The technologies used in cellular telephony can be divided into two major families. These are the ANALOG TECHNOLOGIES,which are FDMA(Frequency Division Multiple Access) based, and the DIGITAL TECHNOLOGIES, which are usually TDMA(Time Division Multiple Access) OR CDMA(Code Division Multiple Access) based.
- Satellite Technology :-
A satellite is a specialised wireless reciever/transmitter that is placed in the orbit around the earth. Currently there are hundreds of satellites operating in orbit. These satellites are used for a wide variety of purposes such as weather forecasting,television broadcasting, radio communications,Internet communications and the Global Positioning System (GPS).
The first satellite ever launched did nothing more than transmitting Morse code signals over and over again.In contrast,satellites today can transmit and receive thousands of signals simultaneously from simple digital data to complex television programming.
There are three types of communications satellite systems and that are categorized according to the orbit they follow.
A Geostationary satellite orbits (GEO) the earth about 22,000 miles up directly above the equator. At this altitude , one trip around the earth takes 24 hours. Thus,the satellite remains over the same spot at all the times. Weather satellites are considered geostationary satellite. A single geostationary satellite can see approximately 40% of the earth's surface. Thus, three geostationary satellites placed at equal.intervals (120 degrees apart) can provide coverage of the entire earth's surface.
A low-earth-orbit(LEO) satellite system use a large number of satellites,each in a circular orbit at a constant altitude of a few hundred miles. The orbits takes the satellite over the geographic poles. Each revolution take anywhere from 90 minutes to a few hours. The fleet is arranged in such a way that at any point and time at least one satellite is on a line of sight. A well designed LEO system is what makes it possible for anyone to access the Internet via wireless communication.
Analog :-
The analog system in use today are frequency-modulated systems the frequency of the carrier wave while data are carried by a variation in the modulation that is proportional to the original signal. This means that conversation is carried by modulation of original signal.
FDMA :-
FDMA(Frequency Division Multiple Access) is e division of the frequency band allocated for wireless cellular communication iit 30, channels each of which can carry a voice conversation or with digital service,carry digital data. FDMA is the basic technology in the Advanced Mobile Phone Services (AMPS), most widely installed cellular phone system installed in North America. With FDMA, each channel can be assigned to only one user at a time. FDMA is also.used in the Total Access Communication System (TACS).
Digital :-
All digital cellular service is use multiplexing to enable devices to share wireless channels. Digital cellular services are being differently in the U.S. than in Europe. In 1987, European Union choose a standard called GSM(Global System for Mobile Communication) for delivering new digital wireless telephony. In the U.S., the TIA(Telecommunications Industry Association) settled on a similar standard using Time Division Multiple Access (TDMA). However, shortly thereafter , many of the Bell telephone companies decided to use a newer method of multiplexing, code division multiple access. CDMA has greater capacity then time division multiple access.
TDMA AND GSM:-
There is more than one TDMA technology currently available in United States. The one most commonly associated term "TDMA" is the North American TDMA or NA-TDMA ( which has also come to be known as D-AMPS). Both of these technologies operate under IS-54 or IS-136 as their governing standard. The GSM (Global System for Mobile TDMA) was originally developed for general European use. But it has been adapted for use in certain markets in the US as PCS 1900, and is now offered in the PCS or 1.9 GHz frequency allocation.
NA-TDMA or DAMPS employs what are described as the slots to put many calls in the same channel. In fact, the same channel scheme used in the 800 MHz AMPS is used for this technology, the difference being that each channel is composed of six time slots, each of which is capable of carrying a conversation. This allows for a six-fold increase in capacity over AMPS service.
Unlike (pre-1995) IS-54. The new IS-136 (PCS) versions of TDMA standards requires unbanding to the PCS frequency space. IS-136 also includes the digital control channel (DCCH) which is an addition to the AMPS control channels used in IS-54. Finally, IS-136 has the capacity to support new user services.
CDMA:-
The newest of the digital technologies discussed here, CDMA differs from the previous technologies in a number of ways. No "channel" is assigned to the conversation; instead, it is broadcast over a spread spectrum (1.25 MHz wide). He signal is prepared by first being digitalized and then multiplied by a wide bandwidth pseudonoise code sequence. This sequence is recognized by both the phone and the cellular base station.
Assigning an unique code to each conversation allows many users simultaneously to share the same RF spectrum region. Ideally, calls will not interfere with one another because all other conversation taking place will be treated as noise and ignored.
Another difference lies in the fact that the phone is in contact with up to four cellular base stations at once. This is achieved by a means of a device known as a rake reciever. The rake reciever is capable of receiving multiple copies of the same signal, regardless of fading or delay. This enables a soft conversation to be transferred from cell site to cell site by means of a "soft" handoff.
A soft handoff means that the phone maintains a conversation on more than one cell site and transfers control of the call to a stronger base station before breaking the connection to the weaker base station. This differs from the analog and TDMA "hard" handoff in which the call is dropped by the weaker base station beforebeing reestablished on the stronger one.
CDPD:-
Several technologies are used for PCS in UnitedStates, including Cellular Digital Packet Data and Global System For Mobile (GSM) communication. GSM is more commonly used in Europe and elsewhere.
CDPD is a specification for supporting wireless access to the internet and other public packet-switched networks. Cellular telephone and modem providers that offers CDPD support to make it possible for mobile users to get access to the Internet at up to 19.2 kbps. Because CDPD is an open specification that adheres to the layered structures of the Open System Interconnections (OSI) model, it has the ability to be extended in the future. CDPD supports both the Internet IP protocol and the ISO connectionless network protocol (CLNP).
CDPD also supports IP multicast service. With multicast a company can periodically broadcast company upstes to the sales and service people on the road or a news subscription service can transmits its issues as they are published. It will also support the next level of IP. IPv6.
For the mobile user, CDPD's support for packet-switching means that the persistent link isn't needed. The same broadcast channel can be shared among a number of users at a same time. The users modem recognises the packets intended for its users. As data such as email arrives, it is forwarded immediately to the user without a circuit connection having to be established.
There is a circuit-switched version, called CS CDPD that can be used when traffic is expected to be heavy enough to warrant a dedicated connection.
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