What is OSI layer?
As per
“geeksforgeeks” website: OSI stands for Open Systems Interconnection. It
has been developed by ISO – ‘International Organization of Standardization‘, in
the year 1984. It is 7 layer architecture with each layer having specific
functionality to perform. All these 7 layers work collaboratively to transmit
the data from one person to another across the globe.
Layers of OSI:
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
At receiver
side Physical layer will be the first layer and data link layer will be second
layer and so on
But at sender side Application layer will be
the first layer and Presentation layer will be the second layer
We can
remember the layers name by below line:
Please do
not touch Stephen pet animal (PDNTSPA)
Each work
first letter start with each layer name:
1. Physical Layer:
This layer
is the first layer at receiver side and last layer at sender side, as the name
indicate it is responsible for actual physical connection with the device.
It converts
the signal in bits and sends by physical medium
This layer
converts the digital bits into electrical, radio, or optical signals
Responsibility of physical layer:
Topology
management: The topology like Bus topology, Star topology etc are managed by
physical layer.
Data Flow
control: The Physical layer also defines the transmission rate i.e. the number
of bits sent per second.
Synchronization:
It is responsible for bit synchronization
Device use
at physical layer: Cables, Hubs
Protocols
used at this layer: The major protocols used by this layer include Bluetooth,
PON, OTN, DSL, IEEE.802.11, IEEE.802.3, L431 and TIA 449
Data Link Layer:
- The main
use of data link layer is to control error. Ensures that the data received
is free of any errors, and to do that it also monitors flow control of
packets. It sends data as per acknowledgement of receiver and vise versa.
Data link
layer has two parts:
1. Media
Access control MAC
2. Logic
Link control LLC
Ø The packet received by Data link
layer divided into frames.
Ø After framing it add MAC address on
header of each frame it is also called physical addressing
Ø It encapsulates Sender and Receiver’s
MAC address in the header
Ø It uses ARP (Address Resolution
Protocol) protocol to get receiver MAC address.
Ø Switch & Bridge are Data Link Layer
devices
Protocol used: ARP, CSLIP, HDLC,
Network Layer:
It also
takes care of packet routing i.e. selection of the shortest path to transmit
the packet, from the number of routes available
It uses
sender and receiver IP address are added to its header for routing
It provide
logical addressing for routing address
Device used
in network layer is routers.
Protocols: Routing Protocols, IP, ICMP
Transport layer:
Transport
layer is responsible for segmentation and flow control error to ensure proper
data transmission. Each segment has its header which contains basic information
which helps in reassembly of segments. It also adds Source and Destination port
numbers in its header and forwards the segmented data to the Network
Layer.
Transport
Layer is called as Heart of OSI model.
It provides
connection oriented connection in such connection below steps take place:
1. Stable
a connection
2. Transfer
data in segments and do acknowledgement
3. Once
sender get acknowledgement it disconnect the connection
Protocols:
TCP (TCP stands for transmission control protocol)
, IP, UDP, DCCP
and SCTP
Session layer:
This layer
is responsible for the establishment of connection, maintenance of sessions,
authentication, and also ensures security. It is end to end layer which stabiles
connection and disconnect only when data get transfer and session layer get
confirmation for same
It also
provide logical ports for data transfer
It support
communication between two device by half and full duplex model
PPTP, SAP,
L2TP and NetBIOS
Presentation layer:
This layer
is responsible for Translation, Encryption/
Decryption, Compression:
We can understand this with an example.
Suppose a user send a message then he send message by any application
like Gmail, facebook etc that application work on Application layer but when we
send message it get encrypted before it send to someone else this encryption
take place at presentation layer.
Or suppose we receive any file (for ex. MP3 file) when we try to open
that we get option to open in VLC media this selection option take place at
presentation layer.
And we open file in an application this application work at application
layer.
Protocols: XDR, TLS,
SSL and MIME
Application
layer:
At the very top of the OSI Reference Model stack of layers, we find the
Application layer which is implemented by the network applications
Ex: Application – Browsers, Skype Messenger, etc.
Protocols: HTTP, SMTP, DHCP, FTP, Telnet, SNMP and SMPP.