2.6. OSI Model Layers

2.6 The 7 Layers of the OSI Model

Layer 7: Application

The top, or seventh, layer of the OSI model is the Application layer. The Application layer provides interfaces to the software that enable it to use network services. Some of the services provided by the Application layer include file transfer, electronic mail and web services.

Layer 7 PDU: Data

Layer 6: Presentation

The Presentation layer serves as a translator between the Application and the Session layer. At the Presentation layer data are formatted in a representation that the network can understand, eg: ASCII. The Presentation layer also takes care of data encryption and decryption, such as password encryption.

Common protocols found at the presentation layer include ASCII, EBCDIC, JPG and MIDI.

Layer 6 PDU: Data

Layer 5: Session

The Session layer allows two applications on different computers to open, use, and close a connection, ie: a session. A session is a highly structured dialogue between two workstations. The session layer is responsible for managing this dialogue. It performs name recognition and other functions, such as security, that are needed to allow two applications to communicate over the network.

The Session layer synchronises user tasks by placing checkpoints in the data stream. The checkpoints break the data into smaller groups for error detection. This layer also implements dialogue control between communicating processes, such as regulating which side transmits, when, and for how long. Common protocols used by the session layer include RPC, SQL and NetBIOS names.

Layer 5 PDU: Data

Layer 4: Transport

The Transport layer is primarily responsible for ensuring that data is transferred from one point to another reliably and without errors. For example, the Transport layer makes sure data are sent and received in the correct sequence.

The Transport layer provides flow control and error handling, and participates in solving problems concerned with the transmission and reception of packets. Common examples of Transport layer protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP) and Sequenced Packet Exchange (SPX).

Layer 4 PDU: Segment

Layer 3: Network

The Network layer handles addressing and routing of data based on logical addressing. Routers belong to the Network layer because they use logical addresses to intelligently direct data from sender to receiver. A router determines which path the data should take based on network conditions, priority of service, and other factors. It also manages traffic problems on the network, such as switching and routing of packets and controlling the congestion of data.

Examples of Network layer protocols are Internet Protocol (IP) and Internetwork Packet Exchange (IPX).

Layer 3 PDU: Packet

Layer 2: Data Link

The data link layer sends data frames from the network layer to the physical layer. It is responsible for the control of the signals that enter and leave the network cable. On the receiving end, the data link layer packages raw bits from the physical layer into data frames. The electrical representation of the data (bit patterns, encoding methods, and tokens) is known to this layer only.

Layer 2 PDU: Frame

Layer 1: Physical

The bottom layer of the OSI model is the physical layer. This layer transmits the unstructured raw data, ie: bits, over the physical medium. The physical layer is totally hardware based and deals with all aspects of establishing and maintaining a physical link between communicating computers.

The physical layer defines how the cable is attached to the NIC. For example, it defines how many pins a connector has and the function of each. It also defines which transmission technique will be used to send data over the network cable.

This layer provides data encoding and bit synchronisation. The physical layer is responsible for transmitting bits (zeros and ones) from one computer to another and because different types of media physically transmit bits differently, the physical layer also defines the duration of each impulse and how each bit is translated into the appropriate electrical or optical impulse for the network cable.

Layer 1 PDU: Bit

Next: 2.7 OSI – Human Context