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Home , Almon Brown Strowger, Rotary dial

Basic

This chapter explains how the instrument was invented and developed and how automation began.

1. How the Telephone was Invented

In 1876 the Scottish born speech therapist and engineer developed the first working telephone instrument. It consisted of a 'microphone' connected to a 'receiver'. In fact both devices were identical. To talk both ways you needed another 'telephone' in the other direction.

It was not very efficient and the following year another well known inventor, made improvements to the microphone. Edison also put a microphone and Bells' receiver into one 'box' to produce a workable device. The microphone and Bell's first telephone receiver in Edison's telephone have remained virtually unchanged to this day! Learn more about how the microphone and earpiece

MORE work. 2. The Early Days

In the early days in homes and businesses were all separately connected with wires to a switchboard. You lifted the receiver and turned a handle to 'call' the operator who then manually connected your call. The operator could call you by ringing a bell associated with the telephone.

Telephones became popular at both home and in businesses. Larger businesses would have their own switchboard operators to allow employers to make and receive calls from their office and out onto the 'public network'.

As more and more telephones were installed the wiring and number of switchboards became very complex. It was clear some form of central automation was needed but no one had been able to work out how. It was a Kansas City undertaker by the Early production telephone name of who solved the problem.

This chapter explains how automation in connecting calls began. 1. Why automation was needed!

Almon Brown Strowger was an undertaker working in Kansas city in the 1880's. He noticed that his business was getting slow and found out that the wife of one of his competitors was the local and whenever someone asked for an undertaker she put them through to her husband's business! Strowger was also an amateur engineer so he sat down and produced the first automatic telephone switch!

The switch - to become known eventually as the Strowger Two Motion Selector - was the mainstay of all telephone exchanges around the world until the early 1970's.

2. How did the automatic exchange work? Early 'candlestick' phone from The automatic allowed callers to dial numbers the 1920's with . themselves and were soon installed in towns and cities all over the The bell was separate. Click world. Exchanges were also produced for private business and to enlarge. became known as Private Automatic Branch eXchanges - (PABX) the word Branch indicating they had a connection from the office to the Public Telephone system. The word PABX is usually shortened to PBX. The public telephone exchange is also known as the Local Exchange, Central Office or simply Exchange.

Strowger systems were mechanical. The rotary dial on the telephone, when rotated and released interrupted the steady current flowing in the . These 'pulses' were counted in the exchange and used to move the mechanical arm of the up to a 'Modern' rotary dial phone, in contact. Further pulses rotated the switch across a bank of contacts. It use up to the 1980's. The bell would need several such switches to handle just one call. These is now inside the phone. This exchanges were fitted in large racks and took up a lot of room space. is an American '500' type. Automation speeded up the time it took to connect a call. Click to enlarge.

By the early 1960's it was possible to dial automatically to many parts of the world.

3. The digital revolution

With the advent of the microchip in the early 1970's it was obvious the telephone exchange would modernise. Two ex British Telecom engineers, Terry Matthews and Mike Copeland founded Mitel in Canada and in 1973 produced the world's first automatic PBX using digital circuits to switch calls. This PBX, the SX-200, was the size of Modern DTMF phone. Click a small refrigerator and could handle nearly 200 users. From then on to enlarge manufacturers used digital processing to make both public and private telephone switches. The mechanical Strowger era was at an end!

With digital switching it was possible to move away from the rotary dial on the telephone to a keypad which sent out audio tones for each digit. Each digit is two tones and is called Dual Tone Multi Frequency (DTMF). Exchanges read these tones and process the call. DTMF is much faster than rotary dial. By the 1990's most public and virtually all private exchanges were using DTMF.

Chapter 4A: PBX Telephone And Building Wiring - Krone IDC And Test Jack Frames

1. How wires are terminated

In the 'old' days each wire was terminated by hand on either a screw terminal or wrapped around a metal tag. In the 1980's a method of terminating called Insulation Displacement Connection or IDC became available. Krone is perhaps the most well known of companies offering products using IDC. With IDC the wire is pushed firmly between two metal blades using a special tool (a Krone tool). The blades cut through the plastic sheath and make contact with the copper core inside. This method is both fast and reliable. It is the preferred method for installations today of both telephone and data networks.

