Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0007—Construction
  • H04Q2011/0016—Construction using wavelength multiplexing or demultiplexing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0007—Construction
  • H04Q2011/0024—Construction using space switching
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0007—Construction
  • H04Q2011/0032—Construction using static wavelength routers (e.g. arrayed waveguide grating router [AWGR] )
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0037—Operation
  • H04Q2011/0039—Electrical control
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0037—Operation
  • H04Q2011/005—Arbitration and scheduling
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
  • H04Q11/0005—Switch and router aspects
  • H04Q2011/0052—Interconnection of switches
  • H04Q2011/0056—Clos

Definitions

• One or more aspects of embodiments according to the present invention relate to optoelectronic switches, and more particularly to multi-dimensional optoelectronic switches and switching elements used therein.
• optical switching offers many of the desired properties but optical devices need to be controlled by and interfaced with electronic devices including traditional electronic data servers.
• Optical devices themselves do not necessarily reduce the size or complexity of a switch.
• One way of doing this relates to the topology of the components within the switch network. It is desirable to produce a highly scalable optical switching unit.
• the gross structure of the present invention involves an array of leaf switches (which are client-side switches, to which input devices and output devices can be connected) which includes a number of sub-arrays, each associated with a different dimension, and wherein switching can occur within a given sub-array in the "direction" of that dimension, via a spine switch (which are fabric-side switches, with no external connections).
• leaf switches which are client-side switches, to which input devices and output devices can be connected
• a spine switch which are fabric-side switches, with no external connections.
• the signal will then be switched in two hops: a first hop takes the signal from the source leaf switch to another intermediate leaf switch in the same row, which is in the same column as the destination leaf switch. Then, in a second hop, the signal is switched along the column from the intermediate leaf switch to the destination leaf switch.
• the signal (which may be a packet of data, for example) can be switched from any source leaf switch to any destination leaf switch in a maximum of L hops, each hop confined to a single sub-array.
• an optoelectronic switch such as this to cater for as many external devices as possible.
• Increasing the number of dimensions necessarily requires the inclusion of larger leaf switches, in order to cater for switching in more than one dimension.
• Increasing the sizes R t of the dimensions may mean that the spine switches, which are used to control the switching from e.g. a source leaf switch to an intermediate leaf switch within a given dimension, need to have a large radix in order to accommodate for switching between all of the leaf switches in that sub-array, preferably in a non- blocking fashion and to preserve bisection bandwidth.
• a first aspect of the present invention provides an L-dimensional optoelectronic switch for transferring an optical signal from an input device to an output device, the optoelectronic switch including:
• each leaf switch having C client ports for connecting to an input device or an output device, and F fabric ports for connecting to spine switches;
• each leaf switch in the sub-array is connected to each spine switch via an optical active switch.
• Insertion of optical active switches in between the leaf switches and spine switches means that fewer spine switches can be used to provide switching between the same number of leaf switches.
• the number of leaf switches is greater than the number of spine switches. There may be more leaf switches, in a given sub-array, than the number of fabric ports per spine switch. This can be understood from a consideration of the radix R, which is the same for both the leaf switches and the spine switches.
• the leaf switch requires at least one fabric port for switching in each dimension, and accordingly, there are fewer fabric ports available for switching within any given sub-array.
• the fabric ports on the spine switches are concerned only with switching in the dimension associated with that sub-array.
• L is equal to two or more, and may take any of the following values: 2, 3, 4, 5, 6, 7, 8, 9, 10.
• leaf switches are provided between leaf switches within a given sub-array by spine switches, which may be electronic packet switches, and may be referred to as EPSs or AOBMs. It should also be noted that throughout this application, leaf switches may be referred to as "optical packet processing modules" or "OPPMs".
• EPSs electronic packet switches
• AOBMs optical packet processing modules
• an “active switch” is a switch wherein the path of a signal through that switch, e.g. from which input to which output, can be actively controlled, and varied, in effect providing full-mesh capability without the need for a full mesh of links within the switch.
• the optical active switch may be an optical circuit switch or OCS (the terms being used interchangeably herein. More specifically, each optical active switch may have a plurality of inputs and a plurality of outputs, the number of inputs being the same as the number of outputs.
• the "radix" of the optical active switches refers to the number of inputs or the number of outputs, and not to the total number of inputs and outputs. Accordingly, outputs of a given optical active switch may be connected to a plurality or set of spine switches in the sub-array.
• a given leaf switch may be connected to a cluster of optical active switches, each connected to a set of spine switches. More specifically, each of the cluster of optical active switches to which a given leaf switch is connected may be connected to different, disjoint sets of spine switches (by "disjoint", we mean that no two optical switches in a given cluster are connected to the same spine switch, i.e. the sets of spine switches have no overlap).
• the leaf switch in question is connected to all of the spine switches in the sub-array, but via a smaller radix optical active switch.
• the optical active switches also have a plurality of inputs, these inputs can be shared between a plurality of leaf switches.
• each input of each optical active switch in the cluster may be connected to a respective (i.e. different) leaf switch.
• the leaf switches may be divided into a plurality of clusters, and in particular: in a given sub-array the leaf switches may be divided into a plurality of clusters, the clusters each containing a plurality of leaf switches.
• Each cluster may have its own associated cluster of optical active switches, the optical active switches providing connectivity between each of the cluster of leaf switches and each of the spine switches in the array, via the cluster of optical active switches, in other words each cluster of leaf switches may be associated with a cluster of one or more fabric-entry optical active switches to form a line card assembly, each leaf switch in the cluster may be connected to each fabric-entry optical active switch in the line card assembly, and the fabric- entry optical switches may be arranged so that optical signals pass through said fabric-entry optical active switches during transfer from a leaf switch to a spine switch.
• each spine switch may be connected to a fabric-entry optical active switch in a line card assembly located in that sub-array, and no more than one fabric-entry optical active switch in that line card assembly is connected to a given spine switch.
