An add and drop node for an optical wdm network having traffic only between adjacent nodes

(54)

PATENT INVENTOR INVENTOR'S OFFICE NAME

Telefonaktiebolaget LM Ericsson, 126 25 Stockholm SE Lars Egnell, Saltsjöbaden SE

Bergenstråhle & Lindvall AB

ADD / Drpo node for an optical WDM network that has traffic only between adjacent nodes (56) (57)

CALLED PUBLICATIONS: - - SUMMARY:

A WDM-type optical fiber network comprises two fibers (7e, 7w) which transmit light signals propagating in opposite directions and arranged in annular form. In the ring, a link between two neighboring nodes is always inactive and constitutes a standbv link, soot is used in the case of errors in another link, in which case the previously inactive link is made active. An add / drop node (1), used in the network for only traffic between neighboring nodes, has band-blocking filters (31a). 31w) connected in a fiber (7e, 7w) between a drain coupler (17e \ 17w ') and an auxiliary coupler (23e'_t 23W '). A tapping switch takes out equal parts of light power via receivers (33e, 33w) and bandpass filters (35e, 35w) to be received in receivers (Ile, 11w). The additional switches add new wavelength channels, which are generated in transmitters (13e, 13w) in the node via multiplexers (37e, 37w) and switches (39e, 39w). The switches (33e, 33w; 39e, 39w) are used to receive and transmit in the wavelength channels in the correct directions depending on the position of the inactive link. Thus, the position of two of the switches must be changed when the inactive link is one of the two links which are directly connected to the node. Such a node has a minimum of component connected in-line and especially a minimum number of filter components connected in-line.

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

520 876

SUMMARY

A WDM-type optical fiber network comprises two fibers (7e, 7w) which transmit light signals propagating in opposite directions and arranged in annular form. In the ring, a link between two adjacent nodes is always inactive and constitutes a standby link which is used in the event of errors in another link, in which case the previously inactive link is made active. An add / drop node (1) used in the network for only traffic between neighboring nodes has band blocking filters (31a, 31w) connected in a fiber (7e, 7w) between a drain coupler (17e ', 17w') and an auxiliary switch (23e ', 23w'). A drain switch takes out equal parts o of light output via receiver (33e, 33w) and bandpass filter (35e, 35w) to be received in receivers (Ile, 11w). The added switches add new wavelength channels, which are generated in transmitters (13e, 13w) in the node via multiplexers (37e, 37w) and switches (39e, 39w). The switches (33e, 33w; 39e, 39w) are used to receive and transmit in the wavelength channels in the correct directions depending on the position of the inactive link. Thus, the position of two of the switches must be changed when the inactive link is one of the two links directly connected to the node. Such a node has a minimum of component connected in-line and especially a minimum number of filter components connected in-line.

520 876 i

TECHNICAL FIELD

The invention relates to an add / drop node for an optical fiber network using wavelength multiplexing (WDM).

BACKGROUND OF THE INVENTION

Multi-channel optical systems, which use wavelength multiplexing, are used both in new networks and to increase the transmission capacity of existing optical fiber networks. Thus, information channels, which must previously be transmitted on a plurality of separate fiber pairs, are transmitted on a single fiber pair in WDM networks. Using optical wavelength multiplexed channels means that a plurality of serial information signals, i.e., a plurality of serial binary signals, are transmitted on the same optical fiber by modulating such a serial signal on a light signal, also referred to as a carrier having a particular wavelength, and then combining the modulated light signals in an optical coupler or optical multiplexer into a composite light signal on the considered optical fiber. The primarily modulated signal on a monochromatic light signal or carrier together with the carrier can be referred to as a channel or traffic channel.

Ice Ring self-healing optical fiber networks are disclosed in U.S. Patent No. 5,442,623 but are not particularly suitable for WDM signaling. A similar network designed for WDM traffic is shown in International Patent Application PCT / SE98 / 00136. The networks described in these writings use an extra protective fiber pair between each node pair.

