Telecom Information Kit No. 3

THE SWITCHING PLACE

THE STORY OF TELEPHONE EXCHANGES

SECTION 1

FROM TOY TO EXCHANGE-BASED SERVICE

When Alexander Graham Bell invented the telephone in 1876, it was at first greeted sceptically. The head of the telegraph monopoly in the United States, Western Union, labelled it an "electrical toy". But before the end of the century every major city in the world had a telephone service - and the telegraph companies were competing for ownership.

The crucial element in this backflip: the invention of the telephone exchange.

Bell envisages the telephone exchange

Bell himself saw the need for exchanges. The evidence is an early letter to supporters, in which he lists all the key functions of present-day telephone exchanges:

"... cables of telephone wires could be laid underground, or suspended overhead, communicating by branch wires with private dwellings, counting houses, shops, manufactories, etc., etc., uniting them through the main cable with a central office where the wire could be connected as desired, establishing direct communication between any two places in the city...".

Other ideas covered in Bell's letter are curiously familiar: an operator, placed in a central office, to connect telephone wires as needed. A fixed annual rental for the use of wires. And, he pointed out, since all connections would pass through a central office, the connection times for calls could easily be noted - and bills sent out periodically.

Networks vs direct wiring

Now, it is taken for granted that we need only as many telephone lines as there are telephones. But without exchanges, telephones could only operate if they were wired to each other - a situation of mind-boggling impracticality. For example: five telephones could be interconnected using 10 wires - but 1000 telephones would need nearly half a million criss-crossing wires.

NOTE: The connection between a telephone and an exchange is referred to as a line, regardless of the number of wires it includes - at times, as many as five wires (see section 2).

Subscribers - or "subs"

In the early days, callers were known to telephone companies as "subscribers" - people who subscribed to an exchange rather than installing their own lines and telephones. While vestiges are still to be found in acronyms such as STD (Subscriber Trunk Dialling), the term subscriber has now gone out of fashion.

Manual exchanges: the pioneers

The first telephone exchange opened in New Haven, Connecticut, in January 1878. Others - including Melbourne, in August 1880 - were hard on its heels. Also among the first wave were Brisbane (less than two months after Melbourne) and Sydney (1881).

As Bell envisaged, these exchanges were manual. All employed operators - mostly young women - who, on request, connected the caller's line to another line and noted the call (for later billing). In other respects, the exchanges differed, each finding its own way of meeting the requirements of a public exchange.

These requirements included:

- A way for callers to alert the operator

- A way for operators to alert the call recipient

- A simple, practical method for connecting pairs of lines

Some early solutions:

Alerting the operator

When a caller lifted the receiver, an electrical signal was transmitted down the line, causing a small metal shutter on the operator's board to drop down. A quick scan of the board then showed her which lines needed attention.

Alerting the person being called

Many shutters and bells were tried. The eventual choice was a magneto bell, devised by Bell's assistant Thomas Watson.

Connecting the two lines

Again, many systems were tried. Thomas Edison (who also invented the carbon telephone transmitter), devised one of the simplest and best. It had a series of horizontal metal bars, insulated from an equal number of vertical bars.

Each line was connected to one horizontal bar and one vertical bar. At every crosspoint, each bar had a hole. To connect (say) line 3 to line 14, the operator would push a metal peg into the hole where bar 3 crossed bar 14. That allowed the current to flow between the two lines, and the conversation could begin.

Edison's limits were reached as the volume of calls grew. As a single operator could not handle the calls, several switchboards were needed, each connected to a sub-section of the total lines. But that entailed a new problem: how to connect a call on one board to a line on another?

Various solutions were tried, all less than ideal. At around 1000 lines, the thickets of criss-crossing wires resembled a forest and it became evident that the switchboard had to be re-designed.

Patch cords and multiple switchboards

The arrangement that replaced Edison was as follows:

Instead of the system of crossed bars, each line had its own socket. The sockets were arranged in a large grid, and the operator used a patch cord - an insulated wire with a plug at each end - to connect any two of them.

Multiple switchboards replicated this arrangement, and each board could access every line. Since the work could be shared by as many operators as needed, exchanges could have thousands of lines - and hundreds of calls every minute.

