WO2013149660A1

WO2013149660A1 - Apparatus and Method For Switching Information Transported Using a Hierarchical Data Structure

Info

Publication number: WO2013149660A1
Application number: PCT/EP2012/056214
Authority: WO
Inventors: Francesco Testa; Roberto Sabella; Mauro Rudi Casanova; Marco Listanti; Vincenzo ERAMO
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2012-04-04
Filing date: 2012-04-04
Publication date: 2013-10-10
Also published as: EP2834990A1; CN104365114A

Abstract

A switching apparatus is provided for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows, The switching apparatus comprises an input stage (210) comprising at least one input switching unit (21 1₁ to 21 1 _H), and an output stage (230) comprising at least one output switching unit (2311 to 231 H). A central stage (220) is coupled between the input stage (210) and the output stage (230), the central stage (220) comprising a central switching unit (221 ). The at least one input switching unit (21 1₁ to 21 1_H) and the at least one output switching unit (231₁ to 231_H) are configured to switch first order data flows contained within second order data flows. The central switching unit (221 ) is configured to switch only second order data flows, wherein the second order data flows have a higher order than the first order data flows.

Description

APPARATUS AND METHOD FOR SWITCH ING INFORMATION

TRANSPORTED USING A HIERARCHICAL DATA STRUCTURE Technical Field

The present invention relates to an apparatus and method for switching information transported using a hierarchical data structure, for example optical channel data unit flows, ODU flows. Background

Existing packet-optical transport platforms (POTPs) are based on integrating packet, optical transport network (OTN), and wavelength division multiplexing (WDM) switching capabilities in the sa m e n ode. Different types of node architectures are known in the prior art. Most node architectures use all-optical switches, for exam ple based on wavelength selective switching (WSS) technology. Such nodes achieve wavelength routing of bypass traffic, and OTN or packet (or both) to process local traffic which is being added/dropped at the node, or local traffic which is being aggregated/disaggregated. Some POTP architectures realize OTN switching using a packet switching fabric as an "agnostic switch". Other recent architectures are based on electrical conversion of incom ing traffic (from both l ine and cl ient interfaces) and subsequent OTN processing and switching. The latter is usually regarded as the digital photonics or the digital optical transport approach, in the sense that all the traffic is converted into electrical format in each node and then digitally processed, as carried out in other digital transport networks such as packet or circuit based networks.

When the digital photonics approach is used, this enables WDM and OTN switching layers to be integrated within the same node. Since WDM and OTN switching layers share transceivers and cross-point switches, this enables cost and energy consumption to be lowered. Exam ples of such an integrated approach are shown in a paper by Kish et al. entitled "Current Status of Large- Scale InP Photonic Integrated Circuits", IEEE Journal of Selected Topics in Quantum Electronics, April 201 1 , and in US Patent 7782843B2 by Zou et al. These prior art solutions are based on strictly non-blocking high-capacity OTN switches that are able to switch ODU sub-flows ranging from low order ODU flows (for example ODU0) to high order ODU flows (for example ODU3, ODU4). I n these cases the OTN switches (or cross-connect switches) are fully accessible, in the sense that each sub-flow being received from any input port can be routed to any output port at any ODU level. This means that the architecture of the OTN switches are quite complex, include large switch fabrics, and thus are quite costly and power consuming.

The main problems and disadvantages relating to current solutions are discussed below, grouping current product families in three main categories:

1 . POTP incorporating transparent wavelength switching, OTN, and packet switches. These solutions illustrate all the advantages and disadvantages associated with a transparent optical transport layer. In particular, the main disadvantages are high cost, low flexibility (constraints in the ROADM section), and low manageability in terms of signal quality monitoring. Examples of such solutions are disclosed in a paper by Eilenberger entitled "Integrated Electrical/Optical Switching for Future Energy Efficient Packet Networks", paper OTuP1 , conference OFC201 1 , and in a paper by Zhang entitled "Novel Multi-granularity Optical Switching Node with Wavelength Management Pool Resources", Communications and Photonics Conference and Exhibition (ACP), 2009 Asia.

2. POTP incorporating packet and OTN switching in the same agnostic switching platform, and transparent wavelength switching. The advantages and disadvantages are similar to the ones above, but the same switching platform is used for both OTN and packet switching, at the expense of higher power consumption and less optimized costs. These higher costs are sometimes justified because of the flexibil ity of the platform to be used in m ultiple applications (regardless of whether or not OTN switching is needed). An example is discussed in a paper by Belotti et al. entitled "Transport Networks at a Crossroads, the roles of MPLS and OTN in packet transport networks", paper OTuG1 , conference OFC201 1 .

