Open System Interconnection Model
The International Standards Organization, in standard ISO/IEC 7498, defines a 7 layer model for describing interconnected systems. It is referred to as the Open Systems Interconnection (OSI) model.
The layers are:
1. The physical layer describes the medium used to connect the systems. Examples are coax cable and fiber optic cable.
2. The data link layer describes the representation of bits on the physical medium and the format of messages on the medium. The data link layer may do some checking of messages and error recovery, but does not guarantee delivery of a message. Many descriptions of link layer systems break the link layer into a number of sublayers.
3. The network layer describes how to transmit messages between connected network segments. The network layer does not guarantee delivery of a message.
4. The transport layer multiplexes message streams used by various applications. It may guarantee in order delivery of the data.
5. The session layer describes how a session is terminated and whether both ends can transmit at the same time or they must transmit one at a time.
6. The presentation layer describes the format of the data.
7. The application layer describes messages for an application.
Each level in the model may define a packet format. In most cases the packet format will consist of a header, a data area, and a trailer. The data area will usually contain a packet for the next higher level.
Ethernet is an example of a protocol at levels 1 and 2. IP is a level 3 protocol. TCP is usually considered a level 4 protocol, but it also includes some features that OSI includes in level 5. In the TCP/IP protocol family, the part of level 5 not in TCP, level 6, and level 7 are combined into an application layer. SMTP and telnet are examples.
The ISO has defined the OSI family of protocols that follows the model, but the OSI family of protocols is not widely used.

Open System Interconnection (OSI) Model
– developed by ISO for interoperability between equipment designed for networks
Layer 7
Application Layer
Performs information processing such as file transfer, e-mail, and Telnet
Layer 6
Presentation Layer
Defines the format of data to be sent:
ASCII, data encryption, data compression, and EBCDIC
Layer 5
Session Layer
Sets up a session between two applications by determining the type of
communication such as duplex, half-duplex, synchronization, etc.
Layer 4
Transport Layer
Ensures data gets to the destination. Manages error control, flow control,
and quality of the service
Layer 3
Network Layer
Sets up connection, disconnects connection, provides routing and multiplexing
Layer 2
Data Link Layer
Manages framing, error detection, and retransmission of message
Layer 1
Physical Layer
Electrical Interface (type of signal), Mechanical interface (type of connector).
Converts electrical signals to bits, transmits and receives electrical signals