FI91691B - Hierarchical synchronization method - Google Patents

Description

91691

Hierarchical synchronization method

The invention relates to a hierarchical synchronization method according to the preambles of the appended claims 5 1 and 4, which is used in a communication system using message-based synchronization.

In this representation, the intersections of the transmission links of the system are referred to as a node. The node may be any device or apparatus capable of interfering with the clock, e.g. a branch or cross-connect device.

The nodes of a system using message-based synchronization are interconnected by the transport connections they use to transmit data. The connections used also forward the sender's clock frequency to the receiver. Each node selects either the frequency of a signal from a neighboring node or the frequency of its own internal clock source as its own clock frequency source. In order to make all the nodes of the system operate at the same clock frequency, the aim is usually to get the system to synchronize to one clock source, the so-called the master source. In this case, all nodes in the system directly connected to the selected main source are synchronized to this main source, and nodes connected to them but not directly connected to the main source 2J5 are synchronized to these nodes next to the • main source. Correspondingly, nodes that are always farther away from the main source are always synchronized to those nodes that are one connection distance closer to the main source.

In order to build a synchronization hierarchy 30 as described above within the system, the nodes of the system transmit synchronization messages to each other. These messages contain information that allows individual nodes to select the timing source. The nodes of the system are prioritized and the system tries to synchronize to the clock frequency of the node that is at the highest priority level of 291691. At the same priority level, there is normally only one node in the system. Normally, the synchronization messages contain information on who the clock frequency of the node sending the message originates from, which is the

5 node priority, and a value describing the quality of the clock signal. This allows an individual node to select the clock frequency of a neighboring node that originates from the desired node and is of the best quality as the source of its own clock frequency.

10 During the system start-up phase, each node selects its own internal clock source as its clock frequency source because no incoming synchronization message has been processed. When the first incoming synchronization messages have been processed, the clock frequency of the neighbor with the highest priority is selected as the source of its own clock frequency. Once the system has reached a stable state for synchronization after all the messages have spread to the system, the system is hierarchically synchronized to the clock frequency of the main source.

Figure 1 shows a system using message-based synchronization in a stabilized situation. The priorities defined for the nodes are marked with numbers inside the circles describing the nodes. The lower the number, the higher the priority of the node. Synchronization messages sent by node n (n = 25 1 ... 6) are marked with the reference number MSGn. The synchronization message sent by each node is generally different and depends on the message-based synchronization method used. The propagation of the clock frequency from the master clock (Node 1) to the other nodes in the system is shown by solid lines. Dashed connections between nodes are not normally used to synchronize the system, but are available in change situations.

A simple principle in message-based synchronization is that the user defines the sync of the nodes.

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91691 3 roaming hierarchy, by assigning each node its own identifier indicating the node level in the hierarchy, and the system synchronizes to the defined master clock independently, utilizing all existing connections between the nodes as needed (cf. Figure 1). If the connection to the master clock is lost and there is no alternate connection, or the master clock fails, the system synchronizes to the next highest level node. Fig. 2 shows a situation when the main clock fails in the system 10 according to Fig. 1. The response to the change in synchronization is through inter-node messaging. When the incoming timer is interrupted, the synchronization hierarchy is rebuilt from the point of interruption onwards (away from the system master). This happens e.g.

15 so that the node that first notices the interruption first goes to the internal timing state for a period of time and forwards the information about the change, at which point the formation of a new synchronization hierarchy begins. In general, the end result is a hierarchical structure similar to the original, in which the faulty-20 connection is replaced by a functioning connection, while the rest of the structure remains almost unchanged.