While that arrangement solved the traffic problem, another difficulty arose: a socket could be free on one switchboard but busy on another. If the operator were not to interrupt conversations, she needed advance warning of the line's status.

Again, different solutions were devised. At some exchanges, connecting a line on one board took all the corresponding sockets out of circuit. At others, the operator tested the socket and received a status signal (such as a buzz or a light).

Going long-distance

Eventually, exchanges were connected by trunk lines. While it was then fairly simple for operators to connect calls to other exchanges, it was practicable only if long-distance calls remained a small proportion of the traffic. Accordingly, a surcharge was imposed, making trunk calls much more expensive than local calls.

The service tradition

While December 1991 saw the last of Australia's manually operated exchanges (at Wanaaring, N.S.W.), many of the services that grew up around manual exchanges remain. Some, such as time requests, are now Dial-It services. Others, such as call redirection, have become programmable options on modern telephones.

Today, operators still connect some calls - such as overseas calls for customers without access to International Direct Dialling (IDD). And people are still employed for certain service tasks, such as fault reporting and directory assistance. But it is now a century since machines began to share - then take over- the work of connecting local calls.

A wired country

The TV series A Country Practice holds in a time capsule the part played by manual exchanges in Australia's not-so-distant past. "Beverley", the manual operator, is invisible but ever-present. She personally connects every call, knows every inhabitant of Wandin Valley by name, and keeps tabs on the leading characters' movements for the purposes of call redirection. And - when cars crash or children are lost in the bush - she remains on duty, personally masterminding the town's relief effort.

The next chapter concerns that initially controversial replacement of the manual exchange: A.B. Strowger's automatic exchange.

THE SWITCHING PLACE

SECTION 2

STROWGER INVENTS:THE BIRTH OF THE

AUTOMATIC EXCHANGE

The automatic telephone exchange was - improbably enough - invented by an American funeral director called Almon B. Strowger.

The grapes of wrath

Strowger's transition to technological inventor is explained by a story which may well be apocryphal.

When his funeral business - based in Kansas City, Kansas began to flounder, Strowger blamed the local telephone exchange. Callers who gave the operator his name, he alleged, often found themselves talking to the city's only other funeral director.

He laid the blame at the feet of one of the manual operators, his rival's wife, and vowed to find a way to eliminate operators from exchanges. Eventually, he did - almost.

Strowger's collar box

By the late 1880s, Strowger had concocted a model of an automatic exchange. It consisted of:

- A cylinder - made from a collar box

- A spindle - made from a pencil

- A perpendicular arm - made from dressmaking pins.

Strowger's idea was to install a cylinder like this at the telephone exchange. It would be lined with rows of electrical contacts - one contact for each telephone line.

To reach a particular contact, the spindle - manipulated by an apparatus attached to each telephone - would slide in and out of the cylinder. It would then stop and rotate, allowing the arm to sweep along one of the rows of contacts. Finally, it would stop at the one the caller required.

The upshot: callers could connect their line to any other line on the same exchange without the help of a manual operator.

Strowger patents his idea

Strowger's first system, patented in 1891, was a cumbersome affair. Each telephone was connected to the exchange by five wires, of which only one (using an earth return) was for the conversation itself. The other four, connected to four buttons on the telephone, transmitted the number to the exchange.

To dial a number - say 324 - the caller had to press the buttons as follows: Button 1 - three presses, Button 2 - two presses; Button 3 - four presses. To disconnect the call, the caller pressed Button 4.

At the exchange, 1000 electrical contacts (one for each line) were arranged inside a cylinder in ten rows, each with ten groups of ten contacts. The call was connected by operating bi-motional selectors - spindle-and-arm mechanisms like that in the early collar-box model.

With each button press, the selector took another step, as follows:

- Each press of Button 1 moved the selector shaft UP one row.

- Each press of Button 2 rotated the selector arm ALONG the row between one block of ten contacts and the next.

- Each press of Button 3 stepped the selector arm to the NEXT CONTACT inside the selected group.

Finally, at the end of the button presses, the selector arm rested against the contact representing the required number.

Into the marketplace

The first Strowger exchange opened in La Porte, Indiana, in l892. While it worked, it was a less-than-perfect solution. The main problems:

The multiplicity of wires, when skies around telephone exchanges were already dark with wires.

