Synchronisation of a network element in a synchronous digital communication network

[0001] [Technical field to which the invention belongs] The present invention relates to a method for synchronizing network elements in the synchronous digital communication network according to the preamble of claim 1, a network element for the synchronous digital communication network according to the preamble of claim 6, and a network according to claim 7. Regarding management equipment. [0002] [Conventional technology] The network elements of a synchronous digital communication network operate according to the ITU-T recommendations for Synchronous Digital Hierarchy (SDH) or Synchronous Optical Network (SONET). Add / drop multiplexers, cross-connects, or line multiplexers are understood as "network elements." It is essential that all network elements operate synchronously during the transmission of information in such a communication network. This is achieved by mutual synchronization of network elements or master-slave synchronization. At that time, each network element extracts a clock signal from the received message signal and synchronizes the internal clock generator with this external clock signal. One or more primary clock generators provide a very accurate reference clock for the network. [0003] Synchronous status messages (SSMs), which are sent as part of the message signal, have been introduced to improve the selection of the message signal as a timing criterion. The SSM indicates the quality of the reference clock to which the sending network element is synchronized. The choice of timing criteria is based on the SSM as follows: The network element selects the information signal that SSM shows the best clock quality as the timing reference. As an SSM, a message "DNU" ("do not use for synchronization") was defined in which the information signal should be sent back to the network element selected as the timing criterion. [0004] Care must be taken to ensure that no timing loops are generated during synchronization, that is, the two network elements do not select each other as the synchronization clock source. Especially when there are two or more parallel transmission paths between two network elements, there is a high risk that a timing loop will be created. Such parallel transmission paths are also called "bundles". Using only the SSM "DNU" does not prevent the generation of timing loops in all possible situations. [0005] To avoid timing loops, European patent application EP0723344 proposes to define two classes of interface devices. Only the information signal received by the first class interface device can be selected as the timing reference, while the information signal received by the second class interface device must be ignored when selecting the reference clock. Must be. Timing loops can be avoided by properly configuring the network, for example, connecting only the first class interface to the second class interface. However, this solution is error prone because it relies on the correct network configuration. In particular, if there is a bundle of parallel transmission paths, errors can get into the configuration, resulting in a timing loop. [0006] European patent application EP0849904 proposes another way to avoid timing loops. The selection of the reference clock is made in the central clock generator of the network node. The selected clock source is transmitted to the central network management facility, which SSM in which of its outputs is sent to each network element of the node that receives its reference clock from the central clock generator. To instruct. In this way, the SSM "DNU" can be transmitted at all outputs, including the risk of timing loop generation, thereby effectively preventing timing loop generation. However, this method is complicated because network management equipment must explicitly assign an SSM to each output of each network element. Also, this method basically depends on the correct network configuration. In addition, multiple clock interfaces must be provided between the clock generator and the network element with the means to transmit the SSM. This increases the complexity and cost of the equipment and cannot be supplied in currently available network elements. [0007] [Problems to be Solved by the Invention] An object of the present invention is to provide a method of synchronizing network elements that can be managed by simple technical means, while reducing the risk of creating timing loops, especially in the presence of parallel transmission paths. Another object of the present invention is to provide network elements and network management equipment suitable for carrying out this method. [0008] [Means for solving problems] The first stated object is achieved by the characteristics of claim 1, and another object is achieved by the characteristics of claims 6 and 7. Another advantageous aspect of the invention is set forth in the dependent claims. [0009] The present invention will become clearer with reference to the description of the following two embodiments relating to the accompanying drawings. [0010] The basic idea of the present invention is to select the timing reference by the central network management equipment, not by the network element or the central clock generator. Anyway, the existing network management equipment stores all the information about the network configuration and configuration. Another basic idea is that when a network element determines the need for a new selection, for every message signal received at its input, each sync message in the message signal is sent to the network management