CS3591 COMPUTER NETWORKS
UNIT I
INTRODUCTION AND APPLICATION LAYER
Data Communication - Networks – Network Types –
Protocol Layering – TCP/IP Protocol suite – OSI Model –
Introduction to Sockets - Application Layer protocols:
HTTP – FTP – Email protocols (SMTP - POP3 - IMAP MIME) – DNS – SNMP
DATA COMMUNICATIONS
 Data refers to information. Telecommunication means
communication at a distance.
 Data communications are the exchange of data (0’s and
1’s) between two devices via some form of transmission
medium such as a wire cable.
Characteristics of a data communications system
 Delivery
 Accuracy
 Timeliness
Components of a data communications system
• Message
• Sender
• Receiver
• Medium
• Protocol
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 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.
• Computer network is a group of interconnected nodes or computing
devices that exchange data and resources with each other. A network
connection between these devices can be established using cable or
wireless media.
NETWORKS
Network Criteria
 Performance:
Performance can be measured by Transit time and Response time
• Transit time is amount required for a message to travel from one device to another
•
Response time is the elapsed time between an Enquiry and a Response
Performance can be measured by
 Number of Users
 Type of Transmission media
 Hardware and Software
Network Criteria
 Reliability:
Failure rate of network components. Measured by
• Frequency of a Failure
• Availability/robustness
• Recovery time of a Network after a failure
• Catastrope-Natural Disorders like fire, Earthquake, Theft
 Security
Data protection against corruption/loss of data due to:
• Unauthorised Access
• Viruses or Malicious users
Physical Structures
Line Configuration or Type of Connection
It refers to attachment of communication devices to a link.
Types of Line Configuration


Point to Point configuration

dedicated link between two devices and

single transmitter and receiver
Multipoint configuration

multiple recipients of single transmission

More than two specific devices share a single link
Types of connections: point-to-point and multipoint
Transmission Mode
It defines the direction of information flow between two
devices. Types of Transmission Mode:



linked
Simplex: Keyboard gets the input and Monitor displays the output
 Communication is Unidirectional
 Only one device can transmit and other can only receive
Half-duplex: Walkie-Talkie
 Communication is Bi-directional
 Each device can both transmit and receive but not at the same time
Full-duplex: Telephone network
 Communication is Bi-directional
 Each device can both transmit and receive data simultaneously at same
time
Data flow (simplex, half-duplex, and full-duplex)
TOPOLOGY
Topology is defined as the Physical or Logical arrangement
of links in a network.
Categories of Topology
• Mesh
• Star
• Bus
• Tree
• Ring
Type of transmission - unicast, mulitcast, broadcast
Mesh Topology:
Every device has a dedicated point to point link to every other
devices
A fully connected mesh topology (five devices)
Star Topology
Each Device has dedicated point to point link only to a central
controller called hub
A star topology connecting four stations
Bus Topology
It is a multi point configuration. One long cable acts as a
backbone to link all the devices.
A bus topology connecting three stations
Ring Topology
Each device has dedicted point to point line configuration only with the
two devices on either side of it. Signal is passed along the ring in one direction.
Each device has repeater which strengthens the data signals
A ring topology connecting six stations
Tree Topology
Tree topology is a network topology in which all the nodes
are directly or indirectly connected to the main bus cable. Tree
topology is a combination of Bus and Star topology.
Figure A hybrid topology: a star backbone with three bus networks
Categories of Networks

Local Area Networks (LANs)

Metropolitan Area Networks (MANs)

Wide Area Networks (WANs)

Personal Area Networks (PANs)
Local Area Networks (LANs)
•Short distances within Building, Office, College, Hospital etc
Designed to provide local interconnectivity
Metropolitan Area Networks (MANs)
Provide connectivity over an entire city
Example:
• Cable Television network and Local Telephone Company
Personal Area Networks (PANs)
Network range within a person's range within a range of 10 meters
Example:
• Bluetooth, Wifi (Wireless PAN)
• USB (Wired PAN)
Wide Area Networks (WANs)
WANs provides long distance transmission of data, voice, image and
video over large geographical area
Long distances with country or continent or over the entire World
Figure WANs: a switched WAN and a point-to-point WAN
PROTOCOLS
A protocol is 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
 Timing
Elements of a Protocol

Syntax



Structure or format of data, meaning the order in which they are presented
Semantics

