Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-01-09
2001-02-06
Chin, Wellington (Department: 2735)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S403000, C370S503000
Reexamination Certificate
active
06185216
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an SDH (synchronous digital hierarchy) network. Communication systems using optical networks, and to a lesser extent radio networks are increasingly employing digital communication patterns. The European systems are being standardized by the CCITT on SDH whereas the American systems are known as SONET (synchronous optical networks) and operate on the ANSI standard. The SDH system uses a network-node interface and is compatible with SONET. With the network node system it is necessary to drive information around a network using a clock pulse and this is normally obtained from an external timing source. The invention is applicable to any such system.
Within such a network it is important that the clock pulse used within a node for synchronization purposes is of as high a quality as possible—i.e. that it has not deteriorated to the extent that there is uncertainty in its value. One problem where such deterioration can occur is when a closed timing loop forms such that the same timing signal is sent repeatedly around a loop and where deterioration occurs in the clock signal each time it passes through a node. It is one object of the invention to provide a means for preventing the occurence of such closed timing loops.
SUMMARY OF THE INVENTION
In Globecom '92: Orlando, US, vol. 1-2-3, 6.12.1992, Institute Of Electrical And Electronics Engineers, pages 821-828, J. A. Crossett et al: “SONET/SDH NETWORK SYNCHRONIZATION AND SYNCHRONISATION SOURCES” the clock references for SONET and SDH network synchronization and distribution of these reference signals in the synchronous optical network are addressed. These signals provide transport equipment and office synchronisation. The paper discusses synchronization quality levels found in the network. The synchronization network is presented from a SONET view point with comparisons made to the SDH network. In WO-A-94-11966 is disclosed a hierarchical synchronization method for a telecommunications system employing message-based synchronization and comprising a plurality of nodes interconnected by transmission lines. In the method the nodes interchange signals containing synchronization messages with information on the priority of the respective signal, in the form of node signatures, within the internal synchronization hierarchy of the system. In addition a transmission line between two nodes is monitored to verify its bidirectionality, and as soon as the bidirectionality of the line cannot be verified, the use of the line for synchronization is prohibited.
In one aspect the invention provides a synchronous digital hierarchy (SDH) network comprising a plurality of nodes interconnected by bidirectional links, an external clock signal being present at an input of one of said nodes which are arranged to pass the clock signal from node to node for synchronization, each node comprising means for stamping the clock signal passing through the respective node with data identifying that node as part of a means for preventing the occurrence of closed timing loops. Once ensuing nodes have knowledge of where the clock signal has been, a decision can be made as to whether a closed timing loop would occur.
In one embodiment each node comprises means for replacing any node identifying data from preceding nodes through which the signal has passed.
Such an arrangement provides an effective means, in particular for preventing the occurence of closed timing loops between adjacent nodes.
Preferably in such a case each node comprises means for reading the identity of the node from which an incoming clock signal has arrived and for transmitting, in the event that the clock signal is to be returned to that same node, a signal indicative that the clock signal is not to be used for synchronization.
Within SDH information is carried in a framework comprising 270 bytes of information. These are arranged in columns in nine rows. The first nine columns are used for creating an overhead known as the section overhead (SOH). Synchronization status indication (SSMB) is included as part of the section overhead. The byte of information referred to in a section overhead for synchronization is known as S
1
and is used to indicate the quality of the timing source that supports the traffic to which it is attached. In the embodiment described above it is preferably the S
1
byte of the SSMB that is adapted to carry the signal indicative data that the clock signal is not to be used.
REFERENCES:
patent: 5355362 (1994-10-01), Gorshe et al.
patent: 5367395 (1994-11-01), Yajima et al.
patent: 5473640 (1995-12-01), Bortolini et al.
patent: 5682408 (1997-10-01), Tanonaka
patent: WO 94/11966 (1994-05-01), None
ITU-T Recommendation G.803, Mar. 1993, Geneve, CH, pp. 1-57, XP002010580, NN: “Architectures of Transport Networks Based on the Synchronous Digital Hierarchy(SDH))”.
GLOBECOM '92; Orlando, U.S., vol. 1, 2, 3, Dec. 6, 1992, Institute of Electrical and Electronics Engineers, pp. 821-828, XP000357679, Crossett, J.A., et al., “Sonet/SDH Network Synchroization and Synchronization Sources”.
TELCOM Report, vol. 17, No. 3, May 1994, Munchen DE, pp. 115-117, XP000451993, Stummer, B.; Volejnik, W.; “Taktkonstanz Sichert Ubertragungsqualitat”.
Chin Wellington
Jones Prenell
Kirschstein et al.
Marconi Communications Limited
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