facility and sent. Based on the synchronization status message and information about the network configuration and configuration, one of the received message signals is selected as a new timing criterion and the network element is notified of this selection. [0011] As is well known, the transmission of information between a network element and a central network management facility over an existing Q interface can take seconds to minutes. However, the present invention recognizes that information transmission in a communication network does not deteriorate significantly even if the network elements are not immediately resynchronized after the reference clock source is lost or failed. Based on. In that case, according to the present invention, the network element switches to a free asynchronous mode, the so-called holdover mode, and waits until the network management facility notifies the new selection. The maximum delay of several minutes caused by the transmission of information via the Q interface, and the waiting time and processing time in the network management equipment are not fatal to the operation of the network. [0012] Alternatively, after the reference clock source is lost, the network element itself may select a new reference clock source, as is commonly done in the prior art. However, this choice is only temporary to provide a time tie for the network management facility to select a new reference clock source. When the network management facility transmits a new reference clock source to the network element, the network element switches to the new reference clock source. [0013] BEST MODE FOR CARRYING OUT THE INVENTION A synchronous digital communication network management facility NET based on the recommendations for Synchronous Digital Hierarchy (SDH) is shown in Figure 1. NET consists of several network elements NE1 to NE5 and central network management facility TMN interconnected in both directions by optical or electrical transmission medium LINK. Network management equipment TMN works by switching logical connections between network elements, monitoring error messages and alarms, and configuring network NETs. Further, according to the present invention, the network management equipment TMN is designed to assign a reference clock source to each network element. Network element NE1 supplies the network with a highly accurate reference clock signal REF from the primary reference clock PRC. [0014] The network elements NE1 to NE5 exchange synchronous message signals configured in the form of the synchronous transport module STM-N via the transmission medium LINK. Synchronization selects the received message signal as a timing reference for each network element, extracts the clock signal from the selected message signal in the network element, and adjusts the internal clock generator of the network element to match this clock signal. It is achieved by that. Resynchronization may be required if the link between the two network elements fails. This requires the selection of another received message signal as the new timing criterion for the two network elements. [0015] The selection of the reference clock source is made by the synchronous status message SSM, which is included in the synchronous transport module, is transmitted with the synchronous transport module, and indicates the clock quality of each of the transmitting network elements. The messages listed in Table 1 are defined in SSM. [0016] [table 1]<img file="JP4358397B2_D0001.tif" />When a network element selects another network element as the reference clock source by SSM, the network element informs the selected network element that the message signal should not be used as a timing reference in the return direction. SSM "DNU" must be sent in the return direction. This is because otherwise a timing loop will be created. On the bundle BUNDLE of the parallel transmission path between the network elements NE2 and NE3 shown in Figure 1, the SSN "DNU" must be transmitted on both paths. Timed loops may be generated on multiple network elements interconnected in the form of rings, such as the network elements NE2, NE3, NE4, and NE5 in Figure 1. In such a configuration, the generation of timing loops cannot be avoided simply by using SSM. However, according to the present invention, the selection of the reference clock source for each network element is made in the network management equipment TMN, and the network management equipment TMN has all the necessary information regarding the network configuration. The generation of timing loops during synchronization or resynchronization after a failure can be effectively avoided by taking into account the information about each received SSM and network configuration. Advantageously, in addition to the network configuration, information about the selected reference clock source for each network element is taken into account during the selection. [0017] FIG. 2 shows an embodiment of a network element according to the present invention. The network element NE has m inputs for receiving the message signals STM-N1 to STM-Nm, respectively. The message signal is configured as a multi-level N (N = 1, 2, 4, 16, ...) synchronous transport module STM-N for SDH systems. Q Interface Through Q, the network element communicates with the central management facility TMN. The block diagram shows only the equipment and connections of the network element NE, which is essential for synchronization. [0018] At each of the m inputs, the network element NE has interface circuits IN1 to INm. In each interface circuit, clock signals CLK1 to CLKm are extracted from the received message