WO1998037651A1

WO1998037651A1 - Synchronization of telecommunications network

Info

Publication number: WO1998037651A1
Application number: PCT/FI1998/000099
Authority: WO
Inventors: Jukka Kainulainen
Original assignee: Nokia Telecommunications Oy
Priority date: 1997-02-19
Filing date: 1998-02-03
Publication date: 1998-08-27
Also published as: FI970700A7; FI102442B1; FI102442B; FI970700A0; AU5867698A

Abstract

The invention is related to a synchronization method in a telecommunications network comprising several nodes (A...D) which have been interconnected by using transmission links. Network nodes transmit synchronization messages which are used to state the quality level of the corresponding signal as regards synchronization. A priority list is formed in each node, the list containing node interfaces on different priority levels. The node uses as its synchronization source the one signal, from among the possible synchronization sources, which has the highest quality level. In order for the synchronization to utilize the backup clocks more efficiently, information is stored to the node for an individual priority level of an individual interface, the information stating the quality level that the signal coming from the interface in question must have in order for the interface in question to be on the priority level in question, and the node searches for the highest quality level that the signals received in the node have, and selects from the interfaces receiving signals of that quality level the one which has the highest priority on the quality level in question and uses that as its synchronization source.

Description

Synchronization of telecommunications network

Field of the invention

The invention relates to synchronization of telecommunications networks in general, and in particular to synchronization methods in which the network nodes indicate the synchronization status of the transmitted signal. The synchronization status indicates the signal quality level relative to synchronization, and thus the node can decide, on the basis of the quality levels received, which signal it is to use as its synchronization source.

Background of the invention

In this description, the term node (or node equipment) is employed for the intersection point of links in a telecommunications network. A node may be any device or equipment, for example a branching device or a cross- connection device.

In present-day telecommunications systems, synchronization may be performed either by means of separate synchronization connections or by utilizing the normal data connections between the system nodes. Separate synchronization connections are used only in isolated cases and very seldom to synchronize an entire network. When data links are used for synchronization, the line code must be such that the nodes are also capable of recognizing the clock frequency from the incoming data signal. Synchronization of the network nodes from these clock frequencies can be achieved by two basic methods: mutual synchronization and slave synchronization. In mutual syn- chronization, each node forms its own clock frequency from the mean value of the incoming signal frequencies and its current clock frequency. Hence, all nodes in the network drift towards a common mean frequency and in a steady state have reached said frequency. However, a network employing mutual synchronization cannot be synchronized with a desired source, and thus it will be difficult to interconnect different networks, as in that case the operating frequency of the entire network cannot be precisely determined in advance. In slave synchronization, on the other hand, all network nodes are synchronized with the clock frequency of one master node of the network. Each node selects one incoming signal frequency as the source for its clock frequency. The node seeks to select a signal having the clock frequency of the master node of the network. In independent slave synchronization, each node makes its decisions about synchronization without receiving any external information to support the decision-making. When the nodes make their decisions on synchronization independently, each node must determine with which node it is syn- chronized. These determinations are often made in the form of a priority list, and thus the node selects from valid incoming signals the one having the highest priority, i.e. the one highest on the list, as its synchronization source. If this signal is lost or its quality deteriorates so that it is no longer acceptable as a synchronization source, the node selects from the list the signal having the next-highest priority. The priority list must be compiled in such a way that all nodes on the list are located between the node in question and the master node of the network, and thus synchronization is distributed from the master node to the lower levels.

However, independent slave synchronization poses limitations to network synchronization: in looped networks, all links cannot be used for synchronization, and hence the dynamic adaptability of the network in different situations is limited. Communication must be present between the nodes in order for the information possessed by an individual node to be sufficient for decision-making in all situations without any need to strongly limit the number of links utilized for synchronization, in which case the clock frequency of the master node could not be distributed as easily to the network nodes. There are two methods for such communication, which will be described in the following.

