall clock time than in the above described prior art solution based on time stamp messages, because the network elements obtain more accurate information of the difference of the operation stages between the clock machineries of the network elements.       
 
         [0014]    In the present document, both data packets, frames and cells are called by the general term Protocol Data Units, PDU. 
         [0015]    An arrangement according to the invention for transmitting time stamp information from a first network element to a second network element, said arrangement including:
       in said first network element a transmission buffer that is arranged to buffer the protocol data units to be transmitted,   in said first network element a multiplexer that is arranged to generate a combined bit stream by connecting to a first bit stream, read from said transmission buffer and representing said protocol data units, a second bit stream,   in said first network element transmitter that is arranged to transmit said combined bit stream to a data transmission link,   in said second network element a receiver that is arranged to receive said combined bit stream from said data transmission link,
 
is characterized in that the system also includes:
   in said first network element a writing unit that is arranged to place the time stamp information in the bits representing said second bit stream, and   in said second network element a reading unit that is arranged to read said time stamp information from the bits representing said second bit stream.       
 
         [0022]    The method according to the invention for transmitting time stamp information from a first network element to a second network element, in which method:
       in said first network element, there is read from a transmission buffer that contains protocol data units to be transmitted, a first bit stream representing said protocol data units,   in said first network element, there is generated a combined bit stream by multiplexing the second bit stream in said first bit stream,   in said first network element, said combined bit stream data is transmitted to a transmission link, and   in said second network element, said combined bit stream is received said data transmission link,
 
is characterized in that in addition, in the method:
   in said first network element, the time stamp information is placed in the bits representing said second bit stream, and   in said second network element, said time stamp information is read from the bits representing said second bit stream.       
 
         [0029]    The network element according to the invention for transmitting time stamp information to a data transmission link, said network element including:
       a transmission buffer that is arranged to buffer the transmitted protocol data units,   a multiplexer that is arranged to generate a combined bit stream by connecting to a first bit stream, read from said transmission buffer and representing said protocol data units, a second bit stream, and   a transmitter that is arranged to transmit said combined bit stream to the transmission port of the network element,
 
is characterized in that the network element also includes a writing unit that is arranged to place the time stamp information in the bits representing said second bit stream.
       
