Telephone - Wikipedia
A telephone, or phone, is a telecommunications device that permits two or more users to .. The circuit diagram of the model shows the direct connection of the After World War II, the telephone networks saw rapid expansion and more. supervisee. ER schema diagram for the COMPANY database ER Entities and Attributes. • Entity, which is a “thing” in the real world with an independent. diagram (ER Diagram) and maps it to its Relations, finds its Functional .. functional dependencies database designers can easily and realistically model the real world as a Scroll to the bottom and select About phone.
The first successful telephone transmission of clear speech using a liquid transmitter when Bell spoke into his device, "Mr. Watson, come here, I want to see you. Edison files for a patent on a carbon graphite transmitter.
The patentwas granted 3 Mayafter a year delay because of litigation. Edison was granted patentfor a carbon granules transmitter in Early commercial instruments Early telephones were technically diverse. Some used a water microphonesome had a metal diaphragm that induced current in an electromagnet wound around a permanent magnet, and some were dynamic — their diaphragm vibrated a coil of wire in the field of a permanent magnet or the coil vibrated the diaphragm.
The sound-powered dynamic variants survived in small numbers through the 20th century in military and maritime applications, where its ability to create its own electrical power was crucial. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument. Early telephones were locally powered, using either a dynamic transmitter or by the powering of a transmitter with a local battery.
One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, telephones powered from the telephone exchange over the same wires that carried the voice signals became common.
Early telephones used a single wire for the subscriber's line, with ground return used to complete the circuit as used in telegraphs.
The earliest dynamic telephones also had only one port opening for sound, with the user alternately listening and speaking or rather, shouting into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but also more expensive. At first, the benefits of a telephone exchange were not exploited. Instead telephones were leased in pairs to a subscriberwho had to arrange for a telegraph contractor to construct a line between them, for example between a home and a shop.
Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Unionalready using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Franciscoand Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operatorby whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell in a ringer boxfirst operated over a second wire, and later over the same wire, but with a condenser capacitor in series with the bell coil to allow the AC ringer signal through while still blocking DC keeping the phone " on hook ".
Telephones connected to the earliest Strowger switch automatic exchanges had seven wires, one for the knife switchone for each telegraph keyone for the bell, one for the push-button and two for speaking.
Large wall telephones in the early 20th century usually incorporated the bell, and separate bell boxes for desk phones dwindled away in the middle of the century. Rural and other telephones that were not on a common battery exchange had a magneto hand-cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
Some local farming communities that were not connected to the main networks set up barbed wire telephone lines that exploited the existing system of field fences to transmit the signal. Cartoon by journalist Marguerite Martyn shows a man using a candlestick telephone, In the s a new smaller style of telephone was introduced, packaged in three parts.
The transmitter stood on a stand, known as a " candlestick " for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook".
Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". They, for example, can determine the boundary of the system and the details of the interaction between the systems and the users.
Why flowchart is not popular in system analysis However, there are several reasons that most system analysts actually do not use detailed flowcharts for process specifications. Using flowcharts required users to deeply understand the systems and frequently modified the flowcharts to the simpler way. Flowcharts will be the time consuming and tedious to redraw the flowchart each time because it requires system analysts to redraw it whenever the detailed policy changes.
New diagramming method such as Data Flow Diagrams are more effective way to illustrate a multidimensional reality. Flowcharts are composed of many symbols but the common flowcharts are composed of three basic symbols: Arrows, Diamonds and Rectangles. Arrows represent flow of control. It must be draw from one symbol and end at another symbol. Diamonds represent decision points.
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Nevertheless, it is possible that there are more than two Arrows coming out of it. In that case, it will be the clear indicator of the complex decision and it is usually required to be broken down.
Rectangles represent tasks or activities that occur in the processes. Unlike Data Flow Diagrams which are used to describe data flow within the system, flow charts are typically used to describe the detailed logic of a business process or business rule.
The example of the Flow Chart. Rather than showing the strict order of execution of steps, it shows how processes depend on one another for information. System analysts use it as a tool for modeling and analyzing the processes in the system.
It helps analysts to visualize the data processes since data enter to the system, and then they are used by the system until they are returned to the environment. Analysts also use DFDs to study alternative information handling procedures during the process of designing new information services. In addition, Data Flow Diagrams can be used for compared the new system and the old system. With this comparison, system analysts can find the gap between two systems and the effectiveness of the improved system.
