FI102442B - Synchronization of a data communication network - Google Patents

Description

102442

Telecommunication network synchronization

FIELD OF THE INVENTION The invention relates generally to the synchronization of telecommunication networks, and in particular to synchronization methods in which the nodes of the network indicate the synchronization status of the signal they transmit. The synchronization status indicates the signal quality level with respect to synchronization, so that the node can decide which signal to use as its synchronization source based on the quality levels it receives.

BACKGROUND OF THE INVENTION In this presentation, the intersections of transmission links in a telecommunications network are referred to as a node (or node hardware). The node can be any device or hardware, e.g. a branch or cross-connect device.

In current communication systems, synchronization can be performed either by means of separate synchronization connections or by utilizing normal data connections between the nodes of the system. Separate synchronization connections are used only in isolated cases and very rarely for synchronizing the entire network. When using data connections for synchronization, the line code must be such that the nodes can also distinguish the clock frequency from the incoming data signal 20. Of these clock frequencies, synchronization of network nodes can be achieved by two different basic methods: mutual synchronization and slave synchronization. In mutual synchronization, each node generates its own clock frequency from the average of the frequencies of the incoming signals and its own current clock frequency. In this way, all nodes in the network drift towards the common average frequency and have reached it in the steady state. However, a network using mutual synchronization cannot be made to synchronize to the desired source, which makes it difficult to connect different networks, because in this case it is not possible to precisely determine the operating frequency of the entire network in advance. In submissive synchronization, all nodes in the network are synchronized to the clock frequency of one master node instead. Each node selects the frequency of one incoming signal as its own clock frequency source. The node tries to select a signal that has the clock frequency of the main node of the network.

In independent submissive synchronization, each node makes its own decision to synchronize without receiving any ♦ 35 decision-supporting information from the outside. When nodes make their decision to synchronize 102442 2 independently, each node has to determine to which node it will synchronize. These determinations are often made in the form of a priority list, in which case the node selects the one with the highest priority, i.e. the one with the highest in the list 5, as the source of its synchronization. If this signal is interrupted or degraded so that it can no longer be qualified as a synchronization source, the node selects the signal with the next highest priority from the list. The priority list shall be constructed so that all nodes on it are between that node and the main node of the network, so that synchronization spreads to levels lower than the main node.

10 However, submissive stand-alone synchronization imposes limitations on network synchronization: in a loop network, not all connections can be used for synchronization, which limits the dynamic adaptability of the network in different situations. Communication must be established between the nodes so that the amount of data available to a single node is sufficient for decision-making in all situations without having to severely limit the number of connections used for synchronization, so that in the event of a fault the main node's clock frequency cannot be allocated to network nodes. There are two methods for this communication, which are described below.

A simple method of extending independent submissive synchronization 20 to communicate is the so-called LP (Loop Protected synchronization). LP synchronization seeks to prevent timing from drifting into an inoperative state in loop networks by using the two status bits mcb and Icb, which are transmitted between the nodes of the network, with the help of the above-mentioned priority lists. The first status bit mcb (master control bit) indicates whether the synchronization originates from the master node of the network 25. The master node defined for the network transmits this bit as a logical zero in its outgoing signals and the other nodes forward it if they are synchronized to a signal where the value of the mcb bit is zero. The second status bit Icb (loop control bit) indicates whether there is a loop in synchronization. Each node in the network • sends this bit as a logical one in the direction in which it is synchronized and as a logical zero in the other directions.

Each node uses its own priority list when selecting the synchronization source, but checks not only the signal status but also the mcb and Icb bits before making the selection. The node primarily tries to find a connection whose clock frequency originates from the main node of the network (mcb = 0). If «35 such a connection is not found (due to a fault condition), the node normally selects 102442 3 the highest working connection in the priority list. However, the selected connection (timing source) is always required to have its timing not in the loop (lcb = 0), even if the signal itself is otherwise valid for synchronization.

