Abstract:
An optical system is provided comprising a first node and a channel drop add device. The first node is configured to transmit data onto an optical fiber in a first line direction. The channel drop add device ( 501 ) is adapted to receive and add channels onto the optical fiber thereby transmitting the data into the first and a second line direction. The network further comprises a second node configured to form a transmitter/receiver function. The second node is configured to receive data on said optical fiber from said first and second line directions. Further, the second node is adapted to synchronize received data from said first and second line directions by delaying the data signals seeing the shortest delay, by a delay device.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical network and devices and methods related thereto. 
       BACKGROUND 
       [0002]    In radio access networks (RAN), traffic can be conveyed using the Common Public Radio Interface (CPRI) format, from e.g., a Base band unit (BBU) (containing a Radio Equipment Control (REC)) to a remote radio head (RRH) where the Radio Equipment (RE) is located. 
         [0003]    When the Radio Equipment Control and the Radio Equipment are located at a distance from each other, CPRI supports that data on layer  1  (physical layer) can be transmitted from the REC to the RE using an optical interface. In other words, an electrical signal can be modulated onto an optical channel and transmitted as an optical signal over the optical interface, e.g. fiber, to the receiving side where it is demodulated into an electrical signal again, which is conveyable to a client interface. When the RE is located remote from the REC, the REC and the RE can be interconnected via an optical transmission line. 
         [0004]    There is a constant desire to improve the performance and robustness in networks. This is also the case for radio access networks. 
       SUMMARY 
       [0005]    It is an object of the present invention to provide an improved network. 
         [0006]    This object and/or others is obtained by the method, devices and system as set out in the appended claims. Systems and methods are provided for enabling protection of, for example, the CPRI channels without interfering with the CPRI protocol itself. Hence, data transmission on a channel can be protected from interruption. 
         [0007]    In accordance with one embodiment an optical network comprising a first node and a channel drop add device is provided. The first node is adapted to form a first transmitter/receiver function and configured to transmit data onto an optical fiber in a first line direction. The first node can comprise a transmitter adapted to perform the transmission of data onto an optical fiber in the first line direction. The transmitter/receiver can for example be a muxponder or a transponder. The channel drop add device is adapted to receive and add channels into a first and second line direction. The network further comprises a second node configured to form a transmitter/receiver function. The second node being configured to receive traffic from said first and second line directions. The second node can comprise a receiver adapted to receive data from said first and second line directions. The transmitter/receiver can for example be a muxponder or a transponder. The second node is adapted to convey data to a client interface for the data received for either said first or second line direction. The client interface can be the interface between the Radio Equipment (RE) and the Radio Equipment Control (REC). The second node is configurable to transmit data into the first and second directions using two line interfaces. The second node is configured to convey client data into one direction, the active line direction, selected from said first and second directions and the second node is configured to not convey client data into the other line direction, standby line direction, selected from said first and second directions. 
         [0008]    In accordance with some embodiments the second node can be adapted to synchronize received data from said first and second line directions by delaying the data signals seeing the shortest delay by a delay device. 
         [0009]    In accordance with some embodiments the second node is adapted to delay both data signals from the first and second line directions by an additional delay. 
         [0010]    In accordance with some embodiments the data from the chosen active line is conveyed to the client interface, and the data from the client interface aimed for the shortest path, through the first or second line as stated above, is delayed with the first delay. The data from the client interface aimed for the longest path, through the first or second line as stated above, is only delayed with a second, shorter, delay. 
         [0011]    In accordance with some embodiments the first node is detached from the channel drop add device. In some embodiments the first node and the channel drop and add device are co-located or integrated. 
         [0012]    In accordance with some embodiments the second node is configured to comprise a WDM (wavelength division multiplex) filter for the first and second line side. 
         [0013]    In accordance with some embodiments the channel drop add device is configured to only drop and add a parts of the used channels. 
         [0014]    In accordance with some embodiments multiple channel drop add devices are configured in a ring configuration. 
