WO1998034363A1

WO1998034363A1 - Wdm traffic protection

Info

Publication number: WO1998034363A1
Application number: PCT/SE1998/000136
Authority: WO
Inventors: Bengt Johansson; Lars Egnell; Robert Batchellor
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1997-01-31
Filing date: 1998-01-30
Publication date: 1998-08-06
Also published as: AU5889498A

Abstract

A fiber-optical network using wavelength multiplexing has a ring configuration comprising a plurality of nodes (1) connected by regular communication links (3) for transferring information between neighbouring nodes. A standby link (5') is provided in parallel to each regular link (3). In the case of a failure of a regular link (3) switches (17, 27) in the nodes (1) switch the information to be transferred over the failed link to travel through all standby links except the standby link parallel to the failed regular link, so that this information will have to travel through all nodes in a direction opposite to that which it would have regularly taken. Optical amplifier assemblies (19, 29; 35) are used for the protective links (5') at both ends thereof. Control units (35) of the amplifiers use signals from light sensors (33, 39) sensing the input and output light power of the amplifiers and adapt the output power of the amplifiers stepwise in accordance with the sensed input power. An intermediate state of the amplifier output power is used for supervising the protective links and then normally light carrying no signals will propagate around the network in two closed loops, the light power therein being considerably lower than that used for regular signal transmission. In this way a simple supervision of the protective links is obtained, resulting in a simple architecture of the nodes requiring little management functions.

Description

WDM TRAFFIC PROTECTION TECHNICAL FIELD

The present invention relates to methods and devices for protecting optical links in a fiber- optical network, in particular for protecting communication between nodes that are directly connected to each other or are separated by only one or a few other nodes, and particularly to a network having protection and the configuration of nodes and links in a network having protection.

BACKGROUND OF THE INVENTION

In telecommunication optical fibres have been used for several years, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibres in their networks as efficiently as possible, in particular for communication over long distances, since such fibres obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibres a plurality of individual wavelength channels can be transmitted on the same optical fibre and thus the information transmitted over the fibre can be multiplied. Thus the need for installing more optical fibres can be postponed. Also, the telecommunication operators of course want to utilize their existing transmission equipment if possible also when changing to WDM systems or at least to utilize their existing equipment to the highest possible extent. When multiplexing channels previously transmitted on a plurality of separate fiber pairs onto a single fiber pair, also fiber pairs can possible be released in order to be used for other network enhancements.

Since more traffic will be concentrated on a single fiber pair, when using wavelength multiplexing, the ability to rapidly restore this traffic in the event of a route failure is critical. Thus, some protection of optical links is in many cases required. An optical fiber cable can like any other cable be broken by construction machines, etc. A protection must then comprise or be contained in another fiber cable that is installed at a physically or geographically different location. The simplest solution is then to duplicate each fiber link, i.e. for each node-to-node connection there are two differently located fiber connections, the regular or working one and the protection or stand-by connection, see Fig. 1. However, this is a costly solution.

STATE OF THE ART

A ring network having protection is disclosed in U.S. patent 5,442,623, Figs. 2 - 4. Each link between neighbouring nodes has a parallel or standby link. However, there are no devices provided for monitoring the state of the standby link. In the case one of the standby links is in a non-operable state, the protective function will in most cases disappear. DESCRIPTION OF THE INVENTION

It is an object of the invention to provide methods and devices for efficiently protecting optical fiber links in a fiber-optical network.

It is another object of the invention to provide devices for automatically switching traffic on an optical fiber link to protective or standby links.

It is another object of the invention to provide methods and devices for monitoring protective or standby optical fiber links in regard of their operative state.

It is another object of the invention to provide nodes in an optical fiber network having amplifiers for optical fiber links allowing a simple monitoring of protective or standby links.

The problem to be solved by the invention is thus how to protect optical fiber links in a fiber-optical network in a simple way requiring a minimum amount of management functions and little additional electrical hardware and software, i.e. how to configure the network in a suitable manner and how to design the nodes thereof, so that a minimum of electrical control devices and control lines have to used in the nodes, the protection being reliable and also subject to monitoring.

Thus, basically a 1:N protection is used, i.e. a plurality (N) of point-to-point systems share the same protection equipment what is beneficial from a fiber and amplifier saving point of view. This is achieved by using a ring-like network in which all nodes are only connected to exactly two neighbouring nodes, as in the cited U.S. patent 5,442,623. In such a ring network the information between two nodes will then be switched, in the case of a failure of the link between these two nodes, to protective links arranged between all nodes of the network except that protective link which is arranged in parallel to the failed link. Each link between two neighbouring link used for normal signal transmission between the links is called a regular or working link and is supposed to comprise two fibers for communicating signals in two opposite directions. Also, each protective link which is always arranged in parallel to a regular or working link comprises two fibers for letting light or signal propagate in the two opposite directions.

In each node of the network preamplifiers and booster amplifiers are provided for fibers carrying incoming signals or light and for fibers carrying outgoing signals respectively, both for the regular and the protective links. The protective links form in the normal state, where all regular links are operative or non-defective, a closed loop, in particular one closed loop of fibers for letting light propagate in one direction, which e.g. can be taken to be the clockwise direction, and another closed fiber loop for light propagating in Ihe opposite direction, thus in the counter-clockwise direction respectively. The amplifiers arranged for protective links are normally in a "glowing state" corresponding to a considerably lower signal output power of the amplified light than that used for signal transmission which is normally made on the regular links. Further there is basically no input signal power to these closed loops, i.e. no signals or no external light at all are/is injected therein. The output light from a protective link as sent from the booster amplifier at one end of the link is detected at the input of the preamplifier at the other end of the link. If intermediate amplifiers are arranged, they must also have such a "glowing state".

