OSI a network model proposed by the International

OSI Model


The Open System Interconnection Model is a
network model proposed by the International Organization for Standardization
(ISO). It is reference model that consists of seven layers that specify
particular network functions with the aim of allowing us to better understand
intricate networking processes. Each layer of the model has its own functions
and protocols and uses protocol data units which is the information passed to
each layer.

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The layers and the functions of these models
will be described below from the top layer, including the protocols and
protocol data units of each layer.

A Protocol Data Unit (PDU) is information which
is added to the overall data unit at each layer of the OSI model, this data can
consist of protocol information and user data. The layer processes the data
unit and encapsulates the information which it intends to add to the data unit
and then adds the current layers header to the original PDU.

Application Layer – The Application layer is
the seventh and top layer of the OSI model. Its purpose is to support the
application processes and provides an interface between the users and end to
end communication. It is the only layer of the OSI model that interacts with
the end user. This layer is responsible for many essential services such as
email, file sharing, remote access, web browsing and similar network or web
based services. Because of these services it also uses protocols such as SMTP,
HTTP, DHCP, SNMP, FTP, TFTP and Telnet. In layer seven the protocol data unit
is data.

Presentation Layer – This layer is accountable
for ensuring effective communication to the application layer and end users as
the data it receives can be received in many different formats, it then formats
it so it can be presented in an accurate format to the application layer, it is
sometimes referred to as the syntax layer. The direct functions of this layer
are data encryption/decryption, the handling of different graphic formats as
well as data compression and conversion. The protocols used on this layer
include SSL, ASCII and MIME. The PDU on this layer is also data like the
application layer.

Session Layer – The session layer is the fifth
layer of the OSI model, it is essentially tasked with the management of the
connections and communications between computers and applications. It
establishes, coordinates and terminates the connections between the
applications and computers, which are called sessions. It is also in
responsible for the authentication of these sessions. The session layer communicates
directly with the presentation and transport layers, it also supports full and
half duplex operations. Protocols used in this layer consist of PPTP, SAP, SQL,
NetBIOS and also the TCP protocol although this also operates on layer four. As
with the above layers the PDU on the session layer is also data.

Transport Layer – The transport layer is
responsible for end to end communication over networks, including segmentation
of data, error checking and error recovery. It ensures data integrity as the
data is transmitted using checksums to guarantee that the data sent is the same
as it received. It provides end to end acknowledgement of message delivery,
which is done via a handshake protocol. Multiplexing and De-Multiplexing is
also carried out on the transport layer. Multiplexing is when every packet is
encapsulated with a transport header and then passed on to the network layer.
De-Multiplexing is the return journey, so the transport layer receives the
data, examines it and transports it to the correct application. Arguably some
of the most important protocols are used in the transport layer, these consist
of TCP, UDP, SPX, DCCP and SCTP. The PDU on the transport layer is a segment.

Network Layer – The network layer is the third
layer of the OSI model, it provides routing and switching for network
communications. It finds the best logical path possible for the transfer of
data. It communicates directly with the surrounding layers. The hardware linked
to the network layer consists of routers, switches, firewalls and other
networking and intermediary devices. Every host or router on a network uses
networking protocols. These protocols would include the likes of IP, ICMP, RIP,
IGRP, and EIGRP amongst others. Packets are the PDU for the network layer.

Data Link Layer – The Data Link layer is
responsible for encoding and decoding bits into and from frames, and providing
error-free transfer of these frames between nodes as it checks the integrity of
them when it receives them. The frame in this layer includes information to
make communication easier, such as source and destination addresses. The
protocols at this layer include ARP, IEEE.802.3, CSLIP, HDLC and the PDU is a

Physical Layer – The Physical layer, or layer
one, is the bottom layer of the OSI model. It is responsible for providing the
hardware a way of transmitting or receiving data. The protocol data unit in the
physical layer are bits. This would include cabling, NIC’s or any other physical
medium. Some of
the most current protocols used at this layer include fast Ethernet, CSMA/CD,
RS-232C and ATM amongst others. Devices
at this layer are part of common network topologies such as ring, bus and mesh.



TCP/IP Model


The TCP/IP Model is a suite of specific network
protocols developed by DARPA, which is the Defence Advanced Research Projects
Agency. It is a four layer network model that is made up of group of protocols
named after its two most important, the Transition Control Protocol and the
Internet Protocol, as they are the main ones used in network communication. The
TCP/IP model is effectively the implementation of the OSI model, but it doesn’t
use all the layers of the OSI model as it does not follow the models strict

Application Layer – The application layer is
the fourth and top layer of the TCP/IP model provides applications and
processes which allow hosts to communicate and send data over a network and
provides network services to user applications. There are a number of
application layer protocols that work at this layer but some of the most
popular are HTTP, SMTP, SNMP, FTP and TFTP.

Transport Layer – The transport layer is the
third layer of the TCP/IP model and focuses on the transmission of data and
uses flow control, error checking and acknowledgment to ensure the successful
transfer of data. The protocols used at this layer are TCP and UDP. TCP is used
when a connection is required and UDP is used when a connection isn’t required.

Internet Layer – The internet layer is the
second layer of the TCP/IP model. It has two main functions which are
addressing and routing. The internet layer is concerned with data packets,
which contain information such as source and destination address and the data
which is being delivered. Routing is then used to transfer the packets to their
destination. The main protocol used at this layer is IP but other’s such as
ICMP are also used.

Network Access – The network access layer is
the first layer of the TCP/IP model. The network access layer provides a
physical interface for how data is sent through a network. It provides IP
mapping between physical address and the network addresses and the data
transfer to it via other physical mediums such as Ethernet or coaxial cables.
The common protocols found at this layer include but are not limited to
Ethernet, Token Ring and FDDI.



Relationship between the OSI and TCP/IP models


The TCP/IP model is essentially the OSI model
but put into practice, however it doesn’t use all the layers of the OSI model.
It summarises certain layers of the OSI model into one with it resulting in
four broad layers. The Application, Presentation and Session layers combine to
be the application layer in the TCP/IP model, with the protocols of the three
layers also encapsulated into the application layer. This is because the TCP/IP
model assumes anything beyond the transport layer will be taken care of by the
application layer. The transport layer remains the same on both models and the network
layer is renamed to the internet layer, these almost identical layers have the
same functions and services in both models. It is important to note that the
TCP/IP model supports only connectionless communication in the network layer,
however it does support both in the transport layer. The Data Link and Physical
layers combine to become the Network Access layer because again the TCP/IP
model doesn’t concern itself with the actual physical hardware as long as there
is network connectivity.

The TCP/IP model does not require stringent
layering, evidenced by how the layers have been combined and mapped to the OSI
model. It is possible for protocols and services to operate across multiple
layers if it is considered reasonable and simplifies the implementation of the
protocol. The TCP/IP model has become more prevalent as the internet that has
become a huge part of the world is built and based around TCP/IP.





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