• a fabric-entry optical active switch in a line card assembly located in that sub-array, and no more than one fabric-entry optical active switch in that line card assembly is connected to a given spine switch.
• all, or substantially all of the optical active switches which are employed in embodiments of the present invention have the same radix, or more specifically the same number of inputs and outputs.
• the same number of spine switches it is possible for the same number of spine switches to support, without prejudice to the bisection bandwidth, a number of leaf switches which is increased by a factor of that radix.
• the sub-arrays may be three times the size. In other words, the opportunities for upscaling the array are greatly increased, without the need to increase the size of the spine switches which are employed.
• each leaf switch in the sub-array may be connected to a second, or "fabric-exit" cluster of optical active switches.
• fabric-entry clusters of optical active switches.
• the fabric exit clusters have a mirror image configuration to the fabric entry clusters.
• the inputs of a given optical active switch in a fabric exit cluster are respectively connected to outputs of a set of spine switches, and the inputs each optical active switch in the cluster are connected to the outputs of different, disjoint sets of spine switches.
• each of the outputs of a given optical active switches in the fabric exit cluster are connected to the fabric ports of each of the leaf switches in the cluster of leaf switches which are associated with that fabric exit cluster, and accordingly each of the fabric ports on a given leaf switch in the cluster is connected to a different optical active switch within the fabric exit cluster associated with that cluster of leaf switches.
• Other clusters of leaf switches may be defined, e.g. there may be clusters containing one leaf switch from each fabric-entry cluster.
• Each cluster of leaf switches may be further associated with a cluster of one or more fabric-exit optical active switches to form a fabric-exit arrangement, each leaf switch in a given cluster may be connected to each fabric-exit optical active switch in the fabric-exit arrangement, and the fabric-exit optical active switches may be arranged so that optical signals pass through said fabric-exit optical active switches during transfer from a spine switch to a leaf switch.
• Each spine switch may be connected to a fabric-exit optical active switch in a fabric-exit arrangement located in that sub-array, and no more than one fabric-exit optical active switch in that fabric-exit arrangement is connected to a given spine switch.
• each leaf switch in a given sub-array may be connected to each spine switch via an optical active switch only in respect of sub-arrays associated with M dimensions, where M ⁇ L.
• the other optional features presented above may apply to any or all of the dimensions in which switching between leaf switches goes via an optical active switch.
• the optoelectronic switch according to the first aspect of the invention may be arranged on an optical backplane. Accordingly, the constituent components (i.e. the leaf switches, the optical active switches, and the spine switches) may be located on cards.
• a card may be a printed circuit board onto which electronic components, optical components and the control components (i.e. arbiters) are formed.
• a card may also house the optical and electronic components therebetween.
• embodiments of the invention may include two types of card: line cards and fabric cards. More specifically, the components may be located on either line cards or fabric cards. Line cards are "client-facing" cards, and fabric cards are “fabric-facing" cards.
• Leaf switches and fabric-entry optical active switches i.e. line card assemblies may be located on respective line cards. More specifically, a single line card includes at least one leaf switch and at least one fabric-entry optical active switch. In some embodiments, a single line card may include a plurality of leaf switches and/or a plurality of fabric-entry optical active switches. In embodiments in which there are clusters of leaf switches, and associated clusters of fabric-entry optical active switches, a cluster of leaf switches and its associated cluster of fabric-entry optical active switches are preferably mounted on the same line card. In N-dimensional embodiments of the present invention, each leaf switch is a member of N sub-arrays, as discussed earlier in the application.
• each cluster of leaf switches and its associated cluster of fabric-entry optical switches may be located on their own line card.
• the spine switches and fabric-exit optical switches may be located on a fabric card.
• the fabric card may also include an arbiter for controlling the path of a signal through a fabric-exit optical active switch located on that fabric card.
• the fabric card may also include a plurality of arbiters, each configured to control the path of a signal through a respective fabric-exit optical active switch on the fabric card. Not all of the spines need to be on the same fabric card.
• the packet processor makes a routing decision based on the destination address of the packet and on the current location.
• the routing decision selects a spine to route to (generally attempting to balance the load across the available spines), which in turn determines a specific output port on both the local leaf switch and the fabric-entry optical active switch.
• the output identifier of the fabric-entry optical active switch is communicated to the arbiter, so that it can determine which of its inputs need to be connected to which of its outputs.
• the routing decision selects the appropriate leaf according to the packet's destination, which in turn determines the local output port on the spine and the fabric-exit optical active switch.
• Arbitration is performed by arbiters, and is the process by which it is determined which path a signal should take through an optical active switch, i.e. from which input to which output, in order to ensure that all of the signals incident upon that optical active switch are directed towards the correct next switching element (which may be a spine switch or a leaf switch, depending on which "stage" the signal is at).
• an optical active switch i.e. from which input to which output, in order to ensure that all of the signals incident upon that optical active switch are directed towards the correct next switching element (which may be a spine switch or a leaf switch, depending on which "stage" the signal is at).
• the line card may include an arbiter for controlling the path of a signal through a fabric-entry optical active switch included in the line card assembly located on that line card, or there may be a plurality of arbiters, each configured to control the path of a signal through a respective fabric-entry optical active switch.
• the optical active switches may have a small radix (e.g. 2, 3, 4, 5, 6, 7 or 8)
• the arbitration process is relatively simple (e.g. as compared to a radix 24 switching element), and so a plurality of arbiters may be combined into a single arbitration component, which may be an ASIC.
• the control performed by the arbiters is constrained within the boundaries of the line card in question. This minimizes latency and synchronization issues associated with the control plane: the distances/flight times on the card can be controlled to far tighter tolerances within the physical dimensions of a single card that across cards that may be located at a substantial distance from each other. Furthermore, by having several arbiters, each associated with a small number (e.g. one) of optical active switches, a large number of small problems may be solved swiftly and in parallel, as opposed to having to centrally solve a more complex problem relatively slowly.
