Optical add drop multiplex (oadm)

(54) (56) (57)

PATENT HOLDER Telefonaktiebolaget LH Ericsson, 126 25 Stockholm SE

INVENTOR'S OFFICE NAME

CALLED PUBLICATIONS:

Erland Almström, Solna SE

Dr. Ludwig Brann Patentbyrå AB

Node Architecture for Application of Optical Optimization (OADM)

US 5 003 531, JP 6 252 890

SUMMARY: The present invention discloses a method for configuring sub nodes, or configuring a system of sub nodes, in an opton network ring against both node and fiber interruptions using OADM, optomultiplexer for adding or removing nodes. The network includes a working ring and a dormant ring, and each sub node comprises selective opto filter means, 2x2 opto-coupler means and opto-amplifier means. The method and the system further comprise the steps of monitoring the inputs and outputs of each sub-node in the working and the dormant ring by means of monitoring means, which generate alarm signals when detecting signal loss at a sub-node. In response to the alarm signal, the state of the sub-node that caused the alarm signal is transmitted from a first state to one of a number of possible states as a function of the generated alarm signal, and thus selects the sub-mode switch configuration in accordance with the new state to eliminate the error detected.

The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

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TECHNICAL FIELD

The present invention relates to a method or system for a self-healing node architecture in a fiber network ring, and more precisely an optomultiplexer (OADM) for adding or removing nodes.

BACKGROUND OF THE ART

A fiber network ring is a collection of nodes that form a closed loop, where each node is connected via a bi-directional communication facility. Multiplexers used in the ring architecture SDH / SONET are adding or removing multiplexers (ADM), which add and remove local channels and pass through transit channels. A self-healing ring is a network ring that provides redundant bandwidth so that interrupted services can be automatically restored following a network interruption.

Current technology, as disclosed in US-A-5,185,736, assigned to Tyrrel et al., Can only provide protection against fiber breakage, but not node breakage. In US-A-4,704,713, Haller et al. a procedure for handling node interruptions is shown, but no fiber interruptions. Furthermore, this solution is not transparent for maintenance, bit rate and code format, depending on the electro-optic conversion in every other node.

Wavelength division multiplexing has so far been focused on packet-switched networks as shown in US-A-4,979,879 assigned to Habbab et al., US-A-4,797,879 to Eda, US-A-5,208,692 assigned to McMahon. All of these solutions are for local area network type 'Local Area Network' (LAN), and are not competitors to the SDH / SONET system.

A ring for SDH / SONET is expensive to upgrade. If changes are made to a sub node, e.g. to increase the bit rate, changes must also be made to all the other sub nodes around the ring. However, with the introduction of a multi-wavelength network layer, flexibility can be extended even further. New transmission formats can be introduced at different wavelengths and in the same fiber network, for example, in a physical ring.

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When the total traffic flow is concentrated in the same fiber, the requirement for protection of the ring increases. A desirable protection feature is a simple, fast and efficient handling whenever an error occurs.

SUMMARY OF THE INVENTION

In accordance with a first object of the present invention, there is shown a method of configuring sub nodes, or configuring a system of sub nodes, against both node and fiber interruptions in an opton network ring, which network comprises a working ring and a dormant ring, and each sub node includes monitoring points, selective optofilter means, 2x2 opto-coupling means and optical amplifier means, and further comprises the steps of monitoring the inputs and outputs of each sub-node of the working and dormant rings by means of a monitoring device which monitors the monitoring points, generating by means of the monitoring device means an alarm signal when detecting signal loss at a sub-node, such as a gag. the alarm signal, the state of the sub node that causes the alarm signal from a first state to one of a number of possible states, as a function of the generated alarm signal, as well as selecting a switching configuration for the sub node in accordance with the new state.