Optical wavelength multiplexing can generally be used in various optical fiber measurement designs or architectures, which have, for example, only a single fiber pair between a node pair. One such architecture is the concept of FlexBus ™, as described in BS Johansson et al., Flexible Bus: A Self-Restoring Optical ADM Ring Architecture, Electronics Letters, Dec. 5, 1996, Vol 32, No. 25, and U.S. Patent Application 08 / 421,734, this architecture comprising optical links forming a ring shape and connecting multiple nodes. The Be25 concept FlexBus ™ has arisen from the need to protect ring networks against fiber cuts and defects in optical amplifiers and to solve the problem often associated with ring network architectures, with circulating signals and noise. In the FlexBus ™ architecture, a section of the fiber ring has always been made passive or inactive by means of optical switches or amplifiers. This intentionally interrupted interruption actually eliminates all problems associated with circulating signals and thus permits fewer circuit components to be used and circuitry of lower performance to be used while still maintaining the shortest possible longest path. For a real error in a link, a link that was previously intentionally rendered inactive is made active and the incorrect link now becomes the inactive link, which can be described as moving the inactive link from its previous position to the incorrect section. This process is called flexing the bus, thereby restoring traffic.

In the concept of FlexBus ™, channel blocking or channel selection filters placed in-line are not necessary, which alleviates the problems associated with successive filter steps. The signal from a transmitter can be transmitted simultaneously in both directions without giving rise to

520 876 interferences and the same wavelength can be used in both directions, thus allowing the same number of bi-directional connections to be established as the number of wavelengths used in the network.

However, with increasingly developed filtering and switching methods, it would be advantageous to be able to reuse wavelengths more than once to be able to establish more connections and thus increase the network's capacity for the limited number of wavelengths possible in a network with respect to gain / bandwidth. of available optical amplifiers, realistic filter bandwidths and frequency stability of filters and light sources. Thus, another design of a node architecture has been invented which is based on the concept of FlexBus ™, but which includes a plurality of interlocking filters and switches connected in-line, i.e. in the direct fiber path of the network or bus through the node, and shown in the published international patent application WO 96/31025 and is called Switchable FlexBus ™ (Rearrangeable FlexBus ™). This design is capable of very efficient use of the wavelengths. In the published international patent application WO 96/24998 an algorithm diagram for the wavelength allocation in networks of the type Switchable FlexBus ™ is shown.

The general architecture of the concept of Switchable FlexBus ™, see also the published international patent application WO 96/24998 and the Swedish patent application An optical WDM network having an efficient use of wavelengths and a node 2o therefor (Optical wdm network with efficient use of wavelengths and nodes for such), submitted at the same time as the present application, is very efficient in using the available wavelengths and is also very flexible in the number of connections that can be established in the network. Partly due to the actual way in which networks have often been designed, partly because of the needs of true nearby traffic in the networks, there often seems to be an interest in finding good network solutions for needs with nearby traffic, ie for traffic between neighboring stations or nodes. For a network with only such a type of traffic, configurability based on a channel pre-selection is not required and it is therefore of interest to search for a simpler design of an add / drop node for this type of traffic pattern, compared to the one is shown in the aforementioned, 3o simultaneously filed Swedish patent application.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide an add / drop node for a network of the type Switchable FlexBus ™, as described above, and for traffic only between neighboring nodes, the node having a minimum of components connected in-line and reliability of the network is increased but it still has the good features of the concept of FlexBus ™ and allows for efficient use of wavelengths.

The problem to be solved by the invention is how, for a network of the type Switchable FlexBus ™, which has traffic only between neighboring nodes, a node structure must be achieved, which works essentially the same as the nodes in this bus and allows a

520 876 efficient wavelength allocation in the network and allows the network and nodes to operate reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described as a non-limiting embodiment with reference to the accompanying drawings, in which

Fig. 1 is a general schematic view of a WDM-type optical fiber network using the flexible bus architecture;

Fig. 2 is a block diagram of a prior art simple add / drop node for the network of Fig. 1; Fig. 3 is a block diagram of an add / drop node designed for communication between adjacent nodes in the network. 2,

Fig. 4 is a view showing an alternate configuration of a switching element used in the node of Fig. 3, and

Figures 5a and 5b are graphical images showing the assignment of wavelengths in a network of the type shown in Figure 1 having nodes according to Figure 3 for two respective different locations of an inactive link.

