WO1994011964A1 - Procede de synchronisation hierarchique - Google Patents

Procede de synchronisation hierarchique Download PDF

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Publication number
WO1994011964A1
WO1994011964A1 PCT/FI1993/000457 FI9300457W WO9411964A1 WO 1994011964 A1 WO1994011964 A1 WO 1994011964A1 FI 9300457 W FI9300457 W FI 9300457W WO 9411964 A1 WO9411964 A1 WO 9411964A1
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WO
WIPO (PCT)
Prior art keywords
node
signature
synchronization
nodes
time period
Prior art date
Application number
PCT/FI1993/000457
Other languages
English (en)
Inventor
Jukka Kainulainen
Original Assignee
Nokia Telecommunications Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nokia Telecommunications Oy filed Critical Nokia Telecommunications Oy
Priority to AU54221/94A priority Critical patent/AU5422194A/en
Priority to GB9509026A priority patent/GB2287158B/en
Priority to DE4395760T priority patent/DE4395760T1/de
Publication of WO1994011964A1 publication Critical patent/WO1994011964A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0679—Clock or time synchronisation in a network by determining clock distribution path in a network

Definitions

  • the invention relates to a hierarchical syn ⁇ chronization method for a telecommunications system employing message-based synchronization and comprising a plurality of nodes interconnected by transmission lines wherein the nodes interchange signals containing synchronization messages with information on the pri ⁇ ority of the respective signal in the internal syn- chronization hierarchy of the system, and the node is forced to enter into a predetermined standard state for a preset time period in a change situation, such as a failure situation, in order to prevent the selection of faulty synchronization messages.
  • a change situation such as a failure situation
  • a node may be any device or equipment capable of affecting clock synchronization, such as a branch ⁇ ing or cross-connection means.
  • Nodes in a system utilizing message-based synchronization are interconnected by transmission lines which the nodes use for data transmission. These lines also forward the clock frequency of the trans ⁇ mitting party to the receiving party.
  • Each node selects the frequency of a signal from a neighbouring node or the frequency of its own internal clock source as the source of its own clock frequency. In order that all nodes in the system would operate at the same clock frequency, one usually attempts to make the system to synchronize itself with a single clock source called a master source.
  • All system nodes connected directly to the selected master source are thus synchronized with the master source while nodes connected to the nodes adjacent to the master source but not directly connected to the master source are synchronized with these adjacent nodes. Accordingly, each node at a greater distance from the master source synchronizes itself with a node one node spacing closer to the master source.
  • the system nodes interchange synchronization messages. These messages contain information by means of which individual nodes are able to select a timing source. The system nodes are prioritised and the system tends to synchronize itself with the clock frequency of a node having the highest level of priority. Normally each priority level is assigned to a single system node.
  • Synchronization messages normally contain information about the origin of the clock frequency of the node transmitting the message and the priority of the node as well as a value describing the quality of the clock signal. Accordingly, a neighbouring node clock frequency which originates from a desired node and which is of the highest quality can be selected by an individual node as the source of its own clock fre ⁇ quency. At the system start-up each node selects its own internal clock source as the source of its clock frequency as it has not yet processed any incoming synchronization messages. After the node has processed the first incoming synchronization messages, it selects the clock frequency of a neighbouring node, having the highest level of priority as the source of its clock frequency.
  • Internodal connections drawn by broken lines are not used in a normal situation for system syn ⁇ chronization, but they are available in change situations.
  • Message-based synchronization is based on a simple principle that the user defines the synchron ⁇ ization hierarchy of the nodes by assigning each node a dedicated signature indicating the hierarchical level of the node and the system synchronizes itself with the defined master clock independently by util ⁇ izing, if required, all existing internodal con ⁇ nections (cf. Figure 1). If the connection to the master clock breaks, and no alternative connection exists, or if the master clock fails, the system synchronizes itself with a node of the next highest level of hierarchy.
  • Figure 2 shows a situation where the master clock fails in the system according to Figure 1.