A simple method for expanding independent slave synchronization to be communicative is loop protected synchronization (LP). LP synchroniza- tion seeks to prevent the timing from drifting into an inoperative state in looped networks by using two state bits, mcb and lcb, as an aid to the above priority lists, the bits being transmitted between network nodes. The first state bit, the master control bit (mcb), indicates whether synchronization is derived from the master network node. The master node defined for the network sends this bit as a logical zero in its outgoing signals, and the other nodes relay it further, if they are synchronized with a signal in which the mcb bit has the value zero. The other state bit, the loop control bit (lcb), indicates whether there is a loop in the 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 other direc- tions. Each node uses its own priority list when selecting the synchronization source, but it also checks the mcb and lcb bits, in addition to the signal state, before making the selection. Primarily, the node seeks a connection which has a clock frequency originating from the master node of the network (mcb=0). If this kind of connection cannot be found (due to an error situation), the node selects the functioning connection listed highest on the priority list in the normal way. Even though the signal may be otherwise suitable for synchronization, it is always required that the timing of the selected connection (timing source) is not in a loop (lcb=0). Thus, in LP synchronization the node strives to use for synchronization the source which has a suitable signal and which transmits state bits mcb=0 and lcb=0. If there are several sources of this kind, the node selects the one which is higher on the priority list than the others. If there are no sources having mcb=0 and lcb=0, the node checks whether there are sources having mcb=1 and lcb=0 and selects from these the one which is highest on the priority list.

Figure 1 illustrates, as an example, a telecommunications network comprising five nodes A...E and using the LP synchronization described above. The mcb and lcb bits sent by each node in different directions are marked next to nodes by using reference marks M (mcb bit) and L (lcb bit). In the figure, the priority lists used by the nodes are marked by using reference mark PL. Reference marks P_A, P_B and P_c mark the ports of each node. The priority list PL of the master node has no incoming signals as the master node always uses its internal oscillator as the synchronization source. Links not used by synchronization at the moment are shown with dotted lines.

On the basis of the above-mentioned rule, node B is synchronized with node A, because that is the only direction from which state bits having values mcb=0 and lcb=0 are received. Node C is synchronized with node D, because, on the priority list, port P_B is the higher one of the ports (P_A and P_B) from which state bits mcb=0 and lcb=0 are received. Node D is synchronized with node B, because port P_A is higher on its priority list than port P_c. Node E is synchronized with node C, because port P_A is higher on its priority list than port P_B (and state bits mcb=0 and lcb=0 are received from both of them).

When using only the priority list, a synchronization tree is formed from tree-like hierarchical structures, whereas in LP synchronization the synchronization tree is formed by using loops. First, a master loop including the master node of the network is formed and after this, nodes are added to the synchronization- tree one chain at a time until all nodes are included. Priority lists are formed in accordance with the loops and the chains. In the example in Figure 1, the master loop consists of nodes A, B, D and C, in this order. A chain of one node (node E) has been connected to this master loop.

Figures 2a and 2b illustrate the behavior of a network using LP synchronization (Figure 1) when an error occurs. During the first stage, illustrated in Figure 2a, the connection between master node A and node B is lost. After this, the network receives synchronization from master node A via node C. During the second stage, illustrated in Figure 2b, the connection between nodes A and C is also lost and node C becomes the new master node as it was the last one to relay master node frequency to the network. The mcb bit transmitted by node C is now a logical one stating that there is no connection to the official master node of the network. Another way in which independent slave synchronization has been expanded to be communicative is to use synchronization status messages (SSM) in accordance with the ITU-T recommendations G.704 and G.708. Recommendation G.704 defines the frame structure of a digital transmission system operating at a rate of 2048 kbit/s. In accordance with the recommen- dation, bits 4-8 in every second frame are spare bits and may be used e.g. to transport the above synchronization status messages. Only one of bits 4-8 in a frame can be used for this purpose, and thus a four-bit synchronization status message is made up of a selected bit (4-8) in frames 1, 3, 5, and 7 and in frames 9, 11 , 13, and 15 of the multiframe. The same synchronization status messages (SSM) are defined in recommendation G.708 for SDH networks. In an SDH network, the synchronization status messages are transported in bits b5...b8 of byte S1 in the section overhead (SOH) of the STM-N frame.

