Abstract:
A method and apparatus are provided for transmitting and receiving a plurality of individual tributary signals in multiplex form via a common line. At the transmitting end, the tributary signals, each of which has a similar initial frequency, are converted into a compound signal having a frame structure with a common data rate. At the receiving end, each individual tributary signal is retrieved from the compound signal with its initial frequency. A phase information signal portion including a respective phase difference between each tributary signal and the compound signal is formed and inserted into the compound signal in the shape of respective coded bits. The initial frequency of each tributary signal is recovered from the phase information signal portion included in the respective coded bits belonging to the respective tributary signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims the benefit of priority from corresponding European Application Serial No. 01305212.1, filed Jun. 15, 2001.  
           [0002]    1. Technical Field  
           [0003]    The invention relates to a method and an apparatus for transmitting and receiving a plurality of individual tributary signals in multiplex form via a common line.  
           [0004]    2. Background of the Invention  
           [0005]    A data line can carry a plurality of signals originating from a plurality of individual sources. In practice, a plurality of signals of nominally the same frequency termed “tributary signals” are multiplexed and transmitted via the common line as a “compound signal”. The multiplexed signals are mapped into the compound signal that has a frame structure and is of a higher data rate than the sum of the tributary frequencies. The compound signal is received at the receiver and is demultiplexed. The individual tributary signals so obtained should be identical to the original tributary signals before being multiplexed at the transmitter. This means that the frequency of each demultiplexed tributary signal (“the recovered clock”) should be identical to the frequency of the original signal.  
           [0006]    In order to adapt to the common data rate of the compound signal, additional bits are used. This offers the possibility to transmit initial tributary signals of somewhat different frequencies. Some of the additional bits are used to transmit control information needed for the rate adaptation of these tributary signals. Some of the additional bits can also be used to transmit some other additional information. The additional bits are put in a fixed position into the framed compound signal. Rate adaptation is made by a stuffing procedure. To that end, gaps are provided in fixed frame positions, wherein information data can be inserted, or which can be left empty. When the initial tributary frequency is lower than the nominal rate, these gaps remain empty (positive stuffing). When the initial tributary frequency is higher than the nominal rate, some of the bits are inserted in the empty positions (negative stuffing). The tributary signals which are adapted in rate, are multiplexed, that is, the bits or bytes of the signals are interleaved and transmitted to the receiver through the common line. For recovering the tributary signals at the receiver, the signals are demultiplexed. For recovering the frequency or clock, phase information transmitted with the compound signal is used, namely the phase difference between the compound signal and the tributary signal. This phase difference is transmitted in the gaps provided in the fixed frame positions and causes no significant harm. However, the stuffing information results in a rough quantization of the phase, which causes wander and jitter of the recovered frequency or clock.  
         SUMMARY OF THE INVENTION  
         [0007]    Wander and jitter in a compound signal are reduced according to the principles of the invention. According to one illustrative embodiment, the phase difference between the compound signal and the tributary signal is accurately calculated in the transmitter. This calculated phase difference is coded preferably by a binary number and is transmitted in dedicated bytes of the compound line signal. In the receiver, the initial frequency of each tributary signal is recovered using the transmitted phase information. The accurate calculation of the phase difference is obtained by using an auxiliary clock at the transmitter. Furthermore, the mean value of the phase difference is calculated for a fixed time interval where the mean value is obtained by an integrator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    A more complete understanding of the invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, with like elements referenced with like reference numerals, in which:  
         [0009]    [0009]FIG. 1 is a block diagram of a transmitter according to an illustrative embodiment of the invention;  
         [0010]    [0010]FIG. 2 is a block diagram of a synchronizer according to an illustrative embodiment of the invention;  
         [0011]    [0011]FIG. 3 is a block diagram of a receiver according to an illustrative embodiment of the invention; and  
         [0012]    [0012]FIG. 4 is a block diagram of a desynchronizer according to an illustrative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 shows a transmitter for inputting four tributary signals  1 ,  2 ,  3 ,  4  and outputting a line compound signal  5  which includes all data of the tributary signals and some control data and may have a frequency of 10 GHz, by way of example. Each tributary signal is delivered to a respective synchronizer  6  which prepares a rate adapted tributary signal  61  that is then interleaved by multiplexer  7  with the remaining rate adapted tributary signals  62 ,  63 , and  64 . Multiplexer  7  composes the data of the tributary signals  61  to  64  and delivers such composed signal to a frame constructor  8  which finally outputs the line compound signal via line  5 . The frame constructor is controlled by a frame counter  9  and a system clock  10  having the frequency of the line compound signal. The system clock  10  is also delivered to the frame counter  9  and a phase-locked loop  11 , which outputs an internally generated auxiliary clock. The output of the phase-locked loop  11  and a further auxiliary clock  12  are delivered to a gate  13  so that the auxiliary clock  12  can be made effective for each of synchronizers  6 . The auxiliary clock  12  is an uncorrelated cycle to the writing cycle and the reading cycle and is used to obtain a higher resolution of the phase difference between signals.  
