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The historical and technical standard of the Internet is
the TCP/IP model. The U.S. Department of Defense (DoD)
created the TCP/IP reference model, because it wanted to
design a network that could survive any conditions,
including a nuclear war. In a world connected by
different types of communication media such as copper
wires, microwaves, optical fibers and satellite links,
the DoD wanted transmission of packets every time and
under any conditions. This very difficult design problem
brought about the creation of the TCP/IP model.
The TCP/IP model has the following four layers:
• Application layer
• Transport layer
• Internet layer
• Network access layer
Although some of the layers in the TCP/IP model have the
same name as layers in the OSI model, the layers of the
two models do not correspond exactly. Most notably, the
application layer has different functions in each model.
The designers of TCP/IP felt that the application layer
should include the OSI session and presentation layer
details. They created an application layer that handles
issues of representation, encoding, and dialog control.
The transport layer deals with the quality of service
issues of reliability, flow control, and error
correction. One of its protocols, the transmission
control protocol (TCP), provides excellent and flexible
ways to create reliable, well-flowing, low-error network
communications.
TCP is a connection-oriented protocol. It maintains a
dialogue between source and destination while packaging
application layer information into units called
segments. Connection-oriented does not mean that a
circuit exists between the communicating computers. It
does mean that Layer 4 segments travel back and forth
between two hosts to acknowledge the connection exists
logically for some period.
The purpose of the Internet layer is to divide TCP
segments into packets and send them from any network.
The packets arrive at the destination network
independent of the path they took to get there. The
specific protocol that governs this layer is called the
Internet Protocol (IP). Best path determination and
packet switching occur at this layer.
The relationship between IP and TCP is an important one.
IP can be thought to point the way for the packets,
while TCP provides a reliable transport.
The Network Access layer is concerned with all of the
components, both physical and logical, that are required
to make a physical link. It includes the networking
technology details, including all the details in the OSI
physical and data link layers. |
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A comparison of the OSI model and the TCP/IP models will
point out some similarities and differences.
Similarities include:
• Both have layers.
• Both have application layers, though they include very
different services.
• Both have comparable transport and network layers.
• Both models need to be known by networking
professionals.
• Both assume packets are switched. This means that
individual packets may take different paths to reach the
same destination. This is contrasted with
circuit-switched networks where all the packets take the
same path.
Differences include:
• TCP/IP combines the presentation and session layer
issues into its application layer.
• TCP/IP combines the OSI data link and physical layers
into the network access layer.
• TCP/IP appears simpler because it has fewer layers.
• TCP/IP protocols are the standards around which the
Internet developed, so the TCP/IP model gains
credibility just because of its protocols. In contrast,
networks are not usually built on the OSI protocol, even
though the OSI model is used as a guide.
Networking professionals differ in their opinions on
which model to use. Due to the nature of the industry it
is necessary to become familiar with both. |
