GB2072464A - Line supplied telephone interface - Google Patents

Abstract

A telephone subscriber's terminal equipment for use in a digital communication system is provided which is remotely supplied with power from the exchange via the line, at least for the conventional telephone portions of the terminal. To avoid power consumption peaks, the codec facility (35) and parts of the central unit of the terminal equipment are turned off until the handset is off hooked, and until the terminal equipment is synchronized to the exchange. This uses a switching device (233) which responds to the coincidence of a signal (SYN) from the local synchronized circuit (197) which indicates that synchronisation condition, and a signal from the switch hook (35). This device (233) when it responds switches on the power supply unit (35). <IMAGE>

Description

SPECIFICATION Line-supplied telephone interface The present invention relates to a subscriber terminal equipment for use in a digital communication system.

A communication system is known in which such a terminal equipment can be used, and in which different speech and data channels are transmitted over a two-wire line between the terminal equipment and the line circuit at the associated exchange using time-division multiplexing (TDM) (DE-OS 27 58 109). If the terminal equipment is to be remotely supplied from the exchange, and the line between the subscriber and the exchange is comparatively long, the transmitted electrical power may be inadequate to supply all circuits of the terminal equipment with the necessary energy.

An object of the invention is to provide a subscriber terminal equipment which can be remotely supplied from the exchange even if the subscriber and the exchange are separated by large distances.

According to the invention, there is provided a subscriber terminal equipment for a digital communication system, which equipment when in use is connected to the line circuit of an exchange via a subscriber line, includes a central unit controlling the transmission of digital intelligence signal over the line, a ringing device, a dialling device, a switchhook, a speech network, a conversion unit connected to the speech circuit and providing analog to digital conversion of the speech signals, and a switching device one input of which is presented with a first signal, indicating the condition of the switchhook, and the other input of which is presented with a second signal, indicating the synchronization of the terminal equipment, and in which when the handset is off the hook and in the presence of synchronization, said switching device provides an output signal which turns on the power supply for units of the terminal equipment.

Since the power needed for ringing is relatively high, the maximum power demand of the terminal equipment can be reduced considerably by disconnecting circuits not needed during the ringing. Since the ringing device is disconnected when the handset is taken off the hook, the available power can be used for sending the speech.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically the time-division multiplex channels formed in a system in which a terminal equipment embodying the invention can be used; Figure 2 shows the assignment of the bits transmitted over the line to the individual channels of Fig. 1; Figure 3 shows the structure of the TDM frame used in the system of Fig. 1; Figure 4 is a block diagram of a subscriber terminal equipment embodying the invention, and as used in the system of Fig. 1; Figure 5 shows the terminal equipment of Fig. 4 with the individual data and control lines; Figure 6 is a block diagram of the line circuit at the exchange; Figure 7 shows the line circuit of Fig. 6 with the individual data and control lines;; Figure 8 shows schematically additional facilities connectable to the terminal equipment for data transmission over the main channel and for data transmission over the additional channel; Figure 9 shows the central unit and the conversion unit of the terminal equipment of Figs. 4 and 5; Figure 10 shows the central unit of the line circuit of Figs. 6 and 7, and Figure ii shows another subscriber terminal equipment embodying the invention.

We now refer to Fig. 1, in which a subscriber terminal equipment 1 of a digital communication system contains a telephone set 2, in which the analog speech signals are digitized by a suitable method---e.g., by pulsecode modulation (PCM)-, and in which the digital signals are gated into a main channel HKA. The digital signalling and synchronizing information needed to set up and operate a communication link are gated into an additional channel ZKS for signalling and synchronization. The terminal equipment may also have a data terminal 3, which, as indicated by a switch 5, can be connected to the system, and whose digital signals are then transmitted over the main channel HKA in place of the speech signals, the signalling and synchronizing information being also entered into or read from the channel ZKS.

The terminal equipment may include an additional data terminal 7, which delivers digital data singals at a low bit rate. Its bit stream is gated into an additional data channel ZKD, while the signalling bits are entered into the channel ZKS. While speech and and data can be sent over channel HKA only separately, data transmission can take place over channel ZKD simultaneously and independently of the transmission over the main channel.

In the embodiment shown, the gross bit rate of the main channel HKA is 64 kbits/s, that of channel ZKS for signalling and syn chronizatiot is 8 kbits/s, and that of data channel ZKD is also 8 kbits/s.

The different bit streams are interleaved in time in a TDM circuit 9 and sent as an 80 kbit/s stream over a subscriber line 11 to a demultiplexe;{ 1 3 at the exchange, in which the bit stream is of the channels HKA, ZKS, and ZKD are separated. During data transmis sion in the opposite direction, i.e., from the exchange to the terminal equipment, the circuit 1 3 is used as a multiplexer, while the circuit 9 acts as a demultiplexer.

The line 11 and hence the equipment 1 is connected via a subscriber line circuit 1 5 to the associated exchange (not shown), and via the exchange to the remainder of the telephone network. From the equipment 1 the information and control signals are transmitted over the main channel HKA and the signalling and synchronizing channel ZKS to the line circuit 1 5. The latter is of a basic design 1 6 and may have with an additional service facility 1 7, which establishes the connection to a data network, such as the integrated data network IDN or data-packet switching network. The exchange of data between the terminal 7 and the facility 1 7 takes place over channel ZKD, signalling and synchronization being effected over channel ZKS.