In telephone wiring the Krone strip is one of the most popular methods. Two such Krone strips are shown on the right. Each one has 20 connections (10 pairs). Each strip has a row at the top (each connection marked with a small black square) and row below (numbered 1, 2 etc to 0). Between these two rows are two metal contacts allowing the top pair of wires to connect to the bottom pair. The technician can use a test telephone and plug it into these central connectors to test telephone circuits.

IDC Termination Krone Connection Strip

2. How the circuits on the PBX are connected

The PBX has many circuit cards in the cabinet, each one handling a number of extension or trunk circuits (typically a card would handle 16 extensions or trunks). The circuits on each card are collected at the back of the cabinet and brought out on a number of multi-pair cables. In the Mitel SX-2000 PBX these cables each have 25 pairs of wires and there are 8 cables in all. The cables plug into the bottom of a Test Jack Frame or TJF. This frame presents the individual extension and trunk circuits in a logical layout from top to bottom. Each circuit from the PBX is connected to the top row of the Krone strips. The technician connects another pair of wires to the bottom row of the Krone strip and that wire is connected to the Building Distribution Frame (see below).

You can see that with a large installation the TJF frames will occupy a lot of space!

Test Jack Frame

3. How do you know which wire is which?

Telephone cables use a colour code. Pairs 1 to 25 are uniquely identified. The sequence then repeats itself every 25 pairs. So, in a 100 pair cable you have 4 'blocks' all the same. In the manufacture of the cable a 'marker' tape (green and orange) is wrapped around these 'blocks' so you know which is which.

Colour Code

4. How the wiring goes around the building

Each circuit on the TJF must now end up on a socket under a desk. It is not practical to run individual pairs of wires from the TJF so 'multi-pair' cable is used again. Next to the TJF is another frame (of similar design). This is called the Building Distribution Frame or BDF. From the BDF large multi-pair cables are run to each floor of the building and terminated on Distribution Points (DP's). From each DP smaller multi-pair cables are run to each group of offices and again terminated on DP's. From these DP's individual cables (usually with 3 pairs) are run to each desk and terminated on a socket (in the UK the 'BT' style socket is used). We only use 1 pair of wires in this last cable but there are instances when another pair may be needed - for example if we connect a second phone to the socket.

To make the connections the technicians connects 'jumper' wires from each circuit on the TJF to each pair on the BDF. This way an extension in someone's office is connected to the circuit card which is programmed with their number. If someone moves offices and wants to take their extension number with them the technicians moves the jumper from the current BDF circuit to the new one for the new office. It is very important that good records are kept of where each circuit is going to and the socket and DP numbers, otherwise the technician will spend a lot of time tracing out the jumpers trying to find where each one is going!

Building Distribution

5. The 'BT' telephone socket

In the UK there is a standard socket used. There are two basic versions, the 'Master' and 'Secondary'.

The 'Master' socket has pins 2 and 5 for the telephone line and a bell capacitor across 2 and 3. There is also an 'out of service' resistor which lets the exchange 'see' a termination when the phone is unplugged. There is also a lightening surge protector. Master sockets are used directly on the incoming telephone line (in your home). A version of the 'master' socket is available for PBX's.

The 'Secondary' socket is used to connect a second telephone to the same line. It has no bell capacitor, resistor or surge protector. The secondary socket is wired to the master by simply connecting pins 2 to 2, 3 to 3 and 5 to 5. Both phones will ring when a call comes in.

Pin 4 in these sockets is only for earth recall an option not used very much now on PBX's.

Telephone socket types

Chapter 5: Trunks - Connecting PBX's To The Public Network And To Each Other

This chapter explains how PBX's are connected to the public and to each other so that extension users can make calls outside of their office to other businesses and to other countries.

1. Why do we need trunks?

It would not be practical to connect every telephone in your business to the public telephone system (PSTN - Public Switched Telephone Network). Instead we make a number of dedicated connections between the PBX and the Local Exchange called Trunks. For example your business may have 100 telephone extensions but would have only 10 trunks. This may seem a problem but of course not everyone uses a trunk at the same time. The ratio of about 10 to 1 for extensions to trunks works for most Trunk circuits and the PBX - click businesses. to enlarge

2. How do we use trunks? Outgoing, Incoming and Direct Dial In (DDI)

Trunks are used to connect your PBX to the public network (a Local Exchange or Central Office trunk) and to connect your PBX to another PBX as part of a private trunk network (Private Trunks, Tie Trunks or Private Wires).

