Telecommunications

INTRODUCTION

Welcome to the Information Factory's course "Introduction to Telecommunications."

If we attempted to define Telecommunications, it would probably be "Communications by Electronic Means." This communications could be voice, computer data or paper records (telegram, telex, etc.).

Twenty-five years ago we didn't hear the word Telecommunications and now it's one of the most important parts of any business. Although the term telecommunications may be new, the ideas have been with us a long time. Telecommunications used to be concerned with whether or not your phone rang when a customer called or did you even get a phone on your desk. Maybe you got to the big time by having a telephone with multiple buttons and lights (then we all learned what "please hold means). Now we talk about Telecommunications as a field of specialization, a growth industry and finally we recognize it as a business resource.

Have you ever heard someone say "I'm in business for myself!" If you stop and think about that statement it's impossible unless you sell a product or service to yourself and buy your materials from yourself. Business, or the flow of goods and services doesn't just happen, it requires communications between two or more people. In response to the communications the goods and services will flow.

Here are some ways in which telecommunications can affect your life:

• Airlines reservation systems allow customers to call in, book flights, arrange seating, meals and even rent cars at their final destination. These reservations can be made through telephone calls or via direct personal computer connections.

• Large stock traders such as mutual funds use automatic computer programs to buy and sell stock. These programs track the stock prices and by using automatic margins know when to buy up and sell the stock. Some of the wild fluctuations in the 1987 market were blamed on these types of transactions.

• Mail order catalog services accept orders using 800 type toll free telephone numbers. At the telephone centers the orders are accepted and entered directly into computers that may be miles away.

Before you can understand how the telephone or computer communications work, you need to understand some basic concepts. We will begin this course by introducing you to the basic ideas and concepts of telecommunications in some simple telegraph networks. Although the telegraph is not widely used today, it is the grandfather of our present technology network concepts.

Learning the terminology (or "buzzwords") can be half the battle and the key to understanding any subject. In this section we are going to teach you how to understand the telecommunications dialect and use it as a bridge to more complex subjects which come later in this course.

Telegraph circuits do not have much application in modern voice and data transmission but they are a basic form of communications. Because they are simple, telegraph circuits will help you to understand the flow of information and operation of telecommunications circuits. As you learn the concepts involved in the simple communications network you will build your confidence to tackle the larger more complex networks in the later parts of this course.

TELEGRAPH CIRCUITS

The telegraph system was the earliest form of electrical communications and from it evolved many of our modern day communication systems terminology and concepts.

After studying this section you should see that the only difference between the old and new systems is the technology used to implement them and the speed with which they operate. After studying this section you should be familiar with basic communications concepts and the terminology associated with telecommunications.

CIRCUIT COMPONENTS

As you examine the different forms of telegraph circuits notice that each is made up of various combinations of the same components. These components are used to send, receive or carry the message. When you look at the circuit drawings, it is easier to use symbols to represent the actual components than it is to try and draw a key, sounder, circuit or battery each time. The following is a description of the components found in a telegraph circuit and the symbols used to represent them in our drawings.

Figure 1.1 Key or Transmitter

The key is the unit used to transmit or enter information into the communications circuit. Pressing the key causes electrical current to flow in the circuit. Releasing the key will open the circuit and stop the current flow.

Figure 1.2 Sounder or Receiver

The sounder is used to receive the data sent via the telegraph circuit. When an electric current flows through the circuit, the magnet pulls the clapper down and a "click" is heard. When the current in the circuit stops, the magnet releases the clapper and the spring restores it to the open position.

Figure 1.3 Circuit or Transmission Path

The circuit or transmission path connects the sounder and key to complete the circuit. The actual circuit in the telegraph system is a pair of wires (or electrical conductors). In some cases the wire pair will be shown on the drawings as a single line.

Figure 1.4 Battery or Signal Source

The battery provides the power or electrical signal for the telegraph circuit.

COMMUNICATIONS MODEL

Before studying the basic circuit configurations it would help if we studied the basic communications model (Figure 1.5). This model is supposed to outline the concept of how communications flows between two points. The model is used by students of communications technology as a basic starting point to examine communications circuits (and eventually networks)

Figure 1.5 Communications Model

In the model a message flows from the "Source" to a "Sink." The source is can also be called the sender. The sink can be referred to as the receiver. The information or message flows between the source and sink through a transmission path or circuit.

If we were to examine a letter sent via mail between you and a friend, you would be the source and your friend would be the sink. The letter would be the message and the mail would be the transmission path. If your friend wrote a response, the model would reverse and you would be the sink and your friend would be the source.

