Abstract:
The invention relates to a method for setting up a data transmission link between xDSL transceivers in which the period for setting up the data transmission link is minimum. The method exhibits a very low susceptibility to interference. An XDSL transceiver with an echo cancellation circuit and an equalizer is also dislcosed.

Description:
TECHNICAL FIELD 
     The invention relates to a method for setting up a data transmission link between xDSL transceivers with a minimum warm start period. 
     BACKGROUND ART 
       FIG. 1  shows a data transmission system according to the prior art. Between a transceiver at the switch end LTU (Line Terminating Unit) and a transceiver at the subscriber end NTU (Network Terminating Unit), the signaling data are first transmitted via a data transmission line for setting up a data transmission link for user data. The transceiver at the subscriber end NTU is a connecting socket which is installed in customer or subscriber premises by the network operator. The transceiver at the subscriber end NTU is connected to a data terminal, for example a computer. The transceiver at the switch end LTU is connected to a network, for example the telephone network. As a rule, the data transmission line is a copper telephone pair. 
     To set up a data transmission link, the two transceivers must be brought from a standby mode into an activated operating mode for the data transmission. In xDSL data transmission methods, the twisted analog telephone pairs are used as wide-band connections in the local loop. The most well-known xDSL data transmission methods are ADSL (Asymmetric DSL), VDSL (Very-High-Data-Rate DSL), SDSL (Symmetrical Single-Pair DSL). 
     After the completed cold start and subsequent deactivation, the two transceivers are initially in a power-saving or standby mode. In the power-saving mode, the transceivers consume minimum power so that the heat generated is minimum and no cooling is necessary. 
       FIG. 2  shows a link between two transceivers LTU, NTU according to the prior art in detail. To set up a data link, the transceivers are reactivated in a warm-start sequence. 
       FIG. 3  shows a possible warm-start sequence according to the prior art, which is described in German Patent Application DE 101 39 779.8. 
     Initially, one of the two transceivers sends a wake-up signal via the data transmission line to the remote transceiver in order to activate the latter. In the sequence shown in  FIG. 3 , the transceiver at the subscriber end NTU sends a wake-up signal W WUN  to the transceiver at the switch end LTU. After a predetermined interval T WS , the transceiver at the switch end LTU also sends a wake-up signal W WUL  back to the modem at the subscriber end NTU via the data transmission line. With this wake-up signal, the modem at the switch end LTU acknowledges reception of the original wake-up signal W WUN . 
     After the wake-up sequence, it is determined in a line probing phase whether the line parameters of the data transmission line have changed since the last activation. For this purpose, the transceiver at the subscriber end NTU sends a line-probing signal W LPN  via the transmission line to the transceiver at the switch end LTU in the example shown in  FIG. 3 . The transceiver at the switch end evaluates the received signal. After a predetermined waiting time T WS , the transceiver at the switch end LTU conversely also sends a line-probing signal W LPL  to the transceiver at the subscriber end NTU. The modem at the subscriber end NTU evaluates the received signal. 
     After the line parameters have been checked, a complete echo signal elimination may take place in a further phase. Since the last activation of the two transceivers, the line parameters of the data transmission line may have changed. This leads to a residual echo signal of the inherent transmit signal of a transceiver. The residual echo signal may impair the detection of a data signal received from the remote transceiver. The echo signal must, therefore, be canceled by echo signal elimination in such a way that it is below a predetermined threshold value. To eliminate the echo signal, the modem at the subscriber end NTU in the example shown in  FIG. 3  sends out a signal for echo cancelation and the remote modem at the switch end LTU does not send out a signal. The echo cancelation circuit in the modem at the subscriber end NTU is set with the aid of the W ECN  signal. After a predetermined waiting time t WS  [sic], the modem at the switch end LTU then also sends out a signal W ECL  for echo signal cancelation. During this time, the other modem NTU does not send out a signal. The echo cancelation circuit in the transceiver at the switch end LTU is adjusted to minimize the residual echo signal with the aid of the adjustment signal W ECL . 
     After the echo signal elimination, synchronization is effected between the two transceivers in a synchronization sequence. The modem at the subscriber end NTU sends out a synchronization signal W SN  for synchronizing the transceiver at the switch end LTU and conversely the modem at the switch end LTU sends out a synchronization signal W SL  for synchronizing the modem at the subscriber end NTU. As soon as the two transceivers are synchronized, they in each case send an indicating signal to the other transceiver which indicates the completed synchronization. Following this, user data are transmitted between the two transceivers. 
