US2030419A

US2030419A - Telephone exchange system

Info

Publication number: US2030419A
Application number: US749692A
Authority: US
Inventors: Samuel B Williams
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1934-10-24
Filing date: 1934-10-24
Publication date: 1936-02-11
Anticipated expiration: 1953-02-11

Description

Feb.. M9 wf s. B. WILLIAMS TELEPHONE EXCHANGE SYSTEM 2 Sheets-Sheet l Filed Oct. 24, 1934 ATTORNEY Feb., il, i936. s. B WILLIAMS SAE@ TELEPHONE EXCHANGE SYSTEM' Filed Oct. 24, 1934 2 Sheets-Sheet 2 m ml F/Rsr 056005@ E I l 303 /NTERMED/ATE DECODER LAST DECODER /NVENTOR 5.5. W/L L /AMS ATTORNEY Patented Feb. 11, 1936 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 24, 1934, Serial No. 749,692

9 Claims.

This invention relates to telephone exchange systems and more particularly to systems in which connections between subscribers lines terminating in different offices of the exchange area are established by automatic switching mechanism over groups of interoce trunks.

In exchange areas having a large number of local oflices it is not economical to provide direct groups of trunks for interconnecting each oce with every other oice, particularly where the oices are widely separated and the traic load is heavy. In some exchange areas therefore it is more economical to route *traino between certain orlices through a tandem oiice which serves as a common intermediate distributing point.

It has heretofore been proposed to render more eiiicient the trunks in a group directly interconnecting any two oiiices by limiting the number of the direct trunks to the maximum required for handling normal traiic between such oices and by automatically rerouting the abnormal or overflow trafc between such oflices to a tandem oice where the traiiic may be directly routed to the desired oiice. This proposal also enables existing direct trunk groups between oces to be employed without the addition of further trunks thereto when traic loads increase beyond the capacity of such trunk groups by rerouting calls through a -tandem oiice. To accomplish the rerouting of calls through a tandem oflice in systems employing central office senders which control the setting of selector switches to establish connections, the sender must receive a signal when all trunks of a direct trunk group extending to an office are busy in order that the sender may take the necessary steps to reroute the next connection for that trunk group over a different trunk group extending to the tandem oice which in turn has direct trunks also extending to the desired oince. This signal depends upon the closure of a chain circuit controlled by the trunks of the direct group when all trunks of the group become busy.

A sender may be given information to route a call to a trunk of a direct trunk group extending to a desired office and proceed to cause the selection oi a trunk in such group when there is only one idle trunk in the group and since an interval of approximately 4.5 seconds may elapse before a selector switch controlled by the sender is actually positioned onthe terminals of the idle trunk, the trunk may during this interval become seized in a previously initiated call and the later initiated call will not then be capable of completion as the selector switch would be driven to the direct trunk group overflow position. To guard `against this condition, it is the object of this invention to signal the sender when there is still one available idle trunk and also when there is no idle trunk in a trunk group in order that rerouting to a tandem trunk group may be effected without the hazard of encountering a false overflow condition. It is a further object of the invention where traffic conditions are such as to warrant it to signal the sender when there are still two available idle trunks, again when there is only one available idle trunk and again when there is no idle trunk in a trunk group in order that rerouting may be made at a time when it will be most accurately eiective.

In systems of the type disclosed in Patent 1,862,549 granted June 14, 1932 to R. Raymond and W. J. Scully the establishment of connections frorn an originating ofhce to any other cnice of the exchange area is made possible by district and oice selectors which are set to select an idle trunk of the group of trunks extending to the ofice in which a desired subscribers line terminates by a sender common to all subscribers lines of the originating oiice. A plurality of senders, any of which may be taken for use by a calling line, is provided, each sender having registers for registering both the ofce and numerical designations of a desired line dialed by a calling subscriber, and controlling apparatus for controlling the setting of a district and an office selector to extend a connection to the oce in which the desired line terminates and for controlling other apparatus in the selected olce for completing the connection. A plurality of decoders is also provided common to all of the senders, an idle one of which becomes associated with a sender taken for use by a calling subscriber. The decoder is provided with ofce code registers to which the oice code is transferred from the registers of the associated sender and which is also provided with as many route relays as there are trunking points to which calls may be trunked from the originating office. The combined setting of the oice code registers of the decoder determines which one of these route relays shall be operated for any particular call. The operated route relay is then instrumental in establishing in the associated sender a registration of the class of the call and a decoded oce code registration for directing the sender to control the setting of the district and oice selector switches or the setting of the district selector alone to select a group of trunks extending to the cnice in which the desired line terminates.