Thus, in LP synchronization, the node tends to use for synchronization the 5 sources whose signal is valid and whose transmitted status bits are mcb = 0 and lcb-0. If there are more than one such source, the node selects the one that is higher in the priority list than the other sources in question. If there is no source with mcb = 0 and lcb = 0, the node checks for sources with mcb = 1 and lcb = 0 and selects the one with the highest priority list from such sources.

Figure 1 shows, by way of example, a telecommunication network consisting of five nodes A ... E using the LP synchronization described above. The mcb and Icb bits transmitted in different directions by each node are denoted by the symbols M (mcb bit) and L (Icb bit), respectively, next to the nodes. In the figure, the priority lists used by the nodes are marked with the reference symbol PL. The ports PA, PB and Pc 15 denote the ports of each node. The priority list PL of the main node 1 has no incoming signals at all, but the main node always uses its own internal oscillator as the source of its synchronization. Connections that are not currently being used for synchronization are indicated by dashed lines in the figure.

Based on the above rule, the Node B synchronizes to the Node 20 A because it is the only direction from which the incoming state bits have the values mcb = 0 and lcb = 0. Node C synchronizes to Node D because port PB is higher in the priority list than the ports (PA and PB) from which the status bits mcb = 0 and lcb = 0 are received. Node D synchronizes with Node B because port PA is on its priority list higher than port Pc. Node E synchronizes to Node C because port PA is higher on its priority list than port PB (and status bits mcb = 0 and lcb = 0 are received from both).

When using a mere priority list to form a synchronization tree from tree-like hierarchical structures, in LP synchronization, a synchronization tree is assembled using loops. First, the main loop in which the main node of the network is located is formed, and then nodes are added to the synchronization tree chain by chain until all nodes are included. Priority lists are formed according to loops and chains. In the example of Figure 1, the main loop consists of nodes A, B, D and C, respectively. A chain of one node (node E) is connected to this main loop.

Figures 2a and 2b illustrate the behavior of a network 102442 4 using LP synchronization (Figure 1) in the event of a failure. In the first step shown in Figure 2a, the connection between the main node A and the node B is broken. The network then receives synchronization from master node A through node C. In the second step shown in Figure 2b, the connection between the nodes A and C 5 is also broken, whereby the new master node becomes the node C which had last forwarded the frequency of the master node to the network. The mcb bit sent by Node C has now changed to logical one to indicate that there is no longer a connection to the official master node on the network.

Another way in which independent submissive synchronization has been extended to communicate is to use the Synchronization Status Message (SSM) according to ITU-T standards G.704 or G.708. Standard G.704 defines the frame structure of a digital transmission system operating at 2048 kbit / s. According to the recommendation, bits 4-8 of every other frame are free (spare bits) and can be used e.g. to transmit the above-mentioned synchronization status messages. Only one of bits 4-8 of a frame can be used for this purpose, so a four-bit synchronization status message consists of the selected bit (4-8) in frames 1, 3, 5, and 7 of superframe and in frames 9, 11, 13, and 15. The same synchronization status messages (SSM) are defined in SDH for network G.708. In the SDH network, synchronization status messages are transmitted in bits b5 ... b8 of the S1 (Section Overhead) S1 byte of the STOH-N-20 frame header area.

The following table shows the synchronization quality levels (QL, Quality Level) indicated by the bit patterns formed by these selected bits San1-San4 (n = 4, 5, 6, 7 or 8) or S1 (b5 ... b8). In the last column, the terms corresponding to the recommendations (in English) are used.

[QL San1-San4 Synchronization Quality (QL) Description or S1 (b5 ... b8) 0 0000 Quality unknown (Existing Sync. Network) - - - _ __ _____ 9 1 0001 Reserved 2 0010 G.811 3 0011 Reserved 4 0100 G.812 Transit 5 0101 Reserved Π 102442 5 I 6 0110 Reserved 7 0111 Reserved.

8 1000 G.812 Local 9 1001 Reserved 10 1010 Reserved 11 1011 Synchronization Equipment Timing Source (SETS) 112 1100 Reserved 13 1101 Reserved 14 1110 Reserved 15 1111 Do not use for Synchronization

As can be seen from the table, four levels of synchronization have been decided within the ITU-T and in addition two other levels have been given importance; one indicates that the level of synchronization is unknown and the other that signal 5 should not be used for synchronization (QL = 1111).