The facility 1 7 can also establish connections to a local viewdata center, communications with which can use channel ZKD while a long-distance call is in progress on channel HKA. Unlike the conventional analog local network, a viewdata call does not block the telephone line. In addition, there is sufficient capacity for sending signalling information for further services to be implemented in future.

Fig. 2 shows the word structure of the digital signals sent over the line 11, i.e., the assignment of the bits to the channels. Bits 1 to 8 of each word are assigned to the main channel HKA, bit 9 is assigned to channel ZKS (signalling and synchronization), and bit number 10 is assigned to channel ZKD (data).

The word duration is 1 25,ups, and the word repetition rate is 8 kHz.

While the bit numbers within a word identify the channels, the frame structure shown in Fig. 3 resolves the bit sequence in the transmission channel into words. A frame contains 32 1 0-bit words, so the frame length is 320 bits.

The frame period is 4ms. At a word repetition rate of 8 kHz, this results in a frame repetition rate of 250 Hz. As can be seen from Fig. 3, bit 9 of words 1 to 8 of a frame is used for frame synchronization, while bit 9 of words 9 to 32 of a frame are used for signalling. Signalling information formed from these bits serves for network control, i.e., for the exchange of signals with the switching equipment and for the exchange of information with special-service facilities and additional services.

Whereas in a conventional analog local network, states are usually signalled, in the present system events, i.e., changes of state, are signalled. The signalling information is sent over channel ZKS, i.e., outside the voice band. Audible tones are the only exception. A signalling word is divided into three fields of 8 bits each (octets): a header, an information field, and a check field.

The terminal equipment 1 (Fig. 4) includes a telephone set 2 which has a speech network with a receiver 21 and a transmitter 23, a switchhook 25, a "dialling" device 27 with 1 6 push buttons, a 1 6-digit numeric display 29, and a ring-tone generator 31.

The speech network 20 (see Fig. 5) contains a handset (not shown) with a dynamic receiver 21 and dynamic transmitter 23, and a matching circuit 33, which represents the interface to a conversion unit 35. The circuit 33 maintains the transmission values needed by the telephone adminstration, such as reference equivalent, sidetone reduction, frequency response, etc., and the transfer level at the interface.

The switchhook 25 signals the subscriber status by turning the power supply on and off and thus initiates the synchronization phase in the digital line circuit.

The conversion unit 35 contains a codec (coder-decoder), a low-pass filter, and a buffer memory. The speech picked up by the microphone 23 is band-limited by the filter, digitized, and coded into 8-bit words in accordance with CCITT recommendation. The coded words are called up at the correct time by a central unit 37 at the subscriber's end and gated into the outgoing line of a serial data bus 39, hereinafter also referred to as "10-bit bus".

In the receive direction, the 8-bit words are gated out of the 10-bit channel and written into the buffer memory the conversion unit 35 under control of a clock from the unit 37.

There the 8-bit PCM words are available for continuous digital-to-analog (D/A) conversion.

The analog signal is then voice-band-limited and applied to the receiver 21. In the rest condition and in the ringing condition, unit 35 is disconnected from the power supply.

In addition to the above functions, when the handset is off-hooked, the switchhook 25 disconnects a call receiving circuit in the unit 37, and turns on the speech network 20 and the conversion unit 35; when the handset is replaced, the speech network 20 and the unit 35 are turned off.

The device 27 has, in addition to the usual push buttons 1 to 0 for entering the call numbers directly into the central unit 37, a button for charge indication, a button for time display, and four additional special function buttons.

The display 29 is a 1 6-digit display with a suitable drive circuit, which is driven with code signalling words evaluated in the central unit 37. The display may show, for example, the time, the charge, the "dialled" number or the number of the caller.

The equipment 1 includes a direction-separating unit 41 and a power supply unit 43 having a seizure detection circuit. When at rest, the circuits of the equipment are current less, i.e., the power supply is off. The power supply is turned on by the seizure detection circuit, a so-called bit-stream detector, which requires very little power in the rest condition.

It turns on the power supply in the event of a seizure, the turning on being initiated (a) by the bit stream sent by the exchange, or (b) by actuating the switchhook upon removal of the handset, or (c) by turning on an additional facility connected to the equipment.

The tasks of the central unit 37 are: synchronizing the terminal equipment 1, receiving and transmitting signalling information, controlling the unit 35 and particularly its codec, and controlling the additional facilities 3, 7, to which only the interface is shown in Fig. 4 (see also Fig. 8).

Details of the seizure detection and the power supply unit 43 are disclosed in German Patent Application P 29 40 617.5.

The unit 37 and the other circuits of the equipment, including the telephone set 2, are interconnected by data bus 39, control lines 47, on/off lines 49, and priority lines 51, (see Fig. 4 below).

The generator 31 operates on direct current. After the remotely controlled power-supply and seizure-detection unit 43 has been activated, the ring-tone generator 31 is turned on and off by a ring control signal sent over the channel ZKS and decoded in the central unit 37.

The equipment 1 is prepared for modular extension, which is achieved, inter alia, by designing the data channel 39 as a serial 10bit bus. At an interface I, formed by the data bus and by control and signalling lines, additional facilities 3, 7 and an add-on unit or "luxury" equipment 53 with additional features can be added. Examples of "luxury" features are an extended alphanumeric keyboard, an alphanumeric display, and a special keyboard (not shown).

