OSI Reference Model

Any networking topic will not be complete without discussing the OSI model. The OSI is an acronym for Open Systems Interconnection – a worldwide standard created by the ISO – Industry Standard Organization. The aim of the OSI is to layout a guide for all manufacturers, vendors, designers, software people etc. making the networking industry compatible to each other. The OSI layered approach explains how information travels and encapsulated from one node to another. Each layer explains its relation to the layer above and the layer below it.

This section is not actually a practical DIY but my aim here is for you to understand travel of information in networking. Having this understanding will help you in troubleshooting network problem in the future.

There are seven layers of the OSI model namely:

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

The application layer is the 7th layer while the Physical is the 1st layer. One way to remember all these layers from top to bottom are through the use of mnemonic:

All People Seem TNeed Data Processing

Or it can be from bottom to top:

Please DNot Take Sale’s People’s Advice

Each layer of the OSI has its own function needed in communication and as I said before, each layer provides relationship to layer above and below it.



Application Layer

This layer provides services enabling network applications to communicate to each other. These services may include synchronization, determining resource availability and requests to Presentation Layer. Data encapsulation in this layer is called Data.

Some examples are:

FTP – File Transfer Protocol

SMTP – Simple Mail Transport Protocol

HTTP – Hypertext Transfer Protocol


TFTP –  Trivial File Transfer Protocol


Presentation Layer

The presentation layer is the sixth layer of the OSI reference model. Its function deals with the conversion of data to a required format for the application layer. Data encapsulation in this layer is called Data.

Some examples are:

EBCDIC (text)

ASCII (text)

JPEG (image)

MPEG (Audio/Video)

GIF (image)

PNG (image)

TIFF (image)

QuickTime (Audio/Video)



Session Layer

This layer deals with establishment and maintenance of session as well as terminating such sessions. Data encapsulation in this layer is called Data.

Examples are:

AppleTalk ZIP

Decnet  Session Control Protocol

Unix Remote Procedure Call (RPC)


Transport Layer


This fourth layer of the OSI reference model deals with the reliable and unreliable end to end delivery of data. Reliable delivery uses connection oriented while non-reliable uses connectionless transmission. Function includes multiplexing, windowing, flow control and error checking-recovery mechanism. Data encapsulation in this layer is called Segment.

Examples are:

TCP – Transmission Control Protocol (connection-oriented)

UDP – User Datagram Protocol (connectionless)


Network Layer


This layer 3 deals with the delivery of packet across the internetwork including routing while choosing the best path. Data encapsulation is this layer is called Packet.

Examples are:

IP – Internetworking Protocol






Data-Link Layer

This layer consists of two sub-layers. The Logical Link Control (LLC) which handles error and flow control over the physical medium. The MAC or the Media Access Control handles physical address or hardware address burned into every network device. This address is commonly called MAC address – a 48 bit address comprises of unique ID and manufacturers ID. Data Encapsulation in this layer is called Frame.

Examples are:

Fast Ethernet

Gigabit Ethernet

Token Ring





Frame Relay

Physical Layer


This is the first layer of the OSI which defines the transmission of bits into the physical hardware. This includes all electrical characteristics such as voltage, clock rates, timings, maximum cable length etc. Data encapsulation in this layer is called bits.

Examples are:



Fiber Optic

CAT 6 Cabling

CAT 5e Cabling

BNC connector

Well that’s it. I have explained the 7 OSI layers as simple as I can. Also, you may have noticed that for every layer, I mentioned about its data encapsulation. As the information travels for each layer, a new encapsulation is being introduced. This way, we could simply know what kind of information we are dealing with whether a packet, a frame or a data. So you could easily remember the data encapsulation, you use the mnemonic:

From top to bottom:

Do       –           Data

Some   –           Segment

People-            Packet

Fry      –           Frame

Bacon –            Bits