Further objects and steps according to the method and system according to the present invention are set out in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, can best be understood by referring to the following detailed description made with the accompanying drawings in which:

Fig. 1 is a simple block diagram illustration of an optical self-healing ring according to the invention;

Fig. 2 is a block diagram of em optomultiplexes for adding or removing nodes in the present invention;

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Fig. 3 is a table representation of the emergency state, switching configuration and monitoring signals corresponding to OADM of Fig. 2,

Fig. 4 demonstrates the sequence of events in a symbolic manner when folding two nodes designated Node 1 and Node 2,

Fig. 5 similarly demonstrates the schematic of line switching for three nodes designated Node 2, Node 3 and Node 4,

Fig. 6 is a flow chart demonstrating folding of a self-healing WDM ring, as well

Fig. 7 is a flow chart demonstrating line switching for a self-healing WDM ring.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Fig. 1 demonstrates a self-healing ring comprising two sets of optic fibers 10 and 11 which connect a plurality of sub-nodes 21-27 to a ring structure, which in turn is coupled to a main node 20 (OXC) and to a DXC / HUB 16. But reference to Fig. 1 each sub-mode is referred to as a number of n optomultiplexers for adding or removing nodes (OADMs). An OADM comprises at least two monitoring points 31 and 32, two selective opto filters 33 and 34, a 2x2 opto coupler 35, and two opto amplifiers 37 and 38, as illustrated in Figure 2. Two passive optocouplers can be used to provide access to both fibers if only one transmitter and a receiver are used, e.g. 'Line TeRMinal'. Furthermore, in accordance with the state of the art, each sub-mode comprises its own processing facility (not shown) which handles the exchange of signals to and from the network.

Each node 21-27 is capable of generating alarm signals I_x and in₂ which serves as a basis for switching decisions necessarily made after the breakdown of a filter or node. A portion of the signal is output at two monitoring points 31, 32 near each node. Both the working ring 10 and the resting ring 11 are monitored by at least one monitoring point as demonstrated in FIG.

2.

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The alarm signals I_T and in₂ derived from surveillance agencies and M₂representing simple detectors, which assess whether there is signal on the fiber or not, ie whether or not the particular fiber is unbroken. When detecting a signal loss on the working ring 10 it generates and sends an alarm signal Ij. Similarly, M₂ an alarm signal I₂ when loss of signal occurs on the dormant ring 11.

The nodes appear in different states, characterized by the way the opto switches are configured and the way in which the opto filter is activated, for example, which of the inputs and outputs have been connected. The four basic node states are:

The entire system is intact and traffic flows on the working ring.

Line switching and traffic are moved to the dormant ring.

Folding after the node. A fiber outage has occurred somewhere after the node (on the fiber length between this node and the next). Outgoing signals may be sent back, i.e., on the dormant ring.

Fold in front of the node. A fiber break has occurred somewhere between the node and the previous one. Outgoing signals are forwarded on the working ring.

Under normal conditions (unbroken fiber and no malfunction of the nodes), the nodes transmit on the working ring 10. Still, the monitoring system requires a signal on the fiber to tell whether it is broken or not. Therefore, for example, we can use a distributive signal or the amplified spontaneous emission from the optical amplifiers 38 in a further manner by transmitting this on the dormant ring 11 for monitoring purposes.

Fig. 2 shows a more detailed block diagram of a sub-mode with opto filter 33, 34, an overall block as well as a 2x2 opto-switch block 35 with two inputs and two outputs. The opto-switch block will be controlled by the various available nodes to the 514 658 stations. Furthermore, a table in Fig. 3 presents a list of the possible node states S1-S4 in combination with their corresponding opto-coupler configurations and monitoring signals.

In accordance with the present invention, it is possible to choose either of two measures in the case of a fiber break such as

(a) folding of the ring; or

b) 1inj switching.

(a) Folding

Fig. 1 shows a block diagram of a self-healing ring. A fiber break occurring on the working ring 10 between sub node 21 (OADMJ and sub node 22 (0ADM₂) gives rise to the following events:

M_x at node 22 (0ADM₂) detects a signal loss (no signal on the working ring 10) and transmits the alarm signal I_x to the switch which folds the ring forward, i.e., the node switches to state S4. With node 22 in state S4, no signal is transmitted on the dormant ring at node 21, referring to the table in Fig. 3. This causes M₂ at node 21 to detect a signal loss whereby it sends the alarm I₂ to a processor card in the node

21st As a result, the node 21 drops behind, i.e., switches to state S3. The other nodes detect no difference. Fig. 4 shows the sequence of events in a symbolic way.