A network using message-based synchronization is described, for example, in U.S. Patents 2,986,723 and 4,837,850. Both patents disclose methods that use time limits depending on the size and shape of the system for system failures, during which nodes are forced to enter a predetermined constant state to prevent incorrect synchronization in the event of failures. In the event of a fault, information about the fault-30 is transmitted as described above by means of system messages. Once the information about the changed situation has spread throughout the system or over a sufficiently large area, the synchronization is rebuilt at the change point and possibly further afield, if necessary. The deadlines will ensure that information on the change is spread over a sufficiently wide area of 4 91691. Upon detecting a change / failure, the node forwards this information, starts its own timer, and enters a predefined state. When the time limit has elapsed, the node restarts its normal actions to obtain the timing, and the system begins to resynchronize those parts that were affected by the change / failure. The solution according to the present invention is specifically intended for systems such as those described in the aforementioned U.S. Patent 2,986,723, in which a change in the system is reflected in a change in the synchronization identifier entering the node. In the system disclosed in U.S. Patent 2,986,723, said constant state is an internal timing state in which a node uses its own internal clock as the source of its timing. The method according to the method disclosed in this U.S. Patent 15 will hereinafter be referred to as Self-Organizing Master-Slave Synchronization (SOMS) and will be used as an example in the further description of the invention.

It should also be noted that when we refer to a deadline in this context, it refers to the predetermined time by which the system is intended to prevent the selection of erroneous / outdated synchronization messages.

In a system using the SOMS synchronization method, a failure of the synchronization connection or failure of a node in the middle of the hierarchical structure is caused by the breakdown of the synchronization structure in a part of the system. Once the synchronization structure is broken, some of the nodes in the system use their own internal clock for timing 30 until the synchronization structure is rebuilt.

However, the internal clock of a node is usually never qualitatively as good as the main clock of the system. As a result, it is difficult to maintain the required 35 quality levels in synchronization, especially in the synchronization of larger systems.

Il 91691 5

It is an object of the present invention to provide a method in which the disadvantages described above are reduced and in which resynchronization can be accelerated. This is achieved by a method according to the invention, which in its first embodiment is characterized by what is described in the characterizing part of the appended claim 1, and in its second embodiment by what is described in the characterizing part of the appended claim 4.

The idea of the invention is, instead of automatically going to e.g. the internal timing mode, to select a new synchronization identifier of the old source immediately when the incoming synchronization identifier currently selected deteriorates. The selection is performed at least 15 whenever said degraded identifier has a higher priority level than the intra-node synchronization identifier. A degraded synchronization identifier means that the corresponding synchronization message is still qualitatively acceptable, but the identifier within it has become lower in priority. Deterioration of the incoming sync tag indicates a change / failure on the connection to the main source of the system.

Thanks to the solution according to the invention, unnecessary visits in the internal timing (or corresponding transitions to the constant state) are avoided, and the synchronization tree of the system is better preserved in resynchronization, whereby the interruption in the frequency transmission of the main node is shorter. In the case of interruptions, one or a few nodes 10 are resolved by resynchronization, instead of all the nodes in the synchronization tree below the interruption point running in internal timing, as in the known SOMS method. The total time for synchronization is thus made even shorter. Solution 15 also reduces the importance of the location of the breakpoint in the 6,91691 synchronization time, which makes it possible to better define in advance the synchronization behavior of the system in different situations. (A synchronization tree refers to the hierarchical tree-like structure that is created in a subordinate 5 synchronization from the main node to the network.)

In the following, the invention and its preferred embodiments will be described in more detail with reference to the examples of Figures 3-8 in the accompanying drawings, in which Figure 1 shows a message-based synchronization system in general form synchronized to the main source clock frequency, Figure 2 shows the network of Figure 1 Fig. 3 shows a network using self-directed slave synchronization (SOMS) in its initial state, Fig. 4 shows the network of Fig. 3 in a stable state, Fig. 5 shows the resynchronization of the network of Fig. 4 after its main node fails, Fig. 6 shows the resynchronization of the network of Fig. 4 between two nodes. Figures 7a-7g show, as a series of events, the application of the method according to the invention in a SOMS system, and Fig. 8 shows the elements of a single node in which the method according to the invention is implemented.