In accordance with one modication of the invention, each trunk group extending directly from one ofiice to another oi'rice, to which it is desirable to apply alternate routing through a tandem office, is provided with a. chain circuit having two relays associated therewith, one `of which operates only when all but one of the trunks of the group are busy and the other of which operates when all trunks of the group are busy. Associated with each decoder are as many reroute relays individualized to each direct trunk group as there are direct trunk groups to which the rerouting feature is to be app-lied.

When there is only one idle trunk in the` direct trunk group extending to a desired oice, the chain circuit associated with such trunk group is so. established through contacts of operated relays of the trunks of that trunk gro-up as to cause the operation of the first of the relays associated with the chain circuit to signal all decoders that there is but one idle trunk. When thereafter a decoder is taken for use by a sender and the registration transferred thereto from the associated sender indicates that a call is to be routed to this trunk group, the route relay of the decoder which is instrumental in causing the routing of the calls to this trunk group will be operated and the sender will be caused to set selector switches to seize the remaining idle trunk of the trunk group. As soon as the sender has received the translated code from the route relay of the decoder, the decoder is dismissed and then the reroute relay in each decoder allotted to the particular trunk group is operated preparatory to rerouting the next call incoming to any decoder for that particular trunk group to a tandem trunk group extending to a tandem oice through which calls may be extended to the oiice in which the desired line terminates. As soon as the last idle trunk of the direct trunk group has been seized by a selector switch, the chain circuit is altered in such a manner that the second relay associated therewith signals all decoders that there are no more idle trunks in the direct trunk group and maintains the reroute relays of all decoders appertaining to such trunk group operated. When thereafter any decoder is seized by a sender, with the reroute relay operated, the oice code registers of the decoder cause the operation of a tandem route relay for routing the calls through a tandem oice to the oice indicated by the office code rather than the o eration of the direct route relay. Should one or more trunks of the direct trunk group thereafter become idle the reroute relay appertaining to that trunk group in each decoder is` released and the next subsequent call for that group of trunks will again be routed thereto. Thus ythe relays associated with the chain circuit of a trunk group are instrumental in counting one idle trunk and informing all decoders when such condition exists. This insures satisfactory operation of the rerouting feature of decoders when direct trunk groups do not have more than ten trunks and the proportion of the traflic to be rerouted through a tandem oince will not exceed 20 per cent.

Where direct trunk groups have more than ten trunks and the proportion of the traffic to be rerouted through a tandem oice exceeds 20 per ce-nt, a more satisfactory operation of the rerouting feature of decoders may be made by employing a circuit such as disclosed in the second modification of the invention. In accordance with this modication, each direct trunk group to which it is desirable to apply alternate routing is provided with a chain circuit having three relays associated therewith, one of which operates only when all but two of the trunks of the group are busy, a second of which operates only when all but o-ne of the trunks are busy, and a third of which operates when all trunks are busy. This circuit is therefore enabled to count two idle trunks and to prepare all decoders sui'iciently in advance of the actual seizure of the last two idle trunks of the group for rerouting calls through a tandem oflice when there are no available idle trunks in the direct trunk group to a desired oiiice.

For a clearer understanding of the invention reference may be had to the following detailed description taken in connection with the accompanying drawings of which:

Fig. l shows schematically a calling subscribers line, a line nnder, district selector, oice selector, link circuit, sender and .decoder-connector of an originating oice; a subscribers line, incoming selectors and a nal selector of a distant oice; a group of direct trunks extending between the oices; and a tandem oilice indicated by a rectangle through which connections may also be established between the originating and distant oices;

Fig. 2 shows a circuit for counting when all but one of the trunks of the direct group of Fig. l are busy and for counting when all trunks of the group are busy;

Fig. 2A shows a modication of the circuit of Fig. 2 for counting when all but two, all but one and all trunks of a. ldirect group are busy; and

Fig. 3 shows such portions of one decodery accessible from the decoder-connector of Fig. 1 as are necessary to an understanding of the invention and other decoders schematically illustrated b-y the dot-dash rectangles also accessible from the decoder-connector.

