The Seven Layers of The OSI Model

OSI Model
Posted by Systech Admin on July 5th, 2010 | No Comments

The OSI Reference Model is founded on a suggestion developed by the International
Organization for Standardization (ISO). The model is known as ISO OSI (Open Systems
Interconnection) Reference Model because it relates with connecting open systems ? that
is, systems that are open for communication with other systems.
OSI Model is a set of protocols that try to identify and homogenize the data
communication practices. The OSI Model has the support of most computer and network
vendors, many big customers, and most governments, including the United States.
The OSI Model is a model that illustrates how data communications should take place. It
segregates the process into seven groups, called layers. Into these layers are integrated the
protocol standards developed by the ISO and other standards organization, including the
Institute of Electrical and Electronic Engineers (IEEE), American National Standards
Institute (ANSI), and the International Telecommunications Union (ITU), formerly
known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone).
The OSI Model affirms what protocols and standards should be used at each layer. It is
modular, each layer of the OSI Model functions with the one above and below it.
The short form used to memorize the layer names of the OSI Model is ?All People Seem
To Need Data Processing?. The lower two layers are normally put into practice with
hardware and software. The remaining five layers are only implemented with software.
The layered approach to network communications gives the subsequent advantages:
Reduced intricacy, enhanced teaching/learning, modular engineering, accelerated
advancement, interoperable technology, and standard interfaces.
The Seven Layers of the OSI Model

The seven layers of the OSI model are:
Layer Name
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
The easiest way to remember the layers of the OSI model is to use the handy mnemonic
“All People Seem To Need Data Processing”:
Layer Name Mnemonic
7 Application All
6 Presentation People
5 Session Seem
4 Transport To
3 Network Need
2 Data Link Data
1 Physical Processing
The functions of the seven layers of the OSI model are:
Layer Seven of the OSI Model
The Application Layer of the OSI model is responsible for providing end-user services,
such as file transfers, electronic messaging, e-mail, virtual terminal access, and network
management. This is the layer with which the user interacts.
Layer Six of the OSI Model
The Presentation Layer of the OSI model is responsible for defining the syntax which two
network hosts use to communicate. Encryption and compression should be Presentation
Layer functions.
Layer Five of the OSI Model
The Session Layer of the OSI model is responsible for establishing process-to-process
commnunications between networked hosts.
Layer Four of the OSI Model
The Transport Layer of the OSI model is responsible for delivering messages between
networked hosts. The Transport Layer should be responsible for fragmentation and
reassembly.
Layer Three of the OSI Model
The Network Layer of the OSI model is responsible for establishing paths for data
transfer through the network. Routers operate at the Network Layer.
Layer Two of the OSI Model
The Data Link Layer of the OSI model is responsible for communications between
adjacent network nodes. Hubs and switches operate at the Data Link Layer.
Layer One of the OSI Model
The Physical Layer of the OSI model is responsible for bit-level transmission between
network nodes. The Physical Layer defines items such as: connector types, cable types,
voltages, and pin-outs.
The OSI Model vs. The Real World

The most major difficulty with the OSI model is that is does not map well to the real
world!
The OSI was created after many of todays protocols were already in production use.
These existing protocols, such as TCP/IP, were designed and built around the needs of
real users with real problems to solve. The OSI model was created by academicians for
academic purposes.
The OSI model is a very poor standard, but it’s the only well-recognized standard we
have which describes networked applications.
The easiest way to deal with the OSI model is to map the real-world protocols to the
model, as well as they can be mapped.
Layer Name Common Protocols
7 Application SSH, telnet, FTP
6 Presentation HTTP, SMTP, SNMP
5 Session RPC, Named Pipes, NETBIOS
4 Transport TCP, UDP
3 Network IP
2 Data Link Ethernet
1 Physical Cat-5
The difficulty with this approach is that there is no general agreement as to which layer of
the OSI model to map any specific protocol. You could argue forever about what OSI
model layer SSH maps to.
A much more accurate model of real-world networking is the TCP/IP model:
TCP/IP Model
Application Layer
Transport Layer
Internet Layer
Network Interface Layer
The most significant downside with the TCP/IP model is that if you reference it, fewer
people will know what you are talking about!
For a better description of why the OSI model should go the way of the dodo, disco, and
DivX, read Kill the Beast: Why the Seven-Layer Model Must Die.

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OSI Model

Posted by Systech Admin on July 5th, 2010 | No Comments

The Seven Layers of the OSI Model

The OSI Model vs. The Real World

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