WO2011058547A1

WO2011058547A1 - Optical communication network having flexible operability

Info

Publication number: WO2011058547A1
Application number: PCT/IL2010/000899
Authority: WO
Inventors: Eyal Lichtman; Avi Levy; Amir Peled; Uri Mahlab
Original assignee: Eci Telecom Ltd.
Priority date: 2009-11-12
Filing date: 2010-10-28
Publication date: 2011-05-19

Abstract

To arrange a dedicated communication path in an optical hub network at low cost, it is suggested to select one or more specific optical channels that can be dropped at one node while are not presently dropped, to ensure that these specific channels are not in use in a unidirectional section between that node and another node and to add these specific channels in a colorless manner at the another node. The technique is effective and advantageous for optical networks comprising a hub and for optical networks utilizing several grids of optical channels.

Description

Optical communication network having flexible operability

Field of the invention

The present invention relates to a technology for increasing flexibility and quality of transmission in optical communication networks, especially in hub networks, to the obtained flexible network and to elements of the network.

Background of the invention

The invention specifically relates to optical networks utilizing multiple optical channels (wavelengths) for communication in the network; the network may be a mesh network, a ring network, a hub network such as WDM and DWDM hub networks, etc.

Flexibility is one of the most critical issues in the modern communication networks. Allocation of added and dropped optical channels at specific nodes of_. an optical network usually requires thorough planning in advance. In case the network needs to be modified, enlarged, upgraded so that optical channels are to be reallocated and/or more optical channels are required for the modified network, the flexibility problem always arises.

A Hub optical network is a kind of optical ring networks wherein one of the network elements - a so-called hub node- is a central transmitting and receiving node of the network; at the hub, all optical channels of the network are terminated by a demultiplexer and then regenerated and returned to the network via a multiplexer. All other nodes in the hub network communicate with the hub and cannot communicate with one another without passing the relevant optical channel(s) via the hub. Allocation and re-allocation of the network channels is usually performed in advance for a long period of time, so that the aiTangement can hardly be changed by request of a user.

The problem of flexibility in many cases can be resolved by utilizing new and expensive network elements, such as Reconfigurable OADMs (ROADM), Wavelength Selective Switches (WSS), etc. ( see Fig. 1 ). With appearance of reconfigurable optical add drop multiplexers (ROADMs), designers of optical networks (including hub networks) have been given a possibility to arrange flexible communication between network nodes, since now any optical channels could be selectively added and/or dropped at any ROADM-node of the network.

However, due to the high cost of ROADMs and WSSs, both designing/ deployment of a new optical network, and upgrading of an existing optical network still constitute a problem.

There is a long felt need in practical, non-expensive solutions which would allow increasing flexibility of optical networks, without degrading their characteristics.

US2004165891A describes a technique for eliminating extinguish losses in a hub ring network that includes Optical Add Drop Multiplexers (OADMs). The need for a wavelength extinguishing function is eliminated by ensuring that when a wavelength is added to the optical path at a certain node in the ring, the arriving traffic in the path before the addition does not contain that wavelength. The methods include using different wavelengths for the drop and the add operations, ensuring that a through traffic does not include local ADD wavelengths which are subsequently combined with the through wavelengths, and bi-directional transmission on one or two separate fibers of identical dropped and added wavelengths.

The US2004165891 A strictly requires that the added channels must be different than the dropped channels and the channels comprised in the arriving traffic in the path before the addition. Even more important is that the approach of US2004165891 A is considered only for a specific case of providing traffic between OADM nodes and Hub, namely - the solution does not get rid from the restricted flexibility of hub networks.

US7106969B describes an optical network terminator for terminating and reducing the accumulated noise in optical networks, particularly ring based networks. The terminator eliminates problems of noise accumulation from amplifier spontaneous emission (ASE), thermal noise, etc., while providing bi-directional communications in the optical network. The optical network may have any topology including ring, star, mesh, point-to-point, etc. In the case of an optical ring, the ring is broken and an optical terminator is placed in line therewith. The optical network terminator includes a filer such as an optical demultiplexer/multiplexer or Fiber Bragg Grating (FBG) based filter. Each individual wavelength of light is filtered and a multi-wavelength optical output is generated whereby the noise accumulation is removed. Each channel is adapted to only pass a band-limited signal around the center frequency corresponding to the wavelengths supported by the particular optical ring network. Channel equalization uses variable optical attenuators and monitors in line with each channel. Channels currently not in use may be disconnected from the ring remotely by setting the corresponding optical attenuator to a low enough level.

Still, neither of the above-cited prior art references solves the task of flexibility so as to allow bidirectional communication of each node with any other node in a hub optical network in a simple inexpensive manner and without side effects in the optical network.

US2004165891A strictly requires that the added channels must be different than the dropped channels. Even more important is that the approach of US2004165891 A is considered only for a specific case of providing traffic between OADM nodes and Hub, namely - the solution does not get rid from the restricted flexibility of hub networks.

US 2004/0109635 Al describes a WDM add drop module. The drop portion in the module can be implemented by using thin film interleavers. The add portion can also use interleavers, for example the fused fiber ones. In a final stage, the fused fiber interleavers can be placed in series. The described interleavers are arranged in cascades, and thus handle optical channels with different spacing between their wavelengths. The arrangement of US 2004/0109635 Al enables reduction of cross talk at a channel edge. US2006/034610 A describes an OADM which comprises a core unit utilizing various technologies for add units and drop units. One of the discussed technologies is using interleavers at a drop side of the core and/or at an add side of the core. At the add side the interleaver can be used as a grouping filter.

Still, neither of the above-cited prior art references tries to solve the task of flexibility, i.e. to allow communication of each node with any other node in the optical network in a simple inexpensive manner.

Also, neither of the above-mentioned interleavers is utilized as the core unit in a node performing OADM functions.

Object and Summary of the invention

It is therefore the object of the invention to provide an inexpensive technique which would allow improving flexibility of optical networks, in a manner allowing easily arranging bidirectional communication from any node to any node, and without noise accumulation in any direction.

It is another object of the invention to improve flexibility of optical networks from the point of easily upgrading and reconfiguring the networks.