Users' objections to the need to count and remember the number of button presses.

Recognising these difficulties, Strowger kept working on improvements to his system. By l896, he had reduced the wires to three, and replaced the push-button arrangement by a dial.

Eventually, Strowger's system needed only one pair of wires and a metallic return circuit between the telephone and the exchange - to carry both conversation and dialling information. That arrangement worked so well that it is still in use today.

Strowger's telephone dial

While the first telephone dial looks awkward, it works on the same principle as present-day versions. The user, by placing a finger into a numbered slot, rotates the dial until it reaches a stop. The dial is then released - and, while flipping back into its resting position, transmits pulses back to the exchange. These pulses signal the wanted number.

Enhancements to Strowger

But while those improvements solved the two main hitches to full public acceptance, exchanges themselves needed to become more streamlined before they could comfortably assimilate growth.

One impediment: Each line needed its own selector - and for exchanges with more than 1000 lines, that made switching a complex and expensive business.

To solve that problem, Strowger staged the selectors (see below).

Selector staging

Consider a selector that can choose any of ten lines. Originally, such a selector would only suit a 10-line exchange - but selector staging changes the possibilities.

- With a two-stage system, each telephone line has its own primary selector. When the caller dials the first digit, this selector contacts not an outgoing line, but a bank of secondary selectors.

- Dialling the second digit then controls one of these selectors, which can choose between ten groups of ten outgoing lines. The caller's choice now extends to 100 outgoing lines.

-With three stages, the choice extends to 1000 lines; with four, 10,000 lines, and so on.

A difficulty with selector staging was that the number of selectors needed for each line rapidly became impractical. For example, with a three-stage system each line would need one primary, ten secondary and 100 tertiary selectors - a total of 111, of which only three would be used for any particular call.

Strowger's solution was to make all selectors (other than the first) available to every line. The provisos:

- Each stage must have enough selectors to cover peak demand.

- Primary selectors must be able to hunt for idle secondary selectors, bypassing those already in use.

Strowger founded a company which incorporated these ideas, and the company's engineers solved the remaining technical problems. In 1897, a Strowger exchange using two-stage switching opened in Augusta, Georgia.

Three years later, that was topped by one in New Bedford, Massachusetts. It could not only handle 10,000 lines, but incorporated automatic hunting for idle secondary selectors.

Traffic wins the day

Despite Strowger's advances, automatic exchanges were not without problems. The complex equipment used in such exchanges made them notoriously prone to breakdown. Another difficulty despite the invention of the dial telephone - was that some users still objected to setting up their own calls.

Two lobby groups also championed the manual exchange in its own right. One group not sharing Strowger's antipathy to manual operators - appreciated the personalised services provided by manual exchange. The other argued that manual exchanges were more economical. Most telephone operators were women - and women's pay, in those days, was around half that of men's.

But eventually, as telephones became more popular, manual exchanges were simply unable to handle the volume of calls. Automation was on its way.

THE SWITCHING PLACE

SECTION 3

STEP-BY-STEP EXCHANGES: THE PRINCIPLES

Consider a small Australian exchange built on the Strowger model - the type generally known as step-by-step, because each successive dialling of a digit set up another step of the call.

The uniselector: a small, significant change

Invented by American engineer Alexander Keith, the sole task of the uniselector was to hunt for a free bi-motional selector - a task which it carried out with a sweep like a clock arm along a single row of electrical contacts.

While simple in concept, uniselectors gave automatic exchanges a new economic edge: first selectors - complex and expensive pieces of electromechanical equipment - could be placed in a common pool, instead of one being permanently attached to a single line.

Widely used in the early days of automatic telephony, such an exchange - using telephone numbers of only four digits - could handle up to 10,000 lines. And, as early as 1904, switching in such exchanges had been streamlined by the new uniselector.

To understand these early exchanges, let us follow, step by step, the setting up of a call.

The step-by-step process

The call begins when the caller lifts a telephone handset. This action closes a spring-operated switch in the telephone, completing an electrical circuit. The exchange receives the signal that a caller wishes to dial a number.

On receiving the signal, the arm on that line's uniselector begins wiping across a row of contacts. It passes those linked to busy first selectors, stopping only when it finds a contact whose first selector is free.

