Monarch 120-A New Digital PABX

POEEJ Vol 73 Part 1 April 1980

A. R. POTTER, B.SC., C.ENG., M.I.E.E.

UDC 621.395. 2: 621.374

Monarch 120 is a digital-switching telephone-exchange system for use at customers' premises. The system, which uses stored-program control techniques, provides a greater range of customer facilities than that provided by existing PABXs. Also, the system is designed to provide data-transmission facilities to meet the present-day and future needs of customers who require a modern business communications centre.

INTRODUCTION

A new private automatic branch exchange (PABX) has been developed by a joint British Post Office (BPO) and industry team . The system, designed for the telephone-equipment rental market, has a capacity of 24-120 telephone extensions.

Although its name during the development phase of the project was Customer Digital Switching System No. I (CDSSl), the system will be marketed under the title of Monarch 120.

The Monarch 120 system exploits the techniques of stored program control (SPC) and digital switching by using A-law pulse-code modulation (PCM), to provide a product that has the following major features:

(a) it has an advanced range of customer facilities;

(b) the equipment is compatible with modern office accommodation, because it is small in size and operates from the public mains-supply;

(d) in addition to the standard telephony services, the system has the ability to handle the future demands of modern businesses for a range of data facilities; and

(e) the central processor aids in the diagnosis of faulty units.

In many ways, the design of the Monarch 120 system is broadly compatible with the design of the System X family of public exchanges and has taken account of the future evolution of BPO telephone network. In particular, the integration of digital transmission and switching functions has been a major factor in controlling costs and has enabled an advanced services capability to be provided.

OVERALL SYSTEM ORGANISATION

The Monarch 120 system consists of a central equipment unit, which is a single free-standing cabinet approximately 1 ·7 m high and 0·6 m2 in cross-section, and an operator console, which can be positioned up to 300 m away from the central equipment unit. The operator console will be the subject of a later article in this Journal and is therefore mentioned in this article only where it is relevant to the description of the system. The equipment cabinet is fitted with 2 shelves of common equipment: the control and switch hardware is mounted at the top of the cabinet and the power supply unit is mounted at the bottom. The intervening 5 shelves house the analogue-digital line interfacing circuits; the number of shelves provided is dependent on the number of extensions provided at a particular installation. In addition, space exists above the control shelf for electro-mechanical meters for recording calls made, or other miscellaneous equipment. The equipment cabinet (with front cover removed) is shown in Fig. 1(a).

The control Shelf

Occupying 4 printed-wiring boards (PWBs) on the control shelf, a microprocessor and various memory circuits provide the central control function for the system. Two other PWBs provide buffer storage for interfacing signalling between the control and the peripheral line-interface circuits.

On the other PWBs within the control shelf are provided the central digital switch, conferencing circuits, tone generation, and oscillator and waveform generation for the system. No replication of equipment on the control shelf is provided, because the inherent high reliability of the digital integrated circuits provides an acceptable failure rate for this small size of PABX.

Line Unit Shelves

Because the Monarch 120 system is a digital switching system, each analogue line terminating on the exchange has to have an analogue-digital interface. A variety of line units are used to cater for the different types of circuits terminating on an exchange; however, in each case, a standard interface is used between the line unit and the remainder of the exchange. Up to 32 individual line units can be accommodated on each line shelf, these being provided by either 2 or 4 units to a PWB. The first 6 positions on the line shelf accommodate the 4-port PWBs-these are generally extension-line interfaces; the 4 remaining positions are available for 2-port PWBs. At one end of the line shelf is a multiplexing card, which interfaces digital speech and signalling highways between the line units and the control-shelf equipment. A further degree of provisioning flexibility is provided, since 2-port and 4-port PWBs are electrically compatible. Finally, a central position in the line shelf can accommodate a PWB of fallback relays, which, under conditions of system failure, connect exchange lines through to a number of designated extensions; if this facility is not required, a simple by-pass PWB is provided instead.

