Common medium multichannel exchange system

Dec. 22, 1936.

N. MONK

2,064,938

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ATTORNEY

INVENTOR

Patented Dec. 22, 1936

2,064,938

UNITED STATES PATENT OFFICE

2,064,938 COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM

Newton Monk, New York, N. Y,, assignor to American Telephone and Telegraph Company, a corporation of New York

Application May 26, 1934, Serial No. 727,799

Claims. (Cl. 250—6)

This invention has for its object the provision of an exchange system in which communication between subscribers is carried out over channels of different frequency derived from a common 5 transmission medium. More particularly, the invention is designed to utilize as a substitute for the many subscriber pairs which are employed in the ordinary telephone exchange system the many channels which are obtainable in the high 10 frequency art as it is now unfolding and to provide means whereby these channels may be employed by subscribers for interconnection.

In the telephone exchange systems which have heretofore been provided it has been the practice io to group subscribers in central office areas. Each subscriber is connected to· his own central office by means of an individual wire circuit or by a party line arranged for non-simultaneous use by a few subscribers. When a subscriber desires to 20 call another subscriber he utilizes his own line to the central office, and upon passing the desired number either to an operator or to an automatic switching mechanism is connected to another line which extends either directly or via another office 25 or offices to the called party. For a system of this kind many thousands of subscriber circuits are necessary in each central office area. These are commonly provided in the form of a network of multiple pair cables extending over the area.

In accordance with the present invention, it is proposed to substitute for such individual wire circuits extending to each subscriber a transmission medium which is common to all subscribers and which will accommodate a range of 35 frequencies sufficiently wide for a large number of telephone channels. Interconnection between subscribers is then carried out over these high frequency channels.

The essence of the invention resides in making 40 available to a large number of subscribers a transmission medium capable of accommodating a wide range of frequencies so that the subscribers may utilize, for the purposes of intercommunication, a plurality of signaling channels de45 rived from the wide band medium. A transmission medium capable of handling the large number of channels required for the exchange system of the invention may be obtained by utilizing either radio transmission in space or high 50 frequency transmission over a conducting or guiding medium.

Considering, first, the use of radio as the medium for transmitting the wide band of frequencies contemplated by the invention, the inven55 tion is designed to employ whatever part of the frequency range may be available. Heretofore, however, the radio frequency spectrum has scarcely afforded sufficient frequency space to yield the channel capacity required for a local telephone exchange system. With the opening 5 up of the field of ultra-short waves this limitation as to channel capacity is removed. Thus, for example, between one and two meters there is a total frequency range of 150 megacycles while the range between 10 and 20 centimeters is 10 1500 megacycles. The frequency ranges obtainable through the use of still shorter waves are much greater.

With these ultra-high frequencies, however, nature imposed another limitation, i. e., the 15 tendency of the waves to take on quasi-optical characteristics, so that they project only in straight lines and do not bend around corners to any considerable extent. Thus, it becomes necessary that subscribers utilizing such waves 20 have a fairly clear line of sight between their antennas. Some diffraction around obstacles may, of course, be obtained, but, in general, reasonably clear line-of-sight projection without intervening obstacles appears to be called for. 25 This, of course, has the advantage of making it possible to utilize the same ultra-high frequencies in different areas.

The line-of-sight characteristics of the ultrashort waves must be recognized in the design of 30 a telephone exchange system utilizing this technique. In one form of the invention it is proposed to provide in the center of a community a central station with antennas mounted sufficiently high above the surrounding buildings to 35 make possible the necessary directness of transmission between these antennas and practically any point in the territory to be served. One subscriber’s station is then placed in connection with another by transmitting first to the central sta- 40 tion and thence through central station to the desired other subscriber located in another part of the community. The central station acts as a means for catching the waves from one subscriber at a relatively high point and transmit- 45 ting them down again to a second subscriber, thus lifting the transmission path above any obstacles lying between the subscribers’ antennas.

In order that such a central station may be economical, it is proposed to make it capable of 50 handling simultaneously a wide band of frequencies such as would accommodate hundreds or perhaps thousands of channels. The particular arrangement of the central station will be hereinafter described. 55

2,064,938

As an alternative to the use of a radio transmission medium, the invention contemplates the employment of a guiding or conducting transmission path suitable for the required band of 5 frequencies. Such a transmission path would be provided in the form of a network, extending to all the subscribers within a given area. Conceivably, such a network might be composed of ordinary wire circuits branched and intercon10 nected so as to make the common circuit available to each subscriber. Thus, the wire network might be somewhat similar to the network employed for distributing light and power currents, except that a simple two-wire network without 15 multi-phase connections, voltage transformation arrangements, etc., would suffice. It would, however, be possible to carry out the invention by employing the light and power network itself and this is contemplated within the scope 20 of the invention.

