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Introduction to Communications

Overview of Communications

The need to communicate is part of human’s inherent being. Since the beginning of time human beings have communicated using different techniques and methods. Circumstances and available technology have dictated the method and means of communications.

Many early forms of communication were writing, depicted on cave walls. Then communication advanced by the development of language and the use of symbols. Papyrus and paper were used to record communication for later use. Smoke signals of the early American Indians; the drums of African tribes; and the towers of the Chinese wall are indications of the desire to communicate beyond the immediate physical boundaries of space. Story tellers around the camp-fire are a good example of communication, using animation, gestures and sound to communicate their message to other members of the tribe.

In 1948, a model of communication was proposed by Claude Shannon. Shannon worked for the Bell Telephone Company in USA, and was concerned with the transmission of speech across a telephone line. Shannon's model of communication has served as a basis for explaining communication.

In terms of oral communication between two people, the model is applied as follows.

MessageThe idea/ thought
SourceThe brain
SenderThe transmitting device, the mouth
ChannelThe medium the message travels over, air
ReceiverThe receiving device, the ear
DestinationThe brain

In any communication there is noise, which affects the message as it travels across the channel from the sender to the receiver. Shannon proposed building in redundancy, which was added to the transmitted message in order for it to be reliably detected at the receiver.

Let us apply this model to a telephone conversation between two people. The person that initiates the call by lifting the telephone handset and dialing a number is the SOURCE, their telephone the SENDER, whilst the person who answers the ringing telephone is the DESTINATION and their telephone the RECEIVER. The CHANNEL is the Public Telephone Switched Network (PTSN), and the MESSAGE is the topic of conversation [speech] that was the reason for the call being made.

Problems of Shannon's Model

However, Shannon's model has a number of problems as a model for explaining communication.

  • it is one way (from source to destination)

  • there is no feedback between the sender and receiver, it is non-interactive

  • it does not translate appropriate to groups with many interactions

  • it does not explain the process of how the message is generated by the source, or interpreted by the destination

Evolution of Modern Communications Networks

Data communications concerns itself with the transmission (sending and receiving) of information between two locations. In the information age, this means sending information between machines that are connected together by physical wires or radio links.

The history of modern electronic communications began with Alexander Graham Bell’s telephone experiments, where speech was able to be converted into electrical energy, transmitted along physical wires and reconstructed at the receiver. Speech, which is actually vibration of the air, vibrated a paper cone to which a small coil was attached. This induced an electrical signal into the coil, which was proportional to the vibration of the paper cone.

The sound waves caused by speech make the paper cone move. The paper cones movement is directly proportional to the strength of the air waves caused by speech.

As the cone moves, the coil of wire also moves, being attached to the cone. Inside is an iron bar which helps to increase the effect.

A corresponding electrical signal is created in the coil, which can then be sent along a pair of wires to a receiving device that would convert the electrical signal back into sound waves (vibration of the air).

By 1878, Bell set up the first telephone exchange in New Haven, Connecticut, and in 1884, long distance connections were made between Boston, Massachusetts and New York City.

In these early experiments at transmitting speech, copper wire was used as the connection media over which the signals traveled. This is due to copper wire being a very good conductor, which lets electrical signals flow down the wire easily.

As the telephone became popular, more and more people wanted to communicate with each other, so a switching center (telephone exchange) evolved. Each customer was connected to the telephone exchange via a pair of wires, which carried the signal from their telephone.

As the need to inter-connect telephone customers grew, they were connected via overhead wires to a central switching center, where the physical wires from each customer was connected to the physical wires of another customer via a manual operator.

Ancient Switching Center

As more and more customers were connected, the need for more and more operators to connect calls was felt. This quickly became unworkable, so development began on automating the connection process between customers, hence, automatic telephone switching exchanges became a reality and replaced local operators, who were still used to connect long distance calls.

The customers wires from their home to the telephone switching center were overhead, installed in the same way as a lot of existing electrical (power) wires are today, on top of long poles inserted into the ground, about 15 or so feet high. This quickly became very cumbersome and difficult to manage. Transmission quality was affected by the weather, as the rain created leakage paths for the signals to go to ground instead of along the wire.

Wellington Manual Telephone Exchange, 1894

At the same time, demand grew to connect customers who lived further and further away, in rural areas. Electrical signals can only travel limited distances, and to provide service to rural customers meant using better cable that allowed the signals to travel longer distances. Unfortunately, this was costly, so something had to be done to provide them with service. In addition, customers also wanted to be able to talk to other people in different cities, so there developed a need to interconnect telephone exchanges together.

Rural customers began sharing cables, so that one cable supported a number of customers, either one at a time, or at the same time using radio signals to separate each conversation. To interconnect telephone exchanges in distant cities together, work began on using different cable and the use of radio signals over cable such as coax.

