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Design of 4 Line Private Exchange Box

Info: 5498 words (22 pages) Dissertation
Published: 6th Dec 2019

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Tagged: Computer Science



Private branch exchange system (PBXs) operates as a connection within private organizations usually a business. Because they incorporate telephones, the general term “extension” is used to refer to any end point on the branch. The PBX handles calls between these extensions. The primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone services. The private branch exchange (PBX) provides internal station-to-station communications for a well-defined set of users. Three distinct generations of private branch exchanges have appeared. In the first generation (1900-1930), a human operator manually set up calls. Second-generation private branch exchanges (mid-1930s to mid-1970s) used mechanical relays to establish the call path. The third generation of private branch exchanges is the stored-program microprocessor-controlled system. Introduced in the mid-1970s, these systems use computer instructions to perform the call set-up and tear-down. The third-generation private branch exchange is physically much smaller than electromechanical models, uses less power, and generates less heat.(Brooks, 1999)

In this project, the design of a 4 line telephone systems with full signaling and switching functions similar to those of the central office systems was embarked upon. Dial tone, busy tone, and ring tone are provided during call process. Switching employs integrated circuit (IC) matrix switches on four buses. Thus, this system is expandable to 8 lines (4 pairs) if more hardware is added. This system is switching on the Dual Tone Multi Frequency (DTMF) dialing signal.


The major problems this project intends to deal with are:

  • Cut down cost of internal calls made within a company.
  • Eliminate the need for a central telephone company to help you monitor your internal calls.
  • Eliminate Stress of notification of telephone company each time you need a new extension and thereby reducing cost.
  • Ensure security of your internal calls which otherwise can be tapped by company operating it.
  • Eliminate the need for a manual switchboard and subsequently an operator to connect the calls.
  • Reduce man-hours lost through staff walking about in an office in order to pass information to each other.


The main aim of this project is to design and implement a 4 line private exchange box that is able to create connection between four different telephone lines internally without having to connect to an external or trunk line.

The objectives include:

  • Establishing connections between the telephone sets of any two users. (e.g. mapping a dialed number to a physical phone)
  • Maintaining such connections as long as the users require them. (i.e. channeling voice signals between the users)
  • Creating an easy means of communication in an office without getting to spend money for their internal calls.
  • To switch between telephone users thereby creating connections.
  • To make sure the connection remains in place as long as it last, by keeping its resources.
  • To properly end the connection when a user hangs up.


The ability or concept of providing an easy and less expensive way of communication within a small office or organization without having to pay for your internal calls or having limits to the rate or length of calls within the office. Also it is not necessary to go from office to office when something is needed, information is to be passed; a call to a colleague saves stress of walking about.


The Private Exchange System in this project is limited to a four lines which means that internal calls can be made from only four nodes. As such, it is only suitable for very small organization.


The review of existing and related works to source appropriate information on how to go about the implementation of the project will be carried out. Information shall be gathered from text books, magazines, journals, and World Wide Web to provide answers in relation to the study. Based on the review, the design and implementation of a four line private exchange box system shall be carried out.


There are several factors that could contribute to the group not delving deeper into this project which could have resulted in a more comprehensive work. Constraints are unavoidable in any system, be it a natural system or a computer system. Due to the extensiveness of this project topic, limitations were encountered some of which include:

  • Time constraint.
  • Financial constraints.
  • Inadequate facilities to work with.


In chapter one, the research topic is introduced, which is followed by the statement of problem after which the aims and objectives of the study are stated, significance of study, scope of study and research methodology are all identified. The second chapter gives us a view of the related works which have been done and how they are related to our work. The third chapter is about our design methodology and this emphasizes on how the whole private exchange system works and its components. The quality of the system is tested and documented in chapter four. Also in chapter four, an in-depth manual of the system functions and contents is given. A summary of all chapters, a conclusion is outlined in chapter five.