  • a network utilizing message-based synchroniza ⁇ tion is described e.g. in US Patents 2,986,723 and 4,837,850. Both patents disclose methods in which time periods depending on the size and configuration of the system are used in the case of system failures. During the time periods the nodes are in a predetermined forced standard state in order to prevent in ⁇ appropriate synchronization in failure situations. Information about of the failure is forwarded as described above by using the messages of the system. After information about the changed situation has been distributed throughout the system or over a suffi ⁇ ciently large area, the synchronization is re- established around the point of change or possibly also at a greater distance, if required. The time periods ensure that information about the change will be distributed over a sufficiently large area.
  • the node On detecting a change/failure, . the node forwards information about " it, starts its own timer, and enters into a predetermined state. After the time period has expired, the node reverts to its normal procedures for obtaining timing, and the system starts to resyn- chronize itself within areas which were affected by the change/failure.
  • the solution of the present invention is especially intended for systems of the type disclosed in the above-mentioned US Patent 2,986,723, where a change in the system appears as a change in the synchronization signature entering the node.
  • the standard state is a state of internal timing, in which the node uses its own internal clock as the source of timing.
  • SOMS Self-Organizing Master- Slave Synchronization
  • the time period refers to a preset period of time intended to prevent the acceptance of faulty/outdated synchronization messages in the system.
  • a failure of a synchronization connection or of a node located centrally in the hierarchical struc ⁇ ture causes the synchronization structure to break down in a system portion.
  • part of the system nodes use their own internal clock for timing until the reestablishment of the synchronization structure is started.
  • the internal clock of the node is usually inferior in quality to the master clock of the system. For this reason, the maintenance of the required quality is difficult in synchronization of larger systems in particular.
  • the object of the present invention is to provide a method Ttfhich alleviates the above-described disadvantages and speeds up resynchronization. This is achieved by a method according to the invention, the first embodiment of which is characterized in that when there occurs deterioration in the selected syn- chronization signature of a node, the node immediately selects the deteriorated signature as its new syn ⁇ chronization signature and forcedly retains it for a preset time period at the most.
  • the second embodiment is characterized in that when there occurs deterioration in the selected synchronization signature of a node, the node compares said deter ⁇ iorated synchronization signature with its internal synchronization signature, and immediately selects the better one as its synchronization signature, and if said deteriorated signature is selected as the new synchronization signature, the node forcedly retains it for the preset time period at the most, and if said internal synchronization signature is selected as the new synchronization signature, the node forcedly retains it for the entire preset time period.
  • the invention rests on the idea that, instead of automatically entering into the state of internal timing, for instance, the new synchronization sign ⁇ ature of the old source is selected immediately when the incoming current synchronization signature deteriorates.
  • the selection is carried out at least always when this deteriorated signature has a higher level of priority than the node's own internal syn ⁇ chronization signature.
  • a deteriorated synchronization signature means that the respective synchronization message is still acceptable in quality but the level of priority of the associated signature has decreased.
  • Deterioration in the incoming synchronization signature is indicative of a change/failure that has occurred in a connection to the master source of the system.
  • the solution according to the invention enables any unnecessary transitions to internal timing (or any respective transitions to a standard state) to be avoided, and a greater portion of the synchronization tree of the system will remain unchanged in resyn- chronization, which reduces the duration of the break in the distribution of the frequency of the master node.
  • a break occurs, a single or only a few nodes have to be resynchronized instead of forcing all nodes below the point of break to change over to internal timing as in the prior art SOMS method.
  • the total time required for synchronization will thus be shortened.
  • the solution also reduces the significance of the location of the point of break for the syn ⁇ chronization time, wherefore the synchronization behaviour of the system in different situations is more predictable.
  • the synchronization tree means a hierarchical tree-like structure which is established in a network below the master node in master-slave synchronization.