     For a warm start between two transceivers, it is desirable that the period for the warm start t warm start  is as short as possible. In the warm start sequence according to the prior art, the probing of the data transmission line, the echo signal elimination and the synchronization take place after one another or serially. A serious problem in this is that a predetermined worst-case time is in each case provided for probing the data transmission line and for eliminating the echo signal in the standard protocol for setting up the data transmission link. For this reason, the period for the warm start is relatively long in the warm start sequence according to the prior art. 
     SUMMARY OF THE INVENTION 
     It is, therefore, the object of the present invention to create a method for setting up a data transmission link between xDSL transceivers in which the period for setting up the data transmission link is minimum. 
     According to the invention, this object is achieved by a method according to the features specified in claim  1  and by an xDSL transceiver having the features specified in claim  14 . 
     The invention creates a method for setting up a data transmission link between xDSL transceivers comprising the following steps: sending of an echo adjustment signal (W ECL ) by a first xDSL transceiver ( 1 - 1 ) to a second xDSL transceiver ( 1 - 2 ) until an echo cancelation circuit ( 21 - 1 ) of the first xDSL transceiver ( 1 - 1 ) is set, the second xDSL transceiver ( 1 - 2 ) synchronizing to the echo adjustment signal (W ECL ) and the second xDSL transceiver (NTU) not sending; sending of a first synchronization signal (W SL ) via the first xDSL transceiver ( 1 - 1 ) to the second xDSL transceiver ( 1 - 2 ) for synchronizing the second xDSL transceiver ( 1 - 2 ), the first synchronization signal (W SL ) being different from the echo adjustment signal (W ECL ); sending of a second synchronization signal (W SN ) by the second xDSL transceiver ( 1 - 1 ) to the first xDSL transceiver ( 1 - 2 ) for synchronizing the first xDSL transceiver ( 1 - 1 ) when the second xDSL transceiver ( 1 - 2 ) is completely synchronized by the first synchronization signal (W SL ), the second synchronization signal (W SN ) being sent by the second xDSL transceiver ( 1 - 2 ) to the first xDSL transceiver ( 1 - 1 ) until an echo cancelation circuit ( 21 - 2 ) of the second xDSL transceiver ( 1 - 2 ) is set, sending of a first indicating signal (W OKN ) by the second xDSL transceiver ( 1 - 2 ) to the first xDSL transceiver ( 1 - 1 ), which indicates to the first xDSL transceiver ( 1 - 2 ) that the echo cancelation circuit ( 21 - 2 ) of the second xDSL transceiver ( 1 - 2 ) is set and the second xDSL transceiver ( 1 - 2 ) is synchronized; and sending of a second indicating signal (W OKL ) by the first xDSL transceiver ( 1 - 1 ) to the second xDSL transceiver ( 1 - 2 ) which indicates to the second xDSL transceiver ( 1 - 2 ) that the first xDSL transceiver ( 1 - 1 ) is synchronized when the first xDSL transceiver ( 1 - 1 ) has received the first indicating signal (W OKN ). 
     An advantage of the method according to the invention consists in that it exhibits a very low susceptibility to interference because the various steps for setting up the data transmission link, namely probing of the data transmission line, echo signal elimination and synchronization in each case take place sequentially at the two transceivers and thus interfering mutual influence during the adjustments of the echo cancelation circuits of the equalizers and of the synchronization circuits are avoided. 
     Advantageous embodiments of the method according to the invention are specified in the subclaims. 