Interprets the meaning of the bits

what action is to be taken
Timing

When data should be sent and

How fast data should be sent or speed at which it is being received.
PROTOCOL LAYERING
Figure Tasks involved in sending a letter
THE OSI MODEL
• OSI is Open Systems Interconnection (OSI) model.
• OSI Model is an ISO standard that covers all aspects of
network communications
• Established by ISO (International Standards Organization)
in 1947, ISO is a multinational body dedicated to
worldwide agreement on international standards.
ISO is the standard organization.
OSI is the model.
LAYERS IN THE OSI MODEL
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
Figure Seven layers of the OSI model
Please Do Not Tell Secret Password To All
Interaction between layers in the OSI model
Interaction between layers in the OSI model
Figure An exchange using the OSI model
Functions of Physical layer
• Physical Characteristics of interfaces and Media – Type of
Transmission Media
• Bit Representation – 0’s and 1’s
• Data rate - Number of bits transmitted per second
• Synchronization of Bits – Sender and receiver clocks must be
Synchronized
• Line Configuration – Point to point and Multi point
• Physical Topology - Bus, Ring, Star, Tree, Mesh
• Transmission Mode – Simplex, Full Duplex or Half-Duplex
Figure Physical layer
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Functions of Datalink layer
The Datalink layer is responsible for
• Framing
• Process of converting stream of bits into manageable data
units called Frames
• Physical Addressing – 48 bit Physical address
• Flow Control - node to node delivery
• Error Control – node to node delivery of the message
• Access Control – Controlling which device has control over the
link at a given time
Figure Data link layer
The data link layer is responsible for moving
frames from one hop (node) to the next.
Figure Hop-to-hop delivery
Functions of Network layer and Transport Layer
The Network layer is responsible for
• Logical Addressing – IP addressing
• Routing – Internetwork connecting devices such as routers or
gateways route the packets to Final Destination
The Transport layer is responsible for
• Service Point Addressing – Port address
• Segmentation and Reassembly – Message is divided into segments,
each segments contain segment number
• Connection Control – Connection Oriented or Connectionless
• Flow Control – End to End rather than across a single link
• Error Control – End to End rather than across a single link
Figure Network layer
Network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Figure Transport layer
The transport layer is responsible for the delivery
of a message from one process to another.
Figure Source-to-destination delivery
Figure Reliable process-to-process delivery of a message
Functions of Session layer and Presentation Layer
The Session layer is responsible for
•Dialog control – Half Duplex ( One way at a time) or Full Duplex
(Two ways at a time)
•Synchronization – Adding Check points (Synchronization Points) into
the Stream of Data
The Presentation layer is responsible for
•Translation – Messages into Bitstreams
•Encryption–Converting Original Message(Plaintext) to Ciphertext
•Compression – Reducing the number of bits to be transmitted
Figure Session layer
The session layer is responsible for dialog
control and synchronization.
Figure Presentation layer
The presentation layer is responsible for translation,
compression, and encryption.
Functions of Application Layer
The Application layer is responsible for
•Network Virtual Terminal – allows user to log on to a remote host
•FTAM – File Transfer Access and Management
•Mail Services – Email Forwarding and Storage
•Directory Services - Distributed Data Sources and access for global
information
Figure Application layer
The application layer is responsible for
providing services to the user.
Functions of OSI layers
TCP/IP PROTOCOL SUITE
TCP/IP protocol suite is made of five layers:
1.Physical Layer
+
2.Data Link Layer
3.Network or Internet Layer
4.Transport Layer
5.Application Layer
Network interface Layer
Layers in TCP/IP
Peer-to-peer processes
An exchange using the Internet model
Physical layer
Physical layer is responsible for transmitting
individual bits from one node to the next
Data link layer
Data link layer is responsible for transmitting frames from
one node to next
Node-to-node delivery
Network Layer
Network layer is responsible for the delivery of packets from
the original source to the final destination
Source-to-destination delivery
Transport layer
Transport layer is responsible for delivery of a message from one process to another
Reliable process-to-process delivery of a message
Summary of TCP/IP MODEL
TCP/IP PROTOCOL SUITE
TCP/IP and OSI model
ADDRESSING IN TCP/IP
Four levels of addresses are used in an internet employing TCP/IP
protocols:
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
Physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Figure Relationship of layers and addresses in TCP/IP
Physical address
Physical addresses is a 48-bit (6-byte) address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is separated
by a colon, shown below:
07:01:02:01:2C:4B
IPv4 Address
An IPv4 address is a 32-bit address that uniquely
defines the connection of a device (computer or router)
to the Internet.
The IPv4 addresses are unique and universal.
An IPv4 address is 32 bits long
Address space is divided into five classes:
A, B, C, D, and E.
Port address
A port address is a 16-bit address represented by one
decimal number
753, 8080, 80, 21, 25, 53 etc.,
A 16-bit port address represented as one single number.
Physical addresses change from hop to hop,
Logical and Port addresses remain the same from the
source to destination.
Figure Port addresses
SOCKET ADDRESS
• Sockets facilitate communication between two computers.
• Socket consists of IP address and port number that machines use to transmit the
data
Socket types
•Datagram Sockets:
• It allow processes to use the
User Datagram Protocol (UDP)
• Stream Sockets:
• It allows processes to use
Transfer Control Protocol
(TCP) for communication
• Raw Sockets:
• It provide user access to
Internet Control Message
Protocol (ICMP)
Multiplexing and demultiplexing