Abstract:
A method and system are provided for allowing time-alignment of teleprotection measurements of power signals. Teleprotection observations are communicated between teleprotection ends through a packet switched network. At each end of a teleprotection segment, a teleprotection device communicates with the network through a router providing CES and located at the edge of the network. Clocks within the two routers are synchronized using IEEE 1588v2 signals. Using this synchronization, accurate one-way latency of data through the network between the two routers in each direction can be determined. The router at the incoming end of the faster path delays playout of packets by the difference between the two one-way latencies, thereby ensuring time-alignment of observations as they are sent from the routers to the teleprotection devices.

Description:
FIELD OF INVENTION 
     This invention relates to teleprotection in power grids, and more particularly to symmetric communication of teleprotection signals. 
     BACKGROUND 
     Teleprotection is an essential requirement for operating and maintaining a reliable, robust, and safe electrical power grid. The current of a power signal is measured at a first location and transmitted over a communication channel to a second location. Simultaneously, the current of the power signal is measured at the second location and transmitted over a communication channel to the first location. Comparison of time-aligned observations of the power signal may reveal different measured values of the current at each location. This is usually indicative of a fault in the power grid, and action can be taken to remedy the fault. 
     The current is measured several times per cycle of the power, and therefore time-alignment is very important. There is inevitably some delay in communicating between the two locations, but TDM networks offer a very symmetric communication channel. Delays introduced by the communication channel in one direction are generally the same as delays introduced by the communication channel in the other direction. The delays are effectively the same in each direction, so time-alignment of observations is still possible. 
     Nowadays core networks are evolving to packet switched networks. However legacy systems still require TDM services. Circuit Emulation Services (CES) are used to provide TDM services, as are required by legacy teleprotection systems, over packet switched networks. The routers at the edge of the packet switched network provide CES, and the devices of the teleprotection system which measure the current of the power signal send their measured observations to the routers over T1/E1 lines. The routers use their CES to transmit the measured observations to each other as packet data. The observations are then converted back into TDM format and sent to the teleprotection devices over T1/E1 lines, where they are used in teleprotection analysis. 
     However packet networks are asymmetric, in that the delay introduced by the network in one direction is not necessarily the same as the delay introduced by the network in the other direction. Differences between the delays may arise for example because different paths are used in the directions, or because of differences in store-and-forward techniques along the paths. The asymmetry in communications makes time-alignment of the power observations difficult, and therefore teleprotection analysis more difficult. 
     A system and method which allowed improved time-alignment of observations even when transmitted over asymmetric communication networks would allow teleprotection systems to better use packet switched networks. 
     SUMMARY 
     According to one aspect, a method of providing Circuit Emulation Service over Packet (CESoP) over a packet switched network is provided. The packet switched network includes a first router in TDM communication with a first device and a second router in TDM communication with a second device. The first router and the second router are separated by the packet switched network. The one-way latency from the second router to the first router is determined, this latency being termed the inbound latency. The one-way latency from the first router to the second router is determined, this latency being termed the outbound latency. The inbound latency and the outbound latency are compared. If the outbound latency is lower than the inbound latency, TDM playout to the first device is initiated when a jitter buffer reaches a playout level. If the inbound latency is lower than the outbound latency, TDM playout to the first device is initiated after the jitter buffer reaches the playout level and then a duration equivalent to the difference between the inbound latency and the outbound latency has elapsed. 
     According to another aspect, a first router providing Circuit Emulation Service over Packet (CESoP) to a first device is provided. The router includes a CESoP processor for receiving packets received over a packet switched network and playing out the packets into a TDM bitstream to the first device. The router also includes a jitter buffer. The router also includes a symmetry enforcer for determining a difference between (1) the one-way latency from the first router to a second router providing CESoP to a second device and with which the first router is in communication over the packet switched network and (2) the one-way latency from the second router to the first router, and for initiating TDM playout to the first device after the fill level of the jitter buffer reaches a playout level and then a duration equal to the difference between the one-way latencies has elapsed. 
     According to yet another aspect, another method of providing Circuit Emulation Service over Packet (CESoP) over a packet switched network is provided. The packet switched network includes a first router in TDM communication with a first device and a second router in TDM communication with a second device. The first router and the second router are separated by the packet switched network. At each router, the one-way latency from the second router to the first router is determined, this latency being termed the inbound latency for the first router and the outbound latency for the second router. At each router, the one-way latency from the first router to the second router is also determined, this latency being termed the outbound latency for the first router and the inbound latency of the second router. At each router, the inbound latency and the outbound latency of the router are compared. At each router, if the outbound latency of the router is lower than the inbound latency of the router, TDM playout to the device in TDM communication with the router is initiated when the jitter buffer of the router reaches a playout level. At each router, if the inbound latency of the router is lower than the outbound latency of the router, TDM playout to the device in TDM communication with the router is initiated after the jitter buffer of the router reaches the playout level and then a duration equivalent to the difference between the inbound latency and the outbound latency has elapsed. 