In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make phone calls “work” in the sense of making connections and relaying the speech information. Early telephone exchanges are a suitable example of circuit switching; the subscriber would ask the operator to connect to another subscriber, whether on the same exchange or via an inter-exchange link and another operator. In any case, the end result was a physical electrical connection between the two subscribers’ telephones for the duration of the call. The copper wire used for the connection could not be used to carry other calls at the same time, even if the subscribers were in fact not talking and the line was silent.

The first telephone exchange opened in New Haven, Connecticut in 1878. The switchboard was built from “carriage bolts, handles from tea pot lids and bustle wire” and could handle two simultaneous conversations. Later exchanges consisted of one to several hundred plug boards staffed by telephone operators. Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of a subscriber’s telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit. When a calling party lifted the receiver, a signal lamp near the jack would light. The operator would plug one of the cords (the “answering cord”) into the subscriber’s jack and switch her headset into the circuit to ask, “number please?” Depending upon the answer, the operator might plug the other cord of the pair (the “ringing cord”) into the called party’s local jack and start the ringing cycle, or plug into a trunk circuit to start what might be a long distance call handled by subsequent operators in another bank of boards or in another building miles away.


PBX is a telephone exchange serving a single organization and having no means for connecting to a public telephone system it serves a user company which wants to have its own communication branch to save some money on internal calls. This is done by having the exchanging or switching of circuits done locally, inside the company. There are some important components which play a major role in the implementation of an effective PBX system.

Some of the Component

  • The PBX’s internal switching network.
  • Central processor unit (CPU) or computer inside the system, including memory.
  • Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
  • Stations or telephone sets, sometimes called lines.
  • Outside Telco trunks that deliver signals to (and carry them from) the PBX.
  • Console or switchboard allows the operator to control incoming calls.
  • Uninterruptible Power Supply (UPS) consisting of sensors, power switches and batteries.
  • Interconnecting wiring.
  • Cabinets, closets, vaults and other housings.


There are essentially three different types of PBXs that could be deployed within an organization infrastructure. It is necessary to be certain of type in use, so as to be able to identify the essential numbers.

There are currently three different PBX classes:

Centrex; Direct Inward Dialing (DID)/Direct Outward Dialing (DOD) and Megalink.


Centrex is the easiest of the PBX types. This PBX, unlike other types is installed within the telephone company’s Central Office (CO) and does not require dialing an extension code (normally 4 numeric characters) after having dialed the 7 to 10 digit number to connect a call to an individual. In a simplistic manner, it could be considered similar to the telephone used at home. It has an area code (NPA), an Exchange (NXX) and a Unique Number, (0000 to 9999) and does not require the dialling of another number after it in order to place a call. These numbers may be entered through a PAD.

2.2.2 Direct Inward Dialing(DID)/ Direct Outward Dialing (DOD)

Unlike a Centrex, these types of PBXs is not installed within the telephone company’s Central Office. Secondly, if a cut of the telephone wire occurs outside the building, individuals are still able to dial within it to talk to colleagues by simply dialing their extension number (normally a number between 0000 to 9999) lastly; this PBX is controlled via a computer interface at a control console. Since the PBX requires constant power to function, it may be necessary to hook it with generating plant, in the absence of power from electricity company.

Direct Inward Dialing (DID) and Direct Outward Dialing (DOD) are simply features of an Automated PBX which require that you dial the company’s general telephone number followed by the entry of the individual’s extension number when prompted to do so. DIDs allow you direct dialing (seven digits) to locate an individual within an organization’s PBX. It is a trunk phone number that must be entered into the PAD program and flagged as a PBX to ensure that the outgoing line(s) get priority.

PBXs may be privately owned or telecommunication company owned. If PBX is programmable it is possible to assign specific trunk lines to specific numbers. These trunk line numbers may then be entered on PAD thus providing dial tone protection.


