TELEPHONY FUNDAMENTALS

The word "Telecommunications" is nothing but a term which defines devices and systems which is used to transmit electronic or optical signals across long distances.It is only because of this telecommunication, people around this world are able to contact one another,and they are able to access the information at an instant,and they are also able to communicate from remote areas.

The process of telecommunications usually involves a sender, who transmits the information and one or more recipients, and both of them are linked by a technology.Usually this technology which is used to connect thesender and a receiver includes a telephone system,for example,which transmits the reqired information from one place to another.

This process of transfering message is not only confined to a very small are but telecommunications enables people to send and receive personal messages across town, between countries, and to and from outer space. It also provides the key medium for delivering the news, datsa, information, and also for entertainment purpose,such as broadcasting live sports programmes.

Usually the telecommunications devices convert different types of information, such as sound and video, into electronic or optical signals.The reason why we use electronic signals for transmission is that it typically travel along a medium such as copper wire or are carried over the air as radio waves.

Apart from this, Optical signals are specially used at present as they typically travel along a medium such as strands of glass fibers or optical fibers.Normally when a signal reaches its destination, there will be a device on the receiving end, and this converts the signal back into an understandable message, such as sound over a telephone(where electrical is converted into sound), moving images on a television, or words and pictures on a computer screen.

For the transmission of the telecommunication message, there are a variety of ways used.The number of ways of sending a message and it also depends on number of sender and receiver,for example

1)One sender to a single receiver - point-to-point

2)One sender to many receivers - point-to-multipoint

In the case of the personal communications, such as a telephone conversation between two people or a facsimile (fax) message (see FacsimileTransmission),point-to-point type of transmission is used  transmission.

For broadcasting purposes we generally use Point-to-multipoint telecommunications, and this type provides the basis for commercial radio and television programming.

 

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://www.boddunan.com/education/19-Engineering/5313-telecommunication-basics.html

TELEPHONY FUNDAMENTALS

Telephone-Line Basics For Modems

Modem connections to the telephone service are made using two wires (ring and tip) that are used in a standard telephone jack. The wires are named for the plug wires used in the original telephone lines by which telephone operators would manually connect two telephones at the phone company switchboard. There are two versions of the telephone jack:

• Half-duplex: The RJ-11 has only two wires, which make up one line. Therefore, only one signal can be sent or received at a time.
• Full-duplex: The RJ-12 uses four wires to make up two lines; it can be used to simultaneously send and receive.

Multifunction Modems

Most modems offer some form of fax capability, along with software that adds functions beyond the average, small, stand-alone fax machine. Such a modem is usually labeled a fax/modem. They can store faxes, both incoming and outgoing, for reference or online reading. Most allow direct faxing of a document from a word processor, generally by using the print command to send the pages to the modem, where they are converted on the fly to the bitmap form used to send and receive fax transmissions. Many programs let you to automatically attach a predesigned cover sheet with each fax.

Another addition to the basic data out/data in modem is voice mail. Here, the PC and telephone work just like an answering machine. If the phone rings and the modem does not detect either a data or fax tone, it switches modes and streams a recorded message (the outgoing message). The caller can be prompted to record a message for the owner, and in some cases the modem will even forward a pager call or fax with the message contents.

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://www.brainbell.com/tutors/A+/Hardware/Modem_Basics.htm

TELEPHONY FUNDAMENTALS

IP telephony testbed

Within Mitretek Systems' Advanced Telecommunications Laboratory (ATL), a VoIP testbed has been set up. The testbed consists of two PC clients, two IP gateways, four analog phones, and a PC-based private branch exchange (PBX) (see Figure 1). The purpose of the testbed is to measure and calibrate traffic generated from the voice conversations on the PCs and the analog phones. To collect this information, a packet sniffer, Observer from Network Instruments, has also been installed in the VoIP testbed.

Figure 1: IP telephony testbed connectivity

The IP gateways use the standard H.323 call control protocol and several standard codecs: GSM120, G.711, and G.723.1. The PC-based PBX served as a voice switch in the testbed. The PC clients were connected to a hub with private IP address space. Different codecs were used in measuring the packet delay, jitter, packet loss, and bandwidth utilization.

