Telecommunication Networks

Categories

We can divide telecommunications networks into categories in many different ways:

two broad categories, public networks and private (or dedicated) networks.

Up to now we explained the operation of the public switched telecommunications

network (PSTN) and used the conventional telephone network as an example.

Public network include many other networks that are optimized to provide

services with different characteristics.

Telephone Network (PSTN) Private Networks:

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Public Networks:

Telephone Network (PSTN)

Mobile Telephone Networks

Telex Network

Paging Networks

Public Data Networks

Internet

ISDN

Radio and Television Networks

Private Networks:

Professional mobile radio (PMR)

Virtual Private Networks

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PSTN

Public switched telephone network (PSTN) in the most ancient TLC network

Involves analogue voice transmission over the frequency band 300–3400 Hz: this

was the only service it was originally designed to carry on it, called POTS (Plain

Old Telephone Service)

When digital data have to be transmitted over the PSTN they must be treated as

analogue in nature

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The PSTN consists of local networks interconnected by a long-distance network45

ISDN

ISDN: all information is transmitted in

digital form from end-to-end

Requires replacement of analog subscriber

interface units with digital ones

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ISDN basic rate interface

Ordinary two-wire subscriber loop of the

telephone network (PSTN) is upgraded to

the basic rate access of ISDN by a network

termination (NT) at the subscriber

premises and by a basic rate interface

(BRI) unit and ISDN software in the local

exchange

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ISDN Access Types

ISDN provides two types of access, called basic access and primary access, respectively:

• Basic access: is the typical access for residential users that offers three digital channels

on a single twisted pair, two 64 kbit/s channels can be used as independent phone lines,

referred to as B-channels, and one 16 kbit/s channel (optional), called D-channel, used for

signaling and for the transmission of packet data. Therefore, the basic access,

characterized by a total speed of 144 kbit/s in each direction of transmission, is also

known as 2B + D access.

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• Primary Access (PRI): scheduled for business users, offers 31 channels of which thirty 64

kbit/s, with the addition of one D-channel at 64 kbit/s used only for signaling between the

user and the network. The primary access, also called 30 B+D access , is realized with a

conventional 2 Mbit/s line.

Intelligent Network (IN)Conventional telephone network able to establish a connection only to a socket

identified by the number of a B subscriber: dialing a certain number makes every

time a connection to a certain socket.

IN is an ordinary digital telephone network with some additional capabilities like

flexible routing of calls and voice notifications

In an IN the physical number and service number have no fixed relation and may

change with time.

The basic structure of an IN, illustrated in Figure below, is based on centralized intelligence.

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IN: Distributed Intelligence

Examples of supplementary services:

• Call forwarding permits to direct incoming calls to another telephone

• Call waiting means that, during a call in progress, a subscriber is

notified of an incoming call

Implementation of supplementary services in local exchanges

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• Automatic callback can be used when the number you are trying to call is busy

• Abbreviated dialing permits a subscriber to specify short numbers that

correspond to complete telephone numbers

Screening of incoming and outgoing calls allows a subscriber to specify which

telephone numbers he does not want to receive calls from, or make calls to.

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Internet

Internet is a worldwide packet-switched

network

It is a computer “network of networks”

having public access

It realizes the global interconnection

between networks of different nature and

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ARPANET, the Internet ancestor, was

developed in the late 1960s by the U.S.

Department of Defense

In the 1990s the user-friendly graphical

user interface “World Wide Web” (WWW)

was introduced50

The Internet as of 2003

Source: http://www.opte.org/maps/tests/

between networks of different nature and

extent, made possible by a common OSI

Layer 3 network protocol called “IP”

(Internet Protocol)

Circuit-switching vs. packet-switching

In circuit-switching, the path is decided before the data transmission starts. The system

decides which route to follow, and transmission goes according to the path. For the whole

length of the communication session between the two parties, the route is dedicated and

exclusive. Therefore, it is released only when the session terminates.

Let us first clarify what a packet is. IP breaks data into chunks

and wraps them into structures, the packets. Each contains,

along with the data load, information on the IP address of

Circuit-switching

Packets

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along with the data load, information on the IP address of

source and destination nodes, sequence numbers and some

control information. Once packets reach destination are

reassembled to make up the original data flow. Obviously, to

transmit data in packets, it has to be in digital form.

In packet-switching, packets are sent towards the destination irrespective of each other.

Each packet has to find its own route to the destination. There is no predetermined path;

the decision as to which node to hop in the next step is taken only when a node is

reached. Each packet finds its way using the information it carries, such as the source and

destination IP addresses.

Packet-switching

Public Telecommunication NetworksWorldwide public TLC networks, even if based on different standards and able to carry

different services are generally interconnected: to day, more and more they are based on the

common fabric represented by the global Internet. This worldwide TLC network contains the

Public Switched Telephone Network (PSTN) and the ISDN which together provide basic

analog/digital telephony, the Public Land Mobile Network (PLMN), which provides access for

cellular subscribers, as well as many other networks.Internet users are connected to the global Internet via the hosts of their Internet Service

Provider (ISP).

Shown two main methods for

accessing the Internet:

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52Overview of the public TLC network

accessing the Internet:

a) telephone or ISDN network used

for dial-up connections

b) ADSL providing permanent

higher data rate Internet service.

Digital PBX/PABX connected to a local

exchange with a 1,544/2,048-kb/s

digital line with capacity of 23/30

simultaneous calls. This connection is

called the primary rate interface in

the case of ISDN.