4-port PWB

A 1981 GEC Extension Line Unit

ASU1A1 SA20043

Later models of Monarch squeezed 8 Extension onto 1 card.

See BT Monarch Line Cards for more information

Monarch 120 PSU

The power supply unit provides conversion between the public AC mains supply and the 4 DC voltages used by the system. An installation comprising a fully-equipped cabinet and 2 operator consoles, and operating at the design traffic maximum of 32 simultaneous calls, requires approximately 600 W of mains input power. Power is distributed to the control and line shelves via a bus bar, and the built-in overvoltage and over-current protection circuitry obviates the need for output fusing of the supply. An electronic ringer circuit, housed in the power shelf assembly, provides the standard 75 V RMS 25 Hz ringing signal. If required, on site replacement of the power supply unit can be speedily effected, since the unit can be slid out from the front of the cabinet.

A view of the rear of the equipment cabinet, displayed in Fig. 1 (b), shows how the interconnexion of the various assemblies of the Monarch 120 PABX is achieved. External lines from the distribution frame terminate on plug-ended cables on the line shelves. Between the control shelf and the line shelves, a ribbon cable, consisting of standard lengths between each shelf, carries the multiplexed digital speech and signalling highways and timing waveforms used by the system. In addition to this main cable, a small ribbon carries the ringing signal and a small number of common services signals from the power supply. A number of connectors are provided on the control shelf to give access between an external data link and the central control, to enable interconnexion between the digital switches and controls of two Monarch 120 cabinets and for the termination of a 2·048 Mbit/s PCM line system in a future development.

DIGITAL SWITCH AND SIGNALLING FUNCTIONS

A block diagram of the architecture of the switch and signal handling circuits is shown in Fig. 2. All line units in the exchange operate synchronously and produce 8 bit PCM samples every 125 J.Ls. During this time interval, each line unit sends to its shelf multiplex 9 bits, comprising the 8 PCM bits and 1 bit of signalling information. Thus, the data rate between a line unit and the shelf multiplex is 72 kbit/s and an 8 bit signalling word is used with a repetition period of 1 ms, as illustrated in Fig. 3.

The shelf multiplex assembles 32 of these 72 kbit/s streams in a fixed order and in a bit-interleaved fashion onto an internal highway running at 2·304 Mbit/s. The shelf multiplex then separates the speech and signalling data for onward transmission to the input time-switch and signalling-input cards at 2·048 Mbit/s and 256 kbit/s respectively, reformatting the speech and signalling into 32 individual slots of 8 bits with the appropriate repetition rate.

The reverse direction of transmission is handled in an opposite fashion and can be considered as completely separate from the transmit direction of transmission; however, within the shelf multiplex, some common hardware is used for the storing and reformatting of the information. The digital switch for the Monarch 120 system is a single-stage time switch whch is fabricated on 3 PWBs: the input time-switch, the cabinet interface and the output time-switch. The digital switch has been structured in this manner to permit future flexibility, in that the system hardware may be readily adapted to enable 2 cabinets to operate alongside each other as a single exchange unit. This option has not been exercised in the initial development and, at present, the cabinet interface board is a passive link between input and output time-switch PWBs, and the output time-switch performs only demultiplexing of the PCM streams. In addition to the individual line units on the exchange, the digital switch provides access to the tone generator and conference units provided on the control shelf. A further PWB position is also left for future enhancement of the system (for example a digital 2·048 Mbit/s line system interface) and, to this end, the unit has external access via a socket on the control shelf back-plate.