  • Figure 1 illustrates the general configuration of a system employing message-based synchronization when the system is in synchronization with the clock frequency of the master source
  • Figure 2 illustrates the network of Figure 1 when the master node has failed
  • Figure 3 illustrates a network employing self- organizing master-slave synchronization in an initial state
  • Figure 4 illustrates the network of Figure 3 in a stable state
  • Figure 5 illustrates the resynchronization of the network of Figure 4 when the master node has failed
  • Figure 6 illustrates the resynchronization of the network of Figure 4 when a connection between two nodes has failed
  • Figures 7a to 7g illustrate the procedural stages of an application of the method according to the invention in a SOMS method; and Figure 8 illustrates means provided in an individual node for realizing the method according to the invention.
  • Figure 3 illustrates a system employing self- organizing master-slave synchronization (SOMS), de ⁇ scribed in US Patent 2,986,723 referred to above.
  • the system comprises five nodes (or devices) assigned SOMS addresses indicated by the reference numerals 1...5 according to their level of hierarchy.
  • the master node of the system has the smallest SOMS address.
  • the nodes interchange messages containing such SOMS addresses. In this way the nodes are able to identify each other by means of the address numbers and establish a synchronization hierarchy so that the whole network can synchronize itself with the master node.
  • a synchronization message contains three differen parts: a frame structure, signature and check sum.
  • the SOMS signature is the most important part of the SOMS message. It comprises three consecutive numbers Dl to D3:
  • Dl is the origin of the synchronization fre ⁇ quency of a node transmitting a SOMS message, i.e. the SOMS address of a node appearing as a master node to the transmitting node.
  • D2 is a distance to a node indicated by Dl. The distance is given as the number of intermediate nodes.
  • D3 is the SOMS address of a transmitting node.
  • Each node compares continuously incoming SOMS signatures with each other and selects the smallest amongst them.
  • the different parts Dl, D2 and D3 are combined into a single number by placing them in succession (D1D2D3) (for the sake of clarity, a dash will be inserted between the different parts in the text below as follows: D1-D2-D3).
  • Dl SOMS address
  • the whole network is thus synchronized with the same master node (as the master node of the whole network has the smallest SOMS address). If two or more of the incoming signals are syn ⁇ chronized with the same master node, the one arriving over the shortest path (D2) is selected.
  • the last criterion for selection is the SOMS address (D3) of the node transmitting the SOMS message, which is used for the selection if the incoming signals cannot be distinguished from each other in any other way.
  • the new SOMS signa ⁇ ture can be derived from the selected smallest SOMS signature as follows: the first part (Dl) is left intact; the second part (D2) is incremented by one, and the third part (D3) is replaced with the node's own SOMS address.
  • Each node also has its own internal SOMS signature X-O-X, where X is the SOMS address of the node. If none of the incoming SOMS messages contains a signature smaller than the internal signature, the node uses its own internal oscillator or possibly a separate synchronization input as the source of clock frequency. Of course, the outgoing SOMS message there ⁇ by employs the internal SOMS signature.
  • the nodes transmit continuously SOMS messages in all directions in order that any changed data in the SOMS signatures would be distributed as rapidly as possible and that they would know the current operat ⁇ ing condition of neighbouring nodes.
  • the SOMS signa ⁇ tures cannot be compared with each other until the incoming SOMS messages have been accepted and the SOMS signatures have been extracted from the messages.
  • the SOMS signature con ⁇ tained therein is accepted immediately for comparison if the message is faultless.
  • the SOMS message is found to be faulty, the current SOMS signature is retained until three successive faulty SOMS messages have been received. At this stage the old SOMS signature is no longer accepted for com ⁇ parison. Waiting for three successive SOMS messages aims at eliminating temporary disturbances. If no SOMS message is received from the line and there is no line failure, the current SOMS signature is rejected only after a period of time corresponding to three successive SOMS messages.
  • the SOMS signature is rejected immediately. If no appropriate SOMS signature is available for comparison due to disturbances in the incoming signal, the SOMS signature of ' the transmission line is rejected.
  • a constant-value signature where all parts (Dl, D2, D3) have their maximum value (MA -MAX-MAX) is thereby used in the comparison as the SOMS signature of this incoming transmission line.