Figure 3 shows a system using Self-Organizing Master-Slave synchronization (SOMS) according to the aforementioned U.S. Patent 2,986,723, comprising in this case five 30 nodes (or devices) denoted by reference numerals 1 to 5 thereof. according to the level of the hierarchy. (The main node of the network has the smallest SOMS address.) The nodes forward messages containing the above SOMS addresses. This allows the nodes to identify each other using these address numbers and to build a synchronization hierarchy, allowing the entire network to

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91691 7 is able to synchronize with the master node.

As mentioned above, the messages that are sent continuously on the network depend on the message-based synchronization method used. In addition, the messages are 5 unique for each sending node. In a SOMS network, a synchronization message comprises three different parts: a frame structure, an identifier, and a checksum. The SOMS identifier is the most important part of the SOMS message. It consists of three consecutive numbers D1 ... D3: 10 D1 is the origin of the synchronization frequency of the sending node of the SOMS message, i.e. the SOMS address of the node visible to the sending node as the main node.

D2 is the distance to the node expressed by D1. This distance is expressed as the number of nodes in between.

15 D3 is the SOMS address of the sending node.

Each node (or device) continuously compares incoming SOMS tags with each other and selects the smallest of these. In the identifier, the parts D1, D2 and D3 are directly connected into one number by putting them in succession (D1D2D3) 20 (for the sake of clarity, a dash will be written in the following to separate the different parts; Thus, the primary selection criterion for the smallest address becomes the SOMS address (D1) of the node visible to the previous nodes as the main node, i.e. the node tends to synchronize to a signal whose frequency is originally from the node whose address is as small as possible. In this case, in a stable situation, the entire network is synchronized to the same master node (because the master node of the entire network has the smallest SOMS address).

If two or more of the incoming signals are synchronized to the same master node, the one with the shortest path is selected (D2). the last selection criterion remains the SOMS address (D3) of the node sending the SOMS message, on the basis of which a selection is made if no difference is otherwise obtained between the incoming signals.

35 Once a node has accepted one of the neighboring nodes 8,91691 as its new synchronization source based on the incoming SOMS ID, the node has to re-establish its own SOMS ID. The new SOMS identifier can be derived from the selected smallest SOMS identifier as follows: the first part (D1) is left intact, the second part (D2) is incremented by one and the third part (D3) is replaced by the node's own SOMS address.

Each node also has its own internal SOMS identifier X-O-X, where X is the one in question. the SOMS address of the node. If none of the incoming SOMS messages contains an identifier that is less than the internal identifier, the node uses its own internal oscillator or possibly a separate synchronization input as the source of its clock frequency. The outgoing SOMS message naturally uses 15 internal SOMS identifiers.

The nodes continuously send SOMS messages in each direction so that the changed information in the SOMS identifiers spreads as quickly as possible and the operational status of the neighboring nodes is constantly known. Before SOMS identifiers can be compared, incoming SOMS messages must be accepted and SOMS identifiers must be separated from them.

When a SOMS message is first received from a particular transmission link, its SOMS identifier is immediately accepted for comparison if the message was error-free.

25 When an incoming transmission link has an accepted SOMS identifier and an error-free message with the same identifier arrives continuously, the situation remains unchanged. If an SOMS message is found to be invalid, the old SOMS identifier is retained until 30 consecutive SOMS messages are received as invalid. In that case, the SOMS tag for comparison. Waiting for three consecutive SOMS messages is intended to eliminate momentary interference.

If no SOMS message is received from the connection, even if the connection 35 otherwise works, three consecutive li 916 * 1 9 SOMS messages are waited until the current SOMS identifier is rejected. If the connection is completely lost, the SOMS identifier is rejected immediately. If, due to interference in the incoming signal, 5 valid SOMS tags are not obtained for comparisons, the the SOMS identifier of the transport connection. In this case, the as a SOMS identifier of the incoming transmission link, a constant identifier in which all parts (D1, D2 and D3) get their maximum value (MAX-MAX-MAX).

