A networking model categorizes and provides structure for networking protocols and standards.
Open Systems Interconnection (OSI) Model
The OSI model is a networking model that categorizes and standardizes the different functions in a network. It was created by the International Organization for Standardization (ISO).
The OSI model divides network functions into seven layers:
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
Layer 7 - Application
This is the layer closest to the user. It interacts with software applications like web browsers. HTTP and HTTPS are considered layer 7 protocols. Layer 7 is responsible for identifying communication partners and synchronizing communication.
Layer 6 - Presentation
Data in this layer is in an “application format”, i.e., in a format that only applications can understand. It needs to be “translated” in order to be sent over the network. The layer’s job is to translate between application and network formats, as in the encryption of data when it’s sent and its decryption when it’s received.
Layer 5 - Session
This layer controls sessions (i.e., dialogue) between communicating hosts. It establishes, manages, and terminates connections between the local application and the remote application.
Layer 4 - Transport
The transport layer segments and reassembles data for communication between hosts. Large pieces of data are broken into smaller segments that are easier to send over the network and are less error prone. It also provides host-to-host communication. A header is added onto to these segments of data, and at this point, one piece of data is called a segment.
Layer 3 - Network
This layer provides connectivity between end hosts on different networks and logical addressing (i.e., IP addresses). It provides path selection between source and destination. This is the layer that routers operate from. Another header is added at this layer, and at this point, one piece of data is called a packet.
Layer 2 - Data Link
The data link layer provides node-to-node connectivity and data transfer (e.g., switch to router). It defines how data is formatted for transmission over a physical medium and can detect and possibly correct physical layer errors. It uses different addressing than layer 3. This is the layer that switches operate from. Another header as well as a trailer are added to the packet when data passes through this layer, and at this point, the piece of data is called a frame.
Layer 1 - Physical
The physical layer defines the physical characteristics of the medium used to transfer data between devices like voltage levels, max transmission distances, cable specifications, etc. Bits are converted into electrical or radio signals at this layer.
TCP/IP Suite
The TCP/IP suite is another network model which includes a set of communications protocols used in the Internet and other networks. It gets its name from its two foundational protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP). It was developed by the United States Department of Defense through the Defense Advanced Research Projects Agency (DARPA). It is similar to the OSI model, but has fewer layers, and is still in use in modern networks.
The differences are
- The Data Link and Physical layers are combined into a single Link layer
- The Application, Presentation, and Session layers are all combined into a single Application layer