Improvements in or relating to data transmission systems

Abstract

Abstract 1,168,476. Digital transmission systems. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. 15 May, 1967 [17 May, 1966], No. 21829/66. Heading H4P. In a digital transmission system in which a plurality of stations are connected to a common line which is in the form of a ring circuit each station has a different priority. If two stations simultaneously wish to transmit their priorities are compared and the station of higher priority is allowed to transmit. A signal can be transmitted only when the preceding signal has passed completely round the ring and agrees with the originally transmitted signal. The ring consists of ten data wires for parallel-bit signals and of four control wires which are selectively energized to indicate, for example, whether the ring is free or busy and whether a called station is free or busy. If station A wishes to transmit and if the ring is not busy it breaks the ring and sends its priority number. At B, if B does not wish to transmit, the number is passed on to C. If B also wishes to transmit, the priorities of A and B are compared. If that of A is greater, transmission from B is suppressed and A's priority number passes on to C. However, if B's priority is greater B's priority number is sent on instead of A's, and the continuity of the line at A is restored. If no station, other than A, wishes to transmit, A's priority eventually returns to A, having completed the ring. Station A now seizes the ring by energizing one of the control wires and transmits the address of the required station. At B the called address is compared with B's address and if it is not the same the called address passes on to C. If it is the same, i.e. A wishes to communicate with B, then if B is busy B is made to take up a waiting condition after it ceases to be busy so that A can make another attempt to establish communication, which it does after a preset time. If B is free the address eventually returns to the transmitter which then transmits a data signal to the selected receiver. The signal passes round the ring to station A where it is compared with the received signal. If it is the same, the next signal is sent. If not, an error signal (all ones) is transmitted to inform the receiver of the fault. Should the receiver detect a fault a repetition requesting signal is returned to A. The end of message code is an all zero signal. Two or more ring circuits are provided. These may be used sequentially. If the error rate becomes too high the presently used ring is replaced by another. A station may transmit data to more than one other station.

Inventors

• 👤 John Michael Cotton
• 👤 Peter Anthony Lloyd

Bibliographic Details