System and equipment for quality checking of a digital connection circuit
CROSS-REFERENCE TO RELATED APPLICATION 
     The Applicant hereby makes cross-reference to his French Patent Application Ser. No. 76-05615 filed Feb. 27, 1976 and claims priority thereunder following the provisions of 35 USC 119. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system for checking the quality of a digital connection made by means of a telephone circuit. 
     It is known to check the quality of a digital connection by transmitting a check message at one end of a telephone circuit using this connection and by comparing the message received at the other end with the check message and counting the errors. The same operations are repeated in the opposite direction after the connection is made. 
     2. Description of the Prior Art 
     French Patent Specification No. 2,130,063 filed on Dec. 10, 1971, by the so-called ALITALIA Company describes a device for making a check of the mentioned kind. FIG. 1 of the said Specification, which is also used as FIG. 1 of the present Application, shows a digital link circuit M between ends A and B, comprising a transmission line and two modems (modulatordemodulators). 
     In order to check the quality of the connection in FIG. 1 two identical devices 1 and 2 each comprising a transmitter STT 1  or STT 2  and a receiver TR 1  or TR 2  are connected to the ends A and B of a telephone circuit. The devices are put into operation by an operator, e.g. operating at end A, who causes the check message to be transmitted. The device at end B compares the received message, bit by bit, with the simultaneously-produced check message. The first error results in a stoppage in the comparison in progress in the device at end B of the circuit, so that the operator can note the error. Next, the operator restarts the device. Correct transmission of the check message in one direction causes transmission of the same message in the opposite direction, and so on. 
     In the event of transmission errors, however, the devices described in the above cited patent require the operator to make certain that the error is systematic. Such an operation is slow, since it requires manual intervention. Stoppage of the device as soon as a received bit is incorrect entails the necessity that a signal returning to the beginning of the message, for the purpose of resetting the device, be transmitted from the end of the circuit where the check was made to the end where the check message was transmitted. 
     The invention relates to a system for checking the quality of a digital connection link whereby the quality can be ascertained at each end of the telephone circuit constituting the said link. 
     SUMMARY OF THE INVENTION 
     The invention relates to a system for checking the quality of a digital connection link between two stations whereby a check message stored in the two stations is repeatedly transmitted by a first device connected to one end of the telephone circuit, the message being received by a second device which is identical with the first and connected to the other end of the circuit and vice versa, the operation of the first and second devices being periodically switched over, characterized in that the check message comprises a synchronization character having i bits repeated n times in succession and a pseudorandom sequence of p bits, and is immediately followed by a quality character stored and displayed by the device which receives it. 
     According to an essential feature of the system of the invention, the quality character is built by means of the following method operations, which are performed during the reception of the check message formng part of the just preceding operating cycle: 
     Identification of the synchronization characters among the data at the beginning of the check message received from the distant device; 
     Counting the number of identified synchronization characters at a counter at the local device; 
     Coding a first number resulting from the said counting in a first number coder of said local device; 
     Storing the first coding result; 
     Comparing the received pseudo-random sequence received in the local station from the distant station with the sequence stand in the local station; and counting the number of identical bits found in the resulting comparison; 
     Coding the last named identical bits number to form a second coding result; and 
     Storing the second coding result beside the first in order to form the quality character. 
     The arrangement according to the invention has a number of advantages, as follows: 
     The presence of synchronization characters in the check message repeated a number of times means that the check device can be reliably synchronized; the transmission of the number of recognized characters ensures that the equipment is properly synchronized; 
     The pseudo-random binary sequence used in the check message is sufficiently long to reproduce nearly all the successive combinations of binary states which may be encountered during the transmission of data. The check made by the method according to the invention is therefore very thorough. 
     The quality character transmitted after the check message can be used to find the quality of transmission in both directions at each end of the circuit. 
    
    
     Other features and advantages of the invention will better appear from the following description of FIGS. 1 to 4, which are given by way of non-limitative illustration and in which: 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a telephone circuit connected to two identical devices for checking the quality of the connection; 
     FIG. 2 shows the operations performed during a cycle by the check devices in the method according to the invention; 
     FIG. 3 is a block diagram of a device for working the method according to the invention; 
     FIGS. 4A and 4B show two cycles of counting time slots and the signals exchanged between the device and the modems terminating the link; and 
     FIG. 5 depicts the data in the check message including the synchronization characters, the pseudo-random sequence and the quality character. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 has already been described. 
     FIG. 2 shows the sequence of operations for checking the quality of the connection according to the invention, performed by one of the devices 1 and 2 of FIG. 1. 
     Operation 3 is the transmission of the synchronization character, which is repeated a number of times in succession by device 1 to form the first part of the check message. 
     Operation 4 is the transmission of a pseudo-random sequence of bits forming the second part of the check message. 