• a small number e.g. one
• the optical active switches are controlled by an arbiter, configured to control the action of at least one of the optical active switches and spine switches within a given sub-array, based on the destination information stored in the packets of data to be switched. This then allows the provision of a route which ensures that all data reaches the appropriate leaf switch in a non-blocking fashion, and which minimizes the occurrence of bottlenecks.
• the packet processors in the leaf switches may each be connected to an arbiter. When a packet of data is received at the
• the transmission side packet processor may send a request to the arbiter to which that packet processor is connected, the request preferably identifying the destination leaf switch, or alternatively identifying the next leaf switch to which the packet of data should be sent (which may be the destination leaf switch).
• the arbiter is then able to establish a scheme which ensures that to the greatest extent possible, each packet is able to perform its next hop.
• the structure of the leaf switches is described in more detail below.
• the arbiters may be connected to other components such as packet processors and optical active switches using dedicated control channels. They may also be connected to a driver chip, which is configured to control the action of the optical active switches.
• a second aspect of the present invention provides a leaf switch which may be used in an
• the leaf switch may include:
• a client port for connecting to an input device or an output device and a client portion connected to the client port; a first fabric portion and a second fabric portion, connected respectively to a first fabric port and a second fabric port for processing signals and communicating with spine switches or optical active switches, the first fabric portion having a transmission side and a receiving side,
• transmission side includes:
• a transmission side input for receiving a first electronic signal carrying information including information about a destination leaf switch of the first electronic signal, the first electronic signal being received from either:
• a modulator for converting said first electronic signal into a first plurality of optical signals containing the same information
• a transmission side multiplexer for converting the first plurality of optical signals into a multiplexed fabric output signal for transmission to an input of an optical active switch
• receiving side includes:
• a receiving side demultiplexer for receiving a multiplexed fabric input signal from an output of an optical active switch, and converting said multiplexed fabric input signal into a second plurality of optical signals
• a photodetector for converting said second plurality of optical signals into a second electronic signal
• a receiving side output for sending the second electronic signal to either:
• the term "port" in the above need not necessarily refer to a physical hole, or socket. Rather, the "client port” and “fabric port” of the leaf switches refer to the regions of the leaf switch which are responsible for preparation or processing of signals which are to be sent to/received from the clients and fabric respectively.
• Leaf switches according to the second aspect of the invention are able to act as intermediate leaf switches, since the second electronic signal may be sent to another fabric port (which may also be referred to herein as a "fabric portion") for retransmission along a different dimension, or to a client port, for transmission to an output device.
• the second fabric port is therefore preferably able to perform the same functions as the first fabric portion, and have equivalent features.
• the second fabric port may be substantially identical to the first.
• Leaf switches according to the second aspect of the invention provide the functionality required to construct the scalable, multi-dimensional optoelectronic switches of the first aspect of the invention. Due to modulation of the first electronic signal, the majority of data transfer is able to take place in the optical domain, rather than the electronic domain. This means that data can be transmitted at high data rates, and over long distances, at a lower power and with lower power loss than would be the case in the electronic domain. Additionally, the use of the optical domain enables wavelength division multiplexing (WDM) to be used. A further important advantage is bit-rate independence, wherein switch plane data operates at packet- and not bit-rate.
• WDM wavelength division multiplexing
• each leaf switch preferably includes a transmission side packet processor, which is configured to receive the first electronic signal in the form of a packet, having a packet header containing the destination information.
• the information included in the packet may include information relating to the destination of that packet, e.g. the client port/destination leaf switch.
• the packet header may include further various pieces of information, including source and destination address, packet length, protocol version, sequence number, payload type, hop count, quality-of-service indicator, and others.
• the transmission side packet processor may be configured to perform packet fragmentation, wherein packets of data having the same destination leaf switch are arranged into frames having a predetermined size, and wherein the packets of data may be split up into a plurality of packet fragments, arranged in a corresponding plurality of frames, and wherein one frame may contain data from one or more packets of data.
• Each packet fragment may have a packet fragment header, including information at least identifying the packet to which that packet fragment originally belonged, so that the packet may be reconstructed after subsequent processing and transmission. For example, consider the case where the packet processor is configured so that the frame payload size is 1000B, and three packets of 400B, 800B and 800B are input into the leaf switch.
• a first frame may include the 400B packet, and 200B of the first 800B packet, and then a second frame may include the second 800B packet and the remaining 200B of the first 800B packet. This leads to an efficiency of 100%.
• the frames that are constructed by this process represent packets of data in their own right, and so further fragmentation may occur at intermediate leaf switches, when the packet undergoes more than one hop in order to reach the destination leaf switch. It should be noted, that even when a signal passes through a spine switch and two optical active switches in order to reach an
• a hop may be defined as a transfer of a signal from one leaf switch to another.
• subsequent processing of a frame may not occur until the filling proportion of a frame reaches a predetermined threshold, preferably more than 80%, more preferably more than 90%, and most preferably 100%.
• the packets may alternatively be sent for subsequent processing after a predetermined amount of time has elapsed. In this way, if packets of data for a given leaf switch cease to arrive at the packet processor, a frame which is still below the threshold filling proportion may still be sent for subsequent processing rather than lying stagnant on the packet processor.
• the predetermined amount of time may be between 50 and 1000ns, but is preferably between 50 and 200ns. Most preferably, the time interval is around approximately 100ns.
• the transmission side packet processor may include or be associated with a
• transmission side memory in which to temporarily store incomplete frames during their
• the elapsed time may be varied depending upon traffic demand; typically, the higher the rate of traffic flow, the shorter will be the elapsed time and lower rates of traffic flow may lead to increased time intervals.