The preferred form of guiding transmission medium, however, is one which is designed for the transmission of a wide band of frequencies with comparatively low attenuation and prefer25 ably also with shielding to minimize the effect of external disturbances. Such a transmission medium may be found in a circuit consisting of two conductors disposed coaxially with respect to one another. In a properly designed coaxial circuit, 30 low attenuation at high frequencies may be attained by the employment of conductors of suitably low high frequency resistance and the use of a substantially gaseous dielectric between the conductors. With this type of circuit the outer ³⁵ conductor serves also as a shield whose protective effect becomes more nearly perfect as the frequency is increased, so that at high frequencies practically complete immunity from external disturbances may be obtained.

The invention contemplates also as an alternative to the coaxial circuit a high frequency transmission medium comprising a circuit consisting of two conductors surrounded by a shield. Here, again, the high frequency attenuation may be 45 minimized by proper design with substantially gaseous insulation and the thickness of shield may be determined so as to minimize interference from external disturbances.

Yet another form of transmission medium 50 suitable for carrying out the invention is a dielectric wave guide comprising a cylindriform dielectric material which may or may not be surrounded by a conductor. An advantageous form of such a transmission path is a hollow 55 cylindrical conductor containing air or some other gas as the dielectric. This type of circuit is particularly adapted to transmit waves of very high frequencies, for example, of the order of a few centimeters or less in wave length.

Using any of the above types of transmission media, the interconnection of subscribers at will from the available channels involves a number of new and difficult problems which are solved by the various features of the invention as here65 inafter disclosed.

In general, the invention contemplates the assignment to each subscriber of two particular frequencies which might preferably be related to his telephone number. The assignment of 70 frequencies should be such as to facilitate the interconnection of subscribers and one scheme of frequency allocation is suggested in the invention for this purpose.

The foregoing outline having indicated some 75 of the principles of the invention as well as a few of its major objects, the complete invention with its various details, features and purposes will now be understood from the following description when read in connection with the accompanying drawings, in which Fig. 1 shows a 5 schematic representation of the principal idea of the invention wherein a plurality of stations are connected to a common transmission medium, Fig. 2 shows in schematic form a system for connecting subscribers through a central office using 10 space radio transmission; Fig. 3 shows one possible allocation of frequencies for a multi-channel exchange system; Fig. 4 shows a subscriber station arrangement which may be used in the system of Fig. 2; Fig. 5 shows a modulating device 15 which may be used in various applications of the invention; Fig. 6 shows a central office arrangement which may be used in association with the subscriber station arrangement of Fig.

4; Fig. 7 shows an antenna arrangement for use 20 in a system of the type described; Fig. 8 is a schematic diagram indicating how connections between stations in different areas may be completed through central stations over trunk circuits; and Fig. 9 is a schematic diagram of a 25 guiding or conducting network which is available in common to a central station and a number of individual stations.

Referring to Fig. 1, there is illustrated one of the principal aspects of the invention. In this 30 figure is shown a common medium CM, to which are connected a plurality of stations Si, S2, S3, etc., each of which includes transmitting, receiving and associated apparatus. This apparatus, which will be hereinafter described, is arranged 35 to permit intercommunication between different pairs of stations over the common medium CM employing different frequency bands to permit simultaneous communication between different pairs of stations. Connection between any pair 40 of stations, for example, Si and S3, takes place through the common central station CS.

Fig. 2 shows in schematic form a telephone exchange system in which a number of subscribers may be interconnected through a central office, 45 the connection between each subscriber and the central office being accomplished by radio transmission through space. Each subscriber is provided with transmitting apparatus including an oscillator-modulator and receiving apparatus in- 50 eluding a demodulator, together with the necessary selecting circuits. At the central office a plurality of circuits such as the one shown are provided for interconnecting between subscribers. Each connecting circuit includes two; separate 55 sets of apparatus, each of which comprises a radio transmitter and receiver, and a connection, to which the operator has access, between the two sets of apparatus.