     According to yet another aspect, a system for providing CESoP between two devices is provided. A first router is located at the edge of a packet switched network and is in TDM communication with a first of the devices. The first router has a jitter buffer, and initiates TDM playout of packets to the first device after the fill level of its jitter buffer reaches a playout level. A second router is located at the edge of the packet switched network and is in TDM communication with the second of the devices. The second router has a jitter buffer, and initiates TDM playout of packets to the second device only after the fill level of its jitter buffer reaches the playout level and then a duration equal to the difference between the two one-way latencies between the routers has elapsed. 
     The methods of embodiments of the invention may be stored as logical instructions on a non-transitory computer-readable storage medium in a form executable by a computer processor. 
     Embodiments of the invention allow teleprotection communications to occur over a packet switched network. By synchronizing the routers and by using different playout times in each router, differences in one-way latency over the packet switched network can be compensated for, allowing accurate teleprotection to be carried out even over a packet switched network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of embodiments of the invention will become more apparent from the following detailed description of the preferred embodiment(s) with reference to the attached figures, wherein: 
         FIG. 1  is a block diagram of a portion of a teleprotection system according to one embodiment of the invention; 
         FIG. 2  is a block diagram of a portion of the teleprotection system of  FIG. 1 , showing a router in greater detail, according to one embodiment of the invention; 
         FIG. 3  is a block diagram of parts of either router of  FIG. 1  according to one embodiment of the invention; 
         FIG. 4  is a flowchart of a method carried out by either router of  FIG. 3  according to one embodiment of the invention; and 
         FIG. 5  is a block diagram of a computing environment according to one embodiment of the invention. 
     
    
    
     It is noted that in the attached figures, like features bear similar labels. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a block diagram of a portion of a teleprotection system according to one embodiment of the invention is shown. A first teleprotection device  10  communicates with a second teleprotection device  12  through a packet switched network  14 . Each teleprotection device  10  and  12  provides teleprotection services, including measurement of power signals. The teleprotection devices  10  and  12  exchange the measurements as packets over the packet switched network  14 . The first teleprotection device  10  accesses the packet switched network through a first router  16 , and the second teleprotection device  12  accesses the packet switched network  14  through a second router  18 . Each router  16  and  18  provides Circuit Emulation Services, encapsulating TDM signals into packets. This allows the teleprotection devices  10  and  12  to communicate with the respective router  16  and  18  using TDM, such as through a T1 line or an E1 line, while the routers  16  and  18  communicate with each other using packets over the packet switched network  14 . In this way, from the point of view of the teleprotection devices  10  and  12  the power signal measurements are transmitted in accordance with TDM, yet much of the exchange of these measurements is packet form over the packet switched network  14 . 
     Each router  16  and  18  is in communication with a reference clock  20 . The reference clock  20  is usually on a telecommunications node within the packet switched network  14 , but is shown in  FIG. 1  as separate from the packet switched network  14  for clarity. The reference clock  20  uses IEEE 1588v2 signals to synchronize the clocks of the routers  16  and  20 . Alternatively, a different timing over packet technology system can be used to synchronize the routers  16  and  18  using the reference clock  20 . 
     Being a packet switched network  14 , the path followed by packets from the first router  16  to the second router  18  may be different from the path followed by packets from the second router  18  to the first router  16 . Even if the same path is used, the store-and-forward mechanisms used throughout the packet switched network  14  may result in different transit times in each direction. The travel time for a packet to leave one router and a second router is referred to as the one-way latency. The one-way latency for packets travelling from the first router  16  to the second router  18  is referred to herein as L AB . The one-way latency for packets travelling from the second router  18  to the first router  16  is referred to herein as L BA . 
     Referring to  FIG. 2 , a block diagram of a portion of the teleprotection system of  FIG. 1 , showing a router in greater detail, according to one embodiment of the invention is shown. Details of router  16  are shown, but router  18  contains similar components. The router  16  includes a packetization processor  30 , a jitter queue  32 , and a packet to TDM interworking function  34 . Measurements  36  made by the teleprotection device  10  arrive at the router  16  in the TDM bitstream. The measurements are packetized by the packetization processor and sent as outgoing packets  38  to the rest of the packet switched network  14  (and ultimately to the router and teleprotection device at the other end of the teleprotection system). Measurements from the other teleprotection device arrive at the router  16  via the packet switched network  14  as incoming packets  40 . The incoming packets  40  are placed in the jitter buffer  32 , and then sent to the packet to TDM interworking function  34  where the measurements are played out in the TDM bitstream and sent to the teleprotection device  10 . 
