Abstract:
The invention relates to a method for optimizing the optical power in an optical network that has a plurality of network nodes each having a transmitter and a receiver. The method comprising generating an optical signal at a first network node, receiving the optical signal at a second network node, detecting the optical power of the optical signal at the second network node, determining whether the optical power detected is outside a defined range, and in this case, generating, for the first network node, a control signal for increasing or decreasing the optical power, sending the control signal to the first network node, and increasing or decreasing the optical power of the optical signal emitted at the first network node. The invention further relates to an optical network having network nodes which are operable to implement this method.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of the priority date of European patent application 04 090 375.9, filed on Sep. 24, 2004, the contents of which is herein incorporated by reference in its entirety. 
   FIELD OF INVENTION 
   The invention relates to a method for optimizing the optical power in an optical network that has a plurality of network nodes each having a transmitter and a receiver, and to a corresponding optical network. 
   BACKGROUND OF THE INVENTION 
   It is a known practice to transmit data using an optical network. An optical network has a plurality of network nodes which are arranged in accordance with a particular topology, for example a ring topology and a star topology. The individual network nodes each have a transmitter and a receiver. The receiver converts the optical signal emitted by another network node into an electrical signal. An intelligent unit in the network node evaluates the electrical signal and checks, in particular, whether the information contained in the signal is intended for its own network node or is to be forwarded. In the latter case, the electrical signal is converted into an optical signal again by the transmitter and is sent to a further network node. 
   The serial communications system MOST (Media Oriented System Transport) has become established in recent years for transmitting audio, video, voice and control data using optical waveguides in multimedia networks and, in particular, in the automotive sector. MOST technology has, in the meantime, become the standard for present and future requirements in multimedia networking in motor vehicles. A MOST system provides a bandwidth of up to 24.8 Mbit/s. The network topology in MOST systems is generally a ring topology or a bus topology. 
   The optical power in an optical network is determined by the optical power levels emitted by the individual optical transmitters of the network nodes. In this case, the optical power is selected in such a manner that, taking into account the path attenuation between a transmitter under consideration and the associated receiver, the optical power detected at the receiver is high enough for reliable signal detection. 
   SUMMARY OF THE INVENTION 
   The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
   The present invention is directed to a method for optimizing the optical power in an optical network and also for providing a suitable optical network wherein this goal may be achieved. 
   Accordingly, the present invention comprises initially receiving and detecting the optical power of the optical signal at a network node. Then, it is determined whether the optical power is within or outside a defined range. If the optical power is outside the defined range, a control signal for either increasing or decreasing the optical power is generated for that network node which emitted the optical signal. The control signal is sent to the emitting network node and the latter then increases or decreases the optical power of the emitted optical signal in accordance with the control signal received. The method may be repeated within a regulating operation until the optical power detected at the second network node is within the defined range. 
   The method of the present invention makes it possible, during operation, to set the optical power emitted by an optical transmitter of a network node to a suitable value as a function of the path attenuation between the transmitter and the associated receiver, thereby optimizing the optical power in the optical network. In this case, the invention ensures, on the one hand, that the signal received by the receiver is large enough to enable reliable signal detection. On the other hand, inefficient operation and an “overload” in the receiver in the case of excessively high optical power levels are prevented. 
   The optical power that is available to a receiver is dependent on various influencing parameters. Examples of such influencing parameters are, for example, the length and attenuation of the optical fiber through which the light is transmitted, the ambient temperature, ageing processes in the optical fiber and the transmitter, production-dictated discrepancies and any optical coupling that may possibly occur. Despite this variety of parameters which influence the path attenuation between a transmitter and a receiver, the method according to the invention makes it possible to always set that ideal optical power at the transmitter which leads to an optical power level in the receiver that is within a desired ideal range. 
   In particular, partial failure of the optical system, which may occur when the optical power in the receiver is no longer sufficient for signal detection because of high path attenuation, is also reliably prevented. The possibility of reducing the optical power of the transmitter in the case of low path attenuation also increases the lifetime of the optical transmitter and thus the overall reliability of the component. This is highly advantageous, in particular, in semiconductor lasers, for example in vertically emitting laser diodes (VCSELs). 
   The optical power of the respective transmitters of the network nodes of the optical network is naturally dependent on the path attenuation of the light transmission path between a respective transmitter and receiver. The optical power levels (which have been set) of the transmitters and the optical power levels detected at the receivers thus reflect the path attenuation between a respective transmitter and receiver. Accordingly, another advantage of the present invention is that the path attenuation on the individual paths of the optical network and changes in the path attenuation may be detected using the optical power levels which have been set and detected and may be used to diagnose the optical network. 
