Patent Application
20110243555
The present invention relates to method and apparatus for controlling the power of a channel in an optical link, including methods and apparatus for controlling the power of a channel in a node of an optical link comprising a plurality of nodes. Embodiments of the present invention are particularly suitable for controlling the power of a channel introduced along an optical link. Embodiments of the present invention are also particularly suitable for, but not limited to, controlling the operation of variable optical attenuators in an Automatically Switched Optical Network (ASON).
Wavelength division multiplexing is the transmission of several different signals via a single optical transmission medium (e.g. fibre), by sending each signal (“channel”) at a different optical frequency or wavelength. A multiplexer is used to combine the different channels together for transmission, and a demultiplexer is used to separate the channels following transmission. WDM optical transmission systems are typically composed of a number of spans of optical fibre linking together the network nodes.
Early WDM networks used simple, fixed optical filters to route the optical signals between the network nodes. Such networks were therefore essentially “static” i.e. the channel configuration (number of channels being transmitted, and the routing of the channels through nodes of the network) did not change, except during fault conditions or due to human intervention to upgrade or alter the network configuration.
More recent WDM networks can include reconfigurable optical network nodes, which allow remote reconfiguration of the channels, faster provisioning of new channels and improved network resilience. Such reconfigurable optical network nodes commonly employ integrated optical devices, such as ROADM (Reconfigurable Optical Add-Drop Multiplexer) or WSS (Wavelength-Selective Switch) devices or similar, in order to control and route the optical signals. Although discrete optical devices can be used to provide the same functionality, generally it is cheaper to utilise integrated devices for high channel-count WDM networks.
Autonomous rerouting of traffic is relatively well known in single-channel optical networks such as SDH (Synchronous Digital Hierarchy) where it is commonly known as ASTN (Automatically Switched Transport Network). In the more advanced versions of WDM networks, the network control software can also autonomously reroute traffic according to the demands on the network e.g. faults, changing bandwidth requirements etc, thus reducing the need for human intervention. Such a WDM network is commonly referred to as ASON (Automatically Switched Optical Network).
In general, signal regeneration using optical-electronic-optical conversion is not performed at every node, but only where necessary to restore signal quality. Hence, the routing and transmission of signals from node to node is generally performed in the optical domain.
It is desirable to control the power levels of each of the channels in an optical system. Optical signals experience wavelength dependent effects as they are transmitted across the network, including fibre and optical device attenuation, Raman scattering within the optical fibre, optical amplifier wavelength dependent gain etc. It is particularly desirable to control the optical signal power as channels are added to the network, to prevent disruption to existing established channels or radiation power surges which can interfere with existing channels e.g. due to the channels being amplified by non-linear devices such as erbium doped fibre amplifiers.
The device 10 comprises a port selector switch 12 which is operable to couple any one of a plurality of input ports 14a, 14b . . . 14m to a Variable Optical Attenuator (VOA) 16. The port selector switch 12 thus acts to select an optical signal path by switching to a particular port, thus directing an optical signal along a desired route through the network. The VOA 16 controls the power of the optical signal transmitted to the output port 18, keeping the channel power within an acceptable dynamic range.
In order to keep the optical signal within the acceptable dynamic range, the VOA 16 will be coupled to a controller 20. A power monitor function is provided at each node which continuously measures the individual channel powers at each line port (e.g. at line port 18). Such monitoring is usually carried out only after the VOA for cost reasons. The controller 20 determines an error signal based on the difference between the measured channel power and a target channel power (e.g. a transmission power level suitable for normal transmission of the optical channel within the network), with a fixed multiple of the determined error signal (the gain) then used to control the VOA attenuation. The controller 20 is often implemented as a cyclic control algorithm. Adjustments are made to the appropriate VOA attenuation at predetermined regular intervals. The period between one VOA update and the next is termed the control cycle.
