COMPUTER NETWORKS UNIT 1

COMPUTER NETWORKS
NAME OF THE STAFF : M.Jancy Priya
NAME OF THE STUDENT : S.Shobana
REGISTER : CB17S 250431
CLASS : III BCA ‘B’
BATCH : 2017-2020
YEAR : 2020
SUBJECT CODE : 16SCCCA8

UNIT-1
OVERVIEW AND PHYSICAL LAYER

INTRODUCTION
1.3
1-1 DATA COMMUNICATIONS
The term telecommunication means communication at a
distance. The word data refers to information presented
in whatever form is agreed upon by the parties creating
and using the data. Data communications are the
exchange of data between two devices via some form of
transmission medium such as a wire cable.
 Components of a data communications system
 Data Flow
Topics discussed in this section:

1.4
Figure 1.1 Components of a data communication system

1.5
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

1.6
1-2 NETWORKS
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a
computer, printer, or any other device capable of sending
and/or receiving data generated by other nodes on the
network. A link can be a cable, air, optical fiber, or any
medium which can transport a signal carrying
information.
 Network Criteria
 Physical Structures
 Categories of Networks

1.7
Network Criteria
 Performance
 Depends on Network Elements
 Measured in terms of Delay and Throughput
 Reliability
 Failure rate of network components
 Measured in terms of availability/robustness
 Security
 Data protection against corruption/loss of data due to:
 Errors
 Malicious users

1.8
Physical Structures
 Type of Connection
 Point to Point - single transmitter and receiver
 Multipoint - multiple recipients of single transmission
 Physical Topology
 Connection of devices
 Type of transmission - unicast, mulitcast, broadcast

1.9
Figure 1.3 Types of connections: point-to-point and multipoint

1.10
Figure 1.4 Categories of topology

1.11
Figure 1.5 A fully connected mesh topology (five devices)

1.12
Figure 1.6 A star topology connecting four stations

1.13
Figure 1.7 A bus topology connecting three stations

1.14
Figure 1.8 A ring topology connecting six stations

1.15
Figure 1.9 A hybrid topology: a star backbone with three bus networks

1.16
Categories of Networks
 Local Area Networks (LANs)
 Short distances
 Designed to provide local interconnectivity
 Wide Area Networks (WANs)
 Long distances
 Provide connectivity over large areas
 Metropolitan Area Networks (MANs)
 Provide connectivity over areas such as a city, a campus

1.17
Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet

1.18
Figure 1.11 WANs: a switched WAN and a point-to-point WAN

1.19
Figure 1.12 A heterogeneous network made of four WANs and two LANs

1.20
1-3 THE INTERNET
The Internet has revolutionized many aspects of our daily
lives. It has affected the way we do business as well as the
way we spend our leisure time. The Internet is a
communication system that has brought a wealth of
information to our fingertips and organized it for our use.
Organization of the Internet
Internet Service Providers (ISPs)

1.21
Figure 1.13 Hierarchical organization of the Internet

1.22
1-4 PROTOCOLS
A protocol is synonymous with rule. It consists of a set of
rules that govern data communications. It determines
what is communicated, how it is communicated and when
it is communicated. The key elements of a protocol are
syntax, semantics and timing
 Syntax
 Semantics
 Timing

1.23
Elements of a Protocol
 Syntax
 Structure or format of the data
 Indicates how to read the bits - field delineation
 Semantics
 Interprets the meaning of the bits
 Knows which fields define what action
 Timing
 When data should be sent and what
 Speed at which data should be sent or speed at which it is being
received.

2.25
2-1 LAYERED TASKS
We use the concept of layers in our daily life. As
an example, let us consider two friends who
communicate through postal mail. The process
of sending a letter to a friend would be complex
if there were no services available from the post
office.
Sender, Receiver, and Carrier
Hierarchy

2.26
Figure 2.1 Tasks involved in sending a letter

2.27
2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body
dedicated to worldwide agreement on
international standards. An ISO standard that
covers all aspects of network communications is
the Open Systems Interconnection (OSI) model.
It was first introduced in the late 1970s.
Layered Architecture
Peer-to-Peer Processes
Encapsulation

2.28
ISO is the organization.
OSI is the model.
Note

2.29
Figure 2.2 Seven layers of the OSI model

2.30
Figure 2.3 The interaction between layers in the OSI model

2.31
Figure 2.4 An exchange using the OSI model

2.32
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions
of each layer in the OSI model.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

2.33
Figure 2.5 Physical layer

2.34
Figure 2.6 Data link layer

2.35
Figure 2.7 Hop-to-hop delivery

2.37
Figure 2.9 Source-to-destination delivery

2.38
Figure 2.10 Transport layer

2.39
Figure 2.11 Reliable process-to-process delivery of a message

2.40
Figure 2.12 Session layer

2.41
Figure 2.13 Presentation layer

2.42
Figure 2.14 Application layer

2.43
Figure 2.15 Summary of layers

2.44
2-4 TCP/IP PROTOCOL SUITE
The layers in the TCP/IP protocol suite do not
exactly match those in the OSI model. The
original TCP/IP protocol suite was defined as
having four layers: host-to-network, internet,
transport, and application. However, when
TCP/IP is compared to OSI, we can say that the
TCP/IP protocol suite is made of five layers:
physical, data link, network, transport, and
application.
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer

2.45
Figure 2.16 TCP/IP and OSI model

2.46
2-5 ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical, logical,
port, and specific.
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses

2.47
Figure 2.17 Addresses in TCP/IP

2.48
Figure 2.18 Relationship of layers and addresses in TCP/IP

2.49
In Figure 2.19 a node with physical address 10
sends a frame to a node with physical address
87. The two nodes are connected by a link (bus
topology LAN). As the figure shows, the
computer with physical address 10 is the
sender, and the computer with physical address
87 is the receiver.
Example 2.1

2.50
Figure 2.19 Physical addresses

2.51
As we will see in Chapter 13, most local-area
networks use a 48-bit (6-byte) physical address
written as 12 hexadecimal digits; every byte (2
hexadecimal digits) is separated by a colon, as
shown below:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical
address.