The line finder district selector |02, oice selector |01, district nder |03 and sender selector |04 of the link circuit, sender |05 and decoder-coni ector |06 being o1 the well-known type and functioning in the usual manner have, for the purpose of simplifying the drawings, been disclosed only schematically herein. For a complete disclosure and description thereof, reference may be had to Patent No. 1,862,549, granted to R. Raymond and W. J. Scully, hereinbefore referred to. The decoder 300, only such portions of which as are deemed necessary to an understanding of the present invention have been disclosed, is of the type disclosed in the aforementioned patent. The tandem oilce equipment indicated by the rectangle |08 of Fig. l is or" the type disclosed in detail in Patent No. 1,840,132, granted January 5, 1932 to T. H. Roberts. The incoming selectors |09 to ||2, inclusive and the nal selector H3 are also of the well-known type and therefore have been only schematically disclosed.

In order that the invention may be more clearly understood, it will be assumed that the subscriber at substation |00 of an originating oice of an exchange area desires to establish a connection with a subscribers line ||4 which terminates at a distant oice of the same exchange area and, upon removing his receiver from the switchhook, obtains a connection with an idle sender in the originating oice through the operation of the line finder |0|, district nder |03 and sender selector |04 in the well-known manner and then dials the once code letters and numerical digits of the directory number of the wanted line |4.

acca-1e It willwfirst be assumed that atithe time decoder 300 is seized by the decoder-connector |06,

In response to dialing, the office code and numerical designations are registered in the sender in the well-known manner and following the registration of the letters of the oiice code designation the decoder-connector |06 is operated to associate an idle one or" a plurality of decoders with the sender. Three of the plurality of decoders are indicated by the rectangles 300, 30| and 302 of Fig. 3. Assuming that the first decoder 300 is at the time free, the decoder-connector |06 causes the association thereof with the sender and operates the decoder start relay 3|5.

Each decoder is provided with the usual receiving registers to which an associated sender transfers the registered office code designations, a route relay corresponding to each route over which connections may be extended from the originating' office and transmitting registers which are selectively set by any operated route relay. The transmitting registers are employed to transfer routing infomation to registers of the associated sender which in turn control the functioning of the sender to establish the desired connection over -district and office selectors, or over a district selector only, by the route indicated by the operated route relay of the decoder. Four of the route relays of decoder 300 are disclosed, of which

relays

303 and 305 designate direct routes over which connections may be established from the originating office directly to different distant or terminating oflices and

relays

304 and 300 designate tandem routes extending from the originating oflice to different tandem offices. If it be assumed that there are two hundred local terminating ofces in the exchange area, there would be two-hundred direct route relays such as 303 and 305 and as many tandem route relays such as 304 and 300 as there are tandem oihces in the area to which the originating ofce has access.

In addition to the usual equipment the decoder is also provided with a reroute relay corresponding to each direct route to which it is desirable to apply alternative routing. Two of these reroute

relays

301 and 308 have been disclosed.

With the recoder idle, the operating winding of each reroute relay is normally connected to the trunk group busy conductor of the group of direct interofce trunks forming a part of the corresponding direct route, over the back contact of a cut-'off relay. Nine groups of such cut-off relays of four relays each may be provided, or thirty-six relays, over the back contacts of which the trunk group busy conductors of as many as two hundred direct groups of trunks may normally extend to the two hundred reroute relays, if it should be required that alternative routing be applied to all direct routes. Two of these cut-olf relays 309 and 3|0 have been disclosed. The nine groups of cut-off relays are in turn controlled by a smaller group of three relays 3| i, 3i? and 3|3 which are in turn controlled by relay 3|4 and the decoder start relay 3|5. Thus, when the decoder is seized and the` start relay 3|5 operates,