In this section of the book we will be unconcerned with the content of messages only with the method of sending and receiving. In the following telegraph circuits, the key will be the source (sender) and a sounder at the far end will be the sink (receiver).

The model only explains one way flow of information and does not verify that the information is sent or received correctly. The content of messages between the sender and receiver confirm the delivery or acknowledge reception of information.

BASIC CIRCUIT CONFIGURATIONS

With the few components described above we can now assemble most of the basic circuits used in communications. As you read this part of the text pay close attention to the key words in quotations. These are terms and definitions still in use in modern communication networks. Also note that the basic circuit types are identical in operation to modern data and voice communications circuits.

Figure 1.6 Simplex Circuit

If we place a key at one end of a transmission line and a sounder at the other end, messages can be sent along the circuit. This simple form of circuit is called a "Simplex" circuit (shown in Figure 1.6).

When the key is at rest, the circuit is "broken" or "open" and no electrical current flows. When the key is "closed" or "depressed" it completes the circuit and the current flows from the battery or power source. The power energizes the electromagnet in the sounder at the other end making an audible click. By opening and closing the key an operator can send a message to a listener at the far end of the circuit.

The simplex circuit is limited in usage because you have no direct way of knowing if the message is being received. For reliable communications there must be a way to send back a message saying "I got it!" or "please send again!"

The simplex circuit can be used in communications systems where a direct response is not required. Some examples of modern simplex circuits are:

Alarms - Where a signal is sent to a security service or the fire department.

Signals - The circuit can turn on and off traffic lights, railroad signals, etc.

Timing - In limited use today are clocks which get signals from master clocks. These are used in schools and factories to insure precise beginnings and endings of classes or work shifts.

Stock Tickers - The message boards at brokers and exchanges are driven through simplex circuits.

Although simplex circuits do not acknowledge their messages directly, there is a "cause and effect" type of feedback. For example if the stock ticker was not working, a broker would call the service company. For telegrams and other person to person communications we need an immediate response to the quality and reception of the message and the cause and effect type of feedback is not acceptable.

Figure 1.7 Half Duplex Circuit

In the "Half-Duplex" circuit, there is a key and a sounder at each end of the transmission line. Depressing either key will cause the sounders at both ends to operate. With an operator at each end of this circuit, messages can be sent in either direction. The name "Duplex" indicates the capability of two way transmission, while "Half-Duplex" indicates the circuit can be used in only one direction at a time. There is only one physical path between the two points.

If both operators attempt to send a message on a half-duplex circuit at the same time, their transmissions will mix and become unintelligible. To prevent chaos the half-duplex circuit requires a set of rules that determine:

• Who can transmit

• How messages are formatted

• How to tell if a message was received

These rules were given to the operator as a "line discipline" or "protocol." The line discipline told the operator how they would ask to use the line and after a message was received to acknowledge it or ask for a retransmission. Line disciplines are still in use today as protocols on the modern data transmission systems used by computers.

You should also note that the half-duplex circuit forms the beginning of a simple communications network where:

• There is the ability to send messages in both directions

• Reception of messages can confirmed

• Operators require sophistication and training to use the circuit

As the circuits become more complex the operation and use will also become more complex. Later when we examine multipoint circuits we will also examine line disciplines in depth.

Figure 1.8 Full Duplex Circuit

The "Full-Duplex" circuit is capable of two way, simultaneous transmission. we again use the term duplex to indicate that the circuit can transmit in both directions. The prefix full indicates that transmission can be in both directions at the same time. This circuit can actually be viewed as two simplex circuits. Each of the simplex circuits carries a message in opposite direction.

Because there are separate paths for the messages, two way simultaneous transmission can occur. In reality the two way transmission would be limited because it would require two operators at each end, one transmitting and one receiving. This brings up an interesting distinction between operation of a circuit and the physical design. If we only had one operator at each end of the full-duplex circuit, they would have to operate the circuit in a half-duplex manner because they could not send and receive simultaneously. So in this case the circuit would be full-duplex but operating in a half-duplex line discipline. Even today we will see computer systems connected over full duplex circuits that are limited to half duplex transmission by either their hardware or software. Think about the standard telephone call where both sides can hear each other (full duplex) but it is impractical for both to talk in anything but a half duplex manner.

If there is a problem operating the full-duplex circuit, why do we use it? In the telegraph networks, it was sometimes used over high traffic routes where multiple operators could be justified. The full-duplex circuit allowed one location to send a continuous stream of messages without stopping to wait for an acknowledgement (these came over the return circuit instead). In modern computer networks, full duplex circuits will be seen as a method of improving line throughput.

