Telecommunication Networks
Categories
1
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.
We can divide telecommunications networks into categories in many different ways:
two broad categories, public networks and private (or dedicated) networks.
Telephone Network (PSTN)
Private Networks:
ISDN
Professional mobile radio (PMR)
Mobile Telephone Networks
Virtual Private Networks
Telex Network
Public Networks:
Paging Networks
Public Data Networks
Internet
Radio and Television Networks
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2
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
The PSTN consists of local networks interconnected by a long-distance network
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3
ISDN
ISDN: all information is transmitted in
digital form from end-to-end
Requires replacement of analog subscriber
interface units with digital ones
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
ISDN basic rate interface
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4
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.
• 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.
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5
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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6
IN: Distributed Intelligence
Implementation of supplementary services in local exchanges
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
• 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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7
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
extent, made possible by a common OSI
Layer 3 network protocol called “IP”
(Internet Protocol)
ARPANET, the Internet ancestor, was
developed in the late 1960s by the U.S.
Department of Defense
The Internet as of 2003
In the 1990s the user-friendly graphical
user interface “World Wide Web” (WWW)
was introduced
Source: http://www.opte.org/maps/tests/
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Circuit-switching vs. packet-switching
8
Circuit-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.
Packets
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
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.
Packet-switching
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.
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Public Telecommunication Networks
9
Worldwide 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:
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.
Overview of the public TLC network
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