Basic Telephony This chapter explains how the telephone instrument was invented and developed and how automation began. 1. How the Telephone was Invented In 1876 the Scottish born speech therapist and engineer Alexander Graham Bell 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, Thomas Edison 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 telephones 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 Almon Brown Strowger 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 switchboard operator 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 telephone exchange allowed callers to dial numbers the 1920's with rotary dial. 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 telephone line. These 'pulses' were counted in the exchange and used to move the mechanical arm of the Strowger switch 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.