         [0015]    In accordance with some embodiments the system is adapted to perform transmission bidirectionally with upstream and downstream traffic in a single fiber. The channels can be closely separated. For example the channels can be separated at 50 GHz, 100 GHz, or 200 GHz. 
         [0016]    In accordance with one embodiment the transmission is conducted in a dual fiber configuration with upstream and downstream traffic in separate fibers. The upstream and downstream channels can have the same wavelength 
         [0017]    In accordance with some embodiments a CPRI channel is carried in the first and second line direction. 
         [0018]    In accordance with some embodiments several CPRI channels are multiplexed into one channel in the first and second line direction. 
         [0019]    The system can also be used for a client format other than CPRI having delay and delay variations. 
         [0020]    In accordance with some embodiments several formats having delay and delay variations are multiplexed into one channel. 
         [0021]    In accordance with some embodiments the first node comprises a line and client interface. 
         [0022]    The invention also extends to methods and devices used for implementing the above system and for transmitting data in the system. 
         [0023]    Using the systems, methods and devices as set out herein it is possible to provide a protected optical transmission path from a baseband unit located at a distance from the radio equipment that is able to switch from an active transmission path to a protecting stand-by transmission path at a very short time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where: 
           [0025]      FIG. 1  shows the principal layout of a modern radio access network (RAN) network. 
           [0026]      FIG. 2  shows a CPRI interface embodiments of the present invention. 
           [0027]      FIG. 3  illustrates the whole network including a protected ring. 
           [0028]      FIGS. 4 a  and 4 b    depict different implementations. 
           [0029]      FIGS. 5 a  and 5 b    show embodiments of a protected link system with at least two active transmission paths for the CPRI link. 
           [0030]      FIG. 6  shows a detailed data path. 
           [0031]      FIG. 7  is a flow chart illustrating some steps performed when transmitting data in a protected optical fiber system. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    In the following description, it is understood that all recited connections between structures can be direct operative connections or indirect operative connections through intermediary structures. A set of elements includes one or more elements. Any recitation of an element is understood to refer to at least one element. A plurality of elements includes at least two elements. Unless otherwise required, any described method steps need not be necessarily performed in a particular illustrated order. A first element (e.g. data) derived from a second element encompasses a first element equal to the second element, as well as a first element generated by processing the second element and optionally other data. Making a determination or decision according to a parameter encompasses making the determination or decision according to the parameter and optionally according to other data. Unless otherwise specified, an indicator of some quantity/data may be the quantity/data itself, or an indicator different from the quantity/data itself. Computer programs described in some embodiments of the present invention may be stand-alone software entities or sub-entities (e.g., subroutines, code objects) of other computer programs. Computer readable media encompass non-transitory media such as magnetic, optic, and semiconductor storage media (e.g. hard drives, optical disks, flash memory, DRAM), as well as communications links such as conductive cables and fiber optic links. According to some embodiments, the present invention provides, inter alia, computer systems comprising hardware (e.g. one or more processors and associated memory) programmed to perform the methods described herein, as well as computer-readable media encoding instructions to perform the methods described herein. 
         [0033]    The following description illustrates embodiments of the invention by way of example and not necessarily by way of limitation. Elements from different embodiments can be combined or substituted. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0034]      FIG. 1  shows the principal layout of a modern radio access network (RAN) network  100 . Traffic in RAN is conveyed in the Common Public Radio Interface (CPRI) format from e.g. a Base band unit (BBU)  101  (containing a Radio Equipment Control (REC)) to a Remote radio head (RRH) with Radio Equipment (RE)  103 . The Base Band Unit is in turn connected backwards in the Network via a network interface to e.g. a Mobile Switching Center (MSC). Depending on the RAN type the Base Band Unit can be connected to e.g. a Base Station Controller (BSC), a Radio Network Controller (RNC) or a similar higher level control node. 
