What is OSI Model
OSI (Open Systems Interconnections) model is a universal set of specifications that enables computer platforms across the world to communicate openly, therefore all the network devices can communicate with each other regardless of Operating systems. Hence, the model, divides network communications into seven layers starts from Physical, Data Link, Network, Transport, Session, Presentation and Application. These layers are discussed in brief below from highest to lowest with Application layer where the information flow is initiated.
Application Layer
The top or seventh layer of OSI is the Application layer. Services in Application layer enable communication between software applications and lower layer network so that network can understand an application’s request and in turn, the application can read data sent from network.
For example, when you choose to open a web page in web browser, an Application layer protocol called HTTP (Hypertext Transfer Protocol) formats and sends you request from your computer’s browser to server. It also formats and sends the web server’s response back to your computer’s browser.
Application layer protocols in OSI Model
| PROTOCOLS | DESCRIPTION |
| BOOTP | Boot Strap Protocol |
| DHCP | Dynamic Host Configuration Protocol |
| DNS | Domain Name System |
| FTP | File Transfer Protocol |
| HTTP | Hypertext Transfer Protocol |
| HTTPS | Hypertext Transfer Protocol Over Secure sockets layer |
| IMAP4 | Internet Message Access Protocol |
| PING | Packet Internet Groper |
| POP3 | Post Office Protocol Version 3 |
| NSLOOKUP | |
| NTP | Network Time Protocol |
| SFTP | Secure File Transfer Protocol |
| SMTP | Simple Mail Transfer Protocol |
| SNMP | Simple Network Management Protocol |
| TELNET | Terminal Emulation Protocol |
| TFTP | Trivial File Transfer Protocol |
Presentation Layer
Protocols at Presentation Layer accept data from Application layer and format it so that one type of type of application and host can understand data from another type of application and host. Presentation layer services also manage data encryption such as scrambling of passwords and decryption. For example, if you look up your bank account via the internet, you are using a secure connection and Presentation layer protocols will encrypt your account data before it is transmitted. Likewise on your network end, Presentation layer will decrypt the data as it is received.
Session Layer
Protocols in Session layer coordinate as well as maintain communication between two computers on a network. For example, when you dial your ISP to connect to internet, the Session layer services at your ISP’s server and on your computer negotiate the connection. If your phone line accidentally falls out of wall jack then Session layer protocols on your end will detect the loss of connection and initiate attempts to reconnect. If they cannot reconnect then they will close the session and inform your dial-up software that communication has ended.
Transport Layer of OSI Model
Protocols in Transport layer accept data from the Session layer and manage end to end data delivery therefore such protocols area called connection oriented.
Functions of Transport layer in OSI Model
Flow control
Flow control is the process of gauging the appropriate rate of transmission. In other words, how fast a recipient can accept data.
Checksum
Checksum is a unique character string that allows the receiving network device to determine that arriving data unit matches exactly as the source has sent it. Thus, this function ensure reliable delivery of data.
Segmentation
Data received from Session layer’s protocols breaks in smaller units in Transport layer hence this process is called Segmentation.
Reassembly
This is the process of reconstructing the segmented data units therefore this process is known as Reassembly.
Sequencing
Sequencing is a method of identifying segments that belong to the same group of subdivided data. Furthermore, for sequencing to work properly, transport layer protocols of two network devices must synchronize their timing and agree on a starting point for the transmission.
Transport layer Protocols in OSI Model
TCP (Transmission Control Protocol)
TCP is a connection-oriented protocol hence to ensure accurate delivery of data, TCP first sends a SYN (Synchronization) packet to web server from your network device when you open web page on your computer. The web server then sends a confirmation packet known as SYN-ACK (Acknowledgement), to indicate that it is willing to make a connection. Then, your computer responds with ACK (Acknowledgement) packet. Thereafter, connection between your computer and web server is established after this three-step process.
UDP (User Datagram Protocol)
UDP is a connection-less transport layer protocol. In other words, UDP offers no guarantee that packets will be received at all. Moreover, UDP provides no error checking or sequencing like TCP.
In addition, UDP is considered useful in situation in which great volume of data needs to be transferred quickly, like video or audio transmission over the internet.
Network Layer of OSI Model
Network layer is the third layer of OSI model. The primary function of Network layer protocols is to translate network addresses into their physical counterparts as well as decide how to route data between sender and receiver. Besides, protocols in Network layer accept the Transport layer segments and add logical addressing information. Additionally, data unit becomes packet in Network layer.
Network Layer Protocols in OSI Model
| PROTOCOL | DESCRIPTION |
| ARP | Address Resolution Protocol |
| ICMP | Internet Control Message Protocol |
| IGMP | Internet Group Multicast Protocol |
| IP | Internet Protocol |
| IPSec | Internet Protocol Security |
| RARP | Reverse Address Resolution Protocol |
Data Link Layer
Data link layer packages data in Frames suitable to network transmission method through bridges, switches and WAPS (Wireless Access Points). A Frame is a structured package for moving data that includes not only the raw data (payload) but also the sender’s and receiver’s network addresses, error checking and control information.
IEEE has divided Data link layer into two sub-layers called LLC (Logical Link Control) and MAC (Media access control).
Protocols used in Data link layer of OSI Model
| PROTOCOL | DESCRIPTION |
| L2TP | Layer 2 Tunneling Protocol |
| PPP | Point-to-Point-Protocol |
| PPTP | Point-to-Point-Tunneling Protocol |
| SLIP | Serial Line Internet Protocol |
Physical Layer
Physical layer is the first or lowest layer of OSI model. Protocols at Physical layer accept frames from the Data link layer and generate voltage through connectivity devices such as hubs and repeaters to transmit signals.
Network standards used in Physical layer are as follow.
| Ethernet | IEEE 802.3 |
| Token Ring | IEEE 802.5 |
| Wireless | IEEE 802.11 |
In conclusion, OSI model was developed to understand computer to computer communication. Therefore, this communication was divided into seven different layers as mentioned above.