Both PC-to-PC and phone-to-phone testing was conducted in an intranet environment. In the phone-to-phone experiment, the end-to-end delay was measured as 274 ms. A large portion of the delay is due to the analog/digital conversion, compression, packetization, and OS overhead in the IP gateways. When faster digital signal processors (DSPs) become available, the delay introduced by the equipment should be significantly reduced. The purpose of conducting intranet testing is to provide a performance baseline. When this experiment is expanded to the Internet, the additional delay elevated by Internet can be better understood.

Table 1: Packet size distribution over 30 seconds

		Packet Size (Bytes)
		<=64	65-84	85-128	129-512	513-1024	>1024
G.723.1 6.4 kbps	Listener	0.20%	81.20%	0.70%	0.30%	0.00%	17.50%
	Talker	0.10%	98.30%	0.90%	0.70%	0.00%	0.00%
G.723.1 5.333 kbps	Listener	0.00%	81.40%	1.10%	0.30%	0.00%	17.20%
	Talker	0.10%	98.10%	1.30%	0.50%	0.00%	0.00%
G.711 64 kbps	Listener	2.30%	0.30%	1.90%	50.60%	0.00%	44.80%
	Talker	4.20%	0.60%	3.50%	91.70%	0.00%	0.00%

Table 1 represents the results from a 30-second half-duplex voice session. The columns are packet sizes. With the G.723.1 coder, 81.2% of the packets from the Listener were 64 to 84 bytes in size. When using G.711, the talker sends data out in 310 frames. The remaining packets from the talker to listener are called control packets from the application ranging in size from 64 to 128 bytes. When using the G.723 codecs, the data are in 78-byte frames. When the listener sends data to the talker, the packet size is between 84 and 100 bytes, regardless of the codec. Other packets are from the Observer software or are call control packets from NetMeeting.

Additional tests will be taken in a mixed voice and data environment. The data behavior is highly application dependent. Instead of using a generic data generator to produce the necessary data load, a script language, SILK by Sagueway, which emulates the application transactions, will be programmed to produce Microsoft Word, Microsoft Excel, Microsoft PowerPoint, and Web traffic. It is felt that combinations of these application transactions represent the actual network data behavior better than the traffic generated by the network management device. However, the emulation approach is appropriate only for a reasonable system size. For a large-network system, simulation and analytic models are required to facilitate the analysis.

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://www.isoc.org/inet99/proceedings/4p/4p_2.htm

TELEPHONY FUNDAMENTALS

Anatomy of a Cordless Telephone

To illustrate the parts of a cordless telephone, we will show you the inside of this one from General Electric (GE). It was made in 1993 and operated in the 43-50 MHz range.

GE cordless phone, including handset and base unit

As mentioned above, all cordless phones have a base and a handset. Let's look at these parts individually.

Base
The base unit of the cordless phone is plugged into the telephone jack on your wall.

Base unit components

If you open up the base and expose the circuit board, you see several components that carry out the functions of the base:

• phone line interface - receives and sends telephone signals through the phone line
• radio
  • amplifies signals to and from phone-line interface, user controls and speaker phone (if present)
  • broadcasts and receives radio signals to and from the handset
• power - supplies low voltage power to the circuits and recharges the battery of the handset

Circuit board in the base of the GE cordless phone

Phone Line Interface
Phone line interface components do two things. First, they send the ringer signal to the bell (if it's on the base) or to the radio components for broadcast to the handset. This lets you know that you have an incoming call. Second, they receive and send small changes in the phone line's electrical current to and from the radio components of the base. When you talk, you cause small changes in the electrical current of the phone line, and these changes get sent to your caller. The same happens when the caller talks to you.

Radio Components
The radio components receive the electrical signals from the phone line interface and user controls (keypads, buttons). The radio components convert the signals to radio waves and broadcast them via the antenna. Radio components use quartz crystals to set the radio frequencies for sending and receiving. There are two quartz crystals, one for sending signals and one for receiving signals. Remember that the base and handset operate on a frequency pair that allows you to talk and listen at the same time (duplex). The radio components include an audio amplifier that increases the strength of the incoming electrical signals.