         [0014]    Cycle adaptation, which is aimed at, makes it necessary to use a plurality of gate functions. For this reason, cycle adaptation is realized in CMOS technology, which allows a relative low frequency of 78 MHz, by way of example only (in relation to 10 GHz of the compound signal  5 ). Therefore, the serial data are transformed to parallel data and are written with this low frequency into a memory and read out with a similar low frequency from the memory.  
         [0015]    [0015]FIG. 2 shows particulars of each synchronizer  6 . Input data from one of the tributary signals  1  through  4  is delivered to a FIFO register  14 , which is controlled by a write counter  15  and a read counter  16 . The write counter  15  is operated by a write clock  12   a  and receives the numbers of the bits in the tributary signal via line  9   a . The read counter  16  is operated by a read clock  12   b  and receives the number of the bits from the compound signal through line  9   b . The register  14  is an elastic store which provides write-in positions (write address) for the data bits of the respective tributary signals  1  to  4 , and read-out positions (read address) for reading out these data bits together with bit gaps as provided by the frame structure of the compound signal. A phase difference unit  17  is provided which, by the operation of the write counter  15  and the read counter  16 , forms or calculates a phase difference between each tributary signal and the compound signal.  
         [0016]    In detail, the phase difference is formed between write and read address of register  14 . The resolution obtained with this measurement corresponds to the cycle time of the writing cycle or the reading cycle, that is, phase difference measurement is made synchronously with one of these cycles. However, this resolution is not sufficient to fulfill the requirements as to jitter at the output of the tributary signal. Furthermore, the phase difference between write and read address is changing continuously, and measurement is only a rough quantization of this phase difference. This is the reason why the auxiliary clock  12  is used which is uncorrelated to the writing and reading cycle and allows measurement at fine stepped times. The auxiliary clock  12  is drifting slowly so that, in a measuring period, the clock shifts through all possible positions during a cycle time period of the writing or reading cycle. Additionally, an average value is formed for a defined measuring period which corresponds to the distance between two stuffing positions, e.g., the measured values are integrated across the measuring time. The average value obtained allows for one of the following decisions: stuffing positively, stuffing negatively, or no stuffing. Formation of such average value allows for calculating the influence of the gaps which, due to the frame construction, occur regularly.  
         [0017]    The phase difference unit  17  makes a binary number from the average phase difference and delivers such coded phase information to a data output gate  18 . The coded phase information is also delivered to a stuff decision unit  19  which has outputs connected to the read counter  16  and the output data gate  18 .  
         [0018]    The auxiliary clock  12 , with its portions write clock  12   a  and read clock  12   b , allows the accurate calculation of the phase difference between the line signal  5  and the tributary signals  1 ,  2 ,  3 , and  4 , respectively. The phase difference unit  17  includes the integrator referred to above which, for a fixed time interval, forms the mean or average value of the phase difference that is the basis for calculating the phase difference between line signal and tributary signal.  
         [0019]    [0019]FIG. 3 shows a receiver for four tributary data outputs  21 ,  22 ,  23 ,  24 . These output lines  21  through  24  belong to respective desynchronizers  26 , which are connected to demultiplexer  27 . Demultiplexer  27  is controlled by a frame alignment circuit  28 , which is interconnected with a frame counter  29 . System clock  30  is connected to frame counter  29  and gate  25 , which is also connected to the frame alignment circuit through a recovered clock line  25   a . Gate  25  generates internally an auxiliary clock which is delivered to a phase-locked loop  31  which outputs to a gate  33  having a second input connected to a further auxiliary clock  32 . The gated auxiliary clock is also connected to each desynchronizer  26 .  
