What Is the OSI Model Characteristics, Functions ?
The Open Systems Interconnection (OSI) model describes seven
layers that computer systems use to communicate over a network. It was the
first standard model for network communications.
The modern Internet is not based on OSI, but on the simpler TCP/IP
model. However, the OSI 7-layer model is still widely used, as it helps
visualize and communicate how networks operate, and helps isolate and
troubleshoot networking problems.
OSI was introduced in 1983 by representatives of the major
computer and telecom companies, and was adopted by ISO as an international
standard in 1984.
The OSI 7 Layers
network layer of the osi model
1. Physical Layer
The physical layer is
responsible for the physical cable or wireless connection between network
nodes. It defines the connector, the electrical cable or wireless technology
connecting the devices, and is responsible for transmission of the raw data,
which is simply a series of 0s and 1s, while taking care of bit rate control.
2. Data Link Layer
The data link layer
establishes and terminates a connection between two physically-connected nodes
on a network. It breaks up packets into frames and sends them from source to
destination. This layer is composed of two parts—Logical Link Control (LLC),
which identifies network protocols, performs error checking and synchronizes
frames, and Media Access Control (MAC) which uses MAC addresses to connect
devices and define permissions to transmit and receive data.
3. Network Layer
The network layer has
two main functions. One is breaking up segments into network packets, and
reassembling the packets on the receiving end. The other is routing packets by
discovering the best path across a physical network. The network layer uses
network addresses (typically Internet Protocol addresses) to route packets to a
destination node.
4. Transport Layer
The transport layer
takes data transferred in the session layer and breaks it into “segments” on
the transmitting end. It is responsible for reassembling the segments on the
receiving end, turning it back into data that can be used by the session layer.
The transport layer carries out flow control, sending data at a rate that
matches the connection speed of the receiving device, and error control,
checking if data was received incorrectly and if not, requesting it again.
5. Session Layer
The session layer
creates communication channels, called sessions, between devices. It is
responsible for opening sessions, ensuring they remain open and functional
while data is being transferred, and closing them when communication ends. The
session layer can also set checkpoints during a data transfer—if the session is
interrupted, devices can resume data transfer from the last checkpoint.
6. Presentation Layer
The presentation layer prepares data for the application layer. It
defines how two devices should encode, encrypt, and compress data so it is
received correctly on the other end. The presentation layer takes any data
transmitted by the application layer and prepares it for transmission over the
session layer.
7. Application Layer
The application layer is used by end-user software such as web
browsers and email clients. It provides protocols that allow software to send
and receive information and present meaningful data to users. A few examples of
application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office
Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System
(DNS).
Advantages of
OSI Model
The OSI model helps
users and operators
·
Determine the required
hardware and software to build their network.
·
Understand and communicate
the process followed by components communicating across a network.
·
Perform troubleshooting, by
identifying which network layer is causing an issue and focusing efforts on
that layer.
The OSI model helps
network
·
Create devices and software
that can communicate with products from any other vendor, allowing open
interoperability
·
Define which parts of the
network their products should work with.
·
Communicate to users at which
network layers their product operates
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