The individual lines at the interfaces are apparent from Fig. 5, where the units are designated by the same references as in Fig.

4. Unlike the basic equipment, which is remotely supplied from the exchange, the facilities 3, 7 and the equipment 53 are powered locally from the 220-V line.

At the interface to the facilities 3, 7 and equipment 53, the following lines are provided: active AK(for signalling from the additional facilities), release FR (for signalling from the additional facilities), bit-rate clock TB, frame-rate clock TR, word-rate clock TW, incoming data DE, (binary) outgoing data DA (binary), disconnection of the codec CA, and power supply on/off, STZ.

At the interface II, for a balanced two-wire line, Fig. 5, the a- and b-wires of the line 11 are present. For a balanced four-wire line, as may be used if the subscriber and the exchange are far apart, the interface is formed by the c-, d-, e-, and f-wires of the line 11.

The interface III between the telephone set and the rest of the equipment 1 contains the following lines: analog input AE, analog output AA, power supply on/off (from the switchhook) STG, start/stop for tone ringing R, a four-wire line DTfor data input from the keyboard 27, a control line labelled "Strobe" from the keyboard 27, a four-wire data line DD, and a control line labelled "S.robe" to the numeric display 29.

The interface IV contains the following lines: bit-rate clock TB, system clock TS, word-rate clock TW, incoming binary-coded data DE, and outgoing binary-coded data DA.

The interface V contains the following lines: unbalanced connecting line g, on which the data are present in line code; power supply on/off STZ from the additional facilities, and from the central unit 37, power supply on/off STG, from the switch hook. For a four-wire line, this interface contains in place of the line gan unbalanced line ifor incoming data in line code, and an unbalanced line jfor outgoing data in line code.

The interface VI contains the following lines: codec on CE, read (PCM word) L, write (PCM word) S, bit-rate clock TB, converter clock (e.g., 1 28 kHz), TK, and word-rate clock TW.

The operation of the equipment 1 shown in Figs. 4 and 5 will now be explained. The operating current needed for the basic equipment is coupled out between the a-wire and the b-wire of the line by a coupling transformer in the power supply unit 43, a constant operating voltage being adjusted by a shunt regulator.

As mentioned, the unit 43 contains a seizure detector which detects a bit stream on the line 11. This detector is a receiver with a high-impedance input and a response threshold. It draws the necessary low closed-circuit current from the coupling device, which is still unloaded in this condition, the shunt regulator being not effective.

At rest, no current flows through the line 11 except for the low closed-circuit direct current for the seizure detector. The power supply for all other units is off. It can be turned on by the subscriber, facilities 3, 7, or the bit-stream detector (interface lines STG and STZ).

The coupling transformer of the unit 43 provides the necessary potential separation and signal level adaptation between the line 11 and the equipment, within which the transmission path is unbalanced (interface line g in Fig. 5).

The equipment 1 is protected against any over-voltages on the line 11 by a protective device in the unit 43. The power supply unit for a four-wire line differs from the unit 43 in that the current is coupled out of the c-, d-wires and the e-, f-wires via two coupling transformers, and that the bit-stream detector monitors the bit stream on the c- and d-wires.

The direction-separating unit 41 may be for a two-wire line, in which case it operates in time- or frequency-division multiplex or singlefrequency duplex mode, or for a four-wire line, in which case both directions of transmission are separated in space.

Line coding is also performed in the unit 41. The incoming bit-rate clock needed for line coding is derived by bit synchronization and is also available to the unit 37. The outgoing bit-rate clock is provided by the unit 37.

For a two-wire line, the directions of transmissions are separated as follows. At the interface V (Fig. 5), one must distinguish three types of direction-separating techniques: TDM single-frequency duplex, and two-frequency duplex. In the first type, the signals are in bursts; in the other two types, they are in continuous form.

The unit 41 is connected to the line 11 via the interface line g and the transformer in the unit 43. In addition to the actual separating device, it contains line coders and decoders as well as line drivers and receivers with the necessary pulse-shaping or equilizing devices.

The bit-rate clock needed in the equipment 1 is derived in the direction-separating unit 41 from the bit stream arriving over the line, and is made available to the unit 37 via the line TB.

Incoming data extracted from the line 11 are so conditioned in the unit 41 as to be available on the line DE in binary form and synchronously and in phase with the incoming bit-rate clock TB. The system clock is also applied from the unit 41 to the unit 37. Via the line TW, the out-going word-rate clock is made available continuously to the unit 37.

In the equipment 1 or in the additional facilities 3, 7, inserting outgoing data into the 10-bit channel is so controlled by the central unit that the data are available on the interface line DA synchronously and in phase with the bit-rate clock.

If communication takes place over a fourwire line, the unit 41 operates on a spacedivision multiplex basis. It differs from the corresponding unit for the two-wire line in that a continuous data stream is present at the interface at all times.

The codec and filters in the unit 35, which perform the analog-to-digital and digital-toanalog conversion of the speech signals, are connected to the transmitter and receiver of the speech network 20 on the analog side, and to the 1 0-bit channel on the digital side.

The unit 37 provides the clocks for codec and to write and read the 8-bit PCM words into and from the 10-bit data bus 39 (Fig. 4): the bit-rate clock TB, the converter clock TK, and the word-rate clock TW.