       CPRI Channel Protection 
       [0035]      FIG. 2  shows a CPRI interface. The goal is to convey CPRI traffic from the REC  201  to the RE  203  without affecting performance, i.e. to transparently transfer one or several CPRI channels from the REC  201  to one or several remote RE nodes  203 . Several CPRI channels may be multiplexed into one proprietary channel. 
         [0036]    In earlier traditional networks the Base band unit (BBU) was located just below the antennas—Remote radio heads (RRH). Nowadays modern networks can have detached the BBUs to be sitting at a Base band station Hotel (BSH) that contains several BBUs. The distances between the BSH where the REC is located and the RRH where the RE is located can be up to ˜15-25 km or even more. 
         [0037]    The BBU contains a Radio equipment controller (REC) that manages the link. The protocol between the BBU and the RRH is in some embodiments CPRI. See CPRI Specification 6.1. In accordance with other embodiments other formats can be used. For example OBSAI (Open Base Station Architecture Initiative) format can be used in some embodiments. 
         [0038]    As has been realized by the inventors it would be advantageous if data from the REC to the RE could be transmitted in a protected manner so that it is ascertained or at least more likely that all data transmitted between the REC and the RE will arrive with acceptable time delay so that there will be no (or at least less) interruption in the data traffic between the REC and the RE is the currently active path is disconnected for some reason. This can be achieved by providing a network connecting BBUs in the Base band hotel (BBH) to one or several CPRI drop node(s)—here also called DU node. The DU node drops, in a protected way, the CPRI channels and then convey the CPRI channels to the RRH. 
         [0039]    The DU node is likely to be sitting at a location close to the RRH, e.g. on a roof, and be transferring CPRI channels to the RRH. However, it can be hard and costly to protect the CPRI channels going up the last meters to the roof, which is why these meters may be left unprotected in some embodiments. 
         [0040]    In accordance with one embodiment the protected network is built by a system where upstream data is transmitted from a node, e.g. the DU node described above or an add/drop node as described below, in two separate optical transmission paths to a node where the base band unit is located and downstream data is transmitted along one of the two separate optical transmission paths. Upstream is a direction from an RRH towards a BBU, whereas downstream is a direction from a BBU towards an RRH. The REC needs to know and compensate for the latency in a signal between the BBU and the RRH. This can be performed by a synchronization process between REC in the BBU node and RRH. The synchronization process can be based on round trip measurement where a symmetrical delay for down and uplinks transmission time is required to achieve a good accuracy. (See CPRI Specification 6.1 Delay calibration) An asymmetrical delay between these will decrease the accuracy to a point where it is unable to be used by the CPRI protocol. 
         [0041]    The maximum distance between BSH (&amp; REC, BBU) and RRH can be in the order of 15 km or more. Longer distances can be envisaged and the size of a FIFO buffer (or similar delay devices) used to compensate for different transmission lengths can be dimensioned accordingly. The transmission time can typically be about 75 μs over an optical transmission line. The refractive index in an optical fiber differs up to 0.5% for different fiber types. However, in all normal cases the same fiber type is used for upstream and downstream traffic. Nevertheless, the refractive index can differ even for the same fiber type in a two-fiber solution. A 0.1% difference is quite likely to happen in real networks. This difference would give 75 ns difference between up and down link assuming a two fiber solution. This would alone lead to 37.5 ns error in the delay calculation. In addition, a two fiber solution might have different fiber lengths, especially in passing fiber distribution panels or WDM filters. 
         [0042]    The needed delay difference accuracy is (see CPRI Specification 6.1) about 8 ns which corresponds to less than 2 m fiber. Altogether, to guarantee a good accuracy the up and down CPRI links are advantageously transmitted in the same medium, i.e. a bidirectional scheme in a single fiber is preferred. 
         [0043]    Optical ring topology is supported in the CPRI specification. However this is done using traditional Time Division Multiplex (TDM) rings. 
