No. 39 PAGE No. 14

E. C. DYSON - Circuit Development Engineering Department
JULY 1959

A technical description of an extensible P.A.B.X. with a cordless manual board appeared in Bulletin No. 33, July 1956. The following article describes a recently developed version of this P.A.B.X. employing floor pattern switchboards with press-button controls. The facilities provided conform to the requirements far use in the United Kingdom and the system has received British Post Office approval.

P.A.B.X. type ET.4 is also suitable for overseas Administrations due to the wide range of facilities available.

In a manual private branch exchange the onus of restricting the service given to an extension line is vested in the operator, but a P.A.B.X. allows any extension to be provided with service facilities suited to the status of the user. P.A.B.X. type ET.4 gives complete flexibility so that every extension can be given any of the following facilities by adding or removing wire straps on the extension line circuit connection strips.

Fig 1
Typical P.A.B.X. ET4 Trunking

1. Barred Access to or from the Public Exchange
Extensions in this category are unable to be connected to the public exchange under any circumstances whatsoever, but calls to and from other extensions can be dialled in the normal manner. Access to private wires and the P.A.B.X. manual board can be permitted or barred in accordance with the customer’s wishes.
This classification is sometimes required for extension telephones accessible to the general public where stringent measures are necessary to prevent fraudulent exchange calls. In some countries, a reduced rental is charged for extensions in this category.

2. Barred Direct Access to the Public Exchange
These extensions have all internal services, but exchange calls must be extended via the operator or transferred from another extension.

3. Direct Access for Public Exchange Local Calls
A considerable proportion of lines usually fall into this category which provides all the facilities of (2) above, with the addition of direct access to the public exchange by dialling ‘ 9’. The barring of trunk calls is effected by segregation of lines in the public exchange or, alternatively, by the provision of equipment in the P.A.B.X. exchange line circuit which checks the code dialled. The call is disconnected if the code is barred and ‘number unobtainable’ tone is transmitted to the caller.

4. Direct Access for Public Exchange Local and Trunk Calls
In this case the user has all the facilities of (3) and is not prevented from dialling trunk calls or the public exchange operator.

5. Priority Extensions
This classification is usually reserved for Executives. Such extensions have all the facilities of the preceding classification and the ability to intrude into existing extension-to-extension calls, but not into public exchange calls. If the need to contact an extension engaged on an exchange call is sufficiently urgent, the P.A.B.X. operator must be called and she will either pass a message or request the engaged extension to clear. When specially required, it is possible to arrange for an extension to intrude into exchange line calls, but this facility is not usually permitted by the British Post Office.

Fig 2A
Operator connected to Exchange
Fig 2B
Operator connected to Extension (Exchange held)
Fig 2C
Extension connected to Exchange

The manner in which traffic is routed is shown in Fig. 1. Conventional Strowger switching via group and final selectors is used for extension-to-extension traffic exchange line calls are routed to final selectors via transfer finders and call back group selectors. These selectors are taken into use only when the operator actually extends a call to the local extension.
The connecting circuits are of a type which replace conventional cord circuits and they serve as control links between the operator and the relay sets.

Fig. 2A shows the condition existing when the operator is answering or originating an exchange line call. When the call is to be extended, her speech circuit is diverted to the call back selector and, as illustrated in Fig. 2B, by using the key-sender or dial, she may establish contact with an extension. In this condition a lamp, designated ’internal‘, glows on the switchboard. Since the speech path between the exchange line and the extension is split, the operator has the benefit of both lamp and tone supervision over the internal call, and neither tones nor speech between operator and extension can be overheard by the public exchange.

The next stage is shown in Fig. 2C where the extension is connected to the exchange line. Normally the connecting circuit is released at this juncture but if the operator so desires, she may momentarily operate a ‘hold’ button and the connecting circuit will remain held for the duration of the call. This is shown by dotted lines. Outgoing trunk calls may then be timed, and on exchanges provided with Subscriber Trunk Dialling (S.T.D.) facilities, the call charge can be ascertained from a connecting circuit meter on the switchboard. Extensions can still transfer exchange line calls even though they may be held, and the operator receives an indication to enable her to allocate call charges appropriately.

