Patent GB-1115520-A (GB)
Title: Apparatus for finding a free path through a switching network
Inventor: BASTIAN MICHEL
IPC: H04Q3/00
Filed: 1966-08-08
Abstract: 1,115,520. Automatic exchange systems. INTERNATIONAL BUSINESS MACHINES CORPORATION. 8 Aug., 1966 [30 Aug., 1965], No. 35362/66. Heading H4K. To connect a calling inlet to an accessible outlet across a network of the kind in which there is only one path between any given inlet and any given outlet, the identity IJKL of the calling inlet is set up in a register 3, Fig. 5, and the first identity ABCI in numerical order of an outlet which is put forward as a candidate for connection to the calling inlet is set up in a register 8; a store 7 taking the addresses abci of all outlets in numerical order and the identity of any inlet connected to such outlet being set up in a store 6 as obtained from a busy record in store 5; any interference found between the proposed assocition of the inlet/outlet pair IJKL/ABCI and the busy routes indicated by the pairs ijkl/abei resulting in advancing the identity ABC to the head of the class of remaining outlets suitable as candidates for connection. As shown in Fig. 1 the network has a first stage of eighteen 3 x 2 matrices divided into two groups of three subgroups of three switches each. The three inlets to each first stage switch is identified by coordinate l, the group is identified by co-ordinate i, the sub-group by co-ordinate j, and the switch in the sub-group by co-ordinate k. The outlet of each switch is identified by co-ordinate a and as can be seen from the trunking one outlet gives access to one group of 2 x 2 link matrices in a stage ST M  while the other outlet of each first stage switch gives access to the second group of link matrices in the stage ST M . These link matrices are identified by the co-ordinate a as regards their grouping, a co-ordinate b identifying the sub-groupings of three in each group of six, and a co-ordinate c identifying each link switch in each sub-group. The second stage of 3 x 2 switches and the third stage of 3 x 3 switches are grouped and trunked as shown. The manner in which the co-ordinates ijk of the switch serving a calling inlet, and abc of a link switch STM, define the unique path between them is shown in Fig. 3 where each switch is identified by three of the co-ordinates which coordinates also indicate the inlets and outlets employed in each such switch. The stage STM of so-called link switches is in fact the median stage of a seven-stage network, the stages to the left of ST M  being duplicated on its right and each of the 54 inlets having appearances on both the first and last of the seven stages. Returning to Fig. 5 the network is to be seen at 1 with the inlets identified by a scanner 2 when a calling condition is found, the scan being halted by a control device 4 which sets up the calling line identity in store 3. Since there are 24 outlets to the median stage ST M , the store 5 has 24 storage locations addressed by the corresponding outlet identities abci. Should an outlet be busy in the service of an inlet ijkl, the identity of that inlet is recorded in the corresponding storage location. To find an inlet ABCI of stage ST M  available to a calling inlet IJKL of stage ST 1  the identity IJKL is set up in store 3 and the stores 6, 7 and 8 are cleared. The co-ordinate I in store 8 is the same as that in store 3 and is not a variable in the search. The 24 values of the coordinates abci are run through in sequence in store 7 and address the store 5 to elicit ooordinates ijkl, if such exist, of corresponding connected inlets. A logic circuit 9, described more fully with reference to Fig. 6, (not shown), analyses the four sets of co-ordinates to find conditions of interference and advance the coordinates ABC accordingly. The logical functions of circuit 9 are as follows: (a) If a path is busy that originates from the same group of switches in stage 1 as the calling line, (i = I) and terminates on the same link switch as is written in store 8 as a candidate, (a, b and c are the same as A, B, and C) then that link switch is barred so that C is incremented to the next link matrix in the same subgroup. (b) If a path is found busy from the same group and subgroup of the first stage to a link switch in the same group and subgroup as the switch ABC offered for use, (i and j are the same as I and J and a and b are the same as A and B) then there is no available link between stages ST 2  and ST 3  giving access to the proposed sub-group of link switches and B must be incremented and at the same time C reset to identify the first link in the next subgroup. (c) If a path is found busy from the same switch of the same subgroup in the first stage (i, j, and k all equal to I, J and K), to the same group of link switches in stage ST M  (a the same as A), then that whole group of link switches is inaccessible and A must be incremented and B and C be reset to identify the first link in the first sub-group of the second group. (d) If during the search a is found to exceed A no further interference is possible so the search is halted and the identity standing in register 8 is seized for the connection. (e) If both groups of link switches are found busy (the condition i, j, k all equal to I, J. K and a equal to A for the second time) the search is halted and all trunks busy condition is signalled. (f) If a does not exceed A and all values of dbci have been gone through the search is completed and the identity standing in register 8 indicates a link switch inlet which can be seized for the connection. When a link inlet is seized the address of the calling line connected to it is written in store 5 in the position provided for that link inlet. The cross-points identified from the set of coordinates IJKL and ABC are operated in conventional manner to set up the connection.