First of all, to achieve the first object, there is proposed a network section in an optical network (which may be a ring network, a mesh network, a hub network), said network section starting at a first node and terminating at a second node of the network;

the first node comprising a colorless add unit provided (may be especially provided) for node-to node dedicated communication, and intended for adding one or more optical channels to said network section,

the second node comprising a colored drop unit being operative to drop at least one of said one or more optical channels whenever added by the colorless add unit, said at least one of the channels added at the first node and dropped at the second node forming a dedicated node-to-node communication path in the network section between the first node and the second node in the network section.

The dedicated communication path should be understood as carrying dedicated communication which is established in the network in addition to conventional communication between the nodes. It should be provided that any specific channel of said one or more optical channels to be added are not in use in the unidirectional section of the network before adding that specific channel to said section (which is a regular rule for optical networks).

Actually, there is also proposed a method for arranging a dedicated communication path between a first node and a second node in an optical network, the method comprises:

ensuring that one or more specific optical channels can be dropped while are not presently dropped at the second node (i.e., ensuring that there are technical resources allowing such channels to be dropped, and thus selecting such channels as candidates for dedicated communication);

ensuring that said one or more specific optical channels are not in use in a specific direction along a network section between the first node and a second node (i.e., in one or more fiber spans and at a node, if any, between the first and the second nodes in that specific direction);

at the first node, adding said one or more specific optical channels in a colorless manner to the network section;

dropping, at the second node, said one or more specific optical channels in a colored manner, thereby establishing the dedicated communication path (for dedicated communication in addition to conventional communication in the network).

The dedicated communication may be also understood as flexible, not preplanned, node-to-node communication . . For optical networks comprising a hub node; the method comprises:

ensuring that said network section includes the hub node and that one or more through passages exist in the hub node for said one or more respective optical channels;

dropping, at the second node, said one or more specific optical channels in a colored manner. In a version of the above method, it may comprise controllably providing said through passages/connections in the hub node upon allocating said one or more optical channels for dedicated communication in the hub optical network. (The hub may be controlled by an operator, by NMS, etc.).

Further, the method may comprise a step of providing a second, alternative path in the hub network in a similar way (by adding channels in a colorless manner), thereby forming a protected path. The second path may cross the hub (say, by using an alternative wavelength/channel), but may pass via the network in the opposite direction without crossing the hub.

Still further, the method may comprise ensuring absence of through passages in the hub for those optical channels which are (presently) not in use and cannot be dropped at any of the nodes in the hub network. In other words, such channels should be disconnected in the hub.

The meaning is that an optical channel which is not provided with a drop point at any of the drop units in the network, will circulate in the network endlessly and even if it does not carry data it will accumulate white noise and other random noise.

In practice, the dedicated communication path may be provided for dedicated communication between a specific first node and a specific second node, in addition to conventional communication in the network, which is usually established to serve all nodes in the network.

It would then be comfortable to organize the dedicated communication between the first node and the second node (i.e., between the colorless add unit and the colored drop unit as will be described below) using so-called dedicated or reserve optical channels, and utilizing them only when needed for the dedicated communication.

In hub networks, each node may be assigned to dropping specific optical wavelengths/channels being different from those assigned to dropping at other nodes of the nub network. In this case, the step of ensuring that the selected one or more specific optical channels are not in use in a specific direction along a network section between the first node and a second node becomes automatic, i.e., becomes ensured by default. In the optical network comprising a hub, the above-mentioned network section starts at a first node and terminates at a second node of the hub network;

wherein

said network section passes through the hub ensuring through passage for said one or more optical channels.

The colorless add unit may comprise one or more tunable lasers, so as to enable addition of practically any desired optical channel to the network section. Selection of the channel for adding depends on availability of a presently free channel in the colored drop unit of the second node.

In one embodiment, said hub is capable of controllably forming a through passage (say, in the form of internal or external connections) via the hub for said one or more optical channels. The hub may be configurable and controllable by an operator or by a central entity such as a Network Management System (NMS). In addition, the hub may be adapted to controllably block through passage for channels being presently not in use in the network and having no possibility for being dropped in the colored drop units of nodes in the network.

It should be explained that typically, a conventional hub node of hub networks does not allow through passage of optical channels carrying data. Indeed, the dropping and then the regeneration of optical channels that pass via the hub intentionally create a "physical gap/cut" in a ring-like fiber carrying the optical channel, and that fact serves for preventing noise circulation and accumulation along ring-like optical channels of the ring network. Therefore, the proposed network section (and, consequently, the proposed network comprising such a network section) guarantees elimination of noise in the dedicated optical channels without forming the "physical gap/cut" in the hub, since the dedicated communication channels are a- priori "open" by having their termination point, and thus noise cannot endlessly circulate along such channels. On the other hand, those optical channels which are currently not in use (not carrying data) in the section/network and do not have "drop points" at nodes of the section/network, should be ensured to be broken by the HUB and thus prevented from accumulating noise. On the other hand, the proposed feature of providing through connections in the hub node allows arranging the dedicated communication in any desired direction in the ring network (since said dedicated communication must not obligatory be interrupted at the hub node). That possibility of arranging the dedicated communication in any desired direction allows easily providing the traffic protection for such communication.

The above-described (first) section of the hub network may be fulfilled by another (second) section which is built between the same two nodes for transmission (usually in the opposite direction along the ring) in order to protect the traffic passing via the first section. The second section may or may not pass via the hub.

Of course, it should be ensured that any specific channel of said one or more optical channels to be added are not in use in the relevant section of the network before adding that specific channel to said section (which is a regular rule for optical networks).

According to one preferred implementation , the "colorless add unit" should be understood as a passive unit (such as an optical coupler, a multiplexer or an optical interleaver) which does not require separation of different optical channels before adding; to the contrary, the colorless add unit simply adds the channels in the mixed, combined ("colorless") form to the network section. The drop unit at the second node is called "colored" since it needs to be provided with suitable filter means for dropping specific optical channels.

The colorless add unit may be a dedicated add unit, provided in a conventional network node in advance, for example at the time of deployment of the network. The dedicated add unit should be understood, first of all, as differing from a conventional add unit of a conventional OADM node. Further, the dedicated unit may be an additional/reserve unit to the conventional add unit. Such a reserve, add unit may be added to the node later, when upgrade of the network becomes necessary. And finally, the dedicated colorless add unit may just form the add unit of a newly proposed OADM node. Therefore, the Inventors actually propose a novel OADM node which is formed/assembled from the described colorless add unit and a colored or tunable drop unit. In general, the colored drop unit of the second node can be a conventional drop unit incorporated in the conventional OADM, intrinsically adapted to drop some optical channels (i.e., already provided with specific "colored" optical filters). In this case, in order to reach the second node from the first node via any specific optical channel which is known as definitely "droppable" at the second node, the first node of the invention adds that specific optical channel via the colorless add unit and starts the dedicated communication with the second node via the specific channel.

