This 2000 word essay will be on the OSI and TCP/IP Models, what they are
and how some parts of them can be compared.

During  and before the 1980s, there were only a
handful of protocols and standards that were around and they belonged to
different manufacturers who didn’t have much dialogue with each other. Though eventually
computer science and technology continued to be further innovated and become
more readily available to companies and the public, it became necessary that a
widespread standard would be needed in place to ensure compatibility between
all machines. This was especially true about networks, and networking
technology. A network is designed to share data and information between
machines, a standard that dictates how this information is formatted,
transmitted, and received would make it easy for data to be shared openly, even
when sending or receiving from networks that are not similar. Requirements
for a new standard of implementing open communications led the International
Organization for Standardization (ISO) and American Nation Standards Institute
(ANSI) to develop a 7 layer network communications model known as Open Systems
Interconnect, or the OSI. The OSI became a link that allows data to be reliably
exchanged and transmitted since guidelines were created to set a standard in
how network equipment is manufactured and network OS’s communicate to each other
on a network. The OSI model doesn’t actually perform any tasks or functions but
it dictates HOW the work should be performed by other hardware or software
between networks so that communication can occur.
OSI model is made up of these seven layers; physical layer, data link layer,
network layer, transport layer, session layer, presentation layer and
application layer. These 7 parts of the model are called a ‘stack’.

Here are the 7 layers in further detail:

Physical Layer
The physical layer defines all the mechanical, procedural,
functional and electrical specifications for activating, deactivating and
maintaining the link between each system . Such characteristics as voltage
levels, timing of voltage changes, physical data rates, maximum transmission
distances, and physical connectors, are defined by physical layer

Data Link Layer
The data-link layer provides error-free transfer of data
frames from one computer to another over the physical layer. The layers above
this layer can assume virtually error-free transmission over the network. The
data-link layer provides the following functions. 
– Establishing and terminating a logical link between two computers
identified by their unique network interface card. 
– Controlling frame flow by instructing the transmitting
computer not to transmit frame buffers 
– Sequentially transmitting and receiving frames 
– Providing and expecting frame-acknowledgment, and detecting
and recovering from errors that occur in the physical layer by retransmitting
non-acknowledged frames and handling duplicate frame receipts 
– Managing media access to determine when the computer is
permitted to use the physical medium 
– Eliminating frames to create and recognize frame boundaries 
– Error-checking frames to confirm the integrity of the
received frame 
– Inspecting the destination address of each received frame
and determining if the frame should be directed to the layer above 

Network Layer

network layer knows the address of the adjacent nodes, controls the subnet
operations, packages output with the correct network address information,
selects routes and quality of service, and recognizes and forwards to the
Transport layer incoming messages for local host domains.
– Controlling subnet traffic to allow an intermediate system
to instruct a sending station not to transmit its frame when the router’s
buffer fills up. If the router is busy, the network layer can instruct the
sending station to use an alternate destination station.
– Resolving the logical computer address with the physical
network interface card address. 
– Keeping an accounting record of frames forwarded to produce
billing information 

Transport Layer

Transport layer makes sure the arrival of messages goes well and provides error
checking and data flow controls such as stopping duplication or loss. It removes the
concern from the higher layer protocols about data transfer between the higher
layer and its peers. The size and complexity of a transport protocol depends on
the type of service it can get from the network layer or data link layer. For a
reliable network layer a minimal transport layer is required. Functions of the
transport layer include the following.
– Accepting messages from the layer above and, if necessary,
splitting them into frames 
– Providing reliable, end-to-end message delivery with
– Instructing the transmitting computer not to transmit when
no receive buffers are available 
– Multiplexing several process-to-process message streams or
sessions onto one logical link and keeping track of which messages belong to
which sessions 

Session Layer
The session layer establishes a communications session
between processes running on different computers, and can support message-mode
data transfer. 
Functions of the session layer include:
– Allowing application processes to register unique process
addresses. It provides the means by which these process addresses can be
resolved to the network-layer or data-link-layer NIC addresses, if necessary.
– Terminating, establishing and monitoring a virtual-circuit between
two processes identified by their specific addresses. A virtual-circuit session
is a direct link that exists between the sender and receiver to add header
information that indicates where a message starts and ends. The receiving
session layer can then refrain from indicating any message data to the
overlying application until the entire message has been received. 
– Informing the receiving application when buffer space is
insufficient for the entire message and that the message is incomplete. The
receiving session layer may also use a control frame to inform the sending
session layer how many bytes of the message have been successfully received.
The sending session layer can then resume sending data at the byte following
the last byte acknowledged as received. When the application provides another
buffer, the session layer can place the remainder of the message in that buffer
and indicate to the application that the entire message has been received. 