The major difference between this and a Centrex PBX is that the exiting trunk lines from a building to the telephone company central office are comprised of fibre optic cables and not through twisted pair wiring. Another difference is that unlike a Centrex that is identified by its’ ten digit telephone number (NPA, NXX, and Unique), Megalinks are identified by a circuit ID number. This number may contain characters and may even resemble a telephone number, however, PAD does not allow for the entry of the circuit switch identifier. The reason is quite simple, fibre optic cabling circuits can handle far more traffic than twisted pair PBXs.


Interfaces for connecting extensions to a PBX include:

  • POTS (Plain Old Telephone System) – the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension.
  • Proprietary – the manufacturer has defined a protocol. One can only connect the manufacturer’s sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons.
  • DECT – a standard for connecting cordless phones.
  • Internet Protocol – For example, H.323 and SIP.

Interfaces for connecting PBXs to each other include:

  • Proprietary protocols – if equipment from several manufacturers is on site, the use of a standard protocol is required.
  • QSIG – for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits.
  • DPNSS – for connecting PBXs to trunk lines. Standardised by British Telecom, this usually runs over E1 (E-carrier) physical circuits.
  • Internet Protocol – H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.

Interfaces for connecting PBXs to trunk lines include:

  • Standard POTS (Plain Old Telephone System) lines – the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call.
  • ISDN – the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections.
  • RBS – (Robbed bit signaling) – delivers 24 digital circuits over a four-wire (T1) interface.
  • Internet Protocol – H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers.

Interfaces for collecting data from the PBX:

  • Serial interface – historically used to print every call record to a serial printer. Now an application connects via serial cable to this port.
  • Network Port (Listen mode) – where an external application connects to the TCP or UDP port. The PBX then starts streaming information down to the application.
  • Network Port (Server mode) – The PBX connects to another application or buffer.
  • File – The PBX generates a file containing the call records from the PBX.

The call records from the PBX are called SMDR, CDR, or CIL.  (Micheal, 1999)


Telephone is one of the most amazing devices ever created.  Although most people take it completely for granted, the telephone is one of the most amazing devices ever created. To talk to someone, just pick up the phone and dial a few digits; connection will be established with the person and a two-way conversation can take place. It is an instrument designed for simultaneous transmission and reception of the human voice. It works by converting the sound waves of the human voice to pulses of electrical current, transmitting the current, and then retranslating the current back to sound. The U.S. patent granted to Alexander Graham Bell in 1876 for developing a device to transmit speech sounds over electric wires is often called the most valuable ever issued. Within 20 years, the telephone acquired a form that has remained fundamentally unchanged for more than a century. The advent of the transistor (1947) led to lightweight, compact circuitry . Advances in electronics have allowed the introduction of a number of “smart” features such as automatic redialing, caller identification, call waiting, and call forwarding. The figure 2.1 shows the major components that makes up a telephone set.


When a person speaks into a telephone, the sound waves created by his voice enter the mouthpiece. An electric current carries the sound to the telephone of the person he is talking to. A telephone has two main parts: (1) the transmitter and (2) the receiver.
The Transmitter of a telephone serves as a sensitive “electric ear.” It lies behind the mouthpiece of the phone. Like the human ear, the transmitter has 14 eardrum.” The eardrum of the telephone is a thin, round metal disk called a diaphragm. When a person talks into the telephone, the sound waves strike the diaphragm and make it vibrate. The diaphragm vibrates at various speeds, depending on the variations in air pressure caused by the varying tones of the speaker’s voice.  Behind the diaphragm lies a small cup filled with tiny grains of carbon. The diaphragm presses against these carbon grains. Low voltage electric current travels through the grains. This current comes from batteries at the telephone company. The pressure on the carbon grains varies as sound waves make the diaphragm vibrate. A loud sound causes the sound waves to push hard on the diaphragm. In turn, the diaphragm presses the grains tightly together. This action makes it easier for the electric current to travel through, and a large amount of electricity flows through the grains. When the sound is soft, the sound waves push lightly on the diaphragm. In turn, the diaphragm puts only a light pressure on the carbon grains. The grains are pressed together loosely. This makes it harder for the electric current to pass through them, and less current flows through the grains.