' relays 3|4, 3| I, 3|? and 3|3 and all cut-off relays,

'such as 309 and 3|0, operate in cascade to disconnect the operating windings of all reroute re'- lays of the decoder from the corresponding trunk group busy conductors. An additional relay 3|6 is also provided which operates in parallel with relay 3|4 for providing locking grounds to the locking windings of all reroute relays. The start relay 3|5 also functions in the usual manner to initiate the registration and translation functions ofthe decoder.

the direct route extending to the oflice in which the wanted line ||4 terminates is available and that all but the last two trunks ||5 and IIB of the group of trunks included in this route are busy, as indicated by the operated condition of their sleeve relays such as relay ill. Associated with each sleeve relay is a resistance, the resistances associated with relays IIB and H0 being disclosed at |20, |2| and |22, respectively. Each resistance has a normally open shunt eX- tending over contacts of the sleeve relaywith which it is associated, and all of the resistances, when all of the trunks of the group are idle, are normally connected in a chain circuit extending from grounded battery therethrough in series to conductor |23 and thence in series through the lower operating Winding of relay |24 and the winding of relay |25 to ground. Relay |24 is a polarized relay, electrically biased by the circuit through its upper winding, and so adjusted that it will operate in series wit-h one of the resistances, but will not operate or will release in series with two or more of the resistances. Relay |25 is margined to operate only when all resistances have been removed from the chain circuit. It will thus be apparent that as each trunk of the group is taken for use, one of the serially connected resistances will become shunted by the operation of the trunk sleeve relay and that when all but one of the trunks of the group have been seized, all but one of the resistances will become shunted and relayv Under the assumption that when decoder 300 I is seized bo-th trunks ||5 and ||6 are idle and that consequently neither relay |24 nor relay |25 is operated, there will be no ground potential on the trunk group busy conductor |26 individual to the group of trunks. of the decoder allocated to the route of which this direct trunk group is a part will therefore be unoperated. Upon the seizure of the decoder and the operation of start relay 3|5, the decoder then proceeds to register the ofce code designations of the wanted line transferred thereto from the office code registers of the sender and to translate such office code by marking the code point all. This causes the operation of relay 35S individual to the direct route of which the direct trunks of Fig. 1 are a part and the operation of the proper direct route relay 305 in a circuit extending over the upper back contact of reroute relay 308. With route relay 305 operated the transmitting registers of the decoder are selectively set and the proper routing information is transmitted to the sender in the usual manner. Relay 3|5 also causes the operation of all Vcut-off relays such as 300 and 3|0 thereby opening the operating circuits of all route relays so that no reroute relay may be operated during the time that the decoder is making a routing thereby preventing the mutilation of the routing that is at the time in progress.

The sender after receiving the required rout- The reroute relay 308 ing information, dismisses the decoder by releasing the start relay 3|5 thereby in turn releasing all of the cut-01T relays such as 309 and 3|0. Any reroute relay such as 301 or 308 may now be operated to change the routing on subsequent calls. The decoder may no-w be employed in connection with another call. rIhe sender also proceeds to set the district selector |02 and the office selector |01 to extend the connection from the calling line to the proper trunk group extending to the office of the wanted subscriber, and the office selector proceeds to hunt for an] idle trunk in the trunk group. It having been assumed that all but the last two trunks of the group are busy, the office selector will hunt over the terminals of busy trunks and Will seize the first idle trunk or trunk H5. The sender will thereupon proceed to control the setting of the incoming selector in which the trunk ||5 terminates and the nal selector I I3 to complete the connection to the wanted line H4. Following the completion of its controlling functions the sender is dismissed in the usual manner.

Upon the seizure of trunk ||5 its sleeve relay i IS is operated to shunt the resistance |2 I. Since now all but one of the trunks of the group are busy and therefore only one resistance |22 associated with the sleeve relay H9 of the only remaining free trunk lili, remains connected in the previously traced circuit through the operating windings of relays |24 and |25, relay |24 operates, but relay |25 does not. It will now be assumed that the decoder 300 is again seized due to the initiatio-n of another call to the same distant oflice and that since the reroute relay 308 is still unoperated, there being no operating circuit therefor over trunk group conductor |25, the decoder in again marking the same code point 3|? reoperates the relay 3|8 and the direct route relay 305. As before, the operation of route relay 305 causes the transmission of translated routing information to the sender whereupon the sender proceeds to extend a connection from the calling line to the desired office over the one remaining idle trunk H5 of the trunk group.