• the receiving side may also include a receiving side packet processor, which is configured to recreate the original packets from the packet fragments, when the original packets are spread over more than one frame. This may be done with reference to the packet fragment headers described above.
• a packet undergoes several separate fragmentations by successive intermediate leaf switches on its journey from source to destination final reassembly of the packet by the receiving side packet processor may be delayed until all of the constituent parts of the original packet have arrived at the destination leaf switch.
• the receiving side packet processor may include or be associated with a receiving side memory in order to temporarily store the constituent parts.
• the transmission side may include a plurality of modulators, preferably optical modulators.
• the optical modulators may be phase or intensity modulators, for example electro-absorption
• Each modulator may be associated with a single light source only or may be lit by fewer light sources, where the light sources are shared between modulators.
• Each modulator may be configured to receive an electronic signal from the input or the transmission side packet processor, and
• the modulator By combining the two, the modulator generates a modulated optical signal having the same wavelength as the unmodulated light from the light source, and carrying the information carried by the original (first) electronic signal. This modulated optical signal may then be transmitted to the transmission side multiplexer.
• the light source is preferably in the form of a laser, in order to generate a substantially monochromatic beam of light restricted to a narrow band of wavelengths.
• the modulators are preferably configured to receive light having a wavelength in the C-band or L-band of the electromagnetic spectrum, i.e. from 1 530 nm to 1625 nm. More preferably, the light has a wavelength falling within the C-band or "erbium window", having a wavelength from 1530 nm to 1565 nm.
• the laser may either be a fixed-wavelength laser or a tunable laser.
• the light source associated with each modulator should have a different wavelength, with non- overlapping bandwidths, in order to minimize crosstalk in the multiplexer.
• the modulator may be in the form of an electro-absorption modulator (EAM), which uses a changing electric voltage to modulate the intensity of the laser light to carry the information contained in an electronic signal.
• EAM electro-absorption modulator
• the transmission side packet processor may also be configured to perform packet slicing, wherein a frame (as constructed by the packet fragmentation process described above) or a packet of data is sliced into a first plurality of electronic signals. Each of the first plurality of electronic signals may then be sent to a different modulator in the plurality of modulators, whereby they are converted into the first plurality of optical signals.
• the photodetector which may be a photodiode, may convert the second plurality of optical signals into a second plurality of electronic signals.
• the receiving side may include a plurality of photodetectors, which may be identical to each other or substantially identical to each other.
• the receiving side packet processor may be configured to recombine the second plurality of electronic signals, which represent packet slices, into the second electronic signal.
• the data can be sent using a number of different wavelengths which are multiplexed into a single optical link by the multiplexer. In this way, several pieces of information can be sent in parallel, and leads to increased bandwidth and more efficient data transfer.
• the packet fragmentation step i.e. the formation of frames of data
• the packet processor recombines the second plurality of electronic signals (i.e. packet slices) into a single second electronic signal before the original packets are reconstructed from the frames.
• frames are constructed which each contain data having intended only for a single leaf switch, by which we mean the next leaf switch in the overall transfer, and not necessarily the final destination leaf switch.
• data is converted into the first plurality of optical signals each differing in wavelength, it is multiplexed, specifically wavelength division multiplexed, by the transmission side multiplexer to form the multiplexed fabric output signal.
• the leaf switch is configured to operate in burst mode, wherein the leaf switch is configured to send the multiplexed fabric output signal a series of successive bursts, each burst including packets and/or packet fragments from a single frame of data, and such that each burst includes only packets and/or packet fragments which are to be sent to a single leaf switch in their next hop.
• Each successive burst may comprise a frame of data to be sent to a different single leaf switch in their next hop. Pairs of sequential bursts may be separated by a predetermined time interval, which may be between 50 and 1000ns, but is preferably between 50 and 200ns. Most preferably, the time interval is around approximately 100ns.
• all of the fabric ports which are connected to the active switch in a single sub-array are configured to operate synchronously, i.e. each fabric port sends a burst to an input of an optical active switch at the same time.
• the optical active switch can route each signal to a given spine switch in one switching action.
• the transmission side packet processor may also be configured to carry out error correction on incoming packets of data. This may be done by means such as error detection and retransmission or forward error correction (FEC). Additionally, the leaf switch may also include a management port, which is configured to perform fabric management processes including initialization, programming routing/forwarding tables, fault reporting, diagnostics, statistics reporting and metering.
• FEC forward error correction
• the route may be deduced entirely from a comparison between the coordinates of the source leaf switch and the destination leaf switch.
• the first hop may match the first coordinate of the source and destination leaf switches
• the second hop may match the second coordinate of the source and destination leaf switches and so on, until all of the coordinates match, i.e. until the packet has been transferred to the destination leaf switch.
• the dimension-ordered route might be: (a, b, c, d) -> (w, b, c, d) -> (w, x, c, d) -> (w, x, y, d) - (w, x, y, z).
• the packet processor may compare the coordinates of the source leaf switch against the coordinates of the destination leaf switch, and determine which coordinates do not yet match. Then it will decide to route along the non-matching directions, e.g. with the lowest index, or the highest index.
• the optical active switches or optical circuit switches of the present invention may be based on an arrangement of Mach-Zehnder interferometers (MZIs) and more specifically may be in the form of an MZI cascade switch.
• An MZI cascade switch includes a plurality of MZIs, each having two arms which split at an input coupler, with two arms feeding the split paths into an output coupler where they are recombined, and two output portions.
• the plurality of MZIs are preferably arranged to provide a pathway from each input to each output of the MZI cascade switch. To the greatest extent possible the arms have the same length. Alternatively, where it is desirable to have a default output, the arms may be unbalanced.
• Each MZI may include an electro-optical region at one or both arms, in which the refractive index depends on a voltage applied to the region via one or more electrodes.
• the phase difference of light travelling through the electro-optical region can therefore be controlled by application of a bias via the electrodes. By adjusting the phase difference, and therefore the resulting interference at the output couple, the light can be switched from one output of the MZI to the other.
