an apparatus for transmission between subscriber terminals and digital switches that employ a 4 : 1 time - division - multiplexing scheme includes a main path for carrying signals between the subscriber terminals and digital switches , a control path which is separate from said main path and carries control information for maintenance purpose , and an interface -& amp ;- control unit which converts the control information so as to conform to specifications of the said digital switches wherein the conversion of the control information is based on switch settings made to said interface -& amp ;- control unit .