Understanding
the OSI Model
Professor Messer
Domain 1.1
Understanding the OSI Model
What is the OSI Model?
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Open Systems Interconnection Reference Model, a model used
within IT do describe the process data takes as it traverses
our networks
It’s a guide
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Don’t get wrapped up in the details
This is not the OSI protocol suite
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Most of the OSI protocols didn’t catch on
There are unique protocols at each layer
You’ll often refer to this model for the rest of your career
All People Seem To Need Data Processing
OSI MODEL LAYERS
LAYER 7
LAYER 6
LAYER 5
LAYER 4
LAYER 3
LAYER 2
LAYER 1
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL
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Understanding the OSI Model
Layer 1 – Physical
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This is they physics of the network
o Signaling, cabling, connectors
o This layer is not about protocols
o This layer deals with anything that is physical including
switches, hubs, cables, routers, anything physical can be
found at the physical layer
“You have a physical layer problem”
o Fix your cabling, punch-downs, etc.
o Run loopback tests, test/replace cables, swap adapter
cards
Layer 2 – Data Link
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The basic network “language”
o The foundation of communication at the data link layer,
the layer used for 2 devices to communicate
Data Link Control (DLC) Protocols
o This layer can also be called the Media Access Control
Address (MAC address) because it is commonly associated
with the adapter cards and network cards within our
device. We refer to the physical address as a MAC address
or a DLC.
The Switching Layer
o Since the network switches we use our network determine
how to forward traffic on the destination MAC address, we
refer to this as the Switching Layer.
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Understanding the OSI Model
Layer 3 – Network Layer
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The “routing” layer
o We often refer to this layer as the routing layer because
this is the layer where routers look at the destination IP
address to determine how to forward traffic and know what
the next hop might be for traffic traversing the network
Internet Protocol (IP) Addresses
o This layer also handles IP addresses or Internet Protocol
Addresses
Fragmenting frames to traverse different networks
o This layer can also fragment data into smaller pieces to
send to another network, especially if it’s sent across a
network that may require smaller frames than our local
network
o Any time we are referring to a problem having to due with
IP addresses, routing or fragmenting, we are referring to
the network layer
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Understanding the OSI Model
Layer 4 – Transport
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The “post office” layer
o Referring to the ability to transport data from one device
to another device
o This is responsible for getting your “letter” or
information from one part of a network to another
Protocols
o Transmission Control Protocol (TCP)
o User Datagram Protocol (UDP)
o These 2 protocols are commonly responsible for getting all
of our information within our IP packets from one device
to another, this involves taking apart a large piece of
data into pieces and sending them across the network.
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Understanding the OSI Model
Layer 5 – Session
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Communications Management between devices
o This layer provides communication management from point A
to point B
o This layer also stops, starts and restarts communications
between devices
Control Protocols, tunneling protocols
o If an application is using some type of application
protocol or tunneling information, it is most likely on
layer 5
Layer 6 – Presentation
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Character encoding
o This layer is responsible for putting all this data into a
format we will see with our human eyes.
o It put it in a format we can understand
Application encryption
Often combined with the application layer
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Understanding the OSI Model
Layer 7 – Application
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This is the layer we see
o Anytime we are interacting with an application, we are
working at layer 7
o Common applications that operate at layer 7 are
HTTP/HTTPS, FTP, DNS, POP3 and thousands of other layer 7
applications
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Understanding the OSI Model
LAYER 7
APPLICATION – Your eyes, what
you see on your screen
LAYER 6
The application you see
PRESENTATION – Application
encryption (SSL/TLS)
LAYER 5
SSL Encryption
SESSION – Control Protocols,
Tunneling Protocols
LAYER 4
LAYER 3
LAYER 2
LAYER 1
Link the presentation layer to
the transport layer
TRANSPORT – TCP Segment, UDP
Datagram
TCP Encapsulation
NETWORK – IP Addresses,
Router, Packet
IP Encapsulation
DATA LINK – Frame, MAC
addresses, Extended Unique
Identifier (EUI-48, EUI-64),
Switch
Ethernet
PHYSICAL - Cables, fiber, the
signals themselves
Electrical Signals
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