Alternatively, the colored drop unit of the second node can also be a dedicated drop unit (a unit separate from a core of the second node, for example - from a conventional OADM core which already comprises its drop unit). Such a dedicated drop unit can be installed in the first node as a reserve facility, at the time of deployment of the network, or can be added later on. It goes without saying that the reserve drop unit should be accompanied by one or more optical filters ("colored" - fixed or tunable filters) for dropping one or more optical channels.

Further, the Inventors propose a new optical network element (node) for performing OADM functions, utilizing such a "dedicated" add unit and such a "dedicated" drop unit. That node will be a non-expensive though quite a flexible one, and will be called here a "quasi-OADM node". It will not comprise usual expensive components of a modern reconfigurable OADM node, i.e., no Wavelength Selective Switches (WSS), no Reconfiguratble OADMs (ROADMs).

In one embodiment, the network node (a quasi-OADM node) may comprise a conventional regular/fixed OADM (say, being the pair "demultiplexer & multiplexer" or a pair of a fixed/colored add unit with a fixed/colored drop unit), additionally provided with at least a dedicated colorless add unit, or with two dedicated units: a colorless add unit and a "colored" drop unit. Such a quasi-OADM node becomes suitable for the dedicated node-to-node communication.

In other words, the quasi-OADM node should comprise any drop unit and a colorless dedicated add unit.

However, in one preferred embodiment, the quasi-OADM node does not comprise the regular OADM as a component: the newly proposed quasi-OADM node is formed from the described colorless add unit and the described colored drop unit. Since the colorless add unit may introduce some additional insertion loss in the optical line, the node may be additionally provided with an optical amplifier (a booster) switched at the output of the proposed quasi-OADM node, being capable at for compensating at least the excessive insertion loss introduced by the dedicated colorless add unit.

The proposed new optical network element (node) may be offered to a customer in combination with the above-proposed hub node, as a kit. These two units may be used in combination in an optical network.

According to the described approach, any optical network (newly designed, or being upgraded) may be provided with one or more such "first" nodes and one or more such "second" nodes; owing to that, network sections for the node-to-node dedicated communication can be organized in the optical network quite easily, and in the most preferred case - from any node to any node (when each of the network nodes serves as a first node to one or more such dedicated network sections, and simultaneously serves as a second node to a number of other dedicated network sections).

The proposed network section, method and nodes allow for judicious reuse of optical channels in the networks.

The feature of judicious reuse of optical channels is actually the feature of "knowing" which channels can be added in order not to distort the existing channels of the same wavelength. This problem is solved in the proposed quasi-OADM node if the add unit always adds those channels which are absent in the through pass of the node. In the above-described practical implementation, the problem of the optical channels reuse is resolved, preferably, by utilizing NMS and controlling the quasi- OADM node (and optionally also the hub node).

The second object of the invention (flexibility, easy upgrade and reconfiguring) can be achieved by providing dedicated communication in an optical network by utilizing^' different grids of optical channels.

Namely, there is provided a method for arranging a dedicated communication path between a first node and a second node in an optical network utilizing a first grid of optical channels, the method comprises: assigning for the dedicated communication path optical channels belonging to a second optical grid being different from the first grid;

ensuring that one or more specific optical channels of the second grid can be dropped while are not presently dropped at the second node;

ensuring that said one or more specific optical channels of the second grid are not in use in a specific direction along a network section between the first node and a second node;

at the first node, adding said one or more specific optical channels of the second grid in a colorless manner;

dropping, at the second node, said one or more specific optical channels in a colored manner, thereby establishing the dedicated communication path, utilizing said second grid of the optical channels.

The alternative grid of optical channels may, for example, comprise a portion (sub-grid) of the basic grid accepted in the existing optical network. Say, the network operates at a basic grid 100 GHz, and the alternative grid may be 50 GHz. Other examples of the two grids can be 400/200, 200/100 (GHz) and so on.

Actually, the dedicated communication paths in the network may use more than one additional grids for serving different dedicated communication paths (say, for serving different network sections). For example, grids 400/200/100/50 GHz can be used.

By using different optical grids, the chance of wavelengths conflict in the proposed optical network will become much less (if will at all remain), while the flexibility of the network will be essentially increased. In other words, the dedicated communication paths between nodes may operate at their own grid, independently from the grid at which the conventional OADMs operate.

Actually, different network sections intended for dedicated communication between different nodes, may utilize different quoted portions of the alternative grid, thus different channels will be used at different portions of the network for dedicated communication between different pairs of nodes.

According to the described approach of multiple grids, any optical network

(newly designed, or being upgraded) may be provided with one or more such "first" nodes and one or more such "second" nodes; owing to that, network sections for the node-to-node dedicated communication can be organized in the optical network quite easily, and in the most preferred case - from any node to any node (when each of the network nodes serves as a first node to one or more such dedicated network sections, and simultaneously serves as a second node to a number of other dedicated network sections).

The new optical network element (the so-called "quasi-OADM node") which has been described above, may be implemented at least in the following configurations.

As mentioned above, the proposed network node for performing functions of OADM (optical add and drop multiplexer), may comprise either a regular OADM core or a core based on an optical interleaver, and may (basically or additionally) be provided with a dedicated colorless add unit and a dedicated drop unit, said dedicated units serving for adding and dropping channels to be utilized for node-to node communication wherein such node-to-node communication being arranged in addition to conventional communication handled by the node. (The conventional communication may be_carried for example, by the conventional core OADM).

In the case of the proposed "interleaver core", the conventional communication may be earned out by using one of the grids while the remaining grid(s) may be used for the dedicated communication.

In a basic embodiment, the network node (a quasi-OADM node) may comprise a conventional regular OADM (say, being the pair "demultiplexer & multiplexer"), additionally provided with two dedicated units: a colorless add unit and a "colored" drop unit, which make the node adjustable for the dedicated node-to-node communication (for example, Fig. 4 referred to in the detailed description).

In another embodiment, the network node (a quasi-OADM node) will not comprise a blocking element (say, Fig. 4 without the block 32, so that block 30 becomes the quasi-OADM node). Any desired channels for dedicated communication can be colorlessly added (at block 36).

