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 present invention relates generally to the synchronization of communications networks and particularly to improvement of network synchronization in a communications network employing message-based synchronization.
• node is used to denote a junction point of transmission sections in a communications network. Accordingly, a node may be formed by any apparatus or equipment such as a branching unit or a cross-connection device.
• the nodes of the system are interconnected by transmission links serving for the information transfer needs of the system.
• the same links also carry the clock frequency of the transmitting node to the receiving node.
• Each node selects as the source of its own clock frequency either the frequency from a neighbouring node, the frequency of its own clock source or a frequency brought into the node from an external source through a separate clock input.
• the master clock To force all nodes of a system to operate at the same clock frequency, in most cases the system is preferentially synchronized to a single clock source called the master clock.
• all the nodes of the system having a direct connection with the selected master clock will be synchronized with said master clock, while more remote nodes connected with said nodes of direct connection, however, not having a direct connection with the master clock, will be synchronized with said nodes situated closer to the master clock.
• the nodes more remote from the master clock will be synchronized to those nodes which are located one link closer to the master clock.
• the nodes of the system exchange synchronization messages with each other. These messages carry information which permit the individual nodes to select the optimal clock source for their clock synchronization.
• the nodes of the system are prioritized, and the system tends to synchronize itself with the clock frequency of the node possessing the highest priority level. Normally, a given priority level can be assigned to one node of the system only.
• the synchronization messages normally contain information on the clock source used by the transmitting node, the priority level of the transmitting node, and a parameter value indicating the quality of the clock signal.
• any single node can select the synchronizing source for its own clock signal to be the clock frequency of that neighbouring node whose clock frequency originates from a desired node and is of the highest quality.
• each node uses its internal clock as its clock frequency source.
• the node selects the clock frequency of the highest-priority neighbouring node as the source of its clock frequency.
• the system runs hierarchically synchronized with the clock frequency of the master source.
• Fig. 1 is shown in steady state a system MS utilizing message- based synchronization.
• the priorities assigned to the nodes are indicated by numerals marked inside the circles representing the nodes. The smaller the value of the numeral the higher the priority of the node.
• the synchronization messages sent by the individual nodes are different from each other and have a format dependent on the message- based synchronization method used in the system.
• Distribution of the clock frequency from the master clock (node 1) to the other nodes of the system is indicated with solid lines. While the internode links indicated with dashed lines are not used for system synchronization under normal conditions, they are available during state change situations.
• the basic concept of message-based synchronization is that the system operator defines the synchronizing hierarchy of the nodes by assigning each node a dedicated signature serving to indicate the level of the node in the hierarchy, and the system is allowed to synchronize itself in a self- contained manner with the defined master clock utilizing all existing internode links as required. If the chain of synchronizing links to the master clock is disrupted and a redundant chain of internode links is not available, or the master clock fails, the system will assume synchronization with a node of the next highest level. Such a reaction to a change in the system synchronization takes place via message interchange between the nodes.
• the synchronization hierarchy is reconstructed from the point of synchronization discontinuity onward (that is, hierarchically outward from the master clock node of the system).
• the end result is a hierarchy structure approximating the original situation, however, having the faulty link replaced by a functional link with the rest of the system configuration remaining almost unchanged.
• the goal of the invention is achieved by the solution specified in the appended independent claims.
• the invention is based on the concept of permitting the system operator to prioritize the different parallel links according to an operator- defined priority categorization. In a normal situation having all the parallel links available, the synchronization signature of the signal received over the link of the highest priority level is selected. The other parallel links are used only when the link of the highest priority level is faulty and the employed synchronization method still prefers to use a signature transmitted over the same internode path.
• the link selections performed by the synchronization method can be affected in the case of multiple parallel links existing between two nodes without compromising the possibility of utilizing any of the parallel links in special situations.
• the synchronization method can be controlled to use a given preferred link always under normal conditions, yet permitting the use of functioning parallel links for synchronization during a fault situation.
• the use of, e.g., an unstable link for synchronization can be inhibited when other parallel links of higher reliability are available.