For rural customers, who lived longer distances from the central switching center, it was too expensive to have a single cable for each customer. The need arose for a single cable to carry the conversations of more than one customer. This way, customers could share a single cable and thus save the telephone company money. So, within a short time, by using radio signals, each open wire cable could carry up to 12 separate speech conversations, with an optional telegraph (telex) signal. Distance then became a limiting factor, so work began on increasing the distance over which the signals could be transmitted and received. One method was to increase the diameter of the cable (which also makes the cable heavier).

The cost of cable was very high, and special equipment was used to carry more than one speech conversation on one pair of wires. This was done by a technique known as multiplexing (specifically Frequency Division Multiplexing, which separates each conversation by frequency).

This bought down the cost of providing speech circuits (one circuit = one speech conversation) to customers. Whereas previously one cable equated to one speech circuit, now a cable could be equated to hundreds of speech circuits.

For a given cable size and type, the speech signal is affected by loss. In other words, as the signal travels down a cable, part of it is absorbed by the cable and thus the signal arrives at the end of the cable with less strength than when it started (this is called attenuation). It thus follows that if the cable is too long, no signal will arrive at the end, or the signal will be so small that we cannot hear it.

A thicker cable affects the signal less than a thinner cable, so you can go greater distances with a thicker cable before the signal becomes too weak. However, thicker cables cost more and are heavier. Open wire systems were originally made from copper, but the cost became very expensive.

Attenuation is a measure of how much loss a signal experiences when it travels down a communications medium. Part of the signal is dispersed as heat, or absorbed by the communications medium. Not all of the transmitted signal arrives at the receiver. Attenuation is measured in decibels (dB). The longer the distance that the signal travels, the greater will be the attenuation that the signal suffers.

Open wire copper systems gave way to open wire systems using aluminum. These cables had to be a great deal thicker because aluminum is not as good a conductor as copper (it affects the signal more), but it was lighter and cheaper. The other problem is that aluminum suffers from oxidization when exposed to the air, and this also affects the signal as it travels along the cable. Aluminum cables began to be used during the second world war, when copper became scarce.

Open wire systems, being exposed to the weather, often failed during wet weather and storms. They were often struck by lightning and falling trees. It wasn't long before open wire systems serving rural customers were replaced by other forms of communication systems.

Cables and Network Communication Systems

Transmission Media

Transmission media refers to the many types of cables and other mediums that carry the signal from the sender to the receiver. Copper based cable is the most common form of medium, and is used for virtually all links except long distance. A cable medium often introduces unwanted changes to the signal, which limits the speed (how fast we can send information) and frequency range of the signals that can be transmitted.

Open Wire

Seldom used nowadays, but was used extensively in rural areas to provide 12 speech channels over a single wire atop telephone poles. Open wire cable has been replaced by micro-wave systems or newer technologies. Being exposed to the weather, dust particles and salt spray were deposited on the insulators that secured the cable to the telegraph pole. When storms and wet weather arrived, the dust turned to mud, and provided a path of least resistance for the signal, thus the signal was diverted down the pole to ground and significantly reduced in strength. Lightning strikes were also a hazard and special ceramic arrestors were used to protect equipment attached to open wire lines.

Twisted Pair Cable

Before long, the open overhead wires servicing users from the telephone centers were replaced by multi-strand cable, which was buried underground and protected by many sheaths of polyurethane plastic. This prevented water seeping into the cable and affecting the signal. It also helped overcome some of the limitations that open wire systems had. These multi-strand cables used a pair of wires for each user, and there were about 500 pairs or more per cable. Each wire is twisted around each other wire in order to try and reduce unwanted noise (hence the term twisted pair).

Cable was laid by the telephone company along each street, from the telephone center, to the customers’ houses. Thus each customer had a physical set of wires which ran from their telephone set all the way into the telephone exchange. The use of multi-strand underground cable, still in use today, made the delivery of low-cost telephone services to the general public.

Twisted pair cable is the most common form of cable today, used to connect telephone subscribers to exchanges (switching centers) and wire buildings. Two insulated wires are twisted around each other, and combined with others into a cable. In general, each twisted pair supports a single voice channel. Twisted pair is also used to interconnect PC's on a Local Area Network (LAN).

Twisted pair used in Local Area Networks has several ratings.

Category 3 - has a speed rating of 10 mbps (the speed of Ethernet).

Category 5 - has a speed rating of 100 mbps.

UTP (unshielded twisted pair) is cable which has no ground shield. Cables are often provided with a ground shield which helps to reduce signal interference from external sources, thus making the signal traveling down the cable less prone to attenuation. Twisted pair cable is provided in two forms, UTP and STP (shielded twisted pair).

