Computer Networks basics and OSI

Basics of data communication and OSI model
Navneet Soni
Asst. Professor
GDRCST, Bhilai

Data communication is the exchange of data between two
devices by making use of some transmission media.
Data Communication may be of two types
Local
Remote
If the communicating devices are in the same building then
it is a type of local communication
If the devices are farther apart then it is a type of remote
communication
Navneet Soni (Asst. Professor) 2

 Effectiveness of data communication depends on 3
fundamental characteristics
Delivery
Accuracy
Timeliness
 Components of Data communication
Protocol Protocol
Sender Receiver
Message

Sender:- Device that sends the message
Receiver:- Device that receive the message
Medium:-It is the physical path by which message travels from
sender to receiver.
Protocol:-It is the exchange of data between two separate entities.
Message:- Information which is to be communicated.
Networks:
A network is a collection of autonomous computer. The computer
which can forcibly start, stop and control another one are not
autonomous. Two computer are said to be interconnected if they are
able to exchange information.

The communication can be done by copper wire, fiber
optics, microwaves and communication satellite.
Networks uses distributed processing in which task is
divided among multiple computers.
Advantages of distributed processing are
Security
Distributed Database
Faster problem Solving
Security Through Redundancy
Collaborative Processing

Performance can be measured in many ways including the Transit Time &
Response Time. It depends on the following factors, they are:
1. Number of users
2. Types of Transmission Media
3. Hardware
4. Software
Navneet Soni (Asst. Professor)
6
Criteria
Performance Reliability Security

 Reliability:- it is measured by frequency of failure, the
time it takes link to recover from a failure & Catastrophe.
 Security:- Network must be safe guarded from:
 Unauthorized Access
 Viruses

 Protocol is a set of rules that govern the exchange of data
between two separate entities.
 A protocol defines: What is communicated, How it is
communicated & When it is communicated.
 The Key Element of protocol is:
 Syntax
 Semantics
 Timing

 Syntax: It refers to the format of the data, i.e. meaning
of the order in which they are presented.
 Semantics:- It refers to the meaning of each section of
the bits.
 Timing:- it refers to when the data should be sent and
how fast they can be sent.

A standard provides a model for development that makes it
possible for a product to work regardless of the individual
manufacturer
Data communication standards fall into 2 categories:
Defacto (By Fact)
Dejure (By Law)
1) Dejure (By Law) : Standards that have not been approved by an
organized body but have been adopted as a standard.
2) Defacto (By Fact) standards are of two types:
1. Proprietary
2. Non Proprietary

Line configuration refers to the way two or more
devices attach to the link.
A link is the physical connection i.e. the pathway that
transfer the data from one device to another. There are 2
possible line configuration
Point-to-Point
Multipoint

Point-to-Point:
It provides a dedicated link between two devices. The entire
capacity of the channel is reserved for transmission between
those two devices.
Multipoint:
It is also known as multi-drop line configuration, it is the one
in which more than two specific devices share a single link.

Transmission mode defines the direction of single
flow between two devices. There are 3 types of
transmission mode, they are:
1. Simplex
2. Half-Duplex
3. Full-Duplex

It is the geometrical representation of how the nodes in
the network are attached to each other. Basically there are
5 types of topologies, they are:
Mesh
Star
Tree
Bus
Ring

A type of network setup where each of the computers and
network devices are interconnected with one another, allowing
for most transmissions to be distributed, even if one of the
connections go down. This type of topology is not commonly
used for most computer networks as it is difficult and expensive
to have redundant connection to every computer.
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There are two types of mesh topology they are
Fully Connected Mesh Topology
Partial Connected Mesh Topology
Advantages:
Eliminating the traffic problems.
Mesh topology is robust.
It provides security.
Disadvantages:
The amount of cabling and the ports required

Also known as a star network, a star topology is one of
the most common network setups where each of the
devices and computers on a network connect to a
central hub. A major disadvantage of this type of
network topology is that if the central hub fails, all
computers connected to that hub would be
disconnected.
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It is a central controlling device in a star
network.

There are 2 types of hub
1. Active Hub
2. Passive Hub
An Active hub contains a repeater which is a
hardware device that regenerates the received bit
pattern before sending them out.
A Passive hub provides a simple physical connection
between the attached devices.

Advantages:
 It is less expensive than a mesh topology because each device needs
only one link and one I/O port to connect it to any number of others.
 It includes robustness means if one link fails than only that link is
affected, all the other link remain active.
Disadvantages:
 If the hub fails then it affects the whole network.

Bus networks use a common backbone to connect all devices. A single
cable, the backbone functions as a shared communication medium that
devices attach or tap into with an interface connector. A device
wanting to communicate with another device on the network sends a
broadcast message onto the wire that all other devices see, but only the
intended recipient actually accepts and processes the message.