DESCRIPTION OF A PREFERRED EMBODIMENT

A network is shown in Figure 1, which uses the basic flexible bus structure for WDM communication over optical fibers. A plurality of optical add / drop nodes are interconnected by links 3 to form a network or bus which includes a physical annular structure having as a base element a pair of optical fibers 7e, 7w which are connected, so that they form two parallel fiber rings. Each fiber ring transmits light which propagates in a particular direction, the propagation directions being opposite to each other. Thus, in one of the fiber directions, light is always propagated in the counterclockwise direction, in the configuration 25 of FIG. 1, the inner ring 7e, this direction being hereafter referred to as the eastern direction. In the other 7w of the rings in the pair of fiber rings, light is always propagated in the opposite direction, ie in the counterclockwise direction, as seen in FIG. 1, this direction being referred to as the western direction. Thus, a node 1 in the bus structure is physically connected only to neighboring nodes, a left node and a right node. Thus, the connections for a considered node 1 comprise a left physical link 31 comprising a western line cable 51 and a right physical link 3r comprising an eastern line cable 5r, the other end of each link 31, 3r being connected to the adjacent left and right nodes respectively. . Each piece 51, 5r of line cables comprises a pair of optical fibers 71w, 71w and 1w, respectively. 7re, 7re, wherein in a 71e, 7re of the fibers of a pair of fibers in a link 31, 3r light is always propagated in one direction, such as in an easterly direction as seen in Fig. 1 and in the other 71w, 7rw of the fibers included. in a pair of fibers in a link light is always propagated in the opposite direction, in the counterclockwise or westward direction, as can be seen in Fig. 1. Further, a node 1 is connected to receiver 11 and transmitter 13 for converting optical signals into electrical signals and vice versa, whereby the electrical signals are transmitted to, received from, other devices, links or networks, not shown.

One of the links 3 in the ring structure is always deactivated, see link 2 in Fig. 1, so that at least no light containing useful information to be transmitted in the network can pass through it, in no direction. This prevents such signals and sASE noise from circulating for several turns in the ring structure, where ASE noise is amplified spontaneous emission generated in optically amplified connected in-line, which is usually included in the nodes

1st When an error occurs in a link between two adjacent nodes, the network can be reconfigured so that this link now becomes the deactivated link while the previously deactivated link (2) is now made active and functions as the other active links (3) in the ring structure io and allows signals to pass in the two opposite directions.

A basic structure of a node 1 in the basic flexible bus structure of FIG.

is shown in the block diagram of Fig. 2. The optical WDM traffic, which comprises a plurality of WDM channels with definite, separate wavelengths, each channel occupying a wavelength band around the channel wavelength, enters the node from the left or east ice direction and from right or in the west direction of the fibers 71e and 7rw, respectively. The incoming signals can be amplified in optional optical preamplifiers 15e and 15w, respectively, in which the light signals are amplified. The incoming light is then divided into drain couplers 17e, 17w. These couplers are optical power dividers which emit a portion of the total power of the light propagated in one direction in the bus, through an optically combining coupler 19 which adds the deflected portions of the power from each direction to one another bank 21 with filters, which may also be referred to as an optical demultiplexer, having one or more bandpass filters for wavelengths used in transmission in the network. Thus, the filter bank 21 filters out channels, each channel carrying information in a particular wavelength band. The filtered light signals are then also conveyed to optoelectric receivers 11, whereby an optical receiver is arranged for each received channel.

The remaining portion of the light's power, which is divided into the drain coupler 17e, 17w is passed through the node 1 and is mixed in additional coupler 23e, 23w with new traffic, which is to be added to the node. This new traffic is obtained from electro-optic transmitters 3013, each of which transmits optical signals within a wavelength band or in a channel separate from the other transmitters. The output signals from the transmitters 13 are joined together in an optically combining coupler or optical multiplexer 25, the resulting combined signal then divided into a dividing coupler 27 into two parts of equal power, one of the two parts being transmitted to one of the additional couplers 23e , 23w and the second portion is transmitted to the other by the additional couplers. The light signals obtained from the additional switches 23e, 23w for each direction are fed to the fibers 7re, 71w contained in the links 3r, 31 connected to the node and transmit light emanating from the node through optional optical power amplifiers 29e. 29W.