     The invention also creates an xDSL transceiver comprising a signal input for receiving digital user data, a signal generator for generating signaling data, 
     a controllable multiplexer for switching through the generated signaling data or the user data in dependence on a control signal, 
     a scrambler for scrambling the data switched through by the multiplexer, 
     a mapping unit for mapping the scrambled data, 
     a coder for coding the mapped data, 
     a D/A converter for converting the coded data into an analog transmit signal, 
     a hybrid circuit for connecting the xDSL transceiver via a data transmission line to a remote xDSL transceiver, 
     an analog/digital converter for converting an analog received signal into a digital received signal, 
     a detection circuit for detecting a received wake-up signal, 
     a synchronization circuit for synchronizing to the received digital signal, 
     an echo cancelation circuit for echo signal cancelation in the received digital signal, 
     an equalizer for equalizing the received digital signal, 
     a descrambler for descrambling the equalized received digital signal, 
     and with a control unit, 
     the control unit, after a received wake-up signal has been detected by the detection circuit or after a warm start request signal has been received, controlling the coder in such a manner that an echo adjustment signal W ECL  is sent by the xDSL transceiver via the data transmission line, the control unit, after receiving a first indicating signal from the echo cancelation circuit (W ECOK ), which indicates that the echo cancelation circuit (EC) is set, driving the multiplexer in such a manner that a generated synchronization data sequence is switched through to the scrambler which is sent as synchronization signal (W SL ) by the xDSL transceiver via the data transmission line for synchronizing the remote xDSL transceiver, 
     the control unit, after receiving a second indicating signal (W SYNCOK ) from the synchronization circuit, which indicates that the synchronization circuit is synchronized to a received digital signal and after receiving a third indicating signal (W EQOK ) from the equalizer (EQ), which indicates that the equalizer (EQ) is set, driving the multiplexer in such a manner that a generated synchronization data sequence is switched through to the scrambler which is sent as indicating signal (W OK ) by the xDSL transceiver via the data transmission line to the remote xDSL transceiver and indicates to the latter that the xDSL transceiver is ready for the data transmission of user data. 
    
    
     
       BRIEF DESCRIPITON OF THE DRAWINGS 
       In the text which follows, preferred embodiments of the method according to the invention for setting up a data transmission link and of the xDSL transceiver according to the invention are described with reference to the attached figures for explaining features which are essential to the invention. In the figures: 
         FIG. 1  shows a data transmission link according to the prior art; 
         FIG. 2  shows a circuit configuration of conventional transceivers; 
         FIG. 3  shows a timing diagram which represents the setting-up of a data transmission link according to the prior art; 
         FIG. 4  shows a block diagram of an xDSL transceiver according to the invention; 
         FIG. 5  shows a flow chart for explaining the method according to the invention for setting up a data transmission link; 
         FIG. 6  shows a state diagram for explaining the method according to the invention for setting up a data transmission link between two xDSL transceivers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows a block diagram of the xDSL transceiver  1  according to the invention. The xDSL transceiver  1  has at least one signal input  2  for receiving digital user data. The digital user data come either from a data terminal or from a data network. The xDSL transceiver  1  also contains a controllable multiplexer  3 . The controllable multiplexer  3  has a first input  4  which is connected to the signal input  2  for the user data via a line  5 . In addition, the multiplexer  3  has a second input  6  which is connected to a signaling data generator  8  via a line  7 . The multiplexer  3  is switched by an internal controller  10  of the xDSL transceiver  1  via a control line  9 . The multiplexer  3  has a signal output  11  which is connected to a scrambler  13  via a line  12 . 
     The output of the scrambler  13  is connected via a line  14  to a mapping unit  15  for mapping the scrambled digital data. The output of the mapping unit  15  is connected via a line  16  to a coder  17  for coding the mapped data. 
     In a preferred embodiment of the xDSL transceiver according to the invention, the coder  17  is a so-called Tomlinson coder. The coded data are passed to a digital/analog converter  19  via a line  18  and to an echo cancelation circuit  21  via a line  20 . The digital/analog converter  19  converts the coded data present into an analog transmit signal which is applied to a driver circuit  23  of the xDSL transceiver  1  via an internal line  22 . The driver circuit  23  amplifies the analog signal present and outputs the amplified signal to the hybrid circuit  25  via a line  24 . The hybrid circuit  25  forms the connection between the xDSL transceiver  1  and the data transmission line  26 . The data transmission line  26  is preferably a copper pair. 
     The hybrid circuit  25  is connected via a line  27  to an analog/digital converter  28  which converts the received analog signal into received digital data and outputs them to a subtracting unit  30  via a line  29 . The subtracting unit  30  subtracts the output signal of the echo cancelation circuit  21 , which is present via a line  31 , from the received digital data. The subtracting unit  30  is followed by an equalizer  33  via a line  32 . The equalizer is used for far-end signal recovery and equalizes the signal distortions occurring. The output signal of the subtracting unit  30  is output to a synchronization circuit  35  of the xDSL transceiver  1  via a line  34 . The synchronization circuit  35  is used for synchronizing the xDSL transceiver  1  to a received signal. The synchronization circuit  35  is followed by a decoding and descrambler circuit  37  via a line  36 . The decoder/descrambler  37  decodes and descrambles the equalized received data and outputs the descrambled data to a signal output  39  of the xDSL transceiver  1  via a line  38 . 