         [0044]    Ring protection supporting several wavelengths is desired feature for these RAN networks. This may be done having two separate CPRI links over a WDM network where the CPRI links are routed in two different paths. However this requires dual CPRI interfaces both at the BBH node and at the RRH and larger internal switching. 
         [0045]    Furthermore, the REC would need to establish and control two CPRI links for each channel. As earlier has been explained the RRH is sitting at the antenna and the DU at the roof. A scheme having dual CPRI links from RRH to RRH would require more equipment at the antenna. This should preferably be avoided. 
         [0046]    As set out above this can be done using a bidirectional scheme having single CPRI interface both at the BBU as well at the RRH. However, switching from a working optical transmission path to a protecting optical transmission path, the CPRI channel has to reestablish the link. A new synchronization with respect to frequency and absolute frame timing accuracy may take up to 10 seconds (see CPRI Specification 3.9.1) plus the time for auto-negotiation of features. 
         [0047]    To enable switching from an active to a stand-by path, the switching can be performed over to an already active protection path. However, this is not supported by the current CPRI standard specification 6.1 from 2014. 
         [0048]    In accordance with one embodiment a ring network containing many CPRI channels aimed for different DUs can be configured to not be connected directly to each DU since a DU is typically sitting at the roof. Thus, a ring configuration with a connection via a DU node would not only increase the transmission link but also increase the risk of a fiber break. In accordance with one embodiment a channel drop/add node (CDAM), detached from DU node, can then be used. The channel drop/add nodes can then be located close to the DU node, but at a location separated from the DU node to reduce the risk of fiber brake. For example the DU node can be located at a roof whereas the drop/add node can be located at ground level below the roof. 
         [0049]    Another desired feature is to have per channel protection (in contrast to link protection). This increase the reliability as well as the flexibility. Moreover, it enables a ring network where the CDAM only drops a band of the channels and another (or several) CDAM drops another band. In case of a fiber break between two CDAMs the traffic still can be carrying traffic. 
         [0050]    A network configuration as set out above will accomplish the requirement of having symmetrical delay for uplink and down link. Protection is accomplished without the REC and RRH having to renegotiate the CPRI channel. Protection with WDM transmission will be achieved well within 50 ms protection time. Channel protection will enable higher reliability and more flexibility. The network configuration will be cost effective, using the same transceiver at DUs both for active and protection path. 
         [0051]    The network configuration provides a protected link carrying CPRI channels in a cost effective way. Latency compensation is done combined with a protection scheme where protection is done dual ended even though the protection only is actively done at one side. 
         [0052]      FIG. 3  illustrates an exemplary complete network including the protected ring. This embodiment provides a protected interface/path at the CPRI interface. The right part of  FIG. 3 , i.e. boxes  311 ,  313 ,  309 ,  307 ,  307   b ,  307   c  is preferably located at a BBH (base band hotel). The CDAM (channel drop add multiplexer)  301  is typically placed in a manhole or in a small distribution site. The DU node  303  is typically placed at the roof, below the antenna  305  or at some location close to an antenna associated with the DU node  303  antenna  305 . It is understood that any suitable number of CDAM  301  and corresponding DU nodes  303  can be provided in the ring, but only one is depicted here. Below some exemplary signaling steps are illustrated. The time delay difference of arrival at the second node between data sent in the working path and data sent in the protected path can be measured by calculating the time difference between, for example, the Start of Frame on the different paths. To know which path is shortest, a simple frame number mechanism can be used. But other methods are envisioned like using known bit-patterns such as start of packet, exemplified by Ethernet Start of Packet or time stamps, exemplified by Ethernet Time of Day.  FIG. 3  further comprises multiplexer/demultiplexer, MUX/DEMUX A side  311 , a MUX/DEMUX B side  313 , a CPRI transponder/muxponder, TP/MXP  309 , and a BBU/REC  307  connected to the CPRI TP/MXP  309 . In the case the CPRI TP/MXP  309  is a muxponder, there may be more than one BBU/REC  307 ,  307   b ,  307   c  connected to the CPRI TP/MXP  309 . In an embodiment, there may be more than one CPRI TP/MXP  309  connected to the MUX/DEMUX A side and B side  311 ,  313 , and in that case, the more than one CPRI TP/MXP  309  may each be connected to one or more BBU/REC  307 . The MUX/DEMUX A side  311  is connected to a first side  302   a , also called first line direction, of the optical cable  302 . The MUX/DEMUX B side  313  is connected to a second side  302   b , or second line direction, of the optical cable  302 . The CDAM  301  is arranged to add and drop optical signals to/from the optical fiber  302 . 
