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Jayaprasad K M
Lecturer ,
Presented by:
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Introduction
What history says
Network structures
Network servicesTerminology
Regulation
Standard
Know_this_Links
HOME_Page
Back_to_Figure
Back_to_Theory
Extra_Information
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Electrical communication began with the invention of the
telegraph independently by two persons, Wheatstone andSamuel Morse in 1837.
Telegraphy system consisted of separate point-to-pointlines, sending information in one direction.
Alexander Graham Bell invented telephone in 1876.
First telephone exchange @ New Haven, Connecticut
opened in 1878.
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If communication is required between n users stations, itcould be provided by a network consisting of a line from
each station to every others.
Different types of network configurations
Mesh network
Bus network
Ring network
Star network
Tree network
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Each station needs lines to n-1 others.
Total number of lines required is
N=1/2n(n-1)
If n>>1, then N n2
Practicable if n is small & the lines areshort.
Used for small system serving a number of
telephones in the same office.
As n increases & the lines become longer,the arrangement becomes too much
expensive. A system serving 10,000 users would nearly require 50
million lines
A
B
C
D
E
F
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Instead of connecting each station toevery other, they can all be connected toa single line as shown in figure.
Useless for normal telephony, since onlyone conversation at a time can take
place.Used for higher speed data transmission,
generated by the individual terminals.
When the common circuit is already in
use, a terminal that needs to send amessage stores it until the circuit
becomes free.
Used for LAN connection over short
distance. RING NETWORK
A
B
C
D
E
H
G
F
A
B
C
D
E
F
BUS NETWORK
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For telephony, two-way
communication is required i.e.
between any pair of stations it
must be possible for many
conversation to take place at thesame time.
This requirement is met by
providing a line from each users
station to a central switchingcentre, which connects the line
together as required.
Number of lines reduced to N=n.
A
B
CD
E
S
STAR NETWORK
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As the area covered by the starnetwork & the number of stations
served by it grows, line cost
increases.
Cont...
..
It becomes economic to divide
the network into several smallernetworks, each served by its own
exchange.
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If an area is served by several exchanges, customers on one
exchange will wish to communicate to customer on otherexchanges.
Hence circuits between the exchanges are provided, these
are calledjunction circuits & they form ajunction network.
Junction networks have mesh configuration.
if the cost of the junction circuits is high, it will be
uneconomic to connect all the exchanges directly.
To make cheaper, connections between the customers local
exchanges via a central switching centre called tandem
exchange is made.
Junction networks then havestarconfiguration.
Cont...
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In practice, direct junctions between two local exchanges
prove economic whenthere is a high community of interest between their customers(resulting in
high traffic)
the distance between them is short(resulting in low transmission cost)
Indirect routing via a tandem exchange is cheaper when thetraffic is small or distance is large.
A multi-exchange area usually has direct junctions between
some exchanges, but traffic is routed via tandem exchange.The network of the area is then a mixture of astarnetwork
joining all local exchanges to tandem exchange, and a mesh
network connecting local exchanges together.
Cont...
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T
L L
L
L
L
L
L L
L
L
L
Fig: Multi-exchange area.
L = local exchange
T = tandem exchange
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Different areas of the country are interconnected by long-
distance circuits, which forms a trunk network or tollnetwork.
As it is usually uneconomic for all the local exchanges in an
area to be fully inter-connected, it is often uneconomic forall the trunk exchanges to be fully inter-connected.
Hence routings between different areas are provided by
tandem connections via trunk transit exchanges.
Cont...
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TREE NETWORK
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Satellite linksSubmarine cables
Local network
Trunk network
International network
Local exchange
Local tandem exchanges
(Primary trunk switching
centers)
Regional tandem exchanges
(Secondary trunk switching
centers)
National tandem exchanges
(Tertiary trunk switching
centers)
International gateway
exchange
(Centre de Transit 3)
Customer lines
Fig: National telecommunications network
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In the network shown in figure, when there is a direct route
between two exchanges at the same level, there is also a possiblealternative route between them via an exchange at the next
higher-level.
If the direct route is not available, it is possible to divert traffic to
the indirect route.In older switching systems, such changes are made by manual
rearrangements.
In modern switching systems it is done automatically, it is
known as automatic alternative routing(AAR).
With AAR, if an originating exchange is unable to find a free
circuit on the direct route to a destination exchange, it
automatically routes the call through the higher-level exchange.
Cont...
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AAR routing takes place not only when there is no free
direct circuits available because of breakdown but also
when they are all busy.
Use of AAR improves the resilience of the network to
withstand both breakdowns and traffic overloads
Cont...
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A nationalpublic switched telecommunication network(PSTN)
consists of the following hierarchy:1. Local networks - connects customers stations to their local exchanges. (also
called as subscribers distribution networks, customer access networks or thecustomer loop)
2. Junction networks - interconnects a group of local exchanges serving an area
and a tandem or trunk exchange.3. Trunk or toll network - provides long-distance circuits between local areasthroughout the country.
The totality of (2) & (3) is sometimes known as core network ,the inner core consisting of the trunk network and the outer
core consisting ofjunction networks. International network provides circuits linking the national
networks of different countries.
National network is connected to international network by one
or more international gateway exchanges.
Cont...
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Below the hierarchy of the PSTN, some customers have
internal lines serving extension telephones.
These are connected to one another and to lines from the
public exchange by aprivate branch exchange(PBX).
A telecommunications network contains a large number oftransmission links joining different locations, which are
known as the nodes of the network. Customer nodes.
Switching nodes. Transmission nodes.
Service nodes.
Cont...
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Each customers terminal is a node.
Switching centers form other nodes.
At some nodes, certain circuits are not switched but their
transmission paths are joined semi-permanently.
Customers require connection to nodes where there aretelephone operators to assist them in making calls and to
public emergency services.
They can also obtain connection to commercial providers ofvalue-addednetwork services (VANS) such as voice mail,
stock-market prices, sports results, etc
Cont...
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In order to set up a connection to the required destination,
and clear it down when no longer required, the customermust send information to the exchange.
For a connection that passes through several exchanges,
such information must be sent between all exchanges on theroute.
This interchange of information is calledsignaling.
A telecommunications network is considered as a system
consisting of the following interacting subsystems:
Transmission systems.
Switching systems.
Signaling systems.
Cont...
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Customers of a public telecommunications operator (PTO)
require many different services which may require different
networks.
Examples of networks Public switched telephone network (PSTN).
Public switched telegraph network (Telex).
Private networks for voice and data (using circuits leased from the PTO).
Cellular radio networks providing mobile communications.
Public data networks (PDN), employing packet data.
Special service networks, to meet specialized demands from customer.
Service providers over telecommunication networks can be
divided into two categories1. Teleservices, in which provision of the service depends on particular terminal apparatus
( e.g. a telephone or a teleprinter).
2. Bearer services, which present the customer with transmission capacity that canbe used
for any desired function ( e.g. private circuits).
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North America British
Customers loop Local network / Access network
Central office Exchange
End office Local exchange
Class 5 office / Inter-office trunk Junction
Junctor Trunk
Toll office Trunk exchangeToll network Trunk network
Table 1.1 Comparison of nomenclature
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Regulation
Standards
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