Telecommunications

TELECOMMUNICATIONS

TELECOMMUNICATION is communication at a distance

by technological means, particularly through electrical signals or electromagnetic waves.

Electrical and electromagnetic telecommunication technologies include telegraph, telephone, and teleprinter, networks, radio, microwave transmission, fiber optics, communications satellites and the Internet.

TelecommunicationNetworks

• collection of terminal nodes, links and any intermediate nodes.

•transmission links connect the nodes together.

•nodes use circuit switching, message switching or packet switching to pass the signal through the correct links and nodes to reach the correct destination terminal.

Examples of Telecommunication Networks

•The Telelphone network

•Computer Networks

•The Internet

•The Global Telex network

•The Aeronautical ACARS Network

Telephone Network

A Telephone Network is a telecommunications network used for telephone calls between two or more parties.

Types of Telephone Networks:

•A fixed line network or PSTN.

•A wireless network 0r Cellular networks.

•A private network such as a private branch exchange (PBX).

pstnThe public switched telephone network (PSTN) is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators, providing infrastructure and services for public telecommunication.

The PSTN consists of :-

•Telephone lines, •Fiber optic cables, •Microwave transmission links, •Cellular networks, •Communications satellites,•Undersea telephone cables.

all interconnected by switching centers.

TrunkLineLocal

ExchangeLocal Exchange

Transit Exchange or TAX

Transit Exchange or TAX

CPECPE

Switching

•A Switch transfers signals from one input port to an appropriate output port.

•Basic function of any switch is to set up and release connections between transmission channels on an “as-needed” basis.

Types of Switching

•Circuit Switching

•Message Switching

•Packet Switching

Circuit SwitchingCircuit-Switched network assigns a dedicated communication path between the two stations. It involves Point to Point from terminal node to network Internal Switching and multiplexing among switching nodes. Data Transfer. Circuit Disconnect.

Advantages connection is established only once Network is transparent.

Disadvantages Connection even if no data is sent. Delay prior to usage of connection

Message Switching

•It was the precursor of packet switching, where messages were routed in their entirety, one hop at a time.

•It was first built by Collins Radio Company, Newport Beach, California.

•Each message contains addressing information, and at each switch this information is read and the transfer path to the next switch is decided.

•Each message is stored before being transmitted to the next switch. Because of this it is also known as a 'store-and-forward' network.

•Email is a common application for Message Switching.

Packet Switching

• All transmitted data – regardless of content, type, or structure – are grouped into suitably sized blocks, called packets.

• Packets are normally forwarded by intermediate network nodes asynchronously using first-in, first-out buffering.

•The best-known use of packet switching is the Internet and most local area networks. 

•Two major packet switching modes exist:1.connectionless packet switching, also known as datagram switching; and2.connection-oriented packet switching, also

known as virtual circuit switching.

EWSD

•One of the most widely installed telephone exchange systems in the world.

•The main hardware units of an EWSD switch are as under:

Digital Line Unit (DLU)

Line/Trunk Group (LTG)

Switching Network (SN)

Coordination Processor (CP)

Common Channel Signalling Network Control Unit (CCNC)

Optical Fiber Communication

Optical fibre

•An optical fiber is a flexible, transparent fiber made of high quality extruded glass (silica) or plastic, slightly thicker than a human hair.

•It can function as a waveguide, or “light pipe”, to transmit light between the two ends of the fiber.

•Light is kept in the core of the optical fiber through TOTAL INTERNAL REFLECTION.

•The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics.

Construction

Core:•Glass or plastic with higher refractiveIndex than cladding•Carries the signalCladding:•Glass or plastic with lower refractive indexBuffer:•Protects the fiber from damage or MoistureJacket:•Holds one or more fibers in a cable

Types of fiber

SINGLE MODE OPTICAL FIBERIt has a core diameter of 8-9 microns which only allows one path for light to travel.It is best for high speeds and long distances.It is used by telephone companies and CATV.

MULTIMODE STEP-INDEX FIBREIt has a core diameter of 50-62.5 microns(sometimes even larger).It allows several light paths which cause modal dispersion.Due to its large core size,power source can be easily coupled to the fiber.High Attenuation(4-6 Db/km)Low bandwidth (50MHz/km)Used in short, low-speed datalinksAlso useful in high-radiation environments

MULTIMODE GRADED-INDEX FIBERThe index of refraction gradually changes across the core.Modes that travel further also move faster reducing the modal dispersion thus increasing the bandwidthThe most widely used core diameter is 62.5 /125 micronIt is useful for premises networks like LAN’s, Saecurity systems etc

LOSSES IN FIBERSIn fibre-optic systems,losses from splices and connections are more than in the cable itself.Losses result from:oAxial or angular misalignmentoAir gaps between fibersoRough surface at the end of fibers

ADVANTAGES OF OPTICAL FIBERSThinnerLess ExpensiveHigher carrying DensityLess signal DegradationLight SignalsNon FlammableLight Weight

AREAS OF APPLICATIONTelecommunicationsLocal Area Networks(LAN)Cable TV (CATV)CCTVOptical fiber sensors

Cellular Networks

•A cellular network provides cell phones or mobile stations(MSs), to use a more general term, with wireless access tothe public switched telephone network (PSTN).

•The service coverage area of a cellular network is divided into many smaller areas, referred to as cells, each of whichis served by a base station (BS).

•The BS is fixed, and it is connected to the mobile telephone switching office (MTSO), also known as the mobile switching center.

•Within a cell covered by a BS, there are multiple MSsthat need to communicate with the BS.

•The methods for MSs to share the air interface in an orderlymanner are referred to as multiple access methods.

•The popular multiple access methods include FDMA, TDMA, and CDMA.

Multiple Access Methods

Frequency Division Multiple Access (FDMA)

•Each mobile is assigned a separate frequency channel for the duration of the call.

•Sufficient guard band is required to prevent adjacent channel interference.

frequency

Time Division Multiple Access(TDMA)

Time is divided into slots and only one mobile terminal transmits during each slot.

Guard time – signal transmitted by mobile terminals at different locations do no arrive at the base station at the same time

Code Division Multiple Access (CDMA)

•Use of orthogonal codes to separate different transmissions.•Each symbol of bit is transmitted as a larger number of bits using the user specific code – Spreading.

Orthogonal among users

Evolution of Cellular networks

GSM(GLOBAL SYSTEM FOR MOBILE COMMUNICATION)

•It is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G ) digital cellular networks used by mobile phones.

•It became the de facto global standard for mobile communications.

•GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands.

GSM Architecture

Wi-Fi Technology

•Wireless Technology is an alternative to Wired Technology, which is commonly used, for connecting devices in wireless mode.

•Wi-Fi (Wireless Fidelity) is a generic term that refers to the IEEE 802.11 communications standard for Wireless Local Area Networks (WLANs).

•Wi-Fi Network connect computers to each other, to the internet and to the wired network.

How a Wi-Fi Network Works

•Basic concept is same as Walkie talkies.

•A Wi-Fi hotspot is created by installing an access point to an internet connection.

•An access point acts as a base station.

•When Wi-Fi enabled device encounters a hotspot the device can then connect to that network wirelessly.

Wi-Fi Applications

•Home

•Small Businesses or SOHO

•Large Corporations & Campuses

•Health Care

•Wireless ISP (WISP)

•Travellers