| OSI Layer | Purpose | Examples |
| Application | Provides services to network applications. This layer is responsible for determining resource availability, identifying communication peers, and synchronizing communication between the applications. | Simple Mail Transport Protocol (SMTP) Telnet File Transfer Protocol (FTP) Trivial File Transfer Protocol (TFTP) HyperText Transfer Protocol (HTTP) |
| Presentation | Provides the coding and conversion functions that are applied to the data to/from the application layer. This layer ensures that there is a common scheme used to bundle the data between the two ends. | ASCII (text) EBCDIC (text) JPEG (image) GIF (image) TIFF (image) MPEG (sound/video) Quicktime (sound/video) |
| Session | Establishing, maintaining, and terminating communications sessions between upper layer applications. | Session Control Protocol (SPC) Remote Procedure Call (RPC) from Unix Zone Information Protocol (ZIP) from AppleTalk |
| Transport | Responsible for end-to-end data transmission. Can be either reliable (connection-oriented) or best effort (connectionless). This layer organizes data from various upper layer applications into data streams, handles end-to-end flow control, multiplexing, virtual circuit management, and error checking and recovery. | Transmission Control Protocol (TCP) from IP User Datagram Protocol (UDP) from IP |
| Network | This layer allows both connection-oriented and connectionless data flows to access the network. The network layer addresses help define a network hierarchy. Network devices are normally grouped together based on their common network layer address. | Internet Protocol (IP) |
| Data Link | Provides either reliable or best effort transmission of data across a physical medium. Most networks use a best effort data link layer, such as Ethernet. The data Link Layer for LANs provides a physical address to each device called a Media Access Control (MAC) address. MAC addresses are typically burned into the network interface card (NIC). The LAN data link layer also uses a Logical Link Control (LLC) to indicate the type of network layer data that is encapsulated inside the frame. | LAN: Ethernet/IEEE 802.3 (include Fast Ethernet) 802.3z (Gigabit Ethernet) Token Ring/IEEE 802.5 FDDI (from ANSI) WAN: High-Level Data-link Control (HDLC) Point-to-Point Protocol (PPP) Frame Relay |
| Physical | Defines the electrical, mechanical, and functional specifications for maintaining a physical link between network devices. This layer is responsible for such characteristics as voltage levels, timing and clock rates, maximum transmission distances, and the physical connectors used. | LAN: Category 3 cabling (LAN) Category 5 cabling (LAN) WAN: EIA/TIA-232 EIA/TIA-449 V.35 |
OSI Model versus TCP/IP Protocol Suite
| OSI Model Layer Number | OSI Model Layer | TCP/IP Protocol Suite Layer | Protocol Data Unit | Network Device |
| 7 | Application | Application | Data | |
| 6 | Presentation | |||
| 5 | Session | |||
| 4 | Transport | Transport | Segment | |
| 3 | Network | Internet | Packet (or Data-gram) | MultiLayer Switch or Router |
| 2 | Data Link | Data Link | Frame | Switch or Bridge |
| 1 | Physical | Physical | Bits | Hub |
TCP/IP Transport Layer Protocols
Transmission Control Protocol (TCP) is a reliable, connection-oriented, protocol that uses sequence and acknowledgement numbers to provide reliability. TCP verifies that the remote end is listening prior to sending data, using a 3-way handshake: SYN, SYN/ACK, ACK.
User Datagram Protocol (UDP) is a best-effort, connectionless, protocol that does not have sequence or acknowledgement numbers, and does not do far-end verification.
Brain Knowledge 