Patent GB-1147414-A (GB)
Title: Path hunting circuit in a telephone network with a centralised control unit
IPC: H04Q3/545
Filed: 1967-07-21
Abstract: 1,147,414. Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP. 21 July, 1967 [26 July, 1966], No. 33573/67. Heading H4K. The central processor of an exchange in an inter-exchange network works out available routes between itself and other exchanges in accordance with orders of choice relating to the directness of the route, the degree of congestion existing in the inter-exchange trunk groups throughout the network, and the priority grading of those trunk groups. Path routings are classed as Rh1 if direct over a trunk group; Rh2 if routed over one tandem exchange; or Rh3 if routed over two tandem exchanges. More elaborate routings are mentioned but the network specifically considered comprises only six exchanges so that a route can employ two tandem exchanges at most. If a connection is required to another exchange network the routings are further classed according to a preference indicated by a translator employed to indicate appropriate own-network exchanges giving access to the wanted other network. The degree of congestion is graded as Qa for trunk groups lightly loaded; Qb for trunk groups moderately loaded; Qc for trunk groups heavily loaded; and Qx for trunk groups that are blocked. Trunk groups may be dedicated to priority use permanently or be so dedicated for busy hour working. The central processor, which may be of the type employed in the Bell System Electronic Switching Systems, has a " junction path store " containing the identity of all junctors and associated trunk groups in use in the network. When the store is updated for junctors taken into and out of use a trunk supervision store having a compartment for each trunk group to record the number of trunks in use and an indication of the congestion level implied by that number, is updated by adding or subtracting one from the number. If the addition or subtraction takes the group from one congestion level to another, the congestion code Qa, Qb, Qc or Qx, is updated accordingly. The congestion levels may be overlapping in that to pass from one level to another requires a higher score when the number of busy trunks is increasing than when the number is decreasing. Such overlapping decreases the number of transitions from one level of congestion to another in fluctuating conditions. Updating the trunk supervision store takes precedence over periodic reading out of the store to update the route search equipment which has a separate store for the codes and also records the priority tags attaching to the trunk groups. Path hunting is pursued in processing cycles of five time slots each and in accordance with a programme having eighteen available phases. Phase P0 is a standby state. Phase P1 governs common functions relating to the compilation of tables of trunk groups of specified grades of congestion. The first such table compiled in phases P2, P4 or P5 confines path hunting to lightly loaded trunk groups Qa; the second table compiled in phases P4 or P5 adds the moderately loaded trunk groups Qb; while the third table compiled in phase P3 adds the heavily loaded groups Qc. Phases P6 to P11  are employed to investigate the routings Rh1, Rh2, Rh3 available with a specified table of trunk groups. Phase P12 is devoted to sending the search result to the processor. Phase P13  is employed to recycle a route search when one trunk table has proved abortive and fresh search is to be made with a bigger table. Phase 14 provides access to the outgoing route translator in obtaining the identity of outgoing exchanges for out-of-network calls. Further phases Pol, Po2 and Po3 are selected for updating the trunk group occupation state data and for inscribing priority codes from keysets at a manual position in the route search circuits. The logical functions governing progression from one phase to any other are set out in a table and are illustrated in Figs. 5 and 6 not shown. For the purpose of compiling trunk group tables the trunk group congestion codes are recorded as binary digits Q1, Q2; Qa being 1,1; Qb being 1,0; Qc being 0,1; Qx being 0,0. The route search circuit trunk store comprises for each digit Q1, Q2, and for each of two further digits L 1, L2 indicating permanent or busy hour priority, a row of bi-stables each one of which relates to a particular trunk group. To obtain a table of trunk groups having congestion codes Qa and not tagged for priority working, the row Q1 of the trunk store is read out in Phase P1 to obtain a table including trunk groups regardless of priority and with congestion codes of both Qa and Qb. In the next cycle with phase P2 the row Q2 of the trunk store is read out to delete the Qb coded trunk groups from the table. In the next cycle in phase P4, and in the following cycle in phase P5, the priority digits are read out of the other two rows of the trunk store to delete priority trunk groups from the table. The residue left in the table represents the lightly loaded trunk groups (code Qa) available to a non-priority call. The table including moderately loaded trunk groups is available from read-out of the row Q1 alone with deletions effected from the priority digit rows. The table including heavily loaded trunk groups is available only to priority calls and is compiled by reading out row Q1 to give the list of trunk groups with codes Qa and Qb, and then reading out row Q2 to add the groups with codes Qc without deleting those with codes Qb. If a path without a priority tag is required by the processor, the programme is directed to the assembly of the table of non-priority lightly loaded trunk groups and paths of the Rh1 (most direct) type are investigated in so far as they may be included in the table. If no Rh1 paths are available the programme recycles and searches for a path of type Rh2 (one tandem exchange). If necessary the programme recycles again to search for a path of type Rh3 (two tandem exchanges). Should none of these searches produce an available route the programme instructs the compilation of the table including the moderately loaded trunk groups and proceeds to investigate routes of types Rh1, Rh2 and Rh3, as far as is necessary to produce a route or prove that one is not available and instruct the central processor accordingly. If the processor in asking for a route attaches a priority tag to the call the complete table of unblocked trunk groups is compiled and routes of type Rh1, Rh2, Rh3 are searched for sequentially as far as may be necessary. If a path is demanded to another interexchange network the code of the wanted exchange is passed through a translator which supplies the identities of own-network exchanges having trunk groups giving access to the other network in which the wanted exchange is to be found. The table of lightly loaded trunk groups, which includes other-network trunk groups, is set up to see if, out of the suitable outgoing network trunk groups, a lightly loaded group is available. If such a trunk group is available a search is made for a path to the corresponding outgoing ownexchange, the orders of preference Rh1, Rh2, Rh3 being taken sequentially as far as necessary as for an intra-network connection. Failing the availability or accessibility of a lightly loaded trunk group the route search table is augmented with moderately loaded trunk groups and, after further search cycles and if necessary, the table may be expanded to include the heavily loaded trunk groups as well. Having obtained access to an exchange in the other network the processor sends routing data to a processor in the first exchange reached in the other network. Should the other network not prove able to complete the call to the wanted exchange a further attempt may be made by approaching the other network from a different direction. The process of finding an out-of-own-exchange connection can be varied by taking one table of trunk groups and running through as many of the alternate routes offered by the translator as may be available and as is necessary; further tables being set up to repeat the process if required.