Abstract:
System for connecting a plurality of digital subscribers to a data network, comprising a local part connected, by an optical fiber link, to a remote part. Said local part comprises a plurality of DSLAM line cards with xDSL over fiber transceiver that, in turn, comprise a plurality of xDSL lines comprising a modified analog front end which comprises an digital-analog converter to transform a transmitted digital downstream signal into an analog downstream signal; an oscillator which fixes an oscillation frequency for the xDSL line; a mixer, directly connected to the output to convert the analog downstream signal into the oscillation frequency; and a band pass filter centered at the oscillation frequency which filters the converted analog downstream signal. And in upstream direction: a band pass filter centered at the oscillation frequency which filters an analog upstream signal; an oscillator which fixes an oscillation frequency for the xDSL line; a mixer, directly connected to the output of the band pass filter to convert the analog downstream signal into the original frequency; a low pass filter to avoid aliasing; an analog front end which comprises an analog-digital converter to transform the converted analog upstream signal into a digital upstream signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase Application of PCT International Application No. PCT/EP2011/073222, International Filing Date Dec. 19, 2011, which claims priority of Spanish Patent Application No. P201031875, filed Dec. 17, 2010, both of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to data networks more specifically to the connection by means of optical fiber and service to digital subscribers. 
     BACKGROUND OF THE INVENTION 
     xDSL technologies are the most widespread broadband technologies. These technologies, which comprises, for example ADSL, ADSL2, ADSL2+ or VDSL2 use Telco&#39;s copper access networks to provide broadband connectivity. xDSL signals are transmitted over metallic pairs from a Central Office to customer premises as it is shown in  FIG. 1 . 
     The downstream traffic, from the network to end users is aggregated by de DSLAM (Digital Subscriber line Access Multiplexer)  1 , where there are several xDSL DSLAM line cards  2 , and transmitted to the CPE (Customer Premises Equipment)  4  located at customer premises  19  over a metallic pair  11 . The upstream traffic, from the end user to the network is collected by the CPE  4  and transmitted over a metallic pair  11  to the DSLAM  1  which typically is located at a Central Office  17  ( FIG. 1 ) but it can also be located out that point, in a remote node  18  ( FIG. 2 ). 
     The xDSL signals share the metallic pair  11  with voice signal from the PSTN (Public Switched Telephone Network) service. In order to share the metallic pair  11  spectrum, splitters  14   a  and microfilters  14   b  are used. For each metallic pair, there is a splitter  14   a  at the Central Office  17  or at the Remote Node  18  (see  FIG. 2 ) side, and there is either a splitter  14   a  or microfilter  14   b  at the customer premises side. Voice splitters  14   a  and microfilters  14   b  divide/combine the xDSL and voice signals. The voice signal is exchanged between the PSTN switch  16  at the Central Office  17 , and the telephone  15  at customer premises  19 . In case of FTTN (Fiber To The Node) deployments ( FIG. 2 ), the PSTN switch  16  can be located at Central Office  17  or if it a small one, it can be located at the remote node  18 . 
     Splitters  14   a  and microfilters  14   b  are only required in case that voice services are provided using circuit switching technology. But the voice service can also be provide by means of VoIP (Voice over IP) using packet switching technology. In case of VoIP voice services, splitters  14   a  and microfilters  14   b  are not required. 
     xDSL technologies provide broadband access over existing metallic (typically copper) pair access network. But there are some constraints for these technologies:
         Metallic pair attenuation. This attenuation increases with the length of the pair. That means that the SNR (Signal to Noise Ratio) decreases as metallic pair length increase. So, in order to keep the BER (Bit Error Rate) below a maximum threshold, the bitrate provided by xDSL decreases as the metallic length increases.   Crosstalk, a disturbing signal which appears when there are several xDSL links over metallic pairs that share the same cable or binder. These parasitic signals appear due to capacitive and inductive coupling between adjacent pairs. Crosstalk signals increase significantly the noise level and thus reduce significantly the SNR of the received xDSL signal in the disturbed pair.   Noise: transient signals electromagnetically coupled which appears randomly and creates burst errors.       