Transmission from subscriber A to subscriber 50 B is effected by means of the transmitter STi, the oscillator-modulator SMi, the tuned circuit or filter SFi, the antenna SAi, the antenna OAi, the tuned circuit or filter OFi, the demodulator ODi, the hybrid coil OHi, the connecting circuit 65 CC (across which is bridged the operator’s set OS), the hybrid coil OHa, the oscillator-modulator OM2, the tuned circuit or filter OFi, the antenna OA2, the antenna SA2, the tuned circuit or filter SFi, the demodulator SD2 and the re- 70 ceiver SR2. Transmission in the opposite direction is accomplished in an analogous manner, employing the transmitter ST2, the oscillatormodulator SM2, the selecting circuit SF3, the antenna SA2, the antenna OA2, the selecting circuit 75·

2,064,838

OFa, the demodulator OD2, the hybrid coil OH2, the connecting circuit CC, the hybrid coil OHi, the oscillator-modulator OMi, the selecting circuit OF2, the antenna OAi, the antenna SAi, the 5 selecting circuit SF2, the demodulator SDi and the receiver SRi.

It is proposed that different frequencies be employed for the oppositely directed transmissions in Fig. 2 and that the frequencies employed 10 between the calling subscriber and the central office be different from those used between the central office and the called subscriber. Transmission might be on either a double or a single sideband basis with the carrier frequency trans15 mitted.

In order that simultaneous conversations may take place between a number of pairs of subscribers, it is necessary that the frequencies employed in the system of Fig. 2 be properly select20 ed. One way of arranging the frequencies would be to assign to each subscriber a separate pair of frequencies, one for transmitting and one for receiving, and to provide apparatus at the central office for receiving and transmitting the various 25 required frequencies. Thus, subscriber A might be assigned a transmitting frequency /1 and a receiving frequency /2, and subscriber B transmitting and receiving frequencies /3 and /4, respectively. This would mean that in Fig. 2 the 30 modulators SMi and SM2 would operate with carrier frequencies fi and /3, respectively, and the modulators OMi and OM2 with carrier frequencies fa and fi, respectively.

With such a scheme it might prove desirable 35 in order to facilitate the allocation of frequencies and the interconnecting of subscribers, to have a constant difference between the transmitting and receiving frequencies of each subscriber or, in other words, to make ⁴⁰ ^-/2=/3-/4=0 (1)

The frequencies /1, /3, etc., might then be grouped in one part of the frequency spectrum and the frequencies /2, /4, etc., in another part. A dia45 gram of a frequency allocation of this sort, assuming a total of 99 subscribers, is given in Fig. 3. It will be noted that the order of the frequency assignments corresponds to that of the subscriber numbers. This scheme, while not essential to all 50 embodiments of the invention, is a very desirable feature.

Detailed arrangements of apparatus which might be used at the subscriber’s station and at the central office in a scheme of this sort are il55 lustrated in Figs. 4 and 6. While some of the apparatus shown in these figures is more particularly adapted to radio frequencies lying below the ultra-short wave range, it will be understood that such apparatus is shown merely to illustrate 60 the principles of the invention, and is not intended in any way to limit its scope.

The operation of the subscriber’s apparatus illustrated in Fig. 4 is as follows: When the subscriber desires to make a call, the receiver SRi 65 is removed from the switchhook SH. Through the contacts of the switchhook this closes a circuit which furnishes power supply to operate the oscillator and modulator OC and MO. The operation of the switchhook also provides D. C. 70 supply for the transmitter STi, this supply being derived through a rectifier RF which is connected to the A. C. supply. The speech currents produced in the transmitter, after passing through the transformer TRi, are modulated 75 upon the assigned frequency, here assumed to be /1. The oscillator and modulator may be separate units of any desired types, or a self-oscillating modulator might be used. The carrier and sidebands in the modulator output, after passing thru the selecting circuit SFi, are radiated by the 5 antenna SAi to the central office. When communication with the central office is established in a manner hereinafter to be described, the number of the called subscriber is passed orally to the operator and after the proper arrange- 10 ments are set up at the central office, communication is established with the called subscriber who is provided with apparatus identical with that of Fig. 4 except for the frequency assignments used. 15

In the case of an incoming call, the carrier frequency /2 and its associated sidebands are received by the antenna SAi, selected by the selecting circuit SF2 and demodulated by the demodulator SDi. This demodulator may be of 20 the vacuum tube type, in which case it-should be kept always in operating condition so that a call may be received. Preferably, however, the demodulator might comprise some passive element or elements, as, for example, copper-oxide units 25 as shown in Fig. 5. Associated with the demodulator is a relay RLi which is operated by the rectified current resulting from the incoming carrier frequency and which in turn operates the call bell SB. When the subscriber answers the 30 call, the switchhook SH is operated, which, in addition to completing the contacts previously mentioned, operates the slow-release relay RL2, which breaks the circuit of the bell SB. The received voice frequency currents, after passing 35 through the transformer TR2, are applied to the receiver SRi and two-way communication is thus established. If desired, a low-pass filter BP (shown dotted in Fig. 4) may be included in the receiver circuit to suppress unwanted high fre- 40 quency components in the demodulator output. It will be understood that a similar filter may be provided in the arrangements hereinafter described. When the subscriber hangs up, the slow-release feature of RL2 prevents his bell from 45 ringing before the other party also hangs up.

