This invention relates mainly to a digital subscriber set, a subscriber terminal for use in a time shared bidirectional digital communication network, such as a time shared two-wire digital communication network.
As described in, for example, an article contributed by Jan Meyer, Terje Roste, and Roald Torbergsen to IEEE Transactions on Communications, Vol. COM-27, No. 7 (July 1979), pages 1096-1103, under the title of "A Digital Subscriber Set," with reference to FIGS. 2, 3, 8, and 14 thereof in particular, a time shared two-wire digital communication network comprises a plurality of master terminals, a plurality of subscriber terminals, and a plurality of conventional two-wire communication or subscriber lines between the master and the subscriber terminals. The master terminal may be a line circuit (subscriber circuit) of a digital telecommunication exchange or a like circuit.
As will become clear as the description proceeds, a subscriber terminal according to this invention is usable in a more general time shared bidirectional digital communication network. The subscriber terminal may be connected to a master terminal through a more general communication channel. Merely for brevity of description, the network and the master terminal will be restricted in the following to a time shared two-wire digital communication network and to a line circuit in a central office of the network. The communication channel will be called a communication line.
Speech and/or data information to be exchanged between a pair of subscriber terminals and consequently between each subscriber terminal and a counterpart master terminal, is bidirectionally transmitted as digital signal bursts through an interconnecting communication line. The data information may be given by facsimile signals. Inasmuch as this invention relates mainly to a subscriber terminal, the signal bursts received thereby from and sent therefrom to the communication line will be referred to as digital "receive" signal bursts and digital "send" signal bursts.
In order to separate the two transmission types on the communication line by time division, the receive and the send signal bursts are alternately received from the communication line and sent thereto by a subscriber terminal at a predetermined repetition frequency, herein called a frame frequency. In other words, the communication line transmits successive receive or send signal bursts, one in each frame period. Each signal burst consists of a predetermined number of consecutive signal bits of a bit rate defined by clocks. The information is encoded into the signal bits and decoded therefrom at the subscriber terminal. Such operation of the subscriber terminal must be timed by the frame periods and the clocks. In other words, the operation must be synchronized with phases of the frame periods and the clocks, herein termed a frame phase and a bit phase.
On initiating a call from a subscriber terminal, a call originating signal is sent to a master terminal as at least one send signal burst. It has been the practice that the central office always delivers receive signal bursts to all subscriber terminals in the network in order to synchronize the call originating signal with the frame and the bit phases. This is objectionable in view of the power consumption at the central office, which usually remotely feeds the subscriber terminals in the network.
An improved subscriber terminal has therefore been proposed to reduce the power consumption. However, the improved subscriber terminal is bulky and heavy and must comprise a hook switch pair as will later be described with reference to one of of the accompanying drawing.
However much the subscriber terminal might be improved, the receive and the send signal bursts will still be out of frame and/or clock synchronism at the beginning of call origination. Even during communication, the signal bursts may go out of synchronism. For the best possible performance of a time shared two-wire digital communication network, such loss of synchronism must be corrected within the shortest possible interval of time.
Hook switch pairs have been used since very early stages of development of telephones (and are still called by the name of "hook" even in a telephone set where an actual hook is no longer used). Although the hook switch pair is highly reliable, it often causes trouble in the telephone network. The trouble occurs when a handset of the subscriber terminal is misplaced on the "hook." In a telephone set in which a microphone and a loudspeaker are substituted for a conventional handset, the hook switch pair is no longer indispensable.