In this nodes are connected to the bus cable by drop lines
and taps. A drop line is a connection running between the
device and the main cable.
A tap is a connector that splices into the main cable to create
a contact with the metallic core.

Advantages:
 Easy to connect a computer or peripheral to a linear bus.
 Requires less cable length than a star topology.
Disadvantages:
 Entire network shuts down if there is a break in the main cable.
 Terminators are required at both ends of the backbone cable.
 Difficult to identify the problem if the entire network shuts down.
 Not meant to be used as a stand-alone solution in a large building.

In this topology each device has a dedicated link only with the two
devices on the either side of it. All messages travel through a ring in
the same direction (either "clockwise" or "counterclockwise"). A
failure in any cable or device breaks the loop and can take down the
entire network.

Advantages:
 It is relative to install and reconfigure
 In this fault isolation is simplified.
Disadvantages:
 Unidirectional traffic may be disadvantage.
 A break in the ring can disable the entire network.

It is a variation of star, nodes in a tree are linked to the
central hub. But in this case not all devices are connected to
the central hub they are connected to the secondary hub after
that all the secondary hub are connected to the central hub.
The central hub is the type of Active hub
The Secondary hub is the type of Passive hub

Advantages:
• It allows more devices to be attached to a single central hub.
• It allows the network to isolate and prioritize communication from
different computers

A network topology that uses two or more
network topologies.

Networks are categorized into 3 categories:
1. LAN (Local Area Network)
2. MAN (Metropolitan Area Network)
3. WAN (Wide Area Network)

When two or more networks are connected together they
become an internetwork or internet.

 An ISO standard that cover all aspect of network
communication is the OSI model.
 The purpose of the OSI Model is to provide open
communication between different system.
 It is a model not a protocol.
 The OSI model is built of seven ordered layer.

 Physical layer
The physical layer coordinates the function required to
transmit a bit stream over a physical medium.
It deals with the electrical and mechanical specification
of the interface and the transmission medium means that
it defines the procedure and functions that physical
devices and interfaces have to perform for transmission
to occur

 Physical characteristics of Interface and media.
 Representation of bits.
 Data Rate.
 Synchronization of bits.
 Line configuration.
 Physical Topology
 Transmission Mode

 It transforms the physical layer a raw transmission facility to a
reliable link and it is also responsible from node to node delivery.
 Specific responsibility of the physical layer are:
Framing
Physical Addressing
Flow Control
Error Control
Access Control

 The network Layer is responsible for the source to
destination delivery of a packet across a multiple links
or networks.
 Specific responsibilities of the network layer are:
Logical Addressing
Routing

 This layer is responsible for source to destination delivery
i.e. end to end delivery of the entire message.
 For adding security the transport layer may create a
connection between the two end ports.
 Specific responsibility of this layer are
Service Point Addressing
Segmentation & Reassembly
Connection Control
Flow Control
Error Control

 This layer establishes, maintains and
synchronizes the interaction between
communicating systems
 Specific responsibilities performed by this
layer is:
Dialog Control
Synchronization

 This layer is concerned with the syntax & semantics
of the information exchanged between the two system.
 Specific responsibilities of this layer is:
Translation
Encryption
Compression

 This layer enables the user to access the
network. It provides user interface and provide
support for services.
 Specific responsibility performed by this layer
is:
Network Virtual Terminal
File Transfer, Access & Management
Mail Services
Directory Services

 The ARPANET was a research network sponsored by the
DOD. It eventually connects hundred of universities and
government installations by using leased telephone line.
 When satellite and radio networks are attached, the existing
protocols had trouble in internetworking with them. For that
a new architecture was made which is known as TCP/IP
reference model.

 Application Layer
 Transport Layer
 Network Layer
 Physical + Data Link Layer---Host-to-Network Layer

 This layer contain some higher level
protocol, out of which some of them are:
Telnet (Virtual Terminal)
FTP
SMTP

 In this layer it make use of two protocol
TCP
UDP
TCP stands for Transmission Control Protocol. It is a
type of connection oriented and reliable protocol.
UDP (User Datagram Protocol) which is a
connectionless and unreliable protocol.

 The job of the network layer is to permit the
host to inject packets into any network and
have them travel independently to the
destination.
 The packets may even arrive in a different
order than they were sent, in this case it is the
job of the higher layer to rearrange them, if
inorder delivery is required.

Data Transmission
Parallel Transmission Serial Transmission
Synchronous Asynchronous

 To alert the receiver that a new group of data is
arriving, two extra bits are added one at the
beginning which is known as start bit and the
other one is at the end which is known as stop
bit . There may be a gap between each byte.
 The start bit is always 0 and the stop bit is
always 1.
 The start bit, stop bit and the gap alert the
receiver to the beginning and end of each byte
and allow it to synchronize with the data
stream

Data can be represented in form of signal and the signal are of
two types:
Analog Signal
Digital Signal
Analog refers to something that is continuous means a set of
specific points of data and all possible points between them. Ex:
Human Voice.
Digital refers to something that is discrete means a set of specific
points of data with no other points in between. Ex: Data stored in
memory.