Fig. 6 illustrates central office apparatus which may be used in conjunction with the subscriber’s apparatus shown in Fig. 4. The apparatus is so arranged that a call coming in at any frequency 50 is automatically picked up and brought to the operator, who completes the connection to the called subscriber. Referring to Fig. 6, the antenna OAo is designed to receive incoming carrier frequencies within a range corresponding to 55 all of the subscribers assigned to the central office or to a group of subscribers. The received frequencies are delivered to the tuned circuit OFo whose frequency of response is being continuously varied by the driving motor DMo. The 60 same motor also rotates the switch SWo which is provided with a number of contacts, one for each subscriber whose incoming frequency may be selected by the selecting circuit OFo. The switch is arranged so that the contact correspond- 65 ing to any given subscriber is made at the same moment that the frequency of that subscriber is picked out by the selecting circuit.

The carrier frequency of an incoming call is selected momentarily by OFo and is rectified in 70 the rectifier ODo. The resultant current operates the relay ORo, which closes a circuit through the winding of the slow release relay ORi, the back contact of the relay OR2 and the switch SWo. The operation of the relay ORi lights the sub- 75

2,064,938 scriber’s line lamp OLi thereby calling the attention of the operator to the incoming call. Similarly, as the selection and switching are being varied, the line lamps corresponding to other in5 coming calls will be lighted.

When the operator observes the lamp OLi she inserts a plug, for example, OPi, which is associated with one end of a cord circuit, in the subscriber’s jack OJi. This action completes a cir10 cuit through the relay OR2, whose operation deenergizes the relay ORi and thus extinguishes the lamp OLi. After insertion of the plug in the jack OJi, the operator adjusts manually by means of the control CTi, the response frequency of the 15 variable selecting circuit OPi to the transmitting frequency of the calling subscriber, which at the same time brings the oscillator OOi and the selecting circuit OF2 to the receiving frequency assigned to the calling subscriber. The incom20 ing carrier frequency of the calling subscriber is rectified in the demodulator ODi and operates the relay OR3, thereby lighting the supervisory lamp OL2 during the time when; the calling subscriber’s receiver is off the hook.

Having ascertained from the calling subscriber the number which is desired, the operator completes the connection by inserting the other cord circuit plug OP2 in the jack OJ2 which corresponds to the number of the called party and at 30 the same time adjusts by means of the manual control CT2 the frequencies of the oscillator OO2 and the selecting circuits OF3 and OFj to the frequency assignments of the called subscriber. The connection established by the jack OJ2 en35. ergizes the relay ORe, thereby preventing the line lamp of the called subscriber from lighting when he answers the call. The supervisory lamp OL3 is lighted when the called subscriber answers, and remains lighted until he hangs up. The con40 versation then proceeds with the apparatus functioning in a manner similar to that described in connection with Fig. 2. When the conversation is completed the lamps OL2 and OL3 are extinguished as the subscribers hang up and the op45 erator takes down the connection. By means of keys such as OK2 the operator may connect her set to different cord circuits and using the key OKi she may split the connection and talk to either subscriber without the other.

CO As noted, the selecting circuit OFo and the switch SWo might serve for all the subscribers assigned to the office, or these subscribers might be divided into groups, with one selecting circuit and switch taking care of each group, and all such 55 circuits and switches driven by a common motor.

It is apparent that the arrangement of Fig. 6 permits the employment of a relatively simple arrangement at the subscriber’s station, inasmuch as the frequencies used by the subscriber are al00 ways the same. The cord circuit arrangement at the central office is somewhat complicated, but the number of cord circuits required will be only a small fraction of the total number of subscribers.

⁶® In the various arrangements shown in Fig. 2 to Fig. 6, radio transmission between the subscribers and the central office may be accomplished directly using any suitable type of antenna. However, in order to permit the use of ultra short waves for which substantially lineof-sight projection may be desired, it may be necessary to carry the transmission path above any obstacles lying between the antennas of individual subscribers. For this purpose an ar rangement of the type shown in Fig. 7 might be employed.