Relay 3|8 upon operating establishes a circuit from battery over its contacts, conductor |21, Winding of relay |28, back contact of relay |25 to ground at the front contact of relay |24, relay |24 having operated as an indication that there is but one available idle trunk in the group. Relay |28 upon operating locks directly toi battery independent of the control of relay 3|8 and connects ground to conductor |25. The operation cf relay |28 therefore makes a count of the fact that there is but one remaining idle trunk in the associated trunk group. Until the decoder has completed its functions and been dismissed by the associated sender, the cut-off

relays including relays

303 and 3|0 will remain operated and therefore the connection of ground to ccnductor |26 will not operate the reroute relay 308, thereby preventing the mutilation of the routing information at the time being established by the decoder.

When the last idle trunk in the group, for example, the trunk! i5 is seized, its sleeve relay |9 shunts the last unshunted resistance from the previously traced circuit through the windings of relays |24 and |25 and relay |25 now operates. With both relays |24 and |25 operated, the locking circuit of relay |28 is opened and ground is connected to conductor |26 independently of relay |28 over the front contacts of relays |24 and |25. In the meantime, the decoder having completed its functions has been dismissed thereby releasing relays 3|5 and 3|8. With relay 3| 8 released relay |28 now releases. Relay 3|5 upon releasing releases the operated cut-off relays such as 309 and 3|0 and, with ground connected to conductor |26 over the front contacts of relays |24 and |25, the reroute relay 308 is operated to transfer the code point 3|] from the direct route relay 305 to the tandem route relay 306. A similar transfer is effected by the corresponding reroute relay in each of the other decoders.

If, now, any of the decoders is seized on another call, as for example decoder 300, and the start relay 3|5 thereof is operated, relays 3| 4 and 3|6 will be operated, relay 3|6 establishing locking circuits for all operated reroute relays of the decoder. Reroute relay 308 will, therefore, be locked operated and, assuming that the information transferred to the decoder from the associated sender signifies that the connection desired is for a line terminating in the same distant oflice, the decoder will, because of the operated reroute relay 358, cause the .ope-ration of the tandem route relay 305 and the transmission of routing information to the sender whereby the sender will cause the setting of the district selector and an office selector to select and idle trunk extending to the tandem oflice |08. The sender will then transmit the necessary routing information to the sender of the tandem office whereby the connection may be further extended to the distant office. The connection is then completed in the distant oflice under the control of the tandem sender, for example over the incoming selector 09 and the final selector I3.

Should more than one .of the decoders be handling a call for the same direct trunk group, for example, the trunk group disclosed in Fig. l,

at the time relay |24 thereof is operated, then as l of the reroute relay of all decoders corresponding L to relay 300 to inform the decoders that any subsequent call to that trunk group should be rerouted. 'Ihese reroute relays will then remain operated under the control of relays |24 and |25 of the trunk group and decoder start relays :s

as long as all the trunks of the group are busy.

If one of the trunks becomes idle', relay |25 will release in turn releasing the reroute relay, such as relay 308, of all busy decoders as soon as those decoders become idle. The circuits are now ready to count another last call for the trunk group. If more than one trunk becomes idle relays |24 and |25 will both release and the reroute relays of the decoders will release as previously described andthe circuits conditioned to count when all but two of the trunks of the trunk group become busy.

From the foregoing it will be apparent that the decoder and any associated sender are given information when there is but one available trunk in a direct trunk group and again when there is no idle trunk, suiciently in advance of the establishment of a possible call to the trunk group, so that rerouting to a tandem route may be effected without the hazard of losing a call.

