Open Systems interconnections (OSI) is a reference model
that tells how applications communicate over network. OSI model refers to
conceptual framework that describes the networking functions or
telecommunication system.
The OSI model uses 7 layers to help give a visual
description in a particular networking system and tells us what is actually
going on in visual format. These OSI layers help network professionals to solve
issues.
The main purpose of the OSI model is that it is the
process of communication between two endpoints in a networking system can be
divided into 7 different layers or groups. Each communicating user is on a
device that can provide those 7 layers of function.
The OSI model is a method that standardizes communication
within network systems by dividing it into different layers. All Cisco hardware
devices are based on this OSI model and you need to know all seven layers of
this model to complete CCNA certification exam:
- Physical layer
- Data link layer
- Network layer
- Transport layer
- Session layer
- Presentation layer
- Application layer
Detailed overview about seven layers of OSI model:
7. Application
layer: these layer servers as the window for users and
application processes used to access network services. This layer is
responsible for initiating the request. It helps to provide platform to send
and receive data over the network. You can identify communication partners in
this layer and this help us to know is there someone to talk to. All
applications and utilities that communicate with networks fall in this
application layer. Such as:
Browsers: Google chrome, Mozilla Firefox, Internet explorer,
etc.
Email clients: Outlook express, Mozilla thunderbird, etc
FTP clients: Filezilla, sFTP, vsFTP,
DNS: Domain Namng System is used to translate the name with
IP address
SNMP: This is used to
control the connected networking devices
DHCP: This is used to assign IP address and DNS information
automatically to hosts
HTTP: This is used to browse web pages
FTP: This is used to send and receive file
SMTP: This is used to send emails
NTP: This is used to synchronizes clocks
6. Presentation
Layer: This layer formats the information so that it is understood by
receiving end. It represent the data. It takes all data from application layer
and marks it with formatting extensions such as .doc, .jpg, .txt, etc. This
layer also deals with compression and encapsulation. Simply you can view this
layer as a translator for the network. Presentation layer can be used to
translate data from a format used in application layer into a simple format at
the sending point, then translate the common format known by the application
layer at the receiving station.
5. Session Layer:
When two computers, devices or servers wants to communicate with another then
session needs to be created, and this is done at the session layer. This layer
setup and terminate the communications . It helps to reconnect and authenticate
after any interruption. Over the internet, TCP and UDP (User Datagram Protocol)
provide these services for most applications. This layer determines the packet
loss, error checking, and other checks to determine whether all packets have
arrived or not. For an example, in a web server, many users communicates with
server in sate time where session helps to keep track of which user
communicates on which path and session layer does this accurately.
4. Transport Layer:
This is most important layer. It provides following services:
·
This layer is responsible for setup and maintain
connection between two devices
·
This layer allows multiple applications to
simultaneously send and receive data
·
Data transmission method can be connection based
or connection less
·
In this layer connection less method is used for
unreliable data delivery
·
Connection less method used UDP protocol
·
In the transport layer for reliable data delivery
connection oriented method is used
·
Connection based method uses TCP protocol
There are five main functions of transport layer:
segmentation, connection management, reliable and unreliable data delivery,
flow control, and connection multiplexing.
3. Network Layer: The network layer
decides best path or route is for packet delivery based in sending and
receiving IP address. This layer is responsible for providing logical address
which is known as IP address. Routers work on this network layer. Route can be
set by Static route or Dynamic route. Network layer also convert network
address and names to MAC address. The main functions of this layer are follows:
·
It define IP address
·
It find route or path based on IP address to
reach its destination
·
It connects different data link type together
like as Token Ring, Serial, etc.
Major protocols used at this layer are IP, ARP, ICMP, IPX,
PING, OSPF, RIP, IGMP, IPsec
2. Data Link Layer:
This layer is responsible to provide node-to-node data transfer. It also
handles error correction from the physical layer. It setup links across the
physical network and putting packets into network frames. Data Link Layer is responsible
for physical addressing, error correction and preparing the data information
for media. The main functions of this layer are as following:
·
Responsible to define MAC or hardware address
·
Responsible to defining physical or hardware
topology for network connections
·
It defines how the network layer protocol is
encapsulated in the data link layer frame
·
It provides connection oriented and connection
less services
·
It define MAC
1. Physical Layer:
This layer deals with electrical signals. It receives data from data link layer
and converts them into bits. Then it loads these bits on real communication
media depending on media type these bits values are converted by this layer in
single. Physical layer works with binary transmission, data rates and voltage
levels. Major protocols used in this layer is IEEE, X.25, EIA/TIA, 802.5.
Physical layer covers different devices and mediums such as
cabling, receivers, connectors, transceivers and repeaters.