The table below presents the synchronization quality levels (QL) indicated by the bit patterns formed by these selected bits San1-San4 (n=4, 5, 6, 7 or 8) or S1 (b5...b8). The last column shows the expressions in accordance with the recommendations.

As will be seen from the table, ITU-T has decided on four synchronization levels, and additionally a meaning has been given to two further levels; one indicates that the synchronization level is unknown and the other that the signal should not be used for synchronization (QL=1111).

Figures 3 and 4 illustrate the operation of the SSM method in a ring- shaped network having five nodes in all, denoted by references N1...N5. Within each node, the quality level of the node's internal clock (QL1011) is indicated at the top of the column. Beneath that the priority list of the node is shown, wherein the selected timing source is indicated in italics. As stated previously, each node selects as its timing source the signal having the highest quality level as indicated by the synchronization message included therein. If several signals have the same quality level, the one highest on the priority list is selected. The synchronization status message transmitted by each node is shown with the reference "Q .xxxx" beside each port of the node. External timing sources S1 and S2 are connected to the master node N1 and to node N3, respectively. The quality levels of the synchronization status messages (QL=0010 and QL=0100) are indicated above the sources. A QL value must be given to each source external of the loop synchronization.

Figure 3 shows the network in a normal situation (no failures). The master node N1 utilizes an external timing source S1 , which in this example has been defined to be a clock having the quality level QL=0010. The master node transmits this synchronization status message in both directions. Slave nodes are synchronized with the signal arriving from the port Pa from the main direction; the synchronization status message included in this signal is QL=0010. In this situation, they transmit the same quality level (QL=0010) forward through port Pb and send the quality level QL=1111 (do not use for synchronization) in the direction from which they are receiving their timing (in the direction of port Pa).

Figure 4 shows a situation in which a failure condition has occurred on the connection between nodes N1 and N2. When node N2 detects this failure, it selects a new timing source. Since it is receiving the quality level QL=1111 from the other direction (from node N3), it cannot use this direction for timing either, and hence it changes to internal timing state and starts transmitting the quality level QL=1011 of its own clock. The next node (node N3) receives this quality level through port Pa, and changes external source S2 for its timing source, as the quality level QL=0100 given by this source is higher than that received through port Pa 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 itself with the signal arriving from node N3, as the quality level included in that signal is higher than the internal quality level (QL=1011) of node N2, and thus it starts transmitting the quality level QL=1111 in the direction of node N3. Node N4 also accepts the quality level transmitted by node N3, because it is receiving the quality level QL=1111 through port Pb. Hence, node N4 transmits the quality level QL=0100 to node N5, which is synchronized in the direction of port Pb, as the quality level QL=0010 is obtained therefrom. In that situation, node N5 returns the quality level QL=1111 to node N1 and transmits the quality level QL=0010 to node N4. The remaining nodes in the loop do the same, that is, transmit the quality level QL=0010-from port Pa and return the quality level QL=1111 to port Pb. Hence, the situation shown in Figure 4 has been reached. The loop has thus been synchronized in its secondary direction. In the previous examples the connection to the master clock was lost (Figure 2b) and thus the network also lost the master level clock, or the connection to the master node was retained all the time and the network was able to adapt to the error situation (Figures 2a and 4).

A disadvantage of the synchronization methods using synchroniza- tion status messages is that the network using the methods cannot flexibly utilize the backup clock in the network when an error occurs, for example, when the connection to the master clock is lost. Thus, the ability of the network to recover from failure situations is inadequate. These failure situations are described in more detail later in this document.

Summary of the invention

The objective of this invention is to eliminate the above-mentioned problem and to improve synchronization methods of the type described above so that a backup clock can be used effectively in the network. This objective can be achieved by using the solution defined in the independent patent claims.