UTP Category 5 cable, suitable for transmission speeds up to 100 mbps

STP Category 3 cable, suitable for transmission speeds up to 10 mbps

Unshielded twisted pair cable is the predominant cable used today. Two conductors are coated with a plastic sheath then twisted around each other. These pairs are then twisted around other pairs to make a multi-pair cable. The twisting of the wires around each other helps to reduce unwanted signals being induced into the wires. It is used for telephone wiring inside buildings, as telephone cables which link customer houses and buildings to telephone switching exchanges, and for implementing local area networks.

UTP has the advantages of

  • a high installed base
  • cheap to install
  • easy to terminate

Its disadvantages are,

  • very noisy
  • limited in distance
  • suffers from interference

Coaxial Cable Systems

There became a need to inter-connect telephone centers in different towns together. The use of open wire or multi-strand cable for this was unsuitable, as the capacity of each cable was too low (number of simultaneous speech conversations per cable) in order to make it economically viable. Hence the introduction of coaxial cable.

Earlier, coaxial cable was extensively used to support long distance links.

Today, it is being replaced by micro-wave, satellite or fiber-optic links. Coaxial cable is a two wire conductor with a larger bandwidth than twisted pair cable. It is used in television, radio, and Ethernet LANs. In voice communication systems, each coaxial cable supports about 60 speech channels.

It has a single core, with an outer conductor which acts as a shield. The signal is transmitted on the inner core. The inner core and the outer shield are separated by an insulator, either plastic or mica. The cable is enclosed in polyurethane to protect it and give it some strength.

It is important not to bend the cable too tightly, as this damages the insulator which separates the inner core from the shield. The transmission qualities of coax are affected by the properties of the type of insulator used. Coaxial cable could carry up to 1200 speech circuits per cable. Coaxial cable operates better than open wire, as it is buried in the ground and not subject to the elements of the environment. The systems used to implement multi-speech channels on coaxial cable are called broad-band systems.

When used in Local Area Networks to interconnect computers, the most popular form is RG-58AU cable, commonly called thin Ethernet. The coax cable connects to each PC using a special T connector, and up to a maximum of 30 connections can be made in tandem, from PC to PC.

Thin Ethernet is cheap to install and is rated at 10 million bits per second. Each end of the cable is terminated using a 50 ohm terminator. Failure to terminate each end of the cable, or a break in the cable, causes the network to fail.

Coaxial cable is used extensively in networking and data communications. A center conductor is separated from an outer conductor by an insulator medium. The cable cannot be crushed or bent sharply, as this damages the insulation between the conductors and thus alters the electrical characteristics of the cable. When used for local area networking, it links PC's together. The networking protocol commonly used with coaxial cable is ETHERNET, which describes how data is formatted and transmitted along a shared cable system.

As open wire gave way to better systems, so coaxial cable has given way to others. The problem with coaxial cable is the number of speech channels available per cable. With the high demands for data communications between computers, increased telephone circuits between cities and countries, not to mention television channels, other mediums have taken over from coaxial cable. Coaxial cable became popular in the 1980's as a method of interconnecting computers, specifically Local Area Networks (LANs), because it was cheap and easy to install. In addition, some cable TV systems use coaxial cable to supply programming content to subscribers.

Coaxial cable has the advantages of

  • cheap to install
  • conforms to standards
  • widely used
  • greater capacity than UTP to carry more conversations

Its disadvantages are,

  • limited in distance
  • limited in number of connections
  • terminations and connectors must be done properly

Micro-wave systems

As the demand for more and more speech circuits grew, by customers wanting to make long distance calls, the telephone companies had to expand the capacity to meet this demand. One such system which was used was Microwave, which does not use cable as a transmission medium, rather it uses the air.

Using very high frequency signals, microwave support thousands of telephone channels and several television channels on the one circuit. Microwave is a radio system which uses very high frequencies to send and receive data. Because of the high frequencies involved, stations are located about 30 kilometers apart and in line of sight (visible to each other). Microwave systems have sufficient bandwidth capacity to support a large number of voice channels and one or two TV channels.

Transmitters and receivers must be located within sight of each other, about 30 kilometers apart. Microwave does not bend round corners or jump over hills.

Dishes and towers are expensive to construct and with the distance limitations, meant it is expensive to go very long distances. Nowadays, many companies use microwave systems to interconnect buildings at high speed digital links of 2 mbps or greater.

Sometimes, this is a cheaper solution than linking buildings using fiber-optic cable, especially in inner cities where cabling is a problem, or across rivers etc. where terrain prohibits the use of existing physical cabling methods. This allows companies to link their networks in different buildings together into one common network, allowing the sharing and accessing of information.

Today, microwave systems are used in a number of areas, such as linking local area networks together between campus buildings, sending radio signals from a radio station to its transmitter site, and the sending of video or audio signals from an outside sports event back to a TV broadcasting studio.

