Patent GB-1187488-A (GB)
Title: Telecommunication System
Inventor: THOMAS DAVID LANE
IPC: H04M13/00
Filed: 1967-10-25
Abstract: 1,187,488. Telecommunication systems; multiplex pulse signalling. INTERNATIONAL STANDARD ELECTRIC CORP. 25 Oct., 1967, No. 48466/67. Headings H4K and H4L. A telecommunication system comprises stations SS, Fig. 1, distributed round a looped unidirectional transmission line LL time division multiplexed from a timing station TS, a calling station being able to seize a free channel for a connection and circulate a code identifying the wanted station. The system employs pulse code modulation with 32 eight bit channels one of which is used for synchronization. During the synch channel the timing station TS, Fig. 3, breaks the loop at A 3 , A 4  to connect a pattern generator PG to one end of the loop and a resistor R to the other end. The synch pattern is all ones. For the 31 channels other than the synch channel a register records idle/busy states. The switches A 3 , A 4 , close to complete the loop for busy channels. Channels occupied with speech have nought in the first bit position while channels occupied with supervisory signals have one in the first bit position. In idle channels the loop is broken and the generator PG injects the code one followed by all noughts. Idle channels are detected by empty channel detector ECD. For calls going outside the loop the timing station has an outgoing number detector OND which connects a channel to a buffer BUF by way of switches A 8 , A 9 . Calls incoming to the loop are detected by IND, the timing station connecting the buffer BUF in a free channel over switches A 8 , A 9 . Synchronization between the framing of signals sent from the timing station and those received after the delay consequent on one circuit of the loop is effected by a variable delay D adjusted frame by frame as described later with reference to Fig. 10. Station equipment, Fig. 2, comprises a microphone and transmitter with associated decoder and coder which can be connected by switches A 1 , A 2  into one limb of the loop as a receive channel and the other limb of the loop as a send channel. With the handset onhook a station number detector SND looks out for the code identifying the station. Should the station code be received a ringing tone generator and bell are activated, the timing circuits detecting the calling channel to operate switches A 1 , A 2  so as to relay a ringing tone code to the calling station. To originate a call the off-hook condition connects up free channel detector ECD which seizes the first channel containing the code 10000000 and operates switches A 1  and A 2  accordingly. The failure of the code 10000000 to complete the loop and return to the timing station signals the timing station that the channel has been seized so that switches A 3 , A 4  can be operated to close the loop. The code of the wanted station is produced by a called number generator CNG, described later with reference to Fig. 8. If the wanted station is idle it responds to the code to break the loop and return ringing tone code in the calling channel. If the station is busy the loop is not so broken and the wanted number code will circuit the loop and be detected at the calling station. Synchronizing station clocks.-A master clock MC divides the bit rate by 16 to obtain channel widths and by 256 to obtain frame lengths. The bit rate is obtained from a free running multivibrator triggered by bit pulses on the line LL. As seen in Fig. 5 the station clock MC produces a synch pulse of the same duration as the synch channel on LL. The synch pulse leading edge sets bi-stable B5/1 to gate the line LL to a ¸ 8 counter. If the synch pulse is in synchronism with the synch channel eight consecutive ones are received and the bi-stable B5/1 is reset; a bi-stable B5/2 in a carry over circuit also being reset. If the synch pulse is not in synchronism with the synch channel the first appearing nought from the line LL resets the counter over inverter 15/1 and B5/1 is not reset from the counter. In case the failure to receive the synch code from LL is due to a transmission fault the carry over circuit responds to the trailing edge of the synch pulse from MC to reset B5/1 and set B5/2, an inhibit circuit H5/2 being open so long as B5/2 is reset and the ¸ 8 counter gives no output. If, however, at the next attempt to match the synch pulse with the synch channel, the ¸ 8 counter fails to find the synch code, the bi-stable B5/1 is not reset by the trailing edge of the synch pulse as B5/2 is set. B5/1 stays set, therefore, while ¸ 8 counter scans a frame in search of the synch code. When the code is found the ¸ 8 counter sets MC to its all ones synch pulse state, sets B5/2, and resets B5/1. Seizure of free channels.