Figures 3 and 4 illustrate the operation of the SSM method in an annular network with a total of five nodes, denoted by reference numerals N1 to N5. Within each node, the quality level of the intra-node clock is marked as the highest (QL: 1011). Below this is the Node Priority-10 list, where the selected timing source is shown in italics. As mentioned above, each node selects the signal with the highest quality level of the synchronization message as the source of its timing. If several signals have the same quality level, the one with the highest priority list is selected. Node: · 15 synchronization status messages sent by the node are marked with the reference uQL: xxxx ”next to each port. External timing sources S1 and S2, respectively, are connected to the main node N1 and the node N3. The quality levels of external sources (QL = 0010 and QL = 0100) are marked above the sources. Each source outside of loop synchronization must be assigned a QL value.

Figure 3 shows the network in its normal situation (no faults). The main node N1 20 uses an external timing source S1, which in this example is defined as a clock at the level QL = 0010. The master node sends this synchronization status message 10442 in both directions. The slave nodes are synchronized to a signal from the main port (Pa) with a synchronization status message QL = 0010. In this case, they forward the same quality level (QL = 0010) from port Pb and send the quality level QL = 1111 (“do not use 5 for synchronization”) in the direction from which they receive their timing (in the direction of port Pa).

Figure 4 shows a situation in which a fault has occurred in the connection between nodes N1 and N2. When Node N2 detects this fault, it selects a new timing source. Since it receives 10 quality levels QL = 1111 from the second direction (node N3), it cannot use this direction for timing either, in which case it enters the internal timing mode and starts transmitting its own clock quality level QL = 1011. The next node (i.e., node N3) receives this quality level from port Pa, changing the external source S2 to its timing source because the quality level QL = 0100 provided by this is better than the quality level received from port Pa-15, and port Pb cannot be used for timing (QL = 1111 ). Node N3 starts transmitting the quality level QL = 0100 in both directions. Node N2 synchronizes to the signal from node N3 because the quality level it contains is better than its internal quality level (QL = 1011), at which point it starts transmitting the quality level QL = 1111 in the direction of node N3. Node N4 also accepts the quality level sent by node N3 because it receives the quality level QL = 1111 from port Pb. Thus, the node N4 forwards the quality level QL = 0100 to the node N5, which is synchronized in the direction of the port Pb, because the quality level QL = 0010 is obtained from there. In this case, the node N5 returns the quality level QL = 1111 to the node N1 and forwards the quality level QL = 0010 to the node N4. The rest of the nodes in the loop do the same, i.e. forward 25 quality level QL = 0010 from port Pa and return quality level QL = 1111 to port Pb. This has led to the situation shown in Figure 4. The loop is thus synchronized in its secondary direction.

In the previous examples, the connection to the master clock was lost (Figure 2b), whereby the network also lost the master level clock, or the connection to the master node 30 was maintained at all times and the network was able to adapt to the fault situation (Figures 2a and 4).

The disadvantage of synchronization methods using synchronization status messages is that the network using the methods is not able to flexibly utilize the backup clock in the network in the event of a fault, e.g. when the connection to the main clock 35 is lost. The network's ability to recover from certain failure situations is thus 102442 7 deficient. These fault situations are described in more detail below.

SUMMARY OF THE INVENTION

The object of the invention is to get rid of the above-mentioned drawback and to improve synchronization methods such as those described above, so that a spare clock can be used flexibly and efficiently in the network.

This object is achieved by a solution as defined in the independent claims.

The idea of the invention is to link the location of the source in the priority list and the synchronization status of 10 sources (i.e. the quality level with respect to synchronization). The idea is therefore to add to the priority list information about what synchronization status a particular synchronization source must have in order to be at a certain level in the priority list. The idea can also be implemented by keeping a separate priority list for each synchronization status.

