The document describes the four layers of the OSI model from top to bottom: the Application Layer which decides protocols and interfaces with applications, the Transport Layer which handles error correction and packetization, the Network Layer which adds IP addresses and error correction, and the Link Layer which adds hardware addresses, frames packets and sends them over physical networks.

1. Application Layer
Transport Layer
Network Layer
Link Layer

2. Application Layer
Decides which protocol to use
(eg: http:// ftp://)
Interfaces with the end user applications

3. Transport Layer
Handles error correction
Breaks into packets
Numbers and orders packets
Reassembles packets

4. Network Layer
Adds IP addresses to packets
Handles error correction (2)

5. Adds hardware (MAC) addresses
Breaks packets into frames
Timing and sending onto
Physical network
Link Layer