The channels to be utilized for the node-to-node communication may belong to an optical grid different from another optical grid used for the conventional communication. In one alternative preferred embodiment, the quasi-OADM node comprises a core based on an optical interleaver configurable to selectively handle two different grids of optical channels, the arrangement being such that

the dedicated colored drop unit forms at least part of a drop unit capable of dropping one or more optical channels of any of the two grids, and

the dedicated colorless add unit forms part of a distributed add unit capable of colorlessly adding one or more optical channels of any of the two grids.

One of the grids handled by such a quasi-OADM node is supposed to be utilized for the dedicated node-to-node communication.

When we speak about two different grids of optical signals, we mean two systems of optical channels with different spacing between the channels. For example, these two grids may be a 100GHz grid and a 50 GHz grid, where spacing between optical channels in the first grid is twice narrower than the spacing between optical channels in the second grid. (Other grid examples are possible.)

The above-mentioned core interleaver has three input/output ports; two of said ports are input ports and each of them may receive optical channels of one or both of the grids arriving from outside, but is capable to let into the interleaver only channels of one of the grids while blocking channels of the other grid. The third port is an output port which outputs from the interleaver all the optical channels which have been finally let into the interleaver through the two input ports. Traffic of two opposite directions (East/West traffic) is handled by two respective sets of the described quasi-OADM.

In one specific embodiment, the quasi-OADM node may comprise:

an input line capable of carrying optical channels of two grids,

an output line capable of carrying channels of two grids;

the colored drop unit ( such as an optical splitter) connected to the input line and capable of selectively dropping one or more desired channels of one or both of the grids,

the input line being connected to the interleaver via an optical switch (since the drop unit does not block any of the incoming channels, it further applies all the optical channels received from the input line to the interleaver); the switch being configured so as to allow applying the incoming optical channels to both of the input ports, to one of the input ports or to neither of the input ports of the interleaver);

the dedicated colorless add unit being implemented at least in one of the following manners:

-by colorlessly adding to a specific input port of the interleaver at least one optical channel of the grid which is not blocked by that specific port, (if said at least one added channel is not present among the incoming channels if applied to that specific port),

- by colorlessly adding, to the output line of the node, one or more optical channels of one or both of the grids (if said one or more added channels are not present among the optical channels outgoing via the output port of the inteleaver).

As mentioned above, the dedicated drop unit may preferably be in the form of a passive optical splitter provided with a number of filters for simultaneously serving a number of optical channels of two different grids; the dedicated add unit may preferably be presented as a passive optical coupler or a passive multiplexer.

As has already been mentioned, any specific channel of the optical channels to be added at said quasi-OADM node, should be not in use among the optical channels passing through the interleaver (and going out through its output port).

To arrange flexible dedicated communication between nodes in the network, the dual grid quasi-OADM can be configurable and reconfigurable by changing filters at the dedicated drop unit, by selecting channels to be added via the dedicated add unit, and also by configuring the core (the interleaver with the optical switch).

Depending on the above, the quasi-OADM node which is capable of receiving optical channels of said two grids, can be adapted to various operations, such as:

to drop any desired channels of one or both of the grids,

to apply the optical channels of said two grids to the optical interleaver, to pass optical channels of none, one, or both of the grids through the interleaver, while respectively blocking optical channels of both, the other one, or none of the grids,

to add required optical channels of one or both of the grids colorlessly to the node. Blocking of one of the grids at the interleaver is useful in a case when that gird is utilized for the dedicated communication between some node and the discussed quasi-OADM node. Upon dropping the necessary channels of that grid at the dedicated drop unit, these channels should be terminated at the quasi-OADM node to prevent their further propagation: that can be performed by blocking (in the interleaver) the grid which was used for the dedicated communication.

However, the interleaver may pass there-through all the incoming channels, and thus to perform a so-called bypassing operation. Such an operation may be useful for transparently carrying via the node the dedicated communication which utilizes channels of one of the grids.

The network node, for example that comprising the interleaver, may be adapted to selectively add and/or drop optical channels of the two grids under control of at least one of the following control entities: a central control entity (such as NMS) and a local monitoring & control unit.

Other possible options will be described in the detailed description.

The proposed section, method and node allow for the judicious reuse of optical channels in the networks.

The feature of judicious reuse of optical channels is actually the feature of "knowing" which channels can be added in order not to distort the existing channels of the same wavelength. This problem is solved in the proposed quasi-OADM node if the add unit always adds those channels which are absent in the through pass of the node. In our practical implementation, the problem of reuse is resolved, preferably, by adding channels belonging to the grid which is blocked in the interleaver - therefore reuse of the channels can always be obtained.

In general, for adding a channel belonging to one or another grid, the node may be provided with a control unit which would operate either locally or in cooperation with a network management system (NMS). However, these control entities may serve for monitoring and controlling some other parameters as well.

In view of the above, the idea of using simultaneously two grids of optical channels is advantageous, since it

a) allows adding new optical channels to the existing allocation of channels quite easily, b) allows obtaining certain flexibility in the optical network, say by using one of the grids (the first) for "conventionally" passing its channels through nodes and along the network, while utilizing the second grid for the "local" or "dedicated" communication between adjacent nodes.

c) allows reconfiguring the network according to the current needs by reconfiguring one or more of its quasi-OADM nodes.

The network section may comprise one or more of the above-described quasi OADM nodes.

And finally, there is provided an optical network for inter-node dedicated communication.

From one point of view, it is an optical network (preferably, a ring network, and in a specific configuration - a hub network) comprising interconnected network nodes, wherein each of said nodes (we do not speak now about a hub node) is provided with the above-discussed dedicated colorless add unit and/or the dedicated drop unit, so that the network is adapted for flexibly and selectively establishing dedicated node-to-node communication via respective network sections. It should be kept in mind that the dedicated communication is preferably established in addition to conventional communication which takes place in the network by occupying its OADM and quasi-OADM nodes.

The new proposed network can be defined as a network comprising one or more above-described network sections.

From another point of view, it is an optical network comprising two or more above-described quasi-OADM nodes.

Brief description of the drawings

The invention will be further described in detail and illustrated with reference to the following non-limiting drawings, in which:

Fig. 1 (prior art) presents a table of possible implementations of OADM nodes, which are presently in use in modem optical networks.

