GB1588218A - Digital switching of electronic signals - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO DIGITAL SWITCHING OF ELECTRONIC SIGNALS (71) We, THE POST OFFICE, a British corporation established by Statute, of 23 Howland Street, London W1P 6HQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the switching of digital signals. The invention has application to the switching of pulse code modulated (PCM) signals. The invention also relates to a digital switching device which can be used as a PABX (Private Automatic Branch Exchange) telephone exchange to switch telephonic speech in PCM form and to circuits used in the switching device.

Digital switching of telephonic speech which has been converted to digital PCM form by an analogue to digital converter (coder) is well known. For speech in its analogue form attempts have been made to obtain the benefits of digital switching by positioning one or more coders and associated digital to analogue converters (decoders) next to a digital switch. (The combination of a coder and decoder is known as a "codec"). In the past codecs have been expensive and hence telephone exchanges have used as few as possible. Two methods have been used in such telephone exchanges.In the first speech information from several telephonic terminations is multiplexed using pulse amplitude modulation and time division multiplexing techniques and a single time shared codec is used to convert the multiplexed stream to PCM form thereby keeping to a minimum the cost per termination of the codec. In the second method a stage of analogue switching is used to concentrate the telephonic information before passing it to the codes.

Cheaper codecs are now available and it is possible to provide a PCM switching device which has a codec for each telephone or other analogue input connected to the digital switch and which is economic.

According to the present invention there is provided a digital switcing device for switching digital signals under the control of a central control unit, said device comprising at least one primary selector having a plurality of single channel inputs for receiving non-multiplexed signals, a secondary selector which operates as a space switching stage, each primary selector being arranged to transmit multiplexed digital signals along particular time slots determined by said central control unit on a multiplexed output highway to said secondary selector, a secondary distributor which operates as a space switching stage, a primary distributor which has a plurality of inputs for receiving digital signals, a central highway connecting the secondary selector to a secondary distributor, and said primary distributor being connected to the secondary distributor to receive digital signals in particular time slots determined by said central control unit from said secondary distributor, each input to the primary selector being connectable to any channel on the muliplexed output highway.

The device may include a demultiplexor associated with the secondary selector and arranged to remove signalling information from the signals passing through the secondary selector and a multiplexor associated with the secondary distributor and arranged to insert signalling information at the input of the secondary distributor. The signals may be PCM signals and each primary distributor may receive signals in time slots corresponding to the time slots along which they were transmitted from the primary selector to primary distributor.

The switching device may include a plurality of telephonic line units connected to the inputs of each primary distributor. The switching device may be arranged to operate with two speed signalling, one speed of signalling being tied to dedicated signalling bit times associated with each line unit.

The or each primary selector may be arranged to periodically sample its inputs under the control of said central control unit and the or each primary distributor has pulse stretcher circuits whereby switching occurs by controlling the sampling sequence at the primary selector.

The central control unit may be a microprocessor.

The device may be a PABX.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a block schematic diagram illustrating a switching device in accordance with the present invention Figure 2 is a block schematic diagram illustrating part of the device of Figure 1; Figure 3 is a block schematic diagram showing in more detail the present switching device; Figure 4 shows one form of line feed arrangement which can be used in the switching device; Figure 5 shows another form of line feed arrangement which can be used in the switching device; Figure 6 shows the signalling logic of Figure 2 in more detail; Figure 7 shows the power switch Figure 2 in more detail;; Figure 8 illustrates the extraction of signalling information by the central control, and Figure 9 illustrates how the primary and secondary selectors and distributors are controlled.

The switching device to be described is a digital PABX (private automatic branch exchange) which is a stored programme machine using an 8-bit microprocessor as the central processor unit. Switching between lines, trunks, auxiliary units and an operator console is via pulse code modulation (PCM) techniques. All speech signals are encoded to a digital representation prior to their transmission to a digital switch.

