This layer defines the electrical, mechanical, procedural, and

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.

In the early years of computer and network research and development, many
systems were designed by a variety of competing companies. Although it became
apparent as network usage grew, that it was difficult to enable these systems
to communicate with each other.
In the early 1980s, the International Organization for
Standardization (ISO) recognized the need for a network model that would help
companies create common network implementations. The OSI reference model,
released in 1984, addresses this need. The OSI reference model became the
primary architectural model for communications. Although other architectural
models have been created, most network vendors relate their network products to
the OSI reference model.
The OSI model describes the processes necessary for effective
communication in terms of a seven-layer model.

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The seven layers are:

Physical Layer
The physical layer defines the electrical, mechanical,
procedural, and functional specifications for activating, maintaining, and
deactivating the physical link between end systems. Such characteristics as
voltage levels, timing of voltage changes, physical data rates, maximum
transmission distances, and physical connectors, are defined by physical layer
specifications.
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
The network layer controls the operation of the subnet. It
determines which physical path the data takes, based on the network conditions,
the priority of service, and other factors.
The network layer provides the following functions. 
– Transferring the frame to a router if the network address
of the destination does not indicate the network to which the station is
attached 
– 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
The transport layer makes sure that messages are delivered in
the order in which they were sent and that there is no loss or duplication. 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
acknowledgments 
– 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.
– Establishing, monitoring, and terminating a virtual-circuit
session between two processes identified by their unique process addresses. A
virtual-circuit session is a direct link that seems to exist 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,
DS, CMIP.

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. There are some issues over
for 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 internet architecture refers to
the two models in TCP/IP as 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 Internet layer of the TCP/IP model is compared to the
Network layer of the OSI model. Both models support “Connectionless” network
services, but only the Network layer supports connected-oriented 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.