  • each node employs its own internal synchronization source, and transmits its own internal SOMS signature X-O-X to the other nodes. This signa- ture is also compared with incoming SOMS signatures. If none of the incoming signatures is smaller than the internal signature, the node continues to use its own internal timing.
  • the SOMS network is shown in an initial state when none of the nodes (or devices) has yet processed any one of the incoming SOMS messages. In all nodes, the highest priority is assigned to the internal SOMS signature of the node as no other signatures have yet been processed.
  • the SOMS signatures are indicated beside each node to which they are transmitted, and the selected signature is framed (in the initial situation shown in Figure 3 all nodes employ their internal timing source) . Lines used in synchronization are drawn by a continuous line and standby lines are drawn by a broken line (in the initial situation shown in Figure 3, all lines are standby lines) .
  • node 1 When the nodes start to process the incoming SOMS messages, node 1 retains the use of the internal timing, nodes 2 and 4 synchronize themselves with node 1 on the basis of the signature 1-0-1, node 3 is synchronized with node 2 (2-0-2), and node 5 with node 3 (3-0-3). At the same time the nodes generate their own new SOMS signatures as described above and provide their outgoing SOMS message with the new signature.
  • the network in a stable situation is shown in Figure 3. All nodes have synchronized with the master node 1 over the shortest possible path.
  • the node selects a new synchroniza ⁇ tion direction on the basis of the second smallest SOMS signature. Prior to this, however, the node is forced to change over to internal timing, which it retains for a preset time period in order for any faulty signatures occurring in the network to be eliminated. For instance, if node 1 in the situation of Figure 4 should fail, nodes 2 and 4 would no longer receive the signature 1-0-1, with which they were synchronized. If they now accepted immediately the second smallest SOMS signature, the network would no longer be synchronized with a single master node but a synchronization loop would result.
  • node 1 fails, node 2 still receives the signatures 1-1-4 and 1-2-3, and node 4 receives the signatures 1-1-2 and 1-2-5, as nodes 3 and 5 have not yet responded to the changed situation. If the second smallest signatures were accepted immediately, node 2 would be synchronized with node 4, and node 4 with node 2. This situation is prevented by the above-mentioned forced state of internal timing, in which the nodes start to use their own internal timing source and transmit their own internal SOMS signature (X-O-X).
  • Nodes which were synchronized with the node now in the state of internal timing detect that a change has occurred in the network and that the SOMS message on which the former synchronization was based is no longer valid as it has been changed into the internal SOMS message of the neighbouring node. As a consequence, the nodes also enter into the forced state of internal timing for a preset time period.
  • the nodes 2 and 4 are immediately forced to enter into the state of internal timing when they lose the incoming SOMS signature 1-0-1.
  • the nodes 3 and 5 detect the change that has taken place in the nodes 2 and 4, they are also forced to enter into the state of internal timing.
  • node 2 receives the internal SOMS signatures (3-0-3 and 4-0-4) from the nodes 3 and 4 and retains the use of the internal timing as the SOMS signatures received from outside are not smaller than its own internal signature (2-0-2).
  • Node 4 is then synchron ⁇ ized with node 2. After having stabilized, the network is in the state shown in Figure 5, where node 2 is the new master node of the network.
  • FIGS. 7a to 7g show the procedural stages of an application of the method according to the inven ⁇ tion in a SOMS system portion comprising nodes 15, 17, 18, 19, 27 and 30.
  • Nodes in the forced state for the preset period are indicated by underlining the respective numbers, and connections used for syn ⁇ chronization are indicated by continuous lines.
  • a connection of node 15 towards the master node of the system fails.
  • node 15 is forced to enter into the state of internal timing and starts to transmit its internal synchronization signature.
  • the selected current synchronization signature received at nodes 18 and 19 thereby deteriorates, and the nodes immediately select the changed signature as their new synchronization signature and generate on the basis of it a new outgoing signature of their own ( Figure 7c).
  • Nodes 18 and 19 are forced to retain the new, changed signature for a preset time period before they are allowed to freely select the smallest (best) one amongst all incoming synchronization signatures.
  • the automatic selection of the changed signature according to the invention spreads in the synchronization tree downward without changing it.
  • the selected signature has reached nodes 27 and 30.
  • the time period of node 15 has expired and it is allowed to freely select the source of timing amongst the incoming synchronization signatures. However, as no better synchronization signature is received, node 15 voluntarily retains the internal timing.