Switching between parties is accomplished by sending the encoded signals at the appropriate time to the receiving party and reconverting the digital signals to analogue form. Because all the speech signals are digitally encoded before being applied to the digital switch the PABX can send the speech signals directly to a local exchange over a regenerated line system without the need for expensive PCM multiplexing equipment at the PABX location. The switching device employs time space time switching. The PABX can have up to 160 telephonic terminations for example extension lines. public exchange lines and private wires connected to the PABX, or other auxiliary terminations such as conference circuits and multi-frequency signalling receivers.

Referring now to Figure 1 each telephonic termination (two of which are indicated at A and B) is connected to a line unit 10. A line unit 10 is provided for each telephonic termination and it will be appreciated that many more line units can be used than are shown in Figure 1. Each line unit is connected by a pair of wires 12 to a digital switch 14 and a further pair of wires 16 connect the digital switch to the line unit 10.

Auxiliary units such as that indicated at 18 can also be connected in a similar manner to the digital switch 14. The digital switch 14 is connected by a highway 19 to the central processor or control unit 20.

Each line unit 10 receives audio speech signals from its respective telephonic termination and converts these signals into a PCM representation. The PCM encoded signals are fed to the digital switch 14 by way of wires 12. Each line unit 10 also receives PCM encoded audio signals from the digital switch over the wires 16 and decodes these signals to their analogue form. The signals generated within the line unit are time division multiplexed with the PCM speech signals and passed to the switch 14 over the wires 12 from where they are fed to the central control 20. Signals from the central control 20 are received by the line unit via the digital switch having been time division multiplexed with the speech signals on the wires 16. The digital switch 14 thus provides a signalling capacity between the line units 10 and the central control 20.

The signalling capacity can be considered as having two portions. Each PCM speech word which is fed to the digital switch 14 has an additional bit available for carrying signalling information. Some of the signalling bits are permanently dedicated to parti- cular inlets and outlets of the digital switch 14 whilst others are associated with particular time slots in the digital switch and are made available to whichever inlets or outlets are connected to those time slots. This can be varied as required. This arrangement provides a very high signalling rate which is generally not required for most purposes.

For interrogation purposes one bit time every 8 milliseconds is assigned to each line unit for interrogation purposes etc. This means that during normal running i.e. when a call is not present on a lint unit, information is signalled at a very slow rate in bit times dedicated to each line unit, but when a call is being set up and a high rate signalling is necessary information is signalled at a much faster rate using up to one signalling bit per PCM word. Thus, each line unit has a basic signalling capacity to and from the central control 20 and this signalling capacity can be considerably enhanced when desired, for example when a call is established.

All the signalling bits are combined at the centre of the digital switch and are conveyed as a multiplexed digital stream along highway 19 to the central control.

As well as interconnecting line units the digital switch 14 can establish paths between line units 10 and auxiliary units such as 18.

Turning now to Figure 2 each line unit 10 has a line feed 25 which provides a source of electrical current for energising the microphone of the telephone associated with that line unit. The line feed 25 has an output which is connected to a two wire to four wire converter 26. The two wire to four wire converter 26 is required since the paths through the digital switch 14 of Figure 1 are unidirectional and in general two paths are needed for each telephone call established across the switching machine. There are some exceptions to this, for example when a subscriber is being sent a tone. Under these circumstances only a single path is necessary from the tone generator to the subscriber.

The two wire to four wire converter 26 has a first pair of output wires 28 which are connected to a filter 29. The output of the filter is connected to a coder 30 and the output of the coder 30 is connected to a multiplexor 31. The output of the multiplexor 31 consists of the output wires 12 of Figure 1.

The wires 28, filter 29, coder 30 and multiplexor 31 constitute a path over which signals from the telephone can be transmitted to the digital switch 14. The two wire to four wire convertor can also receive signals from the digital switch over a path consisting of wires 34, filter 35, decoder 36, demulitplexor 38 and wires 16. The filters 29 and 35 are provided to filter out any signals which have frequencies outside the specified audio band.

The coder 30 and decoder 36 are A-law (or ll-law) PCM devices. Devices of this type are known and will not be described in further detail. One form of codec which can be used is described in U.K. Patent Application No. 5014/77 (Serial No. 1588219).

