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OSI (Open System Interconnection) Model
it defines and used to understand how data is transferred from 1 computer to another in a computer network
in its basic form, 2 computers are connected via a LAN (Local Area Network) cable and connector (RJ45)
shares data with the help of NIC (Network Interface Controller) card
this forms the computer network
Problem
if 1 computer uses Windows OS, and another uses Mac OS, how would they communicate?
Solution
OSI Model was introduced by ISO (International Organization for Standardization) in 1984
OSI Model contains 7 layers
each layer is a package of protocols
Application layer (7)
this layer is used by network applications
network application means computer apps that uses internet
computer apps e.g.: chrome, firefox, outlook, skype, etc.
these apps are a network app that runs in your PC and does not reside in the application layer
but uses application layer protocols to use the internet
application layer protocols
examples
HTTP (Hypertext Transfer Protocol)
HTTPS (Hypertext Transfer Protocol Secure)
FTP (File Transfer Protocol)
NFS (Network File System)
FMTP (Flight Message Transfer Protocol)
DHCP (Dynamic Host Configuration Protocol)
SNMP (Simple Network Management Protocol)
Telnet
SMTP (Simple Mail Transfer Protocol)
POP3 (Post Office Protocol version 3)
IRC (Internet Relay Chat)
NNTP (Network News Transfer Protocol)
the application layer protocols forms the basis for various network services
File Transfer: uses FTP
Web Surfing: HTTP/S
Emails: SMTP
Virtual Terminals: Telnet
Summary
application layer provides services for network applications with the help of protocols to perform user activities
Presentation layer (6)
Translation
receives data in the form of alphanumeric characters from Application layer
Presentatin layer then translate the data into binary code format for the machine to understand
e.g.: convert ASCII (American Standard Code for Information Interchange) to EBCDIC (Extended Binary Coded Decimal Interchange Code)
Data Compression
before translated data is transmitted, presentation layer will reduce the number of bits that were used to represent the original data
can be in 2 type of formats
Lossy
Lossless
it reduces the amount of space used to store the original file
e.g.: a 5 MB file into a 2 MB file
when space is reduced, the amount of time required to transmit also reduces, thus allow data transmission to be faster
useful for realtime audio and video streaming
Encryption / Decryption
use to maintain the integrity of data before transmission
it helps to enhance the security of sensitive data
data gets encrypted on the sender side, and gets decrypted on the receiver side
examples:
SSL (Secure Sockets Layer) is used for encryption and decryption
Session layer (5)
it helps in setting up and managing connections enabling sending and receiving of data
followed by termination of connections or sessions
it has its own helpers called APIs (Application Programming Interface)
examples:
NetBIOS (Network Basic Input/Output System)
it is an API example that allows applications on different computers to communicate with each other
Authentication
done before a session or connection is established with the server
it is a process of verifying who you are
for this, server uses username and password
when matched with the server, a connection or session is established between the server and your computer
Authorization
after authentication, authorization is checked
it is a process used by the server to determine if you have permission to access a file
Session Management
session layer keeps track of the files being downloaded
webpage example
it contains text and image files, and are stored as separate files on the web server
when you request a website on the web browser, it opens a separate session to the web server to download each of the text and image files separately
files are received at the computer in the form of Data Packets
Session layer keeps tracks of which data packets belongs to which file and where it goes to (web browser for this case)
the web browser performs all functions of Application, Presentation, and Session layer
Transport layer (4)
controls the reliability of communication
Segmentation
data received from the Session layer is divided into small data units called Segments
each Segment or data unit contains
a source and destination Port number
helps to direct the segment to the correct computer application
a sequence number
helps to reassemble segments in the correct order to form correct message at the receiver
Flow Control
controls the amount of data being transmitted
example: a mobile app connected to a server
server can transmit a max data of 100 Mbps
mobile can process data with a max of 10 Mbps
when mobile is downloading a file from a server
if server sends data at 50 Mbps
it is a a rate the mobile can't process
mobile uses the Transport layer to tell the server to slow the data transmission rate to 10 Mbps so that no data gets lost
if server sends data at 5 Mbps
mobile uses the Transport layer to tell the server to increase the data trasmission rate to 10 Mbps to maintain system performance
Error Control
Automatic Repeat Request
this is used when a data unit did not arrive at the destination