         [0020]    Line  5  delivers the compound signal carrying the data of the tributary signals and also additional bits to the frame alignment circuit  28  which firstly outputs the data of the composed signal and secondly the recovered clock  25   a  of the compound signal. The recovered clock  25   a  is used in the frame counter  29  to decide when a frame begins and ends. Demultiplexer  27  receives the data of the composed signal  28   a  and is controlled by the frame counter  29  so as to deliver the appropriate rate adapted data  71  to  74  to the respective desynchronizer  26  in the adapted rate. The auxiliary clock  32  is used to reconstruct the initial frequency or rate of the respective desynchonizer  26  so that each tributary data output  21 ,  22 ,  23 , or  24  has a frequency that is exactly the same as the initial frequency of the signal.  
         [0021]    [0021]FIG. 4 shows a desynchronizer circuit  26  according to one illustrative embodiment. Data from demultiplexer  27  on line  27   a  is received at FIFO register  34 , to which a write counter  35  and a read counter  36  as well as a phase difference unit  37  are connected. By way of example only, FIFO register  34  is an elastic store having write-in positions (write address) for the compound signal received, and read-out positions (read address) for the data bits of the respective tributary signals. Input line  27   a  is also connected to a phase and stuff information unit  39 , which has a second input  29   a  from compound signal frame counter  29 . Phase and stuff information unit  39  has a first output  39   a  for delivering stuff information to the write counter  35  and a second output  39   b  for delivering phase information to a summing member  40  which has a second input from the phase difference unit  37 . The output of summing member  40  is the input of the phase-locked loop  31  which includes a controller  41 , a numeric controlled oscillator  42 , a phase detector  43 , a filter  44  and a voltage-controlled oscillator  45 . The output of the voltage-controlled oscillator is the read clock  32   b  and is also used as the tributary clock to an output data gate  38 .  
         [0022]    The data of the composed signal reaching demultiplexer  27  from the frame alignment circuit  28  are demultiplexed, so that signals  71  to  74  containing the additional bits in an adapted rate are obtained in succession in the several desynchronizers  26 . Controlled by frame counter  29 , the additional bits in the compound signal are read out from the rate adapted data stream of the tributary signal  27   a  into unit  39 , whereas all bits in the compound signal are written into elastic store  34 . The coded phase information taken up from unit  39  is used for an accurate calculation of the phase difference between the write and read address of the elastic store  34 . The whole phase difference is calculated in phase difference unit  37 .  
         [0023]    The whole phase difference has several portions, including but not limited to: stuffing information (which is a rough quantization of the phase course, and is only transferred when a stuffing operation is actually made); synchronizer phase difference between write and read addresses (which has been calculated at the synchronizer and is transferred to the desynchronizer with specific bytes—this value is transferred regularly, one time per stuffing position independently from whether there is a stuffing operation, or not); and desynchronizer phase difference between write and read addresses (calculated at the desynchronizer as a mean or average value, in the same manner as at the synchronizer).  
         [0024]    In one illustrative embodiment, the phase difference is represented by the addition of these portions and is added to the phase course of the system clock or cycle of the respective channel or tributary signal (when frame gaps removed) so as to yield the original phase course of the respective channel.  
         [0025]    In detail, phase information as well as calculated phase difference is further processed for such clock recovery in the phase-locked loop  31 . The loop includes a numeric controlled oscillator  42  so that the output signal thereof takes the initial frequency of the respective tributary signal  1 ,  2 ,  3 , or  4 . The phase-locked loop  31  is responsive for delivering the clock with the correct phase relation. When recovering the clock on line  32   b , any phase deviation from the phase of an ideal clock of the same frequency is wander and jitter. Wander and jitter are kept low by the procedure described above, since the tributary clock on line  32   b  is recovered from the clock of the demultiplexed signal from which the gaps contained in the compound signals have been removed by virtue of the phase-locked loop  31 . The additional bits in the regular gaps of the frame structure of the compound signal produce only low values of phase deviation since the phase-locked loop  31  has a low cut-off frequency. On the other hand, irregular gaps as occurring with stuffing produce irregular phase steps at the input of the phase-locked loop  31 . This will produce big phase changes at the output of the phase-locked loop. However, the transmitted phase difference is used when recovering the clock in the receiver so that the clock produced in the phase-locked loop  31  is a clock with the desired phase for each tributary signal. The phase at the output of summing member  40  contains no more irregular and big phase steps.