   The method according to the invention thus also improves the ability to diagnose the optical network, implemented, for example, by storing changes in the attenuation of particular paths in a central memory for retrieval and evaluation using an evaluation system. In another aspect of the invention, it is also possible to define suitable interfaces which can be used to detect the information relating to the respective path attenuations. These improved diagnosis capabilities may be implemented solely on the basis of software, that is to say without additional hardware. 
   It shall be pointed out that, within the scope of the present invention, the term “optical signal” is a synonym for an optical data signal, that is to say denotes an optical signal that has been modulated in accordance with data to be transmitted. In this case, the data to be transmitted may comprise both control or signaling data and user data. 
   In one exemplary implementation of the invention, the control signal is transmitted to the first network node via the optical network. The optical network itself thus serves for control signal transmission and feedback to the transmitter of the emitting network node. In principle, however, it is likewise conceivable for the control signals to be sent to the emitting network node via other connections, for example via a local area network and/or the Internet or via a radio link. 
   In another exemplary implementation, provision is made for signal transmission in the optical network to be subject to a transmission protocol that defines at least one user channel for transmitting user data and at least one control channel for transmitting control and signaling data. In this example, the control signal in the control channel is transmitted to the first network node. The control channel of the optical network thus serves to transmit the control signals. 
   In particular, provision may be made for the receiver of the second network node to be used to convert the received optical signal into an electrical signal, then, if the optical power detected is outside the defined range, for an electronic module to add the control signal to the electrical signal, and for the electrical signal that has been changed in this manner to be supplied to the transmitter of the second network node and converted into an optical signal. In this case, the electronic module may be arranged in the network node itself or may alternatively be assigned to a network management unit that communicates with the network node electrically. 
   It shall be pointed out that a control signal is generated irrespective of whether the signals received at the second network node are intended for the second network node or are forwarded to another network node. Thus, respective control signals each relate to the setting of the optical power in a transmission section of the optical network, irrespective of the contents of the transmitted data and their destination. 
   In one exemplary implementation, the optical power at the second network node is detected by measuring the photodiode current after the optical signal has been converted into an electrical signal. This makes it possible to easily and effectively determine the optical power that has been detected. 
   The optical network, in one exemplary configuration comprises a serial network in which optical signals are transmitted between adjacent network nodes. In particular, the network has a ring topology and/or a unidirectional network. In principle, however, other network topologies, for example a star topology or a bus topology, may also be implemented. The optical network may also be a bidirectional network. 
   Further, provision may be made for the control signal generated by the second network node to contain the address of the first network node in the optical network as the destination address. Of course, this is important when the generated control signal passes through a plurality of network nodes on its way to the destination node (e.g., the node that emitted the optical signal). 
   The destination address, in another exemplary implementation, may be specified without any problems when the transmitter contains its own address as sender information. This is because this sender information can then be given as the destination address. However, this is not always the case. In the case of unidirectional networks, provision may be made for the address of that network node which precedes the second network node in the unidirectional network to be given as the address of the first network node (that is to say as the address of the destination node for the control signal). The network nodes are numbered consecutively and each network node (n, 1≦n≦N) knows that it receives optical signals from a particular preceding network node (n−1). 
   In still another exemplary implementation, provision is made for the first network node to increase or decrease the optical power by a defined amount after it has received the control signal. The optical power is thus raised or lowered in a stepwise manner until a suitable value exists. Alternatively, the control signal may specify a particular percentage or value by which the optical power is raised or lowered. This would reduce the number of control loops. 
   In one advantageous embodiment, the optical power levels detected at the individual receivers of the network nodes and/or values derived therefrom (for example attenuation values) are stored in a memory. In this case, the attenuation of the individual transmission paths of the network is determined from the instantaneous optical power levels, for example, on the basis of the data contained in such a memory. The attenuation of the individual transmission paths of the network may be evaluated, for example, as part of a network diagnosis. 
   The optical network according to the invention has at least one network node having a means for detecting the optical power of an optical signal that was emitted by another network node. The optical network also has a means for determining whether the optical power detected is outside a defined range, and a means for generating, for that network node which emitted the optical signal, a control signal for increasing or decreasing the optical power if the optical power detected is outside the defined range. The optical network further comprises a means for sending the control signal to that network node which emitted the optical signal. 
   The optical network also has at least one network node having means for increasing or decreasing the optical power of the emitted optical signal as a function of corresponding control signals. 