         [0053]    In the downstream traffic:
       The BBU/REC  307 ,  307   b ,  307   c  is connected to a CPRI TP/MXP  309 . The CPRI TP/MXP  309  can be a transponder or a muxponder. If CPRI TP/MXP  309  is implemented as a muxponder, several CPRI channels nay be multiplexed by CPRI TP/MXP onto one channel. The former may be a proprietary format or the CPRI format itself. One or several CPRI channels may also be carried by Ethernet. In such a case it is Ethernet.   The CPRI protocol stays untouched in the CPRI TP/MXP  309 . The latency is also constant (apart from any buffer adjustments)   The CPRI TP/MXP is connected to the MUX/DEMUX A  311  and MUX/DEMUX B  313  but may only transmit on the active side, e.g. side A.   The CDAM (channel drop add multiplexer)  301  may drop and add the entire band from A and B side and combine/divide these with an optical coupler. Alternatively it may only drop a certain band or a certain selection of the available channels.   The DU node  303  contains a mux/demux (selecting the channels in use) and one or several CPRI TP/MXPes. The CPRI TP/MXP in DU node  303  receive the channel (only A or B transmitter side is active at the BBU).   The CPRI TP/MXP in DU node  303  receives the channel and demultiplex the channel (if MXP) and transmit the CPRI channel to the RRH. This is preferable done using a bidirectional small form-factor pluggable, SFP/SFP+, optical transceiver over single fiber or a normal SFP/SFP+ optical transceiver over a two fibers. No buffer adjustment is done. The SFP is a compact, hot-pluggable transceiver used for both telecommunication and data communications applications, for data rates below 10 Gbit/s. The enhanced SFP (SFP+) is an enhanced version that supports data rates up to 16 Gbit/s.       
 
         [0060]    In the upstream traffic:
       The CPRI TP/MXP in DU node  303  receives CPRI signal from RRH  305  (using normally a single or a dual SFP/SFP+)   If using a MXP several CPRI channels are multiplexed into a proprietary channel.   The CPRI TP/MXP in DU node  303  transmits the signal to the CDAM  301 .   The CDAM  301  divides the signal and add this to A direction  302   a  and B direction  302   b.      The MUX/DEMUX A  311  and MUX/DEMUX B  313  receive signal from A and B and feed this into the CPRI TP/MXP  309 .   The CPRI TP/MXP  309  receives the signals and compare the delay between A and B path.   The time delay difference D is added to the signal seeing the shortest delay using a FIFO buffer, in the electrical domain.   In addition another delay T is added to signals. T is normally much shorter than D and is used to align the longer link with the shorter when this is the standby link. This to maintain a constant delay at the active link.   If the CPRI TP/MXP  409  is a muxponder, MXP; the format is demultiplexed into several CPRI channels.   The active signal path is fed to the output.   A delay will be added to the downstream traffic. At start up T and D+T will be added to the two different paths.   Once started, the buffer for the active path, e.g.  302   a  may not be adjusted. Instead, both upstream and downstream buffer adjustments shall be done at the standby path, e.g.  302   b . This is done to ensure that the CPRI protocol will not require adjustment (beside what is done in the protocol itself).   The CPRI TP/MXP  309  at the BBH constantly monitor anomalies and defects, e.g. LOF (loss of frame), LOS (loss of signal), SD (signal degraded), etc. In case of e.g. a LOS, a transmitter of the CPRI TP/MXP  309  facing the faulty side, i.e. the active path, where the LOS was detected, shall be turned off and a transmitter facing the standby path shall be switched on. This can be done within a few ms.   The upstream CPRI signal will have a very short interruption caused by the detection and switch over time of the CPRI TP/MXP  309 . The downstream signal will see a small additional delay caused by the transmission time through the fiber  302 . The delay can typically be 75 μs assuming 15 km for the longest path.   It is assumed that the CPRI protocol won&#39;t need to reestablish the link having this short interrupt.       
