Computer Networks unit first chapter for frameworks

RAJEEV GANDHI MEMORIAL COLLEGE OF
ENGEERING & TECHNOLOGY
COMPUTER NETWORKS
IV B.Tech I
SEM
DEPARTMENT OF ECE
DEPT OF ECE. RGMCET J.LEELA
MAHENDRA KUMAR

UNIT-I
INTRODUCTION-Network hardware, Network Software - Reference Models, The
physical layer: Guided Transmission medium- Communication satellites, PSTN-
Local loop: Modem, ADSL, and wireless- Trunks and Multiplexing, switching-
Circuit and Packet.
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NETWORK HARDWARE:
Broadly speaking, there are two types of transmission technology that are in widespread use.
They are as follows:
A. Broadcast links.
B. Point-to-point links.
A. Broadcast links:
Broadcast networks have a single communication channel that is shared by all the machines
on the network. Short messages, called packets in certain contexts, sent by any machine are
received by all the others.
An address field within the packet specifies the intended recipient. Upon receiving a packet,
a machine checks the address field. If the packet is intended for the receiving machine, that
machine processes the packet; if the packet is intended for some other machine, it is just
ignored

Classification of interconnected processors by scale.
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Network Software:
1.Protocol Hierarchies.
2.Design issues for the layers
3.Connection Oriented and Connectionless Services
4.What are Service Primitives?
5.Relationship of Services to Protocol:
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PROTOCOL HIERARACHY
To reduce network software design
complex, most networks are organized as
a series of "layers" or "levels", each one
built upon the one below it. The number
of layers, the name of each layer, and the
function of each layer differ from
network to network. However, in all
networks, the purpose of each layer is to
offer a certain service to the higher
layers, shielding those layers from the
details of how the offered services are
actually implemented.

In some systems, data only travel in one direction; in others, data can go both
ways. The protocol must also determine how many logical channels the connection
corresponds to and what their priorities are. Many networks provide at least two logical
channels per connection, one for normal data and one for urgent data.
2. DESIGN ISSUES FOR THE LAYERS
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Reference Models in Communication Networks
The most important reference models are:
1. OSI reference model.
2. TCP/IP reference model.
Introduction to ISO-OSI Model
There are many users who use computer network and are located all over the
world. To ensure national and worldwide data communication ISO (ISO stands for
International Organization of Standardization.) developed this model.
This is called a model for open system interconnection (OSI) and is normally
called as OSI model.OSI model architecture consists of seven layers. It defines seven
layers or levels in a complete communication system.

Difference between OSI and TCP/IP Reference Model

Diagrammatic Comparison between OSI Reference Model and TCP/IP Reference Model
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Guided Transmission Media: (Transmission Mediums in Computer Networks)
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Guided media, which are those that provide a conduit from one device to another,
include Twisted-Pair Cable, Coaxial Cable, and Fibre-Optic Cable.
A signal travelling along any of these media is directed and contained by the
physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper)
conductors that accept and transport signals in the form of electric current. Optical fibre
is a cable that accepts and transports signals in the form of light.
 A twisted pair consists of two conductors (normally copper), each with its own plastic
insulation, twisted together. One of these wires is used to carry signals to the receiver, and
the other is used only as ground reference. The receiver uses the difference between the
two.
 In addition to the signal sent by the sender on one of the wires, interference(noise) and
crosstalk may affect both wires and create unwanted signals. If the two wires are parallel,
the effect of these unwanted signals is not the same in both wires because they are at
different locations relative to the noise or crosstalk sources. This results in a difference at
the receiver.

Twisted Pair is of two types:
1. Unshielded Twisted Pair (UTP)
2. Shielded Twisted Pair (STP)
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• Outer metallic wrapping is used as a shield against noise and as the second conductor which
completes the circuit. The outer conductor is also encased in an insulating sheath. The outermost part
is the plastic cover which protects the whole cable.
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Optical fibres use reflection to guide light through a channel. A glass or plastic core is
surrounded by a cladding of less dense glass or plastic. The difference in density of the two
materials must be such that a beam of light moving through the core is reflected off the
cladding instead of being refracted into it.
Internal view of an Optical fibre
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Figure : (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

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Figure 2-21. A typical circuit route for a medium-distance call.