     These constraints involve that using xDSL for broadband access, the access bit rate cannot exceed a net bitrate of 6-8 Mbit/s beyond 2.5 km away from Central Office, depending on wire gauges and pair isolation. 
     The introduction of fiber into the local loop, replacing partially or completely the metallic pair improves significantly the xDSL performance due to the low attenuation of fiber and its electromagnetic immunity which avoids crosstalk and noise problems. That is the reason why some Telcos has deployed FTTN xDSL access networks, following the scheme shown in  FIG. 2 . This solution introduces the fiber into the aggregated link between the DSLAM  1  located at the Remote Node  18  and the Central Office  17 . This approach allows using short metallic pairs between each DSLAM  1  port  3  and each CPE  4 . So the attenuation and crosstalk levels decrease and the noise risk also decreases. The FTTN approach could permit to extend both the coverage and the bitrate of broadband access. But it is a very expensive approach because to achieve such goal, it would be necessary to deploy a lot of remote DSLAMs which is very expensive both in capital and operation, apart from other important issues like DSLAMs feeding and the location problem that can be more complex than the base stations or Node B location for mobile services. Some patents propose solutions of this kind. 
     Some alternative solutions can be found looking at the patents WO0245383 A2 “Apparatus for connecting digital subscriber loops to central office equipment”, CA2346573 A1 “Arrangements for connecting digital subscriber loops to central office equipment”, CA2353594 A1 “Extended distribution of ADSL signals” or US2004264683 A1 “Hybrid Access Networks and Methods”. 
     But these solutions entail some other problems as looking for sites to locate the Remote Nodes  18 , either in the street or in buildings; managing and monitoring remotely the equipments, the DSLAMs  1 , located at the Remote Nodes  18 ; or remote powering of a fully equipped DSLAMs  17  located out of Central Office  17  premises. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforementioned problems by disclosing a system which provides an improvement of existing systems. Present invention considers the modification of current DSLAM line cards, with an optoelectronic transceiver required to transport xDSL signals over optical wavelengths included into DSLAM line cards and a simplification of the typical Analog Front End, thus the resulting modified card does not require any hybrids or solid state hybrids transformers to combine/split downstream and upstream xDSL signals because multiplex are created over the fiber before transmission over metallic pairs and the 2 wires conversion is not needed. 
     According to a first aspect of the invention, a system for connecting a plurality of digital subscribers to a data network is provided. Digital subscribers send analogue upstream signals to the data network and the data network sends digital downstream signals to the digital subscribers. The system comprises a local part, located at a central office, connected by an optical fiber link to a remote part, located at an intermediate place between the central office and the digital subscribers. Said local part further comprises a plurality of digital subscriber line access multiplexer line cards with xDSL over fiber transceiver, and said digital subscriber line access multiplexer line cards with xDSL over fiber transceiver comprises:
         a plurality of xDSL lines comprising:
           in downstream direction:
               an analog front end which comprises an digital-analogue converter to transform a transmitted digital downstream signal into an analogue downstream signal at a original frequency and at least one amplifier;   an oscillator which fixes an oscillation frequency for the xDSL line;   a mixer, directly connected to the output of the at least one amplifier to convert the analog downstream signal into the oscillation frequency;   a band pass filter centered at the oscillation frequency which filters the converted analog downstream signal;   
               in upstream direction:
               a band pass filter centered at the oscillation frequency which filters an analogue upstream signal;   an oscillator which fixes an oscillation frequency for the xDSL line;   a mixer, directly connected to the output of the band pass filter to convert the analogue downstream signal into the original frequency;   a low pass filter to avoid aliasing   an analogue front end which comprises an analogue-digital converter to transform the converted analogue upstream signal into a digital upstream signal, and at least one amplifier;   
               
           an adder circuit to combine the analogue downstream signals received from each xDSL line;   an optical upconverter centered at a downstream wavelength, directly connected to the output of the adder, and the output of said optical upconverter is delivered to a wavelength division multiplexer;   an optical downconverter centered at an upstream wavelength, directly connected to a wavelength division multiplexer;   a wavelength division multiplexer which is connected through a point to point single model fiber to the remote part.       

