The TCP/IP model has four layers:
- Application layer (http, data)
- Transport layer (TCP, UDP, segments)
- Internet layer (IP, ICMP, ARP, RARP, packets)
- Network acces layer (frames, bits)
- Application (ftp, http, dns, dhcp, snmp, telnet, smtp, data)
- Presentation (mime, ssl, shells, ascii, .txt, also known as the translator, data)
- Session (SQL, API, RPC, NetBIOS, data)
- Transport (TCP, UDP, segments)
- Network (IP, NAT, packets)
- Data link (MAC, error correction, FDDI, HDLC, Frame Relay, frames)
- Physical (bits)
The OSI upper layers deal with the data's format, organization and communication. The lower layers implement protocols to transport and route data across a network.
Don't forget that a layered model has several benefits: helps design of protocols, interoperability of vendors, changing one technology without affecting other layers, common terminology to teach and learn.
The data link layer has two sublayers to provide physical media indepedence: the upper logical link control (LLC) layer and the lower media access control (MAC) layer. The Ethernet protocol operates at the data link and physical layer of the OSI model!
LLC: what to do with a packet after it is receive.
MAC: how data is placed and transported over the physical wire.
The layers communicate with each other using service access points (SAP) and protocol data units (PDU).
Encapsulation wraps data with the necessary protocol information before network transmission.