1. Telecommunications
1.1. Basic definitions
Telecommunications: Refers to any procedure that allows a user to reach one or more specific users (eg telephony) or eventual users (eg radio, television), information of any nature (written, printed document, fixed or moving image, videos, voice, music, visible signals, audible signals, mechanical control signals, etc.), using for said procedure, any electromagnetic system for its transmission and / or reception (electrical transmission by wires, radioelectric, optical, or a combination of these various systems)
Telecommunications System: It is the set of equipment and links, both physical and electromagnetic, usable for the provision of a certain telecommunications service.
Telecommunications Service: It is the activity developed under the responsibility of a certain company or entity, to offer its users a modality or type of telecommunications, whose use is of interest to said user.
Public Telecommunications Service: It is that service that is provided in a general way to all the inhabitants of a country, the one in charge of providing it is the State, but it can give it in concession to private companies, but always regulating it.
1.2. Classification according to the propagation medium
a) Terrestrial Telecommunications: These are those whose means of propagation are physical lines, these can be copper cables, coaxial cable, waveguide, optical fiber, twisted pair, etc.
b) Radioelectric Telecommunications: These are those that use the earth's atmosphere as a means of propagation, transmitting signals in electromagnetic waves, radio waves, microwaves, etc. depending on the frequency at which it is transmitted.
c) Satellite Telecommunications: They are those radio communications that are made between space stations, between earth stations with space stations, between earth stations (by means of retransmission in a space station). Space stations are at different heights outside the atmosphere.
2. Cell phone
2.1. Basic definitions
Cellular Telephony: It is that telephony in which the coverage area is divided into cells and sectors. The Tx / Rx medium between the subscriber and the exchange is wireless, through radio frequency channels.
Mobile Cellular Telephony: It is that cellular telephony in which the subscriber's terminal can move from one place to another (maintaining an established communication) with a speed of up to 200 km / h.
Low Mobility Cellular Telephony: It is that cellular telephony in which the terminal can move but at a low speed (low mobility), between 10 to 40 km / h. It is actually a wireless local loop system, but it is mobile thanks to a delay time compensation algorithm, and uses the same type of telephone equipment as mobile cell phones, but with time access (TDD).
Wireless Local Loop (WLL): It allows to provide the fixed telephone service, also under the criteria of cellular telephony, but the terminal does not have mobility. The path from the switching center to the subscriber (local loop) is by wireless means (wireless). When the voice is packetized it is called WLL-IP
Fixed Wireless Access (FWA): It is the section between the subscriber (fixed) and the base station, using the radio spectrum as a transmission medium. You can pass any service such as: telephone, internet, broad band, etc.
Personal Communications System (PCS): It is one that provides universal accessibility to services such as: voice, data, video, audio, messages, positioning, Internet, etc., wirelessly, to mobile users. It is commonly associated with cellular mobile telephony.
2.2. Basic structure of a cellular system
A cell phone system consists of four elements:
Mobile cellular terminal
Base station
Control and switching station
Radio channels
- Mobile cellular terminal
It is the electronic equipment that allows a subscriber to make or receive calls, it is composed of: control unit, power supply, transmitter / receiver, antenna. It is portable, transportable, movable from one place to another. Performs a periodic update of the signal received from the base station, sends information to register with the base station.
- Base station (bts)
It is the central station within a cell, known as BTS (Base Tranceiver Station), performs the RF link to the cellular terminals, transmits information between the cell and the control and switching station, monitors the communication of the subscribers. It is made up of: control unit, power unit, sector antennas (which use diversity methods to capture the best signal), TRAU (unit in charge of adapting and converting the code and signal speed), and data terminal.
- Control and switching station
Commonly known as MTSO (mobile telephony switching office), when it applies GMS technology it is called MSC (mobile switching center), and for Wireless Local Loop networks it is called XBS.
It is the central element of the system, its main functions are:
Coordinate and manage all BTS
Coordinates calls between the landline office and subscribers, as well as calls between terminals
cell phones and subscribers, through BTS
It takes care of the billing
Lead the Hand off between cell sites
Has a management software: network management system
It is interconnected to tandem exchanges to communicate with other telephone networks.
It can be of 2 types (according to the geographical area and amount of traffic):
Centralized: a single exchange for the entire operator's concession area, uses star topology.
Decentralized: more than one central, distributed in the concession area.
(«) The BTS, Central and TANDEM are interconnected via fiber optic links, or via microwaves (high speed data links - SDH).
- Radio channels
Radio Channel is understood as the pair of carrier frequencies plus a time slot, which will serve as traffic channels in a communication. Of these 2 frequencies one will be the Tx frequency of the base station and Rx of the terminal, the other frequency will be the Rx frequency of the base station and Tx of the terminal. They carry data and voice between the subscriber and the base stations, each subscriber can only use one channel at a time.
2.3. Types of radio channels
Cellular channels or radio channels are those that will make possible a cellular telephone communication. They can be of 2 types:
a) Control Channel (CCH):
This channel allows data to be sent and received between the BTS and the laptop. These channels can be:
Forward Control Channel (FCC): generally provides basic information about the particular cellular system: system identification number, range of paging and access channels that it can scan.
Paging Channel: These are the channels used to keep a terminal in temporary location. ¬ Access Channel: These are channels used to answer when the terminal is being called, or to initiate a call. It is also used to inform the laptop which TCH to use.
In small, low-traffic areas, a single control channel performs the tasks of all three channels.
b) Traffic Channel (TCH):
Also known as the Voice Channel, it is in charge of conducting the traffic (voice and data) between the base station and the laptop when in a calling process. It is also used to send signaling messages from the BTS to the laptop, also to handle the hand over process, and to control the transmission power of the terminal. The data coming from the BTS is called “data in advance” and the data coming from the terminal is called “reverse data”, both are sent at 10 Kbps.
Bibliography
Bellamy, John. (nineteen ninety six). "Digital telephony" Wiley, 1st edition, New York.
Lati, Robert. (1986). "Comunication system". Mc Graw-Hill,, 1st edition, Mexico.
Praxis (1994). "Communication systems engineering". Prentice-Hall, 1st edition, New Jersey.