signal. The clock signals CLK1 to CLKm are supplied to the selection device SEL via the respective connections. Further, the interface circuits IN1 to INm read the synchronization status messages SSM1 to SSMm included in the overhead area of the synchronous transport modules STM-N1 to STM-Nm, and pass them to the communication unit COM. The communication unit COM is connected to the network management facility TMN via the Q interface Q. The communication unit COM transmits synchronization status messages SSM1 to SSMm to the network management facility TMN together with information on which of the received message signals each synchronization status message originates from. The network management facility TMN then evaluates the synchronization status messages and selects one of the extracted clock signals CLK1 to CLKm as the timing reference based on these messages and the stored information about the configuration of the communication network. To do. The network management equipment TMN notifies the communication unit COM of the network element NE of this selection via the Q interface. The communication unit COM passes the notification to the selection unit SEL, which in response transfers the selected clock signal to the internal clock generator PLL. The communication unit COM may be, for example, a network element control device, a so-called network element manager. [0019] The internal clock generator PLL may be, for example, a digital phase-locked loop that produces a clock signal with the quality defined in ITU-T G.812 in free asynchronous mode. When the clock signal is transferred by the selection unit SEL to the internal clock generator PLL, the clock signal synchronizes the clock generator PLL. The output signal iCLK of the clock generator PLL is distributed as an internal reference clock to all output circuits O1 to Ok. As a result, the message signal tSTM-N to be transmitted is generated and transmitted at the internal reference clock speed iCLK. [0020] In general, at least a portion of the interface circuits IN1 to INm of the network element NE incorporates timing recovery circuits, so that the clock signals CLK1 to CLKm are extracted from the message signals STMN1 to STMNm received by these interface circuits. One of these clock signals is then selected as the reference clock by the selection unit SEL according to the selection by the network management facility TMN. Alternatively, only one timing recovery circuit may be provided that extracts the reference clock signal from each selected message signal. [0021] [0021] In a preferred embodiment, the network management facility also notifies the network element NE via the Q interface Q and the communication unit COM of a synchronization status message tSSM for each output O1 to Ok. This allows the SSM "DNU" ("do not use for synchronization") to be used in all directions with respect to the selected reference clock source, and thus does not use the synchronization method according to the invention. Network elements that themselves select by SSM can also operate in communication networks. Alternatively, the network element NE can use the SSM of the received message signal selected as the timing criterion provided by the standardization. This is because the network management equipment TMN avoids the generation of timing loops by considering the information stored therein regarding the configuration and configuration of the communication network. [0022] The method of this embodiment is shown as a flow diagram in FIG. This method is performed when the network element must be synchronized, or when the network element loses its reference clock source due to a malfunction and must be resynchronized. The method includes the following steps. [0023] Step S1: The network element to be synchronized receives multiple message signals. One of these message signals must be used as a new timing reference. All received message signals include synchronization status messages. [0024] Step S2: The synchronization status message of each received message signal is read by the network element. [0025] Step S3: The network element sends the read synchronization status message to the central network management facility, along with information about which message signal each synchronization status message originated from or the input that each message signal was received. Send. [0026] Step S4: Based on the synchronization status message, the information received about the origin of each synchronization status message, and the stored information about the configuration and configuration of the communication network, the network management facility renews one of the message signals. Select as a timing criterion. [0027] Step S5: The network management facility notifies the network element of the selection made. [0028] Step S6: The network element extracts the clock signal from the selected message signal. [0029] Step S7: Using the clock signal extracted from the selected message signal, the network element adjusts its internal clock generator to synchronize with the new reference clock source. [0030] The network management equipment shown in Figure 4 is a means of receiving synchronization status messages sent by network elements, a means of storing information about the configuration and configuration of communication networks, and a means of storing synchronization status messages sent. Includes means for informed timing criteria for network elements to be selected. Advantageously, the network management facility is a Q interface circuit I / O connected to the Q interface Q specified for the communication network, a semiconductor