Abstract:
There is provided a node for facilitating time distribution in communication networks, and more specifically for time synchronization in digital television (DTV) distribution network. The node comprises an interface, a clock for establishing a local time, and a time-locked loop. The interface is configured for interconnecting the node to at least one neighboring node over an isochronous transport link for transmission and reception of repetitive frames comprising time information. The time-locked loop is configured for, based on remote time information received via the interface and local time information from the clock, synchronizing the clock to the clock of one of the at least one neighboring node. This facilitates that the node, or a corresponding synchronous network comprising nodes according to the inventive concept, is rather insensitive to network delays. In this way the requirements on the network infrastructure are reduced. In particular, there is no need for dedicated networks. Further, a synchronous network, a method for the node and a method for a synchronous network is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Stage of International Application No. PCT/EP2010/058210, filed on Jun. 11, 2010. The content of the above application is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to time distribution in communication networks, and more specifically to time synchronization in digital television (DTV) distribution networks. 
     BACKGROUND OF THE INVENTION 
     Distribution of digital terrestrial television (DTT) and mobile digital television (MDTV) frequently utilizes single frequency networks (SFN). In an SFN, several transmitters simultaneously send the same signal over the same frequency channel. 
     The transmitters in an SFN must be synchronized to send their signals at the same time to avoid interference at the receiving antennas. This is commonly achieved by installing global positioning system (GPS) receivers at all transmitter sites. GPS receivers, however, may be easily intentionally or unintentionally jammed, or fail for other reasons such as equipment failure, and represent an additional cost in the network in terms of equipment and supervision. Further, the military control of the GPS may be an issue. 
     Also known are techniques for time synchronization of network nodes without utilization of GPS. For instance, the network time protocol (NTP) may be used to synchronize the clocks of network nodes to a master node or a reference clock using time stamps. However, the accuracy of NTP, at least in non-dedicated networks, is far too limited for the purpose of time synchronization in digital television (DTV) distribution networks. 
     WO 2008/103170 A discloses a method of a network client for extracting a reference frequency carried in the physical layer of a network signal originating from a server, and for using it to stabilize an oscillator of a clock of the client. The method also includes determining a clock correction value based on a server time stamp and a client time stamp. 
     U.S. Pat. No. 7,535,931 B discloses a two-way time transfer protocol for estimating a time error between the clocks of two network nodes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a more efficient alternative to the above techniques and prior art. 
     More specifically, it is an object of the present invention to provide an improved node for time synchronization in a synchronous network and a method for time synchronization in a synchronous network with integrated Time Transfer functionality, and which is suited for integration in an existing synchronous network. 
     These and other objects of the present invention are achieved by means of a node for a synchronous network having the features defined in independent claim  1 , by means of a method of a node in a synchronous network defined in independent claim  7 , by means of a synchronous network according to claim  6 , and by means of a method of a synchronous network defined in independent claim  12 . Embodiments of the invention are characterized by the dependent claims. 
     According to a first aspect of the invention, a node for a synchronous network is provided. The node comprises an interface, a clock, and a time-locked loop. The interface is configured for interconnecting the node to at least one neighboring node over an isochronous transport link. The isochronous transport link is configured for transmission and reception of repetitive frames. The repetitive frames comprise time information. The clock is configured for establishing a local time. The time-locked loop is configured for synchronizing the clock to the clock of one of the at least one neighboring node. The synchronization utilizes remote time information received via the interface, local time information, and delay compensation from said remote node. 
     According to a second aspect of the invention, a synchronous network comprising a plurality of nodes according to the present invention is provided. 
     According to a third aspect of the invention, a method of a node in a synchronous network is provided. The synchronous network comprises a plurality of nodes. Each node is interconnected via an interface to at least one neighboring node of the plurality of nodes over an isochronous transport link. The isochronous transport link is configured for transmission and reception of repetitive frames. The repetitive frames comprise time information. The method comprises the steps of receiving remote time information, sending local time information, and synchronizing a clock of the node to the clock of the one of the at least one neighboring node. The remote time information is received from one of the at least one neighboring node via the interface. The local time information is sent to the at least one neighboring node via the interface. In the step of synchronizing a clock of the node to the clock of the one of the at least one neighboring node, remote time information and local time information are utilized. 
     According to a fourth aspect of the invention, a method of a synchronous network is provided. The synchronous network comprises a plurality of nodes. The nodes are interconnected in pairs over isochronous transport links. The method comprises the steps of adapting a network synchronization topology and synchronizing the plurality of nodes to a master node. The step of synchronizing the plurality of nodes to a master node utilizes bidirectional exchange of time information over the isochronous transport links. 