Thanks to the solution according to the invention, networks such as those described above are able to flexibly activate the backup clock and re-synchronize it again with the master clock when the connection to the master clock is restored. The solution according to the invention thus improves the fault and interference tolerance of the network, whereby the reliability of the network is also improved. Solution 20 also offers more flexibility in parameterizing the synchronization at the network level, because e.g. the priority list no longer needs to be defined not only according to where the main clock of the network is located, but also possible spare clocks can be taken into account.

25 Pattern list

The invention and its preferred embodiments will now be described in more detail with reference to Figures 5a to 10c in the examples of the accompanying drawings, in which: Figure 1 shows a communication network using LP synchronization; Figures 2a and 2b illustrate the behavior of the network of Figure 1 in fault situations; Fig. 4 illustrates the behavior of the network of Fig. 3a in the event of a fault, Figs. 5a to 5f illustrate a problem related to LP synchronization, 102442 8 when the network backup clock is at the same level as the main clock, Figs. 6a to 6i illustrate a problem related to LP synchronization, when the network backup clock is at a different level than the main clock, Figures 7a and 7b illustrate the basic idea according to the invention, Figures 8a ... 80 illustrate the operation of the method according to the invention, Figure 9 is a flowchart showing the synchronization source selection process performed by a single node, Figure 1 0a is a block diagram illustrating those elements of a node that implement the method of the invention, and Figures 10b and 10c show alternative embodiments of the node.

Detailed description of the invention

In order to clarify the problem underlying the invention, a network using LP synchronization with nodes A ... D according to Fig. 5a will be considered first. The main clock M is connected directly to node A and the backup clock BM to node C. The nodes form a ring network. Next to each node, the priorities of the different sources (ports) are marked with numbers, which are defined in question. using the node's priority list.

20 The sequence assumes that the backup clock is set to the same quality level as the main clock (M = 0). Figure 5a shows the basic situation in which the network is in good condition. The synchronization runs in the “chain” M- »A-» B- »C-> D and the mcb and Icb bits run in the same direction as zeros, indicating that the frequency is from the master source (M = 0) and that there is no loop in the timing ( L = 0) In the opposite direction, the mcb bit also runs as zero, but the Icb bit as one (L = 1), because the synchronization comes from this direction.The synchronization directions are indicated in the figure by arrows in the connections between different devices.

In Figure 5b, the connection between nodes A and B is lost. Node B can no longer use this. connection to the synchronization, but also the connection from the node C 30, because from there the Icb bit comes first. Node B then switches to using its internal clock and sends out the data M = 1 (the clock signal is not from the master) and L = 0 (no loop). The rest of the network has not yet noticed anything about the break between nodes A and B at this stage.

In the situation shown in Figure 5c, Node C has had time to receive 35 data M = 1 and L = 0 from Node B. Since Node B no longer provides a master clock, and 102442 9 from Priority 2, Node D becomes L = 1 (i.e., “not allowed to use ”), The Node C switches to use the backup clock BM (which is of the quality of the master clock) connected with priority 3 nodes. Since node C again receives its timing from the master-level clock (M = 0), it sends the data M = 0 and L = 0 to both node B and node D according to the new situation.

In the step shown in Figure 5d, the Node D has had time to process the bits coming from the Node C. However, since there has been no change in these, Node D does not react in any way. Node D still gets its timing through Node C, but now the timing comes from the backup clock BM. Node B now receives bits M = 0 and L = 0 from node C 10, so Node B has changed its timing source to Node C, where it returns the values M = 0 and L = 1. At this point, the network is thus synchronized to two different clock sources, node A to the main clock M and nodes B, C and D to the backup clock BM. However, the situation is undesirable because a connection to the master clock also exists at nodes B, C, and D. Without the backup clock 15, synchronization would work correctly, as observed from the previous examples.