Fig. 2 schematically illustrates an example of how the proposed dedicated no- to-node communication can be arranged in a proposed hub network. Fig. 3 a, b, - schematically illustrate variants of the proposed network section in an optical network, providing the inventive dedicated node-to-node communication.

Fig. 4a, b, c schematically illustrate some embodiments of the proposed quasi- OADM node.

Fig. 5 a, b, c - schematically illustrate variants of the proposed network section in an optical network, providing the inventive dedicated node-to-node communication.

Fig. 6 a, b schematically illustrate two examples of how the proposed dedicated no-to-node communication can be arranged in a ring network.

Fig. 7 schematically illustrates one embodiment of the proposed quasi-OADM node.

Fig. 8 schematically illustrates another, dual grid interleaver embodiment of the proposed quasi-OADM node.

Fig. 9 a, b, c schematically show how the quasi-OADM node of Fig. 8 operates at differently configured core (or different states of the optical switch), thereby allowing for different functions of the node.

Fig. 10a, 10b schematically illustrate options of central and local control of wavelengths in the proposed node.

Detailed description of the preferred embodiments

Fig. 1 (prior art) is a table which shows choices of a network designer for planning a modern optical network; namely the designer may use known OADM nodes based on:

a) a fixed drop unit + a fixed add unit, for example each comprising an assembly of fixed/colored filters (the first "a" column in the table)

b) MUX-DMUX technique with fixed add and drop channels (the second, "b" column), or

expensive flexible OADM nodes based ROADM (column "c") on WSS switches (column "d"). It can be seen that the possibility of reconfiguration and of flexible use of wavelengths exists only for complex and expensive OADM nodes. It means that for achieving flexibility in the network and for providing dedicated node-to-node communication, a network designer is forced to use very expensive OADM nodes.

If the cost of a conventional OADM=X (say, $500) in a Hub network, then the cost of ROADM=14X (~$7,000), the cost of DOADM=4X (~$2,000). The cost of the proposed modified OADM which will be described with reference to Figs 4 and others may vary from 0.7X (when the proposed OADM is assembled from a colored drop unit and a colorless add unit) to 2.5X (depending on the implementation).

Fig. 2 shows an example of establishing dedicated communication using quasi-OADM nodes Nl , N2, N3 in a hub optical network 21 comprising a configurable hub 22. (We keep in mind that a conventional HUB node usually terminates all optical channels arriving to it.) In the drawing, the ring optical network 21 is shown to comprise two concentric fibers 23 and 24 transmitting traffic between the nodes in two opposite directions. Examples of quasi-OADM nodes will be presented in Figs.4 a-c. The ring network configuration with a hub is especially useful if the ring comprises fixed OADMs of the type "a" (see Fig. 1 , column "a") and/or quasi -OADM nodes shown in Fig. 4a.

The two ring-like fibers 23 and 24 can be used for organizing traffic protection in ring networks. Actually, a single fiber may serve for transmitting bidirectional communication, so it can be used both for bidirectional communication between nodes, and for traffic protection purposes.

Let the dedicated communication is arranged between node Nl and node N3 via the HUB node and using an optical channel having the wavelength Lambdal . We suppose that the channel Lambdal was selected as the channel "droppable" at node N3 and then was colorlessly added at node N l (at unit DAU') to be earned in the clockwise direction towards node N3 via the ring 24. To prevent the dropping of the selected dedicated channel Lambdal at the Hub 22, the configurable HUB was controlled to selectively provide a though connection 25 there-inside for that optical channel. Therefore, the channel safely (and without utilizing excessive resources) reaches node N3 and is dropped there either by the own drop unit of node N3 or by a dedicated drop unit (not shown). The network section 26 (a solid line) is an example of the proposed network section comprising a HUB node with a selectively provided though channel for the dedicated communication. Noise cannot accumulate in the channel since that channel, upon being added, is definitely dropped at the end of the created dedicated network section.

Let us look how a protection path can be arranged in the network 21 for the traffic transmitted via the section 26. At node Nl , the same traffic added to the network section 26, can now be colorlessly added (via unit DAU") to a network section 27 (a wavy line) of the fiber ring 23 and carried in the counterclockwise direction toward the node N3 through node N2. Lambda 1 will pass through the quasi- OADM node N2 as a regular through channel. In the clockwise direction, the traffic will be dropped at node N3 by another drop unit (not shown) provided for serving the ring 23.

According to the example, the protected dedicated traffic can be arranged in the HUB network quite flexibly, without complex OADM nodes, without terminating optical channels at the HUB and without noise accumulation.

Fig. 3a, 3b illustrate the principle proposed by the Inventors for establishing dedicated node-to-node communication in an optical network by utilizing widely used and inexpensive optic elements.

Fig. 3a illustrates how the dedicated communication can be arranged in a network section 30 between a first node Nl and a second node N2 (being both newly proposed quasi-OADM nodes), and via a HUB node 35. Upon selecting a wavelength Lambda 1 which is currently not in use at the dedicated drop unit DDU2 26 of node N2, that wavelength can be colorlessly added via a dedicated drop unit DAU1 of the node Nl , and thus can form a dedicated communication section 30 between the nodes N l and N2 via a Hub 35. To ensure free transit of the channel Lambda 1 through the HUB node 35, a through passage schematically shown as 37 has been provided in the HUB. It can be performed by configuring the HUB either manually, or in a controlled manner automatically, for example according to a control signal from outside (say, from NMS). Alternatively, external jumper(s) can be used to controllably and selectively provide through channel(s) via the HUB. The dedicated path 1 (DPI) is thereby established. It should be kept in mind that the DAU is "colorless", it is capable of colorlessly adding to the network section 30 as many channels as necessary for the desired dedicated communication if they are allowed in the section 30 - i.e., not in use.

Fig. 3b illustrates another example. It show how dedicated communication paths can be arranged between nodes N3 and N4; nodes N3 and N5 via a hub 35'. In this example, nodes N3, N4, N5 comprise conventional OADMs there-inside. Let us suppose that all optical channels in the OADM of N5 booked/planned in advance. Let OADM of N4 is capable of dropping an optical channel of Lambda 1. Though OADM5 is "all booked" and not adapted to drop any more optical channels, the WDM system of the network allows transmitting more optical channels via the section 34.