     The present invention makes use of an understanding that the transport network used to distribute data or contents, e.g., video data in the case of a DTV distribution network, may be used to distribute time information from a master node to all nodes in the network. In that way, a time synchronization of all nodes in the network may be achieved while at the same time eliminating the need for GPS receivers. 
     According to an embodiment of the invention, the local time information and the remote time information are used in a two-way Time Transfer method. In other words, a Time Transfer method based on bidirectional exchange of time information between neighboring nodes in the network is utilized. 
     According to an embodiment of the invention, the time-locked loop is configured for synchronizing the clock to the clock of one of the at least one neighboring node. The synchronization utilizes a time difference calculated from the remote time information and the local time information. To this end, the clock of a network node which is to be synchronized to the clock of a neighboring node, referred to as the source node, is phase-locked to the time difference between the clocks of the two nodes. The time difference between the nodes may be calculated according to a two-way Time Transfer method. By phase-locking the clock of a node to the time difference, a time-locked loop is achieved. The embodiments of the present invention described hereinabove are advantageous since they facilitate a system which is rather insensitive to network delays. In that way the requirements on the network infrastructure are reduced. In particular, there is no need for dedicated networks. 
     According to an embodiment of the invention, the interface is further configured for selecting one of the at least one neighboring node as a source for the remote time information. The selection is performed according to a synchronization topology of the synchronous network. Thus, if a network node is connected to several neighboring nodes, any one of the neighboring nodes may be chosen as the source node for synchronizing the clock of the node. The source node can either be predetermined, e.g., by configuring the node during network roll-out, or may be chosen dynamically. The source may, e.g., be chosen based on the current status of communication links and network nodes, or based on measurements performed on achieved synchronization stability. The source node may be chosen according to a network synchronization topology conveyed by a synchronization protocol. 
     According to an embodiment of the invention, the synchronous network is a Dynamic Synchronous Transfer Mode (DTM) network. DTM networks are based on time division multiplexing and designed to provide a guaranteed quality of service (QoS), e.g., for streaming video, but may also be used for packet based services. All nodes of a DTM network are synchronized with respect to frequency and relative phase. Embodiments of the invention may also employ other synchronous networks, such as SDH/SONET or synchronous Ethernet. 
     According to an embodiment of the invention, a synchronous network is provided. The synchronous network comprises a plurality of nodes. The nodes are interconnected in pairs over isochronous transport links. The nodes are configured for adapting a network synchronization topology for synchronizing the plurality of nodes to a master node. The synchronization is performed by bidirectional exchange of time information over the isochronous transport links. In other words, in a network according to an embodiment of the invention, every node may be synchronized to maintain the same absolute phase, which phase is dictated by a master node of the network. The master node may, e.g., be controlled by a reference clock. 
     The synchronization of the network nodes is achieved using a two-way Time Transfer method such that a source node transfers its local time to its neighboring nodes, which neighboring nodes return their respective local time. A time difference between the source node and the respective neighboring nodes may then be calculated and be used to compensate the clocks of the neighboring nodes such that their respective local time is the same as the local time of the source node. This amounts to maintaining the same absolute phase at the neighboring nodes as the source node. Once a neighboring node is time-locked to its source node, it may in turn distribute its local time to its neighboring nodes such that the neighboring nodes may time-lock to the source node. The process may continue until all nodes in the network have acquired the same absolute phase, i.e., the same local time, as the master node. A network according to an embodiment of the invention may comprise more than one master node to achieve redundancy. If the present master node fails, or a communication link connecting the present master node to the rest of the network fails, a different master may be assigned and the network synchronization topology is adapted for distributing the time of the new master node to the other nodes in the network. 
     According to an embodiment of the method for a synchronized network, the method further comprises to in an individual node time multiplex time information from a plurality of nodes being interconnected with the individual node, and subsequently synchronize the time multiplexed time information to the master node in that individual node. 
     Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, in which: 
         FIG. 1  shows an SFN for DTV distribution using GPS synchronization. 
         FIG. 2  shows an SFN for DTV distribution using Time Transfer synchronization, according to an embodiment of the invention. 
         FIG. 3  illustrates a DTM frame. 
         FIG. 4  shows a network node, in accordance with an embodiment of the invention. 
         FIG. 5  shows a network synchronization topology, in accordance with an embodiment of the invention. 
         FIG. 6  illustrates two-way Time Transfer, in accordance with an embodiment of the invention. 
     