that temporarily stores the intermediate result of the selection request from the network element and the selection algorithm. Memory MEM, for example, data memory DATA with a database in the form of a hard disk, processor CPU that executes selection by a control program, and a bus system that interconnects Q interface circuit I / O, semiconductor memory MEM, data memory DATA, and processor CPU. including. The database embedded in the data memory contains all the information about the configuration and configuration of the communication network. [0031] Similar to the selection of the reference clock source for a single network element, the present invention can be used to select the reference clock source for the network node NODE, which includes multiple network elements NE1 to NE6 and the central node clock generator SASE. This outline is shown in FIG. 5 as a second embodiment. The network node NODE includes six network elements NE1 to NE6 and a central node clock generator SASE. The node clock generator SASE can be installed within the node as an independent device, or can be incorporated into one of the network elements, preferably within the cross-connect. Each of the network elements NE1 to NE6 transmits the clock signals CLK1 to CLK6 extracted from the received message signal STM-N to the node clock generator SASE. The node clock generator synchronizes with one of these clock signals and sends a common reference clock signal REF to all network elements. The network elements NE1 to NE6 are then synchronized with the reference clock signal REF received from the node clock generator SASE. [0032] The network node NODE is connected to the central network management facility TMN. Both each of the network elements and the central node clock generator SASE are linked to the network management facility TMN. The network element transmits the synchronization status messages SSM1 to SSM6 included in the received message signals from which the respective clock signals CLK1 to CLK6 transferred to the node clock generator SASE are extracted to the network management facility TMN. Based on the received synchronization status messages SSM1 to SSM6 and the data related to the communication network configuration and configuration stored in the network management equipment TMN, the network management equipment TMN sets one of the clock signals CLK1 to CLK6 to the network node NODE. Select as the reference clock source for. The network management facility TMN notifies the central node clock generator SASE of this selection via the Q interface Q. The node clock generator SASE is then synchronized with the selected clock signal. Preferably, the network management facility TMN also informs the individual network elements NE1 to NE6 of which synchronization status message to send in their output, based on the quality of the selected timing criteria (Table 1). reference). The existing Q interface between the network element and the network management facility TMN can be used to send the synchronization status messages SSM1 to SSM6. [0033] Network elements NE1 to NE6 are preferably considered as add / drop multiplexers and cross-connects. A network node may consist of network elements installed at the exchange of a synchronous digital communication network for the exchange to route its toll traffic. The node clock generator SASE is preferably incorporated within the cross-connect and is further used as an internal clock generator for the cross-connect. The node clock generator adjusts and distributes the uniform reference clock signal REF of the network node NODE. Like the network element, the node clock generator is connected to the network management facility TMN via the Q interface. If the node clock generator SASE is designed as a self-contained device, it can also be considered a network element within the scope of the invention, in which case its input signal is the clock signal received from the remaining network elements of the node. CLK1 to CLK6. The clock signal is a 2 MHz signal or a 2 Mb signal that can also send a synchronization status message to the node clock generator. [0034] In the event of a failure, one or more network elements NE1 to NE6 can stop each clock signal CLK1 to CLK6 being transmitted to the node clock generator SASE by them. This feature is called "squelching". If this is a clock signal that the node clock generator is synchronizing with, a new selection needs to be made. The node clock generator determines that the synchronized input signal has failed and notifies the network management facility TMN that a new selection is needed. The network management facility TMN then makes a selection using the method according to the invention and notifies the node clock generator SASE of the new selection via the Q interface Q. Meanwhile, the node clock generator switches to the asynchronous mode (so-called holdover mode). [Simple explanation of drawings] FIG. 1 is a diagram showing a synchronous digital communication network. FIG. 2 is a block diagram of a network element according to the present invention and a network management facility. FIG. 3 is a flow chart of a method according to the present invention. FIG. 4 is a block diagram of a network management facility according to the present invention. FIG. 5 is a block diagram of a network node. [Explanation of symbols] BUNDLE Parallel transmission path bundle LINK Optical or electrical transmission medium NE1, NE2, NE3, NE4, NE5 network elements NET synchronous digital communication network PRC primary reference clock signal QQ interface REF common reference clock signal TMN network management equipment