The role of each layer in the OSI Reference Model

In the OSI Reference Model, from bottom to top, each layer accomplishes different, targeted functions.
1. Physical layer (physical layers)
The physical layer specifies the mechanical, electrical, functional, and process characteristics that activate, maintain, and close communication endpoints. This layer provides a physical medium for the upper level protocol to transmit data.
in this layer, the unit of data is called the bit (bit).
Typical canonical representatives belonging to the physical layer definition include: Eia/tia RS-232, Eia/tia RS-449, v.35, RJ-45, etc.
2. Data Link Layer
The Data link layer provides reliable transmission on unreliable physical media. The functions of this layer include: Physical address addressing, data framing, flow control, data error checking, re-sending, etc.
in this layer, the units of the data are called frames.
the Data Link layer protocol includes: SDLC, HDLC, PPP, STP, Frame Relay, and so on.
3. Network Layer
the network layer is responsible for routing data packets between subnets. In addition, the network layer can also realize congestion control, Internet interconnection and other functions.
at this level, the unit of data is called a packet (packet).
Network layer protocol representatives include: IP, IPX, RIP, OSPF, and so on.
4. Transmission Layer (Transport layers)
The Transport layer is the first end-to-end, that is, the host-to-host hierarchy. The transport layer is responsible for segmenting the upper data and providing an end-to-end, reliable, or unreliable transmission. In addition, the transport layer also handles end-to-end error control and flow control issues.
in this layer, the unit of data is called the Data segment (segment).
The Transport Layer protocol includes: TCP, UDP, SPX, and so on.
5. Session Layer
The session layer manages the session process between hosts, which is responsible for establishing, managing, and terminating sessions between processes. The session layer also uses the insertion of checkpoints in the data to synchronize data.
the representative of the Session Layer protocol includes: NetBIOS, ZIP (AppleTalk Zone Information Protocol), etc.
6. Presentation layers (Presentation layer)
the presentation layer transforms the upper data or information to ensure that one host application layer information can be understood by another host's application. The data transformation of the presentation layer includes data encryption, compression, format conversion, and so on.
representatives of the presentation layer protocol include: ASCII, ASN.1, JPEG, MPEG, and so on.
7. Application layer (application layers)
The application layer provides an interface for operating system or network applications to access network services.
the Application layer protocol includes: Telnet, FTP, HTTP, SNMP, and so on.

The role of each layer in the OSI Reference Model