3. D ig ita l reconfi g u rab le add/d rop m u lt i p l exers ( ROAD M s) incorporating OTN and WDM switching based on Optical-Electronic-Optical (OEO) technology, including opto-electronic conversion in each node for all channels, as shown in the paper mentioned above by Kish et al, and in US Patent 7782843B2. The main advantage is full flexibility, manageability, and relaxed transmission requirements, due to the digital optical transport layer. The main drawback, however, is that all incoming traffic has to be OTN processed independently, regardless of whether or not there is any need for that. Such a solution therefore has the disadvantage of consuming more power compared to the solutions mentioned above. The first two categories are based on transparent optical transport, whose switches make use of all-optical costly devices. The main disadvantage with such switches is the need to manage an analogue optical layer with all of the problems which are associated with that. In addition, such switches do not allow any real hardware integration between completely different technologies. Thus, to realize low-cost solutions for these categories, it is necessary to scale down the flexibility features (e.g. colorless, directionless, contentionless, dynamic control, easy management etc.).

Figure 1 shows a known architecture for realizing the third category above, whereby OTN and WDM switching are integrated based on Optical-Electrical- Optical (OEO) technology. Wavelength divisional multiplexed (WDM) input signals 1011 to 101 _N are received by demultiplexing units 103i to 103_N. The WDM input signals 1011 to 101 _N are received from respective optical fibers, not shown. The demultiplexed WDM signals are converted by optical-to-electrical (OE) converters 105i to 105_w into electrical data flows, corresponding to optical channel data unit flows (ODU flows) 107i to 1 07_w. The electrically converted high order ODU flows are demultiplexed into low order ODU flows and subsequently switched by input switching units 106. The switching units 106 receive at their inputs K high order ODU flows 107i to 107_K, electrically demultiplex and OTN switch them to output a number M of low order ODU flows 108i to 108 . For example, M= 4 x K for ODU1 and ODU2 flows. The M low order ODU flows 108i to 108_M are sent directly to a space switch 109, such as a cross connect switch. Since the space switch 109 can receive different order ODU flows from different switching units, or high order ODU flows which have bypassed switching units, the space switch 109 is configured to switch any order data flow from any input port to any output port. Low order ODU flows 1 10i to 1 10M at the output of the space switch 109 are sent to output switching units 1012, whereby the low order ODU flows 1 10i to 1 10_M are OTN switched and multiplexed into high order ODU flows 1 to 1 1 1 K- Electrical-to-optical (EO) converters 1 13i to 1 13_w convert the high order ODU flows into optical data flows. One or more multiplexers 1 15i to 1 15_N are configured to multiplex a group of optical data flows into respective output WDM signals 1 17i to 1 17_N. Each WDM signal 1 17i to 1 17_N is coupled to a respective optical fiber for onward transmission. The switching architecture of Figure 1 has the disadvantage of requiring a complex, fully accessible and strictly-non blocking switching fabric 109, having a high number of ports. This is because, as mentioned above, the switching fabric 109 must be fully accessible in the sense that each sub-flow, being received from any input port, can be routed to any output port at any ODU level. For example, the switching fabric needs to be very large to deal with ODU2 data flows at 10 Gbps, which means that the switch is under utilized when switching only ODU 1 (2.5 Gbps) or ODU0 (1 Gbps) data flows. This means that the architecture of the switching fabric 109 is costly and consumes a relatively large amount of power. Summary

It is an aim of the present invention to provide a method and apparatus which obviate or reduce at least one or more of the disadvantages mentioned above.

According to a first aspect of the present invention, there is provided a switching apparatus for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows. The switching apparatus comprises an input stage comprising at least one input switching unit, an output stage comprising at least one output switching unit, and a central stage coupled between the input stage and the output stage, the central stage comprising a central switching unit. The at least one input switching unit and the at least one output switching unit are

configured to switch first order data flows contained within second order data flows. The central switching unit is configured to switch only second order data flows, wherein the second order data flows have a higher order than the first order data flows.