Power Components
A DC power cube transformer supplies the low voltage required by the electrical components on the circuit board. The power components on the circuit board work with the power cube to supply electrical current to re-charge the battery of the handset.

In addition to the above components, some bases also have audio amplifiers to drive speakers for speaker phone features, keypads for dialing, liquid crystal displays (LCDs) for caller ID, light-emitting diodes (LEDs) for power/charging indicators, and solid state memory for answering machine or call-back features.

Handset
You can carry the handset with you throughout the house or outside within the range of the base transmitter. The handset has all of the equipment of a standard telephone (speaker, microphone, dialing keypad), plus the equipment of an FM radio transmitter/receiver.

 

 

When you open up the handset, you see these components:

• speaker - converts electrical signals into the sound that you hear
• microphone - picks up your voice and changes it to electrical signals
• keypad - input for dialing
• buzzer or ringer - lets you know that you have an incoming call
• radio components
  • amplify electrical signals to and from microphone and speakers
  • send and receive FM radio frequencies
• LCD or LED displays - indicator lights
• re-chargeable battery - supplies electrical power to handset

Parts of the GE cordless phone's handset, showing the fronts of the circuit boards

Parts of the GE cordless phone's handset, showing the backs of the circuit boards, the speaker, microphone, ringer and battery

Speaker
The speaker receives the electrical signals from the audio amplifier in the radio components and converts them into sound. When you remove the cover from the speaker, you see a large round permanent magnet with a hole in the middle and a deep groove surrounding the hole. Within this deep groove is a coil of fine copper wire that is attached to a thin plastic membrane. The plastic membrane covers the magnet and coil.

Close-up view of the speaker in the GE cordless telephone handset

Close-up of the speaker with the top removed

Close-up of the speaker with the plastic membrane and attached coil lifted out. The large metal disc is the magnet.

Close-up of the speaker's plastic membrane with attached wire coil

To hear sounds, the following events happen:

1. Electrical signals come from the radio components.
2. The electrical signals travel in the coil of copper wire.
3. The electrical signals induce magnetic currents in the coil of wire, thereby making it an electromagnet.
4. The electromagnetic coil moves in and out of the groove within the permanent magnet.
5. The coil moves the attached plastic membrane in and out at the same frequencies as the changes in electric currents.
6. The movements of the membrane move air at the same frequencies, thereby creating sound waves that you can hear.

Microphone
The microphone changes the sound waves from your voice into electrical signals that are sent to the audio amplifier of the radio components. A microphone is essentially a speaker that works in reverse. When sound waves from your voice move the membrane, they make tiny electric currents either by moving a coil of wire within a magnet or by compressing the membrane against carbon dust (see How do microphones work? for details).

Close-up of handset's keypad circuit board with attached microphone and buzzer

Keypad

The keypad allows you to dial a number. It transfers the pressure from your fingertip on the appropriate key into an electrical signal that it sends to the radio components. Below the rubber keypad is a circuit board with black conductive material under each button (shown above). The keypad works like a remote control. When you press a button, it makes a contact with the black material and changes its electrical conductance. The conductance sends an electrical signal to the radio components indicating that you have selected that number.

Buzzer or Ringer
When the radio components of the handset receive the ringer signal from the base, they send electrical signals to the buzzer. The buzzer changes those electrical signals into sound much like the speaker does. You hear the buzzer sound and know that someone is calling you. In some phones, the speaker is used to make the ringer sound and there is no need for a separate ringer. 

adio Components
The radio components of the handset are like those of the base -- they convert electrical signals from the microphone into FM radio signals and broadcast them at the same frequency as the receiving crystal of the base unit. The radio components also receive radio signals at the same frequency as the broadcasting crystal from the base, convert them to electrical signals and send them to the speaker and/or buzzer (ringer).

Remember that the base and handset operate on a duplex frequency pair that allows you to talk and listen at the same time.

LCD or LED Displays
Most handsets have one or more light-emitting diodes (LED) that indicate various things, such as when the phone has an open line or when the battery is low.

LED indicator light on the handset of the GE cordless phone

Some handsets have an LCD that can display numbers for caller ID features, similar to a cell phone. The LCD may be reflective or backlit so that you can see it when the room light is low.

