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THE OSI MODEL
What is the OSI Model?
The OSI Model is a universal set of
specifications that enable computer platforms
across the world to communicate openly with
each other.
What does OSI stand for?
OSI stands for Open System Interconnection
It is used to divide networking architecture into
seven different layers.
(the word architecture is used to describe a
networks infrastructure because like a building,
networks are comprised of many integrated
elements: cabling, servers, applications, NIC’s
and many other elements) .
Theses layers are from the top to the bottom:
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Application
Presentation
Session
Transport
Network
Data Link
Physical
Each layer has its own set of functions (chores
that it is responsible for) and interacts with the
layers directly above or below.
Who designed the OSI Model?
The OSI model was designed by are you ready!
ISO (International Organization for Standards)
The reason that ISO is used instead of IOS is
that ISO is not supposed to be an acronym for
International Organization for Standards.
In fact ISO is the Greek word for “equal”.
So who is the ISO and what do they do?
The ISO is a collection of Standards
organizations that represent 130 countries.
Its headquarters are in Geneva Switzerland
ISO’s goal is to establish international
technological standards to facilitate global
exchange of information and barrier-free trade.
So starting at the bottom lets look at the OSI
Model
THE PHYSICAL LAYER
The Physical Layer also referred to as Layer 1 is
where everything connects together and
comprises of devices that do not translate the
information flowing on the network, but only
pass it on.
The Physical Layer consists of things like:
The wires in the network, which could be
Ethernet cables or fiber optic cables
Other technology for sending signals such as
infrared or micro wave
The connectors at the ends of the wires (used to
join devices together)
Repeaters (devices that amplify any signal
coming in without checking the signal)
Hubs (devices used to join multiple computers
together)
Devices that control the speed at which
information is sent so that most of the
information makes it to its destination. These
devices know the speed of the networks they
are connected to as in 10 Mbps or 100 Mbps and
slow down or speed up the packets on the
network.
Without these network devices it would be like
pouring water in a funnel. If you pour too fast it
will overflow and the water that overflows does
not make it to the desired location)
Basically the Physical Layer provides the
hardware means of sending and receiving data
on a carrier.
THE DATA LINK LAYER
The Data Link Layer also referred to as Layer 2
is the layer responsible for breaking raw data
into smaller chunks called frames.
Frames from particular nodes are usually the
same size and used for timing purposes just like
frames in a film.
Layer 2/Data Link Layer is responsible for
making sure the data reaches the desired
destination.
Layer 2/Data Link Layer is also responsible for
making sure data flows at the right speed.
A frame is a structured package for moving data
similar to how a train is configured to move
cargo.
Like the train that has the Engine at the front,
cargo in the middle, and the caboose at the end,
a data frame also has different components.
A simplified Data Frame is comprised of:
Destination
Address
Source
Addres
s
Control
Information
Data sent by
Source
Error Checking
Information
As mentioned before The Data Link layer
interacts with both the Layer below it (The
Physical Layer) and the Layer above it (The
Network Layer)
The Data Link Layer uses the Destination and
Source MAC Addresses
A MAC Address (Media Access Control) is the
unique identifier (address) that every NIC
(Network Interface Card) in the world has
There are 248 or 281,474,976,710,656 different
combinations.
The Data Link Layer provides the control
information (information that it can use to make
sure the whole frame is transmitted)
The Data Link Layer breaks the Source Data into
transmittable frames
The Data Link Layer provides the error checking
information (information that network can use to
make sure the whole frame was delivered)
One way that this is done is by size.
The Data Link Layer knows that a full Data
Frame has 500 (fictitious size) bits of
information.
If the receiving device reads 500 bits then it
assumes all the Frame is present, otherwise it
will respond back to the sending device to
please resend. Yes there is a lot more to it than
that but that is the concept.
Another way of explaining this is
First think that computers communicate like
people do. You might be in a large noisy class
and one student wants to ask the teacher a
question. So this student calls out “Mr.
Forndran, could you explain why do some trains
have engines at the front and back” What
happens if Mr. Forndran only hears part of the
question? For example he might hear: “Why do
trains have engines at the front?” It would be
the Data Links Layer to realize that some data
has been dropped and ask the first computer to
retransmit-just as Mr. Forndran might say “I
didn’t hear you. Can you please repeat the
question?”
Another common problem that the Data Link
Layer is responsible for is sorting out
congestion. Again back to the noisy classroom
what if at the end of class 20 students ask
different questions all at the same time. Mr.
Forndran would not be able to pay attention to
all the questions so he would say “One person at
a time please.” then point to one of the students
who asked a question. This is what the Data Link
Layer does for the Physical Layer.
THE NETWORK LAYER
The Network Layer also referred to as Layer 3 is
primarily responsible for getting packets from
the source node (computer) to the destination
node using the quickest know route.
