OSI was a rival to the TCP/IP model originally. But now the protocol is defunct and TCP/IP dominates the market. However the terminolgy used to describe OSI has survived and is still used to describe the networking and the TCP/IP model even though it doesn’t directly translate.
The OSI layers are:
- Data Link
Contrast this to the TCP/IP model layers:
Most people refer to the TCP/IP by the corresponding OSI layers. Often referring to a “Layer 2 Switch” or “Layer 3 Switch”. Meaning a switch that can only switch Ethernet frames (Layer 2) or do packet forwarding (Layer 3 – IP routing).
The below image shows a table of how the TCP/IP model maps over to the OSI model:
“Please Do Not Throw Sausage Pizza Away” is a neat way to remember the format of the OSI model.
Here is a quick breakdown of the original OSI layers and their function (from the CCNA Official Cert Guide):
7.) Application Layer. Provides an interface from the application to the network by suppling 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.
5.) Session Layer. This layer provides methods to group multiple bidirectional messages into a workflow for easier management and easier back out of works that happened if the entire workflow fails.
4.) Transport Layer. In function, much like TCP/IP’s transport 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 delivering 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, light modulation and so on.
Here is an example of how the OSI model applies to the TCP/IP model:
Application, presentation and session (Layers 5-7): Telnet, HTTP, FTP, POP3, VoIP, SNMP
Transport (Layer 4): TCP, UDP
Network (Layer 3): IP
Data Link (Layer 2): Ethernet (IEEE 802.3), HDLC
Physical (Layer 1): RJ45, Ethernet (IEEE 802.3)