     The system of the invention has an oscillator for each xDSL line. Said oscillator fixes an oscillation frequency for each xDSL line higher Δf than the previous one, being Δf:
         Δf&gt;1.014 Mhz for ADSL and ADSL2;   Δf&gt;2.208 MHz for ADSL2+;   Δf&gt;30.000 MHz for VDSL2;       

     According to a second aspect of the invention, the system of the invention includes a terminal unit into the remote part establishing a link between a port at the digital subscriber line access multiplexer and said terminal unit for remote monitoring using monitoring facilities of xDSL Operation, Administration and Maintenance mechanisms. 
     Finally, the system can include optionally some elements to introduce wavelength division multiplexing in xDSL over fiber. That is:
         including a plurality of switches to assign the xDSL lines to the wavelength pairs and a switch control module implemented to control said plurality of switches.   including a block in the data network to transport a first additional wavelength for digital television distribution and a second additional wavelength for an optical outside plant supervision, said block comprises:
           a wavelength division multiplexer to split wavelengths for xDSL transport from additional wavelengths;   a cyclic Arrayed Waveguide Grating connected to the wavelength division multiplexer to split the wavelengths for xDSL transport into a plurality of output fibers;   a passive splitter connected to the wavelength division multiplexer to divide the optical power of the wavelengths used for additional wavelengths into a plurality of output fibers;   a plurality of wavelength division multiplexer couplers connected to the cyclic Arrayed Waveguided Grating and the passive splitter to combine at each output fiber the wavelength pair for xDSL transport with the additional wavelengths;   
           including in the remote part an Automatic Wavelength Locking module which receives a signal injected in the local part and transmitted through the cyclic Arrayed Waveguide Grating to an automatically tuning of the assigned wavelengths used for the transmission.       

     To conclude with the advantages of present invention it is appropriated to point out that it means an improvement of actual systems providing an increase of the optical budget of 3 dB, which is a coverage radius increase comprised between 3.5 Km and 5 Km. The coverage increase provided by current proposal permits a drastic reduction of the number of Central Offices  17  in high-population urban areas by a factor of thirty. Proposed invention could provide a minimum net access bitrate of 10 Mbit/s up to 20 km away from Central Offices  17 . 
     Current invention permits the transport of multiple xDSL multiplexed over different wavelengths in the same fiber, using a different pair of downstream and upstream wavelengths for each xDSL multiplexed. 
     In case of multiple wavelength pairs for xDSL signals transport, the system may include a module which permits an automatic tuning to the corresponding wavelength. 
     Current invention may includes a mechanism for the remote monitoring of the xDSL over fiber links. 
     And current invention includes a mechanism which permits the coexistence in the optical distribution network of different wavelength pairs for xDSL transmission, digital television transport and optical outside plant monitoring. 
     For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of aiding to better understand the characteristics of the invention according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character: 
         FIG. 1  shows a diagram of xDSL access deployed from a central office in the prior art. 
         FIG. 2  shows a diagram of xDSL access deployed from a remote node in the prior art. 
         FIG. 3  shows a diagram of xDSL over fiber approach of the prior art. 
         FIG. 4  shows a diagram of xDSL over fiber with an integrated xDSL over fiber transceiver—central into DSLAM line cards. 
         FIG. 5  shows a block diagram of a current xDSL port in a DSLAM line card. 
         FIG. 6  shows a block diagram of the DSLAM line card with xDSL over fiber transceiver—central. 
         FIG. 7  shows a block diagram of a terminal unit embedded into xDSL over fiber transceiver—remote for remote monitoring. 
         FIG. 8  shows a block diagram including modifications of the data network required to permit the coexistence of several wavelengths. 
         FIG. 9  shows a block diagram of a xDSL DSLAM line card with tunable xDSL over fiber transceiver—central. 
         FIG. 10  shows a xDSL over fiber transceiver—remote with an automatic wavelength locking system. 