 
         [0076]    Notably, the MUX/DEMUX A side  311  and the MUX/DEMUX B side  313  and the CDAM  301  will preferably act on a single fiber (i.e. bidirectional communication) but may also be acting on two different fibers. The guaranteed latency accuracy is in this case much less. Several CDAM  301  may be sitting at a fiber ring. The CDAM could be implemented for example using an optical coupler  416  (see  FIG. 4 a   ) and alternatively using a band drop/add filter combined with a coupler  422  (see  FIG. 4 b   ). Several fiber rings may be used. 
         [0077]    The proposed solution is most suitable using a DWDM but CDWM can also be used but then with much less capacity. The MUX/DEMUX  311 ,  313  could be implemented using single unit mux/demux with 100 GHz channel spacing, a mux with 100 GHz channel spacing and demux with 100 GHz channel spacing combined with an OIU (optical interleaver unit) or a mux and demux combined with an optical circulator. The mux and the demux at the latter two cases are shifted 50 GHz from each other creating a system with 50 GHz channel spacing where every second channel is going downstream and every second channel is going upstream. 
         [0078]      FIG. 4 a    depicts an 80 channel implementation using bi-directional communications. The CDAM includes in this case of two 3 dB couplers  416 ,  417  adding and combining signals to/from the A and B side of the fiber  402 . The fiber  402  comprises in this alternative four fiber rings  402   a - d . The DU node comprises two MUX/DEMUX units  412   a ,  412   b ,  413   a ,  413   b . Each MUX/DEMUX is built up of an even and an odd channel WDM filter  418   a ,  418   b  e.g. an AWG filter. The MUX/DEMUX  412   a ,  412   b ,  413   a ,  413   b  each has two TP/MXP  432   a ,  432   b  connected to the WDM filters  418   a ,  418   b . The MUX/DEMUX  412   a ,  412   b ,  413   a ,  413   b  are connected to antennas/RRH  405   a ,  405   b , via the TP/MXPs  432   a ,  432   b . The MUX/DEMUX  412   a ,  412   b  further connect to the 3 dB coupler  416  through optical circulators  419   a ,  419   b  allowing the even channels to be transported upstreams over the fiber rings  402   a - d , i.e. towards the BBU, and the odd channels to be transported downstreams. The 3 dB coupler box  416  comprises the actual 3 dB coupler unit  416   a  and a fiber cable connection  416   b  connecting the 3 dB coupler unit to the fibers from the optical circulators  419   a ,  419   b . There are two advantages using this scheme. Firstly, it reduced the loss and cost compared to using an OIU (optical interleaver unit) even if an OIU has some benefits and could be a possible realization of the optical circulator. Consequently, an alternative realization of the optical circulators  419   a ,  419   b  may be an OIU. Secondly, the DU node is likely to be placed in I-temped environment, i.e. placed in an environment fulfilling an industry temperature range such as −40 to 85 degrees Celsius. 