Although Data Flow Diagram is one of the power tools for analyzing the systems, the system analysts must keep in mind that it provides only one view of the system. It focuses mainly on the function-oriented view.
So, if the systems are more concerned with data relationships than the functions, Data Flow Diagram will be less important. Instead, system analysts must concentrate mainly on developing the Entity-Relationship Diagrams. Alternatively, the State-Transition Diagrams will be the most importance if the time-dependent behavior of the system dominated all other issues.
Data Flow Diagram Components A data flow diagram illustrates the processes, data stores, and external entities in a business or other system and the connecting data flows. Squares or Ovals represent external entities, terminators, sources, or sinks. Typically, a terminator is a person or a group of people outside the control of the system being modeled.
It represents where information comes from and where it goes. There are three important things that we must remember about terminators: As a result, neither the system analysts nor system designers are able to change the contents, organization and the internal process within the terminators. Any relationship between each terminator is not a part of the system we are considering, so the system analysts and designers can not include any relationship that exists between terminators.
Thus, in case there are relationships occur between terminators and those relationships are important for system analysts to model a proper system, the terminators are actually part of the system and system analysts should include it as processes in data flow diagram, not terminators.
Circles or Rounded Rectangles represent processes within the system. It shows a part of the system that transforms inputs into outputs. The name of the process in the symbols usually explains what the process does so that it is generally used with verb-object phase.
In some cases, however, the process may contain the name of the person or group of people, or a mechanical device. So, it sometimes tells us who or what is carrying out the process, rather than describing what the process is. Arrows represent the data flows. It can be either be electronic data or physical items or both.
The name of the arrows represents the meaning of the packet data or items that flow along. In addition, like Arrows in flow charts, Arrows in data flow diagrams show direction to indicate whether data or items are moving out or into a process. Open-ended Rectangles represent data stores, including both electronic stores and physical stores. Data stores might be used for accumulating data for a long or short period of times.
Some examples of the procedural questions are what the criteria for moving data from one process to another process and what the order in which the processes are performed. The data output for each process should be modified from the data input. So, the outgoing data should be in the new forms.
Each data store must be involved with at least one data flow. Each external entity must be involved with at least one data flow. A data flow must be attached to at least one process. A data flow can be moved in one direction only. A data flow can not be returned to the process from which it originated. Both branches of a joined data flow must have the same data type.
Data Flow Diagram Levels In the real projects, data flow diagrams are considerably large and complex. So we need to avoid drawing the whole system in only one diagram. System analysts organize the overall DFD in a series of levels so that each level provides successively more detail about a portion of the level above it.
It consists of only one circle representing the entire system, the data flows showing the interaction between the system and the external terminators.
There is no data stores appear on a context diagram because the data stores of the systems are conceptually inside the one process. The context diagrams are useful for showing how a proposed system may behave for a specific example or scenario.
Level-0 diagram is the decomposition of the one process from the context diagram into two to nine high-level processes. Then, each process in the Level-0 diagram can be decomposed into the next level.
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In order to properly create many levels of data flow diagram, you must understand two concepts: Decomposition is the iterative process by which system description has been broken down into deeper detail. One process on a given diagram is then explained in greater detail on a lower-level diagram.
Balancing is the conservation of inputs and outputs to a data flow diagram process when that process is decomposed into a lower level.
The example of the Data Flow Diagram. It shows how a system interacts with the external entities. So, it is relatively sparse about the details of how the system behaves internally and how the external environment is configured. Indeed, Use Case Diagram shows what we want the system to do rather than describe how it can be accomplished.
One of the major benefits of this diagram is communication. Use cases, Actors, and Associations and System boundary. Those actions must provide the measurable value to an actor. It is represented by horizontal ellipse. Furthermore, it can be networks, communication devices, computers, or other programs on the same computer. It is represented by stick figure. It is represented by lines connecting between use cases and actors with an optional arrowhead on one end of the line. Notice that the arrowheads in Use Case diagram is used for indicating the direction of the initial invocation of the relationship or to indicate the primary actor, while the arrowheads of Data Flow Diagram is used for showing the flow of data in the system.