 
         [0033]    The various embodiments of the invention are characterized by what is set forth in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    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 
           [0035]      FIG. 1  illustrates a system according to the invention for transmitting time stamp information between network elements, 
           [0036]      FIG. 2  illustrates an exemplary situation of how the time stamp information is included in the bit stream to be transmitted by a data transmission link in a system according to an embodiment of the invention, 
           [0037]      FIG. 3  illustrates an exemplary situation of how the time stamp information is included in the bit stream to be transmitted by a data transmission link in a system according to an embodiment of the invention, 
           [0038]      FIG. 4  illustrates an exemplary situation of how the time stamp information is included in the bit stream to be transmitted by a data transmission link in a system according to an embodiment of the invention, 
           [0039]      FIG. 5  illustrates a system according to an embodiment of the invention for transmitting time stamp information between network elements, 
           [0040]      FIG. 6  is a flow diagram illustrating a method according to an embodiment of the invention for transmitting time stamp information between network elements, 
           [0041]      FIG. 7  is a flow diagram illustrating a method according to an embodiment of the invention for transmitting time stamp information between network elements, and 
           [0042]      FIG. 8  illustrates a network element according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]      FIG. 1  illustrates a system according to an embodiment of the invention for transmitting time stamp information from a network element  101  to a network element  102 . The network element  101  is a transmission buffer  103  that is arranged to buffer the protocol data units D to be transmitted. The network element  101  is a writing unit  106  that is arranged to place the time stamp information AL in the bit stream  113   a . The dotted line arrow  121  represents an operation where the values of such bits that in a time slot between two points of time represent the bit stream  113   a  or part of the bit stream  113   a  are placed so that the set formed by said bits contains the time stamp information AL. The network element  101  is multiplexer  104  that is arranged to generate a combined bit stream  111   a  by merging the bit stream  113   a  with the bit stream  112   a  read from the transmission buffer  103  and representing the protocol data units. The network element  101  is a transmitter  105  that is arranged to transmit the combined bit stream  111   a  to the data transmission link  107 . The network element  102  is a receiver  108  that is arranged to receive the combined bit stream  111   b  from the data transmission link  107 . The network element  102  is a demultiplexer  109  that is arranged to separate from the combined bit stream  111   c  the bit stream  112   c  and the bit stream  113   c  representing the protocol data units. The network element  102  is a reading unit  110  that is arranged to read the time stamp information AL from the bit stream  113   c . The dotted line arrow  122  represents an operation where the values of those bits that in a time slot between two points of time represent the bit stream  113   c  or part of the bit stream  113   c  are read. 
         [0044]    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  111   a  emitted from the multiplexer  104  that represent the bit stream  113   a  or part of the bit stream  113   a.    
         [0045]    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  111   c  that represent the bit stream  113   c  or part of the bit stream  113   c . Now the demultiplexer  109  is not needed for transmitting the time stamp information AL from the network element  101  to the network element  102 . 
         [0046]    The drawing references  111   a ,  111   b  and  111   c  represent the above mentioned combined bit stream, so that the drawing reference  111   a  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  111   b  refers to the bits transmitted by the data transmission link and representing said combined bit stream, and the drawing reference  111   c  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  112   a  represents in the network element  101  the same bit stream as the drawing reference  112   c  in the network element  102 , and the drawing reference  113   a  represents in the network element  101  the same bit stream as the drawing reference  113   c  in the network element  102 . 
         [0047]    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  113   a ,  113   c  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. 
         [0048]    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. 
         [0049]    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 . 
         [0050]      FIG. 2  illustrates an exemplary situation of how the time stamp information AL is included in the combined bit stream  111   b  transmitted from the data transmission link  107  in a system according to an embodiment of the invention. The areas  201 ,  202  and  203  represent chronologically successive Ethernet protocol MAC layer data frames, of which the data frame  203  is first in the transmission order. The areas  204 ,  205 ,  206 , and  207  represent interframe gaps. In this exemplary situation, the time stamp information AL is included in the interframe gap  205  between the data frames  201  and  202 . 
         [0051]    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. 
         [0052]    In a system according to an embodiment of the invention, the bit stream  113   a ,  113   c  in  FIG. 1  represents the control data of the SDH (Synchronous Digital Hierarchy) frames. Thus, the time stamp information AL is transmitted in frame fields reserved for the control data of the SDH frames. In the present document, all other SDH frame data except payload data is considered to belong to the control data of the SDH frames. Consequently, the control data contains synchronizing words, synchronizing indicators, label data, etc. The protocol data units D in  FIG. 1  can be for example IP (Internet Protocol) data packets. 
         [0053]    In a system according to an embodiment of the invention, the multiplexer  104  in  FIG. 1  is part of a commercially available SDH framing circuit that is arranged to place the protocol data units in the payload frame fields of the SDH frames. The SDH framing circuit also forms the control data, i.e. the bit stream  113   a.    
         [0054]    In a system according to an embodiment of the invention, the demultiplexer  109  in  FIG. 1  is part of a commercially available SDH framing circuit that is arranged to separate the protocol data units from the payload frame fields of the SDH frames. 
         [0055]    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 . 
