The Data Link Layer, handles the functional and the procedural ways to transfer data between network devices and to detect and to eventually correct errors that occur at the Physical layer. The Data Link Layer is concerned with the local delivery of frames between devices on the same LAN. The Data Link Layer has two sublayers: the Logical Link Control (LLC) layer and the Media Access Control (MAC) Layer.
The LLC sublayer handles the multiplexing of the protocols, provides flow control, acknowledgement and error notification.
The MAC sublayer of the Data Link Layer is sometimes referred as the layer which determines who is allowed to access the media at any one time and other times it refers to a frame structure with a MAC address inside. Example of services at this sublayer are: Physical Addressing (MAC addressing), LAN switching, Spanning tree protocol, Quality of Service (QoS), Virtual LANs (VLANs).
The Data Link Layer also handles the encapsulation of the Network Layer packets into frames.
The Network Layer is the third one in the OSI Reference Model. The Network Layer is responsible for end-to-end packet delivery and routing while maintaining QoS and error control functions. The Network Layer also provides the connection model: connection-oriented or connectionless. For example, the telephone system is connection-oriented, because the called user has to pick up the phone before a communication can be established. The logical IP addressing is also handled by the Network Layer for both IPv4 and IPv6. At this layer, data is represented as packets.
The forth layer of the OSI Model is the Transport Layer. The Transport Layer provides transparent data transfer, controls the reliability of a given link through flow control, segmentation/desegmentation, same order delivery, port numbering and error control. Typical example of Layer 4 protocols are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Other examples include AppleTalk Transaction Protocol (ATP), Fiber Channel Protocol (FCP), NetBIOS Frames Protocol (NBF), and Stream Control Transmission Protocol (SCTP). At this layer, data is represented as segments.
Upper in the hierarchy is the Session Layer. This layer is responsible for opening, closing and managing a session between end-user application processes. It also provides full-duplex, half-duplex and simplex operation, session checkpointing an recovery. Example of session protocols are: L2TP (Layer 2 Tunneling Protocol), NetBIOS, PAP (Password Authentication Protocol), PPTP (Point-to-Point Tunneling Protocol), RPC (Remote Procedure Call protocol).
The Presentation Layer is handling the delivery and formatting of information to the application layer for further processing or display and vice-versa. At this layer encryption and data compression are usually made, also. Examples of Presentation Layer protocols are: ASCII (American Standard Code for Information Interchange), EBCDIC (Extended Binary Coded Decimal Interchange Code), LPP (Lightweight Presentation Protocol), RDP (Remote Desktop Protocol), and NCP (NetWare Core Protocol).
The 7th Layer of the OSI Reference Model, the Application Layer, is the closest layer to the user, which means, both the OSI application layer and the user interact directly with the software application. An easy to understand example is Telnet. Telnet is a remote terminal program. When a user tries to enter a command, the application layer will take that command, send it to the lower layers in a format which is understandable, and when the data is received by the other end, the command is executed. Other examples of Application Layer protocols include: HTTP (Hyper-Text Transfer Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), SNMP (Simple Network Management Protocol).
In this lesson, we have tried to explain to you as easily as possible the OSI Reference Model. We hope we did a great job and you found this information helpful in your study for the CCNA exam.