A total of eight 2·048 Mbit/s highways access the input time-switch, 5 from the individual shelf multiplexes and 3 from the miscellaneous PWB positions on the control shelf. Each highway is first converted from serial to an 8 bit wide parallel format and then all highways are multiplexed to-

FIG. 4-Block diagram of the Monarch 120 system time-switch gether and written sequentially into the time-switch speech store (a 256 x 8 bit read/write memory). A block diagram of the digital switch is shown in Fig. 4. The cyclic writing of information into the speech store is controlled by an input counter. To connect one port on the exchange to another (a port can be an individual customer, a tone source, a multi-frequency (MF) signalling receiver, etc.), it is necessary to read out the relevant contents of the speech store at a time corresponding to the output port connexion identity. Because the content of the speech store varies from call to call and during the different phases of a call, a connexion store (also a 256 x 8 bit read/write memory) is used to control this a cyclic reading of information from the speech store onto an 8 bit parallel highway to the cabinet interface and onward to the output time-switch PWB. At this PWB, the speech channels are demultiplexed back onto the 8 highways and are converted to serial transmission for onward transmission to the shelf multiplexes etc. The connexion store contents are output cyclically every 125 fLs to control the reading of data, and the connexion-store contents are updated from the central processor every time a switch path is set up or cleared. Because every port has a time-slot on the highways within the exchange and within the speech and connexion stores, the switch is completely non-blocking. This factor simplifies the overall control and dimensioning of the exchange.

Signalling input and output PWBs provide a buffering and access capability between the shelf multiplexes and the central processor unit (CPU). A 256 kbit/s highway carrying 32 signalling channels (8 bit/channel, repetition rate 1kHz) is converted from serial to parallel transmission, multiplexed with the other highways and written cyclically into a 256 x 8 bit read/write memory. This information is updated at the I ms repetition rate of the signalling and, in this way, the up-to-date signalling status of each port is made available to the CPU at any time. In the reverse direction, the CPU causes an 8 bit data word to be placed into a location within the 256 x 8 bit read/write memory in the signalling output PWB. This data word is then repeated to an individual port every 1 ms until it is updated.

CONTROL

The central control for the Monarch 120 System is provided by a single microprocessor (8085 type) and the memory circuits for holding program and working data information. The organization of the CPU and the means of access to the signalling and time switching PWBs is shown in Fig. 2. Linking between all the units is provided by a parallel bus system consisting of 8 data and 20 address lines. The microprocessor supervises the item of hardware which may access the bus, and a further 3 lines are provided to control the reading and writing of data between the microprocessor and the other PWBs. To read or write data to the signalling and timeswitching memories does not require the full 20 bit of addressing capability that the bus provides. Partial decoding of the address bus is therefore provided on the signalling input PWB and only 8 bits of the address bus are extended beyond this point.

Three types of memory are provided. Programmable-read only memory (PROM) is used for storing the basic control program for the exchange. Two PWBs are used, each with a capacity of 48 Kwords, each of 8 bits. Read/write memory or random-access memory (RAM) is used for storing working data such as information concerning calls in progress. In this case, a single PWB with 16 Kwords each of 8 bits is provided. Finally, a non-volatile memory board is used to store data which needs to be changed during the life of the exchange, but this information must be protected against power supply interruptions. To this end, a form of low-power RAM is used, together with an on-board battery to ensure that information is retained for a protracted period if power is removed from the board. A total of 8 Kwords are provided, of which 4 Kwords are used for fault information, facility useage and individual extension metering records. The remaining 4 Kwords store the database, which is the information describing the basic configuration of the exchange (equipment/directory number translations, location of particular line unit types and facilities available to particular extensions, etc.). In order that this database may be transported to site, 4 Kwords of PROM is also provided on this non-volatile memory PWB to provide an initial database. This is copied into the RAM where, subsequently, it can be updated to meet the ongoing operational needs of the customer.

The heart of the control is the CPU. Apart from the basic microprocessor, a number of other important hardware items are located on the CPU PWB. Three serial data input/output channels are provided for communication with external hardware. One of these channels is available at the front of the CPU PWB, and facilities exist to enable a portable teletype or similar machine to be plugged-in and used to interrogate the exchange. The other two channels are available from the back of the cabinet via sockets on the control shelf assembly, and these are used for data-logging equipment and inter-cabinet communication.

