The first layer of the OSI Reference Model is the physical layer. This layer describes the physical characters of the network. This layer defines the tangible aspects of the network medium, such as types of cables and pin arrangements, but it also defines how much voltage is used in copper cables or what wavelength of light is used in fiber optic cables. Topology is another aspect of the physical layer. Common topologies include star, ring, bus, and mesh.
The data-link layer is the second layer of the OSI Reference Model. It is responsible for transmitting data to the physical layer. Unlike other layers of the OSI Reference Model, the data-link layer is made up of two sub-layers. The Logical Link Control (LLC) sublayer controls access to the physical layer and allows multiple protocols to access the network simultaneously. The Media Access Control (MAC) sublayer controls access to the network and is where the unique MAC address is defined. The data-link layer deals with the logical topology of the network, rather than the physical topology of the physical layer.
The network layer provides a mechanism for data to move between hosts. This layer uses network addresses and routing to control paths between destinations. The network layer does not define a specific method for addressing hosts on the network and many protocols have been created to do that. Although the Internet Protocol (IP) is the most popular protocol in use today, other protocols such as Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) and AppleTalk have been in use.
Layer four of the OSI Reference Model is the transport layer. The transport layer is responsible for inter-host communication. It does not allow applications to communicate with each other, but does ensure that the data flow is formatted in such a way that both hosts can send and receive data correctly. This includes retransmitting data packets that are lost and creating packets of appropriate sizes. Guarantee of packet delivery is a hallmark of connection-oriented protocols. Connectionless protocols exist in the transport layer that do not guarantee delivery. Instead, packets are sent to the destination host and no receipt of delivery is required.
The session layer is responsible for creating sessions between two applications. Whereas the transport layer is responsible for inter-host communication, the session layer provides similar functionality for applications. The three primary functions of the session include the creation of sessions, transferring of data, and destruction of existing sessions. Protocols in this layer may operate in simplex, half-duplex, and full-duplex mode.
The sixth layer of the OSI Reference Model is the presentation layer. The presentation layer formats data so that it can be transmitted across the network. Not all applications store data in a method that is easily transported. Images require specific file formats. Similarly, sound and video files require specific format to ensure the data is properly received. The presentation layer is also responsible for encryption and decryption.
The final layer of the OSI Reference Model is the application layer. The application layer provides network services directly to the applications running on a host. Therefore, applications do not need to know how to access the network; the application layer provides standard interfaces that applications can use to make network access. Additionally, the application layer prepares data to be sent to the lower levels of the OSI Reference Model and prepares incoming data to be delivered to the application.