The operator may monitor the circuit at any time but she cannot effect control without consent of the extension, and her presence on the line is indicated by a rhythmic 400-cycle intrusion tone. Battery feed to the extension and, for all practical purposes, the transmission level of the call are unaffected.

Two other circuits shown on the Trunking Diagram, Fig. 1, may be mentioned. Firstly, the O/G Auto relay set which enables operators to contact extensions and secondly, the Enquiry relay sets which carry traffic in the reverse direction and are mainly used for booking purposes. Unless specially requested, provision is not made for completing calls over the Enquiry relay sets on a demand basis.

On the previous P.A.B.X., small desk type switchboards were used. Due to the greater quantity of face equipment necessary on the ET.4 version, a console switchboard (including provision for eight connecting circuits) as shown in Figs. 3, 4 and 5 has been adopted.

Fig 3
Manual Switchboard
Fig 4
Switchboard Control Panel

The console is constructed of light oak and stands on a plinth faced with black Warerite. The writing shelf (finished in grey Warerite), provides ample space for tickets or writing pads and an extension of the side members separates adjacent positions to prevent papers falling to the floor or being inadvertently pushed on to neighbouring positions. This extension can be provided on cordless positions because operators are not required to reach over to other positions.

A matt dark green finish is normally applied to the steel control and display panel, which is shaped to raise the most frequently used buttons to a convenient level for operation.

The control panel is hinged so that all maintenance work can be undertaken from the front, thereby permitting positions to be installed either side by side or back to back. This freedom of location means that switchboards may be positioned singly, in groups or even backed against a wall. The possible saving of space over cord equipment which requires cable turning sections, or a straight line layout for the benefit of the multiple, with freedom of access to the rear, can be appreciable. Apparatus racks for cordless systems frequently occupy more space than the equivalent racks for cord systems, but an overall saving is usually effected because fewer manual positions are required and less space is required per position.

There are no relays accommodated within the switchboard; only components for the operator’s telephone circuit and an alarm buzzer, thus avoiding the need for frequent maintenance.

Fig 5
Switchboard control panel raised

The Waiting Calls meter shown in Fig. 4 registers the number of incoming exchange calls awaiting answer although it can, in addition, be arranged to indicate other types of traffic. This meter is positioned on the right-hand side of the switchboard and is easily visible from an adjacent position, but if necessary, up to about 15 meters can be connected without circuit modifications. The standard calibration registers from 0 to 10 waiting calls and the circuit adequately compensates for variations in exchange battery voltage.

The indicators in the centre of the switchboard panel, when illuminated, display bold letters and figures 1 in. (25 mm.) in height. The operator brings the indicators into use whenever she wishes to identify a private wire, exchange line, enquiry or
other circuit connected to the banks of the connecting circuit finders. For example, an operator engaged on an exchange line might momentarily press the ‘identify’ button and a display of ‘E.21‘ would appear if she were dealing with Exchange Line 21. To cancel the display, the ‘display cancel’ button is momentarily pressed.

Detailed layout of the keys can be studied more easily from Fig. 4 although the key colouring, an important aspect, is not apparent.

The buttons in the centre are white and, with associated lamps, are arranged in eight vertical columns serving eight connecting circuits. Black denotes buttons common to the position, and includes the keysender controls on the right and the bottom horizontal row on the left of the connecting circuits. Above this latter row will be found fourteen ‘class of call’ buttons used when answering or originating calls.

An incoming call will be signalled by the glowing of a red lamp, there being a different red lamp for each class of call, such as ‘enquiry‘, ‘incoming exchange‘, ‘operator recall‘, etc., and the operator answers the selected class of call by momentarily pressing the red button immediately below the red lamp. To originate an outgoing call the operator presses a white button associated with the appropriate circuit group. Should all circuits in the required group be engaged, a white lamp glows to warn that operation of the button will be ineffective.

