Patent GB-1176514-A (GB)
Title: Improvements in or relating to processes for the finding of free connection paths in telecommunications networks
IPC: H04Q3/00
Filed: 1967-03-28
Abstract: 1,176,514. Automatic exchange systems. SIEMENS A.G. 28 March, 1967 [29 March, 1966; 29 July, 1966], No. 14009/67. Heading H4K. In a multi-exchange system, the number of the called party is transmitted from the calling party's exchange simultaneously to all the adjacent exchanges and from these it is transmitted onwards to all their neighbouring exchanges until the called party's exchange is located, whereafter a path from the latter to the calling exchange is set up. The number of the calling party may be sent with that of the called party so that the backward connection can be set up using a similar technique. Altematively, or additionally, at each exchange on route, a binary data item, which identifies the route via which the called party's number reached this exchange, is added to the called party's number prior to its onward transmission so that the backward connection is setup over the route which was used for the forward connection. The system is applicable to mobile subscribers (see below). Interexchange transmission can be effected using TDM with pulse-phase pulse-amplitude or pulse-code modulation; frequency division multipler; or conventional V.F. junctions. Number sending is effected by multi-frequency V.F. signals. Exchange details.-Each exchange includes a memory in which the addresses of subscribers permanently connected to this exchange are stored. Route selection within an exchange is achieved with the aid of a " memory map," which also provides an indication of the free/ busy condition of the subscribers connected to this exchange. Setting-up a call.-Subscriber 567 (Fig. 1) at exchange A on initiating a call is connected to a register in which the number of the wanted party is recorded. The number is compared with the numbers in the subscriber address memory and if no comparison is obtained it is transmitted over a free junction to adjacent exchange C. A similar comparison takes place at C and if it is successful the junction to A is reseized (the junction having been released as soon as number sending terminated) and the free/busy state of the called party reverted. If the comparison is unsuccessful the number, together with the address of the junction(s) to A is transmitted to adjacent exchanges B, D and E, but not to A. Similar processes occur in each of these exchanges with the exception that the further transmission from B cannot be accepted by E which is inhibited against all direct number-reception from B. Similarly, B is inhibited from receiving numbers direct from E, the exchange C always being used as an intermediary for these two exchanges. If during number transmission an exchange receives identical data from two or more of its neighbours, the data from only one of them is accepted. A timing circuit is included in each exchange for preventing it from processing identical called party numbers twice within a predetermined time period which might happen due to the transmission of these numbers over routes of different length. Assuming now that the called party is at K and that the direct route D, H, K is free, K sets up the connection back to A by reverting the called party's number and the additional route information which was added at each exchange back over the previously used junction (not necessarily the same channel or line within the junction) to H which deletes the information that was previously added at H and extends the connection to D and so on. If the backward route cannot be set-up due to busy conditions now being encountered, K stores the data relating to the call and attempts to complete the connection later. However, if the calling party's number is available to K then an alternative route back to A will be selected in .the same manner that the forward route was selected. (It is not mentioned how the junctions are held in this case-it being remembered that junctions are normally released as soon as number transmission ceases.) The condition of the called party is reverted and the appropriate tones transmitted. If the called party is busy each backward, junction is dropped as soon as it has been used for transmitting this information. Release is under control of the originating exchange but the details are not described. The originating exchange includes a delay line which force releases the calling party-by reverting busy tone-should the called party not be found within a pre-set time. It is not specified whether the calling party is " re-rung " should he release during the period when K is attempting to set up a backward connection to him. Mobile system.-Each exchange transmits on a separate frequency band. In response to the arrival of a mobile called party's numbereither from an own-exchange subscriber or from an adjacent exchange as described above-the exchange transmits the number in a free channel of its frequency band. If there is no response within a pre-set time, the called number together with the incoming route information is sent on to the adjacent exchanger. If there is a response, the back connection to the calling party's exchange is set up. Also if the exchange recognises that the called mobile is busy-from its record of all calls currently existing at itthe backward connection is set-up so that the calling party may be apprised of the situation. The exchange's frequency band is constantly scanned to detect whether any mobile is in a calling condition. If it is, it is treated as though it were a fixed subscriber of this exchange. During a mobile's transition from one exchangearea to another, an alternative connection to the adjacent exchange is set up using similar techniques to those described above so that continuity of the call is maintained. If the adjacent exchange cannot be used because either all junctions to it are busy or all its frequency channels are in use, the call continues through the nearest possible exchange. Each mobile subscriber station includes a transmitter/receiver which automatically tunes itself to the frequency band of the nearest exchange. The band is scanned so as to detect whether the station is being called. Scanning is stopped during a call. Mobile/mobile calls can be made via an exchange(s).