COMMUNICATION TECHNOLOGIES

Archive for the ‘Mobile Communication’ Category

Hi folks!! Due to the extreme growth in technologies there are loads of comforts are available to us. These extreme comforts helped us to enhance our life style. Nowadays everyone of us using mobile phone which is the great invention for us that helps to communicate with anyone around the world in an easy way. There are many features are included in the mobile phone like camera, media player, web browsing and more. Most of the people can throwing the mobile phone if anything happened in the phone. Some people desire to use always new mobiles with some additional features for that they can easily throw away their old mobiles.

Let stop throwing the old mobile phones to trash. There is lots of recycling mobile companies are available in the market. With the help of these companies you can get a reasonable price for your old phones. So never throw away your mobile phone to anywhere, just sell your mobile phone for getting a good price. If you are really looking for such kind of company for the purpose of selling your old mobile phone, I would recommend that you to log on this website www.sellmymobile.com. They give you the most valuable returns for your old mobile phone and provide the best comparing price than the other competitor. They are number one mobile recycling price comparison site in UK and they proud it also. They accept all type of mobile phones like Nokia, Motorola, LG, Sony Ericsson, Samsung and all type of Black Berries. So there is no matter what type of mobile you have. You can get a best price for any kind brand and model of mobile phones. Here you can also get mobile phone trade in for a quick cash return. For more details about that, pay a visit to this website.

Cellular communication is a significant step forward in the provision of communication series. Cellular systems have evolved from military radio design. In cellular mobile telephony, it is necessary to change the operating frequency as one move from one area to another. In cellular communications, a region is geographically divided into cells. The cells are hexagon in shape. Each cell has a base station. The cell phones in a cell communicate with each other through the base station of that cell. All the cells are routed through this base station. Each base station consists of a 800 or 1900 MHZ transceiver with an antenna to provide coverage. Each base station uses carefully chosen frequencies to reduce interference with neighboring cells.

This reduces interference while the same frequency is used by another cell, at the same time. In cellular communications the most commonly used modulation is narrow band frequency modulation. Here the carrier frequencies are separated by about 25 KHZ. The total band width allocated supports about 280 channels. The cell sizes need not be uniform throughout a cellular range. When a subscriber initiates a call at the cellular phone, the instrument sends a message to its base station announcing its telephone number. The base station then allocates an available frequency to the telephone for communication.

Electronic Switching Centers and System Interconnects:

The electronic switching center is a digital telephone exchange located in the MTSO that is the heart of a cellular telephone system. It controls switching between the public wireline telephone network and the cell-site base stations for wireline-to-mobile, mobile-to-wireline, and mobile-to-mobile calls, and it processes data received from the cell-site controllers concerning mobile unit status, diagnostic data, and bill-compiling information. Electronic switches communicate with cell-site controllers using a data link protocol. Fourwire leased lines are generally used to connect switching centers to cell sites and to the public telephone network. There is one dedicated four-wire trunk circuit for each of the cell’s voice channels.

Mobile and Portable Telephone Units:

Mobile and portable telephone units are essentially identical. The only differences are that portable units have a lower output, have a less efficient antenna, and operate exclusively on batteries. Each mobile telephone unit consists of a control unit, a multiple-frequency radio transceiver, a logic unit, and a mobile antenna. The control unit houses all the user interfaces, including a built-in handset. The transceiver uses a frequency synthesizer to tune into any designated cellular system channel. The logic unit interrupts subscriber actions and system commands while managing the operation of the transceiver and control units.

The transfer of a mobile unit from one base station’s control to another base station’s control is called a handoff. Handoffs should be performed as infrequently as possible and be completely transparent to the subscriber. A handoff consists of four stages: initiation, resource reservation, execution and completion. A connection that is momentarily broken during the cell-to-cell transfer is called a handoff. A hard handoff is a break before make process.

With a hard handoff, the mobile unit breaks its connection with one base station before establishing voice communication with a new base station. Has handoff generally occur when a mobile unit is passed between disjointed systems with different frequency assignments, air interface characteristics, or technologies. a flawless handoff called a soft handoff, which is imperceptible to voice telephone users, although the delay maybe disruptive when transmitting data.

• Initiation: Either the mobile unit or the network determines the need for a handoff and initiates the necessary network procedures.
• Resource reservation: Appropriate network procedure reserve needed to support the handoff.
• Execution: The actual transfer of control from one base station to another base station takes place.

Roaming is when a mobile unit moves from one cell to another-possibly from one company’s service area into another company’s service area. As a mobile unit moves away from the base station transceiver it is communicating with, the signal strength begins to decrease. The output power of the mobile unit is controlled by the base station through the transmissions of up/down commands, which depends on the signal strength the base station is currently receiving from the mobile unit.