• the prioritization of parallel links may be utilized in an easy and controlled manner for changing the preferred synchronization link during network reconfiguration, for instance.
• Figure 1 shows a communications system using a message-based synchronization when the system is synchronized with the clock frequency of the master source
• Figure 2 shows a network using a self-organizing master-slave (SOMS) synchronization scheme in its initial state;
• SOMS self-organizing master-slave
• Figure 3 shows the network of Fig. 2 in steady state
• Figure 4 shows a flow diagram of the comparison process according to the invention of synchronization signatures
• Figure 5a shows an apparatus suited for implementing the method according to the invention in single node of the network
• Figure 5b shows an alternative embodiment of node equipment
• Figure 6a shows an example of the structure of the synchronization message
• Figure 6b shows an exemplifying embodiment of the transfer of the synchronization signature and the reserve path information in the syn- chronization message illustrated in Fig. 6a.
• FIG. 2 therein is illustrated a system utilizing Self- Organizing Master-Slave (SOMS) synchronization (which is a well-known message-based synchronization technique), said system in the illustrated case comprising five nodes (or apparatuses) indicated by reference numerals 1-5 according to their assigned level in the synchronization hierarchy.
• SOMS Self-Organizing Master-Slave
• the master node of the network has the smallest-value SOMS address.
• the nodes interchange messages containing said SOMS ad- dresses.
• the nodes can identify each other by virtue of these address numbers and establish a synchronization hierarchy permitting the entire network to be synchronized with the master clock source node.
• the synchronization messages interchanged continuously in the network are dependent on the type of the message- based synchronization method used. Moreover, each transmitting node sends a message with an individual content.
• the synchronization message is comprised of three distinct parts: a frame structure, a signature and a checksum.
• the SOMS signature is the most important part of the SOMS message. The signature is formed by three consecutive numbers D1-D3:
• D1 indicates the origin of the synchronizing clock frequency used by the transmitting node, that is, the SOMS address of the node acting as the master clock source node for the transmitting node.
• D2 is a connection quality parameter, which typically is given a value proportional to the distance of the receiving node to the node indicated by D1. The distance is expressed as the number of nodes between the source node and the receiving node.
• D3 is the SOMS address of the transmitting node. Each node (or device) performs continuous comparison between the incoming SOMS signatures and selects the one with the smallest value. While the fields D1 , D2 and D3 are directly combined in the signature into a single number formed by the direct combination (D1 D2D3) of the fields (in the text, however, a hyphen is used to separate the different parts of the signature, e.g., D1-D2-D3).
• the primary criterion in the selection of the smallest-value address will be based on the SOMS address (D1) of the node appearing to the preceding nodes as the master clock source, which means that any node aims to synchronize itself with a signal whose clock frequency source can be traced to a node of the smallest possible address. Then, the entire network in steady state will run synchronized with the same master node (because the master node of the entire network has the smallest-value SOMS address).
• the receiving node will select the master clock frequency from the transmitting node via which the path to the master node is shortest (i.e., the value of D2 is smallest).
• the last selection criterion is based on the SOMS address (D3) of the SOMS-message transmitting node, whereby the smallest value of this address is chosen if the preceding steps of address selection have not been able to rank the incoming signals.
• the new value of the SOMS signature can be derived from the selected smallest-value SOMS signature as follows: the first field (D1) is left unchanged, the value of the second field (D2) is incremented by one and the third field (D3) is replaced with the SOMS address of the node itself.
• each node has an internal SOMS signature of the format X-O-X, where X is the SOMS address of the node itself. If none of the SOMS messages in the incoming signals has a value smaller than in the internal SOMS signature of the node, the node will select the internal oscilla- tor of the node, or possibly a synchronizing signal present at the external clock signal input of the node, for synchronizing the clock of the node. Obviously, the internal SOMS signature of the node will then be used in the outgoing SOMS message.