This insures satisfactory operation of the rerouting feature of decoders when direct trunk groups do not have more than ten trunks and the proportion of the traiiic to be rerouted through a tandem oi'ce will not exceed 20 per cent Where direct trunk groups have more than ten trunks and the proportion of the traffic to be rerouted through a tandem office exceeds 2O per cent, a more satisfactory operation of the rerouting feature of decoders may be made by employing the modified counting circuit disclosed in Fig. 2A, in lieu of the counting circuit of Fig. 2, conductors |33, |36 and |31 of Fig. 2 being joined to conductors |23, |23 and |21, respectively, of Fig. 1. Relays |34 and |40 are polarized and electrically biased by the circuits through their upper windings and so adjusted that relay |34 will operate when two resistances are included in the chain circuit over conductors |23 and |33 but will not operate or will release in series with three or more resistances and that relay |40 will operate when one resistance is included in the chain circuit but will not operate or will release in series with two or more resistanccs. Relay |35 is margined to operate only when all resistances have been removed 'from the chain circuit. It will thus be 'apparent that the operation of relay |34 is indicative that there are but two idle trunks in 'the group, lthat the operation of relay |40 is indicative that there is but one idle trunk and that the operation of relay |35 is indicative that 'all trunks of the group are busy. i When all but two of the trunks of the group become busy and relay |34 operates, an obvious circuit is establishedA for relay |4|. Should the decoder 300 thereafter 'receive information for routing a call to this trunk group and ground the code point 3|1, relay 3|8 of the decoder will operate and remain operated until the decoder is dismissed and will close the circuit from battery "at its contacts over conductors |21 and |31, upper back Contact of relay |42, inner upper Contact of relay v|4|, lowerv back contact of relay |42 to ground through the winding of relay |43. Relay |43 will operate and lock in a circuit from ground through its winding and contacts, the winding of relay |42, the upper front contact of relay |4|,

to battery, but relay |42 will not operate in this locking circuit since its winding is shunted by battery supplied by relay 3|8 of the decoder. As soon, however, as the decoder is dismissed after having informed the associated sender to route the call to the trunk group, relay 3|8 releases permitting relay |42 to operate. Relay |42 upon operating opens at its lower contact the initial operating circuit of relay |43 and at its upper front contactA prepares the operating circuit for relay When the last call which may be extended over Athe trunk group arrives in a decoder, for example the decoder 300, and the decoder again vgrounds the code point 3H and reoperates relay 318, `the circuit for relay |38 is completed over conductors |21 and |31 and relay |38 operates locking over its lower front contact, the lower contact of relay |4| to battery at the back contact of relay |35. Relay |30 upon operating thus counts the last call that may be extended to the trunk group and places ground on conductors |33 and |26 which is instrumental in operating reroute relay 308 of the decoder as. soon as the decoder is dismissed following its functioning to inform the associated 'sender'to'route a call to the last available trunk of the'trunk group'. Thereafter, any subsequent calls intended for this trunk group will be rerouted through a tandem office as previously described.Y

The arrival of the call to the trunk group at the time there were but two free trunks therein, causes the seizure of one of the remaining idle trunks and the consequent operation of relay |40 which, in turn, closes a shunt around the winding oi relay |43 extending from ground at the contact of relay |40, through resistance |44, front contact and winding or relay |43 to ground. Relay M3 now releases but relay |42 remains operated. This is done so that should another trunk become idle before a call directed to the group has seized the last previously indicated idle trunk and relay |40 consequently releases, relay |42 will have no holding circuit and will release and open the previously traced operating circuit for relay |38. Relay |38 will not therefore operate as previously described and the decoder will thus be informed to route two calls to the trunk group.

The arrival of the last call which the trunk group can handle, assuming that all but one of the trunks are at the time busy, will busy the one remaining idle trunk and thus cause the operation of marginal relay |35. Relay |35 upon operating releases relay |30 but maintains ground on conductors |36 and |26 to hold the reroute relay corresponding to relay 308 operated. If thereafter only one trunk becomes idle, relay |38 will release, removing ground from conductor |26 to release the decoder reroute relay such as relay 308 of decoder 330, and to prepare the locking circuit of relay |38 for counting another last call to the trunk group.

If two trunks l: ecome idle, both relays |40 and |35 will release, the release of relay |40 permitting relay |42 to release, thus preparing the circuit to count two possible vcalls for the trunk group. If more than two trunks become idle all of the relays |34, |35, |40, |4|, |42, |43 and |38 will be released and will remain released so long as more than two trunks of the group remain idle.