At the beginning of the process (state A) the VOA 16 is at maximum attenuation (in the blocking state) causing the channel power at the output of the device (and hence the node) to be negligible. This is typically the quiescent state of the device, in order to minimise the transmission of optical noise power through the node. Such noise can be produced during optical amplification of signals at upstream nodes.
To introduce the channel, the VOA 16 is first adjusted to a predetermined “safe” value (state B in
Once the initial calibration step has been performed by reading the channel output power, the control loop algorithm is activated, and acts to reduce the VOA attenuation, producing the steps seen in time interval C in
An optical channel will typically be transmitted along an optical link comprising several nodes. To prevent disruption to existing established channels or radiation power surges which can interfere with existing channels, a new channel will be iteratively introduced to each successive node. Once the channel power is substantially at the target channel power (i.e. state D in
In a first aspect, the present invention provides a method of controlling a variable optical attenuator. The method comprises receiving a signal indicative of an upstream variable optical attenuator having completed an initial calibration step, and initiating calibration of the variable optical attenuator in response to receipt of said signal.
Such a control system allows the optical attenuator to reduce the time taken for a channel to be introduced along an optical link, by performing the VOA calibration step whilst the upstream node is still introducing the optical channel. Previously, it had been believed that it was necessary for the channel to be fully introduced at each node before the next node downstream could introduce the channel, so as to maintain stability of the optical signal within acceptable limits, and prevent overall power instability in the network. However, the present inventor has realised that, once the initial calibration step has been performed by the upstream node, there is a high level of confidence that the channel will be successfully output at the desired power level from that upstream node, and hence the calibration of the downstream node can be initiated. Such a control system can thus improve the speed with which a chain of reconfigurable optical nodes can establish end-to-end transmission of an optical signal, providing a competitive advantage when employing ASON networks. Further, the corresponding relatively fast restoration of a link that can be provided by such a control system can be used to improve network resilience by quickly restoring back-up (e.g. protection) routes.
The step of initiating calibration of the variable optical attenuator may be performed prior to receipt of a signal indicative of said upstream variable optical attenuator having an optical output signal at a transmission power level.
The signal indicative of an upstream variable optical attenuator having completed an initial calibration step may be indicative of the attenuation of said variable optical attenuator having been set to a predetermined attenuation value, and an output optical signal from the variable optical attenuator having been measured to be within a predetermined power range whilst the upstream variable optical attenuator is at said predetermined attenuation.
The method may further comprise transmitting the signal from an upstream node including the variable optical attenuator indicative of the upstream variable optical attenuator having completed the initial calibration step.
The method may further comprise the steps:
In a second aspect, the present invention provides a method for control of a variable optical attenuator located in an optical link comprising a plurality of optical attenuators. The method comprises determining that an initial calibration step of a variable optical attenuator has been completed, and transmitting a signal indicative of the variable optical attenuator having completed the initial calibration step for receipt by a downstream variable optical attenuator.
Transmission of such a signal allows a downstream node to commence the initial calibration of a VOA at the downstream node, whilst the channel is still being introduced at the current node.
The control system may comprise a processing element arranged to receive a signal indicative of an upstream variable optical attenuator having completed an initial calibration step, and initiate calibration of the variable optical attenuator in response to receipt of said signal.
The processor may be arranged to initiate calibration of the variable optical attenuator by providing a control signal to set the attenuation of the variable optical attenuator to a predetermined attenuation value, and to receive a signal indicative of the optical output power from the variable optical attenuator.
The processing element may be further arranged to determine whether the signal indicative of the optical output power from the variable optical attenuator lies within a predetermined power range, and transmit a signal indicative of the variable optical attenuator having completed an initial calibration step if the optical output signal power lies within said predetermined power range.
The processing element may be further arranged to
In a fifth aspect, the present invention provides a control system for a variable optical attenuator. The system comprises a processing element arranged to determine whether an initial calibration step of a variable optical attenuator has been completed, and transmit a signal indicative of the variable optical attenuator having completed the initial calibration step for receipt by a control system of a downstream variable optical attenuator.