The unit 35 has control inputs CE ("converter on") and CA ("Converter off"). Turning it on and off is necessary to reduce power consumption when on-hook, so that sufficient ringing current is available, and during data transmission over the main channel.

In addition, write and read pulses are generated for the codec of the unit 35, which pulses cause the 8-bit PCM word to be, respectively, written into or read from the 10bit data stream at the correct time. The interface line L is activated during the first 8 bit periods of each 10-bit word on the interface line DE.

For outgoing data, the 8-bit PCM words are gated into the outgoing 10-bit data bus, i.e., the interface line DA, in the first 8 bit periods of each word. The write instruction from the unit 37 marks the positions of the 8-bit PCM words in the outgoing PCM channel. The interface line S is activated during the first 8 bit periods of each 10-bit word on the interface line DA.

On the interface line AE, which runs to the matching circuit 33 of the speech network 20, the incoming signal is available in analog form with a defined level. On the line AA, which comes from the device 33, the outgoing signal is available in analog form with a defined level. The digital-to-analog and analog-to-digital conversion in the unit 35 are performed according to the A-law of CCITT recommendation G 711. The transmission characteristics conform to CCITT recommendation G 712.

The central unit 37 contains a synchronizing device for deriving the frame-rate clock from the bit-rate clock from the unit 41. In addition, it includes a multiplex control for inserting information from the channel HKA and the channels ZKS, ZKD into the 1 0-bit data bus 39 and extracting it therefrom. It provides the necessary write and read clocks to the codec and includes devices for adapting the signalling between the units of the basic equipment-switchhook 25, dialling device 27, numeric display 29, and ring-tone generator 31-and the channel ZKS. The units 25, 27, 29, and 31 are connected to the central unit 37 via interface devices 61, 63, 65, and 67, respectively.

If the equipment 1 has additional facilities, the bit-rate clock, the word-rate clock, and the frame-rate clock are made available by the central unit 37 to the interface I for those facilities 3, 7 via the lines TB, TW, and TR, respectively. Two further signals, transferred over the interface lines FR and AK, control the priority between the telephone set 2 and the facilities 3, 7. In the outgoing direction, signalling information from the telephone set has priority over that from the additional facilities, but a transmission started for the additional facilities is completed in any case. The unit 37 controls all the time sequences between the data bus 39 (interface IV), the unit 35 (interface VI), the signalling devices of the telephone set 2 (interface Ill), and the facility 3 or 7 (interface 1).

In the unit 37, an 8-bit synchronizing word is selected from every ninth bit of the incoming data DE with the aid of the incoming bitrate clock TB. The synchronizing word is inserted into the outgoing data stream DA in the ninth bit of the first eight word periods of each frame period (cf. Fig. 3).

During the seizure phase, synchronisation is supported by gating a bit pattern into those bits of the outgoing data stream which are not used for the synchronizing word. The gate is interrupted as soon as the request to seize has been detected. During the synchronization time, the incoming data stream, too, has a pattern supporting synchronization, which is designed to speed up the synchronization.

Synchronization is considered achieved if the synchronizing word was detected twice in succession. If the synchronizing word was not detected twice in succession, the synchronization is re-initiated. After synchronization, the central unit 37 provides the bit-rate clock TB, the frame-rate clock TR, and the word-rate clock TW, which is derived from the framerate clock. The bit-rate clock is delivered in continuous form.

Bit, word, and frame periods begin at the same time. A frame period is 32 word periods and 320 bit periods long.

After synchronization has been achieved, a continuous check is made to determine whether the code for telephone communication is contained in the header of a frame period of the incoming data stream DE. If the request-to-seize signal is detected together with the header for telephone communication, first the gating of the synchronization-supporting bit pattern into the outgoing data stream is interrupted, and then the condition of the switchhook is continuously checked. Thus, starting from the rest condition, each change of the switchhook from "on-hook" to "offhook" is gated as the seizure signal into the outgoing data stream, and each change of the switch hook from "off-hook" to "on-hook" is gated into the outgoing data stream as the clear signal, in the information field of a frame period, with the header containing the code for telephone communication.The seventh bit of the header serves for informtion-field identification: "0" for control signal, "1" for a symbol from an internationally standardized alphabet (I.A. No. 5).

To control the code of the unit 35, the unit 37 generates a read pulse L, through which the codec is informed when an 8-bit PCM word is available on the data bus 39, or a write pulse S, through which the codec is informed when an 8-bit word is to be gated into the data bus 39.

To alert the subscriber to an incoming call, a memory at an output R to the ringing device is set if, with the header for telephone communication, the character for "connection of ringing tone" is detected in the information field of a frame period of the incoming data stream DE. If the character for "ring-tripping" is detected in the same manner, the memory will be reset.

For the display 29, if, with the header for telephone communication, the international standard character is detected in the information field of a frame period of the incoming data stream DE, the last four bits are transferred in parallel form over the control line DD to the display 29 with a read pulse (strobe).

"Dialling" is performed as follows. If the central unit 37 is selected by the push buttons of the "dialling" device 27 via the interface lines DT, the push-button codes received by the unit 37 are converted into corresponding binary-coded characters, which with the character for "telephone communication" in the header, are gated into the outgoing data stream in the 4th to 7th bits of the information field of a frame period. The first three bits of the field are assigned the bit sequence 110".