    
    
     All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a conventional DTV distribution network  100  for SFN transmission using GPS synchronization. At the headend site  101 , program streams from various input channels may be received and multiplexed into suitable transport streams, e.g., MPEG transport streams in DVB-ASI format (Digital Video Broadcasting—Asynchronous Serial Interface). Timing information retrieved from a GPS receiver  104   1  is inserted in to the transport stream. 
     The transport streams are transmitted through the transport network  102  which provides multicast connections from the headend site  101  to all transmitter sites  103 . The transport network  102  may, e.g., comprise optical fiber or microwave links. 
     At the transmitter sites  103 , the propagation time through the transport network  102  may be compensated by comparing the inserted timing information with a local time reference provided by GPS receivers  104  which the transmitter sites  103  are equipped with. By comparing the received timing information with the local time, an additional delay for synchronization of SFN transmission may be calculated. 
     The timing information may, e.g., be provided by the GPS receivers  104  to the headend site  101  and the transmitter sites  103  using both a 10 MHz frequency reference and a Pulse Per Second (PPS) time reference. The PPS time reference may be divided into 100 ns steps provided by the cycles of the 10 MHz frequency reference. This is used to time stamp the video transport stream in the head end site  101 . The time stamp is embedded into the transport stream and allows the transmitter sites  103  to synchronize the signal with the locally available GPS signal such that the transmitters will transmit the signal at almost the same time. The time stamping of the video stream is typically performed in an SFN Adapter. 
       FIG. 2  shows a DTV distribution network  200  according to an embodiment of the invention. As in a conventional distribution network  100 , discussed with reference to  FIG. 1 , program streams received from various input channels may be multiplexed into suitable transport streams, e.g., MPEG transport streams in DVB-ASI format, at the headend site  201 , together with timing information retrieved from a GPS receiver  204 . 
     The transport streams are transmitted through the transport network  202  which provides multicast connections from the headend site  201  to all transmitter sites  203 , i.e. nodes in the network. The transport network  202  may, e.g., comprise optical fiber or microwave links. 
     In contrast to a conventional DTV distribution network  100 , the distribution network  200  shown in  FIG. 2  does not utilize GPS receivers at the transmitter sites  203 . Instead, the nodes of the distribution network  200 , i.e., the headend site  201  and the transmitter sites  203 , are synchronized by exchanging time information over the transport network  202 . 
     With a DTV distribution network  200  according to an embodiment of the invention, distribution of time information for the purpose of synchronizing all network nodes is accomplished over the same transport network  202  which carries the video signals. At the headend site  201 , the same time reference signals as in a conventional DTV distribution network  100  are used, which is provided by the reference clock  204  in the form of a GPS receiver  204  or any other suitable reference clock  204 . Note that the headend site  201  and the reference clock  204  are not necessarily arranged at the same location. To illustrate that the time stamp for the video signal is not restricted to having the reference clock  204  by its side, the head end site  201  is here divided into two nodes,  201   1  and  201   2 . Node  201   1  provides timestamps for the video signals in the network, and node  201   2  provides the reference clock for synchronizing all network nodes, including node  201   1 . In an alternative embodiment the head end site  201  is a single node providing both timestamps for the video signals, and the time reference from the reference clock  204  for synchronizing all network nodes. In an alternative embodiment several head-end nodes may exist for global or local distribution over network  202  to all or a subset of the transmitter nodes  203 . The time synchronization information is distributed through the transport network  202 , and at the transmitter sites  203  the same synchronization information is provided to the SFN transmitter as is provided by a GPS receiver  104  in a conventional distribution network  100 . 
     A DTV distribution network  200  according to an embodiment of the invention may, e.g., be based on a Dynamic Synchronous Transfer Mode (DTM) network as standardized by the European Telecommunications Standards Institute (ETSI). DTM is designed to provide a guaranteed quality of service (QoS), e.g., for streaming video and audio, but can also be used for packet-based services. The transport mechanism of DTM is based on time division multiplexing and is in this sense similar to SDH/SONET, albeit more flexible and adapted to other types of traffic and applications. The signaling system on the other hand could be compared to what is available in packet-based technologies such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP). 
     As is illustrated in  FIG. 3 , in DTM transport the link capacity is divided into fixed size frames  301  of 125 microseconds (μs) duration, which are further divided into a number of 64-bit time slots. The number of time slots per frame is dependent on the bit rate of the link. Slots can be used either for network internal signaling, i.e., as control slots  302 , or for user traffic, i.e., as data slots  303 . As each slot is repeated 8000 times per second, the transport capacity of a slot is 512 kbps. 
     In  FIG. 4  a node  400  for a DTV distribution network in accordance with an embodiment of the invention is illustrated. Note that node  400  corresponds to a node  203  as described above. Node  400  comprises a network interface  401 , for connecting the node to at least one neighboring node and for sending and receiving data over a pair of unidirectional transport links  402 . The transport links may, e.g., be DTM links. Node  400  is further associated with a transmitter  405  for transmitting synchronized streams, thus the transmitter  405  comprises a synchronization unit  403  for synchronizing the transmission of a transport stream with a time reference received from the node  400 . The node  400  provides the working signal to be transmitted in the form of a ASI-signal, 10 MHz, and a PPS time reference, containing information about the time and frequency by which the working signal is to be transmitted, to the synchronization unit  403  in the transmitter  405 . The timing interface, which comprises a synchronization unit  404 , regenerates the timing in the form of 10 MHz and PPS such that convenient interfacing to the transmitter  405  and synchronizer  403  is achieved. In addition the TV signal transport stream is provided over the interface  405  such as ASI and Ethernet. The transmitter  405  synchronizes the transport stream using the timing signals prior to modulation, amplification and transmission. Further, the time reference in node  400  is provided by the local clock, i.e. the synchronization unit  404 , synchronized to a reference clock of the DTV distribution network. The local clock  404  communicates with interface  401  for exchanging, i.e., sending and receiving, time information with at last one neighboring node connected through node  400  over a transport link  402 . 
