OSI
(Open Systems Interconnection) is reference model for how applications can
communicate over a network. A
reference model is a conceptual framework for understanding relationships. The
purpose of the OSI reference model is to guide vendors and developers so the
digital communication products and software programs they create will interoperate, and to
facilitate clear comparisons among communications tools. Most vendors involved
in telecommunications make an attempt to describe their products and services
in relation to the OSI model. And although useful for guiding discussion and
evaluation, OSI is rarely actually implemented, as few network products or
standard tools keep all related functions together in well-defined layers as
related to the model. The TCP/IP protocols, which define the Internet, do not
map cleanly to the OSI model.
Developed
by representatives of major computer and telecommunication companies beginning
in 1983, OSI was originally intended to be a detailed specification of actual interfaces. Instead, the committee decided to establish a
common reference model for which others could then develop detailed interfaces,
which in turn could become standards. OSI was
officially adopted as an international standard by the International
Organization of Standards (ISO).
OSI layers
The
main concept of OSI is that the process of communication between two endpoints
in a telecommunication network can be divided into seven distinct groups of
related functions, or layers. Each communicating user or program is at a
computer that can provide those seven layers of function. So in a given message
between users, there will be a flow of data down through the layers in the
source computer, across the network and then up through the layers in the
receiving computer. The seven layers of function are provided by a combination
of applications, operating systems, network card device drivers and networking
hardware that enable a system to put a signal on a network cable or out over Wi-Fi or other wireless protocol).
The seven Open Systems
Interconnection layers are:
Layer 7: The application layer.
This is the layer at which communication partners are identified (Is there
someone to talk to?), network capacity is assessed (Will the network let me
talk to them right now?), and that creates a thing to send or opens the thing
received. (This layer is not the application itself, it is the set of services an
application should be able to make use of directly, although some applications
may perform application layer functions.)
Layer 6: The presentation layer. This layer is usually part of an
operating system (OS) and converts incoming
and outgoing data from one presentation format to
another (for example, from clear text to encrypted text at one end and back to
clear text at the other).
Layer
5: The session layer. This layer sets up, coordinates and terminates conversations. Services
include authentication and reconnection after an interruption. On the Internet,
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
provide these services for most applications.
Layer
4: The transport layer. This layer manages packetization of data, then the
delivery of the packets, including checking for errors in the data once it
arrives. On the Internet, TCP and UDP provide these services for most applications
as well.
Layer
3: The network layer. This layer handles the addressing and routing of the data (sending it in the right
direction to the right destination on outgoing transmissions and receiving
incoming transmissions at the packet level). IP is the network layer for the
Internet.
Layer
2: The data-link layer. This layer sets up links across the physical network,
putting packets into network frames. This layer has two
sub-layers, the Logical Link Control Layer and the Media Access Control Layer.
Ethernet is the main data link layer in use.
Layer
1: The physical layer. This layer conveys the bit stream through the network at the electrical,
optical or radio level. It provides the hardware means of sending and receiving data on
a carrier network.
We summarize all the important functions of each layer
in the table below (please remember them, they are very important knowledge you
need to know about OSI model):
Figure 1.1
Example
Figure 1.2
TCP/IP Model
Communications between computers on a
network is done through protocol suits. The most widely used and most widely
available protocol suite is TCP/IP protocol suite. A protocol suit consists of
a layered architecture where each layer depicts some functionality which can be
carried out by a protocol. Each layer usually has more than one protocol
options to carry out the responsibility that the layer adheres to. TCP/IP is
normally considered to be a 4 layer system. The 4 layers are as follows:
1. Application layer
2. Transport layer
3. Network layer
4. Data link layer
1.
Application layer
This is the top layer of TCP/IP
protocol suite. This layer includes applications or processes that use
transport layer protocols to deliver the data to destination computers.
At each layer there are certain
protocol options to carry out the task designated to that particular layer. So,
application layer also has various protocols that applications use to
communicate with the second layer, the transport layer. Some of the popular
application layer protocols are :
§
HTTP (Hypertext transfer protocol)
§
FTP (File transfer protocol)
§
SMTP (Simple mail transfer protocol)
§
SNMP (Simple network management protocol) etc.
2.
Transport Layer
This layer provides backbone to data
flow between two hosts. This layer receives data from the application layer
above it. There are many protocols that work at this layer but the two most
commonly used protocols at transport layer are TCP and UDP.
TCP is used where a reliable
connection is required while UDP is used in case of unreliable connections.
TCP divides the data (coming from the application layer) into proper sized
chunks and then passes these chunks onto the network. It acknowledges received
packets, waits for the acknowledgments of the packets it sent and
sets timeout to resend the packets if acknowledgements are not received in
time. The term ‘reliable connection’ is used where it is not desired to lose
any information that is being transferred over the network through this
connection. So, the protocol used for this type of connection must provide the
mechanism to achieve this desired characteristic. For example, while
downloading a file, it is not desired to lose any information(bytes) as it may
lead to corruption of downloaded content.
UDP provides a comparatively simpler but
unreliable service by sending packets from one host to another. UDP does not
take any extra measures to ensure that the data sent is received by the target
host or not. The term ‘unreliable connection’ are used where loss of some
information does not hamper the task being fulfilled through this connection.
For example while streaming a video, loss of few bytes of information due to
some reason is acceptable as this does not harm the user experience much.
3.
Network Layer
This layer is also known as Internet
layer. The main purpose of this layer is to organize or handle the movement of
data on network. By movement of data, we generally mean routing of data over
the network. The main protocol used at this layer is IP. While ICMP(used by
popular ‘ping’ command) and IGMP are also used at this layer.
4.
Data Link Layer
This layer is also known as network
interface layer. This layer normally consists of device drivers in the OS and
the network interface card attached to the system. Both the device drivers and
the network interface card take care of the communication details with the
media being used to transfer the data over the network. In most of the cases,
this media is in the form of cables. Some of the famous protocols that are used
at this layer include ARP(Address resolution protocol), PPP(Point to point
protocol) etc.
TCP/IP
CONCEPT EXAMPLE
Figure 2.2
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