Patent GB-1152402-A (GB)
Title: Improvements in or relating to Time Division Communication Systems
IPC: H04Q11/04
Filed: 1966-08-04
Abstract: 1,152,402. Telephone exchange systems. WESTERN ELECTRIC CO. Inc. 4 Aug., 1966 [7 Aug., 1965], No. 34950/66. Heading H4K. In a communication system comprising a central office connected with a subscribers' line concentrator over time-division send and receive paths wherein the central office has means for controlling the concentrator and for receiving answer information and wherein the latter receiving equipment is capable of initiating new commands for transmission to the concentrator, the command and answer information is transmitted over the time-division paths in a particular time channel not employed for the transmission of signals from lines engaged in call connections. The subscribers' lines 111, 112, Fig. 2, terminating at a concentrator 101 are scanned by the concentrator under the control of scan number generator (350), Fig. 3 (not shown), at the central office and through a command sender (332) when permitted by a sender controller (402), Fig. 4 (not shown). A cross-correlation operation in which " bits " of the generated number are compared at the central station with those in a long memory (330) to insert a " 1 " in a short memory (401) in the command sender in a position corresponding to the time slot which the line number occupied in the memory (330) is effected at the same time and determines whether or not the scanned line is already active. The stored " 1 " in memory (401) releases a " call progress word " from the long memory for verification at the remote concentrator prior to the next call progress step. A memory 203 at the concentrator comprising seven delay lines each of twenty microseconds delay stores and circulates the seven digits of each terminating subscribers' line, and effects a control of a corresponding line circuit 206 during the time in which the line number appears. Commands, which comprise the designation and current state of a subscribers' line are sent from the central office and stored in a command receiver which verifies the command by comparison with the content of memory 203 before effecting action. Erroneous commands initiate a request for retransmission. Changes in a subscriber's line condition detected by a scan circuit 210 are reported to central office if a " 0 " is present in memory 203 to indicate a request for service, but if a " 1 " is stored indicating an active line the output is coded for transmission to a connected subscriber. Communicating lines are interconnected at a switching network 220 controlled by memories (327), (328) and the arrangement may include pulse shifters (322), (325) for use when the calling and called subscribers are allotted different time slots, the pulse shifters being controlled by a memory (329). Concentrator control units 230-233 each of which includes a long-memory (330) maintaining a record of all active lines and of the activity condition or call progress, act, together with common equipment comprising a register (345), a marker (335) (the master controller) and a scan number generator (350) to establish, and disconnect calls through command sender (332) and answer decoder (333). The information in long memory is stored in distributed serial form in a manner described with reference to Fig. 10 (not shown). Command sender (332) comprises a short-delay circulating-delayline memory (401) storing twenty " bits " each appearing once in each frame interval, and a group of logic gates for directing information, the arrangement also including a shift register (403) providing time slot numbers in coded form corresponding to the particular output of memory (401), a store for call progress words received from long memory (330), a code converter and transmitter, a write-erase control for memory 401, and the cross-correlator. The system uses seven commands having four priority levels, the top priority being unconditional " erase " and " write," in response to a found discrepancy. Erasing a specified number from a specified time slot is of second priority, and third priority commands include the writing of a specified number in a specified time slot and the changing of the call progress in remote memory 203. The lowest priority is given to scanning subscribers' lines. This scanning, if the subscriber is active, results in successive commands from scan number generator, firstly to verify that the active subscriber's number is written in memory 203 and secondly to detect a termination. An answer from a concentrator that a control has been executed prepares the control apparatus to advance the call progress to its succeeding state, and an answer that a command is erroneous causes a retransmission of the same command and if refused again, a command to write in the specified number is transmitted. The marker (335) shown in detail in Fig. 5 is seized by answer decoder (333) in response to a request for service and deals with one call at a time. It allots a time slot which is commonly idle in the highway and register (345) to the calling line and arranges for information regarding the allocation to be written in the long memory. If a commonly idle time slot is not available then pulseshifters are introduced in the switching network. Subsequently when the called line designation is stored in register (345) the marker transfers it to a further store (504) from whence it is fed to a cross-correlator which determines whether the called line is busy and accordingly controls the transmission of busy tone. Finally, the marker effects disconnection after a verification and erases the content of the long memories and is then itself released. The setting up of a call is described with reference to flow diagrams, Figs. 7 and 8 (not shown), and includes description relating to the monitoring of dialling and to restoration in the event of incomplete dialling.