         FIG. 11  shows a block diagram of an automatic wavelength locking system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In present invention, xDSL signals are transmitted over a fiber in order to improve coverage and access bitrate provided by xDSL access technologies. The usage of fiber drastically reduces the attenuation and crosstalk, and improves noise resilience. All these factors improve significantly xDSL performances: bitrate and coverage. 
     The system proposed introduces fiber in the loop, providing the maximum performance increase in both bitrate and coverage, while minimizing the impact in currently deployed xDSL access infrastructure. The only change required is focused on DSLAM line cards  2 , while currently deployed xDSL Customer Premise Equipment  4  (CPE from now on) are kept without any change. The proposal assumes the replacement of legacy voice services based on circuit switching technology by Voice over IP (VoIP) services, as it is been doing now with fiber access based on GPON solutions. That is the reason why voice splitters  14   a  and microfilters  14   b  are no longer required in the xDSL links considered in present invention. 
     Some patents propose solutions, as presented in  FIG. 3 , consisting of an optoelectronic module, that it is called from now on xDSL over Fiber transceiver—Central  5 , hosted in a local part. This optoelectronic module  5  is connected through a point to point Single Mode Fiber (SMF)  7  to a remote optoelectronic module that it is called from now on xDSL over Fiber transceiver—Remote  9  closely located to Customer Premises  19 . The remote part, can be located in the street near the building or even inside the building which is served. 
     Present invention includes the xDSL over Fiber transceiver—Central  5  into the DSLAM line card  2  So the xDSL over fiber scheme is the one shown in  FIG. 4 . 
     The present invention includes a simplification of the Analog Front End  13  (AFE from now on) in the xDSL over fiber Transceiver—Central  5 . 
     A typical AFE  13  of an xDSL DSLAM port  3  is shown in  FIG. 5 . The typical AFE  13  consists of a DAC (Digital to Analog Converter)  58  which transforms the transmitted digital downstream signal into an analogue signal, an ADC (Analog to Digital Converter)  59  which transforms the upstream analogue signal into a digital one, and an hybrid  50  with a matched load  55  used for 2-4 wires conversion. 
     The improvement consists of an AFE  13   b  with no hybrid  50 . The hybrid is no longer needed because of the 2-4 wires conversion does not take place in the local part of the invention. This makes a difference with previous patents. 
     Previous patents use xDSL signals which are sent to the subscribers over a metallic pair in a 2 wires transmission. Thus, an hybrid was required in the local part for splitting signals transmitted in both directions, sharing the spectrum, in order to transmitting the signals coming from the fiber in a 4 wires transmission. However, present invention works on xDSL signals before sending them to the metallic pair, generating a multiplex of signals which are transported by optical carriers in different wavelengths, upstream and downstream. There is a multiplex of signal transmitted by an upstream wavelength and a multiplex of signals transmitted by a downstream wavelength. Both are sent in a 4 wires transmission over a single mode fiber which connects the local part and the remote part. The hybrid is no needed since there is not any 2 wires conversion in the local part. In the remote part an hybrid it is required for the 2-4 wires conversion as usual, but hybrid suppression in the local part adds three additional decibels to the optical link power budget. This increment in the optical link power budget means a coverage radius increase comprised between 3.5 and 5 km, while keeping constant the access bit rate. 
     The elements shown in  FIG. 4  combine and split the N xDSL links in the following way:
         The DSLAM Line card with xDSLoF transceiver—Central  2   b  included in DSLAM  1 , multiplexes N xDSL downstream signals and transport it by means of FDM (Frequency Division Multiplexing) using a wavelength λ DOWN . At the same time it demultiplexes N xDSL upstream signals received by means of a FDM signal carried by a wavelength λ UP .   The xDSL over Fiber transceiver—Remote  9  close to Customer Premises  19 , multiplexes the N xDSL upstream signals from N CPEs  4  into an unique FDM signal transported by a wavelength λ UP . And at the same time it demultiplexes the N xDSL downstream signals received by means of a FDM signal carried by a wavelength λ DOWN .       