         [0079]    The leftmost box  411  of  FIG. 4 a    depicts a 20 channel mux/demux solution without the use of a circulator, i.e. an alternative realization of the MUX/DEMUX  412   a ,  412   b . All channels are even in this case. The system of  FIG. 4 a    further comprises transceiving units  415   a ,  415   b ,  415   c ,  415   d , one for each fiber  402   a - d , the transceiving units including MUX/DEMUX and TP or MXP, the transceiving units being connected to the fiber at a different location of the fiber than the 3 dB couplers  416 ,  417 . The transceiver units may be part of, or connected to, a baseband hotel comprising one or more BBUs. Other filter configurations are of course also possible. One such configuration is depicted in  FIG. 4 b   .  FIG. 4 b    shows a fiber ring  420  to which an CDAM  422  is connected. The CDAM has a 3 dB coupler  429  that receives signals from the fiber ring (downstream) and a band multiplexer  423  connected to the 3 dB coupler. The band multiplexer  423  is in its turn connected to a number of channel filters  424   a, b, c, d, e , the filters being arranged to filter optical signals sent from/to RRHs/antennas  425   a ,  425   b . The RRHs  425   a ,  425   b  are connected to the channel filters via TP/MXPs  428   a ,  428   b . The system of  FIG. 4 b    further comprises a BBU  426  that comprises a transceiving unit  427 . The BBU may be a part of a baseband station hotel. The transceiving unit  427  may be similar to the transceiving units  415   a - d  of  FIG. 4   a.    
       Delay Compensation 
       [0080]      FIG. 5 a    shows one embodiment of the protected link system with at least two transmission paths, a first line direction and a second line direction, for the CPRI link. The embodiment comprises an RE  505  connected to a DU node  503  comprising a transponder or muxponder, TP, and an MDU. The DU node  503  is in its turn connected to a channel drop add device  501  that is connected to an optical fiber. The optical fiber has a first part  502   a  and a second part  502   b  that both extends from the channel add drop device  501  towards a BBU/REC  507 . The first part  502   a  may be seen as a first line direction between the channel add drop device and the BBU/REC. The second part  502   b  may be seen as a second line direction between the channel add drop device and the BBU REC. As a further example, several DU nodes could also be used. 
         [0081]    The signal from the RE (The antenna)  505  is coupled on two fiber parts  502   a ,  502   b . In the BBU/REC node  507 , the TP (Transponder or muxponder) will have two incoming signals, one from the first part  502   a  and one from the second part  502   b , and is using one of them for communication. In case of a fiber break on the working fiber part it switches to the other fiber part. There can be many coupling devices  501  (such as the channel drop add multiplexer described above) provided in the same ring as depicted in  FIG. 5 b   . I.e.  FIG. 5 b    shows another example of a protected link system where there are two REs  505   a ,  505   b , each connected via a DU  503   a ,  503   b  to a CDAM  501   a ,  501   b  coupled to the optical fiber. Further, there may be two TPs and two BBU-units in the BBU node  507 , the TPs and BBU-units handling on RE  505   a ,  505   b  each. Also, as the REs (RRHs)  505   a ,  55   b  has three antennas each in the examples of  FIGS. 5 a  and 5 b   , each DU  503 /BBU  507  may have three TPs, one for each antenna. 
         [0082]    In the opposite direction from the BBU  507  towards the antenna  505 , the TP in the BBU unit is only transmitting on one of the fibers  502   a ,  502   b . The active transmission fiber can be the same fiber as the TP is receiving on. In case of a fiber break on the active fiber TP starts to transmit on the other fiber. This scheme has a drawback and that is that the delay over the two fibers might be different, see  FIG. 6  below. The improved scheme as described herein solves this problem. 
