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Network Models
Table of Contents
Network Models ............................................................................................................................. 3
Why Use the OSI (or TCP/IP) Model? ............................................................................................. 4
OSI Layer ......................................................................................................................................... 5
OSI Layer 1 Physical ...................................................................................................................... 6
OSI Layer 2 Data-link .................................................................................................................... 9
OSI Layer 3 Network .................................................................................................................. 10
OSI Layer 4 Transport ................................................................................................................. 12
OSI Layer 5 Session..................................................................................................................... 13
OSI Layer 6 Presentation ............................................................................................................ 15
OSI Layer 7 Application .............................................................................................................. 16
OSI Model vs. TCP/IP Model ......................................................................................................... 18
TCP/IP Model ................................................................................................................................ 20
TCP/IP Layer 1 Physical .............................................................................................................. 21
TCP/IP Layer 2 Data-link ............................................................................................................. 22
TCP/IP Layer 3 Network ............................................................................................................. 23
TCP/IP Layer 4 Transport ........................................................................................................... 24
TCP/IP Layer 5 Application ......................................................................................................... 25
TCP/IP Model -1 ............................................................................................................................ 26
TCP/IP Model -2 ............................................................................................................................ 27
TCP/IP Model Sender -1 ............................................................................................................. 28
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TCP/IP Model Data Transmitted ................................................................................................ 33
TCP/IP Model Receiver -1 .......................................................................................................... 34
TCP/IP Model Security Considerations -1 ..................................................................................... 39
Notices .......................................................................................................................................... 48
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Network Models
5
Network Models
**005 Nick Flores: So, we'll begin with the first topic right off the bat, which is network models.
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Why Use the OSI (or TCP/IP) Model?
6
Why Use the OSI (or TCP/IP) Model?
Permit exchange of information among systems that are open or compliant with this standard
Reference framework to enable independent work Definitions of security terminology Standard descriptions of security services and mechanisms Identify where services map to OSI model Security management
Information may not be given to, accessed by, nor permitted to be inferred by, nor may any resource be
used by those not appropriately authorized
- General Authorization Policy ISO 7498-2
**006 What are network models, and then specifically what are the OSI and TCP/IP network models? Well, it's important to understand that, first, the Open Standards Interconnection model, the OSI model, is an educational tool. It's meant to help us understand how networks work, how computers communicate with each other, but it's not actually implemented in any live network that you see out there out today. TCP/IP, on the other hand, is becoming and has become the de facto standard for most computer networks. It's what the internet really runs on.
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And we're going to learn how the OSI model, which will help us understand networking and network stacks, relates to the TCP/IP model, how they're different, and how the real- life version of this, the TCP/IP model, actually works in today's networking environment.
OSI Layer
7
OSI Layer
Defined by ISO 7498
Also describes security mechanisms and where they fit in the model.
Physical
Data-link
Network
Transport
Session
Presentation
Application
**007 So, the OSI layer, the OSI model, is consistent of seven layers, and they're defined by the ISO standard 7498. And you see them stacked graphically in this representation because that's how logically they work. You'll hear people refer to a network stack, and the reason they call it a network
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stack is because the layers operate either from the top down, or from the bottom up. And we'll describe these mechanisms and how they work in the following slides.
OSI Layer 1 Physical
8
OSI Layer 1 Physical
Physical
Data-link
Network
Transport
Session
Presentation
Application Transmits logical bits (1s and 0s) over a physical circuit
Electrical and physical specifications
Devices NICs, repeaters, concentrators
**008 So, the first layer of the OSI model, called the lowest layer, or the physical layer, has to do with the transmission of bits across physical media. Now, this physical media may sometimes be wireless, but it's still considered physical media as far as the network stack is concerned. The physical layer is responsible for actually transmitting bits, ones and zeros, from Point A to Point B. Now, these bits may be transmitted as
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electrical impulses across a copper wire; they may be transferred as radio waves over an RF signal; they may be transmitted as light over a fiber optic cable. The physical media doesn't really matter to the networking stack; the idea is that the physical layer is responsible for transmitting this physical information and then decoding it on the other end and translating it back up the stack. Some of the devices that work at this physical layer include things like your network interface card, your NIC-- that card in your computer that's got the computer jack that you plug into. That's a NIC and it works at the physical layer. It's responsible for taking information from the data-link layer higher up and translating that information to electrical impulses over a copper wire. Your wireless network interface card operates at the physical layer. Repeaters and concentrators, that we'll talk about in more detail later, work at the physical layer. Before we get too deep into this stack, a lot of people have problems remembering what the stack is or what the layers are, and what order they come in. A popular mnemonic to use that a lot of people remember, or make it easier to remember-- to understand or remember the layers of the OSI model is: Please Do Not Throw Sausage Pizza Away. And I'll repeat that, because it's something that was kind of drilled into me many, many times: Please Do Not
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Throw Sausage Pizza Away. And if you start at the bottom at the physical layer with the P, the P for Please; the D at the data-link layer for

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