Patent GB-1512440-A (GB)
Title: Subscriber digital multiplexing system with time division concentration
IPC: H04M7/14
Filed: 1975-12-02
Abstract: 1512440 TDM systems INTERNATIONAL STANDARD ELECTRIC CORP 2 Dec 1975 [9 Dec 1974] 49411/75 Heading H4M In a digital multiplex system in which data is conveyed between a central office (Fig. 1A) and one or more remote terminals (Fig. 1B) in alternate mark inversion bipolar form, the central office includes a timing control arrangement 40 generating multiplex frames of n channels, there being m plural bit signalling channels and n-m data channels, and the n-m data channels pass through the timing control for channel alignment before transmission. A bipolar violation is produced within the m channels of certain predetermined frames and each remote terminal is able to detect such violations and to feed a corresponding signal to timing control arrangements in order to synchronize timing. Each remote terminal also incorporates arrangements which in response to the violation signal, reconstitute the original bits in the signalling channels before violation and count the occasions on which consecutive violations have been missed. If a predetermined number of misses occurs, an alarm is given. In an embodiment, 128 lines are connected over 32 duplex channels using continuously variable slope delta modulation by using a concentrater or TDS switching stage. Inputs at a central office (Fig. I A) are coupled to line units e.g. 20-26 containing appropriate modems which are commoned to shelf scanners 30-31. Scanning is under the control of a time division memory 34 and TDS control 36 via an interface circuit 32, such that channel assignments are distributed evenly to the 32 channels by rotation. The memory records line status and channel assignment and status. Timing control for both transmit and receive operations is provided by a master timer 40, which also provides alternate mark inversion and zeros pulse stuffing. Control and digital data are sent from the timer 40 to a remote terminal circuit 62, which is slaved to the timer 40. The timing signals are returned to the receive timing circuit 42 for comparison and for alarm control 44. The remote circuit 62 also forwards required data to the remote TDS circuit 64 for directing shelf unit selection to the shelf scanners 66 to perform further decoding to select a desired line unit 70. Timing signals are sent to framing circuit 72 to provide a pattern match of timing and sync control signals to actuate an alarm circuit 74 when indicated. There are 35 channels per frame, the first three channels, A, B, C, being for timing, framing, and sync. All timing pulses are generated in the master timer 40, and this unit also ensures that data for each channel is transmitted at the proper time, provides stuffing pulses and converts data from unipolar to bipolar AMI (alternate mark inversion) for transmission and sync, and provides line balancing. Units 40, 42, 62 and 72 combine to interchange timing, framing and control information. The transmit timing unit 40 incorporates a master oscillator at 6À176 MHZ and divider circuits to provide frame, superframe, sub-group (40 frames) and group (320 frames) timing signals, channel organizer arrangements and zero counting and pulse stuffing facilities (Figs. 2A and 2B, not shown). Frame sync, performed by controlled bipolar violations, occurs every fifth frame, during which the timing control channels transmit a 101 pattern in which the second 1 is modified for bipolar violation and which is added to the baseband control signal by the channel organizer, zero counter and pulse stuffing. Pulse stuffing is used to prevent a maximum number of consecutive zeros being exceeded - which would endanger satisfactory performance. The line units at the input to the central office (Fig. 1A) are scanned by scanners 30, 31 in response to addresses from the timing unit 40 and enabled when found to be seeking service. Transmission is inhibited by the scanner if alarm conditions exist, otherwise the signals are gated through to the timing unit 40. Zeros are counted and if more than four occur successively, stuffing occurs. The baseband signal then enters an AMI generator which inverts every second mark. Thereafter the signal is applied to a line driver. Receive timing circuiting in the central office (CO) is very similar to that at the transmitter (Figs. 3A, 3B, not shown) with the addition of new waveforms in channel C for status reporting. Shelf scanners (Fig. 5, not shown) are similar at either the CO or remote terminal and are provided one for each 16 line units to provide address decoding circuitry to direct line scan between the system and appropriate line unit, together with power distribution and alarm facilities. The remote timing unit 12 (Fig. 6, not shown) is similar to that at the CO with the addition of an AFC circuit (303) to enable the remote oscillator to be controlled. It acts, in addition to providing basic timing waveforms, as a carrier signal detector, bipolar violation detector, error detector, synchronizer and regenerative repeater. It also provides drop and insert facilities. If four consecutive frame five sync (bipolar violation) pulses are missed an alarm occurs. A remote framing unit (Fig. 7, not shown) operates by detecting a repetitive pattern of a seven bit code in the A channel in every frame except frame five, and which occurs at the beginning of every frame 40 sub-group. If the pattern does not occur in four consecutive sub-groups, an alarm is given.