Telephone system digital channel bank interface

A method of operating a telephone system for communicating an above-voiceband ASK function signal between a central office and a remote terminal, the remote terminal having a plurality of electronic business set telephone instruments connected thereto, each electronic business set telephone instrument including a key pad, a plurality of function keys and means responsive to actuation of each function key to generate a unique ASK function code including a sequence of pulses of a tone at a frequency significantly above voiceband frequency, the central office and remote terminal being interconnected by voiceband telephone lines and being separated by a predetermined distance, the central office including switching means for directing calls to a telephone instrument identified by a dialing sequence, the remote terminal and said central office each including coder-decoder means for, in one direction, receiving a composite signal including voice frequency analog voice signals and the ASK function codes from each said electronic business set and for producing a sequence of digital output words representative of the analog voice signals for delivery to the telephone lines, each digital word produced by the coder-decoder means including a predetermined number of bits, the method including

• determining when an ASK function code is present in a signal to be transmitted over the telephone lines,
• digitizing into a plurality of 8 bit words the signal to be transmitted,
• setting the least significant bit of each word at a first predetermined level when an ASK function code is present and a second predetermined level when no ASK function code is present,
• converting received digital words into analog signals for delivery to designated ones of said electronic business set telephone instruments,
• determining from the least significant bits of all of the received words an ASK function code originated at the transmitting location, and
• reconstructing the ASK function code at the receiving end whereby said ASK function code is transmitted from one instrument to another without increasing the bandwidth of said telephone lines.

An improved telephone system of the type having a remote terminal, a plurality of electronic business set telephone instruments connected to the remote terminal, each electronic business set telephone instrument including a key pad, a plurality of function keys and means responsive to actuation of each key pad key or function key to generate a unique ASK function code including a sequence of pulses of a tone at a frequency significantly above voiceband frequency, a central office including digital multiplex switch (DMS) means for responding to function codes generated by an electronic business set telephone instrument and for directing calls to a selected telephone instrument identified by a unique dialing sequence, and a plurality of telephone lines interconnecting the remote terminal and the central office terminal, the improvement comprising the combination of

• coder-decoder means in said central office including means for receiving 8 bit digital words from said telephone lines, for converting said received digital words into analog signals, for generating ASK signals when the least significant bits of said received words are at a first predetermined level and for coupling composite signals including said analog signals and said ASK signals to the DMS means in said central office so that said DMS means can respond thereto;
• said coder-decoder means further including means for receiving composite signals from said DMS means including voice frequency analog voice signals and said function codes and for producing a sequence of digital output words representative of said analog voice signals for delivery to said telephone lines,
• each said digital word produced by said coder-decoder means for transmission over said telephone lines including 8 bit groups in each of which the least significant bit is set at a first predetermined level when an ASK is present and a second predetermined level when no ASK is present for delivery on said telephone lines to a remote terminal and ultimately to a selected electronic business set telephone instrument, whereby said ASK function code is transmitted to or from said DMS means to or from an electronic business set telephone instrument without increasing the bandwidth of said telephone lines.