  • the time periods of nodes 18 and 19 also expire.
  • Node 18 obtains an signature origin ⁇ ating from the master source of the network from node 17 and is synchronized with it, whereas node 19 retains the former state as it does not receive any better signature than the signature received from node 15.
  • nodes 15 and 19 have already processed the new signature from node 18, which has changed due to changes at the preceding stage. Nodes 15 and 19 are thereby synchronized with the master source of the system through node 18. The time periods of nodes 27 and 30 have expired but they need not change the source of their timing as no better synchronization signature is received.
  • the internodal connections are used for synchronization even when the node is in the forced state for the preset time period.
  • the minimum length of the above-mentioned time period is preferably equal to a time required for the neighbouring nodes of the node to be informed of the transition of the node into the standard state and to respond to the change and for the node itself to be informed by the neighbouring nodes of their response.
  • the time period can be calculated by the following formula:
  • the figure shows two connections A and B between a system node and neighbouring nodes.
  • the transmission line of each connection is connected to a signal transmission and a signal reception means 13a and 13b, respective ⁇ ly, which process the physical signal.
  • the means 13a and 13b forward the synchronization message to an associated synchronization message transmission and reception means 16a and 16b, respectively.
  • the trans- mission and reception means 16a and 16b e.g. check whether the message is faultless and forward the message to a centralized synchronization decision means 23 having a respective input connected to the output of the respective reception means 16a, 16b.
  • the signal transmission and reception means 13a and 13b also supervise the quality of the received signal and store information thereon into interface-specific fault databases 24a and 24b, respectively.
  • the synchronization message transmission and reception means 16a obtains fault data from the database 24a and the transmission and reception means 16b from the fault database 24b, respectively.
  • the signal trans ⁇ mission and reception means monitor failures/changes in the connection in a manner known per se.
  • the decision means 23 compares the messages and stores them in a memory 21, e.g. in priority order so that the selected synchronization signature always has the highest status.
  • the decision means also receives the fault data of a specific signal from an interface- specific transmission and reception block 11a or lib in the form of a synchronization message or as separate fault data.
  • the decision means judges from the supplied data that the node has to enter into the standard state for the preset time period, it selects the source of its timing as defined in the applied synchronization method for this kind of situation; applies an appropriate synchronization signature to the interface-specific synchronization message transmission and reception means 16a and 16b from a memory 22 (where it generates an outgoing signature used in each particular case); and starts a timer means 25.
  • the node informs the neighbouring nodes about the change that has occurred by trans ⁇ mitting the new signature.
  • the timer means 25 indicates that the preset time period K has expired, the decision means 23 is again allowed to select the source of timing by a normal procedure.
  • the decision means When the decision means receives the deteri ⁇ orated synchronization message from the message transmission and reception means corresponding to the selected synchronization signature, it starts the timer means 25, selects the deteriorated signature and generates on the basis of it its own new synchroniza ⁇ tion signature, which it applies from the memory 22 to all synchronization message transmission units.
  • the node after having selected the deteri ⁇ orated signature as a new signature, is again allowed to select a new signature if a better signature is received over the selected connection during the time period. Signatures received over other connections, however, cannot be accepted as new selected signatures before the time period has expired. This additional feature further speeds up the synchronization of the system.
  • the deteri ⁇ orated synchronization signature is selected either for the entire time period or at least for a part of the time period.
  • the node first compares the deteriorated synchronization signature with its own internal signature and selects the better alternative. The comparison is carried out in the decision means 23.
  • the node enters into the state of internal timing in a manner known per se, and if the deteriorated signature is better, the node selects it as the new signature as described above either for the entire time period or until a still better signature is received over the connection.