     Referring to  FIG. 3 , a simplified block diagram of the first router  16  of  FIG. 1  according to one embodiment of the invention is shown. In particular,  FIG. 3  shows the components of the router  16  involved in Circuit Emulation Service over Packet (CESoP) operations. The second router  18  includes the same components shown in  FIG. 3 . The first router  16  includes a teleprotection supporter  50 . The teleprotection supporter  50  is in communication with the reference clock  20  and with an internal clock  52  of the first router  16 . The teleprotection supporter  50  is also in communication with a CESoP processor  53  which controls the CESoP functions of the router. The CESoP processor  53  is in communication with a packet transmit/receive function  54 , including a packet switch and interfaces, which in turn is in communication with the rest of the packet switched network  14  (not shown in  FIG. 3 ). The CESoP processor includes the TDM interworking function  34  and is in communication with a TDM transmit/receive function  55 , which in turn is in communication with the first teleprotection device  10 . The CESoP processor  53  is also in communication with a buffer memory  56 . A portion of the buffer memory  56  comprises the jitter buffer  32 . 
     Broadly, in a teleprotection system in which two routers separated by a packet switched network are each in TDM communication with a respective teleprotection device, one of the routers initiates playout of packets to its associated teleprotection device after the fill level of its jitter buffer reaches a playout level. The other router initiates playout of packets to its associated teleprotection device after the fill level of its jitter buffer reaches the playout level and then a duration equal to the difference between the two one-way latencies between the routers has elapsed. 
     Referring to  FIG. 4 , a flowchart of a method carried out by the teleprotection supporter  50  of  FIG. 3  according to one embodiment of the invention is shown. A similar method is carried out by the teleprotection supporter of the second router  18 . At step  60  the teleprotection supporter  50  synchronizes the first router  16  with the reference clock  20  using IEEE 1588v2, or alternatively using another timing over packet technology, updating the local clock  52 . Since the method is also carried out by the teleprotection supporter  50  of the second router  18 , the local time of the first router  16  is synchronized with the local time of the second router  18  to a degree enabled by the particular timing over packet technology used to synchronize the routers. 
     At step  62  the teleprotection supporter  50  determines the one-way latency in each direction with the second router  18 . Integrated OAM tool capabilities at the IP Layer (Ping, TWAMP), MPLS layer or Ethernet layer (ITU-T. Y.1731) for example, can be used to determine the one-way latency in each direction. While the two routers  16  and  18  are synchronized (in time), the teleprotection supporter  50  instructs the packet transmit/receive function  54  of the first router  16  to send an OAM packet, with a timestamp (T1) indicated by the local clock  54 , to the second router  18 . At the second router  18 , the OAM packet is received and immediately timestamped (T2) This OAM packet is timestamped again (T3) immediately before it is sent back to the first router  16 . When the first router  16  receives this OAM packet, it immediately notes the time (T4). In this way, the first router  16  can deduce the one-way latency from the first router  16  to second router  18  (T2−T1), the one-way latency from the second router  18  to first router  16  (T4−T3), and the round-trip latency (T2−T1+T4−T3). The second router  18  can initiate this same OAM operation as well to determine the one-way latencies. Alternatively other methods of determining each of the one-way latencies can be used. For the first router  16 , the one-way latency from the second router  18  to the first router  16  is termed herein as the inbound latency and the one-way latency from the first router  16  to the second router  18  is termed herein as the outbound latency. Similarly, for the second router  18  the one-way latency from the first router  16  to the second router  18  is termed herein as the inbound latency and the one-way latency from the second router  18  to the first router  16  is termed herein as the outbound latency. 
     At step  64  the teleprotection supporter  50  determines whether it is the destination router of the lower of the two one-way latencies, in other words whether the inbound latency is lower than the outbound latency. If not, that is the one-way latency for packets sent from the first router  16  to the second router  18  is lower than the one-way latency for packets send from the second router  18  to the first router  16 , then the teleprotection supporter  50  waits while the jitter buffer  32  receives packets at step  68  as the second teleprotection device  12  sends messages to the first teleprotection device  10 . Once a playout level of the jitter buffer  32  is reached, the teleprotection supporter  50  initiates TDM playout of the packets in the jitter buffer  32  at step  70 , and the packets are sent to the packet to TDM interworking function  32  for sending as messages to the first teleprotection device  10 . The playout level is typically 50% of the size of the jitter buffer  32 , but of course different values may be used. 