     The xDSLAM Line card with xDSL over Fiber transceiver—Central  2   b  block diagram is shown in  FIG. 6 . Each xDSLAM Line card with xDSL over Fiber transceiver—Central  2   b  handles N xDSL lines. The N digital xDSL downstream signals are injected into their corresponding simplified AFE  13   b  meanwhile the N digital xDSL upstream signals are extracted from their corresponding simplified AFE  13   b . The analog downstream xDSL signal  100  is upconverted by a mixer  80 . For each xDSL line there is a local oscillator ( 71  for the first xDSL line,  72  for the second one, and so forth,  79  for the N-th and last xDSL line). The oscillation frequency of each of the N oscillators is Δf Hz higher than the previous one and Δf Hz lower than the next one. The frequency band Δf must be large enough to span the xDSL spectrum, including both upstream and downstream, and also a band guard. So:
         Δf&gt;1.104 MHz for ADSL (ITU-T G.992.1) and ADSL2 (ITU-T G.992.3).   Δf&gt;2.208 MHz for ADSL2+ (ITU-T G.992.5).   Δf&gt;30.000 MHz for VDSL2 (ITU-T G.993.2)       

     So f 0  is the oscillation frequency of oscillator  71 , f 0 +Δf is the oscillation frequency of oscillator  72  and so forth, f 0 +(N−1). Δf is the oscillation frequency of oscillator  79 . The output signals of each mixer  80  is then filtered by narrowband bandpass filters, each of one is centered at the oscillation frequency of the corresponding oscillator: bandpass filter  91   a  is centered at frequency f 0 , bandpass filter  92   a  is centered at frequency f 0 +Δf and so forth, bandpass filter  99   a  is centered at frequency f 0 +(N−1). Signals  90   a ,  91   a , . . . ,  99   a  are the output signals from the previous bandpass filters. All these signals are combined by an adder circuit  300  whose output signal  110  is delivered to an optical upconverter  320  which works at a centre wavelength λ DOWN . The optical upconverter  320  output signal  120  is delivered to a Wavelength Division Multiplexers  6 , also called WDM. 
     In upstream direction, the WDM  6  of the DSLAM Line card with xDSL over fiber Transceiver—Central  2   b  receives a signal  130 . The part of the signal  130  spectrum centered at wavelength λ UP  is the signal  240  which is the input signal for the xDSL over Fiber transceiver—Central  5  module. Signal  240  is the input signal for an optical downconverter  330  which also works at a centre wavelength λ UP . The output signal  250  from the optical downconverter  330  can be filtered through a bandpass filter  310  and splitted into N signals. Each signal is filtered by a narrowband bandpass filter, and each of these filters is centered at a different frequency. Bandpass filter  91   b  is centered at frequency f 0 , bandpass filter  92   b  is centered at frequency f 0 +Δf, and so forth, bandpass filter  99   b  is centered at frequency f 0 +(N−1). The output signal of each of these filters is an input signal for a RF downconverter based on a mixer  81 . But each mixer  81  uses a different local oscillator. Oscillator  71  generates a carrier at frequency f 0 , oscillator  72  generates a carrier at frequency f 0 +Δf, and so forth, oscillator  79  generates a carrier at frequency f 0 +(N−1). The output of each mixer  81  passes through a low pass filter  60  used to avoid aliasing and the low pass filter output signal  260 , shown in  FIG. 6 , is injected into the corresponding simplified AFE  13   b.    
     WDM  6 , are used to permit both wavelengths λ DOWN  and λ UP  share the same optical fiber  7 , as it is shown in  FIG. 6  and  FIG. 7 . In order to keep backward compatibility with other optical signals that share the same fiber link, Wavelength Division Multiplexers are used at both sides of the fiber link allowing the injection and the extraction of optical signals from other services like reflectometry based optical outside plant supervision, GPON or Digital Terrestrial Television (DTT) overlay over GPON/XG-PON enhancement band. 
     In addition to this, WDM  6  includes an input port  12   a  to inject optical signals for reflectometry based optical outside plant supervision, and Digital Terrestrial Television (DTT) overlay transport over the GPON/XG-PON enhancement band. This port  12   a  is also an output port for optical reflectometry signal echoes.
