Selection of branch lines of multipoint junction circuits

摘要

A multipoint junction unit at a hub office interconnects a main line and a plurality of branch lines, the main line extending to a control station and the branch lines extending to data station line loops, to downstream local offices and to a main line of a multipoint junction unit in a downstream hub office. Normally, the junction unit broadcasts downstream data to all branches and combines upstream data from the branches for propagation up the main line. When a branch line is to be tested, test equipment connected to the upstream main line sends an address sequence to select an upstream branch and, if required, a second sequence to select a downstream branch, whereupon unselected branches are blocked and the selected branches are unblocked to permit passage of upstream and downstream data. The selected branch may thereafter be removed from the network by a "blocking" sequence which blocks the selected branch and restores all unselected branches. The upstream junction unit does not block its selected branch in response to the blocking sequence when a second address sequence has been sent to select a downstream branch.