2.1 Outgoing calls

PBX's are programmed to allow the extension user to simply dial a code (9 for example) and then an external number. The call is routed over one of the trunk circuits to the local exchange. If you had a private trunk network you would probably just dial the extension number of the person you wanted in the other office. The call is routed over one of the PBX's private trunks to the other PBX to ring the extension directly.

2.2 Incoming calls - non-dial in Incoming calls from the public network will route over a trunk into the PBX. They go to a fixed answering point which can be an operator console who will answer the call and then put it through to the extension; or the answer point could be a group of extensions (or key on a Mitel Superset phone); or a series of bells around the office. Then anyone can answer the call by dialling a code from any extension. These are non-dial in trunks - the answer point (console, extension or bell) is fixed and will always answer the call. The call is controlled by the PBX so if there is no answer from the answering point the call can be routed elsewhere. If the call was answered by the operator and put through to an extension that does not answer then the call will 'recall' back to the console.

2.3 Incoming calls - dial in

Today most businesses use Direct Dial In Trunks - DDI (sometimes called Direct Inward Dial - DID). This allows the caller to your business to dial the number and be connected straight through to the extension without going through the operator. For example your office number may be 0129143XXXX where the X's represent the extension number you want. So, to call someone whose extension is 7654 you'd dial 01291437654.

When programming DDI trunks in a PBX you may have to correct numbers from the local exchange service provider (for example British Telecom). In the example above of a DDI number 01291437654 it is most likely you only get the last 3 digits (654) sent from the exchange. The PBX (Mitel SX-2000) can insert the missing digit 7 to complete the number. You may even find that the local exchange cannot provide the correct numbers in which case the PBX will have to convert the incoming range to that of the extensions.

For example: Extension numbers 7400 to 7700. DDI range from the service provider 300 to 600. The caller would dial 01291 437654. The PBX would receive 554 which it would change to 7654.

2.4 Day and Night operation

When a caller rings your business they expect an answer. During working hours this may be the operator console or an extension if DDI. But at night or weekends your business may be closed. The PBX can be programmed to route calls out of hours to an alternative answer point, such as a security guard, voicemail or recording.

2.5 Direct Inward System Access (DISA)

A normal analogue exchange line can be set to DISA operation. When someone makes a call into the PBX on the DISA trunk they hear a quick burst of ring tone then from the PBX. They may then dial any valid number within the PBX. This is potentially a security risk so DISA trunks can be protected with a 'PIN' and limitations on the access the caller has. A problem with DISA trunks occurs when the analogue trunk type is 'Loop Calling' (see below). If the caller dials in and then dials out on another trunk (trunk to trunk connection) the PBX has no way of knowing when the call is eventually cleared down and both trunks can remain 'locked up'. A solution is to make sure your analogue trunks are using 'Earth Calling' rather than 'Loop Calling' signalling.

3. What sort of trunks are there?

There are two basic types: Analogue and Digital. Analogue trunks are slowly disappearing and most businesses today will be using Digital trunks.

What's the difference? Analogue trunks use an electrical current (battery and earth) to signal and can't provide many (if any) features to the caller (such as the number of the person calling). One analogue trunk is one pair of wires between the PBX and the Public Exchange. So if you needed 5 trunks you'd need 5 pairs of wires (5 circuits).

Digital trunks use a completely different method to call and signal. A number of individual trunk channels are multiplexed on one connection known as a digital link. This link could be a single pair of wires, a or fibre optic cable. In the UK and Europe there are 32 channels which is known as an E1 link. Note that 2 of these channels are reserved for timing (Channel 0) and signalling (Channel 16) leaving 30 channels for calls. These 32 channels are multiplexed onto a 2Mbit/sec link (the required). In the US and Canada there are 24 channels multiplexed onto a single 1.54Mbit/sec link which is known as a T1 link.

In the UK and Europe you may have a full 30 channel E1 link or if your trunk traffic is not high you can have a partial link of 8 or 15 channels (availability varies in different European countries).

Another digital trunk available is the Basic Rate Interface or BRI. This is 3 channels multiplexed together. Two channels available for traffic and 1 for timing and control. BRI trunks are used to connect PBX's to the public exchange where only a few trunk channels are needed and a full or partial E1 link would be too expensive. BRI trunks are also available from the public exchange to your home. BRI trunks are also known as ISDN2e.