By means of independently operating control units of the amplifiers a node can autonomously decide whether the protection path is or is not carrying information signals and whether it is in order. If the protection path is utilized for protective purposes, i.e. for transmitting useful information, other nodes are forbidden to switch over to a protection state. This is accomplished by detecting the status of the protection links, using the average light power level or possibly the average light intensity level distinguishing a "working state", in which the protective links are already activated for transferring useful data for a link on failure, from a "glowing state" of the protective links having a lower light power level or lower light intensity level respectively of the transmitted light, the latter state being indicative of an operative, standby function of the protective links. A node and in particular the amplifiers therein are provided with functionality for distinguishing between these light power levels or light intensity levels respectively, a working power level and a glowing power level or a working light intensity level and a glowing light intensity level respectively. There are also other situations, for example planned maintenance, where it must be ensured that the protection path is not occupied.

Basically each node comprises two independently operating half-nodes and it further comprises one amplifier for each fiber part of the links. One side of each half-node is the working or regular one communicating information whereas the other side acts as the start and end of a standby path. Nodes that are not adjacent to a fiber break, in the case where a break exists, act as line amplifiers where the protected traffic flows through the two parts of the communication links. The protection side is activated by setting switches in the node to connect the standby link to the receiver-transmitter side provided in each node and by possibly also isolating the amplifier-pair which serve the failed section.

The integrity monitoring of the protection path comprises that a failed protection fiber must be recognised and reported. Furthermore, a node recognising a fiber failure must not switch over to the protection path if this is already utilised by other nodes for transmitting information. The nodes or more exactly the half-nodes performs this autonomously primarily by using the different power levels of transmitted light in the "working state" and in the "glowing state" as has already mentioned. Thus the links and in particular the amplifiers connected to the links work in three different states: an "off , a "glowing" and a "working" state. The amplifier output state is made to be a reaction to what is coming in to the considered amplifier , i.e. a "working" input level gives a "working" output level, a "glowing" input level gives a "glowing" output level and an input level where substantially no incoming light power can be detected, i.e. an "off input level, gives an "off output level by the fact that then the amplifier is virtually switched off. The link setups and the traffic restoration can then be made in a node autonomous way. After a protection restoration has occurred and fibers have been repaired, the network has to be manually switched back by transmitting particular forcing control signals to various control devices used in the nodes for transferring the nodes into their original state where the "working" links carry the information and light having a "glowing" power level is propagating along the protective links.

As the word "glowing" indicates, the amplifiers can work in an intermediate position between the off-state and a working state in which information is transmitted. The amplifiers are suitably optical fiber amplifiers and they can reach a "glowing" output level by means of only their internal amplification and spontaneous emission without any input signal light power provided to the amplifiers. This function is used when the protection ring is established.

Due to the automatic power shut down feature the amplifiers of both nodes surrounding a broken link will switch to the protection path. This will in particular cause the amplifiers of the respective protection links to switch from a "glowing" to a "working" state and thus the network is restored. The amplifiers in the protection ring in nodes not adjacent to a fiber failure will act as line amplifiers when they are used for the protection function, i.e. for transmitting information.

It is only allowable to make the protection path active for transmitting information in the case where the protection ring is in a "glowing" state, i.e. in the case where the protection path is accessible. A mechanism to ensure that a "former working" pre-amplifier for a protection link is not by mistake switched on while the node is in a protecting state, is also provided.

There are thus two reasons of monitoring the protection ring. Firstly, to ensure that it still is in order and secondly that it is not occupied as protection for another node-pair surrounding a defective link. The former condition corresponds to a "glowing" level and the latter to a "working" level. In the case of a fiber failure on the protection ring it will go from a "glowing" level to an off-level or "LOP" (loss of power) level telling that it is unavailable.

Normally both the working and the protecting fibers break when a cable failure occurs. The preamplifier on the broken protecting fiber then switches off and consequently, owing to the automatic power shutdown feature of the amplifiers also the following booster amplifier downstream in the next node (= normally the node towards the other direction). To suppress this shut off function until the traffic has switched into the protection booster amplifier, this booster amplifier has to have a delay function before it actually switches off in case of a detected loss-of-power-state. The node will during this delay still see the "glowing" status of the booster amplifier and thereby allow the traffic to be switched into the protection ring. The time before the booster amplifier on the protection side actually receives traffic after a single fiber break can in a practical embodiment be of the magnitude of order of 50 ms (20 ms + 5-7 ms per line amplifier).

In the case of a fiber failure on the protection path and for a delay of 1 second, it will then take approximately (N-1) seconds before all nodes are updated where N is the number of nodes of the ring. The risk that a second failure will occur during this time is minimal.

All amplifiers used in the node can have an identical construction, even if some of the functionality thereof is not used for amplifiers in some positions, and they will then have the capability to be only in one of the three operational states described above: "off , "working" and "glowing". The input range and the output level of all three modes is set for the specific position in the node in a suitable way in order to provide the desired characteristics of each state. In order to give an autonomous behaviour of all amplifiers, an input glowing level of an amplifier shall give a glowing output level. Likewise a working input level shall give a working output level. A prerequisite is of course that the amplifiers are controlled so that they in each state have constant output power levels.

