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The OSI Reference model
Open system Interconnection Model
• The OSI Model is a standard developed on the
area of the computer networking
• It enables devices from different vendors to
work together( Interoperability)
• It enables us to visualize the process involved
in data communication
Introduction
• The OSI Reference model divides the process of data communication into 7 layers
• Each layers perform a specific task
• In each layer there is a set of protocols involved, or devices
• The seven layers1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Presentation layer
7. Application layer
Application layer
• It is the layer through which users or
application programs access to the network
• It provides the interface for application
programs to access the network
• Includes programs and protocols like
– FTP: File transfer protocol
– SMTP: Simple mail transfer protocol
Presentation Layer
• Handles formatting of data to common form
• In the sending computer, it translates data from
the format sent down from the application layer
into a commonly recognized, intermediary format
• At the receiving computer, this layer translates
the intermediary format into a format that can be
useful to that computer's application layer
• It handles encryption and decryption,
compression and decompression
Session layer
• open, use, and close a connection called a
session.
• Performs dialog between computers
Transport layer
• Prepare the data for transportation
• Divide message into smaller part that can be transmitted at a time. This process is called segmentation
• The transport layer provides flow control and error handling, and participates in solving problems concerned with the transmission and reception of packets
• Data at this layer is called segment
Network layer
• Responsible for computer’s logical addressing
• It translate logical address to physical address
• Determines the route through which the data
is transmitted
• Devices at this layer can process data using
the logical address(like IP address)
– Router
• Data at this layer is called packet
Data Link Layer
• Adds physical address on the data
• Converts data to a form that can be transmitted into the network media
• It controls the electrical impulses that enter and leave the network cable.
• Devices on this layer can process data by using the physical address(MAC Address)
– NIC, Switch, Bridge
• Data at this layer is called frame
Physical layer
• It is the layer on which the actual data is transmitted
• Defines the medium through which the bits are transmitted
• The physical layer is responsible for transmitting bits (zeros and ones) from one computer to another
• Devices at this layer do not process data but simply receive and pass data in a form of bits
– Cables, connectors, repeaters, hubs
Network Protocol
• Protocols are rules and procedures for
communicating
• Two computers can communicate with each
other if
– They use the same protocol or
– They use compatible protocols
Protocol stack(Suit):
• Set of protocols that work together
• Each layer of the stack specifies a different
protocol for handling a function or subsystem of
the communication process
• Protocol stack defines the process of networking
into layers. And in each layers there is a specific
protocol to perform a specific task on that layer
• Protocols exist at each layer of these stacks,
performing the tasks specified by that layer
Protocol stack examples
• The ISO/OSI protocol suite.
• TCP/IP
• IPX/SPX
TCP/IP
• Transmission control protocol/Internet
protocol
• TCP/IP was developed by the United States
Department of Defense as a set of wide area
network (WAN) protocols
• It is currently the standard protocol in the
Internet
TCP/IP and OSI
• The TCP/IP protocol does not exactly match the OSI reference model
• It divides the process of networking into four layers
– Network interface layer
– Internet layer
– Transport layer
– Application layer
• Each of these layers corresponds to one or more layers of the OSI reference model.
Network Interface Layer
• corresponding to the physical and data-link
layers of the OSI reference model,
Internet Layer
• corresponding to the network layer of the OSI reference model
• Protocols– Internet Protocol(IP): is a protocol that performs
addressing and route selection
– Address resolution protocol(ARP) : The ARP determines hardware address (MAC addresses) that correspond to an IP address
– Reverse Address Resolution Protocol (RARP): changes a MAC address to an IP address
– ICMP(Internet control management protocol : protocols to send and receive status reports about information being transmitted eg: pinging
Transport layer
• corresponding to the transport layer of the OSI reference model
• The transport layer provides acknowledgment of receipt, flow control, and sequencing of packets.
• Transport layer uses either UDP or TCP– TCP- is connection oriented. i.e. each segment is
acknowledged by the receiver (data communication on the internet)
– UDP(user data gram protocol) –connectionless. In this protocol the sender doesn’t need acknowledgement to send the next data(e.g: TV broadcast on the Internet)
TCP VS UDP
TCP
• TCP is connection-oriented protocol. When a file or message send it will get delivered unless connections fails. If connection lost, the server will request the lost part. There is no corruption while transferring a message.
• Reliable
• slower
UDP
• UDP is connectionless
protocol. When you a send
a data or message, you
don't know if it'll get there,
it could get lost on the way.
There may be corruption
while transferring a
message.
• Unreliable
• fast
TCP vs. UDP
Application layer
• Corresponding to the session, presentation,
and application layers of the OSI reference
model
Protocol Data Units(PDUs)
• PDUs is the data with control information at each
layer of the OSI reference model
• The control information is attached to the header
or the trailer
• Each layer protocol data unit is defined as follows
– Transport layer PDUs: Segment
– Network layer PDUs: Packet
– Data link layer PDUs: Frame
– Physical layer PDUs:bit
Data Encapsulation
• Data Encapsulation is a process of taking one Protocol Data Unit (PDU) and enveloping it within a set of protocol header and trailer. So that it can be handled by the next lower layer– It is performed by the sending computer
– Data->segment->packet->frame->bits
• De-encapsulation is the process of taking one PDU and removing the header and trailer of the PDU. So that it can be handled by the next upper layer.– It is the reverse of encapsulation
– it is performed in the receiving computer
– Bits->frame->packet->segment->data