• the nodes send the SOMS messages continuously in each direction in order to assure maximally rapid emission of altered synchronization information in the SOMS signatures and to keep neighbouring nodes continuously informed of each others' operating status.
• the incoming SOMS messages must be accepted and the SOMS signatures must be separated therefrom.
• the SOMS signature of the message is accepted immediately for comparison if the message is faultless.
• the incoming signal from the transmission link has an acceptable SOMS signature and the received messages carrying the same, unchanged signature are faultless, the situation remains unchanged. If the received SOMS message is found corrupted, the current SOMS signature is retained valid until three successive faulty SOMS messages have been received. Then, the SOMS signature is excluded from the comparison process. This kind of waiting for three consecutive SOMS messages serves to eliminate temporary disturbances.
• the comparison process activates a delay corresponding to three consecutive SOMS messages before the currently selected SOMS signature is invalidated. If the link fails entirely, the SOMS signature is discarded immediately. Equally, when interference imposed on the incoming signal makes it impossible to extract an SOMS signature of sufficient quality for comparisons, the SOMS signature of that link is discarded. Instead, the comparison process is carried out using the default SOMS signature for that incoming link having all the fields (D1 , D2 and D3) set to their maximum values (MAX-MAX-MAX).
• each node uses its internal source of synchronization frequency, whereby the node sends the other nodes its internal SOMS signature of the format X-O-X. This signature is also compared with the other incoming SOMS signatures. If none of the incoming signatures has a smaller value than that of the node's internal signature, the node will continue using its internal synchronization source.
• the SOMS-synchronized network shown therein is in its initial state when no node (or device) has yet had time to process the incoming SOMS messages. All the nodes give the highest priority to the internal SOMS signature of the node, because no other signatures have been processed so far.
• the incoming SOMS signatures of each node are marked beside the node, and the selected signature is marked inside the outline frame (herein it must be noted that in the initial state shown in Fig. 2, all the nodes use their internal sources of synchronization).
• Links used for synchronization are drawn with a solid line, and the standby links are indicated by dashed lines (whereby it must be noted that in the initial state illustrated in Fig. 2, all the lines represent standby links).
• node 1 After the nodes have had some time to process the incoming SOMS messages, node 1 stays synchronized with its internal clock source, nodes 2 and 4 will synchronize with node 1 on the basis of its signature 1-0-1 , node 3 will synchronize with node 2 (signature 2-0-2) and node 5 with node 3 (signature 3-0-3). By the same token, the nodes will rewrite their own new SOMS signatures in the above-described fashion and attach the signa- ture to the outgoing SOMS message. After the network has assumed steady state, its configuration will be as shown in Fig. 3. Herein, all the nodes are synchronized with the master node 1 via the shortest possible path.
• a group of parallel links connecting a node to a neighbouring node are provided in the node with parallel signatures (that is, parallel priorities) which are employed in the comparison process of synchronization signatures for ranking the mutual quality order of synchronization signatures received over the parallel links.
• Fig. 4 is shown a flow diagram illustrating the comparison of synchronization signatures occurring in each node of a network using a message-based SOMS synchronization method.
• a "new" synchronization signature received from one interface is compared with a signature received from some other interface called the "old" synchronization signature in the figure.
• step 41 the values of the signature parameters D1 are compared. If the value of parameter D1 in the new signature is better (having a smaller value) than in the old signature, a direct decision is made that the new signature is of higher quality. Correspondingly, if the parameter D1 of the old signature is better, a direct decision is made in favour of the old signature.
• the process performs comparison of parameters D2 in step 42, where the signature of a better value in parameter D2 is selected. In the case that the values of parameters D2 are equal, the process performs comparison of parameters D3 in step 43.
• This comparison step is carried out in the same manner as above, whereby the signature having the better value of compared parameters is directly selected. If even the comparison of parameters D3 fails to find a quality difference between the signatures, the process performs in step 44 the comparison of the parallel priorities. Here, the signature with the higher parallel priority is selected. Thus, the comparison of parallel priorities is carried out only in the case that the "SOMS portions" (D1- D2-D3) of the signatures are equal. This is a result therefrom that the both signatures are passed from the same node over parallel links.