Even in the telegraph circuits, the full duplex circuit could be used to improve throughput. As an example, if an operator missed part of a message, he could send a request on the return channel for the originating site to start the message over again immediately. Time could be saved by having the message restarted without waiting for the operator to get to the end as would be the case in the half duplex circuit.

Figure 1.9 Multipoint Circuit

In the early telegraph circuits a large portion of the cost was the transmission path. Remember that the telegraph was the first practical use of electricity in modern society (predating the electric light). The manufacture of wire was expensive and not yet large industry. Consider that the telegraph companies were a new business and every route had to be pioneered with land use and right-of-ways purchased, poles put in place and wire strung. Finally the route had to generate enough traffic to repay the investment.

Messages were moving at the speed of light between cities and the first information revolution was beginning. If a city was not big enough to support its own telegraph line it could have been cut off from joining the 19th century.

A city could justify its own circuit if it could share it with other cities similar to an old party line telephone. By routing a single circuit through multiple cities the construction and operating costs could be shared. This type of shared circuit is referred to as a "Multipoint" circuit.

All of the circuits covered up to now have been "point-to-point" meaning they connected one single point to another single point. In this circuit multiple points will be connected (hence the name multipoint).

An example of the multipoint circuit is shown in Figure 1.9. In this circuit a half-duplex line has been extended to include transmitters and receivers in four cities. When any key is depressed, all of the sounders in each city will actuate. Now the cities could take turns sending messages over the common transmission path.

This multipoint circuit has large cost saving advantages. If each of the remote cities was connected directly to New York (see Figure 1.10), the mileage would be:

Since the circuit was made multipoint, the actual mileage was:

In terms of transmission facilities, the telegraph company can save 592 miles of cable with this example. It must be remembered that if any city in the network has a large volume of messages, it may require a separate circuit or it could tie up the line preventing the other cities from using it.

Our example shows telegraph offices in New York (NY), Buffalo (BF), Niagara Falls (NF) and St. Catharines (SC). The two letter codes are station addresses used for directing messages. By adding the addresses to the line discipline, the operators can direct their messages to individual cities on the circuit. If the operator in Buffalo wished to send a message to New York , he would prefix the message with "NY" and all other operators would ignore the transmission.

The common transmission path also created a new need. If all of the operators attempted to send at the same time, the result would be chaos at each receiver. Rules had to be set down that told who could transmit first, who would listen and how messages would be formatted. Telegraph operators were trained in the operating procedures of the of the circuit under a set of rules called a line discipline (or protocol).

Figure 1.10 Comparison of Multipoint and Point to Point Circuits

There a two basic types of line disciplines which could be used to run the multipoint circuit. One is "Polling" and the other is "Contention."

POLLING OVERVIEW

In the polling protocol there must be a "Master" station. The master will be the station that tells everyone when they can use the circuit. All of the other stations on the circuit are referred to as "slaves" or "tributaries." The "master" controls the message (or traffic) flow by setting up a roll call of the stations called a polling list. This roll call (or polling) sends a message to each station asking it if it needs to use the circuit to send a message. Looking at Figure 1.11 we can see an example of how the polling takes place. The master station has been designated as New York (NY). "NY" sends a short polling message to "NF?" ( Niagara Falls). This poll message is the request for traffic on the circuit. If "NF" did not have a message it would respond "NO" and the master (NY) would poll the next station. This polling would continue around the network until the end of the polling list was reached, then "NY" would start at the top again.

NY Polls Each Station

Figure 1.11 Telegraph Multipoint Polling Protocol

If during the polling a station did have a message, it would wait for its poll from the master. As an example in Figure 1.12, when Buffalo is polled, it responds by sending the traffic it is holding. When New York receives the message, it will send a response to Buffalo . The response to the message is called an "Acknowledgment." In telegraph protocols the acknowledgment may be simply the letters "RRRR" which mean "I received everything." If the message was not received the master may send a negative acknowledgement such as "Please resend everything after......" or even a request to resend the entire message.

NY Polls Traffic From NF

Figure 1.12 Traffic Response to Polling

Once a message is sent and acknowledged, the master station will return to polling. In this type of protocol, the general rule for the tributaries is "speak when requested".

Variations on the polling protocol would include a technique called "downstreaming" which allows a station on the line to talk directly to another station. Usually polling networks send the messages to the master who resends them to other stations. In the case of downstreaming, when the station is polled it responds by temporarily becoming the master and sending a poll message to another slave station, in this way downstream stations communicate directly with each other.

CONTENTION OVERVIEW

With a contention protocol all of the stations on the network are "peers." That is each station has the same status with no master or slaves.

