Patent GB-1270891-A (GB)
Title: Data transmission system
IPC: H04B/
Filed: 1970-02-11
Abstract: 1,270,891. Digital transmission systems. INTERNATIONAL STANDARD ELECTRIC CORP. 11 Feb., 1970 [14 Feb., 1969], No. 6632/70. Heading H4P.  In a system in which errors detected during reception are corrected by repetition, each terminal has clock defining time slots reserved for different digits of a watch, one clock e.g. at the transmitter acting as a pilot terminal, each information word CCr has ul information digits and n2 parity digits and the transmitted words may include synchronization words Csy repetition request a start words Crd, Crs each of which also has n1 + n2 digits. Each terminal has a word analyser delivering one of signals Cr, Sy, Rd or Rs depending on whether Ccr, Csy, Crd or Crs are received, synchronizing words Csy being transmitted when other words are absent. At the transmitter m last sent words comprising n + n2 bits are retained in a memory. At the received terminal transfer of received words is blocked on an error being detected and a repetition request Crd is returned to the transmitter and at the same time alarm and repetition counters are advanced by one position. When Crd is received and decoded at the transmitter, transmission is stopped and a repetition start signal Crs is sent to the receiver followed by the contents of the memory. Reception of Crs stops the alarm counter and the words are retransmitted at the end of which normal operation is resumed. The transmitters and receivers are substantially similar and may communicate solely to each other or form part of a system.  Terminal TA is connected through channels Dab and Dba and modems DR, DS which control in both directions of transmission the conversion of PCM signals into modulated signals which may be of frequency shift form. In each unit, information is stored in register SR of block SL where it is processed then passed to DS. On reception after decoding in DR it is passed through register RR of block RL and applied to a word analyser WZ which identifies the type of word. A word may comprise ten digits of which nine and ten are parity digits. Data words from unit QPA are transmitted only if a word Crd has not been received thus repetition has priority over normal transfer of information. Of lesser priority is the word Csy constituted by an alternation of "0" and "1" digits which furnishes the maximum number of transmissions, which is only sent when there is no other data to be transmitted. Synchronization of signals Csy is carried out in DR which does not form part of the present invention. Signal SY from analyser WZ adjusts the phase of the reception clock CR such that a signal R1 appears exactly at the reception time of each first digit. Transmission terminal TA has a transmission clock CS and a reception clock CR synchronized to receive the signals. Clock CR is retarded relative to clock CS by an interval equal to the go and return transmission time plus a data processing time in terminal TD. Reception of an error say in word five at the unit TD causes a detector in unit RL to deliver a signal which supresses the signal Cr so that the word five and following words are not forwarded to unit QPB. The reception signal is received at translator TA probably during the time of transmission of word eight hence at the next word time TA transmits a repetition start word Crs followed by words which 1-8 are kept in the memory formed by a shift register with a capacity of eighty digits. The parity computing circuit. (Fig. 10 not shown) operates so the first parity digit b9 is the normal parity of digits b1-b8 its value being equal to 1 if there is an even number of 1's or equal to 0 in the opposite case. The second parity digit b10 gives the parity of the other digits b1-b9. This parity two digit code may have any of four different values of which only one is correct but the circuits detect without ambiguity errors on three consecutive digits. It is considered in the specification that errors occur generally in bursts.