CN unit 1- part 1- Computer networks introduction

Unit -1
Introduction and Physical Layer
Data Communication
Data communications are the exchange of data
between two devices via some form of transmission
medium such as a wire cable.
Characteristics:
1.Delivery - correct destination
2.Accuracy – accurate data
3. Timeliness- delivered in time.
4. Jitter – variation in the data arrival time

3. Data Representation:
•Text - Text is represented as a bit pattern, a
sequence of bits (Os or 1s), Different sets are used.
Each set is called code. Process of representing
symbols are called coding.
Numbers:Numbers are also represented by bit
patterns
Images:represented by bit patterns. image is
composed of a matrix of pixels (picture
elements).
Audio:Audio refers to the recording or
broadcasting of sound or music.

Video:
Video refers to the recording or broadcasting of a
picture or movie.
4. Data Flow:
Communication between two devices can be
i)simplex,
ii)half-duplex, or
iii)full-duplex.

Networks:
-A network is a set of devices or 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.

1. Distributed Processing:
Most networks use distributed processing, in which
a task is divided among multiple computers.
2. Network Criteria:
Performance – users, transmission medium,
connected hardware and
efficiency of software.
Reliability - measured by the frequency of
failure,

Security - unauthorized access,
-damage and development,
-implementing policies and
procedures for recovery from
breaches and data losses.
3. Physical Structures:
 A network is two or more devices connected
through links.
A link is a communications pathway that
transfers data from one device to another.

Two types of connections or link,
1. point-to-point and
A point-to-point connection provides
a dedicated link between two devices.
2. multipoint.
A multipoint connection is one in
which more than two specific devices
share a single link

Physical Topology
• refers to the way in which a network is laid out
physically.
• Two or more devices connect to a link;
• two or more links form a topology.
• The topology of a network is the representation of
the relationship of all the links and linking devices
(nodes) to one another.

1.Mesh:
• every device has a dedicated point-to-point link to every
other device.
• mesh network with n nodes has n(n -1) /2 physical channels.

Advantages
• eliminating the traffic problems
• Robust
• Privacy or security.
• Point-to-point links make fault identification and fault
isolation easy.

Disadvantages
• installation and reconnection are difficult.
• hardware required to connect each link is expensive.
2.Star
• each device has a dedicated point-to-point link only to a
central controller, usually called a hub.
• The devices are not directly linked to one another.
• does not allow direct traffic between devices.
• The controller acts as an exchanger
• used in local-area networks (LANs).

Advantages:
1. Less expensive
2. Easy to install and reconfigure.
3. hardware connection is less
4. Robustness.
Disadvatages
1. Whole topology depends on the hub.

3.Bus Topology:
• Multipoint
• One long cable is used to link all the devices in a network .
• Nodes are connected to the bus cable by drop lines and
taps.

Advantages:
1. Ease of installation.
2. Less cabling
Disadvantages:
1. Difficult reconfiguration and fault isolation.
2. Difficult to add new devices.
3.If any fault in backbone can stops all transmission.

4.Ring Topology
• each device has a dedicated point-to-point connection with
only the two devices on either side of it.
• A signal is passed along the ring in one direction, from
device to device, until it reaches its destination.
• Each device in the ring has a repeater.
• When a device receives a signal intended for another device,
its repeater regenerates the bits and passes along them

Advantages:
1. Easy to install.
2. Easy to reconfigure.
3. Fault identification is easy.
Disadvantages:
1. Unidirectional traffic.
2. Break in a single ring can break entire network.

5. Hybrid Topology:
A network can be hybrid( combination of
topologys)

Categories of Networks(types )
1.PAN (Private Area Networks)
2. LAN ( Local Area Networks)
3. WAN ( Wide Area Networks)
4. MAN (Metropolitan Area Networks)

Private Area Networks
• It’s a networks of only two devices.
• Very simple Networks

Local area network:
• privately owned and links the devices in a single office, building,
or campus.
• LAN size is limited to a few kilometers
• Depending on the needs of organization and type of
technology ,the LAN is configured.
• designed to allow resources to be shared between personal
computers or workstations.
• Resources may be hardware, software and data.
• LAN topologies are bus, ring, and star.
• speeds are normally 100 or 1000 Mbps.

Wide area network
• provides long-distance transmission of data, image, audio and video
information over large geographic areas.
• Complex that connect the Internet(switched WAN) or as simple as a
dial-up line that connects a home computer to the Internet. (point-
to-point WAN).
• Switched WAN uses router to connect to LAN or WAN.
• Point-to-point WAN is a line leased from telephone or cable TV.

Metropolitan Area Networks
• a network with a size between a LAN and a WAN.
• covers the area inside a town or a city.
• It is designed for customers who need a high-speed
connectivity, normally to the Internet, and have
endpoints spread over a city or part of city.
• A good example of a MAN is the part of the telephone
company network that can provide a high-speed DSL
line to the customer.

Internetwork
• Interconnection of Networks.
• When two or more networks are connected, they become
an internetwork, or internet.
(Or)
• Networks of Network.
• Consider the example

Network models
 Computer networks are created by different
entities.
• Standards are needed so that these
heterogeneous networks can communicate with
one another.
• The two best-known standards are
1. The OSI model
2. The Internet model

CN unit 1- part 1- Computer networks introduction

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