SYSTEM FOR TESTING A MODEM

Abstract: System for the remote testing of a modem (12) for two bit rates different from one another, the part for the higher bit rate being connected in the state of testing via a speed converter (14) to the part for the lower bit rate. With the known technique errors will not be detected when the test signal is in the "1" state, because use is made of a pulse lengthener, which extends the "high speed" pulses occur­ ring in the "0" state of the signal, but which does not notice the "high speed" interruptions occurring in the "1" state. This is prevented by the invention by means of a number of logic circuits. one circuit (21) of which is or remains conductive owing to a leading edge of the test signal and a second circuit (25) of which is or remains conductive owing to a trailing edge of the test signal. The other logic circuits ensure that said circuits will be reset (23,27) and that the error information will be processed (29) and supplied (17) to the modem part for the lower bit rate.

Description: System for testing a modem. The invention relates to a system for the remote testing of a modem for two bit rates different from one another, particularly for view data traffic, the part for the higher bit rate being connected in the state of testing via a speed converter to the part for the lower bit rate. A system of this kind is described in the European patent application 80.200607.2 At a central station a test signal with a bit rate of 75 bits/s is applied via a scrambler to a speed converter, which converts this rate into a rate of 1200 bits/s. The test signal is applied via a modem and via a subscriber line to the receiving part for the higher bit rate of the modem at the subscriber station, which modem has to be tested. After the modulation the higher bit rate is converted via a speed converter into a lower bit rate, after which the signal is suited to be transmitted via the transmission channel of the modem and via the subscriber line to the part for the lower speed of the modem at the central station. From there the signal is transmitted to the data tester, which detects and records errors in the test signal. If no special measures are taken, however, the errors in the part for the higher speed of the modem to be tested will not be detected. These errors have to be detected at the subscriber station and have then to be "translated" into such errors in the signal with a lower speed that they can be detected at the central station. For this purpose a pulse lengthener extending the "high speed" pulses occurring in the "O" state of the signal after the speed converter, is made use of in the known technique. A drawback of this solution is that errors will not be detected, when the signal is in the "1" state, in consequence of which the observation time needed for determining a certain bit error ratio will be twice as long as in the case when all the errors are detected. The object of the invention is to improve the known system in such a way that errors will also be detected when the signal is in the "1" state, which implies that the observation time will be halved. The system according to the invention is characterized by an error detector between the speed converter and the transmission channel of the modem to be tested, which error detector comprises some logic circuits, controlled by delayed clock pulses derived from a test signal, of which logic circuits a first circuit is or remains conductive owing to a leading edge of the test signal, a second circuit is or remains conductive owing to a trailing edge of the test signal, a third circuit serves for resetting the first circuit and a fourth circuit serves for resetting the second circuit, a fifth circuit has an "AND'function for the outputs of the first and the second circuit, and a sixth circuit has an "EXCL-OR"-function for the output signal of the speed converter and the clocked output of the fifth logic circuit, and the clocked output of the sixth logic circuit is connected to the modem part for the lower bit rate. The invention will be elucidated with the help of the drawing in which Fig. 1 represents a block diagram; Figs.2-6 represent pulse diagrams, in which according to Fig. 2 an error occurs in the "1" state; according to Fig. 3 an error in the "0" state,which coincides with a clock pulse; according to Fig. 4 an error in the "1" state,which coincides with a clock pulse, and in which according to Figs. 5 and 6 trebled errors occur. A central station, which is not shown in the figures,can be connected to subscriber stations by means of subscriber lines. Number 10 indicates such a subscriber line, to which, via a speech/ data switch 11, of a subscriber station modem 12.to be tested can be connected. The output of this modem for the higher bit speed, called 1200 baud in what follows, is connected via a viewdata terminal/test switch 13 either to the input of a speed converter 14 or to a viewdata terminal 15. The converter is controlled by a clock extraction circuit 16 with a clock signal of 1200 Hz. Via the logic circuits 17 and 18 and a transmission/test switch 19 the output of the converter 14 is connected to an input for the lower bit speed, called 75 baud in what follows, of the modem 12 to be tested. In the other position of the switch 19 the modem 12 is connected to a transmitting device 20, equipped with a keyboard. The other position of the switch 13 serves for passing on viewdata information to the terminal 15 via the line 10 and the modem 12. The other position of the switch 11 serves for normal telephone traffic. In the Figs. 2-6 the output signal of the speed converter is indicated by 14, and a delayed clock signal, derived from the converter, is indicated by c. A first logic circuit 21 (a so-called flip-flop circuit) becomes or remains conductive owing to a leading edge 22 (Fig. 2). Under the control of a pulse 24 of the clock signal c a third logic circuit 23 resets the first circuit 21. A second logic circuit 25 (also a flipflop circuit) becomes or remains conductive owing to a trailing edge 26 (Fig. 2). Under the control of pulse 28 of the clock signal c a fourth logic circuit 27 resets the second circuit 25. A fifth logic circuit 29 (an "AND"-gate) has an "AND"-function for the outputs of the first and the second circuit (21 and 25). In case they are both conductive, the clock signal c (e.g. pulse 30 Fig. 2, pulse 31 Fig.3) applied to a logic circuit 32 ensures that a signal will be transmitted to the (sixth) logic circuit 17. The circuit 32 forms a store in which the information is kept till the next clock pulse. The sixth logic circuit 17 has an "EXCI-OR"-function. If no errors occur,the output of the circuit 32 will have the logic value "0". If an error occurs, this output will have the value "1". The output signal inverts the signal which comes from the speed converter 14 in the "EXCL-OR"-gate 17. If only one of the signals from the converter 14 and the circuit 32 has the value "1", the clock signal c applied to the logic circuit 18 ensures that a signal will be transmitted at the right moment to the central station via the switch 19, the modem 12, the switch 11 and the line 10. In the Figs. 2-6 the errors are indicated by f. In the Figs. 2-4 only one of the three erroneous pulses in the signal 14 is shown. It is a known fact that with self-synchronizing scramblers, which are used at the central station, the number of transmission errors is trebled. Fig. 2 shows a signal 14 with an error in the "1" state. The consequence is: one bit error in the signal 18. Fig. 3 shows a signal 14 with an error in the "0" state, which coincides with a clock pulse c. The consequence is: three bit errors in the signal 18. Fig. 4 shows a signal 14 with an error in the "1" state, which coincides with a clock pulse c. The consequence is: three bit errors in the signal 18. Fig. 5 shows a signal 14 with a trebled error in the "1" state. The clock pulse 33 occurs between two errors 34 and 35. The consequence is: two bit errors in the signal 18. Fig. 6 differs from Fig. 5 in that in the former the clock pulse coincides with one of the errors (35). The consequence is: three bit errors in the signal 18. According to calculations the chance of a single bit error is 9/16 on an average, the chance of a doubled bit error is 4/16 on an average and the chance of a trebledbit error is 3/16 on an average. All the errors are observed and some of them are multiplied as will be clear from the above. In the case of random errors the average multiplication factor will be (9/16 x 1) + (4/16 x 2) + (3/16 x 3) = 8 1.6. In practice the modem is found correct if within a certain observation period not more than a certain number of errors occurs. The requirement is for example at mdst 10 errors in an observation period of 104 bits for the 75-baud part (corresponding with 1200 x 104 : 75 for the 1200-baud part) or 133 s = 2 m 13 s.