In a sixth aspect, the present invention provides a device comprising at least one variable optical attenuator, and at least one control system as described above. Said at least one control system is coupled to said at least one variable optical attenuator for controlling the attenuation of said at least one variable optical attenuator.
The device may comprise a plurality of said variable optical attenuators, each arranged to controllably attenuate a different wavelength optical signal.
In a seventh aspect, the present invention provides an optical network comprising a plurality of nodes, with each of said nodes comprising at least one device as described above.
Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
It is known for the network control software to autonomously re-route traffic according to the demands on the network e.g. faults, changing bandwidth requirements etc. For example, it is desirable to restore an end-to-end optical link after a fault in the network, by the creation of a new traffic path along a route which has not been affected by the fault. The faster this re-routing can be performed, the more quickly network traffic and/or resilience can be restored.
The present inventor has appreciated that new challenges are presented by the use of reconfigurable optical devices as may be used within photonic nodes.
For example, in many existing ASTN networks (i.e. non-WDM networks) the time to perform traffic restoration is limited chiefly by the speed of operation of ASTN management software, as the associated electronic switching hardware has much lower reconfiguration times. The general expectation is that when the equivalent functionality is deployed in WDM networks (e.g. to realise ASON), the operation times will be similar. However, the present inventor has appreciated that this is not likely to be the case.
In particular, the present inventor has realised that the reconfigurable optical devices used within photonic nodes cannot generally operate in the same time scales as the electronic equivalents used in single-channel systems. The time taken for such reconfigurable optical devices to perform optical switching (changing the routing of a signal from one port to another) and also the introduction of optical channels, including, optical “levelling” (changing the channel attenuation to adjust the optical output power of a channel to a desired target level), are limited by the optical response times of the technology used and/or the necessary data processing within the device. Thus, the time delay between channel power updates (i.e. the “control cycle” used by the controller) must be relatively long.
For example, typically a control cycle for a controller in a photonic node would be of the order of hundreds of milliseconds, which in turn affects the time taken to introduce a channel at each node. As a link will require the introduction of a channel at a number of nodes, and as each channel must be sequentially introduced to each node before being introduced to the next node in the link, the introduction of channels along a chain of nodes using the prior art method can be unacceptably long (e.g. many seconds).
The present inventor has appreciated that the time taken to introduce a channel to a chain of nodes (e.g. to an optical link) can be reduced by partially overlapping the channel introduction procedures at successive nodes.
Accepted practice is that it is undesirable to overlap the channel introduction process, as that can lead instability of the power level of the optical signal, leading to overall power instability in the network. However, the present inventor has realised that, once the initial calibration step has been performed by a VOA in a node, there is a high level of confidence that the channel will be successfully output at the desired power level from that node, and hence the calibration of the next node in the link can be initiated.
Preferably, to further reduce the time taken to introduce the channel at each node, the control system for the variable optical attenuator at each node utilises at least two gain values. A first (e.g. relatively high) gain value is used to ensure that the initial step-size (used after the calibration procedure) is relatively high, so that the VOA relatively rapidly approaches the target radiation power level (e.g. the transmission power level, which is a power level suitable for normal transmission of the optical channel within the network).
To prevent instabilities which might arise if the first gain value were consistently used along a chain of similar nodes, which could lead to a degradation in the performance of the traffic path (bit errors or complete loss of signal, or even disturbances on other optical channels due to dynamic range effects within optical amplifiers), a second, lower value of the gain factor is subsequently used e.g. after a predetermined time period. The predetermined time period could be a predetermined number of control cycles, or once the optical output power level has reached a predetermined level e.g. after the error signal indicative of the difference between the target radiation power and the optical output power of the VOA has dropped to beneath a predetermined level.