• each optical active switch has K t inputs and K t outputs, which may be made up, for example, of a plurality of I x 2 and 2 x I MZIs, arranged to provide a pathway from each input to each output.
• K t inputs and K t outputs which may be made up, for example, of a plurality of I x 2 and 2 x I MZIs, arranged to provide a pathway from each input to each output.
• a MZI cascade switch is beneficial over a full mesh for connecting K t inputs to K t outputs when K t is 5 or more, since a full mesh requires 'AK ⁇ Ki + 1) optical fibres to connect all of the inputs to all of the outputs, whereas an MZI cascade switch requires only 2K optical fibres.
• An additional port may be supported on each cascade switch by building K t + 1) trees on each side and omitting an internal connection so that an input is not connected to the output which is connected to the same switch as itself.
• An MZI cascade switch such as this is largely wavelength-agnostic, and so is able to switch the whole multiplexed fabric output signal from input to output without first requiring any
• the spine switches may be electronic active switches or electronic packet switches, such as electronic crossbar switches.
• the spine switches may be an electronic shared memory switch.
• An electronic shared memory switch is an electronic crossbar switch which also includes a memory. The presence of a memory within the switch is advantageous since it means that the switch can perform not only switching, but also buffering, i.e. storing a queue of packets when a bottleneck arises at the electronic shared memory switch, as described above. This means that the electronics on the packet processors on the leaf switches can be simplified.
• each of the spine switches may contain the same components as each of the leaf switches.
• all of the leaf switches and spine switches may be identical or substantially identical to each other.
• optoelectronic switches according to the present invention may be constructed by assembling a set of identical or substantially identical elements, and then the functionalities of these elements may be controlled using software, for example.
• Spine switches are entirely "fabric-facing", i.e. they do not connect to external devices, and accordingly they have no client ports or client portions.
• a spine switch may include:
• first fabric portion and a second fabric portion connected respectively to a first fabric port and a second fabric port for processing signals and communicating with leaf switches or optical active switches, the first fabric portion having a transmission side and a receiving side,
• transmission side includes:
• a transmission side input for receiving a first electronic signal carrying information including information about a destination leaf switch of the first electronic signal, the first electronic signal being received from an output of the second fabric portion;
• a modulator for converting said first electronic signal into a first plurality of optical signals containing the same information
• a transmission side multiplexer for converting the first plurality of optical signals into a multiplexed fabric output signal for transmission to an input of an optical active switch; wherein the receiving side includes:
• a receiving side demultiplexer for receiving a multiplexed fabric input signal from an output of an optical active switch, and converting said multiplexed fabric input signal into a second plurality of optical signals
• a photodetector for converting said second plurality of optical signals into a second electronic signal
• a receiving side output for sending the second electronic signal to a transmission side input of the second fabric portion.
• the spine switches may be electronic switches of some kind. Since the outputs from the optical active are optical signals, these must be converted into electronic signals, which are then switched using the spine switches. Accordingly, an optical-to-electronic (herein "O/E”) converter may be located at the inputs, and preferably at every input, of the spine switches in the sub-array, for converting signals from e.g. the fabric entry clusters of optical active switches.
• O/E optical-to-electronic
• E/O converters may be located at the outputs, and preferably at every output, of the spine switches. Between these O/E and E/O conversions, the signal may be switched in the form of an electronic switching signal. It is preferable to have the O/E and E/O converters at the spine switch, rather than at the optical active switches because, as discussed earlier, it is advantageous for the signals to be conveyed optically, rather than
• the O/E converter may include a photodetector or plurality of photodetectors, each of the plurality of photodetectors corresponding to an input of the spine switch.
• the O/E converter may include a demultiplexer for demultiplexing the multiplexed fabric output signal (into e.g. a first plurality of optical switching signals) before converting the demultiplexed signals into electronic switching signal(s), whereupon they are switched by the spine switch.
• the E/O converter may then convert the switched electronic switching signal(s) into a corresponding second plurality of optical switching signals, which may then be multiplexed into a single signal to form the multiplexed fabric input signal, to be transmitted to an optical active switch.
• the E/O converter may also include a multiplexer.
• the electronic active switch may be configured to temporarily store a queue of packets or frames of data when a request relating to said packets or frames cannot be met.
• any or all of the multiplexer, transmission side multiplexer, demultiplexer and receiving side demultiplexer is preferably in the form of an arrayed waveguide grating (AWG), which is a passive device.
• AWG arrayed waveguide grating
• An AWG allows a plurality of optical signals of different wavelengths to be carried along a single optical fibre. Because the wavelengths of the plurality of modulated optical signals produced by the modulators are all different, the multiplexed fabric output signal produced by the AWG suffers from little to no crosstalk, since light of different wavelengths interferes only linearly.
• the multiplexed signal may be broadcast to a number of wavelength-selective filters, each tuned to receive a wavelength of one of the desired split signals.
• bandwidth is used to refer to the maximum rate of data transfer of which a particular portion is capable, and is typically measured in gigabits per second (herein abbreviated to "Gbps") Specifically, it is important to ensure that there is a conservation of bandwidth on both a local and global scale. In order to ensure that it is not possible for more data to enter a given leaf switch in a given time than can be transmitted away from that leaf switch in the same time (i.e. resulting in a bottleneck which is localized onto that leaf switch), the total bandwidth of the client ports on a leaf switch preferably does not exceed the total bandwidth of the fabric ports on the same leaf switch.
• the total bandwidth of the fabric ports on a leaf switch exceeds the total bandwidth of the client ports on the same leaf switch, and most preferably, the bandwidth of each fabric port on a leaf switch exceeds, or is equal to, the total bandwidth of all of the client ports on that leaf switch.
• the leaf switches and spine switches are connected via optical active switches.