According to a second aspect of the present invention, there is provided a method in a switching apparatus for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows. The method comprises the steps of switching first order data flows contained within second order data flows in an input stage of the switching apparatus, the input stage comprising at least one input switching unit. Only second order data flows are switched in a central stage of the switching apparatus, the central stage comprising a central switching unit. The method also comprises the step of switching first order data flows contained within second order data flows in an output stage of the switching apparatus, the output stage comprising at least one output switching unit.

According to another aspect of the invention, there is provided a control apparatus for controlling a switching apparatus as claimed in the appended claims. The control apparatus comprises an input interface for receiving second order data flows and first order data flows. A processing unit is adapted to organise the first order data flows within second order data flows to reduce the likelihood of blocking in the switching apparatus. The control apparatus also comprises an output interface for outputting second order data flows having organised first order data flows therein.

Brief description of the drawings For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Figure 1 shows a switching apparatus according to the prior art;

Figure 2 shows a switching apparatus according to an embodiment of the present invention;

Figure 3 shows a method performed in a switching apparatus, according to an embodiment of the present invention;

Figure 4 shows a switching apparatus according to another embodiment of the present invention; and Figure 5 shows a control apparatus according to another embodiment of the present invention. Detailed description

The embodiments of the invention will be described below in relation to apparatus and methods for switching information transported using a hierarchical data structure, such as optical channel data unit (ODU) data flows. It is noted, however, that the embodiments of the invention are intended to embrace other forms of data flows having a hierarchical structure.

Figure 2 shows a switching apparatus according to an embodiment of the present invention, for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows. For example, the hierarchical data structure may comprise optical channel data unit (ODU) data flows, such as ODU0, ODU1 , ODU2,

ODU3, ODU4 ODUX data flows. The switching apparatus comprises an input stage 210 comprising at least one input switching unit 21 11 to 21 1 _H- The switching apparatus also comprises an output stage 230 comprising at least one output switching unit 2311 to 231 _H. The switching apparatus comprises a central stage 220 coupled between the input stage 210 and the output stage 230, the central stage 220 comprising a central switching unit 221 .

At least one input switching unit 21 11 to 21 1 H and at least one output switching unit 2311 to 231 _H are configured to switch first order data flows contained within second order data flows, wherein the second order data flows have a higher order than the first order data flows. The central switching unit 221 is configured to switch only second order data flows.

Since the central switching unit is configured to switch only second order data flows, this has the advantage of enabling the complexity of the switching apparatus to be simplified, thereby saving cost and power consumption. According to one embodiment an input switching unit 21 1 of Figure 2

demultiplexes K second (or high) order ODU flows 212i to 212_K received at their input into first (or low) order ODU flows, OTN switches the first (low) order ODU flows and then multiplexes the first (low) order ODU flows into second (high) order ODU flows again before outputting K second (high) order ODU flows 213i to 213_K. In this way first order data flows contained within second order data flows can be switched by an input switching unit. In a similar manner, an output switching unit 231 of Figure 2 demultiplexes the K second (or high) order ODU flows 232i to 232_K received at their input into first (or low) order ODU flows, OTN switches the first (low) order ODU flows and then multiplexes the first (low) order ODU flows into second (high) order ODU flows again before outputting K second (high) order ODU flows 233 to 233_K.

The first order data flows may comprise lower order ODU sub-flows such as ODU0, ODU1 , ODU2, for example, while the second order data flows may comprise higher order ODU flows such as ODU3, ODU4, for example. It is noted that the first order data flows and second order data flows may

respectively comprise any combination of ODU flows (any combination of

ODU0, ODU1 , ODU2, ODU3, ODU4, ODUX), provided that the second order data flow has a higher order data flow than a data flow of the first order data flow.

The central switching unit 221 may comprise a space switch matrix, for example, such as a cross-point switch having input/output ports, and later stages having OTN switches of different granularities (ODU0... ODU4).

An input switching unit 21 11 to 21 1 _H is coupled to receive a sub-set "K" of second order data flows 212i to 212_k and output a respective sub-set K of second order data flows 213i to 213_k. The central switching unit 221 is coupled to receive a plurality of second order data flows comprising at least the sub-set of second order data flows 213i to 213_k from each of the plurality of input switching units 21 11 to 21 1 _k, and coupled to output a plurality of second order data flows 232i to 232_k. An output switching unit 2311 to 231 _H is coupled to receive a sub-set of second order data flows 232i to 232_k from the central switching unit 221 , and output a corresponding sub-set of second order data flows 233 to 233_k.

