Patent GB-936419-A (GB)
Title: Method and apparatus for data transmission
IPC: H04L1/18
Filed: 1961-02-24
Abstract: 936,419. Code telegraphy. STANDARD TELEPHONE & CABLES Ltd. Feb. 24, 1961 [March 2, 1960], No. 6871/61. Class 40 (3). In a system for transmitting information by coded combinations of binary pulse-shaped signals, with a theoretically acceptable minimum of errors, over a channel subject to severe bursts of interference, the information is provided as a serial train of information elements forming blocks or groups of the elements of a predetermined size irrespective of their information meaning, e.g. words, and of their normal spacing, but including also redundancy elements derived from the application of a selected error-checking code system so that the blocks constitute errorchecking groups whose total number of elements related to the number of information elements, i.e. redundancy factor, is chosen in accordance with (a) the acceptable probability of error for the block, (b) the error-checking system selected, and (c) the available storage capacity for the blocks of elements at the transmitter, where they are stored for transmission in sequence, or for repeated transmission if requested as a result of faulty reception at a receiver station, at which the received blocks of signals are stored and checked by the checking system employed, and the receiver is controlled to send over a return channel a signal acknowledging the correct reception of each block in turn, or, in a similar transmission manner, to signal a request to the transmitter for the repetition of a block, or blocks, of the signals detected as erroneous by the receiving station. As shown in Fig. 4, signals from a tape reader 1 are fed via a switch S3 and a switch S1, movable under control of a clock pulse generator 5, to apply a number of signals, forming a block or group in sequence to storage and code forming devices TS1, TS2, TS3 which are connected in succession by a switch S2, also controlled by the pulse generator 5, but delayed by one operative position relative to the switch S1 so that, as shown, the group in TS2 is being passed via switch S4 to the transmitting modulator 2, whilst a signal group is being applied from the tape reader 1 to the storage-converter TS3. The signals are frequency-modulated and passed by a transmitter network 3 over a line 4 to a filter network 7 and demodulator 8, and over one arm of a switch S5, operated by a clock pulse generator 9, alternately to receiver storage devices RSa, RSb. The signals fed to the devices RSa, RSb are also applied by the second arm of switch S5 to a testing device 12, and, if received without error, are passed by a switch S6 and a switch S7 to a receiving device 11, such as a reperforator. The switch S6 is also operated by the pulse generator 9 and in opposite phase to the switch S5. During normal, i.e. undisturbed reception, a faultindicating circuit 16 operates a transmitting device 18 to pass a frequency f1 over the line 4<SP>1</SP> to an amplifier 23 and a selective circuit 20 to a fault-alarm device 17. When a faulty signal group has been detected by the test circuit 12 a pulse is applied to a control device 15 to operate the switch S7 to disconnect the receiving device 11, and also to the device 16 to control a transmitting device 19 to pass a frequency f2 over the line 4<SP>1</SP> to the transmitting station where the frequency selective member 21 operates the device 17 to control the circuit 6 to move switch S3 to disconnect the tape reader 1 and to move the switch S4 to connect a store RTS for a predetermined signal block # to the modulator 2. A coincidence circuit 13 detecting the predetermined signal group passes a pulse via gate 14 to synchronize the generator 9 to the group #, and also passes a pulse to the control device 16 to switch in generator 18 for transmission of the frequency f1 to the calling station, which has sent a second block #, and at which the alarm circuit 17 is operated to control the circuit 6 to restore the switches S3, S4 to the positions shown to repeat the transmission of the signal block which was erroneously received. The alarm device 17 is arranged to be operated not only by the frequency f2, but by the absence of f1 and f2, and also by the simultaneous occurrence of the frequencies f1 and f2. The operation of the tape reader is delayed for a period corresponding to three blocks after the frequency f1 has been restored, and at the receiver the three blocks received immediately following the correct reception of the first # block, are withheld from the reperforator 11. The system may be used in duplex operation by choosing the lengths of the blocks or their speed of transmission so that their transmission time is at least equal to the sum of the transit times of both the forward and backward (return) channels, and that during the time interval allowed for the transmission of the " answer-back" signal acknowledging correct reception or requesting a repeat, the next successive block has been transmitted and that upon request for a repeat, or in cases where the answer-back signal has failed to be received, the repeat criterion " # " is transmitted subsequently to the first transmitted block in lieu of the next block.