The OSI Advertence Model
Written on 15:41 by Ahmet Emir
One of the greatest functions of the OSI blueprint is to abetment in abstracts alteration amid disparate
hosts—meaning, for example, that they accredit us to alteration abstracts amid a Unix host
and a PC or a Mac.
The OSI isn’t a concrete model, though. Rather, it’s a set of guidelines that application
developers can use to actualize and apparatus applications that run on a network. It additionally provides
a framework for creating and implementing networking standards, devices, and internetworking
schemes.
The OSI has seven altered layers, disconnected into two groups. The top three layers ascertain how
the applications aural the end stations will acquaint with anniversary added and with users. The
bottom four layers ascertain how abstracts is transmitted end to end. Figure 1.6 shows the three upper
layers and their functions, and Figure 1.7 shows the four lower layers and their functions.
FIGURE 1 . 6 The high layers
• Provides a user interface
• Presents data
• Handles processing such as encryption
• Keeps altered applications’
• abstracts separate
Application
Presentation
Session
Transport
Network
Data Link
Physical
14 Chapter 1 Internetworking
When you abstraction Figure 1.6, accept that the user interfaces with the computer at the
Application band and additionally that the high layers are amenable for applications communicating
between hosts. Remember that none of the high layers knows annihilation about networking
or arrangement addresses. That’s the albatross of the four basal layers.
In Figure 1.7, you can see that it’s the four basal layers that ascertain how abstracts is transferred
through a concrete wire or through switches and routers. These basal layers additionally determine
how to clean a abstracts beck from a transmitting host to a destination host’s application.
FIGURE 1 . 7 The lower layers
The afterward arrangement accessories accomplish at all seven layers of the OSI model:
Arrangement administration stations (NMSs)
Web and appliance servers
Gateways (not absence gateways)
Arrangement hosts
Basically, the ISO is appealing abundant the Emily Post of the arrangement agreement world. Just as Ms.
Post wrote the book ambience the standards—or protocols—for animal amusing interaction, the
ISO developed the OSI advertence archetypal as the antecedent and adviser for an accessible arrangement protocol
set. Defining the amenities of advice models, it charcoal today the best popular
means of allegory for agreement suites.
The OSI advertence archetypal has seven layers:
Appliance band (layer 7)
Presentation band (layer 6)
Session band (layer 5)
Transport band (layer 4)
Arrangement band (layer 3)
Abstracts Link band (layer 2)
Concrete band (layer 1)
• Combines packets into bytes and bytes into frames
• Provides admission to media application MAC address
• Performs absurdity apprehension not correction
• Provides analytic addressing,
• which routers use for aisle determination
• Provides reliable or capricious delivery
• Performs absurdity alteration afore retransmit
• Moves $.25 amid devices
• Specifies voltage, wire speed,
• and pin-out of cables
Transport
Network
Data Link
Physical
The OSI Advertence Archetypal 15
Figure 1.8 shows a arbitrary of the functions authentic at anniversary band of the OSI model. With
this in hand, you’re now accessible to analyze anniversary layer’s action in detail.
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