The line. feed 25 has further outputs 40 which connect the line feed 25 to a signal detector 42. The signal detector 42 is connected by lines 43 to the multiplexor 31. The signal detector 42 is provided because it is necessary to recognise in each line unit those signals from the associated telephone which, because they do not lie within the specified audio band. would not be transmitted through the digital switch 14. These signals are detected in the signal detector 42 and are converted to digital signals which can be transmitted through the digital switch to the central control. This is achieved using the multiplexor 31 and the signalling capacity previously mentioned. The signal detector 42 makes use of the characteristics of the line feed arrangement. Two versions of the line feed 25 have been devised and will be described later.

Each line unit also has a power switch 45 and signal application unit 46. The power switch can be used to isolate voltage supplies from the two to four wire convertor 26, the filters 29 and 35 and the coder 30 and decoder 36 when these are not in use. This arrangement reduces power consumption.

The power switch is operated under the control of the central control unit 20, the necessary instruction being conveyed in the signalling channel.

Each line unit 10 provides access from the PABX to its associated telephone for a variety of purposes in addition to the one of passing decoded speech to the telephone.

Such access is initiated by signals received at the line unit from the central control unit 20 via the digital switch 14 and demultiplexor 38. The signals are decodeed and used to operate devices which connect the telephone line to separate highways which can carry ringing current or test signals or other signals used for operational purposes. Relays are used as the above devices although some versions use silicon controlled rectifiers in place of the relay associated with ringing of the telephone bell.

In use each line unit 10 produces and receives A-law PCM signals at a sample rate of 8,000 samples per second. This means that for correct operation each sample of encoded PCM speech must be passed to, or received from, the digital switch 14 within 125 microseconds. The signalling bits to and from the line unit are time division multiplexed with the PCM speech signals to form a serial digital stream for transmission over wires 12 and 16. Thus, the bit rate on these wires must be adequate to convey 8 speech bits plus X signalling bits in 125 microseconds. It is envisaged that X equal to unity will give sufficient signalling capacity for present needs. However, it will be relatively simple to introduce as many signalling bits as the speed of the technology permits. With the figures quoted above a minimum bit rate of 72 Kbits per second is needed on wires 12 and 16.

The first version of the line feed circuit referred to above is shown in Figure 4. In this circuit the A and B wires of a telephone are connected to the primary of a conventional line current feeding transformer 100.

The secondary of the transformer is connected to the exchange. Relays 101 and 102 are connected to the transformer primary and resistors 104 and 105 and thermistors 1()6 and 107 are connected in series with the relays. The relays are used to detect the loop created when the associated telephone is in use and earth current flowing between the PABX and the telephone. The relays 101 and 102 are operated or released according to the currents flowing in the pair of wires to the telephone. The thermistors 106 and 107 are provided to limit the current flowing to the transformer under fault conditions. Each thermistor is a PTC thermistor and is designed to limit fault currents to less than 200ma within a few seconds.

The signal detector function can alternatively be carried out using a high input impedance amplifier to detect the voltage drop across various resistors in the circuit.

The second version of the line feed circuit is shown in Figure 5. In this circuit the A and B wires of the telephone are again connected to the primary of a line current feeding transformer 120 in a conventional manner. The secondary of the transformer is connected to the exchange. The multiplicity of coils and capacitors serve to match the line feed transformer, which acts as a transmission bridge, to line and is conventional. In this version of the line feed circuit loop detection is not by relays but by a Hall Effect device. A Hall Effect slice represented schematically at 122 is implanted in the magnetic circuit of the transformer. The Hall Effect slice is connected to an operational amplifier 123 which is in turn connected to a Schmitt trigger 124. The Schmitt trigger 124 is arranged to change state at a predetermined level of amplifier output.

The Hall Effect slice detects the changes in the magnetic field of the transformer core introduced by the telephone signalling arrangements. These changes result in triggering of the Schmitt trigger 124 the output of which is processed in a conventional manner. It will be noted that the Hall Effect device can be used to detect the presence of DC on the primary of the transformer whilst being electrically isolated from the primary.