The embodiment of Figure 2 therefore uses OTN switches in the lateral stages (the input and output stages 21 0, 230), which are able to switch sub-flows (lower hierarchies of the incom ing flow), while the interconnection between lateral stages and central stages is implemented using higher-level OTN flows. In this way the complexity of the interconnections between lateral stages and central stage of the fabric can be reduced, given a certain total capacity of the switch. Further details of a switching apparatus according to another embodiment of the invention will be provided later in connection with Figure 4 of the application.

Figure 3 shows a method according to another embodiment of the present invention. The method is performed in a switching apparatus for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows. For example, the hierarchical data structure may comprise optical channel data unit (ODU) data flows. The first order data flows may comprise lower order ODU sub-flows (for example ODU0, ODU1 , ODU2), while the second order data flows may comprise higher order ODU flows (for example ODU3, ODU4). It is noted that the first order data flows and second order data flows may comprise any combination of ODU flows (any combination of ODU0, ODU1 , ODU2, ODU3, ODU4, ODUX) provided that the second order data flow has a higher order data flow than a data flow of the first order data flow. The method comprises the steps of switching first order data flows contained within second order data flows in an input stage of the switching apparatus, the input stage comprising at least one input switching unit (for example at least one input switching unit 21 11 to 21 1 _H of Figure 2), step 301 . In step 303, only second order data flows are switched in a central stage of the switching apparatus, the central stage comprising a central switching unit (for example the central switching unit 221 of Figure 2). In step 305 first order data flows contained within second order data flows are switched in an output stage of the switching apparatus, the output stage comprising at least one output switching unit (for example at least one output switching unit 2311 to 231 _H of Figure 2).

In one embodiment, the method further comprises the steps of partitioning the second order data flows into at least one sub-set of second order data flows (212i to 212_k of Figure 2) for switching by respective ones of the at least one input switching unit 21 11 to 21 1 _H, prior to being switched by the central switching unit, and partitioning the second order data flows received from the central switching unit into at least one sub-set of second order data flows 232i to 232_k for switching by the at least one output switching unit 2311 to 231 _H.

Figure 4 shows a switching apparatus according to another embodiment of the present invention for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows. As with Figure 2, the hierarchical data structure may comprise optical channel data unit (ODU) data flows as described above.

The switching apparatus comprises an input stage 210 comprising at least one input switching unit 21 11 to 21 1 _H- The switching apparatus also comprises an output stage 230 comprising at least one output switching unit 2311 to 231 H. The switching apparatus comprises a central stage 220 coupled between the input stage 210 and the output stage 230, the central stage 220 comprising a central switching unit 221 .

At least one input switching unit 21 1 -i to 21 1 H and at least one output switching unit 2311 to 231 _H are configured to switch first order data flows contained within second order data flows, wherein the second order data flows have a higher order than the first order data flows. The central switching unit 221 is configured to switch only second order data flows. An input switching unit 21 1 of Figure 4 demultiplexes K second (or high) order ODU flows 212i to 212_K received at their input into first (or low) order ODU flows, OTN switches the first (low) order ODU flows and then multiplexes the first (low) order ODU flows into second (high) order ODU flows again before outputting K second (high) order ODU flows 213i to 213_K. In this way first order data flows contained within second order data flows can be switched by an input switching unit 21 1 . In a similar manner, an output switching unit 231 of Figure 4 demultiplexes the K second (or high) order ODU flows 232i to 232_K received at their input into first (or low) order ODU flows, OTN switches the first (low) order ODU flows and then multiplexes the first (low) order ODU flows into second (high) order ODU flows again before outputting K second (high) order ODU flows 233₁ to 233_K.

For example, the first order data flows may comprise lower order ODU sub- flows (for example ODU0, ODU1 , ODU2), while the second order data flows may comprise higher order ODU flows (for example ODU3, ODU4). It will be appreciated that the first order data flows and second order data flows may comprise any combination of ODU flows (any combination of ODU0, ODU1 ,

ODU2, ODU3, ODU4, ODUX), provided that the second order data flow has a higher order data flow than a data flow of the first order data flow. The central switching unit 221 may comprise a space switch matrix, for example, such as a cross-point switch having input/output ports, and later stages having OTN switches of different granularities (ODU0... ODU4), for example lateral stages.