Referring to Fig. 7, it will be seen that in this case a single central office antenna OA serves for receiving all frequencies from subscribers and 5 transmitting all frequencies to subscribers. This antenna may be mounted on a tall building or tower so that substantially a line-of-sight transmission path is obtained between the central office and each subscriber’s antenna within the 10 central office area. The subscriber’s antenna should preferably be designed for radiating a beam toward the central office antenna and for receiving radiation from that antenna. Thus the subscriber’s antenna might be a balanced doublet 15 SA as shown in Fig. 7, placed near the focus of a parabolic reflecting surface, PR, directed toward the central office antenna OA. The central office antenna, however, is designed for radiating to and receiving from all subscribers. For this 20 purpose there might be employed an antenna capable of radiating in all directions and receiving radiations from all directions throughout the range of frequencies utilized. Such an antenna, for example, might be a balanced doubtlet OA as 25 shown in Fig. 7. Another possibility would be to employ a central office antenna, so designed that it is capable of radiating into and receiving from a conical space which comprehends all subscribers’ locations. This might be accomplished by 30 adding the reflector RE above the doublet antenna OA.

It is also contemplated that instead of employing a single central office antenna a multiple antenna arrangement might be used at the cen- ³⁵tral office building, each antenna being designed for handling a part of the total range of frequencies. It would also be possible to employ two antennas for each subscriber instead of one.

Although the arrangements described in con- 40 nection with Figs. 2, 4 and 6 are designed to provide a radio connection between the subscribers within a given area, it is proposed to interconnect such stations with subscriber stations in other areas by providing in each area a central 45 office, so that a radio connection may be established between a subscriber and his central office, using a frequency or frequencies assigned for that purpose, and the connection completed over suitable interoffice trunks. The method is illus- 50 trated schematically in Fig. 8. The central office apparatus to be used in carrying out this method might be patterned after that shown in Fig. 6. The types of trunks which may be used to interconnect the central offices are well known 55 in the art.

In the arrangements thus far described, with the exception of Fig. 1, it has been assumed that free space is employed as the common transmitting medium. It is contemplated, however, that GO transmission over a high frequency conducting or guiding path may be employed instead of radio transmission. Such a path would take the form of a network extending to all subscribers within a given area and to the central office for that ⁶⁵area. A network of this kind is shown schematically in Fig, 9, where each line represents a complete circuit, C designates the central office and S a subscriber’s station.

Probably the simplest form of such a network would be one comprisedof ordinarytwo-conductor circuits suitably interconnected and branched so that the common circuit will be available at all desired points. The circuits comprising the net- 75

2,064,938

Work might, for example, consist of pairs of open-wire or cable conductors.

It is contemplated also in accordance with the invention that the wire network employed for dis5 tributing light and power currents might be used as the common medium from which to derive high frequency channels for the telephone exchange system. The high frequency channels would be superposed upon the power network by 10 carrier methods which are well known in the art.

Another form of transmission medium, and one which is peculiarly advantageous in that it is capable of transmitting a wide band of frequencies with comparatively low attenuation 15 and which may be so shielded as to be practically immune to external disturbances, may be found in a network of circuits consisting of two conductors disposed coaxially with respect to one another. A circuit of this type has been 20 disclosed in the patents to L. Espenschied and H A. Affel, No. 1,835,031, December 8, 1931, H. A.' Affel and E. I. Green, No. 1,781,092, November 11, 1930, etc.

As an alternative to the coaxial circuit, the 25 high frequency transmission medium, might comprise a network of circuits, each consisting of two parallel conductors surrounded by a shield. Circuits of this type are disclosed in the patents to Green, Curtis and Mead, No. 2,034,032, Green 30 and Curtis, No. 2,034,033 and Green and Liebe, No. 2,034,034, all granted March 17, 1936.

The invention contemplates also utilizing as the high frequency transmission medium a dielectric wave guide comprising a cylindriform di35 electric material which may or may not be surrounded by a conductor. Such dielectric guides are disclosed in the patent applications of G. C. Southworth, Serial No. 661,154, filed March 16, 1933 and Serial No. 701,711, filed December 9, 40 1933.

It is contemplated in accordance with the invention that the subscriber set and central office arrangements which have been described may be employed with any of the above types of 45 transmission paths. For this purpose, it is necessary merely to substitute the transmission network for the radio path, replacing the connection of the apparatus to the radio antenna by a connection suitable to the type of medium 50 employed.

It will be obvious from the preceding discussion that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without 55 departing from the spirit of the invention as defined in the following claims.