Such a control system can thus improve the speed with which a chain of reconfigurable optical nodes can establish end-to-end transmission of an optical signal, providing a competitive advantage when employing ASON networks. Further, the corresponding relatively fast restoration of a link that can be provided by such a control system can be used to improve network resilience by quickly restoring back-up (e.g. protection) routes.
An overview of the channel introduction procedure in accordance with an embodiment of the present invention will now be described with reference to
Each node is connected to an adjacent node by one or more respective spans of optical fibre (52, 54). Only the particular optical channel transmitted in the desired traffic direction (along fibre spans 52, from node 50a to destination node 50n) is considered here. The reverse traffic direction (i.e. signals transmitted along optical fibre spans 54) is not considered, and nor are any other wavelength signals (e.g. channels) which may also be transmitted along the same fibre spans 52.
The initial optical signal 60 is input to the first node 50a, and transmitted through each of the adjacent nodes to the destination node 50n, at which it leaves the reconfigurable network as an output signal 62.
The time period T represents the control cycle period. The states A1, A2 represent the relevant VOA being in the blocking (maximum attenuation) state, with states B1, B2 representing the respective VOA being in an initial calibration state, and C1, C2 representing the respective VOA receiving a control signal to reach the desired, target output power. The states D1, D2 indicate the output power of the VOA having reached a suitable transmission power level.
It can be seen that the initial calibration step (B2) of the VOA at node 2 advantageously takes place at the same time as the levelling step (C1) of the VOA of node 1. This overlap can occur by the controller of the VOA at node 1 making a power measurement at the end of the calibration step time period, to determine whether the optical channel power at that VOA output is within acceptable limits whilst the VOA is set to a predetermined (‘safe’) value of attenuation.
If the optical output power from the VOA of node 1 is determined as being within a predetermined power range, then the VOA calibration step has been successfully completed, and the controller of node 1 will transmit a first signal to node 2, indicative of the node 1 VOA having successfully completed its calibration step. This first signal is an additional message type, and could take the form of an instruction to the downstream node (node 2 in this example) to begin its calibration. Should a signal could be transmitted using the Optical Supervisory Channel (OSC).
It should be noted that this first signal does not indicate that the output channel power is actually correct (e.g. at a power level suitable for transmission of the optical channel) at the originating (upstream) node.
Instead, the check of the optical output power being within a predetermined, acceptable output range, allows the prediction to be made that the VOA of node 1 will be able to output an optical signal at its target power (e.g. a transmission power level), given the known dynamic range of the VOA and the measured optical power at a predetermined attenuation value. Therefore, the present inventor has realised, there is no need to wait until the levelling step (i.e. C1) has completed before initiating the setting of the next node's VOA to its ‘safe’ calibration value.
Upon receipt of that first signal indicative of the upstream VOA having successfully completed its calibration step, the controller of the downstream node (node 2) VOA begins its own calibration step (B2) i.e. sends a signal to the VOA to set a predetermined (safe) attenuation value. At the same time, the upstream node VOA that has sent the first signal (node 1) begins its levelling step(s) C1.
It should be noted that it is not usually desirable to set all of the VOAs in the link to the predetermined (“safe”) attenuation value to circumvent the calibration step, because of the build up of optical noise which may then ensue along the link.
In the example shown in
Once the output power from the VOA at node 1 reaches the target power level (e.g. a transmission power level, state D1), a second signal can be sent to the downstream node to indicate this, e.g. using the OSC.
For example, correct channel power can be indicated by subsequent transmission of the original “channel present” message type using the OSC. Provided the OSC message transmission is fast enough, lack of this second message can be used to cause the channel introduction at the downstream node (node 2) to be delayed or aborted in cases where correct channel power cannot be achieved even after a successful calibration stage at the upstream node, e.g. node 2 only commences step C2 (the levelling step), once a signal indicative of the optical signal output from the VOA at node 1 being at a suitable transmission power level has been received.
This “channel present” signal would normally be passed on once it is deemed that the first time period has passed, and as the control loop gain factor in node 1 is therefore reduced to a second, lower value for stability.