The input stage 210 further comprises a first demultiplexer unit 214i coupled to receive a multiplexed input signal 215i, for example a wavelength division multiplexed (WDM) input signal. The first demultiplexer unit 214i is configured to demultiplex the multiplexed input signal 215i into a first input group W of second order data flows 212i to 212_W, the first input group of second order data flows forming at least the second order data flows received by a first group of at least one input switching units 21 11 to 21 1 _H- For a WDM input signal optical-to- electrical converters 236i to 236_w are provided for converting the demultiplexed optical signals into electrical signals for switching by various switching stages.

The output stage 230 further comprises a multiplexing unit 234i coupled to receive a first output group W of second order data flows 233i to 233_w, the first output group of second order data flows 233i to 233_w comprising at least the second order data flows received from a first group of at least one output switching unit 2311 to 231 _H. The multiplexing unit 234i is configured to multiplex the first output group of second order data flows 233i to 233_w into a multiplexed output signal 235i, for example a wavelength division multiplexed output signal. For a WDM output signal electrical-to-optical converters 237i to 237_w are provided for converting the electrical signals into optical signals prior to multiplexing.

The switching apparatus comprises at least a second demultiplexing unit 214_N coupled to receive a second multiplexed input signal 215_N, for example WDM input signal. The second demultiplexing unit 214_N is configured to demultiplex the second multiplexed input signal 215_N into a second input group W of second order data flows 212i to 212_W, the second input group of second order data flows forming at least the second order data flows received by a second group of at least one input switching unit 21 11 to 21 1 H. For a WDM input signal optical- to-electrical converters 236i to 236_w are provided for converting the

demultiplexed optical signals into electrical signals for switching by various switching stages.

At least a second multiplexing unit 234_N is coupled to receive a second output group of second order data flows 233i to 233_w, the second output group of second order data flows 233i to 233_w comprising at least the second order data flows received from a second group of at least one output switching unit 2311 to 231 H. The at least second multiplexing unit 234_N is configured to multiplex the second output group of second order data flows 233i to 233_w into a second multiplexed output signal 235_N, for example a WDN output signal. For a WDM output signal electrical-to-optical converters 237i to 237_w are provided for converting the electrical signals into optical signals prior to multiplexing.

The embodiment of Figure 4 includes N input/output lines (fiber directions), and W WDM channels transmitted in each fiber. The number of directions varies, for example, from 2 to 8 (and beyond in some cases). The overall architecture of the WDM/OTN switch is a three stage switch, in which the lateral stages (input and output stages) consist of OTN switch elements while the central stage is a high capacity cross-point switch that performs a sim ple space switching function. At the input of the switching apparatus, the WDM demultiplexers 214i to 214_N (one per each direction, not shown) are used to separate the different optical channels carrying OTN flows of high order (second order data flows) that are then converted into electrical signals by optical-to-electrical converters 236i to 236_w. The group of OTN electrical signals 212 are processed by the input switching units 21 11 to 21 1 H, for example OTN switching elements. The input switching units 21 11 to 21 1 _H are shown in the embodiment of Figure 4 with a dimension K x K. The input switching units 21 11 to 21 1 _H receive and output second order data flows (high order ODU flows). The input switching units 21 11 to 21 1 H (for example OTN K x K switch elements) are adapted to perform sub-flow switching of low order ODU data flows (for example ODU2/1/0/flex) channels from the flows of high order ODU flows 212i to 212_K received at the input of each switch. Ή" is the number of input switching units shown in the example of Figure 4 for each input/output line 215. As can be seen, not all of the second (high) order ODU channels after the WDM demultiplexer 214 have to be necessarily processed by the input switching units 21 11 to 21 1 H- For example, there may be W-HK second (high) order flows that do not need OTN switching. Only if H=W/K do all the high order flows need OTN switching.

It is noted that after sub-flow switching within the input switching units 21 11 to 21 1 H, the low (first) order ODUs are aggregated into second (high) order ODU flows being output from the various K ports of the respective input switching units 21 11 to 21 1 _H- The aggregated high order ODU flow is then routed by the central switching unit 221 (for example crosspoint switch) towards one of the input ports of the output switching units 2311 to 231 _H. In the output switching units 2311 to 231 _H new sub-flow switching of low (first) order ODU channels is performed. After switching the sub-flows are aggregated again into the second (high) order ODU flows that are output from the output switching units 2311 to 231 H, and converted by electrical-to-optical converters 237i to 237_w into WDM channels. The group of WDM channels transporting the sub-flows are multiplexed by respective multiplexers 234 into the output fibers for transport along the optical network.