     The xDSL transceiver  1  also contains a detection circuit  40  which is provided for detecting a received wake-up signal. The detection circuit  40  outputs a detection signal to the controller  10  via a line  41  when a wake-up signal or wake-up tone signal is received via the data transmission line  26 . The controller  10  controls the signaling data generator  8  via a control line  42 . In addition, the controller  10  controls the coder  17  via lines  43 . The echo cancelation circuit  21  and the equalizer circuit  33  in each case indicate to the controller  10 , via indicating lines  44 ,  45 , that they are set in accordance with their own criteria. The echo cancelation circuit  21  indicates via the line  44  when it is converged, i.e. when the residual echo signal is below a threshold value. 
     The equalizer circuit  33  indicates to the controller  10  via the line  45  when the far-end signal is sufficiently equalized and the equalizer  33  is thus set. 
     In addition, the synchronization circuit  35  indicates to the controller  10  via a line  46  when adequate synchronization to the received data signal has taken place. In addition, the controller  10  can receive a warm start request signal via a line  47 . 
       FIGS. 5 ,  6  show the sequence of the method according to the invention for setting up a data transmission link between two xDSL transceivers  1  according to the invention. 
     Initially, one of the two xDSL transceiver  1  sends a wake-up signal W WUN  to wake up the other transceiver which acknowledges the reception of the wake-up signal by sending out a wake-up signal W WUL . 
     After a certain waiting time T WS , the synchronization sequence according to the invention begins with the setting-up of the data transmission link. In a step S 1 , a first xDSL transceiver  1 - 1  sends an echo adjustment signal W ECL  to a second xDSL transceiver  1 - 2  until the echo cancelation circuit  21 - 1  of the first xDSL transceiver  1 - 1  is set, during which process the second xDSL transceiver  1 - 2  already synchronizes itself to this echo adjustment signal W ECL . During this period, the second xDSL transceiver  1 - 2  is not sending a signal. 
     Once the echo cancelation circuit  21 - 1  is set, the xDSL transceiver  1 - 1  sends a synchronization signal W SL , which differs from the echo adjustment signal W ECL  and which indicates to the second xDSL transceiver  1 - 2  that the echo cancelation circuit  21 - 2  of the first xDSL transceiver  1 - 1  is set. For this purpose, the controller  10  drives the signaling data generator  8  via the control line  42  in such a manner that it applies a different signaling data sequence to the input  6  of the multiplexer  3  via the line  7 . The multiplexer  3  is switched by the controller via the control line  9  in such a manner that the generated signaling data sequence is switched through to the output  11  of the multiplexer  11 . The user data input  4  of the multiplexer  3  is blocked during this sequence. The signaling data sequence generated is scrambled by the scrambler  13  and then mapped by the mapping unit  15 . The mapped signaling data present are coded by the coder  17  and converted into the first synchronization signal W SL  by the digital/analog converter  19 . 
     When the second xDSL transceiver  1 - 2  at the other end of the data transmission line  26  is completely synchronized by the first sychronization signal W SL  and has recognized the first synchronization signal W SL  as such by comparing it with the predetermined data bit sequence, the second xDSL transceiver  1 - 2  sends a second synchronization signal W SN  to the first xDSL transceiver  1 - 1  for synchronizing the first xDSL transceiver  1 - 1 . During this process, the second synchronization signal W SN  is sent by the second xDSL transceiver  1 - 2  to the first xDSL transceiver  1 - 1  until the echo cancelation circuit  21 - 2  of the second xDSL transceiver is set. 
     The second xDSL transceiver  1 - 2  then sends to the first xDSL transceiver  1 - 1  a first indicating signal W OKN  which indicates to the first xDSL transceiver  1 - 1  that the echo cancelation circuit  21 - 2  of the second xDSL transceiver  1 - 2  is set and the second xDSL transceiver  1 - 2  is synchronized. Conversely, the first xDSL transceiver  1 - 1  sends to the second xDSL transceiver  1 - 2  a second indicating signal W OKL  which indicates to the second xDSL transceiver  1 - 2  that the xDSL transceiver  1 - 1  is synchronized when the first xDSL transceiver  1 - 1  has received the first indicating signal W OKN . 