it helps to retransmit the lost or corrupted data unit from the server to the app
Checksum
it is added to each data unit by the Transport layer to find out the received corrupted or missing segment
Connection-oriented Transmission
done via TCP (Transmission Control Protocol)
more reliable than UDP
because it provides feedback
you will know of the data delivery status
data can be retransmitted if lost or corrupted
used when whole data delivery is a must
www (world wide web), email, FTP
Connectionless Transmission
done via UDP (User Datagram Protocol)
UDP (User Datagram Protocol)
faster than TCP
because it provides no feedback
you do not know of the data delivery status
used when receiving of whole data is not necessary
e.g.: online streaming movies, songs, games, voice over ip, TFTP (Trivial File Transfer Protocol), DNS (Domain Name System)
Network layer (3)
Transport Layer sends data segments to the network layer
Network Layer works for the transmission of the recieved data segments from one computer to another located in different networks
Data Units in Network Layer is called Data Packets
The Function of Network Layer are:
Logical Addressing
IPv4 & IPv6 + Mask
every computer in a network has a unique IP address
Network layer assigns sender and receiver IP address to each segment to form an IP Packet
IP address are assigned to ensure each data packet can reached the correct destination
Mask tells that the first 3 combination of an IP address represent the which network
the last combination of the IP address represents which host or computer
Routing
it is a method of moving data packet from source to destination
it is based on the logical addressing format (IP addresses and Mask), routing decisions are made in the computer network
Path Determination
a computer can be connected to an internet server or a computer in a number of ways
choosing the best possible path for delivery from source to destination is called path determination
examples of protocols used to determine the best possible path for delivery
OSPF (Open Shortest Path First)
it is a link-state routing protocol
BGP (Border Gateway Protocol)
it is a standardized exterior gateway protocol
IS-IS (Intermediate System to Intermediate System)
Data Link layer (2)
Data link layer receives data packets from network layer
Data packets contains IP addresses of sender and receiver
There are two types of addressing:
Logical Addressing
done at the Network layer
where senders and receivers IP addresses are assigned to each segments to form a data packets
Physical Addressing
done at the Data Link layer
head is added infront of the IP packet, while the tail is added behind
where MAC addresses of sender and receiver are assigned to each data packets to form a frame
MAC Address is 12 digit Alpha-Character Number Data unit in data link layer is called Frame.
it is embedded into the network card by the manufacturer
Data Unit in Data Link layer is referred to as frame
it is embedded as a software in NIC (Network Interface Card) of the computer
it provide means to transfer data from 1 computer to another via a local media
such as copper wire, optical fiber, and air
media refers to the physical links between 2 computer or networks
allows upper layer of OSI Model to access the media
via techniques such as framing
IP packet gets encapsulated into a frame before sending, upon receiving it gets decapsulated
this process repeats itself until the data reaches its final destination
controls how data is placed and received from the media
via techniques such as media access control, and error detection
multiple devices could be connected via a common media
if 2 or more devices sends data at the same time
there will be a possibility of collision of the 2 messages resulting in a useless message that no recipient can understand
use CSMA (Carrier-Sense Multiple Access) to avoid such a situation
it is a (MAC) Media Access Control protocol
keeps an eye on when the shared media is free so that device can transmit data to the receiver
this helps to control data transmission
each frame contains bits which are used to detect errors in the receiver frame
errors occur due to certain limitations of the media used for transporting data
Physical layer (1)
Till now, Data from Application Layer has been segmented by Transport Layer - placed into Packets by Network Layer
and Framed by Data Link Layer
which is a sequence of binary 0's and 1's
physical layer converts this binary bits into signals and transmit over local media
It can be an electrical signal in the case of Copper cable or LAN cable, Light signal in the case of Optical Fiber, and Radio Signal in the case of AIR
So signal generated by physical layer depends on the type of media used to connect two devices
At the receiver, Physical Layer receives signal and converts to bits and pass it to Data Link Layer as a Frame
Frame is further decapsulated as data moves through higher layers
Finally,data is moved to Application Layer
Application Layer Protocol makes the sender's message visible in the application in the receiver's computer screen
In this way, OSI model is helping to transfer data between different hosts
So, these Seven Layers of OSI model are lying behind the smooth functioning of Internet