In the step shown in Figure 5e, the connection between nodes A and D is also broken. This connection is not in use for synchronization, so disconnecting is not relevant for synchronization. When the connection between nodes A and D is restored (Figure 5f), it has no effect on the synchronization, because node D already gets the best possible status M = 0 and L = 0 in the direction of its first priority. Thus, the network remains synchronized to two different master clocks, even though it should be synchronized to only one master clock. As can be seen from the example, LP synchronization does not support the use of both the main clock and the backup clock if they are set to the same quality level (M = 0).

25 If the master clock and the backup clock are set to different levels (master status M = 0 for the master clock and slave status M = 1 for the backup clock), the problem is that the backup clock is at the same level as the internal clocks. Such a situation is illustrated in Figures 6a ... 6i, where the first two steps (Figures 6a and 6b): correspond to the first two steps in the chain of events of Figure 5 (Figures 5a and 6b).

30 5b). Figure 6c shows the situation when Node C has responded from Node B

to the bits it receives M = 1 and L = 0. A value L = 1 is received from node D, so that the signal cannot be used for timing. The spare clock is of level M = 1 and is only at priority 3 in node C, so it is not used because the connection from node B, which is now of the same level, is at priority 1.

In Fig. 6d, the node D has received the values from the node C M = 1 102442 10 and L = 0 and changed its timing in the direction of the node A, because it becomes the master frequency (M = 0 and L = 0). At the same time, Node D has updated its own outgoing bits.

In the step shown in Figure 6e, the Node C has detected the changed bits of the Node D5, so it is now synchronized via the Node D to the master clock. In addition, Node C has updated its outgoing bits.

In the next step (Fig. 6f), the Node B notices that there is now a connection to the master clock through the Node C, so the Node B synchronizes with the Node C and updates its outgoing bits. The situation has now stabilized and the nodes have been synchronized to the main clock according to the chain M- »A-> D-» C-> B.

If the connection between nodes A and D is then lost (Fig. 6g), node D switches to the internal clock because the value of L = 1 becomes from the direction of node C. Node D updates its outgoing status, sending Node C the values M = 1 and L = 0.

15 Thereafter (Fig. 6h), Node C notices the changed bits. The connection in the direction of the Node B is forbidden (L = 1) and the connection to the backup clock is at the same level as the connection to the Node D. The Node C thus remains synchronized with the Node D because it has a higher priority than the backup clock. Node C updates its outgoing tags.

20 Node B then detects the changed situation and updates the outgoing status. In the final situation (Fig. 6i), the connection to the main clock M is lost, but the backup clock could not be activated at any stage. Thus, LP synchronization like the current one does not support the use of a backup clock, even if the main and backup clocks are set to different levels.

The present invention corrects the drawbacks described above by linking the location of the source in the priority list to the synchronization status of the source. Figures 7a and 7b illustrate this principle using the example network shown above. According to the invention, the master-. for the status (M = 0, L = 0) their own priorities, which may be e.g. shown in Fig. 7a, and for the slave status (M = 1, L = 0) their own priorities, which may be e.g. as shown in Fig. 7b. Thus, when signals with master status are received at a particular node, the priorities shown in Fig. 7a are used when selecting the timing source in question. among the signals with the status. If only slave statuses are received for the node, the priorities shown in Figure 7b are used when selecting the timing source in question. among the signals having the status 102442 11. The idea is thus to define different priorities for different statuses, i.e. to make the location of the source in the priority list of the node dependent on the synchronization status of the source.

Figures 8a ... 8o go through the operation of the network shown above when the priorities shown in Figures 7a and 7b are used in the network 5. The backup clock is assumed to remain at the slave level.

Figure 8a shows the same initial situation as above. In the situation of Figure 8b, the connection between nodes A and B breaks again, whereby Node B switches to the internal clock because it receives the value L = 1 from node C. At the same time, the Node 10B updates the identifier sent to the Node C. Thereafter (Fig. 8c), Node C receives data M = 1 and L = 0 from Node B and data L = 1 from Node D. Since the backup clock BM is connected to the node C at the slave level, the node has two slave level sources. Of these, the backup clock BM is selected because it is at the slave level with first priority (cf. Fig. 7b). Thus, the connection to Node B at the slave-15 level is not one of the sources used. Thus, Node C synchronizes to the backup clock BM and sends data M = 1 and L = 0 to Nodes B and D, thus indicating its new status.

