What is the OSI Model?
The OSI (Open Systems Interconnection) Reference Model is a conceptual framework that describes the functions of a networking or telecommunication system.
The model uses 7 defined layers to help give a visual description of what is going on with a particular networking system. This can help Network IT Professionals narrow down problems (Is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Hardware vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.
OSI Model : Layer Definitions
| 7 | Application Layer | Provides an interface from the application to the network by supplying a protocol with actions meaningful to the application, for example, “get web page object.” |
| 6 | Presentation Layer | This layer negotiates data formats, such as ASCII text, or image types like JPEG and PNG. |
| 5 | Session Layer | This layer provides methods to group multiple bidirectional messages into a workflow for easier management and easier backout of work that happened if the entire workflow fails. |
| 4 | Transport Layer | In function, much like TCP/IP’s tranport layer. This layer focuses on data delivery between the two endpoint hosts (for example, error recovery). |
| 3 | Network Layer | Like the TCP/IP network (Internet) layer, this layer defines logical addressing, routing (forwarding), and the routing protocols used to learn routes. |
| 2 | Data Link Layer | Like the TCP/IP data link layer, this layer defines the protocols for deliverying data over a particular single type of physical network (for example, the Ethernet data link protocols). |
| 1 | Physical Layer | This layer defines the physical characteristics of the transmission medium, including connectors, pins, use of pins, electrical currents, encoding, ligh modulation, and so on. |
OSI Model : Device and Protocol Examples
| Layer Name | Protocols / Specifications | Devices |
| Application, Presentation, Session (Layer 5-7) |
Telnet, HTTP, FTP, SMTP, POP3, VoIP, SNMP | Hosts, Firewalls |
| Transport (Layer 4) | TCP, UDP | Hosts, Firewalls |
| Network (Layer 3) | IP | Router |
| Data Link (Layer 2) | Ethernet (IEEE 802.3, HDLC) | LAN Switch, Wireless AP, Cable Modem, etc. |
| Physical Link (Layer 1) | RJ-45, Ethernet (IEEE 802.3) | CAT5e, CAT6, Fiber Optic, etc. |