         WDM  8  is adjacent to xDSL over Fiber transceiver—Remote  9  and injects into the SMF  7  the upstream multiplex and extracts the downstream multiplex from the SMF  7 .   In addition to this, WDM  8  includes an output port  12   b  to extract the optical signal used to transport Digital Terrestrial Television (DTT) in the GPON/XG-PON enhancement band.       

     In order to monitor the xDSL over Fiber link status, the system can optionally include an embedded ADSL (or ADSL2+)/VDSL Terminal Unit  400 , from now on ATU-R/VTU-R, into the xDSL over Fiber transceiver—Remote  9 , as it is shown in  FIG. 7 . A xDSL Operation, Administration and Maintenance link is established (OAM link) between the corresponding xDSL port  3  at the DSLAM  1  and the embedded ATU-R/VTU-R  400  can be used to monitor the right working of the system, reusing the monitoring facilities supported by xDSL Operation, Administration and Maintenance (OAM) mechanisms, defined in ITU-T Recommendation G.997.1. 
     As it has been mentioned previously, optical reflectometry signals injected through  12   a  port can be used for the remote monitoring of the fiber link. But it is not enough. Fiber link can be working correctly, and however, there can be a failure in the system. Establishing a xDSL link between one xDSL port  3  of the line card  2   b  of the DSLAM  1  at the Central Office  17 , and an embedded ATU-R/VTU-R  400  at the xDSL over Fiber transceiver—Remote  9  provides an xDSL OAM link which is used for system monitoring. 
     The simultaneous usage of multiple optical carrier pairs for multiple xDSL signal multiplex transport provides additional advantages:
         More flexibility because all the xDSL ports of the modified xDSL line card  2   c  can be assigned to different buildings.   This flexibility provided by the new DSLAM line card  2   c  does not cause more complexity at the customer premises side, because the addition of an Automatic Wavelength Locking (AWL) system  401  into the xDSL over Fiber transceiver—Remote  9  avoids any kind of manual in-field configuration and tuning of the xDSLoF remote transceiver.   This flexibility is even higher when each of the xDSL ports  13   b  can be dynamically assigned by means of switches  601  controlled by a central control  600  to one or another wavelength pairs.       

     The invention integrates a plurality of optical modulator/demodulator blocks  401 , as it is shown in  FIG. 8 , into the xDSL DSLAM Line card with xDSLoF transceiver—Central  2   b . Thus, a new DSLAM line card  2   c  is obtained. This new type of DSLAM line card, hereinafter called xDSL DSLAM Line card with tunable xDSLoF transceiver—Central  2   c , is capable to transmit and receive simultaneously multiple xDSL signals multiplexes, each of one is transported by a different wavelengths pair (λ Di  and λ Ui ). So, each wavelengths pair carries a xDSL multiplex, both downstream and upstream, made up from the xDSL signals which correspond to xDSL ports  13   b  different from the ports associated to the remainder wavelengths pairs. 
     Each pair of downstream and upstream wavelengths can be used to reach different buildings, and it provides more flexibility although it reduces the coverage due to the passive optical devices required to split the different wavelength pairs. It can be a solution to increase deployment flexibility in cities downtown. The xDSL lines will be dynamically attached to a specific wavelength pair, which will be used to carry all the xDSL signals to all those customers who live in the same building. 
     In this preferred embodiment, including the introduction of a plurality of optical modulator/demodulator blocks  401  into the DSLAM line cards requires a modification in the passive optical distribution network. This modification is necessary in order to permit the coexistence of several wavelengths for xDSL signals transport with an additional wavelength for digital television distribution and a second additional wavelength for optical outside plant supervision. This modification consists of the block  800  shown in  FIG. 8 :
         A WDM  801 , a passive element which splits the optical wavelengths for xDSL multiplex transport from the wavelengths used for Digital Television distribution and for optical outside plant supervision.   An Arrayed Waveguide Grating  802 , also called AWG, splits the wavelengths assigned for xDSL multiplex transport to different buildings into different fibers. At each output of the AWG there are only two wavelengths, λ Di  and λ Ui , which respectively transport the xDSL downstream and upstream multiplex to those customers who live in the same building.   A passive splitter  803  which equally divides the optical power of the wavelengths used for Digital Television distribution and optical outside plant supervision into as many fibers as buildings are connected to the DSLAM line card.   And a set of WDM couplers  804 , which are used to combine at each output fiber the wavelength pair for xDSL signal transport with the two wavelengths used for Digital Television distribution and optical outside plant supervision.       