Digital trunks offer callers a huge range of features such as calling party identification, callbacks and messages.

For details of exactly how digital trunks work follow the links below.

4. Different Types of Trunks and how they work

Over the years trunks have been developed for all sorts of situations. As analogue trunks slowly disappear and digital trunks become more standardised there are fewer names to worry about! In Europe digital trunks will all eventually conform to the Integrated Services Digital Network standard (Euro ISDN - for PRI and Qsig). An older standard, Communications European Post and Telecoms (CEPT) is still used to define the DPNSS and DASS private trunk standard. DASSII, Euro ISDN PRI and Euro ISDN BRI are all Public Trunk connections from the public exchange to the PBX. DPNSS and the newer Euro ISDN Qsig are private trunk connections between PBX's.

To find out exactly how the trunks work and how the signalling is done, click on the appropriate trunk name in the table below. Please note that the description for DASSII, Euro ISDN PRI, DPNSS and QSig is the same as they are all E1 trunks. The difference is how they signal over Channel 16. This is explained in the text.

Analogue Public Analogue Private Digital Private Digital Public Trunks Trunks Trunks Trunks Digital Access Signalling System type II (DASSII). Digital Private

Loop Start Exchange AC15 (AC15A) Network Signalling

CEPT standard, now being System (DPNSS) phased out by service providers. Euro ISDN Primary Rate (PRI)

Earth Start Exchange E&M (SSDC5) QSig The most common public digital trunk replacing DASSII Euro ISDN Basic Rate

(BRI or ISND2e)

Chapter 6 - PBX Features and Applications

This chapter explains some of the features and applications that PBX's provide for people to use.

PBX manufactures vary in what they offer their customers. This list is not exhaustive but provides the most commonly found features and applications.

Basic Telephone Features When you ring a busy extension you can have the PBX ring you back when it Callback becomes free. Also works if the extension does not answer, you are rung back next time that person uses their phone. Camp On or You ring a busy extension and wait for them to take your call. They will see your Wait on Busy details in their display (Mitel Superset). You take a call and then put in on hold. You can hang up your phone and retrieve Call Hold the call after a few moments or even go to another extension and retrieve the call from there. Call Transfer After taking a call you can transfer it to another extension or to someone's voicemail. A group of telephones that share a common number. Ringing that number makes Hunt Group one of the phones in the group ring. Pick Up A group of telephones. If one of them is ringing, anyone else in the group can Group answer it from their telephone. With a Mitel Superset phone, if you ring someone who does not answer or is busy Messages you can leave a message on their phone. Their message light comes on and when they return they can see who has tried to call them and return the call.

Advanced Features Automatic Route Selection The PBX is programmed to control and restrict the numbers users dial on the Call Barring trunks. It is very flexible and can be set up so that any number can be dialled out but only if you have the right permissions (Class of Restriction). You may have accounts with various service providers for National and International calls. This feature allows the user to simply dial a number and have Call Routing the PBX route the call over the most appropriate route - based on cost or availability. Whatever number the user dials can be 'modified' to send something else. For Number example you may decide to use a specific UK 'Directory Enquiry' service which is modification '118xxx'. Whichever 118 service a user dials the PBX will always dial out the 'default' number you want to use.

Applications A number of telephones are 'grouped' together and become 'agents'. Callers are Automatic routed to an agent based on their skill and availability. You can have multiple Call groups; if all agents in one group are busy the call can be routed to another agent Distribution group. Recordings can inform callers of how long they may have to wait or their position in the queue. Gives telephones user their own 'mailbox' to receive and store voice//email Voicemail messages. The voicemail system can light a lamp on the telephone to indicate a message is waiting. A feature of most voicemail systems whereby incoming callers are routed to a Auto recording telling the caller to dial the number they want. The voicemail auto Attendant attendant then routes the call. Callers are offered the option to dial 0 to speak to an operator. Voice Similar to the auto attendant application, voice recognition system listen to the Recognition words the caller says and routes the call accordingly. For example, to call 'John Smith' you would simply ask for him by name. The application recognises the name and looks up the extension number in its database. Voice recognition can be used to allow users to control their voicemail boxes, email and many other applications.