Thus, the pattern of the sound waves determines the pressure on the diaphragm. This pressure, in turn, regulates the pressure on the carbon grains. The crowded or loose grains cause the electric current to become stronger or weaker. The current copies the pattern of the sound waves and travels over a telephone wire to the receiver of another telephone.

The Receiver serves as an “electric mouth.” Like a human voice, it has “vocal cords.” The vocal cords of the receiver are a diaphragm. Two magnets located at the edge of the diaphragm cause it to vibrate. One of the magnets is a permanent magnet that constantly holds the diaphragm close to it. The other magnet is an electromagnet. It consists of a piece of iron with a coil of wire wound around it. When an electric current passes through the coil, the iron core becomes magnetized. The diaphragm is pulled toward the iron core and away from the permanent magnet. The pull of the electromagnet varies between strong and weak, depending on the variations in the current. Thus, the electromagnet controls the vibrations of the diaphragm in the receiver.
The electric current passing through the electromagnet becomes stronger or weaker according to the loud or soft sounds. This action causes the diaphragm to vibrate according to the speaker’s speech pattern. As the diaphragm moves in and out, it pulls and pushes the air in front of it. The pressure on the air sets up sound waves that are the same as the ones sent into the transmitter. The sound waves strike the ear of the listener and he hears the words of the speaker. (www.howstuffworks.com)


Simply speaking this is a device that alerts you to an incoming call. It may be a bell, light, or warbling tone.   The ringing signal is in an AC wave form.  Although the common frequency used can be any frequency between 15 and 68 Hz.  Most of the world uses frequencies  between  20 and 40 Hz.   The voltage at the subscribers end depends upon loop length and number  of  ringers attached to the line; it could be between 40 and 150 Volts.

The ringing cadence (the timing of ringing to pause), varies from company to company.  In the United States the cadence is normally 2 seconds of ringing to 4 seconds of pause. An unanswered phone in the United States will keep ringing until the caller hangs up.  But in some countries, the ringing will “time out” if the call is not answered. The  most  common  ringing  device is  the  gong  ringer; a solenoid  coil  with a clapper that strikes either a single or double bell. A gong ringer is the loudest signaling device that is solely phone-line powered.

Modern telephones tend to use warbling ringers, which are usually ICs powered by the rectified ringing signal.  The audio transducer is a small  loudspeaker via a transformer.

Ringers are isolated from the DC of the phone line by a capacitor. Gong ringers in the United States use a 0.47 uF capacitor.  Warbling ringers in the United States generally use a

1.0 uF capacitor.  Telephone companies in other  parts  of  the world  use  capacitors  between  0.2  and  2.0  uF.   The   paper capacitors of the past have been replaced almost exclusively with capacitors made of Mylar film.  Their voltage rating is always 50 Volts.  The capacitor and ringer coil, or Zeners in a warbling ringer, constitute a resonant circuit. When phone is hung up (“on hook”) the ringer is across the line; and it has merely silenced the transducer, not removed the circuit from the line. When the telephone company uses the ringer to test the line, it  sends  a  low-voltage, low frequency  signal  down  the  line (usually  2 Volts at 10 Hz) to test for continuity. The company compares result with the expected signals of the line.  This is how it can tell whether an added equipment is on the line. If your telephone has had its ringer disconnected, the telephone company cannot detect its presence on the line.

Because there is only a certain amount of current available to  drive ringers, if ringers are added to phone lines indiscriminately, a point will be reached at which either all ringers will cease to ring, some will cease to ring, or some ringers will ring  weakly. A normal ringer is defined as  a  standard gong  ringer  as  supplied  in  a  phone  company  standard  desk telephone; Value given to this ringer  is  Ringer  Equivalence Number  (REN)  1. It can be as high as 3.2, which means that device consumes the equivalent  power  of  3.2 standard ringers, or 0.0, which means it consumes no current when subjected  to  a  ringing  signal.  If  there is a problem with ringing, it could be that the REN is greater than 5, disconnecting ringers until REN is at 5 or below will usually solve the problem. Other countries have various ways of expressing  REN,  and some  systems  will handle no more than three of  their  standard ringers.  But whatever the system, if an extra equipment was added and the  phones  stop ringing, or the phone answering  machine  won’t pick  up  calls,  the solution is disconnect  ringers  until  the problem  is resolved. Warbling ringers tend to draw less current than  gong  ringers, so changing from gong  ringers  to  warbling ringers may help spread the sound better.