A special logic circuit has been developed to monitor the correct operation of the microprocessor. This circuit, known as the watchdog, expects to receive a signal from the microprocessor every 100 ms. If the watchdog does not receive the appropriate signal when expected, the watchdog forces a warm-start interrupt to be applied to the microprocessor. This warm-start interrupt causes the suspension of the software process running at that' instant and returns control to the operating system software, which activates the scheduling of the next process waiting to be run. The occurrence of a warm start is logged and, in some circumstances, can cause loss of a call. If 13 signals from the microprocessor are missed by the watchdog (each being followed by a warm start) without a 25 s timeout expiring, then a cold-start interrupt is applied. This forces a complete restart of the program which, in turn, clears all calls and working data storage in the exchange before attempting further processing. If a further 3 signals to the watchdog are missed, again without the timeout expiring, then 2 further cold starts are followed by the watchdog halting the processor and forcing the exchange to go into the fallback state. In this latter state, an urgent alarm is produced and a number of designated extensions are connected directly to exchange lines to provide a contingency service.

In addition to the main memory, the CPU is provided with a small memory to enable a wide repertoire of testing of the whole exchange to be carried out with the system off-line (that is, the system is not switching telephone calls). This maintenance and diagnostic program has been provided in addition to program in the main memory that runs maintenance and diagnostic tests on-line (that is, while the system is running in its normal operational mode) and permits fault diagnosis to be carried out even if the main system programs are prevented from running. Finally, the CPU board is provided with a number of switches and a 2-digit hexadecimal display at the front of the PWB. The display is used to indicate those tests which are failing and also the current status of the system; for example, whether the system is running normally or in a restart cycle. Use of the switches enable various operational modes to be invoked and the following facilities are available:

(a) on-line maintenance and diagnostic testing can be inhibited,

(b) the hexadecimal fault display can be cleared and alarms reset,

(d) the watchdog cold-start operation may be inhibited,

and

(e) off-line maintenance and diagnostic testing can be invoked either manually with complete testing and display of results, or by wider interactive teletype control to force particular tests.

LINE INTERFACE CIRCUITS

Since the Monarch 120 system uses a digital switch, it is necessary to provide an analogue digital interface for every analogue line terminating on the exchange. In a fully-equipped unit, 70 % of the cabinet volume is taken up with these circuits, so it is obvious that their most is critical to the viability of the exchange. However, because of the very large number of line interface units required, it is possible to apply modern design techniques to their construction, including the use of custom-designed integrated and thick-film circuits. A block diagram of the extension line interface is shown in Fig. 5, from which it can be seen that the circuitry is divided into two areas: that handling the interfacing of line signalling conditions (including the provision of line-current feeding) and that dealing with the 2-wire-to-4-wire conversion and analogue/digital speech signal processing.

Line current feeding is achieved via an electronic quasiconstant-current circuit; the current varies from 31 mA on a short extension line to 26 mA on a limiting (1200 Ohm ) loop. This technique of current feeding provides for a significant power reduction over the constant-voltage feeds used in conventional transmission bridges. In addition, a much improved balance can be achieved in the 2-wire-to-4-wire converter (because the telephone characteristic of varying its impedance with line current is suppressed) and the reliability of the carbon transmitter is significantly improved.

A miniature transformer provides for the conversion of the balanced AC signal on the line to the unbalanced signal for presentation to the electronic 2-wire-to-4-wire converter. A blocking capacitor is used to prevent DC passing through the transformer, and this design enables a very compact realisation of the transformer to be achieved. Although it does not deal directly with signalling, one very important function implemented in this area of the circuit board is a power-down circuit. This enables power to the 2-wire-to-4-wire and speech signal processing circuitry to be removed when the line is idle and enables further significant power savings, even on relatively-busy PABX installations.