 

Battery

The handset's battery supplies the power for all of the electrical components in the handset. All cordless phone handsets have a rechargeable battery (nickel-cadmium, nickel-metal hydride or lithium). When the battery runs low, an indicator light on the handset usually lights up or flashes. In some phones, a "beeping" sound may also indicate a low battery. You then recharge the battery on the base of the cordless phone.

The GE cordless phone that we dissected was from 1993. Modern cordless phones have the same functions and much of the same hardware. However, many of the electronic circuits that were once achieved with transistors, resistors and capacitors have been replaced with integrated circuits. This advancement allows the handset to be either smaller with the same functions or the same size with more functions.

In summary, a cordless phone is basically a combination of a telephone and an FM radio transmitter/receiver. Because it is a radio transmitter, it broadcasts signals over the open airways rather than specifically between the base and handset.

 JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://electronics.howstuffworks.com/cordless-telephone3.htm

 

TELEPHONY FUNDAMENTALS

Cordless telephones are one of those minor miracles of modern life -- with a cordless phone, you can talk on the phone while moving freely about your house or in your yard. Long before cell phones became so cheap that anyone could afford one, cordless phones gave everyone the freedom to walk and talk within the privacy of their own homes.Cordless phones have many of the same features as standard telephones, and there are many models available. In this article, we will examine how these cordless telephones work and see why there are so many different types on the market today.

The Basics

A cordless telephone is basically a combination telephone and radio transmitter/receiver (see How Telephones Work and How Radio Works for details on these two technologies). A cordless phone has two major parts: base and handset.

• The base is attached to the phone jack through a standard phone wire connection, and as far as the phone system is concerned it looks just like a normal phone. The base receives the incoming call (as an electrical signal) through the phone line, converts it to an FM radio signal and then broadcasts that signal.
• The handset receives the radio signal from the base, converts it to an electrical signal and sends that signal to the speaker, where it is converted into the sound you hear. When you talk, the handset broadcasts your voice through a second FM radio signal back to the base. The base receives your voice signal, converts it to an electrical signal and sends that signal through the phone line to the other party.

The base and handset operate on a frequency pair that allows you to talk and listen at the same time, called duplex frequency.

Diagram showing how the base unit and handset of the cordless phone talk to each other: Each color represents a different frequency.  JOSE LEONARDO MONCADA TORRESC.I 18878408CRF   http://electronics.howstuffworks.com/cordless-telephone1.htm

TELEPHONY FUNDAMENTALS

a technical expert on Asterisk, but being fully aware of its gathering momentum as an is a free software / open-source software implementation of a telephone private branch exchange (PBX), I decided to hang out with some Asterisk code jocks for the better of an afternoon.

I was much looking forward to a walkthrough of Asterisk, which when implemented, allows multiple attached telephones to make calls to each other, as well as to connect to the Public Switched Telephone Network.

The setting was an Asterisk tutorial presented last Monday at the Open Source Conference in my hometown of Portland. The cerebral and witty Brian Capouch, assistant professor and chair of the Department of Computer Science at Saint Joseph’s College in Rensselaer, Indiana, was the MC.

Brian, who is finishing "Inside and Out: Do-it-yourself Open Source Telephony" for Addison-Wesley, walked us through a series of conceptual slides about telephony, IP telephony, and then Asterisk. And as you have probably guessed by now, that is a basic Asterisk schematic at the top of this post.

Now let us look at some of the slides, and how they come together in a code string for a specific Asterisk-enabled application.

This describes the conceptual basics of an Asterisk- configured call.

 

As far as extensions are concerned, they are often, but not always numeric,and can quantify a prioritization insofar as how the call being placed is handled.

 

Applications contained within the code for a specific Asterisk scenario provide instructions for behavior to be executed within that scenario. Here’s how Brian summarizes this:

 

Now, let us take a look at how all of this comes together in what is generally referred to as a "Call Flow:"

 

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://www.zdnet.com/blog/ip-telephony/asterisk-basics-in-six-quick-slides/1197

TELEPHONY FUNDAMENTALS

APLICACIONES TELEINFORMÁTICAS DE LAS REDES TELEFÓNICAS

El teléfono es uno de los instrumentos de tecnología con mayor permanencia, particularmente en los negocios. Todos los días estas entidades realizan literalmente miles de llamadas cuyo costo es realmente bajo en comparación con el volumen de dinero que se maneja a través de ellas.