A second order data flow may comprise an ODU2 flow at 10 Gbps, for example, with the first order data flows comprising 4 x ODU1 flows at 2.5 Gbps each, for example. In such an example an input switching unit 21 1 would receive a second order data flow at 10 Gbps, switch first order data flows at 2.5 Gbps within the input switching unit 21 1 , and output a second order data flow at 10 Gbps. The central switching unit 221 would receive a second order data flow of 10 Gbps at an input port, and switch the second order data flow to one of its output ports. An output switching unit 231 would receive a second order data flow of 10 Gbps, switch first order data flows at 2.5 Gbps within the output switching unit 231 , and output a second order data flow at 10 Gbps. In this way, since the central switching unit 221 is configured to switch only second order data flows, it can be made less complex and more efficient.

It is noted that, among all the WDM channels, some of them may need to be added/dropped, for example because they have to be terminated into client equipment.

The method described above in Figure 3 may further comprise the following steps when used with a switching apparatus as described in Figure 4. The method may comprise the steps of receiving a first multiplexed input signal 215 (for example a wavelength division multiplexed input signal), and demultiplexing the first multiplexed input signal 215 into a first input group of second order data flows 212i to 212_W, the first input group of second order data flows forming at least the second order data flows 212i to 212_k being switched by a first group of at least one input switching unit 21 11 to 21 1 _H.

The method may further comprise the steps of receiving a first output group of second order data flows 233i to 233_w, the second output group of second order data flows 233i to 233_w comprising at least the second order data flows received from a first group of at least one output switching unit 2311 to 231 _H, and multiplexing the second output group of second order data flows 233i to 233_w into a first multiplexed output signal 235 (for example a wavelength division multiplexed input signal).

It will be appreciated that for wavelength division multiplexed signals, the method will comprise optical-to-electrical conversion after the demultiplexing step, and electrical-to-optical conversion prior to the multiplexing step.

By reducing the complexity of the switching apparatus as described in the embodiments above, there is a possibility that a certain blocking probability may be introduced. To compensate for this blocking probability, another aspect of the invention comprises one or more measures for dealing with the blocking probability, or reducing the chance of blocking. These measures may be used alone or together.

According to one embodiment, any of the groups of output switching units 2311 to 231 H can be controlled such that, if an output port of an output switching unit 2311 to 231 _H in that group is blocked, another output port can be used from the same output switching unit, or another of the output switching units in that group of output switching units 2311 to 231 _H.

In this way a low blocking probability is achieved by "pooling" a full group of wavelength division multiplexed signals transmitted in each output fiber (which is the output port of the overall apparatus). The switching function can therefore be set as a point-to-group switch so that, to avoid blocks, any possible wavelength that is available on a given output fiber can be used to avoid blocking. For example, if a sub-flow has to be OTN switched and put on an a given output port of an output switching unit 231 _x, but the specific path towards the relevant output port of the output switching unit 231 _x is blocked, another port of output switching unit 231 _x can be used. Also, if a sub-flow has to be OTN switched and put on an a given output port of an output switching unit 231 _x, but the specific path towards the relevant output port of the output switching unit 231 x is blocked, another port of another output switching unit 231 _Y from the group of output switching units 2311 to 231 _H can be used that is connected to a different wavelength, provided that the wavelength is multiplexed to the same output fiber.

Therefore, the method described above in Figure 3 may further comprise the steps of determining if an output port of an output switching unit 2311 to 231 _H is blocked and, if so, switching a second order data flow to another output port of the same output switching unit, or an output port of another of the output switching units in the first group of output switching units 2311 to 231 _H. Thus, according to another aspect of the invention, the first order data flows contained within second order data flows can be adapted, for example by a control system, such that they are received by the switching apparatus in an organized manner that is better suited for use by the switching apparatus. In other words, a control plane can be adapted to control how the first order data flows are contained within second order data flows, in order to improve (or reduce) the probability of blocking. For example, a path computation engine (PCE) can be controlled to impose certain constraints. In this way the PCE is configured to find the routes for flows and sub-flows taking into account such criteria. According to one embodiment, one or more of the following actions can be taken to reduce the blocking probability:

- A control system, control plane or control apparatus can be configured to separate traffic into first and second types: low order sub-flows that require OTN switching can be multiplexed together in certain high order flows, while sub- flows that do not need OTN switching can be grouped and multiplexed in different high order flows. This has the advantage of optimizing in the high order flows the bandwidth available for OTN switching;