An arrangement of thermistors similar to that described with reference to Figure 4 can be used in the arrangement of Figure 5 for a similar purpose.

Referring now to Figure 3 the digital switch 14 has a number of primary selectors 50 which constitute time switching stages.

Each primary selector has 32 inputs. each input being connected to one of the line units l() or other forms of digital inputs.

Each primary selector is connected by a highway 52 to a secondary selector 54 which constitutes a space switching stage. Each highway 52 operates in time division multiplex with 64 time slots available to the 32 digital inputs on the associated primary selector. The actual time slots used for each call are determined by the central control unit 2() which contains a cyclic store for operating the digital switch accordingly. The bit rate on the highway 52 is very much faster than that generated by individual line units or other forms of digital input, and it is thus necessary to have some means whereby information produced by the line units is returned and this is most conveniently done by the primary selector which can sample the incoming digits at a time corresponding to the chosen time slot on the highway 52.

The secondary selector 54 can select, at each time slot, the information contained in the time slot on any of the highways 52 and pass that information via a signalling demultiplexor 56 to the corresponding time slot on a time division multiplexed central highway 58. The signalling demultiplexor 56 also accesses the signalling bits fed to the secondary selector and passes these over the highway 19 to the central control unit 20.

The central highway 58 is connected to the input of a signalling multiplexor 60 which in turn is connected to a secondary distributor 62. The secondary distributor 62 has a number of outputs which are connected by highways 64 to a number of primary distributors 66. The number of primary distributors 66 corresponds to the number of primary selectors 50. Each highway 64 has 64 time slots available in a similar manner to highways 52.

The signalling multiplexor 60 receives signals from the central control 20 via highway 19 and inserts these signals into the appropriate bit positions on the central highway 58 for transmission via the switch to the desired destination line unit or other form of digital receiver. At each time slot the secondary distributor 62 routes the information in that time slot over an appropriate one of the highways 64 to the appropriate primary distributor 66. The primary distributor, at each time slot and under the control of the central control 20, passes the digital information in that time slot to the desired receiving line unit or other form of digital receiver.

It is necessary to convert from the high bit rate used on the highways 64 to the slower bit rate used by the line units and this is carried out in the primary distributors 66 using a pulse stretching circuit. Details of this pulse stretching circuit are described in our copending Application No. 37659/76.

The primary selectors and primary distributors do not have any storage facilities.

They operate by sampling input lines periodically under the control of the central control 20 to build up the multiplex on the highway 52. The secondary selector 54 carries out space switching on to the single central highway 58. The multiplexor 56 and demultiplexor 60 function to remove signalling data from the incoming speech information and feed it to the central control unit and to insert signalling information into appropriate slots on the speech information for distribution out to the line units associated with the receiving equipment.

The secondary selector and distributor are designed to work with up to 8 input and 8 output highways 52, 64 although in the application shown in Figure 3 a maximum number of five highways are used for connection to primary selectors and distributors. With five primary selectors and distributors the switch can be connected to a maximum of 160 (5 x 32) telephonic terminations or auxiliary terminations such as conference units or multi-frequency signalling receivers. In some applications it may be necessary to use only one primary selector and one primary distributor.

Control of the switch 14 is provided by a stored programme in the central control unit 20. The control unit 20 is an Intel 8080 microprocessor. The operation and programming of such a microprocessor is well known to persons skilled in the art and will not be described here. The central control unit is arranged to carry out all call processing in the PABX, to set up and release paths for signals through the digital switch 14 and to perform scanning operations of lines, trunks, tie lines and data lines from the operator's console. The central control unit 20 has an 8 bit microprocessor chip, a 512 word read-only memory for bootstrap loading, TTL interface logic and up to 30,000 words (8 bits wide) of storage for the control programme.

The switching device is designed with flexibility in mind and it is possible to produce different line units according to the type of telephonic termination connected.

For example, if two pairs of wires are available to the telephone then a four wire telephone may be used in which there is no direct electrical path from the receive to the transmit direction. (This is used for the operator telephone of the PABX and results in improved transmission performance).