Para la mayoría de las compañías, una porción de este costo es evitable, tomando en cuenta que las redes públicas de telefonía poseen un complejo mundo de tarifas y subsidios, que a menudo resultan en situaciones en donde las llamadas salientes forman solo una parte de las llamadas entrantes. Esto hace que las empresas hayan tenido que confiar a la larga con redes privadas.

DISEÑO DE REDES

 

En el diseño de una red integrada de voz y datos, debe existir una diferencia marcada entre el limite que existe en el diseño de redes de voz y datos, ya que ambas tratan de establecer sesiones terminales entre usuarios, debido a que el concepto de señalización, direccionamiento y enrutamiento de las mismas son similares.

Los cambios en el diseño de redes integradas de voz y datos están en comprender como estos elementos son conciliados en una misma red. El retardo y las variaciones de retardo, implican una reducción en su impacto, es decir estudiar redes de voz sensitivas al retardo y redes con trafico de datos insensibles al mismo.

Un punto de peso para el diseño de redes, esta en que no todo el tráfico de voz es necesariamente sensitivo al retardo. Por ejemplo, el fax y el correo de voz, no tienen restricciones en tiempo real, como las conversaciones de voz. Por lo que añadir servicios de correo de voz y fax puede ser una justificación, para soportar "voz" sobre redes de datos.

Para esto podemos seguir ciertos pasos para el diseño:
Auditoria de la red
Objetivos de la red
Revisión de tecnología y servicios
Guías Técnicas
Planificación de la capacidad
Análisis financiero

INTERCONEXIÓN
La red telefónica básica se creó para permitir las comunicaciones de voz a distancia. En un primer momento (1.876 - 1.890), los enlaces entre los usuarios eran punto a punto, por medio de un par de cobre (en un principio un único hilo, de hierro al principio y después de cobre, con el retorno por tierra) entre cada pareja de usuarios. Esto dio lugar a una topología de red telefónica completamente mallada, tal y como se muestra en la Figura 12.

Figura 12: Conexión mediante una red completamente mallada

Si se hacen las cuentas, esta solución se ve que es claramente inviable. Si se quiere dar servicio a una población de N usuarios, con este modelo completamente mallado, harían falta Nx(N - 1)/2 enlaces. Por esa razón se evolucionó hacia el modelo en el que cada usuario, por medio de un par de cobre se conecta a un punto de interconexión (central local) que le permite la comunicación con el resto.

Figura 13: Conexión mediante una red en estrella

De este modo la red telefónica se puede dividir en dos partes. La estructura de la red telefónica mostrada en la Figura 13: Conexión mediante una red en estrella es la que básicamente hoy se sigue manteniendo. Lo único es que la interconexión entre las centrales se ha estructurado jerárquicamente en varios niveles dando lugar a una red de interconexión. De este modo, la red telefónica básica se puede dividir en dos partes: la red de acceso y la red de interconexión (Figura 14).

Figura 14: Estructura de la red telefónica

El bucle de abonado es el par de cobre que conecta el terminal telefónico del usuario con la central local de la que depende. El bucle de abonado proporciona el medio físico por medio del cuál el usuario accede a la red telefónica y por tanto recibe el servicio telefónico. La red de interconexión es la que hace posible la comunicación entre usuarios ubicados en diferentes áreas de acceso (CSAs).

Redes Telefónicas
La red telefónica es una red de conmutación de circuitos, dada su extensión y complejidad, se puede clasificar en lo que constituye las propias centrales de conmutación, la parte de interconexión que las une y la parte de enlace con los usuarios o abonados. Atendiendo a este criterio se tiene:
· Red de enlacesEstá constituida por los circuitos que unen las centrales entre sí, utilizando medios de transmisión diversos, como cables de pares o fibras ópticas, que son los que proporcionan la vía de comunicación con otro que cuelga de una central distinta a la suya. Si las centrales que se unen son urbanas, la red de interconexión se denomina red de enlaces urbanos, y si no, red de enlaces interurbanos.
· Redes de abonados.Es el conjunto de elementos de conexión entre los equipos de abonado y la central local a la que pertenecen, de tal manera que cada uno de ellos tiene asignado un circuito único (bucle de abonado).

