Patent GB-1348260-A (GB)
Title: Automatic telephone exchange with calling line identification  facility
IPC: H04Q3/00
Filed: 1972-07-25
Abstract: 1348260 Automatic echange systems PLESSEY CO Ltd 25 July 1972 [17 Aug 1971] 38639/71 Heading H4K A calling line identification arrangement at a local exchange which is taken into use by a trunk exchange in order to ascertain a caller's directory number for metering purposes, comprises a translation field having inputs connected to the P-wires of the local subscribers line circuits and outputs connectable to an equipment which on a start command applies a positive pulse to the P-wire of the subscriber concerned and notes the result supplied by the translation field in an internal register and then applies a second such pulse to the P-wire whereafter if and only if the results of the two pulse applications are identical does it transmit the result to the trunk exchange. The double pulse interrogation seeks to avoid ambiguities arising from simultaneously applied positive metering pulses. The local exchange may be of cross-bar 5005 or strowger type and is connected by up to three 4-wire order junctions to the trunk exchange for the purpose of transmitting inter-register signals in 2 out of 6 multifrequency format using compelled signalling techniques. The strowger exchange may use D.C. code signalling to its superior exchange in the hierarchy. Three identification equipments allocatable to three different MF senders simultaneouslyare provided although only one equipment at a time is connectible to the translation field thanks to a lock out relay chain (Figs. 9, 10, not shown) which in effect shifts the order of priority so that when one equipment temporarily releases (see below) another waiting equipment can then gain access to the field. The latter, in a fully equipped 20,000 line exchange, comprises twenty sections each consisting of ten, 10 Î 10 diode arrays. The sections are identifiable by particular combinations of operated relays during an identification process thus providing the "10,000" and "1000' digits of a subscriber's identity while the hundreds digits are derived from the particular array used and the tens and units digits from the crosspoint within the array. Each crosspoint consists of a first diode connected between the associated P-wire and two further diodes one of which is connected to a common row wire and one to a common column wire (Figs. 1, 2 and 3, not shown). Communication between an equipment and an MF sender is effected via a set of control wires and by six digit signalling wires on which digits are conveyed by 2 out of 6 D.C. code. The code conversion is effected by a diode array (Fig. 6 not shown). Preferably the array is strapped so that during CLI the exchange digits of the callers identity are automatically transmitted prior to transmission of his directory digits. In operation, an MF sender seized for use in a call is requested by the trunk exchange to seize a CLI equipment. When it has done so, it immediately transmits to 4 digit exchange identity, recorded by strappings in a diode field of the equipment, to trunk one digit at a time in compelled manner (i.e. continuous until acknowledged). Meanwhile the CLI equipment computes for access to the translation field and eventually gets connected to the first section thereof. The equipment observes the condition of the 10 hundreds wires and waits until none of them is exhibiting a positive polarity corresponding to the presence of a metering pulse on the P-wires of one of its array's 100 line circuits. When all the wires are unmarked the equipment immediately despatches an 8 msec. positive pulse over a control wire to the MF sender and thence via a special temporarily seized by path connection and the register allotted to the call to the calling subscriber's P-wire. (This pulse is effective to back off the resistive holding earth for the caller's line and cut-off relays, which however then hold to the pulse. The latter is of course too short to affect the subscriber's meter). If the subscriber belongs to the 1000-line section under consideration the pulse will reappear via the translation field section and will bring up a fast acting relay in the equipment. If the subscriber does not belong, the absence of a pulse steps on a relay counting chain so as to connect the next 1000 line section to the equipment. When again there is no positive potential on a hundreds wire, another positive pulse is sent to the callers P-wire. If this does not reappear the chain steps on but before the next section is interrogated, the equipment temporarily releases from the field so as to allow another waiting equipment access thereto. This courtesy is extended after every two such sectional interrogations. The occurrence of a metering pulse during an interrogation is detected by its longevity so that the result of the interrogation is negated and a new one attempted when the metering pulse ceases. Eventually the correct section is found whereupon the CLI releases from the field and awaits completion of the transmission of the fixed exchange digits. When this is done the trunk exchange, via the MF sender, demands the first i.e. the 10,000th directory number digit. The equipment, when permitted by the other equipments, reconnects to the section of the field which it has just discovered pertains to the caller, awaits cessation of any metering pulse and then transmits a positive pulse to the caller's P-wire. The returning pulse appears at the hundred's terminals of the sections diode array and is then directed via a contact tree controlled by the sectionrelay-counting-chain to a particular one of a first set of storage relays so as to indicate the 10,000th digit value thereat. Thereafter a second positive pulse is sent to the caller's P-wire (as usual after allowing for metering pulses) and the reappearing pulse is effective to bring up an OK relay if and only if it is indicative of the same digit value as the just-preceding pulse. The value is transferred to one of a second set of relays which are checked to ensure that one only is operated and the equipment then withdraws from the translation field. If the P-wire pulse had not returned on the first occasion thus signifying that the wrong section was initially chosen, the relay counting chain would be restarted. If the second pulse leads to a different result from the first a second two pulse sequence is started. When the trunk exchange is ready it demands the stored digit value and this is transferred in 2 out of 6 D.C. code to the MF sender and from there in 2 out of 6 VF code to the exchange. A digit distributor is stepped each time a (fixed or variable) digit is sent to trunk so that upon the latter's next demand, for the 1000's digit, the double positive P-wire pulse is again caused to traverse the relay contact chain but this time in such manner as to identify the 1000's rather than the 10,000's digit. The usual metering and consistency checks are ,made before the digit value is sent out whereafter the other equipments are given access to the field. The next P-wire pulse doublet leaves a particular hundreds terminal and is registered directly on one of the relays of the first set. Similarly, when their turn arrives, are the tens and units digits treated. In any of these 1000'sunits cases if the first pulse does not bring up a first set relay the double pulse test is immediately restarted as it is also of course if a metering pulse intervenes during the actual P-wire pulse interrogation or if one of the checks fails. Further double pulse tests are repeated until such time as they succeed or are timed out by local exchange S and Z pulses. If this occurs, a failure signal is sent to trunk which then clears the call or reinstitutes a new CLI process depending on its constitution. The local equipment is busied out without releasing its operative relay combination and marked for maintenance by a fault alarm and fault lamp. If another equipment fails the first one may be restored to service. Assuming all is well however, the next demand from trunk is effective to return an end-of-identification message and then since the digit distributor is at its last stage the final demand permits the equipment to clear completely from the MF sender.