During signalling, parity checks with acknowledgement are performed. If more stringent requirements must be met, an error detecting code character can be gated into the check field of each frame period of the outgoing data stream, which character is checked and acknowledged at the receiving end.

At the interface I to the facility 3 or 7, the bit-rate, word-rate, and frame-rate clocks are made available (lines TB, TW, and TR), as mentioned. To ensure that signals are processed according to priority, line FR is deactivated as soon as a seizure or clear signal from the switchhook 25 or a "dial" signal from the device 27 is available for transmission over the data bus 39. The signal transmission is delayed until the interface line AK is activated by the additional facility, which ensures that the transmission of a started signal from the additional facility will be completed.

The interface line FR also informs the facility 3 or 7 of the condition of the terminal equipment. It can be activated only if synchronism exists and if a request-to-seize signal was received.

The subscriber line circuit 15, 16, shown in Figs. 6 and 7 is the subscriber-assigned link between his station, i.e. equipment 1, the line 11, and the other equipment of the exchange c.g. control, switching network, etc. (not shownt--and the additional service facility 1 7. The individual units, which are similar to the corresponding units of the terminal equipment 1, are designated by corresponding reference characters supplemented by a hundreds digit. These are essentially a central unit 137, a direction-separating unit 141. and a power supply unit 143. interconnected by a serial data bus 139. control lines 147, on/off lines 149, and priority lines 151 (see Fig. 6).

The basic design of the line circuit 1 5 is such that the circuit can be connected to the exchange and the additional facility 1 7 via a small number of connectors. This simplification is achieved, inter alia, by using the exchange clock as the reference or system clock, and of the serial 10-bit data bus.

The power supply unit 143 may be designed for a two-wire circuit (as shown), or for a four-wire circuit. It contains transformers for coupling the supply current into the line 11, the current being coupled into the a- and bwires in the former case, and into the c-, d-, e-, and f-wires in the latter case. Here, too, the power supply unit has a seizure detection circuit in the form of a bit-stream detector (in the case of four-wire operation only in the incoming direction), which responds when the subscriber line is seized. The seize condition is indicated via a control line.

The current coupled into the wires of the line 11 is maintained at the given constant value by a series regulator. The bit-stream detector, monitoring the state of the line 11 and connected in parallel with the transmission path, is of the same design as the bitstream detector of the unit 41 at the subscriber's end. It consists of a receiver with a highimpedance input and a response threshold. A detected seize condition is indicated via an interface line BK (Fig. 7).

The coupling transformer provides the necessary potential separation and signal level adaptation between the line circuit 1 5 and the line 11. Within the line circuit 15, the transmission path is unbalanced (cf. interface line g in Fig. 7).

The line circuit 1 5 and all following devices are protected against overvoltages on the line 11 by protective devices in the unit 143.

The power supply unit for a four-wire circuit differs from that described in that two coupling transformers are used to couple the current into the c-, d-wires and the e-, f-wires, respectively, and that the bit-stream detector monitors the bit stream on the e- and f-wires.

The direction-separating unit 141 corresponds to that of the equipment 1. It may be designed either for a two-wire circuit, in which case it contains a separating device operating in TDM, two-frequency duplex, or single-frequency duplex mode, or for a four-wire circuit.

The unit 141 contains line drivers and receiv ers, and carries out the line coding. The outgoing bit-rate clock needed therefor is derived from the exchange clock (so called master clock) and is also available to the unit 137.

The unit 141 differs from the directionseparating unit 41 at the subscriber's end only in that the line coder is off in the rest condition and can be turned on from the seizure detector in the unit 143, from the exchange, of from the facility 1 7. To this end, three interface lines BK, BGV, and BGZ are provided which run to a power source to be described below (Fig. 7). So that propagation time delay on the line 11 does not influence the clock assignment, phase compensation is performed in the steady state.

Since the unit 1 37 is clocked by the exchange, the exchange side of the digital subscriber loop operates in the master mode, while the subscriber side operates in the slave mode. By a block 150, constituents of the exchangc c.g. the control and the switching network-are indicated which are supplied with the system clock via a control line TS (Fig. 6).

The central unit 1 37 contains devices for producing the synchronizing pattern, detecting synchronization, and multiplexing to insert information from the main and additional channels into the 1 0-bit data bus or extracting it therefrom. The main channel is permanently assigned to the switching equipment, while the additional channel for data is permanently assigned to the facility 17. The channel ZKS for signalling in the incoming direction, is routed to the telephone exchange or to the facility 17, depending on the header of a signalling word. The data flow is thus controlled unambiguously. At the interface XII to the unit 141, the 10-bit channel is transmitted.At the interface XIV to the exchange 1 50, only the main channel and the signalling assigned therewith are transmitted, and at the interface XV to the facility, the connection is again provided by the 10-bit bus. The interfaces XIV and XV are operated at the system clock rate of 2,048 kHz.

For signalling in the outgoing direction, the unit 1 37 also controls the priority between the exchange 1 50 and the facility 1 7, always in favour of telephone communication. The unit 1 37 controls all time sequences between the data bus 139 (interface Xlil), the exchange (interface XIV), and the facility (interface XV).

The system clock, the word-rate clock, and the frame rate clock are available on the lines TS, TW, and TR of the interface XIV. The bitrate clock (80 kHz) and all write and read clocks necessary for multiplexing and time-slot rearrangement are derived from these clocks.

Bit, word, and frame periods begin at the same time.