Frequency response is the second criterion by which a ringer is described. Because a ringer is supposed to respond to AC waveforms,  it will tend to respond to transients (such as switching transients) when the phone is hung up, or when the rotary dial is used on  an extension phone.  This is called “bell tap” in the United States; in other  countries,  it’s often called  “bell  tinkle.”   While

European and  Asian phones tend to bell tap, or  tinkle,  United States ringers that bell tap are considered defective. The bell tap  is  designed out of gong ringers and fine  tuned  with  bias springs.   Warbling  ringers  for use in the  United  States  are designed  not  to respond to short transients;  this  is  usually accomplished  by  rectifying the AC and filtering  it  before  it powers the IC,  then not switching on the output stage unless the voltage lasts long enough to charge a second capacitor.(Roberts, 2006)


This is a lever that is depressed when the handset is resting in its cradle. It is a two-wire to four-wire converter that provides conversion between the four-wire handset and the two-wire local loop. There are two stages, which are off  hook and on hook

Off hook: The state of a telephone line that allows dialing and transmission but prohibits incoming calls from being answered. The phone is off-hook when the handset is removed from the base unit of a stationary phone or press Talk on a portable phone. The term stems from the days when the handset was lifted off an actual hook. When the handset was removed, a spring caused contacts to press together, closing the circuit from the telephone to the switchboard.

On hook: The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone reciever, i.e., separate earpiec, on a swithch hook. The weigth of the recieved depresses the sping leaded switch hook thereby disconnecting the idle instrument (except its bell) from the telephone line.     (Roberts, 2006)


There are two types of dials in use around the world. The most common one is called pulse, loop disconnect, or rotary; the oldest form of dialing, it’s been in use since the 1920’s. The other dialing  method,  is called Touch-tone, Dual Tone Multi-Frequency  (DTMF)

Pulse dialing  is traditionally accomplished with  a  rotary dial,  which is a speed governed wheel with a cam that opens  and closes a switch in series with the phone and the line.  It works by  actually  disconnecting  or “hanging  up”  the  telephone  at specific intervals.  The United States standard is one disconnect per   digit,   so if a  “1,” is dailled, the telephone  is “disconnected” once. To dial a seven means that it will be  “disconnected” seven times; and dialling a zero means that it will “hang up ” ten times. Some countries invert the system so “1” causes ten “disconnects”  and 0,  one disconnect.  Some add a digit so that dialing a 5 would cause six disconnects and 0, eleven disconnects.  There are even

some systems in which dialing 0 results in one  disconnect,  and all  other digits are plus one, making a 5 cause six  disconnects and 9, ten disconnects.

Although most exchanges are quite happy with rates of 6  to 15  Pulses Per Second (PPS), the phone company accepted  standard is  8  to  10 PPS.  Some modern digital exchanges, free  of  the mechanical  inertia problems of older systems, will accept a  PPS rate as high as 20. Besides  the PPS rate, the dialing pulses have a  make/break ratio,  usually  described as a percentage, but  sometimes  as  a straight  ratio.  The North American standard is 60/40  percent; most of Europe accepts a standard of 63/37 percent.  This is the pulse measured at the telephone, not at the exchange, where  it’s somewhat  different, having traveled through the phone line  with its  distributed  resistance, capacitance,  and  inductance.   In practice, the  make/break  ratio does not  seem  to  affect  the performance of the dial when attached to a normal loop.  However,each pulse is a switch connect and disconnect across  a complex  impedance, so the switching transient often reaches  300 Volts. Usually, a safe practice is not to have fingers  across  the  line  when dialing.