         [0056]    In a system according to an embodiment of the invention, the bit stream  113   a ,  113   c  in  FIG. 1  represents the control data of Sonet (Synchronous Optical Network) frames. Thus the time stamp information AL is transmitted in frame fields reserved for the control data of Sonet frames. In the present document, all other Sonet frame data except payload data is considered to belong to the Sonet frame control data. Consequently, the data contains synchronizing words, synchronizing indicators, label data, etc. The protocol data units D in  FIG. 1  can be for example POS (Packet Over Sonet) data packets. 
         [0057]    In a system according to an embodiment of the invention, the multiplexer  104  in  FIG. 1  is part of a commercially available Sonet framing circuit that is arranged to place the protocol data units in the payload frame fields of the Sonet frames. The Sonet framing circuit also forms the control data, i.e. the bit stream  113   a.    
         [0058]    In a system according to an embodiment of the invention, the demultiplexer  109  in  FIG. 1  is part of a commercially available Sonet framing circuit that is arranged to separate the protocol data units from the payload frame fields of the Sonet frames. 
         [0059]    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 . 
         [0060]      FIG. 3  describes an exemplary situation of how the time stamp information AL is included in the combined bit stream  111   b  transmitted by the data transmission link  107  in  FIG. 1  in a system according to an embodiment of the invention. The areas  301  and  302  represent chronologically successive SDH or Sonet frames, of which the SDH or Sonet frame  302  is first in the transmission order. The areas  311  and  312  represent payload data, and the areas  321  and  322  represent control data. In this exemplary situation, the time stamp information AL is included in the control data  321  field  331 . In  FIG. 3 , SDH or Sonet frames are illustrated in conventional fashion as two-dimensional tables. 
         [0061]    In a system according to an embodiment of the invention, the writing unit  106  in  FIG. 1  is arranged to place the time stamp information 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 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 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 a system according to another alternative embodiment, the writing unit  106  is arranged to divide the time stamp information in at least two parts and to place said parts 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 is constant with the data transmission link  107  in  FIG. 1 . 
         [0062]    In a system according to an embodiment of the invention, the bit stream  113   a ,  113   c  in  FIG. 1  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. Preferably the time stamp information is transmitted in the time slot  0  of the PCM frames. In the present document, all other PCM frame data except payload data is considered to belong to the control data of the PCM frames. The protocol data units D in  FIG. 1  can be for example LAPF (Link Access Procedure for Frame mode services) data frames. 
         [0063]    In a system according to an embodiment of the invention, the multiplexer  104  in  FIG. 1  is part of a commercially available PCM framing circuit that is arranged to place the protocol data units in the payload frame fields of the PCM frames. The PCM framing circuit also&#39;forms the control data, i.e. the bit stream  113   a.    
         [0064]    In a system according to an embodiment of the invention, the demultiplexer  109  in  FIG. 1  is part of a commercially available PCM framing circuit that is arranged to separate the protocol data units from the payload frame fields of the PCM frames. 
         [0065]    The writing unit  106  in  FIG. 1  is advantageously arranged to place the time stamp information AL in those bits of the time slots TS  0  of chronologically successive PCM frames that are independent of the operation of the PCM 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 . Those bits of time slot TS  0 , the values of which are independent of the operation of the PCM protocol in the data transmission carried out between the network elements  101  and  102 , are those bits of the time slot TS  0 , the value of which is not defined in the technical standards dealing with PCM data transmission, and those bits of the time slot TS  0 , 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 . 
         [0066]      FIG. 4  illustrates an exemplary situation of how the time stamp information AL is included in the combined bit stream  111   b  to be transmitted by the data transmission link  107  in  FIG. 1 , in a system according to an embodiment of the invention. The areas  401  and  402  represent chronologically successive PCM frames, of which the PCM frame  402  is first in the transmission order. In this exemplary situation, five bits of the time stamp information AL are included in the bits  4 - 8  of the time slot TS  0 . The drawing reference AL(n) refers to a set of five successive bits in the time stamp information AL, and the drawing reference AL(n+1) refers to a set of the next five successive bits. The drawing reference TS  0  refers to the time slot TS  0  of the PCM frame, to be transmitted first, the drawing reference TS  1  refers to the next time slot of the PCM frame slot, and the drawing reference TS n refers to the last time slot of the PCM frame. 
         [0067]      FIG. 5  illustrates a system according to an embodiment of the invention for transmitting time stamp information between the network elements  501  and  502 . The system illustrated in  FIG. 5  enables a bidirectional transmission of time stamp information between the network elements  501  and  502 . The network element  501  is provided with means  511  for sending time stamp information to the data transmission link  507 . The means  511  correspond to the means  103 - 106  of the network element  101  illustrated in  FIG. 1 . The network element  501  is provided with means  512  for receiving time stamp information from the data transmission link  508 . The means  512  correspond to the means  108 - 110  of the network element  102  illustrated in  FIG. 1 . The network element  502  is provided with means  521  for sending time stamp information to the data transmission link  508 . The means  521  correspond to the means  103 - 106  of the network element  101  illustrated in  FIG. 1 . The network element  502  is provided with means  522  for receiving time stamp information from the data transmission link  507 . The means  522  correspond to the means  108 - 110  of the network element  102  illustrated in  FIG. 1 . 
