COMMUNICATION TECHNOLOGIES

Archive for the ‘Tele Communication’ Category

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Losses transmission line is connected to a source or generator a uniform traveling wave appears along the line in forward direction from source towards load and maximum energy is said to be transferred from a source to load. Instead, the same condition can be achieved if a transmission line of fine length is terminated by a load equal to characteristic impedance of the line at far end from the source end. If the same transmission lien is terminated by a load other than the characteristic impedance mismatch occurs and a backward wave appears on the line in backward direction from load to source end. Due to interference of in these forward and backward waves standing wave occurs which is not desirable and is measure of energy loss.

Thus if the line is losses of infinite length or a fine transmission line is terminated with characteristic impedance, then all the energy sent through forward or incident waves is absorbed by the load and hence no back ward or reflected waves and thereby no loss across the line i.e. all the energy is transferred from source to load without any loss. A single device which is dipole antenna simultaneously possessing the properties of a radiating system, a transmission line and a resonant circuit or resonator. A dipole antenna not only acts as an antenna but also as a resonant circuit or as a resonator, because the energy is reflected from one point to the other of the dipole which gives rise to a standing wave along the length of the antenna.

All LCD displays consists of millions of tiny sub-areas called pixels as in CRT screens. A simple grid is used to supply charge to particular pixel in the display. The grid is formed by two layers called substrates, which are made from a transparent conductive material, usually indium-tin oxide. One substrate is for columns and the other for rows. The rows and columns are connected to integrated circuits that control the charge are connected to integrated circuits that control the charge of particular row or column. The liquid crystal is sand witched between glass substrates, and a polarizing film is added to the other side of each substrate.

To turn ON a pixel, the integrated circuit send a charge to down correct column of one substrate, and a ground that is activated on the corresponding row of other substrate.The rows and columns intersect at the chosen pixel and these deliver a small voltage to untwist the liquid crystals at that pixel. By this effect all the pixel placed in the screen can create black and white colors, when they receive a predetermined charge. If the voltage applied to a liquid crystal is carefully controlled, it can be made to partially untwist to allow some light to pass through it. LCDs can create a grey scale that varies from full brightness to darkness. Most TV screens create 256 levels of brightness per pixel.

Cordless Telephones:

Cordless telephones are simply telephones that operate without cords attached to the handset. Cordless telephones originated around 1980 and were quite primate by today’s standards. They originally occupied a narrow band of frequencies near 1.7 MHZ, just above the AM broadcast band, and used the household power line for an antenna. These early units used frequency modulation (FM) and were poor quality and susceptible to interference from fluorescent lights and automobile ignition systems. In essence, a cordless telephone is a full-duplex, battery-operated, portable radio transceiver that communicates directly with a stationary transceiver located somewhere in the subscriber’s home or office.

The base station is an ac powered stationary radio transceiver connected to the local loop through a cord and telephone company interface unit. The interface unit functions in much the same way as a standard telephone set in that its primary function is to interface the cordless telephone with the local loop while being transparent to the user. Therefore, the base station is capable of transmitting and receiving both supervisory and voice signals over the subscriber loop in the same manner as a standard telephone. The base station must also be capable of relaying voice and control signals to and from the portable telephone set through the wireless transceiver.

Caller ID:

Caller ID is a service originally envisioned by AT & T in the early 1970s, although local telephone companies have only recently offered it. The basic concept of caller ID is quite simple. CallerID enable the destination station of a telephone call to display the name and telephone number of the calling party before the telephone is answered. This allows subscribers to screen incoming calls and decide whether they want to answer the telephone. The CallerID message is a simplex transmission sent from the central office switch over the local loop to a CallerID display unit at the destination. The CallerID information is transmitted and received using bell system 202-compatible modems. This standard specifies a 1200-bps FSK signal with a1200-HZ mark frequency and a 2200-HZ space frequency. The FSK transmitted in a burst between the first and second 20-HZ, 90-vrms ringing signals. Therefore, to ensure detection of the CallerID signal, the telephone must ring atleast twice before being answering.

With The Advent of automated switching centers, a hierarchy of switching exchanges evolved in North America to accommodate the rapid increase in demand for long-distance calling. Thus, telephone company switching plans include a switching hierarchy that allows a certain degree of route selection when establishing a telephone call. A route ids simply a path between two subscribers and is comprised of one or more switches, two local loops, and possibly one or more trunk circuits. The choice of routes is not offered to subscribers.

Classes of Switching Offices:

Before the divestiture of AT&T in 1984, the bell system North American switching hierarchy consisted of five ranks or classes of switching centers. The highest –ranking office was the regional center, and lowest was the end office.

End Office:

A class 5 office is a local exchange where subscriber loops terminated and received dial tone, end offices interconnected subscriber loops to tandem trunks, interoffice trunks and toll-connecting trunks, subscribers received unlimited local call service in return for payment of a fixed charge each month, usually referred to as s flat rate.

Toll Center:

The class 4c toll centers provided human operators for both outward and inward calling service. Class 4p offices usually had only outward operators service or perhaps no operator service at all. examples of operators-assisted services are person-to-person calls collect calls and credit calls, class 4 offices concentrated traffic in one switching center to direct outward to the proper end office.

Primary Center and Sectional Center:

This office provided service to small groups of class 4 offices within a small area of a state. Class 3 offices provided no operator assistance; however, they could serve the same switching functions as a class 4 offices, class 3 office generally had direct trunks to either a sectional or regional center. Sectional center could provide service to geographical regions varying in size from part of a state to all of several states, depending on population density no operator services were provided; how ever a class 2 offices could serve the same switching function as class 3 and class 4 offices.