• the embodiment according to the invention offers the improvement over the synchronization signature comparison of a conventional SOMS method that the step 44 performing the comparison of parallel priorities is added to the comparison process. Figs.
• the node may be comprised, e.g., of a plurality of parallel interface units IU1 , IU2, ..., IUN, each communicating with at least one neighbouring node, and of a control unit CU, common to all the interface units, whereby the control unit performs the decision-making related to node synchronization.
• the control unit and the individual interface units can communicate with each other via, e.g., an internal bus CBUS of the node equipment.
• the figures show the system node communicating with the neighbouring nodes over two incoming links A., and A 2 , each of these terminated at its own interface unit.
• the connections are formed by 2 Mbit/s PCM digital signals compatible with ITU CCITT Recommendations G.703 and G.704 or SDH signals compatible with ITU CCITT Recommendations G.708 and G.709.
• Each interface unit IU may have one or more interfaces through which the node is respectively connected to one or more neighbouring nodes.
• a node can be characterized as comprising N interface units with M interfaces in total (M 3 N).
• M 3 N M interfaces in total
• Figs. 5a and 5b the reference symbols specific to an interface or interface unit are indicated by a subindex, while parts common to all interface units are without a subindex.
• a signal transmit/receive block 13 From block 13 grind the synchronization message extracted therein is further passed to a synchronization message transmit/receive block 16, directly connected thereto.
• the synchronization message transmit/receive blocks 16, check the integrity of the synchronization message and pass the validated message further to a centralized decision-making block 20 of the node via the common bus CBUS.
• the signal transmit/receive blocks also monitor the quality of the receive signal and store the results in interface-specific fault databases 14,.
• each synchronization message transmit/receive block can retrieve fault data from its dedicated fault database. Conventional methods are applied in the signal transmit/receive blocks for monitoring faults or changes in a signal passed over a transmission link.
• the decision-making block 20 of the control unit CU stores the synchronization signatures received from the interfaces in memory area 21 , performs comparison of the stored synchronization signatures, and on the basis of the comparison, selects the signature with the highest priority as the synchronization source for its own clock.
• the decision-making block forms in memory area 22 a priority list in which the available synchronization sources are sorted is descending order according to their priorities resolved from the contents of their synchronization signa- tures so that highest in the list is placed the clock frequency source which is currently selected for the synchronization of the node and whose signature acts as the template on which the outgoing signature of the node is formed.
• the decision-making block From the interface units, the decision-making block also obtains fault information related to the incoming signals of the selected synchronization source, either as a synchronization message or as separate fault data.
• the decision-making block also maintains the currently valid value of outgoing synchronization signature in memory area 24, wherefrom the signature is distributed to the interface-specific synchronization message transmit/receive blocks 16,.
• the parallel priorities can be defined in the node in two different ways depending on the storage location of the priority information.
• Fig. 5a is shown the first alternative arrangement in which priority definitions made by the system operator for the parallel links are stored separately for each interface (memory areas 17,) communicating with the same node, whereby each memory is adapted in conjunction with its respective synchronization message transmit/receive block.
• the synchronization message receive block receives a message over a link of the network and finds therein a changed synchronization signature, it adds to the end of the received message information on the parallel priority of that link prior to forwarding the signature to the decision-making block of the node. This added priority information is then utilized in the manner shown in Fig. 4 in the signature comparison process carried out by the decision-making block.
• Fig. 5b is shown another alternative arrangement in which the operator-entered priority definitions of parallel links are stored in a central- ized manner in one location (memory area 23 of the decision-making block) for common use in the node.
• the decision-making block receives a new synchronization signature from some parallel link, it can identify the priority of the signature-passing link and use it in the comparison process illustrated in Fig. 4. (It must be noted herein that the comparison step of parallel signatures will not be reached if the signatures to be compared are not passed over parallel links.)