         [0068]      FIG. 6  is a flow diagram illustrating a method according to an embodiment of the invention for transmitting time stamp information from a network element A to a network element B. In step  601 , in the network element A there is read, from the transmission buffer that contains protocol data units to be transmitted, the bit stream B 1  representing said protocol data units. In step  602 , in the network element A the time stamp information AL is placed in the bit stream B 2 . In step  603 , in the network element A there is produced a combined bit stream B 3  by multiplexing the bit stream B 2  in the bit stream B 1 . In step  604 , in the network element A said combined bit stream B 3  is transmitted to the data transmission link. In step  605 , in the network element B said combined bit stream B 3  is received from said data transmission link. In step  606 , in the network element B the bit stream B 1  and bit stream B 2  are separated from said combined bit stream B 3 . In step  607 , in the network element B time stamp information AL is read from the bit stream B 2 . 
         [0069]    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 B 3  that represent the bit stream B 2  or part of the bit stream B 2 . 
         [0070]    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 B 3  that represent the bit stream B 2  or part of the bit stream B 2 . Now the separation step  606  of the bit streams B 1  and B 2  is not needed. 
         [0071]    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 B 2  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. 
         [0072]    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. 
         [0073]    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. 
         [0074]    In a method according to an embodiment of the invention, the bit stream B 2  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. 
         [0075]    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. 
         [0076]    In a method according to an embodiment of the invention, the bit stream B 2  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. 
         [0077]    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. 
         [0078]    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. 
         [0079]    In a method according to an embodiment of the invention, the bit streams B 1  and B 2  are multiplexed in step  603  into a bit stream B 3  by using an SDH framing circuit that is arranged to place the protocol data units in the payload frame fields of the SDH frames. 
         [0080]    In a method according to an embodiment of the invention, the bit streams B 1  and B 2  are multiplexed in step  603  into a bit stream B 3  by using a Sonet framing circuit that is arranged to place the protocol data units in the payload frame fields of the Sonet frames. 
         [0081]    In a method according to an embodiment of the invention, the bit stream B 2  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. 
         [0082]    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. 
         [0083]    In a method according to an embodiment of the invention, the bit streams B 1  and B 2  are multiplexed in step  603  into a bit stream B 3  by using a PCM framing circuit that is arranged to place the protocol data units in the payload frame fields of PCM frames. 
         [0084]      FIG. 7  is a flow diagram illustrating a method according to an embodiment of the invention for transmitting time stamp information AL from a network element A to a network element B. In step  701 , in the network element A there is read, from the transmission buffer, the data frame D of the Ethernet protocol MAC (Media Access Control) layer. In step  702 , said MAC layer data frame D is transmitted to the data transmission link. In step  703 , in the network element A the time stamp information AL is placed in that interframe gap IFG of said MAC layer data frame D that is to be transmitted next. In step  704 , said interframe gap IFG is transmitted to the data transmission link. The data frame D and the interframe gap IFG, transmitted in succession, form the bit stream transmitted to the data transmission link. In step  705 , said MAC layer data frame D is received in the network element B from the data transmission link. In step  706 , said interframe gap IFG is received in the network element B from the data transmission link. In step  707 , the time stamp information AL is read in the network element B from said interframe gap IFG. 
         [0085]      FIG. 8  illustrates a network element  800  according to an embodiment of the invention. The network element is a transmission buffer  803  that is arranged to buffer the protocol data units D to be transmitted. The network element is a writing unit  806  that is arranged to place the time stamp information AL 1  in the bit stream  813   a . The dotted line arrow  821  represents an operation where the values of those bits that in a time slot between two points of time represent the bit stream  813   a  or part of the bit stream  813   a  are placed so that the set formed by said bits contains the time stamp information AL 1 . The network element is provided with a multiplexer  804  that is arranged to generate a combined bit stream  811   a  by merging the bit stream  813   a  in the bit stream  812   a  read from the transmission buffer  803  and representing the protocol data units. The network element is provided with a transmitter  805  that is arranged to transmit the combined bit stream  811   a  to the transmission port  850 . 
         [0086]    In a network element according to an alternative embodiment of the invention, said writing unit  806  is arranged to place the time stamp information AL 1  in the bits of the combined bit stream  811   a  emitted from the multiplexer  804 , which represent the bit stream  813   a  or part of the bit stream  813   a.    
         [0087]    A network element according to an embodiment of the invention is provided with a receiver  808  that is arranged to receive the bit stream  811   c  through the receiving port  851 , a demultiplexer  809  that is arranged to separate the bit stream  813   c  and the bit stream  812   c  representing the protocol data units from the bit stream  811   c.    
         [0088]    The network element is provided with a reading unit  810  that is arranged to read the time stamp information AL 2  from the bit stream  813   c . 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  813   c  or part of the bit stream  813   c  are read. 
         [0089]    In a network element according to an embodiment of the invention, the reading unit  810  is arranged to read the time stamp information AL 2  from those bits of the bit stream  811   c  flowing to the demultiplexer  809  that represent the bit stream  813   c  or part of the bit stream  813   c.    
         [0090]    A network element according to an embodiment of the invention is an IP (Internet Protocol) router. 
         [0091]    A network element according to an embodiment of the invention is an Ethernet switch. 
         [0092]    A network element according to an embodiment of the invention is a base station of a mobile phone network. 
         [0093]    A network element according to an embodiment of the invention is an MPLS (Multi Protocol Label Switching) switch. 
         [0094]    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.