The total automatic power shut-down function includes two functions. One is implemented on a node basis and the other is implemented on an amplifier basis. Firstly, in order to enable the protection algorithm to work for even single fiber failures and for eye protection, the booster amplifiers in a node shall turn off in case of LOP (loss of power = no detected input power) in the corresponding preamplifier, i.e. the amplifier serving the same link but receiving and amplifying the information incoming to the node. Secondly, an amplifier always turns itself off in case of LOP on its input. This is to ensure that amplifiers connected downstream are turned off.

All boosters on the protecting ring have the extra feature described above comprising a delay before reacting to a LOP-condition detected in its input line. This delay ensures that the other node sees a glowing state at the booster amplifier for the protection link in case of a fiber break on the working side. All booster amplifiers can be identical and equipped with this possibility. A jumper for enabling/disabling the delay can be arranged. The delay should be in the order of 1 second.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the invention and of the above and other features thereof may be gained from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

- Figs, la and lb are schematic diagrams illustrating one-to-one point link protection,

- Figs. 2a and 2b are schematic diagrams illustrating 1:4 link protection showing ordinary signal paths and signal paths in the event of a link failure respectively,

- Fig. 3 is a schematic picture showing light signal paths in a node but not showing control lines and devices,

- Fig. 4 is a block diagram illustrating the local control of a preamplifier or a booster amplifier,

- Fig. 5 is a block diagram illustrating the logical control of one half of a node,

- Fig. 6 is a schematic picture similar to that of Fig. 3 showing slightly modified, light signal paths in a node, and

- Fig. 7 is a schematic picture showing light signal paths in a node according to a an alternative embodiment.

DETAILED DESCRIPTION

In Figs, la and lb one-to-one point link protection is illustrated. Each node 1 in an optical ring network comprising four nodes is connected to the two neighbouring nodes through two separate, parallel links, a regular link 3 and a stand-by or protection link 5. The nodes 1 comprise switching facilities and have two input/output pairs 7, one pair for signals to be transmitted in and for signals received from the counter-clockwise direction and one pair for signals to be transmitted in and for signals received from the clockwise direction. The parallel links 3, 5 are installed to follow geographically different paths, so that there is always a sufficient geographical distance between the links at locations somewhat remote from the nodes 1. If a regular link 3 breaks down, it is sensed in some manner and then the parallel stand-by link is connected to transfer information in the network. This is illustrated in Fig. lb, where the two nodes 1 at the ends of the failed link 3 have been switched to transfer all signals directed to and to be received from the failed link to the protection link 5. Thus only these two nodes are affected when there is a link failure.

In Figs. 2a and 2b the same basic ring-type network configuration is shown. However, each link 3 between adjacent nodes 1 are only protected by a single parallel stand-by link 5' running directly at the side of the regular link 3, e.g. in the same fiber cable. Each link 3, 5' comprises, like the links 3, 5 of Figs, la and 2b, a pair of optical fibers, one for transmission in each direction. Normally all protection links 5' are connected to form two parallel fiber rings where some idling traffic can be maintained or in any case some lightwaves will propagate for supervising the function of the protection links 5'. When there is a failure or break-down of a normal link 3 between two nodes 1, meaning that also the protective link 5' between the considered nodes is broken and can not be used, the traffic between these nodes is redirected to instead be transmitted along all the remaining protective links 5', i.e. also through all the other nodes. This case is illustrated in Fig. 2b.

Such a system can only protect against a failure of a single fiber pair or duct included in the ring, and the overall ring circumference is limited by the maximum sustainable transmission span obtained using optical amplification but without using extra repeaters. However, to extend the maximum length of the overall ring circumference repeaters may be introduced.

A network having this capability of redirection will also require some special logical solutions for transferring the traffic to the protection links in the case of a fiber failure. In Fig. 3 a block schematic of a node 1 is illustrated. The input/output pairs 7 are here seen to each comprise eight lines 9 for signals incoming to the node to be forwarded on an optical fiber link and eight lines 11 communicating signals received by the node on an optical fiber link in the considered network, the respective signals being received from or transmitted to some other device, not shown, e.g. some standard network like SDH.

The signals incoming on the lines 9 to the node are assumed to be optical signals suitable for wavelength division multiplexing. They are composed to form one composite light signal in the coupler 13 and are therefrom transmitted on single lines to a booster amplifier 15, from which the signal is transmitted on the fiber in a regular link 3. One booster 15 is provided for signals travelling in the counter-clockwise direction and another booster 15 is provided for signals travelling in the opposite, clockwise direction, in the same way as there is one input/output port 7 for each direction. The composite light signal from a coupler 13 is also provided to one input of a two-to-one switch 17 for the protection link. The output of the switch 17 is thus connected to a booster amplifier 19 for the protection link, the output of which is connected to one fiber in the protective link 5'.

Similarly, the optical output lines 11 of an input/output pair 7 receive their signals from a wavelength demultiplexer unit 21, which in turn normally receives its signal from a preamplifier 23, connected to one fiber in the working link 3. The demultiplexer 21 may also, through a two-to-one passive coupler 25 receive its signal from one of the outputs of a one-to-two switch 27, which normally does not transmit any signal through the coupler to the demultiplexer 21. This switch has its input connected to a preamplifier 29 receiving signals from one fiber in the protective link 5'.