The idea of this invention is to interconnect the location of the source in the priority list and the synchronization status (quality level as regards the synchronization) of the source. Hence the idea is to add to the prior- ity list information about the synchronization status that a certain synchronization source must have in order that it could be on a certain level in the priority list. The idea can also be implemented by keeping a separate priority list for each synchronization status.

Due to the solution in accordance with the invention, networks of the type described above can flexibly use the backup clock and synchronize themselves again with the master clock when the master clock becomes available. The solution in accordance with the invention can be used to improve the fault and interference tolerance of the network and thus to improve its reliability. The solution also offers more flexibility to the parameterizing of synchroni- zation on the network level as the priority list does not need to be defined only on the basis of the location of the network master clock, but possible backup clocks can also-be taken into account.

Brief description of the drawings In the following, the invention and its preferred embodiments are described in more detail with reference to the examples in accordance with the accompanying drawings 5a...10c, in which

Figure 1 illustrates a telecommunications network using LP synchronization, Figures 2a and 2b illustrate the behavior of the network in Figure 1 in failure situations, Figure 3 illustrates a telecommunications network using SSM synchronization, Figure 4 illustrates the behavior of the network in Figure 3 in failure situa- tions,

Figures 5a...5f illustrate a problem related to the LP synchronization when the backup clock of the network is on the same level as the master clock, Figures 6a...61 illustrate a problem related to the LP synchronization when the backup clock of the network is on a different level than the master clock, Figures 7a and 7b illustrate the basic idea in accordance with the invention, Figures 8a...80 illustrate the operation of the method in accordance with the invention, Figure 9 is a flow chart illustrating the process of synchronization source selection carried out by an individual node, Figure 10a is a block diagram illustrating those parts of the node which implement the method in accordance with the invention, and Figures 10b and 10c illustrate alternative ways of implementing a node.

Detailed description of the invention

In order to clarify the problem that lies in the background of the invention, let us first study a network in accordance with Figure 5a, using LP synchronization and having nodes A...D. The master clock M is connected via a direct connection to node A and the backup clock BM to node C. The nodes form a ring network. Next to each node there are numbers marking the priori- ties of different sources (ports), said priorities being defined by using the priority list of the node in question.

The figures assume that the backup clock is on the same quality level as the master clock (M=0). Figure 5a illustrates a basic situation in which the network is operating correctly. Synchronization goes in the "chain" M→A→B→C→D, and mcb and lcb bits having value "0" go in the same direction stating that the frequency originates from a master source (M=0) and that the timing has no loop (L=0). The mcb bit going in the opposite direction also has the value "0", but the lcb bit has the value "1" (L=1) as synchronization comes from that direction. The synchronization directions in the connections between different devices in the figure are marked with arrows.

In Figure 5b, the connection between nodes A and B is lost. Node B cannot use that connection for synchronization any longer, and it cannot use the connection from node C as the lcb bit coming from that direction has the value "1". In this case node B starts using its internal clock and transmits the information M=1 (clock signal does not originate from master) and L=0 (no loop). At this stage, other parts of the network have not noticed that there is a break in the connection between nodes A and B.

In the situation illustrated in Figure 5c, node C has had time to re- ceive the information M=1 and L=0 from node B. As node B does not offer a master clock and from the direction of priority 2, node D, comes the information L=1 ("do not use"), node C starts using backup clock BM (which is of the same quality as the master clock) which has priority 3. As node C again receives timing from the master level clock (M=0), it sends the information M=0 and L=0, corresponding to the new situation, to nodes B and D.

