GB1211407A - Digital data message transmission - Google Patents

Abstract

1,211,407. Digital transmission systems; signalling between exchangers. STANDARD TELEPHONES & CABLES Ltd. 28 March, 1968, No. 15050/68. Headings H4K and H4P. In a digital transmission system messages to be transmitted, e.g. telephone network signals, are stored prior to transmission. The signals form bytes of fixed length. The store is scanned prior to the transmission of each byte and data from the message of highest priority currently in the store is read out and assembled in the byte. As described each byte starts with a tag bit and ends with an error-check word added prior to transmission. If a byte relates to a message having different priority from that of the previous byte a signal indicating the priority category is added, such a byte is called a label byte. The tag bit of a label byte differs from that of other bytes. The transmitted bytes are stored so that they may be retransmitted if an error is detected at the receiver. The highest priority message is an answer signal and consists of one label byte. The next lower priority messages are dialling signals. These are sent in a group of bytes, the number of bytes required being indicated in each byte. The next lower priority messages are management signals. These are divided into groups containing a fixed number of bytes. The groups are numbered successively, each byte including the number of its group. The lowest priority messages are test signals. Fig. 2 shows an example of the flow of signals. A block of management signal bytes, numbered 1, is preceded by a label byte. The second block, numbered 2, is interrupted by an answer signal byte, whereafter the label byte of block 2 is repeated and the remainder of block 2 transmitted. Block 3 is interrupted by a dialing signal block of three bytes, preceded by a label byte, after which the label byte of block 3 is repeated and the remainder of block 3 transmitted.