     Operation 5 is the transmission of the quality character relating to the reception of the check message during the just preceding operation cycle. 
     The subsequent operations are performed during the reception of the check message transmitted by device 2. 
     Operation 6 is the identification of the synchronization characters; it is followed by a count of the identified characters (operation 7), coding the number obtained (operation 8) and storing the first result of such coding for forming part of the quality character to be transmitted during the next cycle (operation 9). 
     In operation 10, the received pseudo-random sequence is checked; this is followed by counting the number of errors identified (operation 11), coding the number obtained (operation 12) and storing the second result of the latter coding for forming another part of the quality character to be transmitted during the next cycle (operation 9). 
     Operation 13 is the acquisition and storage of the received quality character; it is followed by display of the same character (operation 14). 
     FIG. 3 is an operational diagram of each of the devices for working the system according to the invention, arranged at the ends of a circuit, the quality of which is to be tested. The diagram shows an instruction generator (control unit) 20 connected to a transmission circuit 21, a receiving circuit 22, to a check message and quality character receiver 24 and a circuit 29 for checking the operation of the device considered. The transmission circuit 21 is connected to a circuit 23 for generating synchronization characters and a pseudo-random sequence and to a circuit 25 for forming the quality character. Circuit 24 is also connected to a circuit 26 for detecting and counting the number of faulty identified synchronization characters, to a circuit 27 for checking the pseudo-random sequence received and adding up the errors, and to a circuit 28 for storing the quality character received. Circuit 26 is connected to a circuit 30 for coding the number of non faulty synchronization characters identified and for storing this number. Circuit 30 is connected on one side to the circuit 25 for forming the quality character and on the other side to a device for displaying the stored number. Circuit 27 is connected to a circuit 32 for coding the number of errors detected in the pseudo-random sequence received and for storing the result of the latter coding. Circuit 32 is connected to circuit 25 and to device 31. Thus, the information from circuit 31 will be transmitted by circuit 21 during the next transmission cycle. A circuit 28 for storing the received quality character is connected to a disply circuit 33. Circuit 29 is connected to a circuit 34 for automatically disconnecting the device for working the method. The instruction generator 20 is also connected to the neighboring modem by the following elements: 
     Conductor 35, which supplies the modem with request to transmit signals DPET; 
     Conductor 36 which supplies the instruction generator 20 with the permit to transmit signal (PAET); 
     Conductor 40, which supplies generator 20 with transmission clock signals HEMT; 
     Conductor 41, which supplies generator 20 with a &#34;presence of detected signals&#34; signal DST; 
     Conductor 37 which supplies generator 20 with reception clock signals HRMT reconstituted from the received signals. Transmission circuit 21 is connected to the modem by conductor 38 and supplies it with the check message followed by the transmission quality character EDT. The receiving circuit 22 is connected to the modem by conductor 39 and receives the transmitted data RDT. 
     Each of the check devices of the invention operates as follows: 
     The instruction generator (control unit) 20 receives from the modem clock signals HEMT via conductor 37. The signals are counted modulo 2p by the counter of generator 20, which can be reset to 0, to i and to (ni) during operation, i representing the number of bits contained in a character and n the number of synchronization characters at the beginning of the message. The clock pulses or bit intervals from the counter in unit 20 are shown along the axis HEMT 1  in FIG. 4. At the beginning of a cycle the control unit 20 in device 1, for example, supplies the adjacent modem, via conductor 35, with a request to transmit signal DPET comprising the +1 state of a binary signal having a duration equal to (p+q) bit intervals lasting from bit interval 0 to bit interval (p+q-1). the signal is represented by the pulse diagram DPET 1 , in FIG. 4. After a delay equal to a variable number of bit intervals, the modem delivers a permit to transmit signal (i.e. a voltage +V) to unit 20 via conductor 36. This signal is represented by pulse diagram PAET 1  in FIG. 4. The positive transition of signal PAET 1  initiates the transmission of the check message represented by pulse diagram ED 1  in FIG. 4 having (ni+p) bit intervals, followed by the transmission of a quality character of i bits formed after the reception of the check message for the preceding cycle, and resets the counter to zero. The signal DPET 1  returns to zero when (p+q) bit intervals have elapsed since the resetting to zero of the counter, and the signal PAET 1 , the reply of the modem to DPET 1 , makes the same slightly afterwards. The counter continues to count the bit intervals. However, the beginnings of two request to transmit signals DPET 1  are separated by a duration equal to 4p bit intervals so that device 2 has a transmission time equal to that of device 1. FIG. 4 shows the information of the counter in device 2 along axis HEMT 2  and also shows pulse diagrams RDT 2 , DPET 2 , PAET 2  and EDT 2 , to which the same remarks apply as to the corresponding pulse diagrams for device 1. 