What is claimed is:

l. In a telephone exchange system, a calling oiiice, called oriices, an intermediate ofce, switching mechanism located in said calling oice, a group of direct paths extending from said switching mechanism to each of said called ofces, a group of indirect paths extending from said switching mechanism through said intermediate office to each called oflice, a register sender for recording the office code designations of wanted vlines and for. controlling said switching mechanism to select a path to the called oice in which a wanted line terminates, translating means associated with said sender for translating office code registrations to enable said sender to control said switching mechanism, reroute relays in said translating means corresponding to the groups of direct paths, each of said vreroute relays being operable to change the translation normally effective for routing a call to the corresponding group of direct paths to a translation effective for routing the call to said groupfof indirect paths, and means effective for operating the reroute relay corresponding to a particular group of direct paths when all but one of that group of paths are actually busy and the translating means has completed the direction of an initiated call to the last idle path in that group.

2. In a telephone exchange system, a calling oiilce, called ofces, an intermediate oce, switching mechanism located in said calling ofiice, a group of direct paths extending from said switchillg mechanism to each of said called oices, a

group of indirect paths extending from said switching mechanism through said intermediate ofce to each called oince, a register sender for recording the oflice code designation of wanted lines and for controlling said switching mechanism to select a path to the called oflice in which a wanted line terminates, a decoder associable with said sender having route relays corresponding to all groups f paths extending from said switching mechanism, operating circuits for said route relays controlled from said sender in accordance with the office code designation registered therein, reroute relays in said decoder corresponding to the groups of direct paths, each of said reroute relays being operable to transfer the circuit normally eective for operating the route relay corresponding to a group of direct paths to a route relay corresponding to said group of indirect paths, and means effective for operating the reroute relay corresponding to a particular group of direct paths when all but one of that group of paths are actually busy and the decoder has completed the direction of an initiated call to the last idle path in that group.

3. In a telephone exchange system, a calling oiice, a called oice, an intermediate omce, switching mechanism located in said calling office, a group of direct paths extending from said switching mechanism to said called oliice, a group of indirect paths extending from said switching mechanism through said intermediate oiiice to -said called oice, a register sender for normally controlling said switching mechanism in response to the dialing of the ofce code of said called oiiice to route a connection to said called oiice over an idle one of said direct paths, a chain circuit for said group of direct paths including as many resistances as there are paths in the group, a relay associated with each direct path and operable upon the seizure of that path by said switching mechanism to short-circuit one of said resistances, a relay in said circuit margined to operate only when all but one of the paths have been seized and all but one of said resistances have become short-circuited, and means associated with said sender controlled by said marginal relay for causing said sender to reroute all calls destined for completion to said called oiiice over said group of direct paths by way of said group of indirect paths only after said sender has completed the rout-ing of a call to the last idle one of said group of direct paths.

4. In a telephone exchange system, a calling oillce, a called oflice, an intermediate oilice, switching mechanism located in said calling ofce, a group of direct paths extending from said switching mechanism to said called olice, a group of indirect paths extending from said switching mechanism through said intermediate olce to said called cnice, a register sender for recording the oiice codes designation of a wanted line terminating in said called ofce and for controlling said switching mechanism to select an idle path to said called'oice, vtranslating means associated with said sender for translating said oflice code registration to enable said sender to control said switching mechanism, a reroute relay in said translating means allocated to said group of said paths, said reroute relay being operable to change the translation normally effective for routing a call to the group of direct paths to a translation eiective for routing the call to said group of indirect paths, a group relay operable when all butlone of said direct paths is busy, a code point relay in said translating vmeans operable whenever said translating meansis functioning to cause a call to be routed to said group of direct paths and an operating circuit for said reroute relay controlled jointly by said group relay and said code point relay and effective only following the complete functioning of said translating means to route a call to the last idle one of said group of direct paths.

5. In a telephone exchange system, a calling office, a called oflice, an intermediate office, switching mechanism located in said calling office, a group of direct paths extending from said switching mechanism to said called omce, a group of indirect paths extending from said switching mechanism through said intermediate oice to said called oflice, a register sender for recording the oice code designation of a wanted line terminating in said called oiiice and for controlling said switching mechanism to select an idle path to said called oi'lice, translating means associated with said sender for translating said office code registration to enable said sender to control said switching mechanism, a reroute relay in said translating means allocated to said group of direct paths, said reroute relay being operable to change the translation normally effective for routing a call to the group of direct paths to a translation eifective for routing the call to said group of indirect paths, a first group relay operable when all but one of said direct paths is busy, a second group relay operable when all of said direct paths are busy, a code point relay in said translating means operable whenever said translating means is functioning to cause a call to be routed to said group of direct paths, a third grouprelay operable under the joint control of said first group relay and said code point relay and maintained operated until said second group relay operates, an operating circuit for said reroute relay controlled by said third group relay and effective only following the complete functioning of said translating means to route a call to the last idle one of said group of direct paths, and a holding circuit for said reroute relay established by the operation of said second group relay.