10 When a new changed SOMS identifier is detected in an incoming SOMS message, it is immediately accepted for comparison if the message was error-free. This does not cause unnecessary delays for network changes.

In the initial situation, each node uses its own internal synchronization source, in which case it sends its own internal SOMS identifier X-O-X to the other nodes. This tag is also compared to incoming SOMS tags. If none of the incoming tags is smaller than the internal tag, the node to use its own internal timing-20.

In Figure 3, the SOMS network is shown in the initial state, in which no node (or device) has had time to process incoming SOMS messages. For all nodes, the highest priority is given to the SOMS tun-75 node within the node, because the others have not yet been processed. In Figure 3, the incoming SOMS tags are marked next to each node, and the selected tag is written inside the frame (in the initial situation of Figure 3, all nodes use their internal timing source). The connections in use in Synk-30 resonance are drawn on a solid line, the spare connections are in a broken line (in the initial situation according to Figure 3, all connections are spare).

When the nodes have time to process the incoming SOMS sa-35 names, the node 1 remains in use of the internal timing, the nodes 2 and 4 are synchronized to the node 1 based on the identifier 1-0-1, the node 3 is synchronized to the node 2 (2-0-2) and the node 5 to node 3 (3-0-3). At the same time, the nodes form their own new SOMS identifiers as described above and exchange the new identifier in their outgoing SOMS message. The state of the network after its stabilization is shown in Figure 4. All nodes are synchronized to the main node 1 by the shortest possible route.

If the smallest of the incoming SOMS identifiers changes or disappears completely when the connection is lost, the node selects a new synchronization direction according to the second smallest SOMS identifier. Before that, however, the node is forced to go into internal timing, where it lingers for a predetermined length of time to "kill" the invalid 15 SOMS identifiers out of the network. For example, if node 1 failed in the situation of Figure 4, nodes 2 and 4 would no longer receive the identifier 1-0-1 that they were synchronized. If, in this situation, they immediately accepted the next smallest SOMS identifier, the network would no longer be synchronized to one master node, but the synchronization would circulate in the loop. By the time Node 1 fails, Node 2 will still receive tags 1-1-4 and 1-2-3 and Node 4 will again receive tags 1-1-2 and 1-2-5, because Nodes 3 and 5 have not yet had time to react to the changed situation. . If the second smallest identifiers were immediately accepted, Node 2 would be synchronized according to Node 4 and Node 4 would be synchronized according to Node 2. This situation is prevented by the above-mentioned forced internal timing mode, whereby the the nodes switch to use their own internal timing source 30 and start sending outwards their own internal SOMS identifier (Χ-0-Χ). In this case, those * nodes that were synchronized to the node that moved to the internal timing will notice that a change has taken place in the network, and the old SOMS-35 message on which the synchronization was based is no longer valid because it has changed

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91691 11 as an internal SOMS message for the node. In this case, said nodes themselves switch to the forced internal timing state for a limited time.

If, in the case of Figure 4, the main node fails, the nodes 2 and 4 immediately forcibly enter the internal timing state after losing the incoming SOMS identifier 1-0-1. When nodes 3 and 5 detect a change in nodes 2 and 4, they are also forced to go to the internal timing. When the node 2 returns to the normal state, it 10 receives from the nodes 3 and 4 their internal SOMS identifiers (3-0-3 and 4-0-4) and stays in the internal timing, because from the outside there is no smaller SOMS identifier than its own internal identifier (2 -0-2). Node 4 in turn synchronizes with Node 2. After stabilizing, the network is in the state 15 shown in Figure 5, where Node 2 has become the new main node of the network.

If, for example, only the connection between nodes 1 and 2 is lost (Fig. 6), only node 2 is forced to enter the internal timing state. When it returns to normal mode, it synchronizes with Node 4, which has a connection to the main node of the network. Once the entire network 20 has stabilized, synchronization is still from Node 1 despite a single outage. This situation is shown in Figure 6.