     Multiple interfaces such as represented by links  402   1  and  402   2  may be time multiplexed in interface  401  prior to being sent to the centralized phase measurement in clock  404 . This represents the fourth aspect of the invention, in that it allows for a simple interface  401  and thus lower implementation complexity when extending the synchronous transport system to include Time Transfer. The synchronous signal reoccurring with the nominal period of 125 μs and having low jitter allows for a time-multiplexing among the interfaces and a single central measurement of the high resolution fractional measure. 
     In  FIG. 5 , synchronization of nodes in a DTV distribution network  500  is illustrated. The network  500  comprises a plurality of nodes  501 . Some of the nodes, e.g., nodes  501   1  and  501   5 , are equipped with reference clocks  502 , e.g., GPS receivers, and may serve as master nodes for time distribution throughout the network  500 , i.e., for providing time information the other nodes of the network  500 . Typically, one master node, e.g., node  501   1 , is used to synchronize all nodes of the network, and other nodes provided with reference clocks, such as node  501   5  in  FIG. 5 , serve as backup nodes. Optionally, each of the nodes  501   1  and  501   5  may serve as a master node for a part of the network. 
     The dotted  503  and dashed  504  lines illustrate possible synchronization topologies for distributing timing information from the master nodes  501   1  and/or  501   5  to the other nodes of the network  500 . If network  500  is a DTM network, the synchronization topology is automatically determined by the DTM synchronization protocol (DSYP). In case of failure of a synchronization path, the DSYP will recalculate the synchronization tree enabling automatic synchronization restoration and avoiding synchronization loops in the network. 
     In a DTV distribution network according to an embodiment of the invention, a two-way Time Transfer method is employed for synchronizing all nodes of the network to a reference clock. Using two-way Time Transfer, a source node transfers its local time to its the neighboring nodes. The neighboring nodes return their time to the source node. The nodes may then calculate a time difference which may be used for synchronizing to the clock of the source node. This process is repeated until all nodes in the network operate on the same time. For instance, with reference to  FIG. 5 , node  502   2  may synchronize its clock with the clock of node  501   1 , which is controlled by a reference clock  502   1 . Then, node  501   4  may synchronize with node  501   2 , and so forth. 
     Two neighboring nodes according to an embodiment of the invention may synchronize their respective clocks by employing bidirectional exchange of time information according to a two-way Time Transfer scheme. This is achieved by transmitting time information to the neighboring nodes, and by receiving time information from the neighboring nodes. If the DTV distribution network is a DTM network, time information may be transmitted over a dedicated slot, i.e., a Time Transfer channel, e.g., slot  302  in  FIG. 3 . The Time Transfer channel may be used for transmission of time stamps, time difference measurements, correction factors, and various statistics between nodes involved in two-way Time Transfer. 
     In an embodiment according to the present invention, instead of performing a separate routing of the synchronization in the network, which typically is a 8 kHz frequency distribution, and a separate routing of the Time Transfer distribution in the network, the routing of both distributions are at all times strictly restricted to a common routing. In addition to the double functionality, separate routing demands for a complex fault management. The common routing may require minor adjustments in the routing algorithm for optimized performance. However, the adjustments for the common routing is associated with a much less demanding operation than the fault management for separate routing. Further, common routing of the distributions provides an increased lucidity for a selected routing. 
     The Time Transfer channels are arranged such that they provide the same information in both directions. Thus, the information is independent of the selected synchronization/Time Transfer routing in the network. As a node is selected as source node for a neighboring node, it has all information, such as time stamps, time difference measurements, correction factors, and various statistics between nodes involved in two-way Time Transfer available. All nodes are thus automatically potential source nodes for their neighboring nodes. Thereby, the actual synchronization/Time Transfer routing in the network is based on local selections at each node. This enables hitless re-routing of Time Transfer as decided by dynamic synchronization routing. 
     The main principle of two-way Time Transfer is illustrated in  FIG. 6 . Time is to be distributed from a source node A, with a local time scale t A , to a slave node B, with a local time scale t B . The source node may retrieve its time scale from a reference clock, e.g., a GPS receiver, or it may be synchronized to a master node of the network. Through the DSYP protocol, node B is configured for receiving time information from node A. 
     It may be further realized that time stamp interchange may be overlapped (t B1 &lt;t B3 &lt;t B2 ) or reversed order (t B3 &lt;t B1 &lt;t B2 ) without changing the functionality as long as the exchange is relatively close in time. Further, node A may insert its local time t A1  into a stream which is transmitted to node B and reaches node B at local time t B2 . 
     A pseudo-range observation p AB =t B2 −t A1  is formed in the receiver at node B. The local clock of node A is then t A2 . In the same way, node B may send a time stamp to node A at local times t B3  and t A3 , respectively, which is received at node A at local times t A4  and t B4 , respectively. A pseudo-range observation p BA =t A4 −t B3  is formed in the receiver at node A. Further, the following relations apply:
 