The caller's line is now connected, through the uniselector, to a selector from the common pool - Selector 1. Special equipment then swings into action, transmitting dial tone down the line. The caller can now dial a number - for example, 4388.

When the caller dials 4, the dial, flipping back to rest, interrupts the current four times. The signal, "four", is transmitted to the exchange.

When these pulses reach Selector 1, its shaft steps up a block of contacts, stopping (in this case) at row 4. Its arm then sweeps across that row, testing each contact in turn. When it finds one with a free second selector, it stops. The caller's line is now connected - through the uniselector and Selector 1 - to

Selector 2.

Selector 2 deals with the block of 100 contacts devoted to numbers beginning with 4. When the signal "three" arrives, the shaft of Selector 2 moves down to row 3, and the arm sweeps across it, hunting for a contact whose selector is free. When that selector - Selector 3 - is found, the hunt ends.

Selector 3 deals with the 100 contacts devoted to numbers beginning with 43. When the signal "eight" arrives, the shaft of Selector 3 steps up to row 8 and sits there, awaiting the last signal. When "eight" arrives, the arm wipes across to contact 8. The call is now connected to the required number - 4388.

The exchange then tests the status of the line. If it is free, equipment to generate the ring tone is connected to the caller's line. At the same time, a current to ring the bell of the called number is transmitted along line 4388. If the receiver is picked up, all equipment EXCEPT the selectors is disconnected from both lines, and the conversation can begin.

If the called line is engaged, the busy tone is transmitted along the caller's line. Indeed, the same happens at earlier stages during the call, whenever a free selector cannot be found - although exchanges typically have enough selectors to make such events rare.

Quick as a flash

While the process of setting up a call takes a long time to describe, it happens in milliseconds. To the caller, there is usually no perceptible delay between picking up the handset and hearing dial tone. similarly, the second selector is chosen in only milliseconds - before the caller begins to dial the second digit.

Modern step-by-step exchanges

To deal with 8 or 10 digit numbers, step-by-step exchanges need many more stages and many more selectors. Nor are calls limited to selectors in the same exchange: long-distance calls may need to pass through several exchanges.

How are such calls connected? Say the number is 555 5678. When 555 (the exchange prefix) is dialled, the caller's line is connected to an outgoing line in another exchange. To make this possible, exchanges are linked by trunk and tandem exchanges.

Step-by-step exchanges were at last count, handling 188,000 lines around Australia in early 1993. They have now been gradually replaced by more complex exchanges, the first of which - crossbar - is discussed in the next section. It is expected that by 1995 there will be no exchanges still using step-by-step selection.

THE SWITCHING PLACE

SECTION 4

FROM COMMON CONTROL TO CROSSBAR EXCHANGE

Any method of automatic switching must have a control system. Its job: to direct the connection and disconnection of lines, and perform other subsidiary tasks, such as checking the status of called lines.

In step-by-step exchanges, control is - in effect - built into the work of selectors. As each digit is dialled, a selector connects a path to another selector, which is then ready to accept the next digit. Then, however long the call, the selectors much remain in place until it is over.

Since bi-motional selectors are complex, expensive and rather fragile, it would plainly be better if they could set up other calls while the first call continued. And for that, the control system needed to be separate from - rather than built into - the switching mechanism.

That is the principle of common control: the basis of all modern exchange systems.

The advantages

Common control has two main advantages:

- The switching mechanisms become simpler and cheaper. While that advantage is partly counter balanced by the need for complex control mechanisms, these are used only briefly.

- Telephone numbers can be more flexibly allocated. In step-by-step exchanges, numbers are "wired in" to the arrangement of selectors. Similarly, in a network of step-by-step exchanges, exchange prefixes are virtually fixed to areas.

While all common control exchanges share these advantages, they apply specifically to a large part if Telecom Australia's switching network: the crossbar exchange.

Early crossbar: Betulander

In 1912, the Swedish engineer Gotthief Betulander patented an automatic switching system based on a grid like that employed by Edison's manual switchboard (described in Section 1). The difference: instead of pushing metal plugs through holes at the crosspoint of vertical and horizontal bars, Betulander's system used relays to make the required connection.