In the stage illustrated in Figure 5d, node D has had time to process the bits coming from node C. However, as these have not changed, node D does not react in any way. Node D still receives its timing via node C, but now the timing originates from backup clock BM. Node B now receives the bits M=0 and L=0 from node C, so node B has changed to node C as its timing source and returns the values M=0 and L=1 to node C. At this stage the network is synchronized with two different clock sources, node A to master clock M and nodes B, C and D to backup clock BM. However, this situation is not desirable as the connection to the master clock also exists from nodes B, C and D. Without the backup clock the synchronization would work correctly as was illustrated in the previous examples. In the stage illustrated in Figure 5e the connection between nodes A and D is also lost. This connection is not used by synchronization and thus the breaking off of the connection has no relevance as regards synchronization. When the connection between nodes A and D is available again (Figure 5f), it has no effect on synchronization as node D already receives the best possible status M=0 and L=0 from the direction of its first priority. Hence, the network remains synchronized with two different master clocks, even though it should only be synchronized with one master clock. As the example shows, LP synchronization does not support the use of both master clock and backup clock, if they are on the same quality level (M=0).

If the master clock and the backup clock are set on different levels (master clock gets master status M=0 and backup clock gets slave status M=1), the problem is that the backup clock is on the same level as the internal clocks of the nodes. This kind of situation is illustrated in Figures 6a...6i in which the first two stages (Figures 6a and 6b) correspond to the first two stages of the series of events in Figure 5 (Figures 5a and 5b). Figure 6c illustrates a situation in which node C has reacted to the bits M=1 and L=0 it has received from node B. The value L=1 is received from node D and thus that signal cannot be used for timing. The backup clock is on level M=1 and it has priority 3 on node C, so it is not used, as the connection from node B, which is now on the same level, has priority 1.

In Figure 6d, node D has received the values M=1 and L=0 from node C and changed its timing in the direction of node A as the master frequency (M=0 and L=0) is coming from there. At the same time node D has updated its outgoing bits.

At the stage illustrated in Figure 6e, node C has noticed the changed bits of node D and synchronizes itself now via node D with the master clock. Additionally, node C has updated its outgoing bits.

At the next stage (Figure 6f), node B notices that there is now con- nection to the master clock via node C and thus node B synchronizes itself with node C and updates its own outgoing bits. The situation is now stable and the nodes are synchronized with the master clock in accordance with the chain M→A→D→C→B.

If the connection between nodes A and D is lost after this (Figure 6g), node D starts to use its internal clock, because it receives the value L=1 from the direction of node C. Node D updates its outgoing status by transmitting the values M=1 and L=0 to node C.

After this (Figure 6h), node C notices the changed bits. Connection in the direction of node B is forbidden (L=1) and connection to the backup clock is on the same level as the connection to node D. Thus, node C remains synchronized with node D, because it has a higher priority than the backup clock. Node C updates its outgoing identifiers.

After this, node B notices the changed situation and updates the outgoing status. In the final situation (Figure 6i), the connection to the master clock M has been lost, but it has not been possible to take the backup clock into use at any stage. Thus, the current LP synchronization does not support the use of a backup clock even in the case where the master clock and the backup clock are set onto different levels.

The invention presented here solves the problems described above by interconnecting the location of the source on the priority list and the synchronization status of the source. Figures 7a and 7b illustrate this principle by using the example network described above. In accordance with the invention, the master status (M=0, L=0) is assigned its own priorities which can be, for example, the ones presented in Figure 7a, and slave status (M=1 , L=0) is assigned its own priorities which can be, for example, like the ones presented in Figure 7b. When signals having a master status are received in a certain node, the priorities shown in Figure 7a are used when selecting the timing source from the signals having said status. If only signals having a slave status are received in the node, the priorities shown in Figure 7b are used when selecting the timing source from the signals having said status. Thus, the idea is to define different priorities to different statuses, that is, make the location of the source on the priority list dependent on the synchronization status of the source.

Figures 8a...8o illustrate the operation of the network described above when the priorities shown in Figures 7a and 7b are used in the network. The backup clock is still assumed to be on the slave level.

