OSI Model Layers Summary

Names, Functions, Protocols

There are many great web resources with different ways of describing the OSI model layers. Here are a couple of good sites that might provide further detail to help you differentiate the layers:

Table 1 - OSI Model Layers and Descriptions

OSI Model Layer Description
Layer 7- Application Layer Provides network services to user applications, and establishes program-to-program communication. Identifies and establishes the availability of the intended communication partner, and determines if sufficient resources exist for the communication.
Layer 6- Presentation Layer Manages data conversion, compression, decompression, encryption, and decryption. Provides a common representation of application data while the data is in transit between systems. Standards include Quicktime, MPEG, MIDI, PICT, TIFF, JPEG, ASCII, and EBCDIC.
Layer 5- Session Layer Responsible for establishing and maintaining communication sessions between applications. In practice, this layer is often combined with the Transport Layer. Organizes the communication through simplex, half- and full-duplex modes. Protocols include NFS, SQL, RPC, AppleTalk Session Protocol (ASP) and X-Windows.
Layer 4- Transport Layer Responsible for end-to-end integrity of data transmission. Hides details of network dependent info from the higher layers by providing transparent data transfer. The “window” works at this level to control how much information is transferred before an acknowledgement is required. This layer segments and reassembles data for upper level applications into a data stream. Port numbers are used to keep track of different conversations crossing the network at the same time. Uses both connection-oriented and connectionless protocols. This layer can provide error correction, if necessary. Protocols at this layer can establish, maintain, and terminate a virtual circuit (connection) between hosts. This is also known as establishing an End-to-End connection between hosts
Supports TCP, UDP and SPX.
Layer 3- Network Layer Routes data from one node to another. Sends data from the source network to the destination network. This level uses a two-part address to establish and manage addressing, to track device locations, and to determine the best path to use for moving data on the internetwork. Responsible for maintaining routing tables. Routers operate at this level. Protocols include IP, IPX, AppleTalk
Layer 2- Data Link Layer Layer 2 Overall- Responsible for physical transmission of data between nodes that are locally attached on a LAN, or between devices that are connected via WAN Links. This layer translates messages from the upper layers into data frames and adds customized headers containing the hardware destination and source address. This layer handles flow control, error detection and notification, and network topology. Bridges and switches operate at this layer. Layer 2 Protocols include HDLC, SDLC, PPP, SLIP, Frame Relay, NDIS, ODI, MAC.
Logical Link Control Sublayer – Acts as a managing buffer between the upper layers and the lower layers. Uses Source Service Access Points (SSAPs) and Destination Service Access Points (DSAPs) to help the lower layers talk to the Network layer. Responsible for timing and flow control.  LLC is the typical protocol listed for this sublayer.
Media Access Control Sublayer – Builds frames from the 1’s and 0’s that the Physical layer picks up from the wire as a digital signal, and runs Cyclic Redundancy Checksum (CRC) to assure that nothing was damaged in transit. Typical protocols for this sublayer include: CSMA/CD, 802.5 Token Ring,
Layer 1- Physical Layer Manages putting data onto the network media and taking the data off. Sends and receives bits. Communicates directly with communication media. Provides electrical and mechanical transmission capability.