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All communications on a network
originate at a source, and are sent to a destination. The information
sent on a network is referred to as data or data packets. If one
computer (host A) wants to send data to another computer (host B), the
data must first be packaged through a process called encapsulation.
Encapsulation wraps data with the
necessary protocol information before network transit. Therefore, as the
data packet moves down through the layers of the OSI model, it receives
headers, trailers, and other information.
To see how encapsulation occurs, examine
the manner in which data travels through the layers as illustrated in
Figure  . Once
the data is sent from the source, it travels
through the application layer down through the other layers. The
packaging and flow of the data that is exchanged goes through changes as
the layers perform their services for end users. As illustrated in Figure
 , networks must perform the following five conversion steps in
order to encapsulate data:
- Build the data.
As a user sends an e-mail message, its
alphanumeric characters are converted to data that can travel across
the internetwork.
- Package the data for end-to-end
transport.
The data is packaged for internetwork
transport. By using segments, the transport function ensures that the
message hosts at both ends of the e-mail system can reliably
communicate.
- Add the network IP address to the
header.
The data is put into a packet or
datagram that contains a packet header with source and destination
logical addresses. These addresses help network devices send the
packets across the network along a chosen path.
- Add the data link layer header and
trailer.
Each network device must put the packet
into a frame. The frame allows connection to the next
directly-connected network device on the link. Each device in the
chosen network path requires framing in order for it to connect to the
next device.
- Convert to bits for transmission.
The frame must be converted into a
pattern of 1s and 0s (bits) for transmission on the medium. A clocking function enables the devices to distinguish these
bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail
message can originate on a LAN, cross a campus backbone, and go out a
WAN link until it reaches its destination on another remote LAN.
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 Lab Activity
Lab Exercise: OSI Model Characteristics and Devices In this lab, the
student will learn the seven layers of the OSI model and the
characteristics, functions and keywords relating to each layer.  |
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Interactive Media Activity
Drag and Drop: Encapsulation Process Flowchart
In this lab, the student will complete encapsulation process
flowchart.
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Web Links