2.52
Figure 2.20 shows a part of an internet with two
routers connecting three LANs. Each device
(computer or router) has a pair of addresses
(logical and physical) for each connection. In
this case, each computer is connected to only
one link and therefore has only one pair of
addresses. Each router, however, is connected
to three networks (only two are shown in the
figure). So each router has three pairs of
addresses, one for each connection.
Example 2.3

2.54
Figure 2.21 shows two computers
communicating via the Internet. The sending
computer is running three processes at this
time with port addresses a, b, and c. The
receiving computer is running two processes at
this time with port addresses j and k. Process a
in the sending computer needs to communicate
with process j in the receiving computer. Note
that although physical addresses change from
hop to hop, logical and port addresses remain
the same from the source to destination.
Example 2.4

2.55
Figure 2.21 Port addresses

MULTIPLEXING
Whenever the bandwidth of a medium linking two
devices is greater than the bandwidth needs of the
devices, the link can be shared. Multiplexing is the set of
techniques that allows the simultaneous transmission of
multiple signals across a single data link. As data and
telecommunications use increases, so does traffic.
Frequency-Division Multiplexing
Wavelength-Division Multiplexing
Synchronous Time-Division Multiplexing
Statistical Time-Division Multiplexing

Frequency-division multiplexing

Assume that a voice channel occupies a bandwidth of 4
kHz. We need to combine three voice channels into a link
with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the
configuration, using the frequency domain. Assume there
are no guard bands.
Solution
We shift (modulate) each of the three voice channels to a
different bandwidth, as shown in Figure 6.6. We use the 20-
to 24-kHz bandwidth for the first channel, the 24- to 28-
kHz bandwidth for the second channel, and the 28- to 32-
kHz bandwidth for the third one. Then we combine them as
shown in Figure 6.6.
Example

Five channels, each with a 100-kHz bandwidth, are to be
multiplexed together. What is the minimum bandwidth of
the link if there is a need for a guard band of 10 kHz
between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This
means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz,
as shown in Figure 6.7.

Wavelength-division multiplexing

Prisms in wavelength-division multiplexing and demultiplexing

Synchronous time-division multiplexing

Figure 6.24 T-1 line for multiplexing telephone lines

Figure 6.25 T-1 frame structure

SPREAD SPECTRUM
In spread spectrum (SS), we combine signals from
different sources to fit into a larger bandwidth, but our
goals are to prevent eavesdropping and jamming. To
achieve these goals, spread spectrum techniques add
redundancy.
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum Synchronous (DSSS)

Frequency hopping spread spectrum (FHSS)

Transmission medium and physical layer

GUIDED MEDIA
Guided media, which are those that provide a conduit
from one device to another, include twisted-pair cable,
coaxial cable, and fiber-optic cable.
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable

Categories of unshielded twisted-pair cables

UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
communication.
Radio Waves
Microwaves
Infrared

Electromagnetic spectrum for wireless communication

Figure 7.21 Unidirectional antennas

CIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches
connected by physical links. A connection between two
stations is a dedicated path made of one or more links.
However, each connection uses only one dedicated
channel on each link. Each link is normally divided into
n channels by using FDM or TDM.
Three Phases
Efficiency
Delay
Circuit-Switched Technology in Telephone Networks

A trivial circuit-switched network

As a trivial example, let us use a circuit-switched network
to connect eight telephones in a small area.
Communication is through 4-kHz voice channels. We
assume that each link uses FDM to connect a maximum of
two voice channels. The bandwidth of each link is then 8
kHz. Figure 8.4 shows the situation. Telephone 1 is
connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of
course the situation may change when new connections are
made. The switch controls the connections.
Example

Circuit-switched network used in Example 8.1

As another example, consider a circuit-switched network
that connects computers in two remote offices of a private
company. The offices are connected using a T-1 line leased
from a communication service provider. There are two 4 ×
8 (4 inputs and 8 outputs) switches in this network. For
each switch, four output ports are folded into the input
ports to allow communication between computers in the
same office. Four other output ports allow communication
between the two offices. Figure 8.5 shows the situation.
Example

Circuit-switched network used in Example 8.2

Delay in a circuit-switched network

DATAGRAM NETWORKS
In data communications, we need to send messages
from one end system to another. If the message is going
to pass through a packet-switched network, it needs to
be divided into packets of fixed or variable size. The size
of the packet is determined by the network and the
governing protocol.
Routing Table
Efficiency
Delay
Datagram Networks in the Internet

A datagram network with four switches (routers)

Routing table in a datagram network

VIRTUAL-CIRCUIT NETWORKS
A virtual-circuit network is a cross between a circuit-
switched network and a datagram network. It has some
characteristics of both.
Addressing
Three Phases
Efficiency
Delay
Circuit-Switched Technology in WANs

Switch and tables in a virtual-circuit network

Source-to-destination data transfer in a virtual-circuit network

Setup request in a virtual-circuit network

Setup acknowledgment in a virtual-circuit network

Delay in a virtual-circuit network

STRUCTURE OF A SWITCH
We use switches in circuit-switched and packet-switched
networks. In this section, we discuss the structures of the
switches used in each type of network.
Structure of Circuit Switches
Structure of Packet Switches

Crossbar switch with three inputs and four outputs

Examples of routing in a banyan switch

COMPUTER NETWORKS UNIT 1

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