• intermediate switches may be electronic active switches, such as electronic packet switches, and may have the same components as the electronic active switches described above, with reference to the spine switches.
• the intermediate switches are optical active switches.
• each of the intermediate switches may have the same components has each of the spine switches and/or leaf switches.
• the definitions of leaf switches and spine switches differ only in that "optical active switch" may be replaced by "intermediate switch".
• An intermediate switch may be defined as a switching element including:
• first fabric portion and a second fabric portion connected respectively to a first fabric port and a second fabric port for processing signals and communicating with spine switches or leaf switches, the first fabric portion having a transmission side and a receiving side,
• transmission side includes:
• a transmission side input for receiving a first electronic signal carrying information including information about a destination leaf switch of the first electronic signal, the first electronic signal being received from an output of the second fabric portion;
• a modulator for converting said first electronic signal into a first plurality of optical signals containing the same information
• a transmission side multiplexer for converting the first plurality of optical signals into a multiplexed fabric output signal for transmission to an input of a spine switch or a leaf switch;
• receiving side includes:
• a receiving side demultiplexer for receiving a multiplexed fabric input signal from an output of a leaf switch or a spine switch, and converting said multiplexed fabric input signal into a second plurality of optical signals
• a photodetector for converting said second plurality of optical signals into a second electronic signal
• a receiving side output for sending the second electronic signal to a transmission side input of the second fabric portion.
• Figs. I to 4 are schematic diagrams showing the connections between the leaf switches, spine switches and optical active switches in a given sub-array, according to three different embodiments of the present invention. The connections are only shown in a single dimension.
• Figs. 5A/B are schematic diagrams showing a line card and fabric card respectively.
• Fig. 6 is a schematic diagram showing the connections between the leaf switches, spine switches and optical active switches in a two-dimensional array of switching elements, the connections in both dimensions being depicted.
• Fig. 7 is a schematic diagram showing an alternative representation of the connections between leaf switches, spine switches and optical active switches in one dimension in a given sub-array. The drawing highlights the difference in the "upward” and “downward” direction, and shows the arrangements of the leaves and spines on fabric and line cards.
• Figs. 8, 9 and 10 show partially folded representations of 2D optoelectronic switches.
• Fig. I I shows an example of a leaf switch which may be used in embodiments of the present invention.
• Fig. 12 shows the relationships between various quantities and parameters of note in optoelectronic switches according to the present invention.
• the leaf switches in each sub-array are connected by spine switches and optical active switches.
• An example of the connections in a given sub-array is shown in Fig. I , in which the radix of the leaf switches (i.e. fabric ports and client ports) and the spine switches (fabric ports only) is four, i.e. each has four inputs and four outputs.
• the eight leaf switches I to 8 on the left-hand side of the drawing are the same as the eight leaf switches I to 8 on the right-hand side of the drawing. They are each shown twice (i.e. in an "unfolded" configuration) because the connections are not bidirectional, and to show them in a "folded” configuration would be confusing.
• a band of eight fabric-entry optical active switches I A to 8A which in this embodiment are optical circuit switches, and are herein referred to as "OCS", and a band of eight fabric-exit OCS I B to 8B. All OCS have a radix of two. Unlike the leaf switches, OCS I A to 8A are not the same as OCS I B to 8B. When connected as shown in Fig. I , the leaf switches, spine switches and OCS form a 5-tier Clos network, in which the five tiers are:
• Fig. I the eight leaf switches I to 8 are arranged into four clusters, each containing two leaf switches, i.e. ⁇ 1 , 2 ⁇ , ⁇ 3, 4 ⁇ , ⁇ 5, 6 ⁇ , ⁇ 7, 8 ⁇ . Each of these clusters of leaf switches is associated with a cluster of fabric entry OCS I A to 8A, as shown below in Table I .
• Each leaf switch in the cluster is connected to each of the fabric-entry OCS in the associated cluster, e.g. leaf switch I is connected to fabric-entry OCS I A and 2A, and leaf switch 2 is connected to fabric entry OCS I A and 2A.
• leaf switch I is connected to fabric-entry OCS I A and 2A
• leaf switch 2 is connected to fabric entry OCS I A and 2A.
• fabric-entry OCS I A is connected to spine switches I and 2
• fabric-entry OCS 2A is connected to spine switches 3 and 4.
• no two fabric-entry OCS in a given cluster of fabric-entry OCS are connected to the same spine switch.
• each of the fabric-entry OCS are connected to a different, disjoint set of spine switches.
• Fig. I the same is true of all of the clusters of fabric-entry OCS.
• the connections between the fabric-exit OCS I B to 8B and the spine switches I to 4 are the same as that for the fabric-entry OCS I A to 8A.
• Fabric-exit OCS I B is connected to spine switches I and 2
• fabric-exit OCS 2B is connected to spine switches 3 and 4.
• no two fabric-exit OCS in a given cluster are connected to the same spine switch.
• each of the fabric-exit OCS are connected to a different, disjoint subset of spine switches.
• Fig. I the same is true of all of the clusters of fabric-exit OCS.
• Figs. 2, 3 and 4 show similar, Clos network arrangements of switching elements within a sub-array, again including clusters of leaf switches which have associated clusters of fabric-entry OCS and fabric- exit OCS.
• the description of Fig. I still applies to these drawings, so they will not be described again in full, only the differences being highlighted.
• the radix of the leaf switches and spine switches is six, and the radix of the fabric-entry and fabric-exit OCS is two.
• Table 3 below sets out the association between clusters of leaf switches, clusters of fabric-entry OCS and clusters of fabric-exit OCS.
• Each of leaf switches I and 2 is connected to each of fabric-exit OCS I A to 3A. Then, fabric-entry OCS I A is connected to spine switches I and 2, fabric-entry OCS 2A is connected to spine switches 3 and 4 and fabric-entry OCS 3A is connected to spine switches 5 and 6. The same pattern applies for each of the clusters of leaf switches/fabric-entry OCS. The same also applies for the clusters of fabric-exit OCS.