The synchronizing pattern is produced in the unit 1 37 by gating the synchronizing word into the ninth bit of the first 8 word periods of each frame period of the outgoing data stream DA. In the opposite direction, the synchronizing word sent back by the equipment 1 is selected from every ninth bit of the incoming data DE.

A bit pattern supporting synchronization is gated into the other bits of the outgoing data stream, which are not used for the synchronizing word, which gating is interrupted as soon as synchronization is achieved. During the seizure phase, the incoming data stream (DA), is also provided with a pattern supporting synchronization. This pattern is evaluated and used to speed up synchronization. Synchronization is considered achieved if the synchronizing word has been detected twice in succession. If the synchronizing word is not detected twice in succession, the synchronization is reinitiated.

After detecting synchronism, a request-toseize signal, together with the code for "telephone communication" in the header of a signalling word, is gated into the information field of a frame period of the outgoing data stream.

The main channel on the interface line DE of the data bus 1 39 is assigned to the 8-bit channel of the exchange 1 50 on the interface line HE. Conversely, the 8-bit channel of the exchange 1 50 on the interface line HA is permanently assigned to the main channel within the 1 0-bit channel on the interface line DA. The logic states of the first 8 bits of each word period in the 1 0-bit channel are transferred into the first 8 bits of a corresponding word period of the 8-bit channel, and vice versa. The unit 1 37 contains the memories necessary for this transfer.

If the code for "telephone communication" is detected on the line DE in the header of a frame period, the logic states of the 8 bits of the information field are transferred into the 2nd to 9th bits of an associated frame period of the signalling channel on the line SE, the signalling condition (logic 1) being indicated in the first bit. If the header for "telephone communication" is not detected, the logic states of the 1 6 bits of the header and the information field are transferred into the 2nd to 17th bits of an associated frame period of the data channel DE of facility 1 7.

In the outgoing direction, the signalling channel is used either by the exchange 1 50 or by the facility 17, according to priority.

If a logic 1 state is detected on the interface line SA in the first bit of a frame period, the states of the 2nd to 9th bits, plus the code for "telephone communication" in the header are gated into the information field of a frame period in the 10-bit channel on the line DA.

Otherwise the states of the 2nd to 17th bits of the data channel on the line DA of the interface XV are transferred into the header and the information field.

The additional data channel in the 10-bit channel on the line DE is permanently assigned to the data channel of the facility 1 7 at the interface XV. Conversely, the data channel of the additional service facility on the line DA of the interface XV is permanently assigned to the additional data channel in the 1 0-bit channel on the interface line DA. The logic state of the 1 0th bit of each word period of the 1 0-bit channel is transferred into the firstebit of a corresponding word period of the data channel of the additional service facility, and vice versa.

During signalling in the 1 0-bit channel, parity checks with acknowledgement are performed. If more stringent requirements are imposed on signal protection, an error detecting code can be gated into the check field of each frame period of the outgoing data stream. This code is then checked and acknowledged at the receiving end.

As soon as logic 1 state is detected in the first bit of a frame period on the line SA of the interface XIV, the line FR at the interface XV is deactivated until the signal made available has been gated into the 1 0-bit channel.

If the interface line AK is active, signal transmission is correspondingly delayed. This ensures that the transmission of a signal already started by the facility 1 7 is completed. The interface line FR can be activated only in the synchronous condition.

At the interface XV, besides the line TB for the bit-rate clock, lines TRE and TRS for the frame-rate clock in the receive and transmit directions, respectively, and lines TWE and TWS for the word-rate clock in the receive and transmit directions, respectively, are present.

The subscriber terminal equipment is supplied with power from a central constant current source 155, which can be turned on and off via lines BK, BGZ, and "out". The constant current from source 1 55 is coupled into the line 11 in the unit 143.

Details of the additional facility of the terminal equipment 1 are apparent from Fig. 8.

There are two different types of additional facilities: 3, 3A for data transmission over the main channel, and 7, 7A for data transmission over the additional channel (see also Fig.

1). The facilities 3, 3A are mainly data transmission facilities with V or X interfaces as specified by CCITT. By contrast, the additional facilities 7, 7A may be various types, such as view-data, teletypewriter or telefax equipment.

The facilities 3, 3A, 7, and 7A are connected to the equipment 1 via interface devices 157, 159, 161, and 163, respectively.

As mentioned, the interface contains the data bus 39, the control lines 47, the lines 49 for turning the equipment 1 on and off, and the lines 51 for controlling priority.

For outgoing signalling, the equipment 1 has priority. If more than one facility 3, 3A or 7, 7A be present, the priority between the respective additional facilities is controlled over the line 1 65 or 167, respectively.

The interfaces 1 61 and 1 63 for data transmission over the additional channel contain devices for gating seize and clear signals into and out of the 1 0-bit data bus in both direc tions For outgoing seizure, the power supply unit 43 in the equipment 1 is turned on. In addition, there are devices for gating into and out of the data bus 39, so that the additional channel for data is available as a transparent 8-kbit/s channel, inciuding the clock.

The write and read clocks necessary for the gating are derived from the bit-rate clock, the frame-rate clock, and the word-rate clock.

The devices 169 and 171, through which the interfaces 161 and 163, respectively, access the interface lines 39, 47, 49, and 51, are indicated by broken lines. The same applies to the access devices 1 73 and 1 75 of the interfaces 1 57 and 1 59, respectively.