- A control system, control plane or control apparatus can be configured to leave in the high order flows some spare bandwidth available for switching low order flows. Simulation results have shown that, by leaving about 40% spare bandwidth available in high order flows for switching for low order flows, a blocking probability as low as 10^"6 is achievable. Figure 5 describes a control apparatus 500 for controlling a switching apparatus as described in the embodiments above. The control apparatus 500 comprises an input interface 503 for receiving second order data flows and first order data flows. A processing unit 505 is adapted to organise the first order data flows within second order data flows to reduce the likelihood of blocking in the switching apparatus. The control apparatus 500 comprises an output interface 507 for outputting second order data flows 506 having reorganised first order data flows. The second order data flows and first order data flows 504 received by the interface 503 may be received as separate data flows which are then organised by the processing unit 505 as described above, and/or received as second order data flows comprising first order data flows, with the first order data flows being reorganized by the processing unit 505.

In the embodiments described above, it is noted that the number of second order data flows forming a sub-set of data flows for one input switching unit and/or output switching unit can be different to the number of second order data flows forming a sub-set of data flows for another input switching unit and/or output switching unit.

Furthermore, the central switching unit 221 may be coupled to receive and/or output at least one second order data flow which bypasses the plurality of input switching units and/or output switching units.

The embodiments of the invention described above have the advantage of providing an integrated WDM-OTN switch with significantly lower complexity, cost, and power consumption. Although a sub-flow blocking probability may be introduced, this can be successfully reduced using control and management procedures at the control plane.

The provision of fully accessible and strictly non-blocking switching fabrics according to the prior art, having a high number of ports, are not necessary in all network segments. The embodiments of the invention therefore provide a simplified switching apparatus architecture by accepting a chance of blocking probability of sub-flows, which can be taken low enough for use in various applications through an efficient use of the control plane.

The embodiments of the invention described above have shown that although the switch architecture does not guarantee that any sub-flow that needs switching can reach the desired output port of the switch, and as a result a blocking probability exists, such a blocking probability can be drastically decreased. If, when blocking occurs, for a certain sub-flow on a certain output port of the switch, a different output port can be used, in which there is room for the sub-flows to be transported. The alternative output ports to be considered for the switching belong to the same WDM multiplexer that aggregate the optical channels on the same fiber. Each particular sub-flow is transmitted along a certain path inside a certain fiber, and can reach the right apparatus or node. To implement the above switching technique, and help reduce the blocking performances of the WDM/OTN switch, embodiments of the invention include an intelligent control system that is able to smartly aggregated the sub-flows into the higher order flow in such a way to minimize the chance of blocking.

The embodiments of the invention have the advantage of providing tight integration of the Optical-Electrical-Optical (OEO) WDM switching layer and OTN switching layer using the same hardware chassis, to rationalize the hardware and thus lower the footprint, costs, and power consumption.

Therefore, given a certain switch capacity, the complexity, number of ports of the switch fabric and number interconnections is greatly reduced by the embodiments of the invention. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

1 . A switching apparatus for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows, the switching apparatus comprising:

an input stage comprising at least one input switching unit;

an output stage comprising at least one output switching unit; and a central stage coupled between the input stage and the output stage, the central stage comprising a central switching unit;

wherein the at least one input switching unit and the at least one output switching unit are configured to switch first order data flows contained within second order data flows, and wherein the central switching unit is configured to switch only second order data flows, wherein the second order data flows have a higher order than the first order data flows.

2. A switching apparatus as claimed in claim 1 wherein:

an input switching unit is coupled to receive a sub-set of second order data flows and output a respective sub-set of second order data flows;

the central switching unit is coupled to receive a plurality of second order data flows comprising at least the sub-set of second order data flows from each of the plurality of input switching units, and coupled to output a plurality of second order data flows; and

an output switching unit is coupled to receive a sub-set of second order data flows from the central switching unit, and output a corresponding sub-set of second order data flows.

3. A switching apparatus as claimed in 2, further comprising:

a first demultiplexer unit coupled to receive a multiplexed input signal, and configured to demultiplex the multiplexed input signal into a first input group of second order data flows, the first input group of second order data flows forming at least the second order data flows received by a first group of at least one input switching units.

4. A switching apparatus as claimed in claim 2 or 3, further comprising:

a multiplexing unit coupled to receive a first output group of second order data flows, the first output group of second order data flows comprising at least the second order data flows received from a first group of at least one output switching unit;

wherein the multiplexing unit is configured to multiplex the first output group of second order data flows into a multiplexed output signal.