Under these circumstances the two wire to four wire converter 34 is not needed in the line unit which may be correspondingly simplified.

If the terminal equipment produces and/ or receives digital information directly (as for example a telephone with integral coders and/or decoders, or a data terminal equipment, or a facsimile machine) the appropriate filters and coders and/or decoders in the line unit can also be omitted.

The PCM switching device has also been constructed with ease of installation in mind. Each primary selector and distributor and its associated 32 line units are mounted on a single shelf which may be installed as a complete entity requiring only plug attachment to the rack in which it is mounted for connexion to telephonic terminations, the secondary selector and distributor, the cyclic store area of the central control unit and the power supplies.

The signal application circuit 46 of Figure 2 is shown in more detail in Figure 6 and the power switch of Figure 2 is shown in more detail in Figure 7. The signal application circuit of Figure 6 comprises two large scale integrated circuits 200, 201 which act as an interface between the demultiplexor 38 of Figure 2 and five NAND gates 203-207. The output of the NAND gate 204 is connected to an inverter 209 the output of which forms an input to the power switch to be described with reference to Figure 7. The gate 204 is arranged to produce an output signal for application to the switch of Figure 7 whenever power to the line unit is required to be switched on or off.

The output of the NAND gate 205 is connected to a further NAND gate 210 the output of which is connected to the base of a transistor 212. The collector of the transistor 212 is connected to a relay 214 which is arranged to be energised whenever ringing signals are to be applied.

The circuit of Figure 6 also includes a loop state detector consisting of two gates 215 and 216. A loop state is detected whenever a switch 218 connected to the input of the gate 215 is closed and an appropriate output is prdduced from the output of the gate 216.

The circuit also includes means for sensing a recall state which occurs when a switch 220 is closed. Closing of the switch 220 is sensed by large scale integrated circuit 224.

Turning now to Figure 7 the power switch 45 comprises an input transistor 225 the base of which is arranged to receive the signal from the output of the inverter 209 of Figure 6. The collector of the transistor 225 is connected to the base of each of three transistors 228, 229 and 230. The collector of the transistor 230 is connected to the base of a further transistor 231. The transistors 228, 229 and 231 are switching transistors which control switching of various power levels of the line unit. Whenever power is required for the line unit of Figure 2 the transistor 225 switches state thereby switching the states of transistors 228, 229 and 231.

This causes the appropriate power to be applied to the circuit. This operation is initiated by appropriate signal from the signalling circuit of Figure 6.

The logic for extracting signalling information at the output of the secondary selector and for re-inserting it at the secondary distributor is shown in more detail in Figure 8. The output of the secondary selector 54 is connected to a demultiplexor 250 which feeds a recirculating memory 252.

The recirculating memory 252 feeds a signalling re-formatting selector 254 which is connected by a data bus 255 to the central microprocessor 20 which as shown in this Figure consists of a central processing unit 256 a read/write memory 258 and a read only memory 259. The central processing unit 256 is connected by an address bus and control line to an address decode logic 260.

The address decode logic 260 is connected to the signalling re-formatting selector 254 and to a further signalling selector 262. The signalling re-formatting selector 262 is connected to a recirculating memory 264 which feeds a multiplexor 265. The output of the multiplexor 265 is connected to the central highway.

The control of the primary and secondary selectors and distributors is illustrated in Figure 9. These circuits are controlled by signalling information from a control store 270 in the central processor 20.

Reference is made to our copending Application No. 37943/77 (Serial No.

1588220), which has been divided out from the present application.

WHAT WE CLAIM IS: 1. A digital switching device for switching digital signals under the control of a central control unit, said device comprising at least one primary selector having a plurality of single channel inputs for receiving non-multiplexed signals. a secondary selector which operates as a space switching stage, each primary selector being arranged to transmit multiplexed digital signals along particular time slots determined by said central control unit on a multiplexed output highway to said secondary selector, a secondary distributor which operates as a space switching stage. a primary distributor which has a plurality of inputs for receiving digital signals, a central highway connecting the secondary selector to a secondary distributor, and said primary distributor being connected to the secondary distributor to receive digital signals in particular time slots determined by said central control unit from said secondary distributor. each input to the primary selector being connectable to any channel on the multiplexed output highway.