Figure 8a illustrates the same start situation as in the previous example. In the situation in Figure 8b, the connection between nodes A and B is lost again and node B starts to use its internal clock, because it receives the value L=1 from node C. At the same time node B updates the identifier it transmits to node C. After this (Figure 8c) node C receives the information M=1 and L=0 from node B and the information L=1 from node D. Because the backup clock BM is connected to node C on the slave level, the node has two sources of slave level. From these, the backup clock BM is selected as it has the first priority on the slave level (see Figure 7b). Thus, the slave-level con- nection to node B is not included in the sources to be used. Thus, node C synchronizes itself with the backup clock BM and transmits the information

M=1 and L=0 to nodes B and D to indicate its new status.

At the stage illustrated in Figure 8d, nodes B and D have noticed the changed status of node C. Node B synchronizes itself with node C, because node C has the first priority on the slave level and thus it is better than the internal clock on the slave level. When node D notices that the status of node

C has changed, it changes node A to its synchronization source as it can get the master clock frequency (M=0) from node A.

In the situation illustrated in Figure 8e, node C receives the changed status of node D and changes from the backup clock to node D, because it can get the master clock frequency from node D. Node C updates its outgoing statuses.

Next (Figure 8f), node B has received the changed status of node C.

Node B remains synchronized with node C, but updates the status it transmits to node C. The situation is stable at this stage.

At the stage in Figure 8g the connection between nodes A and D is lost. When node D has noticed the break-off, it starts to use the internal clock

(the information L=1 comes from node C) and sends the information M=1 and

L=0 to node C. At the next stage (Figure 8h), node C has noticed the change that has taken place. Now node C has two sources (BM and D) available on the slave level and only the backup clock has been assigned priority on the slave level. Thus, node C synchronizes itself with backup clock BM.

At the stage illustrated in Figure 8i, nodes B and D have processed the slave status coming from node C. This status is the best these nodes can get and it has a higher priority than the internal clock which is also on the slave level. Thus, nodes B and D synchronize via node C with the backup clock BM.

At this stage the network is successfully synchronized with the backup clock when the connection to the master clock has been lost.

Next (Figure 8j), the connection between nodes A and D is restored. Node D synchronizes via node A with the master clock after receiving the master clock status from node A. After this (Figure 8k), node C receives the master clock status from node D and changes node D to its synchronization source. In the situation in Figure 81 node B also receives the master clock status (from node C) so it remains synchronized with node C and updates its outgoing status. The situation is stable now. The connection to the master clock has been restored and the nodes have successfully changed the master clock to their synchronization source.

At the stage illustrated in Figure 8m, the connection between nodes A and B has been restored. After this the Master status starts to spread on the network from the direction of the first priority (see Figure 7a). In the situation in Figure 8m, node B has changed node A to its synchronization source. Next (Figure 8n), node C receives master level status from the direction of the first priority and it discards the second priority. In the situation illustrated in Figure 8o, information has spread to node D which also changes its source of secondary priority to a source of the first priority (node A to node C). The situation is now stable and the network has been restored in a preferred manner to the basic situation after errors.

The previous examples concerned LP synchronization. When using a method based on synchronization status messages (SSM) the only difference is that the latter uses more status levels. In this case the method in accordance with the invention gives the above-defined advantages, for example, when there are several backup clocks on the same level in the network.

As the examples described above show, in the known methods the node priority lists have only included different sources (interfaces or ports) in the order of priority, for example, in accordance with the following table:

In each case, the signal with the best status has been selected as the synchronization source for the node. If the same status has several signals, the signal whose interface is highest on the priority list has been selected as the synchronization source. When proceeding in accordance with the invention, the node priority list is formed in such a manner that it includes both the different sources and the synchronization statuses required for a source to have the priority in question. For example, the previous example could in this case look as follows, when using LP synchronization as an example.

The same interface can be on the priority list several times as several different statuses can be received through it. Thus, the node actually has a priority list for each desired status.

When a synchronization source is selected in a node operating in accordance with the invention, the procedure is otherwise the same as in the known methods (i.e. finding the highest possible status and selecting from the sources having this status the one that has the highest priority), but now the node does not have just one common priority list, but each status level can have its own definitions. When the node selects from the sources having the same status the one which has the highest priority, it must use the priority definitions related to this status.