   To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below using an exemplary embodiment with reference to the figures, in which: 
       FIG. 1  is diagram of an exemplary optical MOST network in which the optical power of the individual transmitters is regulated as a function of the path attenuation between the individual network nodes; 
       FIG. 2A  is a diagram of the frame structure of a data frame that is transmitted in the MOST network of  FIG. 1 ; 
       FIG. 2B  is a diagram of a MOST block having 16 data frames as shown in  FIG. 2A ; 
       FIG. 3A  is a diagram of the structure of a control message in the control channel of the MOST network; and 
       FIG. 3B  is a diagram of the structure of the message part of a control message of  FIG. 2A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows, as an example of an optical network, a unidirectional network having a ring topology. The exemplary embodiment illustrated in  FIG. 1  is a MOST ring  100 . The MOST ring  100  has a plurality of MOST devices  1 - 5  as network nodes. Each MOST device  1 - 5  is a physical unit that has at least one MOST transceiver having a transmitter TX and a receiver RX. 
   This is illustrated in detail for two MOST devices  2 , and  3 . The MOST device  2  has a transmitter  21 , a receiver  22  and a first electronic module  23 . Another electronic module which is furthermore provided and forms a microcontroller  24 , may likewise be included in the MOST device  2  or alternatively may be located externally. The microcontroller  24  controls the transmitter  21  and, in particular, sets the optical power of the latter. Further, the MOST device  3  and also the additional MOST components  1 ,  4 ,  5  have corresponding components  31 ,  32 ,  33 ,  34  which are not shown in any more detail for the sake of brevity, in the case of the additional MOST components  1 ,  4 ,  5 . 
   At the application level, the individual MOST devices  1  to  5  may have a plurality of components which are referred to as functional blocks representing, for example, a CD player or a telephone. They are known, as such, to persons skilled in the art and are not discussed in any further detail below. 
   Signals are transmitted in the MOST ring  100  in the following manner. If it is desired, for example, to send a message from the MOST device  2  to the MOST device  5 , this message is first of all sent, by way of the serial structure of the ring, to the device  3 , from the latter to the device  4  and from the device  4  to the device  5 . In this case, the signal intended for the device  5  is transmitted by the transmitter  22  in the form of an optical signal and is passed onto the ring  100 . The emitted signal is detected by the adjacent MOST device  3 , to be precise by its receiver  32 , or converted into an electrical signal. The microcontroller  34  uses signaling information such as the destination address and the message type to check whether the received data are intended for its own node. Since this is not the case in the exemplary embodiment described, the data are passed without any changes, but if appropriate after signal regeneration, to the transmitter  31  and emitted by the latter in the form of an optical signal. 
   The device  4  performs corresponding optical-electrical-optical conversion. The device  5  then detects that the data which have been sent are intended for the device  5  and the data are taken from the ring. An acknowledgement message is used to inform the device  2  (as the transmitting node) of the receipt so that the latter knows that the emitted data have been reliably received. 
   To avoid a problem, the optical power received by a receiving node or a receiver  32  should be within a particular range. For example, while the optical power received should be high enough to enable reliable signal detection, it should also not be high enough to avoid an overload. An excessively high optical power in the receiver also reflects an unnecessarily high optical power in the transmitter, which leads to increased power consumption and accelerated ageing of the transmitter. A defined range within which the optical power received by a receiver of a node  1 ,  2 ,  3 ,  4 , and  5  should be, is thus established. Optical power management, which ensures that the received optical power is always in the defined range is described, with reference to exemplary  FIG. 1 , wherein an optical signal is sent from the MOST device  2  to the MOST device  3 . 
   The optical signal emitted by the transmitter  21  of the MOST device  2  (also referred to below as the transmitting node  2 ) is transmitted to the receiver  32  of the MOST device  3  (also referred to below as the receiving node  3 ) via an optical point-to-point transmission path  101  of the MOST ring  100  of  FIG. 1 . The receiver  32  comprises, for example, a conventional photodiode. Means which determine the received optical power at the receiver  32  are provided in the receiving node  3 . These means are, for example, integrated in the module  33  that additionally performs preamplification. 
   The optical power, in one example, is determined by directly measuring the photodiode current of the receiver  32 . In this case, the photodiode current of the receiver  32  is filtered using a low-pass filter, for example. The current that is then present represents a measure of the input optical power and may be converted into a root-mean-square value of the input optical power using a calibration curve. 
   A check is then, for example, likewise carried out in the module  33  of the receiving node  3  to determine whether the received optical power determined is in the predetermined defined range. If this is not the case, the receiving node  3  forwards the corresponding information to the microcontroller  34 , for example. In this case, the microcontroller  34  may be arranged outside the receiving node  3  or alternatively may also be integrated in the latter. Provision may also be made for the microcontroller  34 , rather than the module  33 , to check whether the optical power received is or is not in the defined range. Furthermore, it is also conceivable, in principle, for the microcontroller  34  to constitute a central microcontroller of the MOST ring  100  rather than being specifically assigned to the receiving node  3 . In the former case, a central unit or a network management unit would thus check whether the optical power received is within the desired defined range. 