If the signal transmission must be delayed to ensure that the terminal equipment 1 has priority, suitable buffer memories are provided. The interfaces 161, 163 also contain devices (code converters, memories, etc.) for matching the data channel of the respective additional facility to the additional data channel available in transparent form from the general portions of the interfaces. In addition, the final adaptation to the respective type of seizure used in the facility 7, 7A takes place in these devices.

The interfaces 157, 1 59 for data transmission over the main channel contain devices (code converters, memories, etc.) for matching the data channel of the additional facility to the 10-bit data bus. The write and read clocks necessary therefor are derived from the bitrate clock and the word-rate clock.

Upon seizure of the main channel HKA by data services via the facility 3, the codec in the conversion unit 35 of the equipment 1 is turned off via the respective interface. The additional facility is released only if synchronism exists.

The design of the unit 37 and the unit 35 of the terminal equipment 1 is apparent from Fig. 9. The unit 37 contains a transmitter 171, a control 173, a clock generator 175, and a receiver 177. The transmitter 171 contains a transmit memory 179, a synchronizing circuit 181, and a signal encoder 183.

The receiver 1 77 contains a receive memory 185, a synchronizing circuit 187, and a signal decoder 189.

The interface lines connecting the central unit to the other units of the equipment 1 correspond to the interface lines of Fig. 5, as indicated by the dash-dotted interfaces I, III, IV, and VI. At the interface VI between the central unit 37 and the unit 35, the lines L and S for transmitting read and write pulses, respectively, are combined into a common line LIS.

Via the interface I the unit 37 can be connected to the facilitates 3 and 7 and to the add-on equipment 53. Via the interface Ill, the unit 37 is connected to the telephone set 2. and via the interface IV, it is connected to the direction-separating unit 41. A line la- belled "out" serves to turn the power supply unit 43 off.

The unit 35 contains a speech-band-limiting filter 191, a codec 193 for D/A and A/D conversion, and a memory 195, in which the digitized outgoing and incoming speech signals are temporarily stored.

Fig. 10 shows the design of the central unit" 1 37 of the line circuit 1 5 at the exchange. It contains a receiver 1 97, a control 1 99, a clock generator 201, and a transmitter 203.

The receiver 1 97 contains a receive memory 205, a transmit memory 207, a signal decoder 209, a synchronizing circuit 211, and a signal decoder 213.

The transmitter 203 contains a transmit memory 207, a receive memory 217, a signal encoder 219, a synchronizing circuit 221, and a signal decoder 223.

The interface lines connecting the central unit 1 37 to the other units of the line circuit 1 5 are as for Fig. 7.

This is also indicated by the dash-dotted interfaces XIII, XIV, and XV. Via the interface XIII the central unit 1 37 is connected to the direction-separating unit 141. Via the interface XIV it is connected to the exchange 150, and via the interface XV it can be connected to the facility 17, which establishes the connection between the main channel-for data transmissionor the additional data channel and the various data networks or, e.g., the viewdata center. A line labelled "out" serves to turn the power source 1 55 off.

The lines at the interfaces Xl to XV of Figs.

7 and 10 will now be explained in perspective.

The interface Xl contains the a- and b-wires of the balanced subscriber line 11. For a balanced four-wire line, c-, d-, e-, and f-wires are present there.

At the interface XII, for a two-wire circuit, there is an unbalanced line g for transmission in line code, and for a four-wire circuit, a unbalanced line for incoming data in line code, and an unbalanced line for outgoing data in line code. In addition, there are a a line BK for incoming seizure, and a line SVfor the constant-current supply.

At the interface XIII the following lines are present: TS for the system clock, TB for the bit-rate clock, TWfor the word-rate clock, DE for incoming binary-co ied data, and DA for outgoing binary-codec data. The interface XIV contains the Ilowing lines: TS, TW, and TR for the system, word-rate, and frame-rate clocks, respectively, SE for incoming signalling information, SA for outgoing signalling information, HE for incoming main channel, HA for outgoing main channel, and BGVfor seizure from the exchange 150.

At the interface XV the following lines are provided: BGZfor seizure from the facility 1 7, DA for outgoing binary-coded data, DEfor incoming binary-coded data, FR for release (for signalling from facility 17), AK for activating (for signalling from the facility 17), TB for the bit-rate clock, TWS for the word-rate clock in the transmit direction, TBS for the bit-rate clock in the receive direction, TWEfor the word-rate clock in the receive direction, TRS for the frame-rate clock in the transmit direction, and TREfor the frame-rate clock in the receive direction.

Both the central unit 37 and the central unit 1 37 are pure hardware circuits which can be built with commercially available active and passive components and are advantageously implemented using integrated circuit techniques.

The data channel ZKD (Fig. 1) can be used either for the transmission of one data service (e.g. viewdata, teletex or telefax) at a time, it being possible, however, to transmit different services at different times, or to establish connections for the different services at the same time, in which case a fixed sub-frame is assigned to the individual services.

In the signalling and synchronizing channel ZKS, in addition to the signalling information for the channels HKA and ZKD, signals can be exchanged between a subscriber and a central facility, such as an alarm center.

The channel allocation for the transmission of information may be as follows: a 64-kbit/s main channel, a 2-kbit/s synchronizing channel, and a 14-kbit/s additional data and signalling channel, the additional data and the signalling data being transmitted as information packets on an equal priority basis and, if necessary, for several services simultaneously.