Most pulse dialing phones produced today use a CMOS IC and a keyboard.  Instead of pushing finger round in circles, then removing finger and waiting for the dial to return  before dialing the next digit, the button can be punched as fast as desired.  The IC stores the number and pulses out the number at the correct rate with the correct make/break ratio and the switching is done with a high-voltage switching transistor.  Because the IC has already stored the dialed number in order to pulse it out at the  correct rate,  it’s a simple matter for telephone designers to  keep  the memory “alive”  and allow the telephone to  store,  recall,  and redial the Last Number Dialed (LND).  This feature enables easy redial by picking up the handset and pushing just one button.

Touch tone is the most modern form of dialing. It is  fast  and less  prone to error than pulse dialing.  Compared to pulse, its major advantage is that its audio band signals can  travel  down phone  lines further than pulse, which can travel only as far  as the  local  exchange. Touch-tone can therefore  send  signals around  the  world via the telephone lines, and can  be  used  to control phone answering machines and computers. 

Bell  Labs developed DTMF in order to have a dialing  system that  could travel across microwave links and work  rapidly  with computer  controlled exchanges.  Each transmitted digit consists of two separate audio tones that are mixed together. The four  vertical columns on the keypad are known as  the  high group and the four horizontal rows as the low group; the digit  8

is  composed  of 1336 Hz and 852 Hz.  The level of each  tone  is within  3  dB  of the other.  A complete touch-tone pad has 16 digits, as opposed to ten on a pulse dial.  Besides the numerals 0 to 9, a DTMF  “dial” has *, #, A, B, C, and D.  Although the letters are not normally found  on consumer telephones, the IC in the phone is capable  of generating them.

The  * sign is usually called “star” or “asterisk.”   The # sign,  often referred to as the “pound sign.” is actually  called an  octothorpe. Although many phone users have never used  these

digits  –  they are not, after all, ordinarily  used in  dialing phone  numbers. They  are used  for  control  purposes,  phone answering machines, bringing up remote bases, electronic banking, and repeater control.  The one use of the octothorpe that may be familiar occurs in dialing international calls from phones.  After dialing the complete number,  dialing  the octothorpe  lets the exchange know you’ve finished  dialing.   It can now begin routing your call; without the octothorpe, it would wait and “time out” before switching your call.

Standard DTMF dials will produce a tone as long as a key  is depressed.   No matter  how long you press,  the  tone  will  be decoded as the appropriate digit.  The shortest duration in which

a digit can be sent and decoded is about 100 milliseconds  (ms).  It’s pretty  difficult  to dial by hand at  such  a  speed,  but automatic dialers can do it.  A twelve-digit long distance number

can be  dialed by an automatic dialer in a little  more  than  a second – about as long as it takes a pulse dial to send a  single 0 digit.(Roberts,2006)


Modular connector is the name given to a family of electrical connectors that were originally used in telephone wiring. Even though they are still used for that purpose they are used for a variety of other things as well. A modular connector’s advantage over many other kinds include; small size and ease of plugging and unplugging. Many uses that originally used a bulkier connector have migrated to modular connectors. Probably the most well known applications of modular connectors is for telephone jacks and for ethernet jacks, which are nearly always modular connectors. Figure 2.2 shows types of connectors commonly used.

Modular connectors were first used in the registered jack system, so registered Jack specifications describe them precisely. These are the specifications to which all practical modular connectors are built.

Modular connectors come in four sizes: 4-, 6-, 8-, and 10-position. A position is a place that can hold a conductor (pin). The positions need not all be used; a connector can have any even number of conductors. Unused positions are usually the outermost positions. The connectors are designed so that a plug can fit into any jack that has at least the number of positions as the plug. Where the jack has more positions than the plug, the outermost positions are unused. However, plugs from different manufacturers may not have this compatibility, and some manufacturers of eight position j

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