-A stepping register SR6/1, Fig. 6, displays and circulates eight consecutive bits from the line LL. Whenever the eight bits complete a channel a sample pulse primes AND gate G6/1. The first bit of each PCM signalling code in a channel is always a one; the corresponding stage of SR6/1 being connected as a direct input to G6/1. When the station handset is off-hook all other stages of the register are connected over inverters to G6/1. When the code marking an idle channel appears gate G6/1 opens to produce pulse P1. A bi-stable B6/1 is set to inhibit the sample pulses at H6/1 and prevent seizure of more than one channel. The pulse P1 gates the master clock MC to a set of bi-stables which record the identity of the seized channel. With the bi-stables set, whenever the clock reaches the position of the seized channel, a pulse is gated to operate station loop contacts A1, A2 (see Fig. 7, not shown). Ringing of wanted station.-With the handset on-hook at a station the register SR6/1 of Fig. 6 is connected to the inverters only in respect of those stages for which a nought appears in the station's code. If a calling station circulates the code, the gate G6/1 opens to produce the pulse P1 at the called station where the channel identity is recorded and loop contacts A 1 , A 2 , operated each time the channel reappears. With the handset on-hook the pulse P1 also triggers the bell and the ringing tone generator RTG. Station key set and connection of busy tone.- While the number of a wanted station could be set up on a decimal keyset and a conversion be made to the corresponding PCM code, the key set shown in Fig. 8 sets up the PCM code direct. As for signals other than speech the first digit in each channel is always a one, the key set has seven keys each of which can set up the corresponding stage of the stepping register SR8/1. The eighth stage of SR8/1 is automatically set up as a one and is read out to line first in the channel seized by a calling station as indicated by gate G8/10. The key settings are read into the register over gates G8/1 to G8/7 and also prime gates G8/11 to G8/17 which latter set of gates open together if the keyed code reappears in the register as read in serially from the line LL. If the code reappears persistently it means that the called station is busy, as it has not responded and broken the loop in the seized channel, and the output of gate G8/8 sets a bi-stable B8/1 to connect busy tone from ENG. If the code does not reappear in the next occurrence of the channel its absence may be the result of propagation time or mutilation and because of this a two-stage counter B8/2, B8/3 is stepped once each frame in the absence of the code until three absences have been detected after which the called station is presumed to be idle, G8/10 closes and the register SR8/1 is stopped. Synchronizing incoming and outgoing sides of timing station.-This involves insertion in the loop of a variable delay in order to make the propagation time round the loop equal an integral number of frame lengths. As shown in Fig. 10 a fixed fraction of the required delay is provided by D10 from which the bit stream on the loop is passed to a stepping register SR10/2 to be tapped off from the stage that introduces the required extra delay. Such tapping is effected by means of one of the gates G10/11 to G10/15 as primed by one of the bi-stables B10/1 to B10/5. One of the bi-stables B10/1 to B10/15 is set according to the distance travelled by a synch pulse PIN, derived from the synch channel by detector SD and stepped along register SR10/1, before the appearance of a synch pulse PREF derived in the timing station. As the pulse PIN may overlap two stages of register SR10/1 the consequent operation of two bi-stables B10/1 to B10/5 will not result in the opening of two gates G10/11 to G10/15 if the dotted line connections are made between bistables and gates. The variable delay is described and claimed in Specification 1,187,489. Multiple loop systems.-Provided the number of loops is small the timing stations of each loop may be connected into a super trunk loop, Fig. 11 (not shown). Otherwise the buffer outlets from the timing station of each loop may be terminated at a central switching station, Fig. 12 (not shown). Provided traffic is light loops may be connected to two other loops to form chains, Fig. 13 (not shown).