Figure 9 is a flow chart illustrating the decision process carried out by the node. First, the node searches for the highest available status (stages 91...93). Then the node searches among the sources transmitting said status for the one which is highest on the priority list (or part of it) related to said status (stage 94). It should be noted that there is always a source with some status as the internal clock of the node is also available. The "Do not use" status is ignored. On the other hand, as the internal clock of the node has higher status than this, the process does not even proceed to the "Do not use" status. If a source is faulty, it is assigned the "Do not use" status.

Thus, in the method in accordance with the invention the highest available status is retrieved first and then, from the sources transmitting this status, the one which has the highest priority on this status. Thus, the primary criterion for selection is the status (the quality level).

Figures 10a...10c illustrate as a functional block diagram the parts which implement the above-described method on an individual node of the network. The general structure of the node is, for example, such that it comprises several parallel interface units IU1, IU2...IUN, each of which is connected to at least one neighboring node, and a control unit which is common to all interface units and which makes the decisions concerning synchronization. The control unit and the different interface units are interconnected, for exam- pie, via the internal bus CBUS of the node.

The figures show two links, A., and A₂, coming from the neighboring nodes to the system node as an example. Both links are connected to their own interface units. The links are, for example, 2 Mbit/s PCM links in accordance with ITU-T recommendations G.703 and G.704. One interface unit IU can have one or several interfaces through which the node is connected to one or several neighboring nodes. Generally, it can be stated that the node has N interface units which have M interfaces (M>N).

In Figures 10a and 10b the interface specific or interface unit specific reference marks have indices and the parts common to all interface units do not have indices.

In an implementation in accordance with Figure 10a, each transmission line has been connected to a signal transmit/receive block 13j (i=1 ,2,...). These blocks perform the physical signal processing. Block 13; relays the synchronization status messages or bits forward to the synchronization block 16_| connected to it. The synchronization blocks 16, perform, for example, the checking of the correctness of the messages, and relay the synchronization status forward to the centralized synchronization control block 20 of the node via bus CBUS. The signal transmit/receive blocks also monitor the quality of the received signal and save this information to interface-specific fault data- bases 14,. Each synchronization block receives the fault information from the corresponding database. Detection of the transmission link errors/changes in the signal transmit/receive blocks is performed in a known way. As a result of a fault, the synchronization block transmits the status "Do not use for synchronization" to the control block. The synchronization control block 20 saves the synchronization statuses of the different sources, received from the synchronization blocks, to memory area 21. When selecting the synchronization source, the control block 20 retrieves from the memory area 21 the highest possible status received at the moment and the sources transmitting it. After this, the control block uses the priority list stored in the memory area 22 to select from the sources trans- mitting said status the one which has the highest priority relating to that status. Outgoing status messages are formed in the synchronization blocks 1β_iτ and when a change in the synchronization source causes changes in the outgoing status information, the control block informs the synchronization blocks of the change. The method can be implemented also by using, for example, the type of node architecture illustrated in Figure 10b. In this case the structure of the node is otherwise the same, but the priority lists have been distributed to different interface units and each interface unit informs the other interface units of its signal status. After receiving this information from others, each interface unit can make a conclusion corresponding to the one made in the centralized control block in the previous example and form the outgoing status information independently.

The idea in accordance with the invention can be implemented either by interconnecting the status and the priority list level or by using status- specific priority lists, in which case each status of a different level has its own list. There can be a list for every possible status or just for selected statuses. In this case, for example, when using the architecture shown in Figure 10a, the control block 20 has a separate priority list for each status and the control block uses the list which is related to the highest status received at the mo- ment. This kind of solution is illustrated in Figure 10c.

The method can be implemented (as described above) in such a way that a source gets a certain priority level when it has exactly the same status as the one defined for the priority level in question. On the other hand, this defined status can be used as a minimum requirement. In this case, the source is always on the priority level corresponding to said minimum value when the quality level of the signal received from the interface is of the minimum value or higher, and no other minimum value of the quality level has been defined for the interface, this other minimum value being higher than said minimum value, but lower or equal to the current quality level (status) of the signal. If the source has not been assigned a priority relating to the current quality status of the source, the system uses the priority which the source gets when it has a status which is the highest of the statuses for which a priority level has been defined and which are lower than the current status of the source.