Finally, there is a through-connection of the protection links. Thus the other one of the outputs of the switch 27 for a fiber receiving signals from a protection link 5' is connected to the other one of the inputs of the switch for 17 for forwarding signals on a fiber in the opposite or other protection link connected to the considered node.

All amplifiers 15, 17, 23, 29 can work in three different states: an "off , a "glowing" and a "working" state. The state of an amplifier is a reaction to what is coming in, e.g. a "working" input signal gives a "working" state of the amplifier and a "working" output. In this way the different amplifiers of the node 1 can control themselves, only sensing the power of the received signal requiring a minimum of logical circuits and all logical circuits being local, i.e. they do not need to communicate with some management system more than send some signal indicating a link failure, as will be described hereinafter.

As the word "glowing" indicates, the amplifiers can work in an intermediate amplifying state between off and on (working). The amplifier can reach a "glowing" state and a "glowing" output level without any input power. This function is used for monitoring the protection ring and sensing the state thereof.

In Fig. 4 a block diagram of an optical amplifier 30 used in the node together with its local control devices is shown. On the input fiber of the amplifier a coupler 31 is connected tapping off some power to a PIN-diode 33 acting as a light power detector. The electrical signal from the diode 33 is provided to a local electronic control unit 35. Also on the output terminal of the amplifier 30 a coupler 37 is connected to deliver some light power to another PIN-diode 39, also acting as a light power detector. The electrical output signal of the downstream PIN-diode 39 is fed to the control unit 35 and in particular to a feedback control block 41 therein. The feedback block 41 operates to maintain the output power of the amplifier 30 at a constant level and is thus also connected to a control input of the amplifier 30. The amplifier can be assumed to be a fiber optic amplifier containing a piece of a fiber doped with rare-earth metals such as erbium. Light from a controllable semiconductor pump laser inside the amplifier is injected in the fiber piece to provide the amplification power. The feedback block 41 has an "on/off control input 43 to shut the

5 amplifier on or off respectively. Also there is a reference voltage input 45 for setting the output level of the amplifier 30. The reference input 45 is connected to a switch 47 that can set or maintain the output power of the amplifier 30 at a "glowing" level when connected to a voltage source 49 providing a fixed DC-level P_Qut GLOW ^anc* ^{t0 a v}°l^tag^e source 51 providing a fixed DC-level of

^{t0 se}t or maintain the output power of ιo the amplifier 30 at a "working" level.

The output signal of the input or upstream light detector 33 is in particular fed to a discriminating control block 53 inside the control unit 35. The discriminating control block 53 comprises first and second comparing circuits 55, 57 for comparing the power level signal and has output lines connected to the exterior and to the feedback block 41. The

15 actual level signal is thus compared in the first comparator 55 to a first threshold level *LOP ^ODtamed fr°^{m a} DC-voltage source 59 defining whether the input power signifies a "loss-of-power" (LOP) condition or not. If the input level is smaller than this threshold value, the output terminal of the comparator 55 is set to a low level value and otherwise a high level value. The output signal of the first comparator is provided to the "on/off input

20 43 of the feedback block 41, so that a low input level to the first comparator 55 results in a signal to the feedback block 41 signifying that the amplifier controlled by the block should be shut down and will not amplify incoming light signals any more.

The incoming power level is in the second comparator 57 compared to a second threshold value P^_j GLOW obtained from a voltage source 63 defining whether the incoming signal

25 has an intermediate value meaning that it was issued by an amplifier operating in a "glowing" state or the incoming signal has a "working" value. If the incoming power is decided to be lower than this second threshold value, there is a "glowing" state of the light power level on the fiber incoming to the amplifier and the amplifier 30 should then be set to this state or maintained in it. Therefor, the signal level of the line connected to the

30 "glowing/working" input of the feedback control block 41 is set to a low level to control the reference value switch 45 to connect to the voltage source providing the desired value P_out GLOW' ^{s0 tnat tne} f^eedback control block 41 will control the amplifier 30 to deliver an output signal having this power level P_out GLOW*

In the case where the incoming signal power is larger than the second threshold

35 P^ GLOW' ^ sig^fies ^a "working" state and then the level of the line connected to the reference value switch 45 is set to a high value by setting the switch to connect to the voltage source 51 providing the high reference value

^Tne f^eedoack control block 41 will then control the output level of the amplifier 30 to have a "working" power level of ]>WORK_' The first threshold value *^{S sma}U^er than the second threshold value P- GLOW ^{an< tne} "gl°^wmg" output level P_out GLOW ^{s sma}^^er than the "working" output level

The switch 47 is controlled by a switch control unit 65, to which the signal from the second comparator 57 is provided through a pulse shaping or differentiating circuit 67, where a positive transition of the input signal is converted to a single pulse. The switch control unit 65 also receives input signals from lines 69 and 71, "force GLOWING mode" and "force WORKING mode" respectively, on which signals in the shape of e.g. single pulses can be received for setting the switch 47 in the position for connecting the reference voltage P_out GLOW ^or ^WORK respectively to the feedback control unit 41. When a pulse is received from the pulse shaper 67, the switch 47 is always set in the position for connecting the voltage source 51 providing the higher reference voltage