As stated earlier, all control buttons including ‘speak’ are non-locking. The circuit arrangements leave the operator’s telephone and the position controls switched to the connecting circuit associated with the ‘speak’ button which was last pressed. The ‘live‘ circuit is indicated by an amber ‘speak’ lamp adjacent to the button and depression of a second ‘speak’ button automatically restores the one previously operated. To effect release during light traffic periods a common ‘restore’ button is provided.

A cord switchboard operator usually has a plug in her hand whilst awaiting the next call. The equivalent to this in the ET.4 system is to have already pressed a ‘speak’ button so that when an ‘exchange call’ lamp glows all the operator has to do is to press the corresponding red ‘answer’ button.

A uniselector associated with the connecting circuit then hunts for the calling line and the connecting circuit ‘busy’ lamp glows on seizure. Next, the operator ascertains the caller’s requirements and if the call is to be extended she presses a ‘transfer’ button to seize a call back selector and switch the connection into the condition shown in Fig. 2B. The wanted extension number is now keyed up and when the necessary digits are stored (indicated by the glowing of the blue ‘send’ lamp) the operator’s duties are normally complete. She is free to attend to the next call whilst the establishment of connection between the exchange and extension, and the ultimate release take place automatically.

If she wishes to introduce the call, the operator remains in circuit and the answering extension is prevented from connection with the exchange line until the operator withdraws.

Should the wanted extension be engaged, the internal switching train need not be released. A ‘park on busy’ facility ensures that the call will mature in the usual manner when the extension does become free. However, the connecting circuit supervisory lamp flashes when a busy line is encountered, and the rate of flashing indicates whether the extension is engaged on an exchange or internal call. Thus operators are restrained from offering public exchange calls to extensions already so engaged. The operator uses her discretion as to whether or not intrusion is warranted and if she decides to intrude she keys ‘0 ‘. A speech circuit with intrusion tone superimposed is then completed between the three parties and the operator may request the extensions to clear.

The wanted extension is re-rung immediately it becomes disengaged and is connected to the incoming exchange line when the handset is again removed.

If the call has not been established and the operator wishes to ring another extension, she operates the ‘transfer’ button twice and then keys the second extension number. The first operation releases the first call back selector and restores her telephone circuit to the exchange line whilst the second operation seizes a second call back selector. The ‘internal’ lamp indicates the routing of the circuit at any instant.

Should the called extension be barred from exchange service, the operator can converse but transfer is impossible. During the conversation a ticker signal warns that transfer is not permissible and the operator’s supervisory lamp flashes as a further warning.

When an extension cannot be obtained immediately in either ‘busy’ or ‘no reply’ circumstances, the operator can advise the public exchange party of the situation on pressing the ‘refer back’ button. This diverts her telephone circuit to the exchange line without releasing the internal switch train.

On all types of call it is fundamental that the operator speaks to one party or the other, but never to both, unless her presence on the line is accompanied by intrusion tone. Thus, the system is inherently secret.

Extensions engaged on exchange lines may call other extensions or the operator without releasing the exchange line. One depression of the instrument transfer button causes a second call back selector to be seized and dial tone returned. The extension may now dial the appropriate number to obtain the necessary information. A further depression of the transfer button restores the connection to the exchange line, releasing the internal connection. But if the first extension should wish to transfer the exchange line to the second extension he should notify the latter of his intention and replace his receiver. The call is transferred automatically and, if necessary, may be transferred again to other extensions.

Busy tone is returned if a number dialled on an enquiry call is busy but, unlike an ordinary internal call, busy tone will change to ring tone and the call will mature when the called line becomes free. A certain amount of repeated dialling of busy numbers is thus avoided, although the originator of the enquiry call must use his discretion and not wait unduly long.