It does this by using the IP address (Layer 3
protocol) together with the physical MAC
address (Layer 2) of both nodes
Packets are formatted blocks of data carried
over a network. Packets vary in length and can
be smaller or larger than the frames they are
carried in
For this reason packets can span more than one
frame
IP stands for Internet Protocol and is a core
protocol of the TCP/IP protocol
A Protocol is the set of rules used to transfer
data
Like every house in the world has a distinct
address so do all computers and network
devices.
How to compare the IP protocol that the
Network layer uses to the MAC address that the
Data Link layer uses is to think in the terms of a
person.
Mr. Smith (IP address) with a Social Insurance
number in Canada of 123 456 789 (MAC address)
In the classroom just calling out Mr. Smith would
get the right person but at Wonderland there
might be many Mr. Smith’s so to make sure you
got the right Mr. Smith you would use Mr. Smith
Canadian Social Insurance Number 123 456 789.
Another task that the Network Layer is
responsible for is either reducing or increasing
the size of Data units (called Segmentation) As
different segments of a network can move data
at different speeds the Network Layer segments
the data to make maximum use of the network..
A way of looking at how different segments
correspond on the Network Layer is to look at
the building of a seven layer Wedding cake. The
cook builds the cake in the kitchen only to
notice it will not fit through the narrow hallway
to the reception hall. The waiters dismantle
(Segment) the cake (Network Layer). They then
carry the different layers out to the reception
hall (Transport Layer) where they rebuild
(reassemble) the cake (Network Layer). The
Data Link Layer would make sure all the pieces
arrived in the reception hall and none of the
waiters had eaten or dropped a piece.
The other important job of the Network Layer is
figure out the right way for data packets to
proceed through the network to get to their
destination known as Routing.
Routers work at the Network Layer and are
devices that store tables known as Routing
Tables. These tables have the directions to
every device connected to that Router.
How that works is the same as you asking for
directions to the closest Ice Cream store. If the
person you ask first knows you get the
directions right away. If the person doesn’t
know they might say, “I don’t know where the
closest Ice Cream store is, but hold on I’ll ask
my father.” This will continue until someone will
be able to point you in the right direction.
TRANSPORT LAYER
The Transport Layer also referred to as Layer 4
is the layer primarily responsible for ensuring
that the data is transferred from point A to point
B
As the Data being transferred might be of a very
large size another protocol is used at this layer
to make sure all of the information is put back
together in the right order when it is received by
the receiving node. One of the most popular
protocols used on the Transport Layer is TCP.
TCP or Transmission Control Protocol
TCP is a protocol whose rules govern how Data
is broken up and reassembled. As Data is
broken into smaller packets for transport over
the network TCP places a sequencer on the data
to denote where in the data stream it should be
placed when received.
This is very important since there are many ways
for data to travel over a network some packets
could be received out of sequence or be lost on
the network.. With this sequencer in place the
receiving node can reassemble the data
correctly.
Also if a particular packet does not make it to
the receiving computer the receiving computer
would be able to request that particular packet
be resent instead of asking for all of the data to
be resent.
TCP also sends a reply saying that the
information was received. This reply is called an
(ACK) or acknowledgement. This ACK is very
important, if the sending PC did not receive an
ACK then its Transport Layer would know that
the receiving PC either did not receive the Data
or could not understand the Data. After a
certain amount of time the sending PC would
resend the Data.
Waiting for an acknowledgement of receipt of
data takes extra time but is critical in making
sure that all the information makes it to its
destination.
There are other Protocols that don’t require
acknowledgement (UDP) but the transmission of
data can not be guaranteed. This guarantee of
transmission is thought to be worth the extra
time needed to transmit.
SESSION LAYER
The Session Layer also referred to as Layer 5 is
primarily responsible for maintaining
communication between two nodes on the
network.
The Session Layers also sets the terms of
communication by deciding which node will
communicate first and how long a node can
communicate similar to a judge at a debate
competition.
PRESENTATION LAYER
The Presentation Layer also referred to as Layer
6 serves as the translator between the
Application Layer and the Network Layer
The Presentation Layer compiles data from the
Network Layer into a format that the Application
Layer can understand.
The Presentation Layer compiles data from the
Application Layer into a format that the Network
Layer can understand for transport over the
network
It is at the Presentation Layer that data is either
encrypted or decrypted
APPLICATION LAYER
The Application Layer also referred to as Layer 7
is where the interaction between certain
programs or the operating system.
This is not where there is interaction between
the computer and humans but where on request
a file is transferred over the network or mail is
read.
An example of how the Application Layer works:
If you are running Microsoft Word on a network
and choose to open a file, your request for that
data is transferred to the network by the
Application Layer.