• the signal transmitted from another node of the communications system to the signal-processing blocks of the interface units IU is, e.g., a 2048 kbit/s signal conforming with ITU CCITT Recommendations G.703/G.704, the frame of the signal comprising 32 time slots (TS0-TS31) and each multiframe being formed by 16 frames.
• the synchronization message can be transferred so that, e.g., the synchronization message reserves two bits in some time slot of the frame structure, advantageously from the bits of time slot TSO (it must be noted herein that while the frame alignment signal occupies the bits of time slot TSO in every other frame, the remaining every other frames have bits 4-8 reserved for national use, whereby they can be utilized for transferring the synchronization message). If the synchronization message transfer is implemented using the bits of time slot TSO, maximally three bits will still remain for other use such as the service channel. Obviously, the bits needed for the synchronization message may also be reserved from some other time slot, but this arrangement bears the penalty of stealing the required transmission capacity from the capacity reserved for the payload.
• the message is sent in the selected channel in a "piecewise" manner (2 bits in each frame).
• the generalized structure of the synchronization message can be such as, e.g., shown in Fig. 6a comprising eight consecutive bytes.
• the actual message begins from the first zero following the string of eight consecutive ones (the messages are sent successively without delay).
• bit 8) the most significant bit of each byte is zero in order to prevent the message byte from ever comprising eight ones, which would cause confusion of the message byte with the start byte.
• bit 2--7 are reserved for header infor- mation and the last bit (x) for user data.
• the next five bytes carry user data in bits 1-7 with bit 8 being zero.
• Bits 1-7 of the last byte contain the check sum of the message.
• the SOMS signature fields D1-D2-D3 can be transferred for example in the manner shown in Fig. 6b.
• Field D3 is transferred in, e.g., bytes 2-4, field D2 in bytes 4 and 5, and field D1 in bytes 6 and 7.
• the length of the transmit buffer in the node is made equal to the length of the message (8 bytes), thus permitting under interference-free operation the receiving node to find the start of the mes- sage always at the same point of the buffer, whereby there is no need to initiate a buffer content scan for finding the start point of each message separately.
• the communications system employing the method according to the invention is formed by, e.g., an SDH network in which the signals present at the input ports of the node conform to ITU CCITT Recommendations G.707, G.708 and G.709
• the method according to the invention can be implemented in a plurality of alternative ways.
• the parallel links may be prioritized individually each to a separate level, or alternatively, the links can be divided into a given number of priority categories comprising, e.g., only two priority categories (defining primary links and secondary links).
• the different parametrizing alternatives increase the system operator options with regard to the network maintenance.
• the node can automatically activate its use, or alternatively, the node can be arranged to wait for a command from the operator to activate the use of the link.
• the parallel priority of a link can be utilized in the comparison process as described in the example of Fig. 4, or alternatively, a list can be compiled in which each of the different synchronization signatures appears only once. After the best of the these signatures is selected, the interfaces receiving said signature are searched for, the parallel priorities of said interfaces are checked, and on the basis thereof, the link most suitable for use as the synchronizing source is selected.
• This implementation requires that the synchronization signatures are unique and node-specific (meaning that a given signature can be transmitted only by one node at a time). If this is not the case, the synchronization signature must be complemented with information indicating the identity of the transmitting node (in the SOMS method, the parameter D3).
• Priority categorization may also be based on other criteria such as the type of the connection (e.g., optical fiber link vs. radio link).

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Data Exchanges In Wide-Area Networks (AREA)
• Synchronisation In Digital Transmission Systems (AREA)
• Computer And Data Communications (AREA)

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Citations (7)

• 1996
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• 1997
  • 1997-09-26 AU AU44613/97A patent/AU4461397A/en not_active Abandoned
  • 1997-09-26 CN CNB971984190A patent/CN1144401C/zh not_active Expired - Fee Related
  • 1997-09-26 EP EP97942971A patent/EP0932950A1/en not_active Withdrawn
  • 1997-09-26 WO PCT/FI1997/000584 patent/WO1998015078A1/en not_active Application Discontinuation

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