     This block  800  introduces approximately an additional attenuation of 8 dB, and it causes a coverage radius decrease in relation to the scenario shown in  FIG. 4  using the DSLAM Line card with xDSLoF Transceiver—Central  2   b  shown in  FIG. 6 . If this block  800  is not introduced, and the xDSL signals from all the N xDSL ports  13   b  of the DSLAM line card  2   c  are carried using a unique wavelengths pair over a unique fiber  7 , the maximum reach will be the same than in the case of the solution described in  FIG. 6 . 
     The xDSL DSLAM line card with tunable xDSLoF transceiver—Central  2   c , described in  FIG. 9 , adds multiple optical modulators/demodulators blocks  401 , switches  601  and a switch control module  600  into the previous xDSL DSLAM Line card with xDSLoF transceiver—Central  2   b  shown in  FIG. 6 . The switches  601  permit the dynamic assignment of each of the N xDSL ports  13   b  to one of the M wavelength pairs. The switch control  600  implements the required logic to control these switches  601 . 
     Apart from the injection of optical carriers for Digital Television (e.g. Digital Terrestrial Television or DTT) distribution and optical outside plan supervision, the input  12   a  of the WDM multiplexer  6  is used to inject an unmodulated broadband light source λ UBLS  that after pass through the cyclic AWG  802 , will be used by the Automatic Wavelength Locking system  901  of each xDSL over Fiber transceiver—Remote  9   b  to automatically tune the wavelength of the optical carrier used for upstream transmission. 
     In the customer side it is necessary to add a block which permits an automatic tuning of the assigned wavelengths, avoiding any kind of in-field configuration. This block is an Automatic Wavelength Locking, or AWL, system  901 , represented in  FIG. 11 , like those used in commercial WDM-PON systems, which will be included into the xDSL over fiber transceiver—Remote with an AWL system  9   b , as it is shown in  FIG. 10 . The xDSL over fiber transceiver—Remote with an AWL system  9   b  is a modified version of the xDSL over fiber transceiver—Remote  9 . The modification consists of the substitution of the upconverter  370  of  FIG. 7  of the xDSL over fiber transceiver—Remote  9  by the optical transmitter  913  of the AWL system  901  shown in  FIG. 11 , and the substitution of the downconverter  360 , shown in  FIG. 7 , of the xDSL over fiber transceiver—Remote  9  by the receiver  912  of the AWL system  901  shown in  FIG. 11 . The xDSL over fiber transceiver—Remote with an AWL system  9   b  tunes itself automatically to the downstream and upstream wavelengths. 
     The Automatic Wavelength Locking system  901  shown in  FIG. 11  consists of a dichroic band-splitting filter  911  which splits the incoming optical signals into the optical carrier λ Di  used for the downstream xDSL multiplex transport and an unmodulated optical carrier λ Ui  at the same wavelength assigned for the upstream xDSL signal multiplex. The λ Di  downstream optical carrier is delivered to a Photo Diode based receiver  912  meanwhile the unmodulated λ Ui  optical carrier is used to tune the Fabry Perot Laser Diode based transmitter  913 . 
     The proposed system is compatible with current xDSL solutions, and it is also fully compatible with current or expected future xDSL improvements like:
         Pair bonding (ITU-T G.998.1/G.998.2).   Level 3 Dynamic Spectrum Management (DSM) based on vectoring (ITU-T G.993.5).   Impulse noise protection (ITU-T G.998.4) based on xDSL frames retransmission.