Thick-film technology has been used to fabricate much of the electronics associated with the line-current feed, loop and called-subscriber-answer detectors and power-down circuitry.

Single-in-line packages provide a very-high circuit-packing density on the PWB and the discrete elements are kept to a relatively-small number, thus minimising assembly, testing and repair costs.

A line unit signalling interface (LUSI) chip provides the necessary logic for formatting the various signalling conditions for forward transmission to the CPU; in the reverse direction, it provides static outputs for driving the various applicators of signalling. The LUSI uses an uncommitted logic array (ULA)2, which is provided by a supplier as a custom-designed digital integrated circuit, fabricated from standard gate cells interconnected in accordance to a specification provided by the customer. This procedure enables custom-designed chips to be made for a relatively-Iow cost and without the delays usually associated with this type of development.

The ULA array used in the Monarch 120 system has 225 cells, of which approximately 180 are used in realising the LUST function. Because this chip has to be powered constantly, a low-power circuit with a typical dissipation of 30 mW is used. Two variants of the LUSI chip are used: the extension line interface circuits use a limited range of signalling function and the chip is contained in a 16-pin dual-in-line package; the exchange line circuits use a 24-pin version with access to all 8 signalling bits for each direction of transmission. The Monarch 120 system uses the standard A-law PCM format for handling speech signals. At present, it is necessary for the PCM encoder/decoder (codec) to have low-pass filters associated to cut-off frequencies above 3400 Hz. These filters and the associated 2-wire-to-4-wire conversion circuits have been realized using resistor/capacitor networks, which are fabricated as single-in-line thick-film circuits, and operational amplifiers.

A PCM codec circuit which exploits modern techniques of signal processing has been developed. Analogue-to-digital conversion is achieved using a delta-sigma modulator, followed by digital circuitry which converts from the delta sigma code, at its sample rate of 2·048 MHz, to PCM at the standard 8 kHz sample rate3. A similar process is used in the reverse direction of transmission. The advantage of this technique is that it minimizes the complexity and precision of the analogue circuit at the expense of digital circuitry, this latter being readily realizable using large-scale integration (LSI) technology. The modulator/demodulator element has been realised using a combination of discrete and thick-film techniques and the code-converter function is achieved using a single LSI chip. This LSI chip was originally designed and fabricated at the BPO Research Centre; however, the requirements of the project were such that commercial versions of the chip were required and two semiconductor manufacturers, General Instruments Microelectronics Ltd, and Ferranti Ltd, now supply compatible versions of the device.

Each line interface unit is mounted separately on the PWB (see Fig. 6), although they share common facilities such as power and waveform distribution. The various interfaces provided as 2-port units differ from :the extension-line interface mainly in the way that signalling is handled.

The exchange-line interface uses conventional earth-calling with loop-disconnect impulsing through to the public exchange, although it is expected that MF signalling will be used when this capability becomes available. A variety of inter-PABX signalling interfaces are possible; units capable of handling SSDC5 are now available, and SSDClO, SSACI3 and SSACI5 units are currently under development. A console line interface and MF signalling receivers also plug into 2-port positions.

TRANSMISSION

The Monarch 120 system uses 4-wire transmission within the exchange and, since connexions between extensions and exchange lines must of necessity be of low loss, it has been necessary to pay particular attention to impedance matching at the 2-wire-to-4-wire conversion points to ensure that good stability margins and sidetone performance are achieved. To this end, as has been already outlined, a constant-current line feed is used for the extension telephones to minimize impedance variations with different line lengths. In addition, the input and balance impedances of the 2-wire-to-4-wlre conversion circuits have been designed to best match the particular type of terminal to which they are connected; the design of the 2-wire-to-4-wire converter used in the Monarch 120 system permits these input and balance impedances to be varied independently.