Regional Center:

Regional centers were the highest-ranking office in the DDD network in terms of the size of the geographical area served and the trunking options available. Ten regional centers were located in the United States and two in Canada.

The Public Telephone Network:

The public telephone network accommodates two types of subscribers: public and private. Subscribers to the private sector are customers who lease equipment, transmission media and services from telephone companies on a permanent basis. The leased circuits are designed and configured for their use only and are often referred to as private-line circuits or dedicated circuits. For example, large bank do not wish to share their communication network with other users, but it is not cost effective for them to construct their own networks. Therefore, banks lease equipment and facilities from public telephone companies and essentially operate a private telephone or data network within the PTN. The public telephone companies are sometimes called service provider, as they lease equipment and provide services to other private companies, organizations, and government agencies.

Instruments and Local Loops:

An instrument is any device used to originate and terminate calls and to transmit and receive signals into and out of the telephone network, such as a 2500-type set, a cordless telephone, or a data modem. The instrument is often referred to as station equipment and the location of the instrument as the station. A subscriber is the operator or user of the instrument. If you a home telephone, you are a subscriber. The local loop is simply the dedicated cable used to connect an instrument at a subscriber’s station to the closest telephone office. In the United States alone, there are several hundred million miles of cable for local subscriber loops. Every one who subscribes to the PTN is connected to the closest telephone office through a local loop.

Trunk Circuits and Exchanges:

A trunk circuit is similar to a local loop except trunk circuits are used to interconnect two telephone offices. The primary difference between a local loop and a trunk is that a local loop is permanently associated with a particular station, whereas a trunk is a common-usage connection. A trunk circuit can be as a pair of copper wires twisted together or as sophisticated as an optical fiber cable. An exchange is a central location where subscribes are interconnected, either temporarily or on a permanent basis. Telephone companies switching machines are located in exchanges. Switching machines are programmable matrices that provide temporary signal paths between two subscribes.

The North American Telephone Numbering plan was established to provide a telephone numbering system for the United States, Mexico, and Canada that would allow any subscriber in North America to direct dial virtually any other subscriber without the assistance of an operator. The network is often referred to as the direct distance dialing network. Prior to the establishment of the NANP, placing a long distance telephone call began by calling the long distance operator and having her manually connect you to a trunk circuit to the city you wished to call. Any telephone number outside the caller’s immediate areas was considered a long-distance call.

North American is now divided into numbering plan areas with NPA assigned a unique three-digit number called an area code. Each NPA is further subdivided into smaller service areas each with its own three-digit number called an exchange code. Initially, each service area had only one central telephone switching office and one prefix. However, today a switching office can be assigned several exchange codes, depending on user density and the size of the area the office services. Each subscriber to a central office prefix is assigned a four-digit extension number. The three- digit area code represents the first three digits of a 10-digit telephone number, the three-digit prefix represents the next three digits, and the four-digit extension represents the last four digits of the telephone number. Initially, within the North American telephone numbering plan area, if a digit could be any value from 0 through 9, the variable x designated it.

If a digit could be any value from 2 through 9, the variable N designated it. The popularity of cellular telephone had dramatically increased the demand for telephone numbers. North America ran out of NPA area codes, so the requirement that the second digit be a 1 or 0 was dropped. This was made possible because by 1995 there were very few electromagnetic switching machines in use in North America, and with the advent of ss7 signaling networks, telephone numbers no longer had to be transported over voice switching paths. The international telecommunications union has adopted an international numbering plan that adds a prefix in front of the area code, which outside North America is called a city code.

Ringer Circuit and Equalizer Circuit:

The ringer circuit, which was originally an electromagnetic bell, is placed directly across the tip and the ring of the local loop. The purpose of the ringer is to alert the destination party of incoming calls. The audible tone from the ringer must be loud enough to be heard from a reasonable distance and offensive enough to make a person want to answer the telephone as soon as possible. In modern telephones, the bell has been replaced with an electronic oscillator connected to the speaker. Today, ringing signals can be any imaginable sound, including buzzing, a beeping, a chiming, or your favorite melody. Equalizers are combination of passive components that are used to regulate the amplitude and frequency response of the voice signals. The equalizer helps solve an important transmission problem in telephone set design, namely, the interdependence of the transmitting and receiving efficiencies and the wide range of transmitter currents caused by a variety of local loop cables with different DC resistances.

Speaker:

In essence, the speaker is the receiver for the telephone. The speaker converts electrical signals received from the local loop to acoustical signals that can be heard and understood by a human being. The speaker is connected to the local loop through the hybrid network. The speaker is typically enclosed in the handset of the telephone along with the microphone.

Microphone and Dialing Circuit:

For all practical purposes, the microphone is the transmitter for the telephone. The microphone converts acoustical signals in the form of sound pressure waves from the caller to electrical signals that are transmitted into the telephone network the local subscriber loop. The microphone is also connected to the local loop through the hybrid network. Both the microphone and the speaker are transducers, as they convert one form of energy into another form of energy. The dialing circuit enables the subscriber to output signals representings, and this enables the caller to enter the destination telephone number.

Hybrid Network:

The hybrid network in a telephone set is a special balanced transformer used to convert a two-wire circuit into a four-wire circuit. Inessence, the hybrid network separates the transmitted signals from the received signals. Outgoing voice signals are typically in the1-v to 2-v range, while incoming voice signals are typically half that value. Another function of the hybrid network is to allow a small portion of the transmit signal to be returned to the receiver in the form of a sidetone.