Contention (Figure 1.13) begins with the line in an idle state (no messages flowing). When a station has a message, the operator listens to be sure the circuit is idle. After a short period the operator will send a "bid" or request to use the circuit. The bid is usually a short transmission similar to a poll message asking a remote station if it is ready to receive. If the addressed (remote) station can receive the message it will respond by acknowledging the bid.

Figure 1.13 Telegraph Contention Protocol

When the bid is acknowledged, control of the circuit belongs to the transmitting station. This is similar to assigning a temporary master status to the successful bidder (station). All of the remaining stations on the line will ignore further transmissions until the transmitting station returns the line to idle.

Once the bid is accepted the transmitting station sends its message to the addressed or receiving station. The receiving station will either acknowledge or request retransmission of the message. If the transmission was successful the transmitting station will send a short "end of transmission" message and return the line to the idle state.

Figure 1.14 Contention Traffic Response

The decision whether to use the polling or contention protocol can usually be made through the analysis of the circuit traffic requirements. If the circuit has high traffic from each city, polling would be better because it gives all stations an equal chance at the network. If the volume is low polling would waste a great deal of time so, contention would be a wise choice. In some cases the protocol can be mixed where the circuit operates contention protocol during low traffic periods and polling protocol in peak traffic periods.

MULTIPOINT SUMMARY

The subsection on the multipoint circuit was longer than the other basic circuits because it is an excellent example of the interdependency of circuit design and operation. It enables you to see how the line discipline (or protocol) fits together with the circuit to form a communications link or network.

There is also the concept of network design to be considered where a cost effective method had to be found to allow a city to join the growing telegraph network. By analyzing traffic requirements an engineer could decide that hooking together certain cities would justify a circuit into a major hub city (such as New York).

In conclusion, the multipoint circuit has cost advantages that can be realized even with several low volume stations. The line protocol is a key element in the operation of the multipoint network.

Morse Telegraph

Most of us know that telegraph systems communicated using a special series of sounds called the Morse Code. This code was named after the inventor of the first practical telegraph Samuel Morse.

When Samuel Morse built his telegraph in 1835, it was not the type seen in western movies where the operator takes down the message while listening to the clicks on a sounder. Instead the Morse Telegraph made long and short marks (dots and dashes) on a moving strip of paper as shown in Figure 1.15. The operator would then transcribe the message above the dots and dashes on the strip of paper.

Figure 1.15 Morse Telegraph

As shown in the drawing, the paper strip was constantly moving under the pen. Whenever the key was closed, current flowed through the circuit and the magnet was energized drawing the pen down. So with the current flowing, the pen made a mark on the paper. With the key released, the current flow stopped, the magnet de-energized and the spring pulled the pen away from the paper so that a space was made.

Because of this type of equipment the terminology of "Marking" and "Spacing" was derived. Marking on the paper meant that current was flowing on the line, while spacing (no marks on paper) meant that no current was flowing. The use of the terms mark and space carried through the telegraph era and are used in modern data communication systems.

The "Morse Code" was one of Samuel Morse's most important contributions to electronic communications. Many inventors in the period built and demonstrated telegraph systems but they were complex and required multiple wires to transmit each character of information. Morse designed a code to represent the characters and numbers using dots and dashes so that they could be transmitted serially over one pair of wires. Morse Code is still in use today by radio operators all over the world but more importantly the same techniques that were used to develop a code for the telegraph system can be applied to developing a code for modern data transmission.

In most modern communications systems we have advanced to digital transmission techniques which are based on the binary numbering systems. In the binary system, numbers can be defined as a series of ones (1's) and zeros (0's). It is easy to see the relationship between our telegraph which has two states "Mark" and "Space," and digital transmission which also has two states "One" and "Zero." This is important because in talking to some technical personnel they may ask "is the line marking or spacing?" which translates to "Are you sending ones or zeros?" In addition communications lines may be shown as a high or low electrical current representing dots and dashes or marks and spaces. As an example of one type of communications link may use:

SUMMARY

Since the telegraph was the earliest form of electrical communications, it gave birth to many of the terms and ideas used in modern communication systems. As the telecommunications industry built new equipment it continued to follow old forms and procedures which were based on proven principles of communications. As you work through this course you will see the various types of circuits and procedures are repeated in modern forms.

It should be noted at this point that various key terms will repeat throughout this text. Modern communications circuits are still described as half or full duplex. The method for systems communicating with each other is still called a protocol or line discipline. The signals on lines are still measured in mark or space conditions.

This section was intended to introduce the various types of basic circuits and some telecommunications terminology. The information in this section will be used as a foundation for the study of the balance of this text.

As you work through the course, periodically you may want to return to this section to review a term or principle of communications. Whenever necessary review the section to help in understanding the more advanced concepts in later parts of this text.