When an interruption or other change occurs in the upper parts of the synchronization tree (closer to the main source of the system) that results in the synchronization identifier of the currently selected timing source deteriorating because there is no longer a connection to the main node of the system along the old route, according to, the new, degraded sync tag of the selected timing source 30 immediately becomes the new sync tag. (Thus, the node does not enter the state of internal timing, as happens in the known solution described above).

Figures 7a-7g show a sequence of events for the use of the method according to the invention 35 in a part of the SOMS system, 12 91 691 comprising nodes 15, 17, 18, 19, 27 and 30. Nodes at a forced time are marked by drawing a line below their number and for synchronization the connections used are still marked with solid 5 lines. In the first step shown in Figure 7a, the connection of the node 15 to the main node of the system is disconnected. In the next step (Fig. 7b), the node 15 forcibly enters the internal timing state and starts transmitting its internal synchronization identifier. In this case, the synchronization identifier entering the nodes 18 and 19 10, currently selected in them, deteriorates, whereby they immediately select the changed identifier as their new synchronization identifier and on this basis form a new own outgoing identifier (Fig. 7c). Nodes 18 and 19 are forced to stay in the new, changed identifier for a predetermined period of time before they are free to choose the smallest (best) of all incoming synchronization identifiers. This forced automatic selection of the changed identifier according to the invention spreads down in the synchronization tree without changing it. In the step shown in Figure 7d, it has reached nodes 27 and 30. In the next step (Figure 7e), the node 15 has expired and is free to choose its timing from among the synchronization identifiers entering the source. However, since there is no better synchronization identifier elsewhere, the node 15 voluntarily remains in the internal timing. In the step shown in Figure 7f, the deadlines of nodes 18 and 19 also expire. Node 18 receives from the node 17 an identifier 30 from the main source of the system and synchronizes based on it, but the node 19 remains in the old state because it does not receive a better identifier elsewhere than the identifier from the node 15. In the step shown in Figure 7g, the nodes 15 and 19 have had time to process the new identifier transmitted by the node 18, which has changed due to changes in the previous step. Nodes 15 and

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91691 13 19 are then synchronized to the main source of the system via node 18. The deadlines for Nodes 27 and 30 have expired, but they do not need to change their timing sources because there will be no better synchronization identifier elsewhere. As can be seen from the series of images (e.g., Figure 7d), connections between nodes are used for synchronization even when the node is forcibly on time.

The minimum length of the above-mentioned period preferably corresponds to the time taken for the node's neighboring nodes to receive information about the node's transition to said constant state, to react to the change, and the node itself has received information from neighboring nodes about their response. This time can be calculated using the following formula: (1) K = 2 x (S + H + V), where S is the maximum duration for generating a synchronization message and its transmission between two nodes, H is the maximum duration for accepting a synchronization message at a node, and V is the maximum comparison of incoming synchronization identifiers duration in the node. By using the length of the formula (1) 20 as the length of the time limit, the synchronization can be further accelerated and the time limit is made constant regardless of the size of the system. The use of said term and the benefits to be obtained therefrom are described in more detail in the co-pending FI patent application FI-92xxxx, to which reference is made for a more detailed description.

Figure 8 shows the elements of the node in which the method according to the invention is implemented. The figure shows two connections to the system node from neighboring nodes, A and B. The transmission line of each connection is connected to the signal transmission and reception section 13a and 13b, respectively, which perform the processing of the physical signal. The section 13a (and 13b, respectively) forwards the synchronization message to the synchronization message transmission and reception section 16a (and 16b, respectively) connected to it. The transmitting and receiving parts 16a and 16b 91691 14 checking the error of the message and forwarding the message to the central synchronization decision section 23 of the node, said input being connected to the output of the respective transmitting and receiving section 16a or 5b. The signal transmitting and receiving sections 13a and 13b also monitor the quality of the received signal and store information about them in the connection-specific fault databases 24a and 24b, respectively. The transmission and reception section 16a of the synchronization message receives the fault information from the database 10a and the transmission and reception section 16b from the fault database 24b, respectively. The detection of a connection fault / change in the signal transmission and reception parts takes place as is known per se.