The division of the terminal equipment 1 and the line circuit 1 5 into several functional units and the clearly defined interfaces permit a modular construction of the system. In particular, the system can be adapted to different direction-separating techniques by replacing the unit 41 or 141, which is on a printedcircuit board. The separation of the basic terminal equipment 1 from the "luxury" equipment 53, the facility 3 for data transmission over the main channel, and the facility 7 for data transmission over the additional channel, which can be connected via the interface 1 (Figs. 5 and 6) using simple connectors, permits the basic telephone set to be remotely supplied via the exchange line. By contrast, the additional facilities and the luxury equipment are supplied locally.

For a transition period which will probably continue for some time, the all-digital telephone network aimed at will not yet be present, so that digital subscriber loops may have to be operated in conjunction with analog local exchanges. In such cases, a suitable matching device, which provides, among other things, D/A and A/D conversion and signalling adaptation, must be inserted between the respective line circuit 1 5 and the analog exchange.

The subscriber terminal equipment 1 A according to the invention, shown in Fig. 11 in a simplified representation, at least has the following devices: the speech network 20 containing a receiver and a transmitter, the switchhook 25, the dialling device 27, and the ringing device 31, containing a ring-tone generator. In addition, the terminal equipment 1A may have a numeric display and a "luxury" equipment (not shown).

The central unit 237, which processes the signals needed to handle a telephone call and controls the transmission of the digitized speech signals over the line 11, contains the receiver 197, which processes the incoming signals, the transmitter 203, which processes the outgoing signals, and the control 1 99 with clock generator 201, which controls these two devices.

A line terminal 231, which establishes the connection to the line 11, contains a power supply unit for coupling out the supply current from the exchange and for maintaining a constant supply voltage. The supply voltage is applied to the aforementioned units of the equipment 1A. The line terminal 231 may include the direction-separating unit 41 of the terminal equipment 1 (Fig. 4).

The ring-tone generator 31 is turned on and off centrally from the exchange by a signalling word which is evaluated in the receiver 1 97.

Since not all units of the equipment 1A of Fig. 11 need to be in operation during ringing, these units or parts thereof can be disconnected from the power supply. The disconnectable units are the conversion unit 35, containing a codec and filters, and the transmitter 203 of the central unit 237. These parts must be on only when the subscriber has removed his handset and when synch ron- ism has been achieved between the exchange and the subscriber terminal equipment 1A.

A switching device 233, in this case an AND gate, has two inputs connected to the output of the switchhook 25 and to the signal output of the receiver 197, respectively. The switchhook 25 provides an output signal G which is in the logic 1 state when the handset is off the hook. The receiver 1 97 provides a signal SYN which is at logic 1 when synchronization has been achieved between the exchange and the equipment 1A. When both signals G and SYN are applied to the inputs of the device 233, the latter provides an output or turn-on signal E, which is applied to the conversion unit 35 and/or to the transmitter 203 of the central unit 237, and turns on the supply voltage in these units.

If the circuits are implemented using CMOS technology,the turning on is effected by enabling all dynamic inputs of these units. In the static state, the power requirement of CMOS integrated circuits is extremely low since most of their power consumption is determined bydynamic processes and particu larly by charge transfer between the capacitances. Consequently, switching from the working condition to a nearly currentless rest condition can be effected by inhibiting the dyanamic signal inputs. To inhibit these signal inputs, simple gates can be used.

The ringing device consumes about 50 mW. By disconnecting the conversion unit 35 and the transmitter 203 or parts thereof, about 50 mW can be saved. If so many units are disconnected during ringing that the power saved is equal to the ringing power, the total power requirement of the subscriber terminal equipment 1A during the ringing process and during the call will be constant.

Thus, there are no consumption peaks which could not be transmitted over the subscriber line if the exchange and the subscriber are separated by large distances.

The subscriber terminal equipment according to the invention increases either the distance capability of the telephone network or the power available for feeding the terminal equipment.

Claims (6)

1. A subscriber terminal equipment for a digital communication system, which equipment when in use is connected to the line circuit of an exchange via a subscriber line, includes a central unit controlling the transmission of digital intelligence signals over the line, a ringing device, a dialling device, a switchhook, a speech network, a conversion unit connected to the speech circuit and providing analog-to-digital conversion of the speech signals, and a switching device one input of which is presented with a first signal, indicating the condition of the switchhook, and the other input of which is presented with a second signal, indicating the synchronization of the terminal equipment, and in which when the handset is off the hook and in the presence of synchronization, said switching device provides an output signal which turns on the power supply for units of the terminal equipment.

2. A terminal equipment as claimed in claim 1, and in which the output signal of the switching device turns the conversion unit on.

3. A terminal equipment as claimed in claim 1 or 2, and in which the output signal of the switching device turns on a transmitting device in the central unit, which transmitting device processes the signals sent out by the terminal equipment.

4. A terminal equipment as claimed in claim 1, 2 or 3, and in which the second signal, indicating the synchronization of the terminal equipment with the line circuit, is provided by a receiving device contained in the central unit and processing the signal received by the terminal equipment.

5 A terminal equipment as claimed in claim 1, 2, 3, or 4, and in which the switching device is an AND gate.

6. A subscriber terminal equipment, for use in a digital communication system, substantially as described with reference to the accompanying drawings.