In the node design shown in Fig. 2, the absence of interlocking filters should be connected

520 876 in-line and switches connected in-line are observed and in particular that light is propagated through or passes through the node in a substantially unaffected or uninterrupted manner. Furthermore, transmitters 13 transmit in and receivers 11 listen to both directions of traffic simultaneously. The left side of the amplifiers 15e, 29w and 15w, 29e, respectively, can be used to deactivate the respective links or segments 31, 3r, which connect the node to the two adjacent nodes. This is done when this link should become the deactivated link, such as when this link becomes inaccurate. Such an error may be caused, for example, by one of the fibers in the pair of the link being interrupted or by one of the optical amplifiers connected to this link being faulty.

The bus structure and node design of Figures 1 and 2 are described in the above article by BS Johansson et al. and in the aforementioned U.S. Patent Application 08 / 421,734. However, this structure only allows for a considered wavelength or channel to be used once in the network in each direction, such as for communication between two nodes. The node architecture which allows reuse of wavelengths, ie that a channel or wavelength is used more than once to transmit information in one direction, is disclosed in the cited international patent application WO 96/31025, which results in more efficient use of available wavelengths. . However, this prior art node design has a plurality of demultiplexers and switches connected in-line, i.e. in the direct path of a fiber in the network or in the bus through the node. Nevertheless, this prior art node allows a very efficient use of the wavelength range or, equally, a very efficient use of available channels.

For the situation with very close traffic, all wavelengths can be reused between all node pairs, see the schematic illustration in Fig. 5a. In this ring network, traffic occurs only between neighboring nodes, where the designation nearby is generally taken into account for the ring design. Thus, all pairs of neighboring nodes, which have an active link or the inactive link between them, communicate with each other. Here is shown a network with six nodes, which uses six different wavelength channels with the wavelengths λ₂, ... λ₆. Wavelengths λ₄, λ₅, λ₆ is used for each node except the easternmost node to transmit on the fiber 7re for traffic going east to the nearest node on the eastern side of the node, these same wavelengths being used by the easternmost node to transmit to the westernmost node using the fiber 71w for light signals going in a westerly direction. Similarly, the wavelengths λ are used_ρ λ₂, λ₃ for each node except the westernmost node for transmitting on the fiber 71w for traffic going in a westerly direction to the nearest node on the west side of the node, the same wavelengths being used by the westernmost node to transmit to the easternmost node using the fiber 7w for light signals going in an easterly direction. In FIG. 5b, the same network is shown after a relocation of the inactive link 2, in which the wavelength assignment is also visible and from which it can be derived, how the transmission and reception of wavelength channels must be changed.

Then, as can be readily seen in Fig. 5a, it is obtained that for Ν_λ wavelengths, where Ν_λ assumed to be one

520 876 even numbers, Νχ / 2 connections between each neighboring node pair can be established, providing N x Νχ / 2 connection for a network with N nodes.

To do this, you must block all reused wavelengths going in the east direction, and everyone going in the west direction in each node. This can be accomplished by providing simple in-line interlocking filters. There is then a possibility of arranging the wavelength assignment so that these blocking filters can have a simple structure, see the block diagram of an add / drop node, which is suitable for traffic between neighboring nodes in Fig. 3. In fig. 3 and 2, the same reference numerals are used for like or similar elements. One possibility of designing such barrier filters may be to arrange, for example, the lower io half of the total wavelength window for easterly directions and the upper half of the window for channels going westerly with a guard interval between the bands, i.e. in Example 1, that all the wavelengths λ_υ λ₂, λ₃ is smaller (or greater) than all wavelengths λ₄, λ₅, λ₆. Then, for example, simple band-blocking filters or even low-pass or high-pass filters can be used as band-blocking filters for each direction.