The decision-making section 23 compares the messages and 15 stores them in the memory 21, e.g. in order of priority, so that the synchronization identifier is always selected at the top. The decision-making section also receives the corresponding signal fault information from the connection-specific transmission and reception block 11a or 11b, either in the form of a synchronization message or as separate fault information. When the decision-making part detects from the information received that the node has to go to a fixed constant state for a specified time, it selects its timing source as defined for the situation in the synchronization method used, issues a corresponding for parts 16a and 16b and starts its timer member 25. With the new identifier, the node indicates the change that has taken place to its neighboring nodes.

30 When the timer elements 25 have given the deadline K

the decision section 23 may again select the timing source according to the normal procedure.

When the decision section receives a degraded synchronization from the transmission and reception part of the message corresponding to the selected synchronization identifier.

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91691 15 message, it starts its timer member 25, selects the degraded identifier and on the basis of it generates its new own synchronization identifier, which it inputs from the memory 22 to all synchronization message transmission units.

According to a preferred embodiment of the invention, after selecting the degraded identifier as a new identifier, the node can again select a new identifier if it still obtains an even better identifier from the selected connection during the time limit. However, tags from other connections cannot be accepted as newly selected tags until the deadline has passed. The aforementioned additional feature further speeds up system synchronization.

In the embodiment described above, the degraded synchronization identifier was selected either for the entire period or for at least part of that period. According to another embodiment of the invention, the node first compares the degraded synchronization identifier with its own internal identifier and selects a better alternative. The comparison is performed in decision making section 23. If the internal identifier is better, the node enters the internal timing in a known manner, and if the degraded identifier is better, the node selects it as its new identifier as described above either for the entire period or until the '25 connection. an even better identifier is obtained.

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. Thus, although the SOMS system has been used as an example above, the solution according to the invention is applicable to all similar types of systems, in which the change in the system is reflected as a change in the synchronization message entering the node.

Claims (6)

16 91691 A hierarchical synchronization method for a communication system using message-based synchronization, comprising a plurality of nodes connected to each other by transmission links (A, B), wherein the nodes send each other signals containing synchronization messages comprising information about the priority of the corresponding signal in the system. in which a node forcibly enters a predetermined steady state as a result of a change condition, such as a fault condition, to prevent the selection of erroneous synchronization messages, characterized in that when the synchronization identifier 15 A method according to claim 1, characterized in that if said degraded identifier becomes better during said period, the node immediately selects the improved identifier as its new synchronization identifier. A method according to claim 1, characterized in that the minimum duration of said period is the time (K) elapsed before the node's neighboring nodes have been informed of the node's transition to said constant state, reacted to the change and the node itself has received from the neighboring nodes information on their response. A hierarchical synchronization method for a communication system using message-based synchronization, comprising a plurality of nodes connected to each other by transmission links (A, B), in which the nodes send each other signals containing synchronization messages comprising information corresponding to signal I! 91691 17 in the internal synchronization hierarchy of the system, and in which a node forcibly enters a predetermined constant state as a result of a change condition, such as a fault condition, to prevent the selection of erroneous synchronization messages 5, characterized in that and if better than said degraded identifier was selected as the new synchronization identifier, the node is forced to remain there for at most a predetermined period of time, and if said internal sync identifier is selected as the new synchronization identifier, about. A method according to claim 4, characterized in that if said degraded identifier is selected and becomes better during said period of time, the node immediately selects the improved identifier as its new synchronization identifier. A method according to claim 4, characterized in that the minimum duration of said period is the time (K) elapsed before the node's neighboring nodes have received information about the node's transition to said constant state, reacted to the change and the node itself has received information from neighboring nodes about their response. 91691 18