     In a preferred embodiment of the method according to the invention, the transmit signals are Tomlinson-coded by the coder  17 . If no Tomlinson preceding takes place, a further synchronization sequence  2  may be required as shown in  FIG. 5 . The signals W SN2 , W SL2 , W OKN2 , W OKL2  are here identical with the non-Tomlinson-precoded signals of the synchronization sequence  1  but the signals are Tomlinson-precoded in the synchronization sequence  2 . 
     As can be seen by comparing the timing diagrams of  FIG. 3  and  FIG. 5 , the warm start period t warm start  required is much shorter in the method according to the invention for setting up a data transmission link, particularly with Tomlinson coding of the signaling data, than in the conventional method shown in  FIG. 3 . 
     In the method according to the invention, the echo adjustment signal W ECL  is structured in such a manner that the remote transceiver  1  can already synchronize to it. In the method according to the invention, the echo adjustment signal W EC  fulfills a dual function. On the one hand, the echo adjustment signal W ECL  is used for adjusting the local echo cancelation circuit  21  and, on the other hand, for synchronizing the other xDSL transceiver  1  through its synchronization circuit  35 . If necessary, the remote transceiver  1  is then completely synchronized by the first synchronization signal W SL  sent which differs from the echo adjustment signal W ECL . In contrast to the previous method, the time t ECL  is not predetermined as a constant time in the method according to the invention. The echo adjustment signal W ECL  is sent out by the xDSL transceiver  1  only until its echo cancelation circuit  21  is completely set and it indicates those to the controller  10  via the indicating line  44 . The controller  10  then drives the signal data generator  8  via the control line  42  in such a manner that it generates another signaling data sequence for the first synchronization signal W SL . In the method according to the invention, the echo signal elimination thus takes place within the synchronization sequence so that the necessary warm start period t warm start  is considerably reduced in the method according to the invention. 
       FIG. 6  is used for explaining the method according to the invention and shows a state diagram of the two communicating transceivers NTU, LTU. The two transceivers are initially in a standby or power-saving mode. If the controller  10  of a transceiver receives a warm start request signal via the line  47  or detects a wake-up signal which originates from the connected other xDSL transceiver  1  via the data transmission line  26 , the transceiver leaves the standby mode. 
     After the exchange of the wake-up signals, the modem at the switch end LTU sends an echo adjustment signal W ECL  in the sequence shown in  FIG. 6 , to which the modem at the subscriber end NTU can already synchronize. The transceiver at the switch end NTU adjusts its echo cancelation circuit  21  by means of the echo adjustment signal W ECL . Once the echo cancelation circuit  21  is set, this is reported to the controller  10  which drives the signal generator  8  to generate another signaling data sequence. 
     The transceiver  1  at the switch end LTU then sends a synchronization signal W SL  via the data transmission line  26 . The synchronization signal W SL  adjusts the equalizer  35  of the other transceiver  1 . After the equalizer  35  is set and the synchronization signal W SL  is detected, the transceiver  1  at the subscriber end NTU changes into another operating mode and now sends a synchronization signal W SN . This signal W SN  is received by the transceiver at the switch end LTU and the transceiver LTU synchronizes to the received synchronization signal W SN . During this sequence, the echo cancelation circuit EC N  of the transceiver at the subscriber end NTU is adjusted. 
     After the echo cancelation circuit EC N  is set, the transceiver at the subscriber end NTU sends an indicating signal W OKN  via the data transmission line  26 . If the equalizer  33  of the modem at the switch end LTU is set and the LTU transceiver detects the indicating signal W OKN  of the other transceiver, it also sends an indicating signal W OKL  to the opposite transceiver. Once the transceiver at the subscriber end NTU has detected the indicating signal W OKL , both transceivers are ready for exchanging user data via the data transmission line  26 . 
     The method according to the invention is a handshake method between the two xDSL transceivers  1 . In this method, no timing conditions are predetermined a priori. The progress of activation of each transceiver is additionally controlled by the control signal from the remote transceiver in addition to the internal conditions achieved. During this process, the unwanted loss of time due to the exchange of information or control signals is minimized by parallel processing of the sequences. Both transceivers can adjust to their own echo cancelation circuit  21  without control signals from the opposite end. At the same time, the opposite end synchronizes and adapts itself.