From the first comparator 55, the output signal thereof is communicated externally through an output control line 73 or "LOP" thus carrying an inverted signal, a high level signifying normal operation where light having at least the minimum level required for a glowing state is received and a low level signifying a loss-of-power state, i.e. that the incoming light power is too low and that for example the respective fiber is defective and cannot be used for forwarding light signals. The signal from the second comparator 57 is available on an output control line 75, also called "GLOW/WORKING". Also there are control inputs of the control block 35 comprising an "Off" -input terminal 77 that can carry a signal forcing the control block 35 to shut down the amplifier 30. This input line is connected to one inverting input terminal of a first OR-gate 79 arranged in the signal path from the output of the first comparator 55 to the feedback control unit 41. To the other inverting input terminal of the OR-gate 79 is thus the output signal of the first comparator 79 provided. The inverting output terminal of the OR-gate 79 is then connected to the input line of the feedback control unit 41 through a second OR-gate 81 having no inverting inputs or outputs. To the other input line of the second gate 81 is a line connected carrying a signal from an input terminal 83, "Force amplif. ON". A signal on this input terminal 83 will then always set the feed control unit 41 in a state where the amplifier 30 is operative, in a "glowing" or "working" state depending primarily on the input power of the amplifier. In the line from the discriminating block 53 to the feedback control block 41 carrying the signal whether a LOP-condition has been detected a delay can introduced which is used in some amplifiers and is symbolized by a delay block 85.

The threshold levels and the reference voltages are set differently for an amplifier used for boosting signals outgoing from the node in which the considered amplifier is located and for an amplifier used for preamplifying signals arriving to the node where the amplifier is located. In a practical embodiment the threshold values and reference voltages can be as follows:

nplifier Booster Amp.

Glowing output level/P_out GLOW -5 dBm -13 dBm Input threshold for Glowing-Working state P_in GLOW -20 dBm -40 dBm LOP threshold PLOP -40 dBm -20 dBm

In Fig. 5 a block diagram is shown, illustrating the logical control of one half of a node, i.e. those parts of a node which are shown in the upper or the lower half of Fig. 3, the layout of the figure corresponding best to the upper half. These two halves operate completely independently of each other and each one comprises the working link in one direction and the protective link in the opposite direction. The amplifier control unit 35 of the preamplifier 23 for the working link 3 has its output terminal 73 carrying the signal "LOP" signifying low received power connected to the "Off -input 77 of the control unit of the associated booster amplifier 15 to switch this amplifier off immediately when there is no input power to the preamplifier. For restart purposes the LOP-output of the booster amplifier 15 control unit is connected to a Prolonged On-unit 87, in which the incoming signal is converted to a single, long period pulse shape by first passing through a pulse detecting circuit 89 sensing a positive edge of the signal. The output signal of the detecting circuit 89 is connected to the input of a single-pulse generator 93 (e.g. a monostable device) providing a pulse having a long period of e.g. 2 seconds on its output. This signal is delivered to the input terminal 83, "Force amplif. ON", for switching on the amplifier 15 during this time period.

When loss of input power is detected by the control unit 35 of a preamplifier 23 for a working link 3, the signal flow must be switched to the corresponding protective link. Therefor, the output terminal 73 carrying the signal "LOP" derived from the preamplifier 23 of a normal link is also connected to a protection control unit 99. This control unit 99 also receives the output signals "GLOWINGAVORKING" on output terminals 75 of the control units 35 of the preamplifier 29 and the booster amplifier 19 of the opposite protective link in the same node. Further it also receives the signals on the output terminals 73, LOP, of the same control units. Thus, when a signal meaning loss of power on an incoming fiber in a working link 3 is received by the protection control unit 99 and the signals of the two amplifiers of the protective link indicate that the protective link is in a "glowing" state, i.e. it is operative and ready to be used for protection services using the alternate path, the protection control unit 99 issues a signal on an output terminal thereof signalling that now the protective links will be used. The condition is thus that at the same all the signals on the terminals "LOP" and "GLOWINGAVORKING" should have low levels. The output signal of the protection control unit 99 is communicated on connection lines from the unit 99 to switch control units 101, 103 controlling the switches 17, 27 respectively of the outgoing and incoming part of the protective link 5' respectively. When these switch control units 101, 103 thus receive signals from the protection control unit 99 on their input terminals 105, 107, also denoted by "force prot. ", they will set the switches 17, 27 to their positions in which the information signals forwarded on the working link 3 from the input coupler 13 and received by the demultiplexer 21 are forwarded and received respectively on the parts of the opposite, associated protection link 5'. The switch control unit 101, 103 also have input terminals 109, 111, also denoted by "force standby". When signals are received on these input terminals, the switches 17, 27 are set to the normal state in which light have a "glowing" intensity is travelling around in all protective links, the information signals being stopped by the switches from entering the closed optical loops formed by all the combined protective links 5'.

The various output terminals and input terminals of the different control units, also those which are not shown to be connected at all in Fig. 5 such as the "force standby" terminals 109, 111 of the switches 17, 27, can be connected to a superior management system, not shown, used for overall control of the nodes and the network, e.g. for performing start-up procedures.