If the fiber break had occurred instead of the dormant ring

II, the same events would have happened, but in the opposite order. Also note that for m nodes in the opton network ring, the sub node OADMn will be equal to OADM₀.

In the case of a node break, the folding of the ring is accomplished in exactly the same way as for a fiber break. In either case, observe Mj or M₂ simply the loss of signal, the reason why it is lost does not matter.

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b) Lini switching

If line switching is the chosen alternative for a fiber break between the sub-modes 21 and 22 of Figure 2 on the working ring 10, an alarm signal I_x to node 22. Node 22 responds by activating optofilter 34 on the dormant ring 11, i.e., switching the sub node to state S2. Node 22 in state S2 gives I_xto the next node 23 which also switches to a state S2. Similarly, this node in state S2 gives I_x to the next node, finally this node also switches to state S2 and so on. A schematic description is shown in Fig. 5.

A fiber break that occurs at another point in the ring will cause the same events in the same order. A fiber break for the dormant ring 11 would be detected by a node and then reported to the wiring system. No further action needs to be taken in this case.

A node function not discussed is the bypass function. This becomes quite important in case of node breakdown when multiple nodes use the same wavelength. If one of these goes down, the others will still be able to communicate. The opto signal should then simply be bypassed by the node. This is easily achieved in the OADM by setting the opto filters in the 'pass through everything' state, see Fig. 2. The normal value (when no voltage is applied) should be 'pass through everything' for filters 33, 34 and node state S1 of the switch, see table in Fig. 3.

In addition to the benefits associated with interruptions, the OADM structure in accordance with the present invention offers a greater simplicity for adding and removing nodes in an existing tone network.

An opt omul iplexing node for adding or removing nodes in an opton network can, of course, be constructed in a myriad of ways using different components than that indicated here in an illustrative embodiment without departing from the spirit, purpose and scope of the present invention.

514 658 method and system defined by the appended claims.

FLOW CHART

Fig. 6 shows a flow diagram demonstrating folding of a self-healing WDM ring. Initially, all nodes are set to state SI and will remain in this state until an interrupt occurs. If an interrupt occurs at node n, the monitoring detects M₂ a signal loss (no signal on the working ring 10) and sends an alarm signal Ι₂ to the node switch 35. The switch folds the node forward, that is, switches the node to state S4.

With the node in state S4, no signal is transmitted on the dormant ring 11 at node n-1. This causes an alarm signal I₂is activated at node n-1, indicating a signal loss, this alarm signal I₂ will be transferred to a processor card in node n-1. As a result, the node n-1 moves behind, i.e., switches to state S3. This is indicated by the part to the right in Fig. 6. The other nodes detect no difference.

If the fiber break had occurred instead of the dormant ring

II, the same events would occur, but in the opposite order, as indicated in the part to the left in Fig. 6. In the case of a node break, the folding of the ring is achieved in exactly the same way as for a fiber break. In either case, observe or M₂ simply the signal loss, the reason why it is lost does not matter.

If two errors occur in the ring on the same fiber, as noted by the monitoring signals marked with an asterisk (*), then at least the node concerned n will be disconnected, but the rest of the network will continue its function. Each event is conveniently reported to a management system.

Finally, Fig. 7 shows a flow chart demonstrating line switching for a self-healing WDM ring. Originally is

514 658 all nodes set to state SI and will remain there until an interrupt occurs.

If a break on the working ring 10 occurs, for this I_x to node n. Node n responds by activating the filter on the dormant ring 11, i.e., switching the node to state S2. The node in state S2 forwards Ι_χ to the next node n + 1, which also switches to state S2. Similarly, this node forwards in state S2 I_x to the next node and so on. As a result, all the nodes in the network will then use the dormant ring 11 instead of the working ring 10, the necessary folding will use the working ring 10 instead of the dormant ring 11. If both fibers of the ring detect errors (monitoring 1 and 2) indicated by the star (*) in monitoring 2 the ring will be broken.

A fiber break at the dormant ring 11 would be reported to the wiring system by the detecting node. No further action needs to be taken in this case.

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