4. Automatic Exchange Implementation

Automatic Telephone Exchanges

The required features of any automatic switching system are as follows :


Performed by
To detect that a caller has lifted his handsetSubscriber Line Circuit
To 'busy' his line so that he is not interruptedSubscriber Line Circuit
To allocate equipment to the caller, if availableLinefinder & Allotter
To indicate to that the caller that he may proceed with diallingDial Tone
To accept digits from the caller and route accordinglyGroup Selectors
To connect the call through to the appropriate subscriberFinal Selector
To either return Busy Tone if busy or apply ring signal to the called party's phone
and ring tone to caller and then cease ringing when the called party answers
Final Selector
& Ring Generator
To detect the answering of the call and register it against the caller's accountMetering Circuits
To alert engineers in case of faultPG/CSH Alarms etc.
Figure 4 - Facilities of an Automatic Exchange

Subscriber's Line Circuit

Every subscriber is connected to his local exchange by one pair of wires; this single pair carries the voice in both directions and the ring current to ring the bell when a call is received. Within the subscriber's premises, in the UK, the line is actually then split into three wires to allow for an anti-tinkling circuit but it's important to bear in mind that only two wires run from exchange to subscriber (known as a & b). Once at the exchange, an additional one or two wires is added to the line. These are used for internal signalling are known as P (or Private) and M (or Meter).

At the exchange, every subscriber's line terminates into its own Subscribers Line Circuit (SLC). This consists of a pair of relays dedicated to that subscriber; if there are 1000 subscribers on that exchange, then there are 1000 SLCs. All other equipment onwards in the chain is shared between all subscribers - otherwise if there were 1000 of everything the cost and size of an exchange would be astronomical and wasteful. It would only be necessary if it was expected that every subscriber was going to place a call at exactly the same time, and that would never happen. As such, when an exchange is designed, cosndideration is given to the maximum amount of traffic that would ever need to be carried at one time, and equipment is therefore provided to allow for that. Overflow meters are also installed so that if it can be seen that all available equipment is regularly in use, additional capacity can be added. Although SLCs normally consist of two relays (called 'LR' & 'K' or 'L' & 'K'), on unattended exchanges (ones without an engineer on-site 24 hours) a third relay (Called 'P') was provided to 'park' the line should a fault occur, to avoid tieing up common equipment for a long period.

Line Finder & Allotter

When the subscriber lifts his handset, current start to flow on the line; this is detected by the SLC. As you will recall, dialling of the digits causes selectors to step up or round the corresponding number of pulses. As there are many subscribers, but only a few selectors, there has to be a method for (1) Finding a free (available) selector and (2) Connecting the calling subscriber to that selector. Step (1) is done by the Allotter. Step (2) is done by the Linefinder.

Local Routing
Figure 5 - Simplified Routing of a Local Call

Although the linefinder is shown looking like a Uniselector in the diagram it is in fact normally a two-motion selector which means that it can serve up to 100 or 200 subscribers. The Allotter, on the other hand is ususally a uniselector, with 25 or 50 outlets, thus allowing access to 25 or 50 first group selectors. n.b. the Subscriber's Line Circuits are not shown in the diagram, but would be in the line between the subscribers' telephones and the linefinders. Another function of the SLC is to 'mark' the caller's line as 'busy' so that incoming calls will detect that the line is in use. A fully working simple Strowger type exchange is shown in the next section.


With the introduction of automatic exchanges, the need arose for automatic charging. Every subscriber was allocated a digit counter (meter) in the exchange. This consisted of an electromagnet which closed with every metering 'pulse'. The electormagnet's armature drove a set of numerical decimal cams (just like a car trip counter). For every meter pulse, the meter clicked one unit. At the end of each billing period, a photographer would take a photograph of all subscribers' meters. These photographs were then sent to the billing department to be read, and bills sent out accordingly. Meters typically had four or five digits, wrapping round to '0000' after '9999'.

The first meter pulse was generated as soon at the called subscriber answers the call. This pulse is generated by the final selector and sent back down the chain of selectors to the caller's meter to register one 'unit'. For local calls, a piece of equipment called a Local Call Timer (LCT) was in circuit between the final selector and previous group selectors. After the initial 'answering' pulse, the LCT starts rotating, clicking round once from each timed pulse. These timed pulses are generated by the Ring Generator. After 10 clicks round, the LCT is back to it's original position and if the call is still active, it sends another metering pulse, and another unit is charged to the caller. The LCT only stops rotating when the calling party clears. If the called party hangs up, then metering continues (See 'CSH', later).

For non local calls, a system of multi-metering was introduced. A meter pulse generator produced different pulse rates and the appropriate one of these would be applied to the call timer depending on the destination of the call.


In order to facilitate efficient operation of an exchange, all Strowger type exchanges are fitted with a number of alarms to alert the engineer to any problems. Some alarms indicate equipment failure, whereas other alarms just indicate unusual operation which might be cause for concern. The most important alarms are as follows :

  • Permanent Glow (PG) : This alarm indicated that a subscriber's phone was offhook and that a call was not in progress. This is not necessarily a fault; a subscriber could have just forgotten to replace their handset properly or deliberately taking their phone off the hook to avoid calls. Doing this meant that the subscriber could be holding onto exchange equipment, thus preventing other subscribers from using it. If enough subscribers left their phone off the hook, no-one else could make any calls. If an engineer spotted a PG alarm, he would work out which selector it was one and release that selector from the subscriber. In unattended rural exchanges(where no engineer was permanently on-site) the subscribers line circuit differed slightly in that a 'parking' relay was provided so that a PG condition was automatically cleared. The term 'Permanent Glow' originates from the days of manual switchboards when a subscriber being off hook would be shown by his light 'glowing' on the board.

  • Called Subscriber Held (CSH) : This condition occurs when the called party hangs up his phone but the caller still remains active. A CSH alarm is not harmful, and is common - for instance, if someone puts the phone down to go and take the call in another room, a CSH condition will occur whilst their phone is back on the hook. As soon as a CSH condition is detected, a timer starts. If a period of (say) 3 minutes elapses without the called subscriber picking their handset up again then the call is cleared down and charging ceases.
  • Release Alarm (RA) : The release alarm is the important one ! The earth supply to most selector electomagnets is connected via the release alarm. As these electromagnets are intended to operate for brief moments only (to drive the selectors), then if current is drawn for more than a few seconds, then it is assumed that the selector has jammed. In order to avoud burning out the selector, the PG alarm is raised, drawing the engineers attention to the particular rack at fault and he can rush round and clear it. In an unattended exchange, circuitry is provided to automatically lower the current to the faulty selector in order to stop it burning out until the engineer arrives.

Details of Michael's Strowger Demo Unit !
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