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Modem is abbreviation for Modulator – Demodulator. Modems are used for data
transfer from one computer network to another computer network through telephone lines.
The computer network works in digital mode, while analog technology is used for carrying
massages across phone lines.
Modulator converts information from digital mode to analog mode at the transmitting end
and demodulator converts the same from analog to digital at receiving end. The process of
converting analog signals of one computer network into digital signals of another computer
network so they can be processed by a receiving computer is referred to as digitizing.

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Digital Subscriber Line (DSL, originally, digital subscriber loop) is a communication
medium, which is used to transfer internet through copper wire telecommunication line. Along
with cable internet, DSL is one of the most popular ways ISPs provide broadband internet
access.
•Its aim is to maintain the high speed of the internet being transferred.
•If we ask that how we gonna achieve such thing i.e., both telephone and internet facility, then
the answer is by using splitters or DSL filters(shown in below diagram).Basically, the use
splitter is to splits the frequency and make sure that they can’t get interrupted.

Digital Subscriber Line Access Multiplexer

The reason that modems are so slow is that telephones were invented for carrying the
human voice and the entire system has been carefully optimized for this purpose. Data have
always been stepchildren. At the point where each local loop terminates in the end office, the
wire runs through a filter that attenuates all frequencies below 300 Hz and above 3400 Hz.
The cutoff is not sharp—300 Hz and 3400 Hz are the 3-dB points—so the bandwidth is
usually quoted as 4000 Hz even though the distance between the 3 dB points is 3100 Hz.
Data on the wire are thus also restricted to this narrow band.
The capacity of the local loop
falls rather quickly with distance
from the end office as the signal
is increasingly degraded along
the wire. It also depends on the
thickness and general quality of
the twisted pair.

To meet the technical goals, the available 1.1 MHz spectrum on the local loop is
divided into 256 independent channels of 4312.5 Hz each. This arrangement is shown
in Fig. 2-34.
The OFDM scheme, which we saw in the previous section, is used to send data over
these channels, though it is often called DMT (Discrete MultiTone) in the context of
ADSL. Channel 0 is used for POTS (Plain Old Telephone Service). Channels 1–5 are not
used, to keep the voice and data signals from interfering with each other. Of the
remaining 250 channels, one is used for upstream control and one is used for
downstream control. The rest are available for user data.

Figure 2-31. Frequency division multiplexing. (a) The original bandwidths. (b) The bandwidths
raised in frequency. (c) The multiplexed channel.
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Here four fibers come together at an optical combiner, each with its energy present at a
different wavelength. The four beams are combined onto a single shared fiber for transmission
to a distant destination. At the far end, the beam is split up over as many fibers as there were on
the input side. Each output fiber contains a short, specially- constructed core that filters out all
but one wavelength. The resulting signals can be routed to their destination or recombined in
different ways for additional multiplexed transport.
Figure 2-32. Wavelength division multiplexing.

When you or your computer places a telephone call, the switching equipment within
the telephone system seeks out a physical path all the way from your telephone to the
receiver's telephone. This technique is called circuit switching and is shown schematically in
Fig. 2-38(a).
Each of the six rectangles represents a carrier switching office (end office, toll office,
etc.). In this example, each office has three incoming lines and three outgoing lines. When a
call passes through a switching office, a physical connection is (conceptually) established
between the line on which the call came in and one of the output lines, as shown by the dotted
lines.
In the early days of the telephone, the connection was made by the operator plugging a
jumper cable into the input and output sockets. In fact, a surprising little story is associated
with the invention of automatic circuit switching equipment.

Figure 2-38. (a) Circuit switching. (b) Packet switching.
MAHENDRA KUMAR

Computer Networks unit first chapter for frameworks

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