Further, let the conventional OADM3 is also booked for some previously existing conventional communication and cannot itself allow new dedicated node-to- node transmission from N3 to N4 using the mentioned optical channel of Lambda 1 even if that optical channel is not presently in use between N3 and N4. The Inventor proposes adding Lambdal via a dedicated add unit DAU3 (which can be just a passive optical coupler) additionally provided in node N3 at the output port of the conventional OADM 3. The channel will be then dropped at N4 by the conventional OADM4,. and the dedicated path DP2 (in network section 38) will be formed.

Further, to arrange a node-to-node communication via a network section 34 between node N3 and node N5, the Inventors propose adding a dedicated "colored" drop unit DDU5 before OADM5 of the node N5.

The DDU5 comprises an optical splitter switched in the incoming line of the OADM3, and is provided with one or more optical filters F for dropping wavelengths selected for the dedicated communication to node N5. The filters of DDU5 may be tunable or fixed. In the example of Fig. 3b, the optical channel of Lambda2, added via the dedicated add unit DAU3 at node N3 is dropped by the dedicated drop unit DDU5 at node N5, thereby the dedicated communication path DP3 is established between N3 and N5 in the network section 34. To ensure transit of the mentioned two optical channels through the HUB 35', respective through passages are selectively provided in the HUB 35'. As noted above, the connections for such passages may be internal or external, configurable and optionally controllable. Fig. 4a illustrates an embodiment 40 of the quasi-OADM node, which does not comprise a conventional OADM core, but is composed from a dedicated drop unit DDU 44 (possibly tunable optical filters) and a dedicated colorless add unit DDU 46 (possibly tunable lasers connected to an optical coupler) which may be integrated with one another or just assembled together in a common block.

Though node 40 comprises add and drop units which are called "dedicated" in the present description, that node may successfully be used for transmitting conventional (basic) communication channels in an optical network and, whenever required, be flexibly utilized for additional node-to-node communication.

The quasi OADM node 40 may be provided with an output amplifier/booster

47, for compensating excessive insertion loss introduced by the DAU block 46 (which added loss may result in 4-6 dB in comparison with insertion loss in the fixed OADM configuration). Naturally, the amplifier 47 may serve for full compensation of the insertion loss in the node and for further amplification of the output signal, if required.

Fig. 4b illustrates another embodiment 40' of the quasi-OADM node, where a conventional OADM core 42 having its drop unit DU and add unit AU is provided with an additional dedicated colorless add unit DAU 46' which may either be integral with the core 42, or constitute a separate add-on to the core 42. In any modification, the node 40' may be equipped with an optional output booster (not shown).

Fig. 4c illustrates yet a further embodiment 40" of the proposed quasi-OADM node, suitable for arranging dedicated node-to-node communication in an optical network. The illustrated node 40" comprises a conventional OADM core 42, both the colored DDU 44" and the colorless DAU 46". It is universal for creating dedicated communication paths in network sections, and also for supporting conventional communication via the core 42.

A local controller of the nodes 40, 40' and 40" (not shown) may be adapted to control the tunable filter/selection of filters in the DDU (or just DU in 40'), to control switches or lasers (not shown) of the DAU locally, and also to control an output booster 47"(if provided). However, the controller may be interconnected with a network management system (not shown) for obtaining information about channels which should be dropped via filters of the DDU/DU and/or about channels which should and can be added via the DAU.

Figs. 5a, 5b, 5c illustrate the principle proposed by the Inventors for establishing dedicated node-to-node communication in an optical network by utilizing widely used and inexpensive optic elements.

Fig. 5a illustrates how the dedicated communication can be arranged in a network section 1 10 between a first node N10 and a second node N20 being both conventional OADM nodes. In this example, no hub exists in the network. Let us suppose that all optical channels in OADMs of N10 and N20 are planned in advance and that OADM of N10 is not configured to add a wavelength Lambda 1. However, the OADM of N20 is capable of dropping the optical channel of Lambdal . The conventional OADM10 cannot allow the node-to-node transmission from Nl to N2 using the optical channels of Lambdal even if that optical channel is not presently in use between N10 and N20. The Inventor proposes adding Lambdal via a dedicated add unit DAU (which can be just a passive optical coupler) to the output through port of the conventional OADM 10. The channel will be dropped at N2 by the conventional OADM20 . The dedicated path 10 (DP 10) is thereby established. It should be kept in mind that the DAU is "colorless", it is capable of colorlessly adding to the network section as many channels as necessary for dedicated communication (and allowed in the section 1 10 - i.e., not in use).

Fig. 5b illustrates an example where, to arrange a node-to-node communication via a network section 1 14 between node N10 and node N30, the Inventors propose adding a dedicated "colored" drop unit DDU 1 16 before OADM30 of the node N30. We suppose that OADM30 is "all booked" and not adapted to drop any more optical channels, while the WDM system of the network allows transmitting more optical channels via the section 1 14.

The DDU 1 16 comprises an optical splitter switched in the incoming line of the OADM30, and is provided with one or more optical filters F for dropping wavelengths selected for the dedicated communication to node N30. The filters of DDU l 16 may be tunable or fixed. In the example of Fig. 5b, the optical channel of Lambda2, added via the dedicated add unit DAU 1 12 at node N10 is dropped by the dedicated drop unit DDU 1 16 at node N30, thereby the dedicated communication path DP20 is established between N10 and N30 in the network section 1 14.

Fig. 5c illustrates an alternative but the preferred way of organizing the required/dedicated node-to-node communication in an optical network.

In Fig. 5c, the nodes N10*, N20*, N30* do not comprise a regular OADM core. These nodes are so-called quasi-OADM nodes based on an interleaver (I) accompanied at least by the dedicated drop unit (DDU). The structure of such a node will be further suggested with reference to Fig. 8. The cost estimate of the quasi OADM shown in Figs 5c and 5b may reach 2.5X when X is the cost of a conventional OADM node. These nodes operate with two optical grids having different spacing between their channels, wherein the two grids (Gridl and Grid2) may coexist in the optical network and form a combined grid in the WDM system. For example, Grid 1 may be a 100GHz grid and Grid 2 - a 50GHz one. In this example, the Inventors propose utillizing Grid 2 for the dedicated node-to-node communication.

In Fig. 5c, node N I O* receives optical channels of Grid 1 from the network. Optical channels of Grid 2 are added via the core interleaver (I) of N 10*. The DDU of node N20* drops one channel of Grid 1 thus participating in the conventional communication of the network. Simultaneously, the DDU of N20* drops two channels of Grid 2 and thus accomplishes the dedicated node-to-node communication via path DP 10 between N10* and N20*. It should be noted that the quasi-OADM node based on the interleaver may either block a specific grid, or pass its channels through. The node N20 * passes through all channels of both grids after performing the required dropping at the DDU (this function is called "drop and continue").