The operation of the network in general and of particularly the half-node of Fig. 5 will now be described in the case of a failure of the working link 3 connected to the amplifiers 15, 23, the working link being located at the left hand side of the considered node and half- node and the half-node corresponding to the top portion of Fig. 3. The optical fiber connected to the input of the preamplifier 23 will then cease to transfer any light and this is detected by the control unit 35 of the preamplifier 23 and in particular by the discrimination block 53 therein. The signal on the output terminal "LOP" of this control unit will go from a high to a low level and this is used for switching off the parallel booster amplifier 15 for the same working link on failure. This is achieved by receiving this signal on the "Off -terminal of its control unit, avoiding that unnecessary light power is injected in the corresponding optical fiber connected to the output of the booster 15, what could be harmful to for instance the eyes of the repair staff.

The low level of the "LOP "-output of the preamplifier 15 control unit is also sensed by the protection control unit 99 which then checks whether the two parts of the opposite protection link 5' of the same half-node, which thus is located at the right hand side of the considered half-node and its node, is in a glowing state. The control unit 35 of the booster amplifier 19 for this protection link has then normally already detected that there is a loss of power for the corresponding fiber connected to its input, since probably also the protection link 5' extending in parallel to the defective working link has been broken and

5 does not forward any light and thus no light will pass therefrom through the switch 17 to the booster 19. Thus the control unit 35 of the booster amplifier 19 would have already switched off the booster amplifier 19. However, only in this control unit 35 controlling the booster amplifier 19 for a protection link 5', the delay 85 inside the control unit is enabled and thus the output "LOP" of this control unit will still have a high level signalling that the ιo input fiber is operative and also the booster amplifier 19 itself has not been switched off.

This output level and also the output levels of the output terminal "GLOWAVORKING" of this booster amplifier 19 control unit and of the output terminals "LOP" and "GLOWAVORKING" of the control unit of the preamplifier 29 for the same protection link 5' at the right hand side are thus checked by the protection control unit 99 and are found to

15 all have a low level, in the case where the protection link 5' is operative and is not already in a "working" state forwarding useful light signal date owing to some earlier fiber failure. Then the protection control unit can issue a signal to the optical switch control units 101, 103 to switch from the state, which they normally adopt and in which the respective fiber parts of the protection links are connected to form two closed links, to a state where useful

20 light data signals are forwarded to the output part of the protection link 5' from the coupler 13 and are received by the demultiplexer 21 from the input part of the protection link.

Then the booster amplifier 29 of the protection link will go to a "working" state since now the input light power level has been increased correspondingly. The output power level of light travelling to the right in Fig. 5 will then be high and it is detected by the preamplifier

25 29 of the next node, which in the case of Fig. 5 is the node located immediately to the right hand side of the node to which the half-node illustrated in Fig. 5 belongs. Then the respective portion of the following protection link located at the right hand side of this next node side will detect the "working" input level and change to a working state, etc., up to and including the node located immediately at the left hand side of the considered node of

30 Fig. 5. Here the LOP-condition has been detected by the control unit of the preamplifier 23 for the working link at the opposite side, see the lower part of Fig. 3, and then here e.g. the switches 17, 27 have been changed to the protection link, see the lower part of Fig. 3.

Then the light signal having a working level will be directed to the respective wavelength demultiplexer 21 and then the whole protective path has been established in one direction.

35 The same procedure is true for the opposite light direction, but then it starts with the detection of the LOP-condition made by the control unit of the preamplifier 23 for the working link at the right hand side, see the lower part of Fig. 3.

It is understood that the control functions described with reference to Figs. 4 and 5 can also be implemented by providing suitable digital circuits having A/D and D/A converters where required. The amplifier control units 35, the control unit 99 for switching when a protection state is to be entered and the switch control units 101, 103 are thus easily transformed to digital circuitry, e.g. implemented in a single ASIC.

In Fig. 6 a block diagram is shown of a modification of the node depicted in Fig. 3. Here the switches 17, 27 are replaced by simple on/off-switches 141, 143, which are connected in the fiber lines connecting the input couplers 13 to the boost amplifiers 19 for the protective links 5' and in the fiber lines connecting the preamplifier for the protective links to the output demultiplexer 21 respectively. Immediately at the on/off switches 141, 143 passive Y-couplers 145, 147 are provided. The Y-coupler 145 at the on/off-switch 141 for signals or light leaving the node has its two inputs connected to the output of the switch 141 and to the fiber line from the output of the preamplifier 29 for the protective link 5' on the opposite side and thus to the one of the outputs of the Y-coupler 147 on the opposite side respectively. Its output is connected to the input of the booster amplifier for the protective link on the considered side. The other Y-coupler 147 at the switch 143 for signals or light incoming to the node has its only input connected to the output of the preamplifier 29 for the protective link 5'. The two outputs thereof are connected to the switch 143 and the fiber line connecting to the input of the preamplifier 19 of the opposite side through the other type of Y-coupler 145 coupled to that amplifier respectively.

Thereby, all the fibers in the protective links 5' are permanently connected through each node. When they are to be used for actual signal communication in the case where the protective function is required the switches 81, 83 are transferred from their normal open states to closed states letting signals through. Then, a larger signal power will be superposed on the "glowing" signal or light normally transferred on the protective links and the amplifiers 19, 29 will raise their amplification to provide output signals of the normal signal level. The amplifiers for the protective side of the broken link must then be switched off since otherwise, in the case where only the fibers used normally for transferring useful signals are broken and the protective fibers of the same link still work, these amplifiers will continue to amplify light, after a while up to the working level and then signals which have entered in a protective link would be transmitted around the respective protective loop and around it again repeatedly. This requires some control functions similar to those described for the embodiment of Fig. 3 with reference to Figs. 4 and 5 and also some control signals from the upper half to the lower half of a node, as seen in Fig. 6, and vice versa. For example, the control functions must as above ensure that the protective links are not used for communicating signals for a normal link which has been detected to be defective, in the case where they are already transmitting signals for another broken link.

