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2.2.4 OSI Model Communications
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OSI Model Communications 0:00-0:16
The OSI model is a theoretical model of describing networking and network communications. In this video, we're going to describe how data
is sent between two devices on a network, breaking down the process by OSI model layers.
Communication Process 0:17-0:35
We're going to use the OSI model to show how a webpage is transmitted from one host to another. We have two devices in a network. This
device is the source device. It's using an application layer protocol (HTTP) to create a web page. It wants to send the page to this computer,
which is the destination device.
Sending Data 0:36-0:43
First, it will compress and encrypt the data at the presentation layer, then append the session ID at the session layer.
The data goes to the transport layer.
Segmentation 0:44-0:56
The transport layer breaks the data into blocks called segments and appends a port number to identify which top-layer application needs to
receive the data on the destination device.
Each segment is passed to the network layer.
Packets 0:57-1:02
The network layer appends the source and destination IP addresses to create a packet.
Frames 1:03-1:19
The packet is then passed to the data link layer which adds the source and destination MAC addresses, as well as the CRC. The packet is now
called a frame. The frame is sent to the physical device where it is translated into some kind of a signal, such as electrical, radio, or light.
Bits 1:20-2:09
This frame then becomes a signal that represents a series of zeros and ones. Each data point in this string is called a bit.
The network interface prepares this signal and sends it out on the transmission medium. The destination device receives this series of bits and
interprets them as a frame. It looks at the MAC addresses and the CRC, and then removes them. It passes the data to the network layer as a
packet. The IP addresses within the packet are examined and removed, and the packet is forwarded to the transport layer as a segment. The
segment is then examined, the port number is looked at, and it is forwarded up the OSI model layer to the appropriate application specified
by the port number. The session ID is used, the encryption is removed, and the data is presented in its original form to the application that
needs to interpret it; in this case, a browser.
Receiving Data 2:10-2:31
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Data flows down through the layers of the OSI model on the sending system, and is transformed at each layer. Data is broken down into
segments, which are then transformed into packets, then frames, and then bits. On the receiving device, the bits are converted back to frames,
then to packets, then into segments, and the segments are reassembled into the original data.
Summary 2:32-3:09
Using the OSI model to describe how devices talk to each other, we see data being transformed at each layer to get it ready to be sent through
the network. At the upper layers, encryption, formatting, and session numbers are added to the data. At the middle layers, data is broken
down into segments associated with a port number, then given the IP address. At the lower levels, packets are transformed into frames that
include the source and destination MAC addresses, and frames are transformed into bits for transmission through the network. Receiving
devices do all this in reverse, converting bits to frames, then to packets and segments, and finally back to the original data.
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