In the above, the invention was implemented by always defining the status level required for a certain priority level. This definition can also be skipped on some priority levels in which case the source in question has the priority defined for the status that is one level lower. If this status has no priority either, the status that is one level lower is used, etc., until the priority defined for the source can be found. If no priority is found even when the lowest al- lowed status is reached, the source is not supposed to be used for synchronization with these priorities. If the implementation is done by using only one priority list into which the required status has been added and not by using priority lists belonging to different statuses, the required status level of a source need not be defined and the source can be put into the priority list without the status requirement. In this case the source always gets this priority in comparisons regardless of its status level.

Even though the example has been described in relation to examples referring to the accompanying figures, it is clear that the invention is not limited to these but that it can be varied within the limits of the idea of the in- vention presented above and in the accompanying claims. For example, quality levels (statuses) are not necessarily received from all of the sources, but the quality level can be defined in the interface in the node so that when the signal is in order, the node assigns this quality level to the signal coming to this interface.

Claims

1. A synchronization method in a telecommunications network comprising several nodes (A...D) which have been interconnected with transmission links (A_1f A₂), according to which - network nodes transmit synchronization messages which are used to state the quality level of the corresponding signal as regards synchronization,

- a priority list is formed in each node, the list containing node interfaces on different priority levels, and - the node uses as its synchronization source the one signal, from among the possible synchronization sources, which has the highest quality level, characterized in that

- in the node, information is stored for an individual priority level of an individual interface, the information stating the quality level that the signal coming from the interface in question must have in order for the interface in question to be on the priority level in question, and

- the node searches for the highest quality level that the signals received in the node have and selects from the interfaces receiving signals of that quality level the one which has the highest priority on the quality level in question and uses that as its synchronization source.

2. A method in accordance with claim ^characterized in that the information about the quality level is stated as an exact value which the signal must have in order for the interface to be on the priority level in ques- tion.

3. A method in accordance with claim 1, characterized in that the information about the quality level is stated as a minimum value of the quality level in which case the interface is on the priority level corresponding to the minimum value always when the quality level of the signal received from the interface is of the minimum value or higher, and no other minimum value of the quality level has been defined for the interface, this other minimum value being higher than said minimum value, but lower or equal to the current quality level of the signal.

4. A method in accordance with claim ^characterized in that in a certain quality level the priority levels are assigned only for part of the node interfaces, whereby the selection is made among those interfaces which have been assigned a priority level.

5. A method in accordance with claim ^ c h a r a c t e r i z e d in that for part of the interfaces the quality level corresponding to the interface is not defined in which case the interface has the priority level assigned to it on all allowed quality levels.

6. A method in accordance with claim 1 , c h a r a c t e r i z e d in that the priority list is implemented as several individual lists and each list relates to a certain quality level.

7. A node arrangement for a telecommunications network comprising several nodes which are interconnected by transmission links, the node arrangement comprising

- means (13,, 13₂) for transmitting synchronization status messages to neighboring nodes connected to the node, the synchronization status mes- sages being used to state the quality level of the corresponding signal as regards synchronization,

- a priority list stored to the node, the list containing node interfaces on different priority levels, and

- selection means (20; 16,, 16₂) used to select the signal to be used as the synchronization source of the node from among other possible synchronization sources in such a way that the selected signal has the highest quality level, c h a r a c t e r i z e d in that

- the node arrangement further comprises information for an individ- ual priority level of an individual interface,, the information stating the quality level that the signal coming from the interface in question must have in order for the interface in question to be on the priority level in question, and that

- the selection means have been configured to select from the interfaces receiving signals of that quality level the one which has the highest priority on the quality level in question as the synchronization source of the node.