Node N30* is also provided with a DDU which performs dropping of one channel of Gridl for the conventional communication, and also of one channel of Grid2 for the dedicated communication between N10* and N30*. However, interleaver of N30* is configured to block Grid2, so the dedicated communication path DP20 is terminated at node N30* (the dedicated communciation is formed between 10* and N30* and does not interfere further in the network).

Figs. 6a and 6b show two examples of various traffic patterns which can be formed using the quasi-OADM nodes in a hub optical network 120, by establishing the conventional and the dedicated communication. Fig. 6a illustrates an example of the dedicated communication between nodes of the hub network 120. Let that such communication is arranged using the 50GHz grid (Grid2). A dedicated path DP30 is formed between the HUB node and the N40 node. A dedicated path DP40 is arranged between the HUB node and the N50 node, without dropping any channels at the intermediate node N60. Node N60 provides bypassing of all channels of Grid 2, i.e. does not drop any of such channels and does not block Grid2. The HUB node automatically terminates all optical channels arriving to it. Nodes N40 and N50 should terminate (block) Grid2 to terminate the dedicated communication. As a result, the optical link between nodes N50 and N40 is not occupied by Grid2, so the dedicated communication is skipped at this network section.

As one can see, arrangement of the dedicated node-to node communication becomes quite flexible in the Hub network.

Fig. 6b illustrates an example of the combined traffic in a hub network 120, where two grids are used (for example, Grid 1= lOOgHZ and Grid2= 50GHz). In the left part of the network (HUB-N60-N50), Grid 1 shown as a waved line, is used for conventional communication and Grid 2 shown as a dashed line is used for dedicated communication. At node N50 the grids are switched and now Grid 2 (dashed line) is used for conventional communication and Grid l (waved line) is used for dedicated communication. Switch of the Grids can be performed a) by blocking both of the grids at the quasi-OADM node such as node N50, and by further adding new optical channels newly distributed between the suitable grids or b) by redirecting both of the grids at the quasi-OADM node such as node N50 (full blocking may not be required, as shown for the dashed line which continues via N50), and by further adding new optical channels newly distributed between the suitable grids. The new optical channels can be added in the colorless manner, for example to the interleaver, or to the output of the interleaver (see Fig. 9d)

Fig. 7 illustrates one embodiment 130 of the proposed OADM node, suitable for arranging dedicated node-to-node communication in an optical network. The illustrated OADM node 130 comprises a conventional OADM core 132, both the colored DDU 134 and the colorless DAU 136. It is universal for creating dedicated communication paths in network sections similar to those shown in Figs 5a and 5b. A controller (not shown) of the node 130 may be adapted to control the tunable filter/selection of filters in the DDU and to control switches (not shown) on the DAU locally. However, the controller may be interconnected with a network management system (not shown) for obtaining information about channels which should be dropped via filters of the DDU 134, and/or about channels which should and can be added via the DAU 136.

Fig. 8 illustrates a schematic block-diagram of embodiment 140 of the proposed quasi-OADM node. The node 140 comprises an interleaver 142 as a core, its two input ports P I and P2 are associated with two Grids (Gridl and Grid2) used in the network. In short, port P I lets in (passes) channels of Grid 1 and blocks channels of Grid2; port P2 behaves in the opposite manner. The interleaver can be reconfigured. The optical switch 144 is schematically shown as two controllable l l switches. Input of the switch is connected to the input line of the quasi-OADM node, via a colored drop unit DU 146. Add unit of the quasi-OADM node can be distributed between two input ports, in the form of an add unit (AU) 143 at Grid 1 and an add unit AU 145 at Grid 2. Alternatively or in addition, the add unit of the node may comprise a mixed add unit AU 147 connected to the output (port P3) of the interleaver 142 before merging the traffic to the output line of the node 140. The drop unit 146, the optical switch 144 and the add unit (143, 145, 147)can be controlled either locally, or via a centralized management entity such as NMS.

Figs. 9a-9d illustrate possible configurations and suitable functions of the quasi-OADM node shown in Fig. 8, at various states of the optical switch 144. Actually, and for the purpose of generalization, the configuring operations can be provided without the switch 144, for example manually. Let Grid 1 is used for the conventional communication and Grid 2 for the dedicated communication.

Fig. 9a illustrates a blocking function of the node with respect to channels of Grid 2 arrived from the input line. It means that if the dedicated communication was carried by Grid 2 channels, a dedicated communication path, if existed, has terminated at the illustrated node. However, another dedicated path may be initiated at the node, for example by adding any channel of Grid 2 to port P2 (shown by a dashed line). In Fig. 6a, Grid l does not allow reuse, while channels of Grid 2 (being blocked), can be reused. Fig. 9b illustrates a function "drop and continue" for channels of the Grid 1 and Grid 2. If no channels of Grid 2 are dropped at the node, the function can be called "bypassing of all dedicated channels". Even if some of them are dropped, they will also be passed through, so the function will be "drop and continue". There is no reuse of channels in the bypassing node.

Colorless adding of channels always exists (if required and allowed). Channels of Grid2 for the dedicated communication can be added via AU145 or via AU 147.

Fig. 9c illustrates the blocking function for basic. (Grid 1 ) channels. Colorless adding of new channels is always possible: new channels of Grid 1 - at the port PI , new channels of Grid 2 - at the port P2, and/or mixed adding of channels - at the output of the interleaver.

Fig. 9d illustrates a grid switching function which is performed firstly by blocking all incoming channels (of both grids), and then by transmitting the necessary information using a newly selected grid. In this case, optical channels of Grid 1 can be selected to perform dedicated node-to-node communication, and channels of Grid 2 - to perform the conventional communication in the network. The information necessary for the dedicated communication will be loaded onto channels of Grid 1 and these channels can be colorlessly added to port P I ( or to port P3). Traffic of the conventional communication will be loaded on Grid2 channels which can be colorlessly added to port P2 (or to port P3).

In Fig. 9c, the optical channels of the blocked Grid 1 may optionally be reused. For Grid 2 passing through the interleaver I, there is no reuse of channels.

