TELECOMMUNICATION is communication at a
distance by technological means, particularly
through electrical signals or electromagnetic
Electrical and electromagnetic telecommunication
include telegraph, telephone, and teleprinter, network
s, radio, microwave transmission, fiber
optics, communications satellites and the Internet.
• collection of terminal nodes, links and
any intermediate nodes.
•transmission links connect the nodes
•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.
•The Telelphone network
•The Global Telex network
•The Aeronautical ACARS 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).
The 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,
•Undersea telephone cables.
all interconnected by switching centers.
•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.
Circuit-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.
connection is established only once
Network is transparent.
Connection even if no data is sent.
Delay prior to usage of connection
•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
•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-andforward' network.
•Email is a common application for Message 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; and
2.connection-oriented packet switching, also known
as virtual circuit switching.
•One of the most widely installed telephone exchange systems in
•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)
•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.
•Glass or plastic with higher refractive
Index than cladding
•Carries the signal
•Glass or plastic with lower refractive
•Protects the fiber from damage or
•Holds one or more fibers in a cable
SINGLE MODE OPTICAL FIBER
It 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 FIBRE
It has a core diameter of 50-62.5 microns(sometimes even
It allows several light paths which cause modal dispersion.
Due to its large core size,power source can be easily coupled to
High Attenuation(4-6 Db/km)
Low bandwidth (50MHz/km)
Used in short, low-speed datalinks
Also useful in high-radiation environments
MULTIMODE GRADED-INDEX FIBER
The index of refraction gradually changes across the core.
Modes that travel further also move faster reducing the modal
dispersion thus increasing the bandwidth
The most widely used core diameter is 62.5 /125 micron
It is useful for premises networks like LAN’s, Saecurity systems
LOSSES IN FIBERS
In fibre-optic systems,losses from splices and connections are
more than in the cable itself.Losses result from:
oAxial or angular misalignment
oAir gaps between fibers
oRough surface at the end of fibers
ADVANTAGES OF OPTICAL FIBERS
Higher carrying Density
Less signal Degradation
AREAS OF APPLICATION
Local Area Networks(LAN)
Cable TV (CATV)
Optical fiber sensors
•A cellular network provides cell phones or mobile stations
(MSs), to use a more general term, with wireless access to
the 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 which
is 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 MSs
that need to communicate with the BS.
•The methods for MSs to share the air interface in an orderly
manner are referred to as multiple access methods.
•The popular multiple access methods include
FDMA, TDMA, and CDMA.
•Each mobile is assigned a separate frequency channel for the
duration of the call.
•Sufficient guard band is required to prevent adjacent channel
Guard time – signal transmitted by mobile
terminals at different locations do no arrive
at the base station at the same time
Time is divided into slots and only one mobile terminal transmits
during each slot.
•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
•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
•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.
•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.
•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.
•Small Businesses or SOHO
•Large Corporations & Campuses
•Wireless ISP (WISP)