Network

To understand network communications, one should use both the OSI and the TCP/IP models.

OSI stands for Open Systems Interconnection. The OSI Model is a logical model created in the 1980s to understand and develop communications between two computers. It divides data communications into layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application - see image below. Each layer comprises particular functions and interacts with adjacent layers.

TCP/IP stands for Transmission Control Protocol/Internet Protocol. The TCP/IP Model is the most popular suite of protocols. By definition, protocols are sets of instructions written by a programmer to perform functions. Computers communicate through these sets of rules.

The OSI Model

OSI Model
 

The OSI Model and Protocols

OSI Layer OSI Layer Function Protocol
Application: Layer 7 – Provides interface between applications and lower level network services by interpreting application requests and requirements. As an example, FTP is an application which transfers files from one computer to another. HTTP
SMTP
POP3
FTP
DNS
Presentation: Layer 6 – Converts data into a common language for hosts and applications; it also performs data formatting, encryption, and compression.
Session: Layer 5 – Establishes, maintains, and terminates communications between two computers.
Transport: Layer 4 – Ensures accurate delivery of data from one computer to another using an appropriate rate of transmission (flow control). It also performs segmentation and reassembly, error correction, and acknowledgement of data. For instance, the TCP protocol is used for communication between the browser and the Web Server in this layer. TCP
UDP
Network: Layer 3 – Translates logical addresses (network addresses) into their physical counterparts - primary function. It also determines routing from one computer to another. As an example, the IP protocol routes the HTTP request, which could be a user request to open a web page. HTTP stands for Hypertext Transfer Protocol, which functions at layers 7, 6 and 5. IP
ICMP
Data Link: Layer 2 – Serves as an interface between the upper layers and the Physical layer by packaging data into frames. Ethernet
FDDI
Physical: Layer 1 – Transfers data to and from the network medium. For instance, if you install a NIC (Network Interface Card – device that enables connection between a computer and the network) on your computer, you’re providing a Physical Layer. Ethernet is a common transmission method.