In the step shown in Figure 8d, Nodes B and D have noticed a changed status of Node C. Node B synchronizes with Node C because node C has first priority at the slave level and thus better than the internal clock at the slave level. When Node O detects that the status of Node C has changed, it changes its synchronization source to Node A, from which it receives the master clock. frequency (M = 0).

«

In the situation shown in Fig. 8e, the Node C receives the changed status of the Node D 25 and changes from the backup clock to the Node D because it receives the main clock frequency thereby. Node C updates its outgoing status.

Next (Fig. 8f), the Node B has received the changed status of the Node C. Node B remains synchronized with Node C, however, updating * the status it sent to Node C. At this stage, the situation has stabilized.

In step 8g of Figure 8, the connection between nodes A and D is lost. When Node D notices an interruption, it switches to the internal clock (Node C becomes data 1 = 1) and sends Node C data M = 1 and L = 0. In the next step (Fig. 8h), the Node C has noticed the change. Now, 35 nodes C have two slave-level sources (BM and D), of which only 102442 12 spare clocks have a defined priority at the slave level. Node C is thus synchronized to the backup clock BM.

In the step shown in Figure 8i, Nodes B and D have processed the slave status from Node C. This status is the best of what. nodes receive 5 and higher priority than the internal clock, which is also at the slave level. Nodes B and D are thus synchronized via node C to the backup clock BM. Thus, at this point, the network is successfully synchronized to the backup clock when the connection to the master clock is lost.

Next (Fig. 8j), the connection between nodes A and D is restored. 10 Node D synchronizes through Node A to the master clock after receiving the master clock status from Node A. Thereafter (Fig. 8k), Node C obtains the master clock status from Node D and changes its synchronization source to Node D. In the situation of Fig. 8I, Node B also receives the master clock status (from Node C), so it remains synchronized with Node C, updating its outgoing 15 status. The situation has now stabilized. The connection to the master clock has been restored and the nodes have successfully switched to the master clock synchronization source.

In the step shown in Figure 8m, the connection between nodes A and B has been restored. The master status then starts to propagate to the network from the direction of the first priority (cf. Fig. 7a). In the situation of Fig. 8m, the Node B 20 has changed its synchronization source to Node A. Next (Fig. 8n), the Node C obtains the master-level status from the direction of the first priority, whereby it discards the second priority. In the situation shown in Figure 8o, the information has spread to Node D, which also switches the source of the second priority to the source of the first priority (i.e., Node A to Node C). The situation has now stabilized and the network has recovered to the baseline situation as desired after the faults.

The above examples were for LP synchronization. When using the Sync Status Message (SSM) method, the only difference is the multiple status levels used by the latter. In this case, the method according to the invention brings the benefits described above e.g. when there are 30 multiple backup clocks of the same level on the network.

As can be seen from the above examples, in the known methods, the priority list of nodes has had different only different sources (interfaces or ports) in order of priority, e.g. according to the following table:

Priority Interface_ 1 External synchronization input_ 102442 13 2__Port 1_ _3__Port 2_ _4__Internal timing_

The signal with the best status is currently selected as the chord synchronization source. If there have been several different signals with the same status, the one with the highest connection in the priority list is selected as the synchronization source.

When operating in accordance with the invention, the priority list of the node is formed in such a way that the priority list contains both the different sources and the synchronization statuses required of them, so that the source is in question. priority. For example, the example above could then look like this if LP synchronization is used as an example.

Priority Port__Required status J__1__mcb = 0_ 2__2__mcb = 0_ _3__2__mcb = 1_ _4__1__mcb = 1_ 5__Internal timing__ 10 The same interface can therefore appear in the priority list several times, as it can receive several different statuses. Thus, the node actually has a priority list for each desired status.

When a synchronization source is selected in a node operating according to the invention, the procedure is the same as in known methods (i.e. the highest possible status is searched for and the source with the highest priority is selected from those with this status), but now there is no longer only one general priority list, but each status level may have its own definitions. In other words, when selecting the source with the highest priority from the sources with the same status, the node has to use the priority number-20 quarrels associated with this status.