When the called line is engaged on an internal call, priority extensions can intrude by dialling ‘0’. They will realize that the call is internal by listening to the cadence of busy tone which is doubled for exchange calls. Some Administrations also allow
priority extensions to gain access to extension lines employed on an exchange call and this facility can be incorporated when required.

Operator recall is a term covering a class of inward call to the operator from extensions engaged on exchange lines or private wires. It includes not only enquiry calls to the operator but also calls diverted to the operator by reason of mis-operation on the part of the extensions. With the previous cordless P.A.B.X., mis-operation, such as clearing from a call-back to an extension by replacing the handset, resulted in recall of the erring extension. On this P.A.B.X. such calls are routed to the operator who is thus brought into circuit by an extension:-

  1. dialling ‘0’ on an enquiry call,
  2. pressing his ‘transfer’ button and immediately replacing the handset,
  3. attempting to transfer a call to a barred extension, or to an extension who also has replaced his receiver, or under other abnormal conditions.

Usually (b) is effected purposely; the extension notifies the other party of his intention, presses the button and replaces the handset, thus avoiding the need to wait until the operator answers. Under both (b) and (c) conditions an operator answering the call is connected to the public exchange.

In case (a), however, the operator is connected to the extension, who still has control of the call and may press his ‘transfer’ button to revert to the exchange line or replace his handset to transfer the call to the operator. If he reverts to the exchange line, intrusion tone will be applied to the circuit until the operator clears.

From the extension user’s point of view, the recall procedure is unaffected whether the connecting circuit is held or not, but operation differs on the manual positions. When the call is not held, the recall produces a general signal which may be answered by any operator, whereas, if the call is held on a connecting circuit, that operator alone is signalled by rapid flashing of the relevant supervisory lamp. In both cases an audible alarm can be given.

Extensions barred direct access book exchange calls over the ‘0’ level enquiry circuits; the operator records the requirements and recalls the extension after having obtained the distant subscriber. It is not usually desirable for extensions to wait on enquiry circuits whilst the operator completes calls on demand, since the practice leads to unnecessary waste of the extension user’s time. Nevertheless, reversion (i.e. the re-ringing of the extension after having seized an exchange line) can be immediate even to the extent of transferring the exchange line to the extension before dialling and allowing him to dial his own number. However, the general procedure will be for the operator to press the appropriate outgoing ‘class of call’ button, whereupon she will receive dial tone and then key or dial the public exchange number. Keysenders may be equipped to meet only the extension numbering scheme or provided with up to 10 digits capacity to cater for national numbers. When the distant subscriber has answered, the operator presses her ‘transfer’ button, keys the extension number and normally retires from the circuit, the procedure being identical for incoming and outgoing calls.

Since the manual positions are identical, any position can deal with any type of call and concentration occurs automatically as positions are vacated in periods of light traffic.

When the last operator leaves the switchboard the removal of the headset plug automatically switches the exchange into the night service mode of operation.

Night service in a cordless P.A.B.X. with transfer facilities can allow a very much improved degree of service over conventional equipment, because the answering extensions have normal P.A.B.X. facilities when not dealing with incoming calls and can transfer incoming calls to the appropriate extension.

The ET4 system permits alternative schemes. Firstly, Class I night service in which any extension may be connected to receive the incoming calls from exchange lines and, if required, calls from private wires or enquiry circuits. All traffic may be routed to one telephone or divided between a number of telephones and, when essential, individual exchange lines may be connected to separate extensions.

Secondly, in the Class II scheme, conveniently situated alarm bells are rung by incoming calls which may be answered by any non-restricted extension dialling a code digit, usually ‘8 ‘.

In both schemes, extension lines used for night service answering can be provided with facilities for intruding into established calls during night service periods when extending incoming calls. They are thus enabled to cope with a considerable volume of traffic and moreover are warned by a suitable tone when further incoming calls await attention.

In this article it is impossible to cover adequately all variations and extra facilities which can be provided but several of the more outstanding features may be mentioned.