Microwave systems have the advantage of

  • medium capacity
  • medium cost
  • can go long distances

Its disadvantages are,

  • noise interference
  • geographical problems due to line of sight requirements
  • becoming outdated

Satellite systems

Ground stations with large dishes communicate with a communications satellite in geo-stationary orbit around the earth. Each channel is managed by a transponder, which can support thousands of speech channels and about 4 TV channels simultaneously. The cost of satellite links is still very expensive (about $4M per transponder). It is primarily used for intercontinental links.

Satellite systems are comprised of ground based transmitter and receiver dishes, with an orbital satellite circuit (called a transponder). Signals are transmitted to the orbiting satellite, which relays it back to another ground station. The footprint coverage of a single satellite system is very large, covering thousands of square kilometers (enough for an entire country that is small in area).

Satellite systems have the advantage of

  • low cost per user
  • high capacity
  • very large coverage

Its disadvantages are,

  • high installation cost in launching a satellite
  • receiver dishes and decoders required
  • delays involved in the reception of the signal

Fiber Optic Cable

This is cable made from fine strands of silica (glass), coated with a plastic sheath. The signals are converted to light pulses using a laser. Each fiber optic strand can support thousands of speech channels and multiple TV channels simultaneously. It is used mainly for long haul links and intercontinental links.

A strand of silica glass fiber (thinner than a human hair), is coated with a reflective surface. When light (provided by a laser) is shown into the strand, it travels along the fiber strand (the reflective layer prevents it from escaping). Fiber optic cable is used for long haul telecommunications links, high speed data communications links for computers, and information services to homes (e.g., Cable TV).

Fiber optic cable has the advantages of

  • high capacity
  • immune to interference
  • can go long distances

Its disadvantages are,

  • costly
  • difficult to join

Cellular Telephone

Also known as mobile phones, cellular telephones are a recent technology designed for mobile users who need to make telephone calls from various locations. The telephone is portable and carried with the user. A cellular phone uses radio frequencies to talk to a nearby cell site (a site that handles cellular calls).

The cellular phone regularly communicates with the nearest cell site to inform the network that it is connected.

A Cell Site is a circular geographical area that handles cellular phones within its defined physical space. Larger coverage is obtained by overlapping cell sites to form a cellular network.

As a user moves location from one cell site to another, the call is transferred to the nearest cell site responsible for that physical area

Each cell site is linked back to a master site that provides an interconnection to the regular telephone network. Calls handled by each cell site are relayed back to the master cell site which then relays it to the telephone network.

Frequencies can be reused by other cell sites making for efficient sharing of facilities. Many calls can be handled by one frequency (especially where digital phones are used).

Cellular telephone suits large geographical areas, including remote sites. It is cheaper to install than traditional copper cable, and is making large inroads into countries with high dense populations that cannot upgrade their existing infrastructures (such as pacific/asia).

Satellite Cellular Telephone

This is cellular phones using low earth orbiting (LEO) satellites. Two such systems in place are - the Iridium and Teledesic satellite networks.

The advantage of a satellite cellular network over a land based one is - much wider geographical coverage, especially in mountainous terrain and at sea. One of the problems of the land based cellular coverage is the amount of cell sites required, and their accurate positioning to avoid blind spots where calls cannot be made.

The Teledesic Network

The Teledesic network is another constellation of low earth orbiting satellites, 288 (plus spares) in total. Its major sponsor is Motorola.

Its purpose is to provide high speed data access to services such as the Internet, video conferencing and high quality voice and data connections.

The Teledesic network can support millions of simultaneous users, with speeds up to 64 Mbps on the downlink path and up to 2 Mbps on the uplink path.


Pagers are small hand held devices that allow one way communication between two parties. Paging was first developed by Charles Neergard in 1949. Neergard, a radio engineer, was annoyed with the way doctors in a hospital continually called over the loudspeaker system. His proposed paging system allowed a much quieter method of informing people that a message was waiting for them.

A ground based radio transmitter sends out a constant stream of messages on a particular frequency. Pagers, which are essentially radio receivers, monitor this stream of messages sent by the transmitter. Each pager has a built-in address code (sometimes called a cap code). When an incoming message is detected that has the pagers built-in address associated with it, the pager decodes and displays the message.

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About the Author
Rajeev Kumar
CEO, Computer Solutions
Jamshedpur, India

Rajeev Kumar is the primary author of How2Lab. He is a B.Tech. from IIT Kanpur with several years of experience in IT education and Software development. He has taught a wide spectrum of people including fresh young talents, students of premier engineering colleges & management institutes, and IT professionals.

Rajeev has founded Computer Solutions & Web Services Worldwide. He has hands-on experience of building variety of websites and business applications, that include - SaaS based erp & e-commerce systems, and cloud deployed operations management software for health-care, manufacturing and other industries.

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