VCE IT Theory Slideshows
The OSI
Model
Open System Interconnection
Reference Model
By Mark Kelly
Vceit.com
With help from howstuffworks.com
VCE Software Development
• SD U3O1 KK03 - a brief overview of the
concept of the OSI model for network
protocols
• SD U3O1 KK04 - purposes and functions of the
physical layer (Layer 1) of the OSI and the
relationship of the physical layer to the
Transmission Control Protocol/Internet
Protocol model
What is it?
• A set of seven layers that define the different
stages that data must go through to travel
from one device to another over a network.
• It’s a guideline for programmers and hardware
developers to use
• Lets their products work predictably with
other software and hardware in networks.
Like an assembly line
• Data travels through the layers
• At each layer, data is handled to prepare it for
the next layer
Higher levels are more
abstract and
sophisticated. Lower
levels are increasingly
specific and ‘primitive’.
Thanks to howstuffworks.com
7 layers, two sets
• Application Set
– Layer 7: Application
– Layer 6: Presentation
– Layer 5: Session
• Transport Set
– Layer 4: Transport
– Layer 3: Network
– Layer 2: Data
– Layer 1: Physical
Mnemonic
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Physical
Data
Network
Transport
Session
Presentation
Application
Mnemonic
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Physical
Data
Network
Transport
Session
Presentation
Application
Mnemonic
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Physical
Data
Network
Transport
Session
Presentation
Application
Programmers
Do
Not
Throw
Stale
Pizza
Away
Mnemonic
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Physical
Data
Network
Transport
Session
Presentation
Application
Programmers
Do
Not
Throw
Stale
Pizza
Away
Layer by layer
Highest to lowest
• Application Set
• The layers where application software works
Layer 7: Application
– Layer 7: Application - This layer actually interacts
with the operating system or application
– E.g. when users transfer files, read messages or
perform other network-related activities.
– The interface between end-user applications and
communications software.
– Protocols such as Telnet, HTTP, FTP, and SMTP
communicate at this layer.
Layer 6: Presentation
• Layer 6: Presentation - Layer 6 takes the data
provided by the Application layer and
converts it into a standard format that the
other layers can understand.
• Handles encryption, formatting, compression,
and presentation of data formats (such as
JPEG) to applications.
• SSL and TLS communicate at this layer
Layer 5: Session
• Establishes, maintains and ends
communication with the receiving device
• responsible for the startup, control, and
teardown of sessions for the presentation
layer.
• handles all transport and data delivery issues
to other systems (a focus on error recovery
and controlling data flow).
• TCP and UDP protocols reside at this layer.
Layer by layer
Highest to lowest
• Transport
• The layers where “behind the scenes”
technical work happens
Layer 4: Transport
• This layer maintains flow control of data and
provides for error checking and recovery of
data between the devices.
• Flow control means that the Transport layer
takes data coming from more than one
application and integrates each application's
data into a single stream for the physical
network.
Layer 3: Network
• The network layer determines the way that
the data will be sent to the recipient device.
• Logical protocols, routing and addressing are
handled here.
• responsible for routing, addressing, and
determining the best possible route.
• ICMP, IP, ARP, and IPSEC reside at this layer.
• IP addresses are found at this layer.
Layer 2: Data Link
• In the data layer, the appropriate physical
protocol is assigned to the data.
• The type of network and the packet
sequencing is defined.
• links the data from one host to another
• MAC addresses are found at this layer.
• Ethernet, FDDI, ATM, and Token Ring reside at
this layer.
Layer 1: Physical
• This is the level where actual hardware is
considered.
• It defines the physical characteristics of the
network such as connections, voltage levels
and timing.
• Provides the physical transportation of data.
• Focuses on connectors, currents, pins, light,
and other specifications that define cabling
standards.
• This layer focuses on binary transmission.
Fregsample…
Putting the OSI to real work
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OSI is only a model
It does not actually do any work
Protocol stacks are actual software tools
They often combine several OSI layers into
one of the stack’s layers.
• Like TCP/IP…
Protocol Stack
• A protocol stack is a group of protocols that
all work together to allow software or
hardware to perform a function.
• (For protocol information, see the Protocol
slideshow)
TCP/IP protocol stack
• TCP/IP is a protocol stack.
• It uses four layers that map to the OSI model
TCP/IP + OSI layer summary
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TCP/IP Layer 1: Network Interface
TCP/IP Layer 2: Internet
TCP/IP Layer 3: Transport
TCP/IP Layer 4: Application
TCP/IP + OSI layer summary
TCP/IP LAYER
OSI LAYERS
Network Interface
Physical
Data
Internet
Network
Transport
Transport
Application
Session
Presentation
Application
TCP/IP + OSI layers
• Layer 1: Network Interface - combines the OSI
Physical and OSI Data layers
• Routes data between devices on the same
network.