Strowger was, by then, well-established. But Betulander's system had the advantage of avoiding the movement of bi-motional selectors. Indeed, the only movement was when the armatures of the relays moved a few millimetres to bring contacts together - and relays could move much faster than selectors.

The system's major drawback was that - as with Edison manual switchboard - it only suited small exchanges. Because the system was arranged on a square grid, the number of crosspoints (and therefore relays), increased as the square of the number of lines. While ten lines have 100 crosspoints, twenty have 400 - quadruple the number.

A second problem: Betulander's relays, while less expensive than selectors, were still not cheap to make. Since even modest exchanges required huge numbers of them, the early Betulander system was used only for small - usually remote - exchanges, where the primary requirement was running for long periods without maintenance or supervision.

Betulander moves on

But next came a string of developments - and substantial reductions in cost.

- Instead of a single large grid, Betulander interconnected many small grids in a series of switching stages. The same number of lines could then be connected using fewer relays.

- In 1936, a newer and much cheaper kind of relay was developed - the reed relay.

- Electronic control systems, developed after the Second World War, meant that such exchanges could be made large enough to handle the telephone traffic of cities.

Reed relay switching became part of the computer-controlled trunk exchanges introduced into Australia in the 1970s. But that development was separate from the crossbar exchanges introduced into Australia some two decades earlier. These employed the crossbar switch, which - designed by Swedish engineers in the 1930s - was based on Betulander's early principles.

The crossbar switch

Unlike Betulander's grids, which had relays at each crosspoint, the crossbar switch uses electromagnets to control the movement of the bars. Each vertical bar ("vertical") has one electromagnet, while each horizontal bar ("horizontal") has two, which can tilt the bar slightly, either up or down.

The movement of the bars controls the opening and closing of various sets of contacts. Each position of the verticals and horizontals is associated with a unique set of contacts. In crossbar exchanges, switches like this are arranged in stages, allowing many pairs of lines to be connected at the same time.

Example of a crossbar switch

With ten verticals and six horizontals, a crossbar switch can connect any of ten input lines to any of twenty output lines.

The steps are as follows:

- The top horizontal tilts up or down, selecting between two groups of ten output lines.

- One of the five remaining horizontals tilts either up or down to select one line from that group of ten.

- A vertical moves, selecting one of the ten input lines.

- The input and output lines are then connected, remaining to until the vertical is released.

- The horizontals are free to make connections to input lines on other verticals.

STD prefixes: the higher code level

In trunk exchanges, crossbar switching made possible a major innovation: Subscriber Trunk Dialling (STD). By dailling an area code, callers could access a more remote area without the help of a manual operator. The STD code is, in effect, a higher-level exchange prefix - one that connects the caller to trunk lines.

What is a relay?

A relay consists of an electro-magnet (a magnet which attracts only when current is passing through the coil) and an armature (a piece of iron which moves when attracted to the magnet). It is arranged so that when a small current passes through the electro-magnet, the armature moves and makes a contact through which another current can flow.

Exchanges in the human mode

In manual exchanges, the operator's intelligence is a control system separate from the switching mechanism - the switchboard with its plugs and sockets. The operator, alerted to an incoming call:

- Listen to, and remembers, the wanted number

- Finds the right way to connect the caller's line to the line being called

- Checks that the wanted line is free

- Makes the connection.

While the first call continues, the operator can connect other calls. In a common control exchange, the control system also (in a sense) sits to one side of the switching system, and "remembers" the called number before going into action.

THE SWITCHING PLACE

SECTION 5

THE TELEPHONE EXCHANGE:

WIRING AND OTHER ELEMENTS

Exchanges do not operate on switching equipment alone. They rely on a network to transmit the signals.

While part of telephone transmission is now carried out by microwave links and optical fibres, a network consisting of thousands upon thousands of tonnes of copper wires -one pair for each line - still radiates out from each exchange to the telephones it serves.

In the early days of telephony, these copper wires were carried on poles. Later, as telephones grew in popularity, so did the weight of the wires. It became necessary to have derricks on the roofs of exchanges to collect and support the wires.

The wired underground

Now, telephone wires are almost invariably cabled in ducts beneath the streets. From there, they run into the exchange's basement and onto a main distribution frame. The wires are then separated into groups of smaller cables and distributed to the switching equipment.

