Patent GB-1185797-A (GB)
Title: Free Path Finding Apparatus
Inventor: MILEWSKI ANDRZEJ TADEUSZ
IPC: H04Q3/00
Filed: 1968-11-06
Abstract: 1,185,797. Automatic exchange systems. INTERNATIONAL BUSINESS MACHINES CORP. 6 Nov., 1968 [20 Nov., 1967], No. 52476/68. Addition to 1,175,846. Heading H4K. The memory map type route finder of the highly symmetrical switching network disclosed in the parent Specification 1,175,846 is modified to allow rearrangement of existing connections so as to remove a blocking condition. The rearrangement is in practice limited to one existing connection at a time, i.e. a rearrangement is not allowable if it can only be achieved as a result of rearranging another coexisting connection. It is assumed that a connection between lines KIJ and K<SP>1</SP>I<SP>1</SP>J<SP>1</SP> is required (K = input level to matrix IJ of the first stage). As disclosed in the parent, the condition of links Aq and Aq + 1 are compared in pairs until a free pair are found (phase 1) whereafter links Bs are similarly compared and a pair selected (phase 2) and then the condition of a so chosen unidirectional junctor AqBs is determined (phase 3), the search reverting to a previous phase if the juctor is busy or a free link pair Bs cannot be found. If a free link pair Aq, Aq + 1 cannot be found it is assumed that the system is fully blocked and consequently busy tone is reverted. If phases 2 or 3 are unsuccessful although phase 1 has proved successful at least once then the phase 1 search is restarted and the addresses of a free link pair Aq, Aq + 1 are noted. During phase 2, the addresses of a free link pair Bs or of a link pair Bs having one link busy and one free are noted (if both links of all pairs Bs are busy, the system reverts to phase 1). In phase 3, the address of a busy junctor (corresponding to a free pair Bs) or of a free junctor (corresponding to a partly free pair Bs) is noted. Again phase 2 may have to be restarted if neither of the just-mentioned alternatives is attainable and busy tone will eventually be reverted unless the necessary condition of there being only one link or junctor busy is achieved. However if the condition is fulfilled then the identity of the connection already using the bush link or junctor is determined and a path finding operation (without the possibility of rearrangement) is initiated for that connection, the proposed path Aq, Aq + 1; Bs; AqBs for the new call being marked busy during this search. If the operation is successful, the existing call is reconnected through the newly found path, the old path for the call is broken down and the thus released link Bs or junctor AqBs is incorporated in the path for the new call. If the operation is unsuccessful, the quest for a path having only one busy link or junctor is recommenced at the point where it left off. The above procedures are then repeated until a rearrangement is accomplished or, all avenues having been explored to no avail, the search is called off and busy tone is reverted. The logic necessary for performing a rearrangement is described in some detail with reference to Figs. 4-6 (not shown). The switching network may be located in a PBX or a concentrator.