Manual and Automatic Switching

8/2/2019 Manual and Automatic Switching

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MANUALAND AUTOMATICSWITCHING

Yoga -- Lanre -- Dotun -- Shahab -- Nijin


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INTRODUCTION

Earliest Telephone Provides two way wiredcommunication between two remote locations.

Limitation: fixed end locations.

To overcome the limitation, each telephone hadlines connected to multiple other telephones

Problemit wasnt practical as the use oftelephone expanded

Solution telephone switching

Image from http://lcweb2.loc.gov


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INTRODUCTION

Telephone switching - a system that connects acaller to a callee when a call is made.

Telephones connected to a central hub.

Central hubs role connect a caller to a callee

Image from http://uva.ulb.ac.be


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INTRODUCTION

Early model Manual switching, in 1878.

Electromechanical exchanges (Strowger exchanges)replaced manual exchanges in early 1990s.

The last Strowger exchange in UK was replaced in June

1995.

Image from http://www.earlypics.com/ Image from http://www.samhallas.co.uk


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MANUAL SWITCHING

Switchboard Exchange

Switchboard consist of subscriber's telephone linelocal termination

Operated manually by actual (women) workers

Image from http://www.portfolioprobe.com/Image from http://people.seas.harvard.edu


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MANUAL SWITCHING

Operation: Operator will be alerted when receiver is lifted signal lamp

would light

Operator plugs the Answering Cord into the callers jack.

Operator will ask the caller for the number to be connected to.

If the callee is someone within the operators local exchange,the Operator will plug the callers Ringing Cord into thecallees jack to start the call.

Images from http://people.seas.harvard.edu


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MANUAL SWITCHING

Manual Switching Operation (cont)

If the callee is someone outside the operators localexchange, she would connect the callers Ringing Cord to a

trunk circuit to connect to another operator at the next localexchange start of a long distance call.

The subsequent operator will repeat the process, and if thecallee is not within the other operators local exchange, the

call will be trunked to another local exchange.

A long distance call involves a chain operators across fewlocal exchanges.

This chain of calls can only be made if the trunk lines areavailable between all the (involved) exchanges at the sametime.

When a call is completed, the operator would unplug thecords from the callees jack.


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AUTOMATIC SWITCHING

Invention Strowgers Exchange Invented by Almon Brown Strowger

A local undertaker in Kansas City

A wife of another local undertaker worked at the local manualtelephone exchange

Whenever a business call is made for Strowger, the wifedeliberately connects the call to her husbandStrowgerscompetitor.

Frustrated - Strowger was driven by the motivation to replacehuman factor in manual exchange

Strowger developed an automatic electromechanicalswitching system that does not require human operator

When a call is made, a moving arm moves around manyother contacts to establish a connection to the callee.

In UK, the first public automatic switching exchange went intooperation in 1912.


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FEATURESOF STROWGERS SYSTEM

In small exchanges (village): 3 digits numbering scheme was used.

In large exchanges (town, city): 4 to 5 digits numbering scheme wasused.

To connect a call to a another exchange, a uniform dialling code wasused.

The exchange-dependant-unique code will identify a given exchange so

that calls can be made to a subscriber from that exchange regardlessof where the call is originating from (the callers location)

A device called the Translator was used to translate the uniform codeand route the calls accordingly.

Uniform codes used 3 digits numbering scheme, often using themnemonic system to help users remember exchange codes easily.

For example: VICtoria (842)

Image from http://rantingravingblog.blogspot.com


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Uses Timed Pulse rotary dialling

Ring Generator is used to produce progress tones Dial Tone Busy Tone - either the callee is busy or if the route to the

callee is congested

Number Unobtainable Tone Number out of service or faulty Ring Tone

Audio files from http://www.seg.co.uk/Image from http://www.telephonearchive.com


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Automatic Charging system was employed byallocating a digit counter meter to each subscriberin the exchange.

A photographer will take the photos of the meterreading and send it to the billing department at theend of a given billing period.

Multi-metering system was used for non-local calls where different pulse rates were produced in the

meter to reflect the charges appropriately.


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HOWITWORKS - UNISELECTOR

A selector starts in 'home' position

Each impulse will move the arm by one position

Each output connected (or lead to) a different subscriber

Image from http://www.seg.co.uk/


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HOWITWORKS? BI-MOTION SELECTOR

Bi-Motion Selector

Image from http://www.seg.co.uk/


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BIGGER PICTURE

1) Receiver is lifted circuit is closed - exchange receives signal that a caller wants to dial a number

2) Uniselector wipes across busy contacts stops at the first contact with a free selector (Selector 1) - each contact isconnected to a selector.

3) Caller will hear a dial tone caller can dial a number - 4388

4) When 4 is dialled, the signal four is transmitted to exchange.

5) Selector 1 will receive the pulse four, and will move vertically to row 4.

6) Its arm will sweep again (horizontally), to search for a contact that is connected to a free selector Selector 2.

7) Selector 2 will receive the pulse three, and move vertically to row 3 Again, its arm will search (horizontally) for acontact that is connected to a free selector the Final Selector.

8) When pulse eight is received, the arm will move vertically to row 8. When the last pulse eight is received, the arm willmove horizontally and stops at contact number 8.

9) The caller will be connected to the callee with the number 4833.

10) Return either Busy Tone or Ring Tone11) Register charging against callers account

Image from http://strowger-net.telefoonmuseum.com


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A TYPICALDAYINASTROWGEREXCHANGE


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SUMMARY

The need for exchanges

Manual Exchange

Highlights:

Manually operated

Switchboard based

Dialling by verbal communication

Automatic Exchange

Highlights:

Operated electromechanically No human operator needed in call making process

Dialled numbers send pulse signals to exchanges

Pulse signals influence selectors to connect to the intendedcallee

Documents

Manual and Automatic Switching