A node can then be divided into a western port and an eastern port, each port having suitable switches which are not connected in-line to allow a change of direction for some of the wavelength channels when the node is changed from being an internal one. node to a easternmost or westernmost node, which is directly connected to the inactive link. The receivers are then divided into east gate receivers 11w for receiving from the east direction, ie from the adjacent node located on the east side of the node, and west gate receivers lie for receiving from the west direction, i.e. from the neighboring node located at the west side of the node. wherein the directions are indicated for a node which is assumed not to be the westernmost or easternmost node, i.e. not a node, which is directly connected to the inactive link 2. For the westernmost node, the west gate receivers II are used instead of receiving from the easternmost node and for the easternmost node, the east gate receivers 11w are used for reception from the westernmost node by changing the setting of the switches, as will be described below, see also Figures 5a and 5b. Similarly, the transmitters are divided into east gate receivers 13e for transmitting in the east direction, to the neighboring node which is located on the east side of the considered node, and west gate receivers 13w for

3o transmitting in a westerly direction to the neighboring node located on the east side of the node, the directions also being indicated in the case when the considered node is connected so that it is an inner node and is not directly connected to the inactive link 2. For the westernmost node, the west gate receivers 13w are also used for transmission to the easternmost node, and for the easternmost node, the eastport transmitters 13e are also used for transmission to the westernmost node.

Since it is not necessary to be able to switch on channel-to-channel basis in the node of FIG. 3, it is sufficient to use a switching element for each group of receivers 11e, 11w and each group of transmitters 13e, 13w, instead of a switch for each received channel and each transmitted channel in the node, as proposed in the aforementioned, simultaneously submitted The Swedish patent application An optical WDM network having an efficient use of wavelengths and a node therefor (Optical wdm network with efficient use of wavelengths and nodes for such).

Thus, in both directions in the middle of the node shown in Fig. 3, simple latch filters 3le, 31w are inserted between the output of the drain switch 17e ', 17w' which transmits the signal to be passed through the node, and an input to the respective auxiliary switch 23e ', 23w'. The drain couplers 17e ', 17w' are similar to the drain couplers 17e, 17w of FIG. 2 but from the light output passing in each direction, two substantially equal parts of power are discharged, each drain coupler 17e ', 17w' thus having three output terminals or lines. The light signal arriving at the node in each direction comprises all wavelengths λ_υ λ₂, ..., λ₆, used in the system, according to the example shown in Figures 5a and 5b. For an inner node, half the number of wavelengths in the node for each direction is terminated so that for the example of Figures 5a and 5b, the wavelengths λ₄, λ₅, λ₆in the east direction terminates, and the wavelengths λ_ρ λ₂, λ₃ in the west direction terminates in an ice inner node. A wavelength channel is said to be terminated in a node if it is received in the node, ie if there is a receiver I1, llw for this channel in the node.

Each latch filter 3le, 31w in the considered node blocks only the wavelengths λ₄, λ₅, λ₆ respectively. λ_ρ λ₂, λ₃, terminated in the node for the respective direction, assuming that the node is connected as an inner node and not as an end node, which is connected to the inactive link. All wavelengths not included in the amount of [λ₄, λ₅, λ₆], may then only pass through the node in an easterly direction in a substantially unaffected or uninterrupted manner, and in the same way all wavelengths not included in the quantity [λ_υ λ₂, λ₃], pass through the node in a westerly direction in a substantially uninterrupted manner, again assuming that the node is connected as an inner node.