6. In a telephone exchange system, a calling oflice, a called oiiice, a group of trunks extend- .ing between said olices, switching mechanism in said calling office for extending connections over said trunks to said called oflice, a chain circuit for said group of trunks including as many serially connected resistances as there are trunks in the group, a relay associated with each group and operable upon the seizure of said trunk to short-circuit one of said resistances, a first relay in said circuit margined to operate only when all but two of the trunks of said group have been seized, and all but two of said resistances have become sho-rt-circuited, and a second relay in said circuit margined to operate only when all but one of said trunks have been seized and all but one of said resistances have become short-circuited.

'7. In a telephone exchange system, a calling ofi-ice, a called oflice, an intermediate oflice, switching mechanism located in said calling ofce, a group of direct paths extending from said switching mechanism to said called oce, a group of indirect paths extending from said switching mechanism through said intermediate onice to said called oiice, a register sender for normally controlling said switching mechanism in response to the dialing of the office code of said called of- -ce to route a connection to said called ofce over an idle one of said direct paths, a chain circuit for said group of direct paths including as many resistances as there are paths in the group, a relay associated with each direct path and operable upon the seizure of that path by said switching mechanism to short-circuit one of said resistances, a first relay in said circuit margined to operate only when all but two of the paths have been seized and all but two of said resistances have become short-circuited, a second relay in said circuit margined to operate only when all but one of the paths have been seized and all but one of said resistances have become short-circuited, and means associated with said sender and controlled by the operation of said rst and second relays for causing said sender to reroute all calls destined for completion through said called cnice over said group of direct paths by way of said group of indirect paths only after said sender has completed the routing of a call to the last idle one of said groups of direct paths.

8. In a telephone exchange system, a calling oice, a called office, an intermediate oice, switching mechanism located in said calling office, a group of direct paths extending from said switching mechanism to said called oice, a group of indirect paths extending from said switching mechanism through said intermediate ofce to said called oice, a register sender for recording the oiiice code designation of a wanted line terminating in said called ofce and for controlling said switching mechanism to select an idle path to said called cnice, translating means associated with said sender for translating said oiiice code registration to enable said sender to control said switching mechanism, a reroute relay in said translating means allocated to said group of direct paths, said reroute relay being operable to change the translation normally effective for routing a call to the group of direct paths to a translation eiective for routing the call to said group of indirect paths, a rst group relay operable when all but two of said direct paths are busy, a code point relay in said translating means operable whenever said translating means is functioning to cause a call to be routed to said group of direct paths, a second relay operable under the control of said rst group relay and said code point relay as soon as said translating means has completely functioned to route a call to one of the two` remaining idle paths of said group of direct paths, a third relay operable jointly under the control of said second relay and said code point relay, and an operating circuit for said reroute relay controlled by said third relay effective only following the complete functioning of said translating means to route a call to the last idle one of said group of direct paths.

9. In a telephone exchange system, a calling cnice, a called office, switching mechanism located in said calling cnice, a group of preferred trunks accessible to said switching mechanism for extending connections from said calling oice to said called office, a group of alternate trunks accessible to said switching mechanism and to the switching mechanism of other calling oices for extending connections from said calling ofces tosaid called omce and to other called ofces, a register sender for controlling said switching mechanism, and means for adjusting the l selection control within said sender to cause said switching mechanism to route one call for said called olice to said preferred group of trunks when all but one of said preferred group of trunks are busy and to route succeeding calls for said called ofce to the alternate group of trunks as long as all but one of said preferred trunks are busy even though the last trunk has not been seized by switching mechanism and made busy by said one call.

SAMUEL B. WILLIAMS.