In a 4-wire environment, a circuit designer has control over transmit and receive gains. (Where PCM coding is being used, 'gain' is a rather loose term, being the effective translation between analogue level and digital numbers and vice-versa.) This facility enables different losses to be achieved on different types of connexion within the exchange and, although a relatively-low loss is used on extension-to-exchange calls, a higher loss (typically 6-9 dB) is present on extension-to extension calls. This is an important feature because, traditionally, internal calls on PABXs have been too loud and have poor sidetone performance.

MAINTENANCE AND DIAGNOSTICS

The Monarch 120 system has a sophisticated range of testing and fault reporting facilities. Under normal system operation,

testing is being carried out regularly under the control of a background program. The control area is largely self testing in

that the background program can cause the memory to be tested, and the watchdog on the CPU ensures that the pro

cessor is running in a correct fashion. Testing of the remaining hardware is achieved with the help of a test-line unit, which

occupies a 2-port position within the system.

The test-line unit sends an alternating digital check pattern which is routed through the digital switch and back for test

purposes. This procedure ensures that the central area of the switch is functioning correctly. A similar test is applied to the

signalling-input and signalling-output PWBs, although in this case the CPU initiates a check pattern which is merely turned

around by the test-line unit. The individual line circuits and the associated highways to them are checked by a 400 Hz tone

from the tone generator, which is routed to the line unit under test. This tone is reflected around the 2-wire-to-4-wire con

verter due to the natural unbalance, and it is then routed to the test-line unit for analysis. Only a simple check of the tone

can be done in this way since the level reflected is rather variable; however, this test ensures continuity of the serial

paths within the system and ensures that the given line circuit is at least operational. Failure of any test causes the test-line

unit to output the appropriate message to the CPU.

A signalling test is also carried out for individual extension line units where one of the 8 signalling bits initiated from the

CPU is turned around; thus, by alternating the pattern, a test of signalling path continuity can be carried out.

A proportion of the equipment cannot be tested easily without the addition of further hardware, notably the MF

receivers and the exchange-line circuits. In these areas, a fault may degrade service, but will not necessarily result in a

complaint from a customer. A statistical check of these circuits is therefore carried out to see whether an individual

unit suffers a significantly greater proportion of calls which clear prematurely. At present, this check is only being applied

to the MF receivers but will be extended to the exchange-line circuits in a later release of the software.

If a test fails, a fault-analysis program is run in an attempt to indentify the faulty area of equipment; for example, a group

of line circuits failing on a given shelf can implicate the shelf multiplex. Once this has been done the fault is logged for

subsequent interrogation and the appropriate alarm given, both to the operator console and to an alarm unit (which is

accessed via the test-line unit). Using the man-machine communication facilities, a technician can then interrogate

any faults present on the exchange, either via the operator console or the front of the CPU.

CONCLUSIONS

Development of the Monarch 120 system began 4 years ago and, following extensive trials, the first production units are

now going into service with the BPO. The system is manufactured by Plessey and GEC Private Systems Divisions at

Nottingham and Coventry respectively.

Development of the system is still not complete however, since the ongoing march of technology will permit major

improvements to be achieved in the coming years. In addition, the existence of a digital switching system at the customers' premises which has the same digital coding format as the System X public exchanges will, in the future, enable a

sophisticated range of data facilities to be added. Customers with the Monarch 120 system will, in fact, find they have a

system capable of providing a true business communications centre for the 1980s.

ACKNOWLEDGEMENTS

Acknowledgement is made to colleagues, in both the BPO and Industry, who have contributed to the success of the Monarch 120 project.

References

1 GRIFFITHS, D. J. Big Advance for Small PABX. POT!, Vol. 30, No. I, p. 26, Spring 1978.

2 TONGE, J. D., GAUNT, D. L., and KENDALL, J. P. Programmable Logic and Microprocessors. POEE!, Vol. 70, p. 136, Oct. 1977.

3 EVERARD, J. D. A Single Channel PCM Codec. IEEE Journal of Solid State Circuits, Vol. SC-14, No. I, Feb. 1979.

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