• the radix of the leaf switches and spine switches is four, as is the radix of the fabric-entry and fabric-exit OCS.
• Table 3 below sets out the association between clusters of leaf switches, clusters of fabric-entry OCS and clusters of fabric-exit OCS.
• Each of leaf switches I to 4 is connected to each of fabric-exit OCS I A and 2A. Then, fabric-entry OCS I A is connected to spine switches I to 4, fabric-entry OCS 2A is connected to spine switches 5 to 8. The same pattern applies for each of the clusters of leaf switches/fabric-entry OCS. The same also applies for the clusters of fabric-exit OCS.
• FIG. 4 A yet further example is shown in Fig. 4, in which the radix of the leaf switches and spine switches is four, the radix of the OCS is two and the sub-array contains eight leaf switches. This drawing also highlights how the various components may be located on line cards and a fabric card.
• a one-dimensional arrangement is shown.
• this invention is by no means restricted to one-dimensional arrangements such as this.
• the arrangement of the connections between the various switching elements which is shown in Figs. I to 4 may be used to interconnect the switching elements of a single sub-array of which a given leaf switch is part, in a higher-dimensional array.
• the leaf switches in a sub-array corresponding e.g. to a given row may be connected as shown in Figs. I to 4.
• each of the sub-arrays associated with a given dimension in the array may be connected in the same manner, or all of the sub-arrays in the whole array.
• Fig. 6 a two-dimensional array of leaf switches is shown, in which the leaf switches and spine switches are organized with the optical active switches on line cards and fabric cards respectively.
• Figs. 5A and 5B respectively show the arrangement of components on the line card and fabric cards in more detail. Before discussing the higher-level of the whole array, these will be described.
• Fig. 5A shows a line card.
• the line card includes two leaf switches L I and L2, and two (fabric-entry) optical active switches, which in this case are optical circuit switches O l and 02.
• Each of the OCS O I and 02 are associated with a different dimension (denoted D I and D2 here, where D I is the horizontal and D2 is the vertical dimension).
• Each of the leaf switches L I and L2 have a connection to each of the OCS O l and 02, in order to facilitate switching in both dimensions.
• the switching in D I is the same as the switching in D2, and so only switching in D I , i.e. only the right hand bundle, or "D l bundle" of connections circled will be considered in this description.
• the D l bundle contains two outputs from OCS O l , and two inputs from the fabric, which pass (one each) to L I and L2 without going via OCS O I and 02.
• Fig. 5B shows a fabric card.
• the fabric card includes two spine switches (or AOBMs) S I and S2, and four (fabric-exit) optical active switches.
• spine switches or AOBMs
• FCI and S2 spine switches
• FC-exit optical active switches
• Each spine switch has a radix of four, i.e. it has four inputs and four outputs. Consider first the inputs. The inputs do not come from the OCS O ⁇ to 04' on the fabric card, since these are the fabric-exit OCS, via which signals exit the fabric, and not from which they enter the fabric.
• each of the spine switches S I and S2 has one output to each of OCS ⁇ ⁇ to 04 ⁇
• Each of the fabric-exit OCS ⁇ ⁇ to 04' (each of which has two inputs, one from each spine switch S I and S2) has two outputs. These two outputs correspond (i.e. are connected) to the two inputs in the D I bundle discussed above with reference to Fig. 5A.
• Fig. 6 shows a diagram of the whole array, unlike Figs. 5A and 5B which show only a single line card and fabric card respectively. From Fig. 6, the "star-like" nature of the topology can be appreciated.
• the sub-arrays in the array e.g. the top row of line cards in the D I direction shown.
• Each of the line cards LC I to LC4 in the sub-array is connected to a central fabric card FC I , through which signals are switched.
• FC I central fabric card
• the signal will be switched in the D l direction first. Accordingly, the signal passes from LC2 to LC4, in the process going via FC I . More specifically, the signal may originate from L I in LC2, from there, since it is switching first in the D I direction, it travels to O l , and then takes one of the connections in the D l bundle to an input of e.g. S2 of FC I , and subsequently switching takes place within the spine switch S2 and the signal is output to 04' via one of the outputs of S2.
• the signal may originate from L I in LC2, from there, since it is switching first in the D I direction, it travels to O l , and then takes one of the connections in the D l bundle to an input of e.g. S2 of FC I , and subsequently switching takes place within the spine switch S2 and the signal is output to 04' via one of the outputs of S2.
• the signal then travels to an input of e.g. L I of LC4 (via the D l bundle of LC4).
• L I the input of e.g. L I of LC4
• the signal is switched internally to an output of L I which is connected to an input of 02 of LC4, i.e. to the fabric-entry OCS which is associated with switching in dimension D2.
• the signal is sent from the D2 bundle to an input of e.g. S I of FC8, which is the fabric card containing the spine switches responsible for switching in the sub-array containing both LC4 and LC I 2.
• S I of FC8 the fabric card containing the spine switches responsible for switching in the sub-array containing both LC4 and LC I 2.
• the signal is switched within S I of FC8 to the output which is connected to e.g. L2 of FC 1 2, and then the signal travels from that output to L2 of FC 12.
• each line card has one or more OCS and a further bundle of connections associated with switching in the third dimension.
• Fig. 7 shows an alternative representation of a one-dimensional case, which highlights the difference between the "upward” and “downward” journeys of a signal as it traverses a given sub-array.
• the "upward” journey refers to the signal travelling from the leaf switch, through a fabric-entry OCS, and into a spine switch.
• the "downward” journey is from the spine switch, through a fabric-exit OCS, and into a destination leaf switch.
• the OCS shown in the "upward” section are the fabric-entry OCS which are located on the line cards with the leaf switches.
• the clusters of leaf switches contain two leaf switches, and the associated clusters of fabric-entry OCS each contain only one fabric-entry OCS.
