Layers in Networking

Open Standard Interconnect (OSI)

• OSI was developed by the International Organization for Standardization (ISO) and introduced in the year 1984.
• It is a layered architecture (consists of seven layers)
• Each layer defines a set of functions which takes part in data communication.

 

Physical Layer: This layer describes stuff like voltage levels, timing, physical data
rates, physical connectors and so on. Everything you can “touch” since it‟s physical.

 Data Link: This layer makes sure data is formatted the correct way, takes care of
error detection and makes sure data is delivered reliably. This might sound a bit
vague now, for now try to remember this is where “Ethernet” lives. MAC Addresses
and Ethernet frames are on the Data Link layer.

Network: This layer takes care of connectivity and path selection (routing). This is
where IPv4 and IPv6 live. Every network device needs a unique address on the
network.

Transport: The transport layer takes care of transport, when you downloaded this
book from the Internet the file was sent in segments and transported to your
computer.
o TCP lives here; it‟s a protocol which send data in a reliable way.
o UDP lives here; it‟s a protocol which sends data in an unreliable way.

Session: The session layer takes care of establishing, managing and termination of
sessions between two hosts. When you are browsing a website on the internet you
are probably not the only user of the webserver hosting that website. This web server
needs to keep track of all the different “sessions”.

Presentation: This one will make sure that information is readable for the
application layer by formatting and structuring the data. Most computers use the
ASCII table for characters. If another computer would use another character like
EBCDIC than the presentation layer needs to “reformat” the data so both computers
agree on the same characters.

Application: Here are your applications. E-mail, browsing the web (HTTP), FTP and
many more.