When the signal drops below a predetermined threshold level, the electronic switching center locates the cell in the honeycomb pattern that is receiving the strongest signal from the particular mobile unit and then transfers the mobile unit to the base station in the new cell. One of the most features of a cellular system its ability to transfer calls that’s are already in progress from one cell-size controller to another as the mobile unit moves from cell to cell within the cellular network. The base station transfer includes converting the call to an available channel within the new cell’s allocated frequency subject.

Each mobile user within a given cell and mobile subscribers in adjacent cells use the same radio-frequency channels. In essence frequency reuse is available in all cells. This is made possible because IS95 specifies a direct sequence, spread spectrum CDMA system and does not follow the channelization principles of traditional cellular radio communications systems. Rather than dividing the allocated frequency spectrum into narrow –bandwidth channels, one for each user, information is transmitted over a very wide frequency spectrum with as many as 20 mobile subscribers units simultaneously using the same carrier frequency within the same frequency band.

Interference is incorporated into the system so that there is no limit to the number of subscribers that CDMA can support. As more mobile subscribers are added to the system, there is a graceful degradation of communications quality. With CDMA unlike other cellular telephone standards, subscribers’ data change in real time depending on the voice activity and requirements of the network and other of the network. CDMA reduces the importance of frequency planning within a given cellular market. The AMPS cellular telephone system is allocated a 50 MHz frequency spectrum. With CDMA many users can share common transmit and receive channels with a transmission data rate of 9.6 kbps.

In the uplink channel subscribers data are encoded using a rate convolution code interleaved and spread by one 64 orthogonal spreading sequences using functions. Frequency hopping spread was first used by the military to ensure reliable antigen and to secure communications in a battlefield environment. The fundamental concept of frequency hopping is to break a message into field-size blocks of data with each blocks transmitted in sequence frequency except on a different carrier frequency.

Space Wave Propagation:

Space wave propagation of electromagnetic electronic energy includes radiated energy that travels in the lower few miles of earth’s atmosphere. Space waves include both direct and ground-reflected waves. Direct waves travel essentially in a straight line between transmitting and receive antennas. Space wave propagation with direct waves is commonly called line-of sight transmission. Therefore, direct space wave propagation is limited by the curvature of the earth. Ground reflected waves are waves reflected by earth’s surface as they propagate between the transmitting and receive antennas. The curvature of earth presents a horizon to space wave propagation commonly called the radio horizon. Because of atmospheric refraction, the radio horizon is approximately four-thirds that of the optical horizon.

Surface Wave Propagation:

A surface wave is an earth-guided electromagnetic wave that travels over the surface of earth. As a surface wave moves over earth’s surface, it is accompanied by charges induced in the earth. The charges move with the wave, producing a current. Since the earth offers resistance to the flow of current, energy is dissipated in a manner very similar to those in a transmission line. Earth’s surface also has dielectric losses. Therefore, surface waves are attenuated as they propagate. Because energy is absorbed from the surface wave, the portion of the wave in contact with earth’s surface is continuously wiped out. The energy is replenished by diffraction of energy downward from the portions of the ground wave immediately above earth’s surface.

Sky Wave Propagation:

Electromagnetic waves that are directed above the horizon level are called sky waves. Typically, sky waves are radiated in a direction that produces a relatively large angle with reference to earth. Sky waves are radiated towards the sky, where they are either reflected or refracted back to earth by the ionosphere. Because of this, sky wave propagation is sometimes called ionospheric propagation. The ionosphere is the region of space located approximately 50 km to 400 km above earth’s surface. The ionosphere is the upper portion of earth’s surface. Therefore, it absorbs large quantities of the sun’s radiant energy, which ionizes the air molecules, creating free electrons. When a radio wave passes through the ionosphere, the electric field of the wave exerts a force on the free electrons, causing them to vibrate.

Global System:

In the early 1980s, analog cellular telephone systems were experiencing a period of rapid growth. Each country subsequently developed its own cellular telephone system, which was incompatible with everyone else’s system from both equipment and an operational standpoint. Most of the existing systems operated at different frequencies, and all were analog. A GSM specification was published in 1990. GSM had the advantage of being designed from scratch with little or no concern for being backward compatible with any existing analog cellular telephone system. GSM provides its subscribers with good quality, privacy, and security. Commercial GSM service began in Germany in 1991, and by 1993 there were 36 GSM networks in 22 countries. GSM networks are now either operational or planned in over 80 countries around the world. GSM was the world’s first totally digital cellular telephone system designed to use the services of SS7 signaling and an all digital data network called integrated services digital network to provide a wide range of network services. GSM is now the world’s most popular standard for new cellular telephone and personal communications equipment.