• the OCS are the fabric-exit OCS, and are arranged on the fabric card with the two spine switches (which are the same spine switches shown in the "upward” section).
• Fig. 8 shows a one-dimensional example of the connections in an optoelectronic switch according to embodiments of the invention as described in greater detail above.
• the leaf switches are only depicted once, in a partially folded configuration. Please note that this is not a true folded-Clos network, because the fabric-entry and fabric-exit OCS are different, so the signal cannot travel "up" and "down” (see Fig. 7) by exactly the same route.
• Fig. 9 shows a partially folded representation of a 2-dimensional optoelectronic switch according to the present invention.
• FIG. 10 shows an alternative, unfolded representation of a 2-dimensional optoelectronic switch including an array of sixty-four leaf switches at the edges of the drawings, and two sets of sixteen spine switches in the centre (one associated with switching in each of the dimensions).
• the dotted lines represent connections in one dimension, and the sold lines represent connections in the other dimension.
• embodiments of the invention are not restricted to arrangements wherein the leaf and spine switches have a radix of four, and the OCS have a radix of two.
• Fig. I I shows a more detailed view of the fabric side F l of a typical leaf switch I which is used in embodiments of the present invention.
• Fabric side F I is divided into two parts, a transmission side Tx and a receiving side Rx.
• Transmission side Tx includes a packet processor PP-Tx, an array of EAMs MOD I , MOD2... MODQ, each of which receives an input from one of an array of light sources LS I , LS2... LSQ.
• Each of the array of EAMs is connected to a single multiplexer MUX, which then outputs its WDM signal to an optical active switch, which can be thought of as "the fabric" which effects interconnection between all of the leaf switches I of the optoelectronic switch of embodiments of the present invention.
• Receiving side Rx has a similar structure. More specifically, the receiving side Rx includes a packet processor PP-Rx, which receives inputs from an array of photodetectors PD I , PD2... PDQ, which each receive input from a single demultiplexer DEMUX. The demultiplexer receives an input from an optical active switch (not shown in Fig. I I ).
• a controller CTRL is also included in the leaf switch I , and is not constrained to either the transmission side Tx or the receiving side Rx.
• the controller CTRL is connected bidirectionally to the two packet processors PP-Tx, PP-Rx and an arbiter, shown by the arrow labelled AR.
• a packet contains information which is to be transmitted from a source leaf switch to a destination leaf switch. Specifically, the packet contains information relating to the intended destination leaf switch. In the following description of the journey taken by the packet, it is assumed that all data which is associated with that packet has the same intended destination leaf switch.
• a packet may be incident on the transmission side Tx of the leaf switch I for example from a client portion which is connected to the client side of the leaf switch I .
• the packet may be received from the receiving side Rx of leaf switch I , (i.e. the same leaf switch) via an integrated switch, so that said packet can be forwarded to another leaf switch (not shown), for transfer to a different dimension.
• the packet incident on transmission side Tx enters the packet processor PP-Tx, where it is sliced into a first plurality of Q electronic signals in the form of packet slices, each with the same destination leaf switch. Each of the electrical signals is then transmitted to one of the Q EAMs MOD I , MOD2... MODQ.
• each of the electrical signals contains information corresponding to the data in the packet slice and information relating to the destination leaf switch of the packet.
• MOD I has two inputs: (a) the electrical packet slice, and (b) light of a given wavelength from a light source LS I .
• the optical channel is chosen to minimize cross-talk and to be relatively easy to manufacture the waveguides in good yield.
• Optical channel spacing of between 0.4nm and 2nm is preferable.
• the laser light would be of as narrow line width as practical for the application and is preferably no less than I KHz. In other configurations, the frequency resolution and spacing will depend on the finesse of the device, and hence the passive components. If there are e.g. 8 wavelengths, then the device may be quite "crude", but if more wavelengths are to be used, a higher specification will be required.
• MOD I then modulates the light from the light source LS I to carry the information contained in the packet slice, to produce an optical signal with a given wavelength. From this point, data transfer is in the optical domain.
• Each modulator operates similarly to produce a first plurality of Q optical signals.
• the Q optical packet slices from each of the EAMs MOD I , MOD2... MODQ are incident on the multiplexer MUX where wavelength-division multiplexing takes place to combine the Q optical signals (one from each EAM) into a single output optical fiber.
• Each of the Q optical signals has a different wavelength, and so crosstalk between signals is minimized.
• the multiplexed signal forming the multiplexed fabric output signal is then transmitted to an optical active switch.
• the optical signals generated in leaf switch I are then transmitted via the fabric to their destination leaf switch or an intermediate leaf switch en route to the destination leaf switch.
• the source and destination leaf switches would not be the same leaf switch.
• the source and destination leaf switches may be the same leaf switch, however, e.g. for testing purposes.
• the source and destination leaf switches are to be substantially identical to each other, so the description based on Fig. I I still applies equally well.
• the optical multiplexed fabric input signal from an optical active switch is incident on demultiplexer DEMUX which is located on the receiving side x of the leaf switch I .
• the multiplexed fabric input signal is demultiplexed by the demultiplexer DEMUX into a second plurality of Q optical signals which are equivalent to those combined at the multiplexer MUX on the source leaf switch I .
• the Q demultiplexed signals are then incident on each of an array of photodetectors PD I , PD2.. . PDQ. Therein, they are converted back into a second plurality of Q electrical signals, again containing the information which was contained in the original packet slices.
• the electrical signals are then transmitted to the packet processor PP-Rx, where, using information contained in the headers of the packet slices, they are recombined into the original packet which was incident on the packet processor PP-Tx of the source leaf switch I .
• Fig. 1 2 shows the mathematical relationship between various quantities involved in optoelectronic switches according to embodiments of the present invention, in which:
• R 2 radix of number of leaves per cluster, number of spines per cluster

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