Like the node of Fig. 2, the optional preamplifiers 15e, 15w each have their outputs connected to a respective drain coupler 17e ', 17w'. The tapping switches couple the two, preferably equally deflected, portions of the power of the received light to 2: 1 switch elements 33e, 33w, so that each such switch element at its two inputs receives a portion of the light power propagated in the two opposite directions. Such a 2: 1 switch element 33e, 33w has its single output terminal connected to the input of a corresponding demultiplexer 35e, 35w and each of its input terminals is thus connected to a separate drain coupler 17e ', 17w'. The demultiplexers or filter blocks 35e, 35w correspond to the filter block 21 of FIG. 2 and performing the necessary demultiplexing or bandpass filtering for the wavelength channels to be received in the receivers I1, 11, the light receivers or light detectors being used in a conventional manner to detect only the power of the received light. Thus, a filter block 35e contains bandpass filters for each of the wavelengths λ₄, λ₅, λ₆, corresponding to eastbound channels terminated in the node, and another filter block 35w contains bandpass filters for each of the wavelengths

520 876 λρ λ₂, λ₃’ ^as corresponds to channels in a westerly direction and terminated in the node, the directions being indicated for the case when the node is an internal node. The output of each filter in such a filter block 35e, 35w, which thus transmits a light signal of a specific wavelength or in a specific narrow wavelength range, is connected to a respective receiver 5e, 11w.

Each transmitter 13e, 13w has its output terminal directly connected to one of the two multiplexer blocks 37e, 37w, these multiplexers corresponding to the multiplexer 25 of Fig. 2. A multiplexer block 37e is arranged for eastbound traffic and a multiplexer block 37w westbound direction, whereby the directions io are indicated for the case when the node is connected so that it is an inner node. A 1: 2 switch element 39e, 39w has its single input terminal directly connected to the output of a corresponding multiplexer 37e, 37w and each of its output terminals is connected to one of the auxiliary switches 23e ', 23w' to add the transmitted combination of wavelength channels in each direction, depending on the position of switching elements. The auxiliary couplers 23e ', 23w', corresponding to the auxiliary couplers 23e, 23w in FIG. 2, thus adding two signals to the traffic passing through the node, and having three inputs.

If in the node of Fig. 3 a wavelength channel is received from one direction and then also blocked in the node in that direction, the same wavelength channel must be able to propagate through the node on the other fiber in the opposite direction without being blocked in the node, see Fig. 5a and 5b. In Fig. 3, the positions of the switching elements 33e, 33w and 39e, 39w are shown for a node which is not connected as an end node, ie when it is not connected to the inactive link at one of its sides. If the bus must flex due to some fault and the node is then directly connected to the inactive link, then one of the receiving switching elements 25e, 33w, 33w must change its position and that of the switching elements 39e, 39w connected via a multiplexer 37e, 37w transmitters 13e, 13w, which are located in the same side or port of the node, must also change their position, that is, when a receiving switch element receiving from traffic going in a first direction has to be changed; the transmitting switch element must also be changed which transmits in a direction opposite to the first direction. Thus, the receiving and transmitting switching elements 33e, 39w and 33w, 39w for changing the path of a correct connection to another node, i.e. for receiving from and transmitting correctly to the second node, can thus be connected to the same triggering point, which is indicated by CP in FIG. 3, which simplifies the procedure which must be performed by any control unit not shown, which is arranged to control the position of the switch elements to allow the bus to flex. It can be observed that the wavelengths blocked by the latch filters 3le, 31w in each direction do not have to be changed for a flexing situation.

Thus, more particularly, the switch elements 33e, 39w on the west gate side of the west gate demultiplexer 35e and the west gate multiplexer 37w should be in the western position, as shown in Fig. 3, unless the considered node is not the westernmost node of the bus, be in the eastern position shown in Figure 3, unless the considered node is not the easternmost node of the bus.

For reasons of reliability, it may be advantageous to replace all switching elements with a unit 41 which is combined by a 1: 2 switch 43 and a simple on / off switch 45, as shown in Fig. 4. On / off switch 45 is arranged in the branch which is most likely to be open, these positions being shown in Fig. 3, and is open in all nodes, except in the westernmost and easternmost nodes, in which it is closed in two switching elements. The link to be deactivated can be rendered inactive by always allowing the west gate amplifiers 15e, 29w in the new westernmost node block to block all signals coming from and going west, and in that node also the on / off switches in the west the door then closes. Correspondingly, in the new easterly node, the east gate amplifiers 15w, 29e can always be intercepted by all signals coming from and going east, and in that node the on / off switches included in the switching elements of the east gate are also closed.

520 876 ίο