In Fig. 7 another embodiment of the nodes is illustrated requiring only six amplifiers in a node. However, it is more complicated in another respect by the use of several switch elements. Thus, at each input/output port 7 there is a booster amplifier 151 having its input connected to the input side of the port and it amplifies signals which are transmitted from the node and enter the node at the input/output port 7. Further, a preamplifier 153 is provided amplifying received signals which leave the node at the input/output port and it has thus its output terminal connected to the output side of the port 7.

The amplifiers 151, 153 are also connected to switches 155, 157 respectively which operate in parallel. Normally, the switches 155, 157 have the positions illustrated in the figure, i.e. they connect the fibers of the normal link to the input/output port. However, the switches have also terminals connected to fiber pieces connecting to the fibers of the protective link 5' located at the opposite side of the considered input/output port. These fiber pieces are thus connected to one of the two branches of Y-couplers 159, 161 which have their other branches connected to amplifiers 163, 165 for the protective links. These amplifiers have their other terminals connected to the Y-couplers 161, 159 respectively for the protective link at the opposite side.

The nodes illustrated in Fig. 7 operate in basically the same way as the node described with reference to Figs. 3 - 6. The upper pair of amplifiers 163, 165 of a node serve only the protection path and they can have basically the same construction comprising a control unit as described with reference to Fig. 4. Their quiescent state of operation is like the embodiments described with reference to Fig. 3 and 6 "glowing", i.e. their pump lasers or fiber amplifiers are operated at such a level that a signal is discernible at the next node in the ring, a signal or at least some light intensity being obtained by the spontaneous emission of the fiber type amplifiers. By this means the integrity of the protection path is continually monitored. In the event of the protection ring being brought into operation, the power detectors associated with each amplifier, see Fig. 4, detect the increased input signal associated with traffic and brings the amplifier to an active state with a normal gain. This operation also generates an inhibit signal to the rest of the node, indicating that the protection path is in use and that in the case where a second failure would occur in the ring, the node is not to attempt any more switch-over to the protection path. Conversely, if a node has switched over, it inhibits by means of a control means, not shown, the activation of its protection path line amplifiers 163, 165 which are turned off, thus preventing recirculation of signals within the protective ring. Change-over from working links 3 to the appropriate protection path including links 5' is implemented using the pair of optical switches 155, 157 at the termination points of each link to be protected. The switches 155, 157 act in parallel and at the same time switch both inbound and outbound traffic fibers to the protective ring. This action is triggered by a loss of power of the wavelength multiplexer, see Figs. 3 and 6, item 21. Whichever end node triggers the switch-over, this action will cause the far end node of the link to switch as well, thus protecting against unidirectional transmission media failures. Both paths of a link are switched at the same time, since this is necessary for protecting phase-sensitive traffic components.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

1. A network, in particular a fiber-optical network using wavelength multiplexing, comprising a plurality of nodes connected by first communication links for transferring information between the links, the nodes and the first links being arranged in an annular configuration, each node being connected to only two neighbouring nodes through a counterclockwise first link and a clockwise first link, and further comprising second links, one second link being provided parallel to each first link,

- the nodes comprising switching means which are arranged in such a way that in the case where all first links are correct and transmit information, the switching means then having such positions that the second links between the nodes all are connected to each other to form a closed loop, and

- the switching means also being arranged in such a way that in the case of a failure of a first link between a first node and a second node the information to be transferred over this first link from the first node to the second node and/or from the second node to the first node is instead transferred through all second links except the second link parallel to the first link having a failure, so that this information will have to travel through all nodes and all these second links, characterized in that in the case where all first links are correct and transmit information light is propagating along the closed loop, the light along the closed loop having an intensity or a power different from the intensity or power respectively of light carrying information on the first links.

2. A network according to claim 1 , characterized in that the nodes comprise optical amplifiers for amplifying light signals input to the amplifiers, each amplifier having control means connected thereto for sensing the power or intensity of light input to the amplifier and adapting the power or intensity of light output from the amplifier in accordance therewith.

3. A network according to claim 2, characterized in that the control means are arranged to sense whether the power or intensity of light input to an amplifier is lower than or not lower than a first threshold level and to adapt the output power to a first value in the case where the power or intensity of light input to the amplifier is lower than the first threshold level and to a second value higher than the first value in the case where the input power is not lower than the first threshold level.

4. A network according to claim 3, characterized in that the control means are arranged to sense whether the power or intensity of light input to the amplifier is lower than or not lower than a second threshold level lower than the first threshold level and to switch off the amplifier in the case where the input light power is lower than the second threshold level and to adapt the power or intensity of light output from the amplifier to the first value" in the case where the input light power is not lower than the first threshold level.