Thus, the next node in the link will begin introducing the channel (the levelling step, C2) as the gain factor of the present node is set to the second, lower value (i.e. in step D1), thus reducing the likelihood of instability in the link. Further, in this way, at any given time, no more than one node in the link will be working at the higher, first gain value.
Preferably, the time taken for the optical power monitoring procedure is small compared to the response of the device to attenuation changes, with the power measurement typically occurring towards the end of each control cycle. If this is not the case it may not be possible to overlap the calibration step of the downstream node VOA with the levelling step of the upstream VOA by a complete control cycle. However, improvements can still be realised, by overlapping the processes by as long a time as is possible, activating the downstream process (e.g. calibration step) as soon as it is known that the target power will be achieved at the upstream node by the time that the power measurement is made during the calibration step of the downstream node.
The above procedure is preferably iteratively repeated at each node (e.g. with node 2, once the calibration step has been successfully completed, sending a first signal to the next downstream node, to allow that downstream node to start the calibration procedure etc), thus overlapping the channel introduction process as much as possible over the optical link.
For clarity, only a single wavelength path is shown i.e. this architecture would be replicated for each separate wavelength. Typically, 40 or 80 wavelength channels may be provided, each with a corresponding device 10′. Optical amplification and other functions may also be provided by the node by known devices, but the relevant devices are not illustrated here. For ease of an explanation, identical reference numerals are used to represent similar features throughout the figures.
It will be seen that the device 10′ illustrated at
In a typical node, such a device 10′ would be used at every line port in the transmit and/or receive direction depending on the node architecture. The transmit architecture is shown (i.e. with a plurality of selectable inputs, and a single output). The receive architecture by contrast comprises a single input port, and a plurality of selectable output ports. Both transmit and receive architectures would include a variable optical attenuator, for controlling the power of the optical signal transmitted through the device 10′, and a corresponding controller 20′.
For transmission and receipt of such signals, the portion 20a can be functionally coupled to a portion (or portions) of the node that transmit & receive the OSC. Functional portion 20a is arranged to initiate calibration of the VOA in response to receipt of the first signal, and to initiate operation of the control loop of the second portion in response to receipt of the second signal from the upstream node. Functional portion 20a is arranged to determine (e.g. by making a measurement of the output power level from the VOA) once the VOA 16 has successfully completed a calibration step, and to output a first signal for the downstream node. Functional portion 20a is similarly arranged to determine (e.g. again, from the optical output power, or alternatively by receiving a relevant signal from portion 20b) once the optical power output from the VOA 16 is at a suitable transmission level, and to then output a second signal for receipt by the downstream node.
The controller portion 20b includes a power monitor element (22) which continuously (or periodically) measures the individual channel power. In the particular embodiment shown in
The output of the ADC 22 is a signal indicative of the radiation power of the optical signal output from the variable optical attenuator.
An error signal is then determined in dependence upon the signal indicative of the radiation power of the output optical signal and a predetermined target radiation power level. The target radiation power level is the desired optical signal output power. Such a predetermined target radiation power level may be set by the network management.
In the implementation shown in
The error signal is subsequently scaled in dependence upon a gain factor by integral functional element 26′. The gain factor is a multiplication factor, and is used to determine the gain of the control loop. In the example implementation shown in
The controller portion 20b is arranged to iteratively repeat the steps of receiving the signal indicative of the radiation power of the optical output signal from the VOA, determine an error signal, and determine the control signal for controlling the attenuation of the VOA 16.
Element 27 in
The operation of the controller will now be described with respect to
Typically, when no optical channel signal is provided to the device 10′, the VOA 16 will be set to maximum attenuation, to prevent the onward transmission of noise. This maximum attenuation state (A1 or A2) is shown in
In step 102, a first signal is received at the node, from an upstream node, indicative that the relevant VOA of the upstream node has successfully completed an initial calibration step. This signal may be used to indicate that a channel is being introduced along the optical link containing both the present node and the upstream node. The channel may be introduced to the link due to the provisioning of a new route through the node, or the restoration of a link through the node.