GSM Services:

The original invention was to make GSM compatible with ISDN in terms of services offered and control signaling. Unfortunately, radio-channel bandwidth limitations and cost prohibited GSM from operating at the 64-kbps ISDN basic data rate. Probably the most bearer service provided by GSM is telephony. With GSM, analog speech signals are digitally encoded and then transmitted through the network as a digital data stream. There is also an emergency where the closest emergency service provider is notified by dialing three digits similar to 911 services in the United States. A wide variety of data services is offered through GSM, where users can send and receive data at rates up to 9600bps to subscribers in pots, ISDN networks, packet switched public data networks, and called circuit switched public data networks using a wide variety of access methods and protocols. GSM is a digital network; a modem is not required between the user and the GSM network. One unique feature of GSM that is not found in older analog systems is the shortest message service which is a bidirectional service for sending alphanumeric messages up to 160bytes in length.

A Cellular telephone system includes all the basic components necessary for cellular communications. A wireless ratio network covering a set of geographical areas inside of which mobile two way radio units, such as cellular or PCS telephones, can communicate. The radio network is defined by a set of radio-frequency transceivers located within each of the cells. The locations of these radio-frequency transceivers are called base stations. A base station serves as central control for all users within that cell.

Mobile units communicate directly with the base stations, and the base stations communicate directly with a mobile telephone switching office. An MTSO controls channel assignment, call processing, call setup, and termination, which includes signaling, switching, supervision, and allocating, radio-frequency channels. The MTSO provides a centralized administration and maintenance point for the entire network and interfaces with the public telephone network over wireline voice trunks and data links. MTSOs are equivalent to class 4 toll offices, except smaller. Local loops do not terminate in MTSOs are connected to the SS7 signaling network, which allows cellular telephones to operate outside their service area.

Base stations can be improving the transmission quality, but they cannot increase the channel capacity with in the fixed band width of the network. Base stations are distributed over the area of system coverage and are managed and controlled by a site computerized cell site controller that handles all cell site control and switching functions. Base stations communicate not only directly with mobile units through the airways using control channels but also directly with the MTSO over dedicated data control links. To complicate the issue, an MTSO is known by several different names, depending on the manufacturer and the system configuration. Mobile telephone switching office MTSO is the name given by bell telephone laboratories, electronic mobile xchange by Motorola, AEX by ericcson, NEAX by NEC, and switching mobile center and master mobile center by novatel. In PCS networks, the mobile switching center is called the MCS.

Each geographical area or cell generally accommodates many different user channels simultaneously. The number of user channels depends on the accessing technique used. With in a cell, each radio-frequency channel can support up to 20 mobile telephone users at one time. Channel maybe statically or dynamically assigned channels are assigned a mobile unit for the duration of call, whereas dynamically assigned channels are assigned a mobile unit when it is being used. With both static and dynamic assignments, mobile units can be assigned any available radio channel.

Principles:

The key principles of cellular telephone were uncovered in 1947 by researchers at bell telephone laboratories and other telecommunications companies throughout the world when they developed the basic concepts and theory of cellular telephone. It was determined that by subdividing a relatively large geographic market area. Called a coverage zone, into small screens, called cells, the concept of frequency reuse could be employed to dramatically increase the capacity of a mobile telephone channel. In essence, cellular telephone systems allow a large number of users to share the limited number of common-usage radio channels available in a region. In addition, integrated-circuit technology, microprocessors and microcontroller chips, and the implantation of signaling system SS7 have recently enabled complex radio and logic circuits to be used in electronic switching machines to store programs that provide faster and more efficient call processing.

Fundamental Concepts:

The fundamental concepts of cellular telephone are quite simple. The FCC originally defined geographic cellular radio coverage areas on the basis of modified 1980 census figures. With the cellular concept, each area is further divided into hexagonal-shaped cells that fit together to from a honeycomb pattern. The hexagon shape was chosen because it provides the most effective transmission by approximating a circular pattern while eliminating gaps inherently present between adjacent circles. A cell is defined by its physical size and, more importantly, by the size of its population and traffic patterns.

The number of cell per system and the size of the cell are not specifically defined by the FCC and have been left to the providers to establish in accordance with anticipated traffic patterns. Each geographical area is allocated a fixed number of cellular voice channels. The physical size of a cell varies, depending on user density and calling patterns. Microcells are used most often in high-density areas such as found in large cities and inside building, by virtue of their low effective working radius, microcells exhibit milder propagation impairments, such as reflection and signal delays.

Microcell may overlay clusters of microcells with slow-moving mobile units using the micro-cells and faster-moving units the macrocells. The mobile unit is able to identity itself as either fast or slow moving, thus allowing it to do fewer cell transfers and location updates. Cell transfers algorithms can be modified to allow for the small distances between a mobile unit and the microcellular base station it is communicating .hexagonal cell grid might look like when superimposed over a metropolitan city.