5. A switching apparatus as claimed in claim 4, wherein the first group of output switching units are controlled such that, if an output port of an output switching unit is blocked, another output port is used from the same output switching unit, or another of the output switching units in the first group of output switching units.

6. A switching apparatus as claimed in any one of the preceding claims, wherein first order data flows contained within second order data flows are received in an organized manner, adapted for use by the switching apparatus.

7. A switching apparatus as claimed in any one of the preceding claims, wherein the number of second order data flows forming a sub-set of data flows for one input switching unit and/or output switching unit is different to the number of second order data flows forming a sub-set of data flows for another input switching unit and/or output switching unit.

8. A switching apparatus as claimed in any one of the preceding claims, wherein the central switching unit is coupled to receive and/or output at least one second order data flow which bypasses the plurality of input switching units and/or output switching units.

9. A switching apparatus as claimed in any one of claims 4 to 8, further comprising:

at least a second demultiplexing unit coupled to receive a second multiplexed input signal, and configured to demultiplex the second multiplexed input signal into a second input group of second order data flows, the second input group of second order data flows forming at least the second order data flows received by a second group of at least one input switching unit; and

at least a second multiplexing unit coupled to receive a second output group of second order data flows, the second output group of second order data flows comprising at least the second order data flows received from a second group of at least one output switching unit, wherein the second multiplexing unit is configured to multiplex the second output group of second order data flows into a second multiplexed output signal.

10. A switching apparatus as claimed in any one of the preceding claims, wherein the hierarchical data structure of data flows comprises optical channel data unit flows, ODU flows.

1 1 . A switching apparatus as claimed in claim 10, wherein the ODU flows form part of wavelength division multiplexed, WDM, signal.

12. A switching apparatus as claimed in any one of the preceding claims, wherein an input switching unit or an output switching unit comprises an equal number of input and output ports.

13. A method in a switching apparatus for switching information transported using a hierarchical data structure, the hierarchical data structure comprising at least first and second order data flows, the method comprising the steps of: switching first order data flows contained within second order data flows in an input stage of the switching apparatus, the input stage comprising at least one input switching unit;

switching only second order data flows in a central stage of the switching apparatus, the central stage comprising a central switching unit; and

switching first order data flows contained within second order data flows in an output stage of the switching apparatus, the output stage comprising at least one output switching unit.

14. A method as claimed in claim 13, further comprising the steps of:

partitioning the second order data flows into at least one sub-set of second order data flows for switching by respective ones of the at least one input switching unit, prior to being switched by the central switching unit; and partitioning the second order data flows received from the central switching unit into at least one sub-set of second order data flows for switching by the at least one output switching unit.

15. A method as claimed in claim 14, further comprising the steps of:

receiving a first multiplexed input signal; and

demultiplexing the first multiplexed input signal into a first input group of second order data flows, the first input group of second order data flows forming at least the second order data flows being switched by a first group of at least one input switching unit.

16. A method as claimed in claim 14 or 15, further comprising the steps of:

receiving a first output group of second order data flows, the second output group of second order data flows comprising at least the second order data flows received from a first group of at least one output switching unit; and multiplexing the second output group of second order data flows into a first multiplexed output signal.

17. A method as claimed in claim 16, further comprising the steps of:

determining if an output port of an output switching unit is blocked and, if so;

switching a second order data flow to another output port of the same output switching unit, or an output port of another of the output switching units in the first group of output switching units.

18. A method as claimed in any one of claims 14 to 17, wherein the

hierarchical data structure of data flows comprises optical channel data unit flows, ODU flows.

19. A method as claimed in claim 18, wherein the ODU flows form part of wavelength division multiplexed, WDM, signal.

20. A control apparatus for controlling a switching apparatus as claimed in any one of claims 1 to 1 1 , wherein the control apparatus comprises:

an input interface for receiving second order data flows and first order data flows;

a processing unit adapted to organise the first order data flows within second order data flows to reduce the likelihood of blocking in the switching apparatus; and

an output interface for outputting second order data flows having organised first order data flows therein.

21 . A control apparatus as claimed in claim 20, wherein the processing unit is adapted to organise the data flows into one or more of:

second order data flows comprising first order data flows that require switching;

second order data flows comprising first order data flows that do not require switching; second order data flows having at least some spare capacity for switching first order data flows.