• Manages the exchange of data between the
network and other devices.
•
Huh?
• SD U3O1 KK04 - purposes and functions of the
physical layer (Layer 1) of the OSI and the
relationship of the physical layer to the
Transmission Control Protocol/Internet
Protocol model
• The importance of the relationship of the OSI
physical layer and the TCP/IP model baffles
me.
TCP/IP + OSI layers
• Layer 2: Internet - corresponds to the OSI
Network layer.
• The Internet Protocol (IP) uses the IP address,
consisting of a Network Identifier and a Host
Identifier, to determine the address of the
device it is communicating with.
TCP/IP + OSI layers
• Layer 3: Transport - Corresponding to the OSI
Transport layer, this is the part of the protocol
stack where the Transport Control Protocol
(TCP) can be found.
• TCP works by asking another device on the
network if it is willing to accept information
from the local device.
TCP/IP + OSI layers
• Layer 4: Application - Layer 4 combines the
Session, Presentation and Application layers of
the OSI model.
• Protocols for specific functions such as e-mail
(Simple Mail Transfer Protocol, SMTP) and file
transfer (File Transfer Protocol, FTP) reside at
this level.
• So, the TCP/IP protocols don’t have
corresponding layers for each function in the
OSI Model.
• But developers use the OSI model to ensure
that a certain level of network compatibility is
maintained.
A simple analogy
• Layer 7: Application
• Mr A of A Corporation wants to send a
message to Mr B of B corporation. He picks up
the phone and calls for his personal assistant
(his PA).
Layer 6: Presentation
• The PA writes down the message in German
(which Mr A speaks) into his notebook.
Layer 5: Session
• The PA rips off the page with the message and
sends it to the mail room with its destination
address attached.
Layer 4: Transport
• The mail room takes the page (along with
many other messages from employees),
checks that the address is present and sorts
messages into departments.
Layer 3: Network
• The mail room manager decides how best to
send the message - does it needs to go
airmail, by container ship, express post,
bicycle courier?
Layer 2: Data
• The mail room manager puts the message into
a standard waterproof envelope with its
destination and delivery rules written on it
(e.g. does the message need to be signed for?
Does it need to be delivered to the recipient in
person? How urgent is it?). Tim, the bicycle
courier is summoned, and told to deliver the
envelope.
Layer 1: Physical
• Tim jumps on his bike and pedals madly across
town to Corporation B, making constant
decisions about steering, changing gear,
jumping gutters and swerving to miss
oncoming traffic.
• He arrives at the front door of Corporation B
and hands over the envelope to the
receptionist.
And now, the process reverses.
Layer 1: Physical
• The receptionist at Corporation B gives the
envelope to corporation B's mail manager,
who opens the envelope to see where it needs
to go in their building.
Layer 2: Data
• The mail manager sees that it needs to go to
Mr B. He puts the message into a standard
Corporation B inter-office memo envelope.
Layer 3: Network
• Mr B's office number is written on the
envelope, which is then given to the work
experience kid. The kid is told where the office
is, that he is to keep left in the corridors, and
not interrupt Mr B if he is in a meeting.
Layer 4: Transport
• The kid trots down the corridors, checking
office numbers and making sure the envelope
is not being damaged by his sweaty hands.
Layer 5: Session
• The kid arrives at Mr B's office, checks the
authenticity of the ID badge of Mr B's
secretary, gets the secretary to sign a receipt
for the message, and hands the envelope over.
Layer 6: Presentation
• The secretary opens the envelope, takes out
the page, translates it into French (which Mr B
speaks) and puts it in Mr B's in-tray.
Layer 7: Application
• Mr B removes the translated message from his
in-tray, reads it and thinks about its contents.
And so it ends
• Note how each step of the process is logically
separated from the others. Each layer
interacts only with the layers directly above
and below. It does not need to know how the
other layers work: it just do its own job within
its own limited walls.
The end
• The courier does not know or care what's in
the envelope: the only thing concerning him is
physically getting it from A to B. Once it's
delivered, his job is done.
The end
• Mr A and Mr B neither know nor care what
sort of bike Tim was riding or what route he
took; they only care that the message arrived
safely and quickly.
• Each step in the process had its role in the
efficient transfer of the information in the
message. Like a factory production line...
VCE IT THEORY SLIDESHOWS
By Mark Kelly
mark@vceit.com
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