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Telecommunications, also called telecommunication, is the exchange of information over significant distances by electronic means. A complete, single telecommunications circuit consists of two stations, each equipped with a transmitter and a receiver. The transmitter and receiver at any station may be combined into a single device called a transceiver. The medium of signal transmission can be electrical wire or cable (also known as “copper”), optical fiber or electromagnetic fields. The free-space transmission and reception of data by means of electromagnetic fields is called wireless. The simplest form of telecommunications takes place between two stations. However, it is common for multiple transmitting and receiving stations to exchange data among each other. Such an arrangement is called a telecommunications network. The Internet is the largest example. Various categories of telecommunication are public switched telephone network, mobile phone system; circuit switched packet telephone, satellite telephone system and fixed wireless telephone system.

 

A) Public Switched Telephone Network (PSTN): The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the wired phone system over which landline telephone calls are made. The PSTN relies on circuit switching. The public switched telephone network (PSTN) is the network of the world’s public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.

To connect one phone to another, the phone call is routed through numerous switches operating on a local, regional, national or international level. The connection established between the two phones is called a circuit. In the early days, phone calls traveled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That is why you needed operators’ assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.

Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching. Digital voice signals can share the same wire with many other phone calls. The advent of fiber-optic cables now allows thousands of calls to share the same line. But fiber-optic and other high-bandwidth cables have not changed the basic nature of circuit switching, which still requires a connection or circuit to remain open for the length of the phone call.

Routing calls requires multiple switching offices. The phone number itself is a coded map for routing the call. In the United States, for example, we have 10-digit phone numbers.

  • The first three digits are the area code or national destination code (NDC), which helps route the call to the right regional switching station.
  • The next three digits are the exchange, which represents the smallest amount of circuits that can be bundled on the same switch. In other words, when you make a call to another user in your same exchange, maybe a neighbour around the corner, the call does not have to be routed onto another switch.
  • The last four digits of the phone number represent the subscriber number, which is tied to your specific address and phone lines.

Within a company or larger organization, each employee or department might have its own extension. Extensions from the main phone number are routed through something called a private branch exchange (PBX) that operates on the premises.

To make an international call requires further instructions. The call needs to be routed through your long-distance phone carrier to another country’s long-distance phone carrier. To signal such a switch, you have to dial two separate numbers, your country’s exit code (or international access code) and the corresponding country code of the place you are calling.

Almost all exit codes are either 00 or 011, although there are a few exceptions like Cuba (119) and Nigeria (009). Country codes are one- to three-digit prefixes that are assigned to specific countries or groups of countries. For example, the country code for the United States is 1, but the United States shares that country code with Canada and several smaller island nations like Jamaica, Puerto Rico and Guam. The country code for Mexico is 52 and Saudi Arabia is 966.

 

 

B) Global System for Mobile communication: Global System for Mobile communication (GSM) is a digital mobile telephony system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephony technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHZ or 1800 MHZ frequency band.

Mobile services based on GSM technology were first launched in Finland in 1991. Today, more than 690 mobile networks provide GSM services across 213 countries and GSM represents 82.4% of all global mobile connections. According to GSM World, there are now more than 2 billion GSM mobile phone users worldwide. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries. SIM cards (Subscriber Identity Module) holding home network access configurations may be switched to those will metered local access, significantly reducing roaming costs while experiencing no reductions in service. GSM, together with other technologies, is part of the evolution of wireless mobile telecommunications that includes High-speed Circuit-Switched Data (HSCSD), General Packet Radio System(GPRS), Enhanced Data GSM Environment (EDGE), and Universal Mobile Telecommunications Service (UMTS).

 

 

C) Satellite Telephone System: Satellite Telephone systems provide fixed, mobile and portable communications to the user. Satellite telephone systems are available for handheld, portable, fixed, mobile or marine use. The handheld devices are like a normal cell phone, but with a larger antenna. The portable systems are usually a suit-case style system that offers greater capabilities than the handheld systems.

The fixed systems actually mount on the outside of a home or business to offer satellite telephone services to standard telephone wiring inside the building. The mobile and marine systems are available as a kit that connects to the handheld system, or as fixed mounted system for inside the vehicle or vessel.

Satellite phones and satellite phone systems are used in a number of applications and the satellite phone sales are still strong despite the fact that they have not developed in the way that was originally anticipated. With cellular technology becoming more widespread, and this occurring more quickly than originally anticipated, the mass market that was envisaged for satellite phones did not emerge.

 

Satellite phone basics

When devising a satellite phone system there are numbers of technical challenges that need to be addressed. The path length between the earth and the satellite introduces significant losses, much greater than those encountered with terrestrial systems. It is for this reason that most of the systems use low Earth orbiting satellite systems. Geostationary satellites are usually considered too high and result in much greater levels of path loss. With the satellites in a low Earth orbit and moving across the sky each satellite will be in view for a certain amount of time. It is therefore necessary even for a stationary phone to be able to handover from one satellite to another.

Phones used for satellites are often larger in size than those used for terrestrial applications. The antenna is often larger to provide and ensure the required level of efficiency. This naturally impacts on the size of the satellite phone.

A further challenge for satellite phones arises from what are termed the backhaul communications and protocol exchanges. Any mobile phone requires to quickly communicate with the network to enable calls to be set up, controlled and finished. In view of the altitudes of the satellites the round trip delay from the mobile to the satellite and back to the earth station are too long to enable rapid communications and exchanges to take place. As a result, much of the intelligence of the system has to be placed within the satellite so that the required protocol exchanges can take place rapidly.

 

 

(d) Fixed Wireless Telephone System: Fixed wireless phones are designed to provide service in rural or underdeveloped areas, where portable, battery-powered handsets often perform poorly. Fixed wireless phones plug into standard AC outlets and resemble a conventional landline phone (fixed) but communicate over existing cellular networks (wireless). They are primarily designed for developing countries, but the phones are also ideal in offices, vacation resorts, and other cost-sensitive or remote locations.

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