     If the teleprotection supporter  50  determines at step  64  that the inbound latency is lower than the outbound latency, that is the one-way latency for packets sent from the first router  16  to the second router  18  is greater than the one-way latency for packets send from the second router  18  to the first router  16 , then the first router  16  must delay transmission of teleprotection messages to the first teleprotection device  10 . The teleprotection supporter  50  waits while the jitter buffer  32  receives packets at step  74  as the second teleprotection device  12  sends messages to the first teleprotection device  10 . Once sufficient packets have been received so that a playout level of the jitter buffer  32  is reached, the teleprotection supporter  50  runs a timer at step  76 . The duration of the timer is equal to the difference in the two one-way latencies. The router  16  continues to accept packets while the timer is running and the jitter buffer  32  may continue to fill. Once the timer is finished, the teleprotection supporter  50  initiates TDM playout of the packets in the jitter buffer  32  at step  70 , and the packets are sent to the packet to TDM interworking function  32  for sending as messages to the first teleprotection device  10 . 
     The method described above with reference to  FIG. 4  is merely one way of causing a delay of transmission of the appropriate duration. Alternatively other methods of delaying transmission of messages to the teleprotection device for the destination router of the faster direction can be effected, as long as the delay accounts for the difference between the determined one-way latencies so that messages reach the teleprotection devices  10  and  12  in synchronicity. For example, the playout level for each jitter buffer can be set to a different value. At the destination router of the faster direction, playout is initiated when the fill level of jitter buffer reaches the normal playout level plus an amount dictated by the difference in the one-way latencies, while playout is initiated at the other router when the fill level of its jitter buffer reaches the normal playout level. 
     In the method described with reference to  FIG. 4 , a step of synchronizing the router using IEEE 1588v2 or other timing over packet technology is used. Alternatively synchronization of the router could be carried out separately from the teleprotection supporter  50 , by another component or functionality. As yet another alternative, the routers could already have been synchronized using IEEE 1588v2 or other timing over packet technology, or by co-located GPS receivers. In any of these alternatives, the method shown in  FIG. 4  would be altered by removing the explicit step  60  of synchronizing the routers with the reference clock. 
     In the method described above with reference to  FIG. 4 , the teleprotection supporter  50  monitors the fill level of the jitter buffer and initiates playout when the fill level reaches the appropriate level. Alternatively these steps can be carried out by the CESoP processor  53 . In such an embodiment, the teleprotection supporter  50  runs the timer before which playout is to be initiated, as described above, and when the duration of the timer expires indicates to the CESoP processor  53  that playout is to be initiated. The method carried out by the teleprotection supporter  50  in such an embodiment can be understood by changing step  70  of  FIG. 4  to notifying the CESoP processor that TDM playout is to be initiated. The teleprotection supporter  50  can still be said to initiate the TDM playout, since playout only occurs upon the teleprotection supporter  50  determining that it is appropriate to do so and then taking an action. 
     The teleprotection supporter described above is preferably implemented as logical instructions in the form of software. Alternatively, the teleprotection supporter may be implemented as hardware, or as a combination of software or hardware. If in the form of software, the logic of the teleprotection supporter may be stored on a non-transitory computer-readable storage medium in a form executable by a computer processor. The logic of the teleprotection supporter may be implemented by a general purpose processor, a network processor, a digital signal processor, an ASIC, or multiple such devices. 
     The symmetric transmission of TDM data can assist applications other than teleprotection. As such, the teleprotection supporter  50  is just one embodiment of a more broadly named symmetry enforcer. The symmetry enforcer establishes the delay required in TDM playout of data to any devices which communicate through a packet switched network 
     A simplified block diagram of one embodiment of the teleprotection supporter is shown in  FIG. 5  as a processor assembly  100 . The processor assembly  100  includes a computer processor element  102  (e.g. a central processing unit and/or other suitable processor(s)). The computer processor element  102  has access to a memory  104  (e.g. random access memory, read only memory, and the like). The processor element  102  and the memory  104  are also in communication with an interface comprising various I/O devices  106  (e.g. a user input device (such as a keyboard, a keypad, a mouse, and the like), a user output device (such as a display, a speaker, and the like), an input port, an output port, a receiver, a transmitter, and a storage device (such as a tape drive, a floppy drive, a hard disk, a compact disk drive, and the like)). In one embodiment, the teleprotection supporter is implemented as software instructions loaded into the memory  104  and causing the computer processor element  102  to execute the methods described above. 
     The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the embodiments described above may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.