         [0083]      FIG. 6  shows a detailed data path. The embodiment shows an RE  601  connected to a first TP/MXP  602 . D1 symbolizes a delay of a signal between the RE  601  and the first TP/MXP  602 . D3 symbolizes the delay of an optical signal transmitted in the first optical fiber path, a working path, from the first TP/MXP  602  towards a second TP/MXP  604  arranged at the optical fiber. D4 symbolizes the delay of an optical signal transmitted in the second optical fiber path, a protection path, from the first TP/MXP  602  towards the second TP/MXP  604 . Further, the second TP/MXP  604  is connected to, or incorporated in, a REC  605 . The first TP/MXP  602  is further connected to an optical splitter  603  arranged for sending an optical signal received from the first TP/MXP  602  into both the working path and the protection path. Further, the optical splitter can receive a signal both from the working path and the protection path but will normally only receive signals in either one of the working path and the protection path at the same time. The optical splitter  603  can be a part of the TP/MXP  602 . The lower part of  FIG. 6  shows the second TP/MUX  604  in a close-up view. A signal received from the REC  605  is received at a splitter  606  that copies the received signal and send it further to optical transceivers  608   a ,  608   b . At any given moment only one of the transmitters in the optical transceivers  608   a ,  608   b  is active. If an error condition is detected in the receive path, e.g. in the working path, by the second TP/MXP  604 , then a switch is made to the protection path and the transceiver  608   a  in the working path is turned off and the transceiver  608   b  in the protection path is turned on.  607  is a selector that selects to receive signals from the working path and the protection path. A WDM filter  614  is arranged as an interface of the TP/MUX  604  to the working path. The WDM filter  614  multiplexes respectively de-multiplexes data from/to the working path. In a similar way, a WDM filter  613  is arranged as an interface of the TP/MUX  604  to the protection path. D2 symbolizes a delay of a signal in and/or between the second TP/MXP  604  and the REC  605 . A total delay from RE  601  to REC  605  would be over the working path D1+D2+D3, and over the protection path D1+D2+D4. 
         [0084]    To compensate for the different fiber delays, D3 and D4, a delay compensation can be done in the TP  604 . This can be performed by putting the Radio data (CPRI) in a TDM frame with a unique Start of Frame (SOF) delimiter. The TP  604  can compare the two incoming links and add a delay (ΔD=D4-D3) via a FIFO (First in First out) buffer (or other delay elements)  609  on the shortest path in order to have the same latency regardless of which fiber is active. In this embodiment, the signals are in the electrical domain when they are delayed. In the transmit direction, towards the BBU/REC node, the same delay, ΔD also denoted D herein and in  FIG. 6 , is added on the shortest fiber and you will have a symmetrical system. The size of the delay element, for example the size of the FIFO buffer will set the limit on how long the difference between the active fiber path and the protection fiber path can be. An equal delay ΔD can be added in both the receiving direction and the transmitting direction on the shortest fiber to achieve a symmetrical system with equal delay on the (at least) two fibers where one is working and the other(s) serve as protection fibers. 
         [0085]    The frames are numbered to be able to calculate which path that has the shortest delay. Since the same data goes out on both paths it is possible to find which one has the shortest delay. 
         [0086]    In accordance with one embodiment an additional typically very small delay T, smaller than ΔD, is added on both paths to enable small adjustments on the non-active path to compensate for latency differences that temperature differences cause. The delay T is preferably variable to allow for the system to compensate for the small differences that could result from e.g. temperature differences. 
         [0087]    In  FIG. 7  steps performed in an optical network comprising one or many remote nodes located in conjunction with a radio equipment are shown. The node(s) are in communication via an optical fiber with a base band node that is located in conjunction with a base band unit remote from the one or many nodes located in conjunction with the radio equipment. There are (at least) two optical transmission paths from the one or many remote nodes to the base band node. The remote nodes can be configured in a ring configuration. The remote nodes can be provided with a channel drop add device. In a first step  701  data from the remote nodes are transmitted in both of the two optical fibers to the base band node. The base band node can receive both data signals in a step  703 . The data received over the shortest transmission line can be delayed in a step  705  with a delay D corresponding to the transmission path difference so that the two data signals are received in a synchronous manner. The base band node transmits data to the remote node(s) via one of the optical transmission paths, the active optical fiber path, in a step  707 . The other optical fiber path serves as a protecting, stand-by, optical fiber path. In case the transmission on the active fiber path is interrupted, the base band node switches to transmit data on the stand-by optical fiber path in a step  709 . 
         [0088]    It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.