What You Should Know About the OSI Reference Model and Layers

The OSI Reference Model is not a physical model. It is a logical model. It has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four  define how data is transmitted, end-to-end.

Layer 7 Application – Provides a user interface.

Layer 6 Presentation – Presents data, Handles processing such as encryption.

Layer 5 Session – Keeps different application’s data separate.

Layer 4 Transport – Provides reliable or unreliable delivery. Performs error correction before re-transmit.

Layer 3 Network – Provides logical addressing, which routers use for path determination.

Layer 2 Data Link – Combines packets into bytes and bytes into frames. Provides access to media using MAC address. Performs error detection, not correction.

Layer 1 Physical – Moves bits between devices. Specifies voltage, wire, speed and pin-out of cables.

 Network devices that operate at all seven layers of the OSI model include:

• Network Management Stations (NMS)
• Web and Application Servers
• Gateways (not default gateways)
• Network Hosts

 Let's dive a little deeper into the layers:

7 - The Application Layer:

This deals with the communication between users and the computer. This is the seventh layer in OSI.

For example:

Browsers :- Mozilla Firefox, IE, Google Chrome, etc.

Email clients :- Outlook Express, Mozilla Thunderbird, etc.

FTP clients :- Filezilla, sFTP, vs-FTP

Protocols:-

* SNMP (Simple Network Management Protocol):- It controls the network devices.

* TFTP(Trivial File Transfer Protocol):- File Transfer.

* DNS(Domain Naming System):- Translates a website name to it’s IP address. Website's names are easy for people to understand and IP addresses are easy for computers.

* DHCP (Dynamic Host Configuration Protocol):- Assign IP, mask and DNS server to hosts.

* Telnet:- Telecommunication Network. It provides a remote terminal connection to manage devices.

* HTTP (Hypertext Transfer Protocol):- For browser web pages.

* FTP (File Transfer Protocol):- Sends/retrieves all file types.

* SMTP (Simple Mail Transfer Protocol):- Used to handle emails.

* POP 3 (Post Office Protocol) v3:- Retrieves email.

* NTP (Network Time Protocol):- It helps to synchronizes network devices clocks.

 

6- The Presentation Layer:

This layer prepares the data. The data from the application layer is marked with formatting codes such as .doc, .jpg, .txt, etc. It also deals with compress and encapsulation. It compresses or decompresses the data file. It converts data into the form the application accepts.

Example protocols: SSL, TLS

 

5 - The Session Layer:

This layer provides its services to the presentation layer. It establishes, manages and terminates sessions between two communicating ends. The OSI model gives this layer the responsibility for graceful close of sessions. For example, simultaneously, many computers can be communicating with server. Therefore, to handle the collision, it keeps track of which computer communicates on which path.

Example protocols: Net BIOS, PPTP

 

4 - The Transport Layer:

This important layer handles the connection between two devices. Data transmission methods can be connection-oriented or connectionless.

Main functions of the transport layer:-

Segmentation, Connection Management, Reliable and Unreliable Data Delivery, Flow Control and Connection multiplexing.

3 - The Network Layer:

This provides a logical address, known as Internet Protocol (IP) address. Routers work on this layer.

Mainly:-

* It defines the IP address

* It connect source (router) to destination (router).

* It connect different data link type together. eg: Token Ring, Serial, FDDI, Ethernet etc.

IP Classes

Class A addresses range from 1-126.

Class B addresses range from 128-191.

Class C addresses range from 192-223.

Class D addresses range from 224-239.

Class E addresses range from 240-254.

Example protocols: TCP (Transmission Control Protocol), UDP (User Datagram Protocol)

0 [Zero] is reserved and represents all IP addresses;

127 is a reserved address and is used for testing, like a loop back on an interface:

255 is a reserved address and is used for broadcasting purposes.

Class A, B and C address are known as public addresses. IANA (Internet Assigned Numbers Authority) are responsible for managing public address.

Private IP address:- Not routeable on a public network:

Class A: 10.0.0.0 – 10.255.255.255

Class B: 172.16.0.0 – 172.31.255.255

Class C: 192.168.0.0 – 192.168.255.255

Example protocols: IP, ARP,ICMP,IPSec

 

2 - The Data Link Layer:

This layer defines the Media Access Control (MAC), hardware address or physical address. It gives both connectionless and connection-oriented services.

 

1 - The Physical Layer:

This layer comes after the data link layer . It receives frames from the upper data link layer and and converts them into bits or binary (10010).

Example protocols: Ethernet, USB (Universal Serial Bus), Bluetooth, IEEE802.11 (WiFi)

Example protocols: PPP, ATM,Ethernet

Summary:

Application: File, print, message, database and application services

Presentation: Data encryption, decryption, compression, decompression and translation services

Session: Dialog control

Transport: End-to-end connection

Network: Routing

Datalink: Framing

Physical: Physical topology