Fig. 9 is a flowchart illustrating a decision process performed by a node. Initially, the node searches for the highest available status (steps 91 ... 93). The node then searches among the sources transmitting that status for the one with the highest priority list (or part of the list) associated with that status (step 94). Note that a source with some status can always be found, because an internal node clock is also available. The “No 102442 14 may not be used” status is not reviewed. On the other hand, because the clock inside the node has a higher status than this, the process does not even reach the “not allowed to use” status. If a source is faulty, it is given the status “not allowed”.

Thus, in the method according to the invention, first the highest available status is retrieved and then the one which has the highest priority level with this status is retrieved from the sources transmitting this status. The primary selection criterion is therefore status (ie quality level).

Figures 10a to 10c illustrate as a functional block diagram-10 those elements which implement the method described above in a single node of the network. The general structure of the node is, for example, such that it comprises a plurality of parallel interface units IU1, IU2 ... IUN, each of which is connected to at least one neighboring node, and a control unit CU common to all interface units, in which synchronization decision-making is performed. 15 The control unit and the various interface units are connected to each other, eg via the node's internal bus CBUS.

The figures show, by way of example, two transmission connections to a system node from neighboring nodes, A and A2, each connected to its own interface unit. Transmission connections are, for example, 2 Mbit / s PCM connections in accordance with ITU-T Recommendations G.703 and 20 G.704. One interface unit IU may have one or more interfaces through which the node connects to one or more neighboring nodes, respectively. In general, therefore, it can be stated that the node has N connection units with a total of M connections (M> N).

In Figures 10a and 10b, an index is also shown for the interface or interface unit-specific reference numerals, and the parts common to all interface units are shown without an index.

In the implementation according to Fig. 10a, each transmission line is connected to a signal transmitting and receiving section 13j (i = 1,2, ...), which performs physical: signal processing. Section 13, forwards the synchronization status messages or bits to the synchronization section 16 connected thereto. The synchronization parts 16 perform e.g. checking the messages for error and forwarding the synchronization status to the node's central synchronization control section 20 via the bus CBUS. The signal transmitting and receiving parts also monitor the quality of the received signal and store the information about them in the connection-specific fault path 35 databases 14j. Each synchronization part receives the fault information from the corresponding database. The detection of a fault / change in the transmission link in the signal transmission and reception parts takes place as is known per se. As a result of the detected fault, the synchronization part informs the control part of the status “not to be used for synchronization”.

5 The synchronization control section 20 stores the synchronization statuses of the various sources received from the synchronization sections in the memory area 21. When selecting the synchronization source, the control section 20 retrieves from the memory area 21 the highest possible status currently received and transmitted by the sources. The control section then selects the priority list 10 stored in the memory area 22 using the of the sources transmitting the status to the one with the highest priority level with that status. The outgoing status messages are generated in the synchronization sections 16, and when a change in the synchronization source also causes changes in the outgoing status information, the control section informs the synchronization sections of the change.

The method can also be implemented using, for example, a node architecture similar to Figure 10b. In this case, the structure of the node is otherwise the same, but the priority lists are divided into different interface units and each interface unit informs the other interface units what the status of its signal (s) is. After receiving this information from others, each interface unit can make the same reasoning as was done in the previous 20 in the centralized synchronization control section and generate the outgoing status information independently.

The idea according to the invention can be implemented either by linking the status and the level of the priority list to each other or by using status-specific priority lists, whereby each different level of status has its own list. These lists can be 25 for all possible statuses or just the ones you want. In this case, e.g. using the architecture of Fig. 10a, the control section 20 has a separate priority list for each status, and the control section uses the list associated with the highest currently received status. Such a solution is # illustrated in Figure 10c.