Presentation Layer
The presentation layer ensures that information sent by the
application layer of one system will be readable by the application layer of
another system. If necessary, the presentation layer translates between
multiple data representation formats by using a common data representation
format. The presentation layer concerns itself not only with the format and
representation of actual user data, but also with data structures used by
programs. In addition to actual data format transformation, the presentation
layer negotiates data transfer syntax for the application layer.

Application Layer
The application layer is the OSI layer closest to the user.
It differs from the other layers because it does not provide services to any
other OSI layer, but rather to application processes lying outside the scope of
the OSI model. Examples include spreadsheet programs, word-processing programs,
banking terminal programs, etc. The application layer identifies and
establishes the availability of intended communication partners, synchronizes
cooperating applications, and establishes agreement on procedures for error
recovery and control of data integrity. Also, the application layer determines
whether sufficient resources for the intended communication exist.

The TCP/IP model
Transmission Control Protocol/Internet Protocol was developed in the 60’s as a
method that connect large mainframes computers together for the simple purpose
of sharing data or information. In the present, most computer operating systems
manufactures incorporate the TCP/IP suit into their software programs allowing
for each individual workstation to encompass the ability to transmit, receive,
and share information through the largest mainframe available, the Internet. The TCP/IP model is made up of 4 layers, a few
layers less than the OSI model. The TCP/IP model consist of from highest to
lowest: The Application layer, The Transport layer, The Internet layer and the
Link layer or Subnet layer.

The Application layer
Just like the OSI model, the Application layer in the TCP/IP
model performs the same sort of function. Only that the Application layer for
the TCP/IP corresponds to the Application layer, Presentation layer and Session
layer of the 7 layer OSI model.

The Transport layer
Transport layers exist in both TCP/IP and OSI model. Even
though both models have Transport layers they differ. The TCP/IP model consist
of two standard transport protocols: Transmission Control Protocol (TCP) and
User Datagram Protocol (UDP). TCP uses a reliable data-stream protocol which is
connected oriented and UDP uses an unreliable data-stream protocol which is
connectionless oriented.

The Internet layer
The Internet layer is a group of protocols and specifications
that are used to transport packets from the host across a network, the host
specified by a network address (IP address).

The Link layer
The lower level layer of the TCP/IP model, this layer is used
by a suite of protocols for the “Internet”. This is used to connect hosts or
nodes to a network. This layer is compared to the “Data Link” layer and the  “Physical ” layer of the OSI model.

TCP/IP Application Layer VS OSI Application, Presentation and
Session layer.
The similarities in both models are comparable but different
at the same time. All though they exist in both, the approach each uses to
construct applications is different. In the OSI model the Application layer,
Presentation layer and Session layer correspond to the Application layer of the
TCP/IP model. They somewhat do the same job but use different protocols, TCP/IP
uses: FTP, SMTP, TELNET, DNS and SNMP where the OSI model uses: FTAM, VT, MHS,

TCP/IP Transport layer VS OSI Transport layer.
UDP and TCP defined by TCP/IP Transport Layer correspond to
many of the requirements of the OSI Transport Layer. Some issues occur over the
requirements in the session layer of OSI since sequence numbers and port values
can help the Operating System to keep track of active sessions. Most of the TCP
and UDP functions and specifications map to the OSI Transport Layer. The TCP/IP
and OSI architecture models both employ all connection and connectionless
models at transport layer. The architecture calls the 2 models in TCP/IP simply
Connections and Datagrams. The OSI model uses the terms “Connection-mode” and
“Connection-oriented” for the connection model and the term
“Connectionless-mode” for the connectionless model.

TCP/IP Internet layer VS OSI Network layer
The Network layer of the OSI model is compared to the Internet
layer of the TCP/IP model. Both models support “Connectionless” network
services, but only the Network layer in the OSI supports connected services.
The OSI layer is a “catch-all” for all protocols that assist in network
functionality, where the “Internet” layer of the TCP/IP model assist in
internetworking using Internet Protocol.

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