OSI Model

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.