Δ T=t   A   −t   B  
 
 t   A4   =t   B4   +ΔT  
 
 t   A2   =t   B2   +ΔT  
 
 t   A2   =t   A1   +d   AB,link  
 
 t   B4   =t   B3   +d   BA,link ,
 
where d AB,link  and d BA,link  are the transmission delays from node A to node B, and vice versa, respectively. The estimated time error ΔTE between the
         nodes A and B can then be expressed as:       

                 Δ   ⁢           ⁢   TE     =           p     BA   ,   link       -     p     AB   ,   link         2     =       Δ   ⁢           ⁢   T     +         d     BA   ,   link       -     d     AB   ,   link         2           ,         
while the Round Trip Time (RTT) can be expressed as:
 
RTT= d   BA,link   +d   AB,link   =p   BA,link   +p   AB,link .
 
     The two-way Time Transfer is based on bidirectional exchange of time information between a pair of interfaces. In a basic mode of operation, the propagation delays over the link, d AB,link  and d BA,link , respectively, may be assumed to be symmetric and may be calculated from the measured round trip time (RTT), which is the sum of the transmission delay of the link connections nodes A and B, d AB,link  and d BA,link , according to: 
     
       
         
           
             
               d 
               
                 AB 
                 , 
                 link 
               
             
             = 
             
               
                 d 
                 
                   BA 
                   , 
                   link 
                 
               
               = 
               
                 RTT 
                 2 
               
             
           
         
       
     
     In case of asymmetric transmission delays, i.e., d AB,link ≠d BA,link , a calibration constant c asym  may be used to take the measured asymmetry into account:
 
 d   AB,link =RTT× c   asym  and  d   BA,link =RTT×(1− c   asym ),
 
where 0&lt;c asym &lt;1, and c asym =0.5 for symmetric transmission delays. The determination of the calibration constant c asym  requires knowledge of the round trip time RTT and the asymmetry error ΔE which is known when the link is operating and both nodes receive correct time through other time sources than the link to be calibrated. The asymmetry error ΔE is formed from the ΔTE expression under the assumption that t A =t B  in which case we get:
 