Abstract

Procédé de synchronisation hiérarchique destiné à un système de télécommunications employant la synchronisation basée sur des messages et comprenant plusieurs n÷uds qui sont reliés entre eux par des lignes de transmission (A, B), lesdits n÷uds échangeant entre eux des signaux qui contiennent des messages de synchronisation renfermant des informations relatives au caractère prioritaire du signal respectif dans la hiérarchie de synchronisation interne du système, le n÷ud adoptant un état standard prédéterminé pendant une durée fixée à l'avance lors d'une situation nouvelle telle qu'une panne, afin d'empêcher la sélection de messages de synchronisation erronés. Afin d'écourter une coupure provoquée par une panne ou tout autre changement de la répartition de la fréquence du n÷ud principal, un n÷ud sélectionne immédiatement, lorsqu'il se produit une détérioration de la signature de synchronisation sélectionnée du n÷ud, la signature altérée comme étant sa nouvelle signature de synchronisation, et par conséquent la conserve tout au plus pendant une durée prédéterminée.
PCT/FI1993/000457 1992-11-09 1993-11-08 Procede de synchronisation hierarchique WO1994011964A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
AU54221/94A AU5422194A (en) 1992-11-09 1993-11-08 Hierarchical synchronization method
GB9509026A GB2287158B (en) 1992-11-09 1993-11-08 Hierarchical synchronization method
DE4395760T DE4395760T1 (de) 1992-11-09 1993-11-08 Hierarchische Synchronisationsmethode

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI925074 1992-11-09
FI925074A FI91691C (fi) 1992-11-09 1992-11-09 Hierarkkinen synkronointimenetelmä

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WO1994011964A1 true WO1994011964A1 (fr) 1994-05-26

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AU (1) AU5422194A (fr)
DE (1) DE4395760T1 (fr)
FI (1) FI91691C (fr)
GB (1) GB2287158B (fr)
WO (1) WO1994011964A1 (fr)

Cited By (1)

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Publication number Priority date Publication date Assignee Title
WO1995024800A2 (fr) * 1994-03-01 1995-09-14 Nokia Telecommunications Oy Procede de synchronisation hierarchisee

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Publication number Priority date Publication date Assignee Title
DE19653261A1 (de) * 1996-12-20 1998-06-25 Alsthom Cge Alcatel Synchrones digitales Nachrichtenübertragungssystem, Steuerungseinrichtung, Netzelement und zentraler Taktgenerator
DE19901588A1 (de) * 1999-01-16 2000-07-20 Alcatel Sa Synchronisation eines Netzelementes in einem synchronen digitalen Nachrichtenübertragungsnetz

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Publication number Priority date Publication date Assignee Title
US4142069A (en) * 1977-06-20 1979-02-27 The United States Of America As Represented By The Secretary Of The Army Time reference distribution technique
EP0242117A2 (fr) * 1986-04-16 1987-10-21 AT&T Corp. Commande de la distribution d'impulsions d'horloge dans un système distribué de communication numérique
EP0435395A2 (fr) * 1989-12-28 1991-07-03 Philips Patentverwaltung GmbH Méthode hiérarchique de synchronisation pour les point de commutation d'un reseau de télécommunications

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US4142069A (en) * 1977-06-20 1979-02-27 The United States Of America As Represented By The Secretary Of The Army Time reference distribution technique
EP0242117A2 (fr) * 1986-04-16 1987-10-21 AT&T Corp. Commande de la distribution d'impulsions d'horloge dans un système distribué de communication numérique
EP0435395A2 (fr) * 1989-12-28 1991-07-03 Philips Patentverwaltung GmbH Méthode hiérarchique de synchronisation pour les point de commutation d'un reseau de télécommunications

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1995024800A2 (fr) * 1994-03-01 1995-09-14 Nokia Telecommunications Oy Procede de synchronisation hierarchisee
WO1995024800A3 (fr) * 1994-03-01 1995-10-26 Nokia Telecommunications Oy Procede de synchronisation hierarchisee
US5838659A (en) * 1994-03-01 1998-11-17 Nokia Telecommunications Oy Hierarchical synchronization method

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FI91691C (fi) 1994-07-25
DE4395760T1 (de) 1995-10-05
FI925074A0 (fi) 1992-11-09
GB2287158A (en) 1995-09-06
FI91691B (fi) 1994-04-15
AU5422194A (en) 1994-06-08
GB2287158B (en) 1996-09-25
GB9509026D0 (en) 1995-06-21

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