Cyclometer type meters with facilities for manual re-setting can be provided on the console positions to register S.T.D. meter pulses, thus enabling individual call charges to be obtained. Meters can also be provided to give a totals indication on each exchange line or may even be fitted on individual extension lines.

Manual extensions arranged to call the switchboard directly for call booking and similar purposes can be provided. Such extensions also have access to the automatic equipment to enable the user to originate extension-to-extension calls and obtain service when the manual board is not staffed. Where the number of manual extensions required is greater than the number of buttons which can be accommodated on the positions, group calling is adopted and a lamp indicator displaying the extension number glows for 3 to 6 seconds when a call is answered.

It is also possible to provide a similar arrangement for answering exchange lines and other services. One common answer button then replaces a number of class of call’ buttons and when the call is answered the class of call is displayed.

The equipment design follows established B.P.O. 2000-type practice. Dimensions of the racks are normally 7 ft. 9 ins. x 4 ft. 6 in. (236 cm. x 137 cm.), although racks of a different height can be supplied to suit building requirements.

Fig 6
Rack for 100 extension lines
Fig 7
Group selector rack with ringing machine
Fig 7
Exchange line and position equipment rack

Rack layout generally conforms to a pattern utilizing three basic rack types, the line rack, group selector rack and relay set rack; typical examples of each type of 7 ft. 9 in. (236 cm.) rack are depicted in Figs. 6, 7 and 8 respectively.

The line rack accommodates 100 extension line circuits, 20 linefinders and 20 final selectors together with connection strips providing for numbering flexibility and extension classification strapping.

At the bottom of the first group selector rack, two battery driven ringing machines can be accommodated. When two machines are fitted, either can serve the exchange and arrangements are provided to change over automatically to the idle machine in the event of failure. The first group selector rack also accommodates the pulse relay sets and these too can be supplied in duplicate with changeover facilities which operate if the pulse uniselector should cease to rotate with a start condition applied. In small installations all alarms have an auxiliary centralized display on this rack but in larger exchanges a fault supervision panel with writing desk can be supplied.

The remaining space on group selector racks is occupied by shelves of call back and local group selectors of which the appropriate quantities are supplied to meet the requirements of individual exchanges.

In designing P.A.B.X. equipment, it is always a problem to attain a satisfactory basis of provision between exchange lines, private wires, extensions and manual positions because the ratios of these circuits vary so much from one installation to another. The system finally adopted allows extension line capacity to be extended without regard to other circuits, and group selector racks to be added in accordance with traffic. With the Exchange Line R/S and Position Circuit racks however, the layout is based on an assumption regarding the number of exchange lines and private wires which an operator can handle. This is dependent on the business conducted by the P.A.B.X. user, the extent to which an operator is expected to answer callers’ queries, and the number of difficult time-consuming calls which an operator must originate on behalf of her extensions. Under optimum conditions an operator may be able to handle 20 exchange lines but in a P.A.B.X. of this size a second position is nearly always specified.

Hence, the basis finally adopted for equipment is a maximum of 15 lines for the first position and an average of 25 lines per two positions in bigger installations where by virtue of the larger circuit groups the traffic per circuit is higher. The Exchange Line and Position rack illustrated has capacity for all relay sets associated with one position (including eight connecting circuits) together with 15 exchange lines and miscellaneous circuits such as night service, O/G Auto and ‘0’ Level relay sets.

Supplementary relay set racks may sometimes be needed for Direct Access O/G Only exchange lines, which add little to the operator’s load and can be provided without regard to the basis of provision mentioned above. Auxiliary private wire relay sets and junction relay sets to satellite exchanges would also be provided on these additional relay set racks.

It will be seen that the ET.4 is a flexible P.A.B.X. system which, although based on well-established principles, provides a number of new facilities, including provision for Subscriber Trunk Dialling.

The new facilities promote speedy and efficient service and the many additional features which can be incorporated should make the system suitable for most Administrations.

Click here for the GPO P.A.B.X. No.4

Click here for more information on the ET P.A.B.X 4


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Last revised: July 20, 2003