Claims: Claim: System for the remote testing of a modem for two bit rates different from one another, particularly for viewdata traffic, the part for the higher bit rate being connected in the state of testing via a speed converter to the part for the lower bit rate, characterized by an error detector between the speed converter (14) and the transmission channel of the modem (12) to be tested, which error detector comprises some logic circuits, controlled by delayed clock pulses derived from a test signal, of which logic circuits a first circuit (21) is or remains conductive owing to a leading edge of the test signal, a second circuit (25) is or remains conductive owing to a trailing edge of the test signal, a third circuit (23) serves for resetting the first circuit and a fourth circuit (27) serves for resetting the second circuit, a fifth circuit (29) has an "AND"-function for the outputs of the first and the second circuit, and a sixth circuit (17) has an "EXCL-OR"-function for the output signal of the speed converter (14) and the clocked output (32) of the fifth logic circuit (29), and the clocked output (18) of the sixth logic circuit (17) is connected to the modem part for the lower bit rate.

Inventors: NUHOFF, PEERKE JAN
Applicants: STAAT DER NEDERLANDEN (STAATSBEDRIJF DER POSTERIJEN, TELEGRAFIE EN TELEFONIE)
Application year: 1981
Main classification: H04L1
IPC codes: H04L1, G01R31, H04L29