Stored program controlled PBX accommodating secondary rate tie trunks using primary rate interfaces

A private branch exchange comprising:

• a time-division switch having a plurality of output ports and a plurality of input ports;
• a controller for controlling the switch to establish time-divided outgoing connections to said output ports and time-divided incoming connections from said input ports according to a stored program;
• a first group of outgoing primary rate interfaces connected to said output ports for respectively receiving a plurality of time-slot sequences in which time slots occur at a primary rate and inserting a frame bit at periodic intervals to each of the received time-slot sequences to form a plurality of outgoing primary rate frames;
• a second group of outgoing primary rate interfaces for respectively receiving the outgoing primary rate frames and removing the frame bit from each of the outgoing primary rate frames to form a plurality of time-slot sequences;
• a time-division multiplexer for multiplexing the time-slot sequences from the second group outgoing primary rate interfaces into a time-slot sequence in which time slots occur at a secondary rate higher than the primary rate;
• an outgoing secondary rate interface for inserting framing bits to the time-slot sequence from the multiplexer to form an outgoing secondary rate frame and transmitting the outgoing secondary rate frame via an outgoing tie trunk to a distant private branch exchange;
• an incoming secondary rate interface for receiving an incoming secondary rate frame from the distant private branch exchange via an incoming tie trunk and removing framing bits from the received secondary rate frame to produce a time-slot sequence;
• a time-division demultiplexer for demultiplexing the time-slot sequence from the incoming secondary rate interface into a plurality of time-slot sequences in which time slots occur at the primary rate;
• a first group of incoming primary rate interfaces for respectively receiving the time-slot sequences from the demultiplexer and inserting a frame bit at periodic intervals to each of the received time-slot sequences to form a plurality of incoming primary rate frames;
• and
• a second group of incoming primary rate interfaces for respectively receiving the incoming primary rate frames and removing the frame bit from each of the incoming primary rate frames to form a plurality of time-slot sequences and supplying the time-slot sequences to the input ports of said time-division switch.