   If the optical power received is not within the desired defined range, the microcontroller  34  or a central network management unit generates, for the transmitting node  2 , a control signal for increasing or decreasing the optical power as a function of whether the optical power received is too low or too high. This control signal is preferably passed onto the control channel of the MOST ring. To this end, the control data of the received optical signal that has been converted into an electrical signal by the receiver  32  are correspondingly overwritten. This is effected, for example, by the microcontroller  34  or additional electronic modules (not illustrated). 
   A corresponding signal whose control data have been changed is then passed onto the ring  100  in the form of an optical signal by the transmitter  31  and forwarded to the device  4 . 
   In this case, the control signal that has been generated comprises, as the destination address of the control signal, the address of the device  2  so that the latter may detect that the control signal is intended for the device  2 . Because the serial ring structure is unidirectional, the device  3  may, in this case, simply give the destination address as the device number of the node that is arranged upstream of it in the unidirectional MOST ring  100 . If, for example, the individual MOST devices have the addresses  1 ,  2 , . . . n, . . . N, the respective receiving node gives the address n−1 as the destination address. 
   The control signal is passed to the device  4 , from the latter on to the device  5 , from the latter on to the device  1  and from the device  1  to the device  2 . The device  2  uses the destination address to detect that the corresponding control signal is intended for the device  2 . It evaluates the control signal to determine whether an increase or a decrease in the optical power is required. The optical power is then increased or decreased accordingly. 
   The power that has been correspondingly changed is again detected by the receiving node  3 , and if the optical power received is still not within the desired defined range, it is changed by emitting a further control signal. The optical output power of the transmitting node  2  is accordingly regulated until the optical power received at the receiving node  3  is within the desired range. 
   Corresponding regulating operations, for example, also take place between the further MOST devices of the ring, that is to say between the MOST device  3  and the MOST device  4 , between the MOST device  4  and the MOST device  5  etc. An ideal optical power level may thus be provided on each transmission path of the MOST ring  100 . 
   The generation of an exemplary control signal in the control channel of the MOST ring  100  is explained, by way of addition, with reference to exemplary  FIGS. 2A ,  2 B,  3 A,  3 B.  FIG. 2B  shows a MOST block having a length of 1024 bytes and containing sixteen frames  6  each having a length of 128 bytes.  FIG. 2A  shows such a MOST frame  6 . The frame  6  has a first region  61  having a length of 1 byte and containing management or administrative information. A second region  62  having a length of 60 bytes contains synchronous data for multimedia applications, for instance audio or video data. A third region  63  contains asynchronous data, for example packetized IP data for time-insensitive applications. A fourth region  64  comprises a length of 2 bytes containing control data. A fifth region has a length of 1 byte that contains CRC and parity information. 
   Since each frame contains 2 bytes of control data, a MOST block contains a total of 16×2=32 bytes of control data which may be regarded as block  8  below  FIG. 2B . The 32 bytes of control data transmit data associated with the control channel of the MOST ring. 
     FIG. 3A  shows an exemplary corresponding control data frame  8  having a length of 32 bytes. A first region  81  having a length of 1 byte contains administrative information. A second region  82  having a length of 2 bytes contains the destination address. A third region  83  having a length of 2 bytes contains the source address. A fourth region  84  having a length of 2 bytes contains the message type. A fifth region (message block)  85  having a length of 24 bytes contains the actual control messages. A sixth region  86  having a length of one byte contains the CRC checksum. A seventh region  87  having a length of 1 byte contains administrative information. 
     FIG. 3B  shows, by way of example, the subdivision of the message block  85  of  FIG. 3A . Various regions  851 - 857  which define particular functions and parameters are again defined. In the context of the present invention, the control signal relating to an increase or decrease in the optical power of an optical transmitter, in one example, is written to the message block  85 . The address “n−1” is given as the destination address in the second region  82 . 
   The configuration of the invention is not restricted to the exemplary embodiment described. Rather, numerous alternative configurations are conceivable. By way of example, the invention may also be implemented in other topologies, for example a bus structure or a star structure. This is possible without problems since the setting of the optical power is accommodated between two adjacent network nodes, independent of the role of the network topology. A bidirectional network may also be used instead of a unidirectional network. One exemplary application of the solution according to the invention is in the automotive sector, in the field of local area networks and in home networking. 
   While the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.