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Switching Technologies
15
Starting at the Application layer, data is converted for transmission on the
network, then encapsulated in Presentation layer information. When the
Presentation layer receives this information, it looks like generic data.
The Presentation layer hands the data to the Session layer, which is respon-
sible for synchronizing the session with the destination host.
The Session layer then passes this data to the Transport layer, which
transports the data from the source host to the destination host in a reliable
fashion. But before this happens, the Network layer adds routing informa-
tion to the packet. It then passes the packet on to the Data Link layer for
framing and for connection to the Physical layer. The Physical layer sends the
data as 1s and 0s to the destination host across fiber or copper wiring.
Finally, when the destination host receives the 1s and 0s, the data passes
back up through the model, one layer at a time. The data is de-encapsulated
at each of the OSI model's peer layers.
At a transmitting device, the data encapsulation method is as follows:
1.
User information is converted to data for transmission on the
network.
2.
Data is converted to segments at the Transport layer, and a reliable
session is possibly set up.
3.
Segments are converted to packets or datagrams at the Network layer,
and routing information is added to the PDU.
4.
Packets or datagrams are converted to frames at the Data Link layer,
and hardware addresses are used to communicate with local hosts on
the network medium.
5.
Frames are converted to bits, and 1s and 0s are encoded within the dig-
ital signal.
Now that you have a sense of the OSI model and how routers and
switches work together, it is time to turn our attention to the specifics of each
layer of switching technology.
Layer 2 Switching
Layer 2 switching is hardware based, which means it uses the Media Access
Control (MAC) address from the host's network interface cards (NICs) to
filter the network. Switches use Application-Specific Integrated Circuits
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