5. A method of protecting a network, in particular a fiber-optical network using wavelength multiplexing, - the network comprising a plurality of nodes connected by first communication links for transferring information between the links, the nodes and the first links being arranged in an annular configuration, each node being connected to only two neighbouring nodes, through a counterclockwise first link and a clockwise first link, and

- the network further comprising a second link parallel to each first link, the method comprising

- that in the nodes, in the case where all first links are correct and transmit information, all the second links between the nodes are connected to each other to form a closed loop,

- that in the nodes, in the case of a failure of a first link between a first node and a second node, the information to be transferred over this first link from the first node to the second node and/or the information to be transferred over this first link from the second node to the first node is instead transferred through all second links except the second link parallel to the first link having a failure, so that this information will have to travel through all nodes and all these second links, characterized in that in the case where all first links are correct and transmit information light is made to propagate along the closed loop of second links, the light being given an intensity or a power different from the intensity or power respectively of light carrying information on the first links.

6. A method according to claim 5, characterized in that light arriving to and transmitted from a node is amplified and that at each amplifying step the degree of amplification is set in dependence of the light power or light intensity before amplification.

7. A method according to claim 6, characterized by sensing whether the power or intensity of input light before amplifying is lower than or not lower than a first threshold level and setting the power or intensity respectively of light output after amplifying to a first value in the case where the power or intensity of input light is lower than the first threshold level and to a second value higher than the first value in the case where the power or intensity of input light is not lower than the first threshold level.

8. A method according to claim 7, characterized by sensing whether the power or intensity of input light before amplifying is lower than or not lower than a second threshold level lower than the first threshold level and by switching off the amplifier in the case where the power or intensity of input light is lower than the second threshold level and setting the power or intensity respectively of light output after amplifying to the first value in the case where the power or intensity of input light is not lower than the first threshold level.

9. A node of a fiber-optical network, characterized by at least one optical amplifier for amplifying light signals input to the node, the optical amplifier having control means connected to it for controlling it and input sensing means connected to the control means for sensing the power or intensity of light input to the amplifier in order to determine whether the power or intensity of light input to the amplifier is lower than or not lower than a first threshold level and to adapt the power or intensity respectively of light output from the amplifier to a first value in the case where the sensed power or intensity respectively of light input to the amplifier is lower than the first threshold level and to a second value higher than the first value in the case where the power or intensity respectively of light input to the amplifier is not lower than the first threshold level.

10. A node according to claim 9, characterized in that the input sensing means are arranged to sense whether the power or intensity respectively of light input to the amplifier is lower than or not lower than a second threshold level lower than the first threshold level and that the control means are arranged, according to a signal provided by the input sensing means, to switch off the amplifier in the case where the sensed power or intensity respectively of light input to the amplifier is lower than the second threshold level and to adapt the power or intensity respectively of light output from the amplifier to the first value in the case where the power or intensity of light input to the amplifier respectively is not lower than the first threshold level.

11. A node according to any of claims 9 - 10, characterized by output sensing means connected to the control device for sensing the power or intensity of light output from the amplifier and providing feedback thereto for adapting the power or intensity respectively of light output from the amplifier correctly.

12. A method of supervising a standby optical link to be used for replacing a regular optical link in the case of failure thereof, characterized in

- that all the time light of a first power or intensity is transmitted through the link, the first power or intensity being considerably lower than power or intensity respectively of light used for signal transmission, and

- that all the time the power or intensity of light received over the standby link is detected and is checked not to be lower than a threshold level.

13. A standby optical link to be used for replacing a regular optical link in the case of failure thereof, characterized by - light generating means for generating all the time light having a power or intensity of a first intensity, the light generating means being connected to the standby link for transmitting this light therethrough, the first intensity being considerably lower than the power or intensity of light used for signal transmission on the regular link, and 5 - light detecting means for receiving all the time the light transmitted over the standby link and for detecting the power or intensity respectively of light transmitted over the standby link, the detecting means being arranged to check that the detected power or intensity respectively of light transmitted over the standby link is not lower than a threshold level.

14. A standby optical link according to claim 13, characterized in that the light generating ╬╣o means comprise an optical amplifier for amplifying incoming light signals and having control means connected to the amplifier for controlling the amplifier, the control means being arranged to sense the power or intensity of light input to the amplifier and controlling the power or intensity respectively of light output from the amplifier in accordance with the sensed power or intensity respectively of light input to the amplifier.

15 15. A standby optical link according to any of claim 13 - 14, characterized in that the light detecting means comprise control means connected to an optical amplifier for amplifying incoming light signals, the control means being arranged to sense the power or intensity of light input to the amplifier and to control the power or intensity respectively of light output from the amplifier in accordance with the sensed power or intensity

20 respectively of light input to the amplifier.

16. A standby optical link according to any of claims 14 - 15, characterized in that the control means are arranged to sense whether the power or intensity of light input to the amplifier is lower than or not lower than a first threshold level and to adapt the power or intensity respectively of light output from the amplifier to a first value in the case where

25 the sensed power or intensity of light input to the amplifier is lower than the first threshold level and to a second value higher than the first value in the case where the power or intensity respectively of light input to the amplifier is not lower than the first threshold level.

17. A standby optical link according to claim 16, characterized in that the control means are arranged to sense whether the power or intensity respectively of light input to the

30 amplifier is lower than or not lower than a second threshold level lower than the first threshold level and to switch off the amplifier in the case where the sensed power or intensity respectively of light input to the amplifier is lower than the second threshold level and to adapt the power or intensity respectively of light output from the amplifier to the first value in the case where the power or intensity respectively of light input to the

35 amplifier is not lower than the first threshold level.