Even in small exchanges, the wiring is astonishingly complex. From the distribution frame, cables run on overhead grids to the switching equipment rooms. From there, they fan out onto different pieces of switching equipment.

In many older exchanges, particularly in cities, step-by-step equipment sits side by side with crossbar equipment, necessitating arrangements that allow the two types of equipment to work together.

The switching equipment

The switching equipment itself sits in vertical racks, separated by aisles to give the technical staff easy access. Step-by-step equipment makes a continuous sound, with selectors moving as each digit is keyed or dialled. Crossbar equipment is quieter and more compact, with relays and switches kept behind glass panels to shield them from dust.

Electricity

Usually, the electricity for operating this equipment comes from a mains supply, although certain areas (such as the Nullarbor) make use of solar power. To operate the exchange's electromagnets and relays, the current must be converted from AC (alternating current) to DC (direct current).

Each exchange has a battery room, which is full of large batteries of the flooded lead-acid or nickel-cadmium type. Mains power keeps them charged, and direct current is drawn from them to operate the exchange. The exchange also supplies direct current to all the telephones it serves.

In a power blackout, while other services stop, telephones operate as usual. The reason is that batteries supply the current for the exchange until they are discharged. Most large exchanges have automatic generators which, triggered by a mains supply failure, keep the exchange batteries charged.

Metering calls: the rationale

Australian telephone bills have two parts: a rental charge, and a charge for each call.

Rental covers the cost of maintenance - of the telephone, the exchange and the network.

Call charges mean that the heaviest users of the system pay the most.

To calculate this second component of the bill, all telephone calls are metered.

The meters are small and resemble water meters. They sit - one for each line - in a meter rack. Every time there is a successful call from that line (that is, the phone rings and the call is answered), the meter registers the call.

For local calls, the meter clicks only once at the end of the call. But for STD or IDD calls, it clicks at a rate that depends on several factors: the distance of the call, the time of day, and the day of the week. For the most expensive IDD calls, the meter clicks every few seconds.

To avoid errors, meters are not just checked and the results recorded, but photographed in blocks with a special camera. The negatives produced this way are then read, and the results entered onto a computer.

Computerised exchanges to not have meters as each call is measured by a special computer program and charged against the caller.

With computerised exchanges (described in Section 6), bills provide full details of STD, IDD and 0055 calls.

Fault testing and checking

To keep exchanges running efficiently, switching equipment is routinely tested for faults - usually late at night, when telephone traffic is at its quietest. When any faulty equipment is found, it is isolated and repaired.

Step-by-step equipment, as previously discussed, is the most prone to failure. Crossbar equipment is more reliable, and computer equipment the most reliable of all. But Telecom Australia still employs technical staff to make regular checks of all exchanges and (according to a fixed cycle) to replace components.

Meters are also regularly checked for accuracy. All customers' accounts are monitored and, if a customers' account is outside its normal range, special checks are made.

Security

A moment's reflection will reveal the central role of telecommunications in every aspect of the nation's functioning, and the immense problems that would result if a city exchange were attacked by - say - terrorists.

Consequently, all exchanges now have a full panoply of security measures, from guards and closed-circuit television to identity cards and computer-coded mil keys.

THE SWITCHING PLACE

SECTION 6

COMPUTERISED EXCHANGES

In computerised exchanges, software carries out the tasks once performed physically by electro-mechanical devices. Such exchanges are sometimes called Stored Program Control (SPC) exchanges.

Not only computerised exchanges, but the computer itself, have very recent origins.

The origins of computers

Computers developed towards the end of World War 2. Their key task was code-breaking.

Gigantic by today's standards, these computers had thousands of large valves - the type used in old-style radios. Valve breakdowns were common, and - to program the computer - it was necessary to re-connect the computer's complicated wiring.

That changed in 1948, when researchers at Bell Telephone Laboratories invented the transistor. The computer's valves - bulky, hot and inefficient - were replaced by compact solid state devices. Computers became faster, cheaper, and more reliable.

Modern computers have many attributes which make them suitable for masterminding telephone exchanges. They can:

Perform multiple mathematical operations in a matter of milliseconds

Accept signals from the outside world

Analyse the signals and send back control signals.