               Δ   ⁢           ⁢   E     =           p     BA   ,   link       -     p     AB   ,   link         2     =         d     BA   ,   link       -     d     AB   ,   link         2             
Since the sum of d AB,link  and d BA,link  is known as RTT, d AB,link  and d BA,link  can be calculated as:
 
 d   AB,link =RTT/2−Δ E  and  d   BA,link =RTT/2+Δ E  
 
Given this value, the c asym  value is easy to calculate form either of the d AB,link  or d BA,link  values to become:
 
 c   asym   =d   AB,link /RTT=1− d   BA,link /RTT=−½−Δ E /RTT
 
The input and output delays of the interface, d A,out  and d B,in , respectively, are used in expressing the transmission delays as:
 
 d   AB   =d   A,out   +d   AB,link   +d   B,in ,
 
where d AB,link  is the transmission delay of the link connections nodes A and B. A corresponding relation applies to d BA . The compensated values d AB,link  and d BA,link  can be calculated from d AB  and d BA  as:
 
 d   AB,link   =d   AB   −d   A,out   −d   B,in  and  d   BA,link   =d   BA −d B,out   −d   A,in .
 
     The observed pseudo-ranges thus becomes after compensation:
 
 p   AB,link   =p   AB   −d   A,out   −d   B,in  and  p   BA,link   =p   BA   −d   B,out   −d   A,in .
 
As described above with reference to  FIG. 3 , fixed size frames of 125 μs duration are used in the DTM transport. Thus, the local time scale of a node may be established by dividing the time scale using a monotonously increasing integer and a value representing the fraction of a 125 μs periodic clock, which may represent a time-scale such as that of international atomic time (TAI) or other suitable time-scale. For node A, e.g., the local time scale can be expressed as:
 
 t   A =( n   A +frac A )×125 μs.
 
A corresponding relation applies to the local time of node B:
 
 t   B =( n   B +frac B )×125 μs.
 
The time stamps taken at the receiver side will consist of the integer fractional value n A  and n B  as well as the fractional resolution values frac A  and frac B  to form a high resolution continuous time scale. The use of the frame start on the transmitting side occurs at frac A =0, so no explicit fractional time needs to be transferred between the nodes. The equipment delay from the node frame start to the frame start on the connector of the equipment is contained in the output delay d A,out  and d B,out  for node A and B respectively. Similarly the delay from the input connector to the actual fine resolution measurement of fractional values is contained in the measurements allow for high resolution and low jitter values compared to time-stamping of packet-based messages as being done in previous art (such as NTP).
 
     Further, the pseudo-range observations p AB  and p BA  are then in fractional form expressed as:
 
 p   AB =( n   B2 +frac B2   −n   A1 )×125 μs
 
 p   BA =( n   A4 +frac A4   −n   B3 )×125 μs
 
     The calculated time error ΔTE would require the node to shift its frequency such that the time error becomes zero. The major part of ΔTE, i.e. multiples of 125 μs, may be adjusted by a coarse adjustment of the local clock  404 . The remaining part of ΔTE could either force the phase of the local clock to become time-aligned such that it operates in an absolute time mode. An alternative approach would be to accept the remaining time error as the offset factor TE 0  (by initiating it with ΔTE when achieving time-lock), and operate the Time Locked Loop on the relative time error TER=ΔTE−TE 0 . The TE 0  for the node needs to be transmitted along with the time from the node, such that any receiver of time may correct for the remote node TE 0 . The complete relative time error becomes TER=ΔTE−TE 0   L +TE 0   R , where TE 0   L  and TE 0   R  are the offset factors for the local node and the remote node, respectively. 
     In a conventional DTM network, the DTM equipment clock (DEC) tracks the 8 kHz synchronization timing received on the incoming interfaces, as selected by the DSYP. In a DTV network using two-way Time Transfer, the phase measurement of the DEC phase-locked loop now needs to include the time difference between the local and the remote node in order to resolve the 125 μs ambiguity between the nodes. This turns the functionality of the DEC into a time locked loop (TLL). 
     The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, embodiments of the invention may be based on other network technologies than DTM.