A calibration of the VOA 16 at the present node is then initiated, with the attenuation of the VOA 16 set to a predetermined “safe” attenuation level that allows the measurement of the optical output power from the VOA for calibration purposes, and the optical output power measured (step 104).
Assuming the output power from the VOA is measured as being within a predetermined range (i.e. indicative that it is possible for the transmission power level to be reached), then the calibration step 104 is deemed to have been completed. Once the calibration step 104 has been completed, a first signal is output from the node, for receipt by a downstream node, indicative that the initial VOA calibration has been completed (step 105a). Also, once the calibration step 104 has been completed, the node waits for receipt of a second signal from the upstream node, indicative of the optical output signal from the relevant VOA (e.g. the channel power) being at a suitable transmission power level (step 105b). Steps 105a and 105b can be performed in either order, or simultaneously.
Once the second signal has been received (step 105b), the control loop for the levelling operation will begin (steps 106-112). The controller will receive a signal indicative of the output radiation power (step 106). In the first iteration of step 106, this signal could simply be the power measurement of the VOA output power made during the calibration step 104, although in later iterations of step 106, new power measurements will be made of the optical output power.
Subsequently, the error signal is determined using the received signal indicative of the output signal power and a signal indicative of the target radiation power (step 108).
The control signal to set the attenuation of the VOA 16 is then calculated from the error signal, using a first gain value or factor (step 110). This first gain value is relatively high e.g. in the example shown in
After the VOA control signal calculated using the error signal and first gain factor has been output to the VOA 16, it is determined whether the first time period has passed (step 112).
Whether the first time period has passed (in which the first, relatively high, gain value is utilised) can be determined in any one or more of a number of ways. It can be determined based upon a predetermined number of control cycles having passed, with that predetermined number depending upon the value of the first gain factor e.g. the higher the gain factor, the fewer control cycles that will be required. Alternatively, whether the first time period has passed could be determined from measuring the optical output power from the VOA 16 and/or the device 10′, and determining whether that optical output power is within a predetermined range of the target radiation power (e.g. the desired transmission power level of the channel).
If the first time period has deemed not to have passed (e.g. not enough control cycles have passed, and/or the output power is not within a predetermined range of the target radiation power), then the control cycle (steps 106-112) is repeated, until the first time period is deemed to have passed.
Subsequently, the gain factor used by the control loop is altered, and subsequent VOA control signals are generated using the second, lower gain value or factor (step 114a). In other words, the control loop gain is reduced to a typical, normal working level (e.g. with the second gain value being less than a third of the value of the first gain value e.g. with the second gain factor being 0.2). The value of the second gain factor may depend upon the number of downstream nodes through which the channel will be transmitted, and can be selected such that the concatenated control loops of all of the downstream nodes do not produce instability in the signal power.
Additionally, once the first time period is deemed to have passed, a second signal is output for receipt by the downstream node, indicative of the VOA having an optical output signal at a suitable transmission power level (step 114b). Steps 114a & 114b can occur in either order, or simultaneously.
It shall be appreciated that the above embodiments of the present invention have been described by way of example only, and various alternatives will be apparent to the skilled person as falling within the scope of the claims.
Whilst
Whilst the above embodiments have been described as using two gain values or factors, it should be appreciated that the present invention could be implemented using three or more gain values e.g. with a first, relatively high gain value used in a first time period, a second, intermediate gain value used in a second time period, and a third, lowest gain value used in a third time period.
Thus, from the above description, it will be readily appreciated that an introduction of a channel in a chain of nodes using a method in accordance with an embodiment of the present invention can greatly increase the speed with which the channel is introduced to the chain, allowing a network to establish end-to-end transmission of an optical signal much more quickly than prior art methods.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/63271 | 10/3/2008 | WO | 00 | 6/13/2011 |