ESTRUCTURA DE LAS REDES TELEFÓNICAS

La conmutación telefónica es el proceso mediante el cual se establece y mantiene un circuito de conmutación capaz de permitir el intercambio de información entre dos usuarios cualesquiera. La imposibilidad de mantener conectados a todos los usuarios entre si, con dedicación exclusiva de ciertos medios para su uso, es lo que hace necesario el empleo de un sistema que permita establecer el enlace para la comunicación solamente durante el tiempo que está dure. Los sistemas que consiguen una mayor eficacia son las centrales telefónicas en sus diversas modalidades.

Estructura
Atendiendo a la distribución geográfica tenemos tres tipos de redes, las llamadas “urbanas” o de corta distancia, las “interurbanas” o de larga distancia y las “internacionales”.

Redes urbanas: Dentro de estas se engloban los circuitos de abonados y los enlaces entre centrales locales, para transmisión en banda base o en baja frecuencia. Normalmente están constituidos por pares de conductores, que al agruparse, forman el llamado “cable de pares”, que puede contener hasta varios cientos de ellos, configurados en simétricos y en cuadretes, para una menor interferencia de unos sobre otros.

Redes interurbanas: Esta es la encargada de proporcionar los enlaces entre centrales localizadas en diferentes ciudades; ello hace que las distancias sean mayores y se deban utilizar cables de distintas características a los antes mencionados, con menores pérdidas y una respuesta plana que se consigue de dos formas diferentes: una cargando los cables de pares, y otra, empleando otros medios distintos de los cables de pares, tales como el cable coaxial, fibra óptica, radio enlaces, etc.; todos ellos con una mayor capacidad de transmisión y una mayor fiabilidad.

Redes internacionales: para dar curso al tráfico entre diferentes países se necesita de la interconexión entre centrales internacionales, encargadas de encaminar el mismo. Esta se realiza mediante enlaces de alta capacidad (varios miles de circuitos full-duplex) y fiabilidad, constituidos fundamentalmente por enlaces terrestres, submarinos o vía satélite, repartiéndose al menos entre dos de ellos por razones de seguridad. Los canales empleados son de tipo analógico (FDM/Multiplexaje por División de Frecuencia) o digitales (TDM/Multiplexaje por División de Tiempo)

Las centrales de conmutación son los elementos funcionales encargados de proporcionar la selectividad necesaria, de forma automática, para poder establecer el circuito de enlace entre dos usuarios que desean comunicarse. En ellas reside además todo el control y la señalización propios de la red.

Central Local: A éstas se conectan todas las líneas de abonado, de tal forma que mediante un par físico se une un teléfono con la central. También, se llama central urbana.

Red telefónica conmutada o red telefónica básica
Depende de la Compañía Telefónica y es la red utilizada en las comunicaciones orales por teléfono.
Puede conectarse un usuario, por medio del correspondiente módem, a cualquier otro abonado, identificándose ambos por su número de teléfono.
Ventajas: amplia cobertura, nacional e internacional, y su precio en comparación con las redes de uso exclusivo, ya que se factura según la duración de la comunicación al igual que las conferencias telefónicas.
Inconvenientes: es su baja calidad, al ser una red para voz con un ancho de banda inferior a lo deseable. Se utiliza principalmente para comunicaciones esporádicas y de corta duración. Las velocidades de transmisión oscilan de 1200 a 2400 bps.

Red Iberpac
Promovida por empresas (bancarias), depende de Telefónica y su objetivo es: una red nacional especializada en transmisión de datos.
Grandes nodos de concentración situados en algunas capitales.
Alta calidad y utiliza la técnica de conmutación de paquetes.
Está conectada a las redes públicas citadas en los apartados anteriores y asimismo a las grandes redes internacionales de transmisión de datos: Transpac en Francia, Tymney y Telenet en Estados Unidos, Datapac e Infoswitch en Canadá.

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://teleinformaticaunerg.blogspot.com/2008/02/aplicaciones-teleinformticas-de-las.html