Miniaturisation in electronics also means that the components which carry out these functions can fit onto silicon chips. Exchanges have become - potentially, at least - less a matter of huge buildings located in a particular place, but software able to be located almost anywhere.

Australian telephone switching first made use of these attributes in the mid-1970s, when computers were added to major trunk exchanges in Sydney, Melbourne and Adelaide. Known as Metaconta 10C, these exchanges used reed relay crosspoint switches to route calls, and - for manual trunk operators - provided much more sophisticated facilities.

The crossbar hybrid: ARE-11

Some crossbar exchanges have also been updated by the addition of electronic equipment. The registers, and some of the markers, are replaced by electronic components on printed circuit boards. A small central computer, dividing its time between them, controls their operations.

ARF crossbar exchanges which have been modified in this way are called ARE-11 exchanges. First installed in Australia in 1977, ARE-11 exchanges work better than the original crossbar exchanges and provide some of the telephone services available on AXE exchanges (see below).

Now a major part of the switching network, ARE-11 exchanges are likely to remain so well into the next century.

The fully electronic exchange: AXE

The first fully dedicated computer-controlled exchange in Australia opened at Melbourne's Endeavour Hills in 1981. Designed by Ericsson Communications Pty Ltd, it was the first of many AXE exchanges.

AXE exchanges have numerous benefits:

Because the equipment makes extensive use of printed circuit boards, they are easy to install, repair or replace, and have lower maintenance costs.

Exchanges can be much smaller, while at the same time having a much greater capacity.

Telephone facilities flourish - including abbreviated dialling, automatic call re-routing, and do- not-disturb.

The network becomes easier to manage, and can be used more efficiently.

Telephone accounts can provide detailed billing.

While the Endeavour Hills exchange had a reed-relay crosspoint switching system, later AXE exchanges have full digital switching in their group selector stages.

Digital switching

In digital switching - unlike step-by-step or crossbar switching - the lines of the two people speaking are not continuously connected. Rather, while a call is passing though a digital switching exchange, the two lines are linked for brief flashes, thousands of times a second.

How does this work? The explanation involves a new kind of telephone transmission: time division multiplexing using pulse code modulation. The principle - despite the jargon - is simple.

NOTE: These terms are explained more fully in Telecom's Information Kit No. I, "From Dots to Data".

Instead of transmitting a continuously varying signal, as traditional systems do, the new method samples the conversation (or other transmitted sound) 8000 times a second. The sound at the instant of sampling is then transmitted digitally - that is, as a binary number in a series of on-or-off pulses.

Mostly, only signals sent between exchanges are digitally converted. Signals between a telephone and a local exchanges are still transmitted in the traditional way - and that is likely to remain so for some time to come.

Computerised mix-and-match

Thanks to the speed of modern computers, a typical digital transmission system can mix thirty telephone calls together. Speech samples, taken from each line in turn, are transmitted one after the other - but the system still returns to the first call within 1/8000th of a second.

At digital exchanges, a specialised computer:

Accepts information from many incoming digital calls

Sorts out where each speech sample needs to go

Briefly sets up a connection path

Sends the sample to the correct outgoing call

Proceeds to the next sample.

When a caller dials a number, a computer calculates the internal operations needed for sending a sample from the caller's line to the line being called. Numbers representing that information are then stored in one or more control stores (or memories) - along with control information for hundreds of other calls.

Queuing and retrieval

When the speech sample comes in, the numbers that represent it are queued in a speech store. But rather than "first in first out", the order in which samples are read out of the speech store, once it is full, depends on the information in the control store.

This, re-ordering of the samples can be thought of as a switching process that functions in time rather than space. For an exchange handling many lines, there will be several such time switches - an internal computer version of the reed relay crosspoint switch.

Remember: to the computer, the speech samples are just numbers. They can be stored, switched about and manipulated, in exactly the same way as ordinary calculations.

Once a sample has been read from the incoming speech store and passed (through a space switch) to an outgoing speech store, another line is sampled. When 512 samples have been transmitted, the computer returns to the first line.

Samples are taken and switched from a particular line 8000 times a second - and users have no idea that their conversation is being carried on intermittently.

All this is far removed from simple switchboards where operators pushed plugs in and out of sockets - a system that, as explained in Section 1, ended as recently as 1991.

Telecom Australia

Education Development Unit

July 1993