     Operation 3 in the system of the invention is performed jointly by control unit 20, circuit 23 and circuit 21. To this end, unit 20 counts (ni) bit intervals from bit interval 0 to bit interval (ni-1) during which it unblocks the output of circuit 23, and the synchronization characters stored therein are transmitted by circuit 21 to the modem (FIG. 3). 
     Operation 4 is carried out jointly by control unit 20, circuit 23 and circuit 21. The previously-mentioned pseudo-random sequence of p bits is formed by circuit 24, on receipt of an enabling signal delivered by unit 20 between the (ni) and (ni+p-1) bit intervals. This sequence is transmitted to the modem by circuit 21. 
     Operation 5 is carried out by unit 20, the circuit 25 for forming and storing the quality character, and circuit 21. During the bit intervals (n+1-1) to (ni+p-1), generator 20 enables the transmission of the quality character delivered by circuit 25 to circuit 21. 
     Device 1 does not transmit any signal between the bit interval of rank [(n+1)i+p] and the (2p -1) rank bit interval and during the 2p subsequent bit intervals. During the same time, device 2 transmits a request to transmit DPET 2  to the modem connected thereto, until the counter associated with its instruction generator passes through 0, and the part of the checking cycle described hereinbefore with reference to device 1 is repeated in identical manner in device 2. As a result, device 1 receives the signals represented by pulse diagram RDT 1  in FIG. 4. 
     Operation 6 in the system according to the invention begins with the reception of data by circuit 22 and the counting by generator 20 of the bit intervals corresponding to the synchronization characters. When the first synchronization character has been correctly recognized, the counter associated with generator 20 is reset to i and, at the end of the reception of the n th  synchronization character, the counter is reset to (ni), as shown along axis HRMT 1  on FIG. 4. In addition, unit 20 identifies the synchronization characters. 
     Operation 7 (counting the identified characters) is performed by circuit 26. 
     Operation 8 (coding the number obtained) is performed in circuit 30 and operation 9 (storage) is performed in circuit 25, which is thus used to form the quality character. 
     Operation 10 (checking the pseudo-random sequence received) is carried out jointly by circuits 22, 24 and 27 during the bit intervals of ranks (ni) to (ni+p-1). Unit 20 enables the formation of the pseudo-random sequence in circuit 24 so as to compare it in circuit 27 with the received sequence delivered by circuit 22. This check is made from next to next bit interval. 
     Operation 11 (counting errors) is also performed in circuit 27, which comprises a shift register. 
     Operation 12 (coding the number obtained) is performed in circuit 32, and the result of coding is displayed in circuit 31. The result of coding is stored in circuit 25, where the arrival of the latter result stops the formation of the quality character and forms part of operation 9, which also comprises the storage of the quality character until the time when it is transmitted. 
     Operation 13 (storing the received quality character) is carried out in circuit 28 by generator 20 during the bit intervals of ranks (ni+p) to [(n+1) i+p-1]. 
     Operation 14 (displaying the received quality character) is carried out in circuit 33. 
     If there is a defect in the transmission link or the modem at one end of the link, the check device at the other end does not receive any DST signal in reply to its message. The circuit 29 controlling the operation of the device adds up the number of successive absences of reply. When the number is equal to several units, circuit 34, actuated by circuit 29, disconnects the check device and reconnects the receiver which is normally in series with the modem. 
     By way of non-limitative example, a device for working the system according to the invention was built, operating with the following parameters: 
     The number i of bit intervals per character was equal to 8; 
     The number n of synchronization characters was 4; 
     The number p of bit intervals in the pseudo-random sequence was 512; 
     The number of bit intervals (p+q) was 576 whereby q=64; 
     The number of bit intervals (ni+p) was 544; 
     The number of bit intervals (n+1) i+p was 552; 
     The permitted number of successive absences of a reply to the check message was 3. Above this value, the device for checking the quality of the link was disconnected and automatically replaced by the conventional receiver. 
     The system for coding the identified synchronization characters used two binary digits a and b having values given in the following table: 
     
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Number of identified characters                                           
                         a     b                                          
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0                        0     0                                          
1, 2, 3                  1     0                                          
4                        1     1                                          
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     The system for coding the number of errors identified in the pseudo-random sequence used two binary digits c and d having values given in the following table: 
     
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Number of errors identified                                               
                         c     d                                          
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0                        0     0                                          
1 to 4                   1     0                                          
5 or more                1     1                                          
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     FIG. 5 represents the check message with its four synchronization characters of 8 bits in bit positions Nos. 0 to ni-1=31, the 512-bit pseudo random sequence in bit positions Nos. 32 to (ni+p-1) = 543, the quality character of 8 bits (only four of which a, b, c, d are significative) in bit positions Nos. 544 to 551. The bit positions 552 to 1023 are unused.