OSI stands for Open Systems Interconnection
and is a conceptual framework
for how applications communicate over a network.
There are seven layers within the model
and the layers depiction is used to help users identify
what is happening within a networking system.
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The OSI model layers are typically described from the top layer down.
The layers are described as Application,
Presentation, Session, Transport, Network, Data Link, and Physical.
These layers are provided by a mixture of network card drivers,
operating systems, applications, and networking hardware
that facilitate the transmission of signals over Ethernet,
Fiber Optic, Wi-Fi, or other wireless protocols.
We'll describe the layers from the top down
as the top layer is the Application layer or layer 7.
This is the layer that most users interact with and will recognize.
The Application layer provides network services to the end user.
These services are protocols
that work with the data the client is using.
One of these protocols may be HTTP
that is used with web browsers such as Google Chrome,
Firefox, and Internet Explorer.
Other examples of applications that use this layer
are Office, Outlook, and Skype.
All of those interactive applications provide a set of services
that allow the application layer to supply data to
and receive data from the Presentation layer.
The Presentation layer, or layer 6,
performs the uncomplicated task of syntax processing
or converting data from one format to another.
For example, say you are ordering something from an online store.
These transactions are typically handled in a secure transmission
which means that the data passing between the "store"
or the Website Application will transmit encrypted data
to the Presentation layer that will need to be decrypted and processed.
This layer handles translating the data from the top layer,
which is presented in application format,
to network format and vice versa.
After the Presentation layer processes the data from one format to another,
the information is then passed to the Session layer
or the Application layer depending on whether the data
is transmitting or receiving.
At the Session layer, or layer 5, the construction,
direction, and conclusion of connections between devices occur.
This layer supports multiple types of connections
as well as being responsible for authentication and reconnection
if a network interruption should occur.
After the session is established the data
then passes to or from the Transport layer.
The Transport layer, or layer 4,
is responsible for the transmission of data across network connections.
This layer coordinates how much data to send, how fast,
where it goes, and these sort of things.
Of the most widely known protocols for internet applications,
these services may be provided by Transmission Control Protocol
or TCP and User Datagram Protocol or UDP.
Other protocols may provide additional capabilities
including error recovery, data flow, and retransmission.
Once the Transport layer has completed its function,
the data is then passed to or from the Network layer.
The Network layer, or layer 3, handles the routing of data.
After the data arrives at this layer,
each frame of data is examined to conclude
if the data has reached its ultimate target.
The layer sends data to the correct destination
on outgoing transmissions and receives incoming transmissions as well.
The IP portion of TCP/IP
is the commonly known network layer for the Internet.
This layer also manages the mapping between logical addresses
and physical addresses,for IP addresses,
this is accomplished through Address Resolution Protocol or ARP.
The data is then passed to the next required layer
which is the Data Link layer.
The Data Link layer, or layer 2,
is considered the most complex of the layers.
This layer is often divided into sublayers
called Media Access Control or MAC and Logical Link Control or LLC.
The layer sets up links across the physical network.
When this layer received data from the Physical layer,
it checks for transmission errors
and then packages the bits into data frames.
From there, this layer manages the physical addressing methods
for the MAC or LLC layers.
An example of the MAC layer includes 802.11
wireless specifications as well as Ethernet.
At the Data Link layer, the data passes to or from the final layer
in the OSI model which is the Physical layer.
The Physical layer, or layer 1, is the electrical/physical layer of the model.
This layer encompasses the network cables, power plugs,
cable pinouts, wireless radio frequencies, connectors,
transceivers, receivers, repeaters, pulses of light, electric voltages, etc.
When troubleshooting problems,
this is typically the first place to start.
Is the device plugged in?
Is the wireless card working?
At this layer, the model transmits the digital data bits
from the source or sending devices Physical layer,
over network communications media,
which can be electrical, mechanical,
or radio, to the receiving or destination devices Physical layer.
The OSI model is a guide for developers and vendors
to smooth the progress of developing communication
products and software programs
that will work in cooperation with a commonly established model.
Once you understand the model,
you can then understand which protocols and devices
will be compatible with one another.
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