Both analog and digital signal can be of two forms:
Periodic Signal
Aperiodic Signal
A periodic signal consist of a continuously repeated pattern.
The period of signal is expressed in seconds. The completion
of one full pattern is known as cycle.
An aperiodic signals have no repetative pattern.

 Simple analog signal is known as sine wave.
 Sine wave is characterized by 3 characteristics:
Amplitude
Frequency
Phase
Amplitude:
It refers to the height of the signal. Amplitude is
measured in volts, amperes & watts.
Period or Frequency:
Period refers to the amount of time a signal needs to
complete one cycle.

Frequency is the number of cycles per second. Period is
measured in sec, millisec, microsec, picosec, nanosec.
Frequency is measured in Hz, KHZ, MHZ, GHZ, THZ.
Phase represents the position of the wave form relative to
time zero (0).
Digital to Analog conversion is the process of changing
one of the characteristics of an analog signal based on the
information in a digital signal.

 Amplitude Shift Keying
 Frequency Shift Keying
 Phase Shift Keying
 Quadrature Amplitude Modulation.

 The Major functions of Data Link Layer are
Framing
Physical Addressing
Line Discipline
Flow Control
Error Control

 Line Discipline determines which device can
send and when it can send.
 Flow control coordinates the amount of data
that can be sent before receiving
acknowledgement.
 Error control means error detection and
correction. It allows the receiver to inform
the sender about the damaging & losting.

 Multipoint Discipline

 Error control method refers to the methods
of error detection and retransmission. Error
control can be done by making use of two
methods:
Stop-and-wait ARQ
Sliding Window ARQ
 Go-Back-n
 Selective-Reject

 In this method retransmission of data is done
in 3 cases:
 Damaged Frame
 Lost Frame
 Lost Acknowledgement

Sender sends frame 0 and waits to receive ACK
1
 When ACK 1 is received, it sends frame 1 and
then waits to receive ACK0
 repeat from step 1 and 2 and so on.
 The ACK must be received before the time
set for each frame expires

If an error is discovered in a data frame , indicating
that it has been corrupted then the receiver sends
an NAK frame which are not numbered that tells
the sender to retransmit the last frame sent.

 A lost or damaged frame is handled in the
same way by the receiver
 When the receiver receives a damaged
frame, it discards it
 The receiver remains silent about a lost
frame and keeps its value of R
 After the timer at the sender site expires,
another copy of frame 1 is sent

 Under this there are 2 methods:
Go Back-n ARQ
Selective-Reject ARQ

 In this method if one frame is lost or damaged,
all frames sent since the last frame
acknowledged are retransmitted.
 Sequence Numbers
 Frame from a sending station are numbered
sequentially. If the header of the frame allows m bits
fro the sequence number, the sequence numbers range
from 0 to pow(2, m-1).
 Sender Sliding Window
 Sender uses window to hold the outstanding frames until
they are acknowledged
 Frames to the left of the window are those that have
already been acknowledged and can be purged
 Frames to the right of the window can not be sent until
the window slides them

 Receiver Sliding Window
Size of the window is always 1
The receiver is always looking for a specific
frame to arrive in a specific order

 Control Variables
The sender has three variables
 S: holds the sequence number of the recently sent
frame
 Sf : holds the sequence number of the last frame
in the window
 Sl : Holds the sequence number of the first frame
in the window
 Size of the window is :
The receiver only has one variable: R, that
holds the sequence number of the frame it
expects to receive

 Only damaged frame is resent
 Sender and receiver windows
 Size of the sender’s window must be one half of
 Size of the receiver’s window must be same as the sender’s
 Negative acknowledgement (NACK) is used to define damaged
frame

 DTE stands for Data Terminal Equipment.
 DCE stands for Data Circuit Terminating
Equipment.
 Both of these equipment is used on both the
side i.e. sender as well as receiver.
 The DTE generates the data and passes them
along with the necessary control character to a
DCE.
 The DCE converts the signal to a format
appropriate to the transmission medium and
introduces it onto the network link.

 A DTE is any device that is the source of or
destination for binary digital data.
 A DTE can be a terminal , micro computer,
computer, printer or fax machine.
 DTE do not often communicate with one
another, they generate and consume
information by making use of an
intermediary device that is DCE.

 A DCE is any device that transmits or receive data in the
form of an analog or digital signal through a network.
 In any network, a DTE generates digital data and passes
them to a DCE, then the DCE converts the data to a
form acceptable to the transmission media and sends
the converted signal to another DCE on the network.
The second DCE takes the signal off the line, converts it
to a form usable by its DTE and delivers it. To make this
communication possible, both sending and receiving DCE
must use the same modulating method.

Computer Networks basics and OSI

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Computer Networks basics and OSI