Fig. 10a illustrates an option of central inband control of the interleaver I. Control signal is transmitted along the same optical fiber OF where both groups of the optical channels (Grid 1 and Grid 2) are transmitted. The control signal may control the tunable filter (λ filter) which in turn may selectively control (the dashed line) inputting of Gridl and/or Grid 2 to the interleaver I.

Fig. 10b illustrates an embodiment of local monitoring and control which may be applied alternatively or in addition to the embodiment shown in Fig. 10a. Optical channels of different groups ( say, of Grid 1 and Grid2) arriving from the optical fiber OF, are monitored by a local monitoring/control block, for example from the point of BER (bit error rate), inter-channel interference, chromatic dispersion, etc. Based on results of the monitoring and upon processing in the local block, the grids entering the interleaver may be controlled (the dashed line), and one or more of the wavelengths of interest may be added (or blocked for adding) by controlling a block of tunable lasers from the local monitoring and control block.

For example, due to effects of inter-channel interference, one or more optical channels may be re-allocated to different wavelengths or even to a different grid as a result of monitoring the incoming channels and controlling the tunable lasers.

It should be appreciated that we have described only specific examples for illustrating technology of arranging node-to node dedicated communication in an optical network. It should be kept in mind that other versions of the method and embodiments of the network portions are possible and that they are to be considered part of the invention whenever defined by the generalized claims which follow.

Claims

Claims:

1. A method for arranging a dedicated communication path between a first node and a second node in an optical network, the method comprises:

ensuring that one or more specific optical channels can be dropped while are not presently dropped at the second node;

ensuring that said one or more specific optical channels are not in use in a specific direction along a network section between the first node and a second node; at the first node, adding said one or more specific optical channels in a colorless manner to the network section;

dropping, at the second node, said one or more specific optical channels in a colored manner, thereby establishing the dedicated communication path.

2. A method according to Claim 1 , for arranging the dedicated communication path in the optical network comprising a hub node, the method comprises:

ensuring that said network section includes the hub node and that one or more through passages exist in the hub node for said one or more respective optical channels.

3. The method according to Claim 2, comprising controllably providing said through passages/connections in the hub node for dedicated communication in the hub optical network.

4. The method according to Claim 2 or 3, further comprising ensuring absence of through passages in the hub for optical channels which are not in use and cannot be dropped at any of the nodes in the hub network.

5. The method according to Claim 1, for arranging a dedicated communication path in the optical network utilizing a first grid of optical channels, the method comprises:

assigning for the dedicated communication path optical channels belonging to a second optical grid being different from the first grid;

ensuring that one or more specific optical channels of the second grid can be dropped while are not presently dropped at the second node; ensuring that said one or more specific optical channels of the second grid are not in use in a specific direction along a network section between the first node and a second node;

6. The method according to Claim 5, wherein different network sections, intended for dedicated communication between different nodes, utilize different quoted portions of the second grid.

7. A network section in an optical network, said network section starting at a first node and terminating at a second node of the optical network;

the first node comprising a colorless add unit provided for node-to node dedicated communication and intended for adding one or more optical channels to said network section, wherein any specific channel of said one or more optical channels to be added in a specific direction are not in use in said section of the network before adding thereof;

the second node comprising a colored drop unit being operative to drop at least one of said one or more optical channels whenever added by the colorless add unit, said at least one of the channels added at the first node and dropped at the second node forming a dedicated node-to-node communication path in the network section between the first node and the second node.

8. The network section according to Claim 7, in the optical network comprising a hub, said network section starting at a first node and terminating at a second node of the hub networks; wherein

9. The network section according to Claim 8, wherein said hub is capable of controllably forming the through passage via the hub for said one or more optical channels.

10. The network section according to any one of Claims 7 to 9, being the first section, fulfilled by a second, alternative section built between the same two nodes for protecting the traffic passing via the first section.

11. The network section according to Claim 7, in the optical network utilizing a first grid of optical channels,

said at least one of the optical channels added at the first node and dropped at the second node forming a dedicated node-to-node communication path in the network section between the first node and the second node, wherein said optical channels belong to a second, alternative grid differing from the first grid.

12. A quasi-OADM node comprising a drop unit and a dedicated colorless add unit.

13. The quasi-OADM node according to Claim 12, formed from the drop unit being a dedicated colored drop, and said dedicated colorless add unit.

14. The quasi-OADM node according to Claim 12 or 13, comprising an output optical amplifier capable of compensating insertion loss.

15. The quasi OADM node according to Claim 12, in a network utilizing a first grid of optical channels, comprising either a core of a regular OADM or a core based on an optical interleaver, and additionally provided with a dedicated colorless add unit and a dedicated colored drop unit, said dedicated units serving for adding and dropping optical channels to be utilized for node-to node communication, wherein said optical channels to be utilized for node-to node communication belonging to a second, alternative grid differing from the first grid.

16. The node according to Claim 15, wherein the core comprises an optical interleaver configurable to selectively handle two grids of optical channels being the first grid and the second grid, and wherein

the dedicated colored drop unit is part of a drop unit capable of dropping one or more optical channels of any of the two grids, and

the dedicated colorless add unit is part of a distributed add unit capable of colorlessly adding one or more optical channels of any of the two grids.

17. The network node according to Claim 15 or 16, being capable of receiving optical channels of said two grids and adapted to the following operations: to drop any desired channels of one or both of the grids, to apply the optical channels of said two grids to the optical interleaver, to pass optical channels of none, one or both of the grids through the interleaver, while blocking optical channels of both, the other, or none of the grids, to add required optical channels of one or both of the grids colorlessly to the node.

18. The network node according to any one of Claims 15 to 17, wherein adding and/or dropping of optical channels of the two grids are selectively controllable by at least one of control entities, being a central control entity and a local monitoring and control unit.

19. A kit comprising a HUB node and a quasi-OADM node according to any one of claims 12 to 18.

20. An optical network adapted for providing inter-node dedicated communication, comprising one or more network sections according to Claim 7.

21. The optical network according to Claim 20, being a hub network.

22. The optical network for inter-node dedicated communication according to Claim 20 or 21 , comprising interconnected network nodes, wherein each of said nodes is provided with a dedicated colorless add unit and/or a dedicated drop unit for channels selected for dedicated inter-node communication, said channels belonging to a grid being alternative to the grid utilized for conventional communication, so that the network is adapted for flexibly and selectively establishing dedicated node-to-node communication in addition to conventional communication in the network.