30 The method can be implemented (as described above) so that the source receives a certain priority level when it has exactly the same status as the one in question. priority level has been defined. On the other hand, this defined status can be used as a minimum requirement. In this case, the source is at a priority level corresponding to that minimum value whenever the quality level of the signal 35 received from the interface is equal to or greater than that minimum value, and no second minimum quality level is defined for the interface 102442 16 that is higher than said minimum value but less than or equal to the current signal quality level. (status). If a source is not assigned a priority by its quality status, the priority that the source receives with the status 5 is the highest of the statuses for which the priority level is defined and which are lower than the current status of the source.

In the above, the invention has been implemented in such a way that the status level required for a certain priority level is always defined. This definition may also be omitted for some priority levels, in which case the the source is equal to the priority defined for the Alem-10 model status. If no priority has been defined for this status either, one lower one is used again and so on until the priority defined for the source is found. If no priority is found, even if you proceed to the lowest allowed status, the source is not intended to be used for synchronization with these priorities. If the implementation is handled using only one of the 15 priority lists to which the required status has been added, and not using the priority lists belonging to different statuses, it is also possible not to define the required status level for the source and put it on the priority list without this status requirement. In this case, the source always receives this priority in comparisons, regardless of its status level.

Although the invention has been described above with reference to the examples according to the accompanying drawings, it is clear that the invention is not limited thereto, but can be modified within the scope of the inventive idea set forth above and in the appended claims. For example, not all sources necessarily receive quality levels (statuses) to the node, but the quality level may be defined for the interface in the 25 nodes so that when the signal is OK, the node provides the this quality level for the signal coming to the interface.

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Claims (7)

1. Telecommunication network synchronization method, comprising several nodes (A ... D) connected to each other by transmission connections (A1, A2), according to which method 5. The network nodes transmit synchronization status messages, with the quality level of the corresponding signal is communicated in relation to the synchronization, - in each node a priority list is formed, which has node interfaces at different priority levels, and 10. the node uses such a signal as a source of synchronization among possible synchronization sources having the highest quality level, characterized by - in the knot, the quality level against each individual priority level is stored by an individual interface that the signal received from the relevant interface should have in order for the interface to exist at that priority level, and - the knot searches for the highest quality level that the signals received in the knot and choose it interface as its source of synchronization among the interfaces that receive signals of the relevant quality level having the highest priority level of the quality level in question. A method according to claim 1, characterized in that the quality level data is given as the exact value that the signal should have for the interface to be at the priority level in question. Method according to claim 1, characterized in that the quality level data is given as the minimum value for the quality level, the interface being at a priority level corresponding to the minimum value always where the quality level of the signal received from the interface is in accordance with the minimum value or greater than this. and no such other minimum value has been defined for the interface which would be greater than said minimum value, but less than or equal to the actual quality level of the signal. Method according to claim 1, characterized in that, at a given quality level, the priority levels are defined for only a part of the node interfaces, the selection being performed only among the interfaces for which a priority level is defined. 35 Method according to claim 1, characterized in that for a part of the interfaces a quality level corresponding to the interface is left undefined, the interface in question being interpreted to be at a priority level defined for the interface at all permissible quality levels. 5 Method according to claim 1, characterized in that the priority list is implemented as several separate lists, each of which is connected to a given quality level. A telecommunications network node arrangement comprising multiple nodes connected to each other by transmission links, the node arrangement comprising - means (131 (132) for transmitting the synchronization status message to neighbor nodes connected to the node with the synchronization status status with a corresponding signal in relation to the synchronization is notified, 15. a priority list inserted in the node, for which node interfaces are defined at different priority levels, and - selector means (20; 161t 162) to select the signal that serves as the synchronization source for the node. among the group of possible synchronization sources in such a way that the selected signal has the highest quality level, characterized in that - the knot arrangement also comprises, against each individual priority level of an individual interface, information about the quality level that the signal received from the relevant gr INTERFACE should have to interface should be on the ifrägavarande prioritetsnivä, and 25. The selection means are arranged to select the interface Tili synchronization ring source pegged among the interfaces that receive signals of the highest kvalitetsnivän, which has the highest prioritetsnivän on the ifrägavarande kvalitetsnivä. • *