2. A digital switching device as claimed in claim l. including a demultiplexor associated with the secondary selector and arranged to remove signalling information from the signals passing through the secondary selector and a multiplexor associated with the secondary distributor and arranged to insert signalling information at the input of the secondary distributor.

3. A digital switching device as claimed in claim 2. wherein said signals are PCM signals and wherein each primary distributor receives signals in time slots corresponding to the time slots along which they were transmitted from the primary selector to primary distributor.

4. A digital switching device as claimed in any preceding claim, including a plurality of telephonic line units connected to the inputs of each primary distributor.

5. A digital switching device as claimed in claim 4, wherein said switching device is arranged to operate with two spaced signalling, one speed of signalling being tied to dedicated signalling bit times associated with each line unit.

6. A digital switching device as claimed in any preceding claim, wherein the or each primary selector is arranged to periodically sample its inputs under the control of said central control unit and the or each primary distributor has pulse stretcher circuits whereby switching occurs by controlling the sampling sequence at the primary selector.

7. A digital switching device as claimed in any preceding claim, wherein said central control units is a microprocessor.

8. A digital switching device as claimed in any preceding claim, wherein the device is a PABX.

9. A digital switching device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   connected by a data bus 255 to the central microprocessor 20 which as shown in this Figure consists of a central processing unit 256 a read/write memory 258 and a read only memory 259. The central processing unit 256 is connected by an address bus and control line to an address decode logic 260.

   The address decode logic 260 is connected to the signalling re-formatting selector 254 and to a further signalling selector 262. The signalling re-formatting selector 262 is connected to a recirculating memory 264 which feeds a multiplexor 265. The output of the multiplexor 265 is connected to the central highway.

   The control of the primary and secondary selectors and distributors is illustrated in Figure 9. These circuits are controlled by signalling information from a control store 270 in the central processor 20.

   Reference is made to our copending Application No. 37943/77 (Serial No.

   1588220), which has been divided out from the present application.

   WHAT WE CLAIM IS: 1. A digital switching device for switching digital signals under the control of a central control unit, said device comprising at least one primary selector having a plurality of single channel inputs for receiving non-multiplexed signals. a secondary selector which operates as a space switching stage, each primary selector being arranged to transmit multiplexed digital signals along particular time slots determined by said central control unit on a multiplexed output highway to said secondary selector, a secondary distributor which operates as a space switching stage. a primary distributor which has a plurality of inputs for receiving digital signals, a central highway connecting the secondary selector to a secondary distributor, and said primary distributor being connected to the secondary distributor to receive digital signals in particular time slots determined by said central control unit from said secondary distributor. each input to the primary selector being connectable to any channel on the multiplexed output highway.
2. 2. A digital switching device as claimed in claim l. including a demultiplexor associated with the secondary selector and arranged to remove signalling information from the signals passing through the secondary selector and a multiplexor associated with the secondary distributor and arranged to insert signalling information at the input of the secondary distributor.
3. 3. A digital switching device as claimed in claim 2. wherein said signals are PCM signals and wherein each primary distributor receives signals in time slots corresponding to the time slots along which they were transmitted from the primary selector to primary distributor.
4. 4. A digital switching device as claimed in any preceding claim, including a plurality of telephonic line units connected to the inputs of each primary distributor.
5. 5. A digital switching device as claimed in claim 4, wherein said switching device is arranged to operate with two spaced signalling, one speed of signalling being tied to dedicated signalling bit times associated with each line unit.
6. 6. A digital switching device as claimed in any preceding claim, wherein the or each primary selector is arranged to periodically sample its inputs under the control of said central control unit and the or each primary distributor has pulse stretcher circuits whereby switching occurs by controlling the sampling sequence at the primary selector.
7. 7. A digital switching device as claimed in any preceding claim, wherein said central control units is a microprocessor.
8. 8. A digital switching device as claimed in any preceding claim, wherein the device is a PABX.
9. 9. A digital switching device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.