OSI Encapsulation

OSI Encapsulation
OSI Layer Name of Protocol Data Units (PDUs)
Transport Segments/Datagram
Network Packets
Data Link Frames
Physical Bits

Starting at the Application layer, data is converted for transmission on the network and 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 responsible 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 information 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. 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 any reliability parameters
required are 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 digital signal.
Now that you have a sense of the OSI model and how routers and switches work together,
it is time to turn your attention to the specifics of each layer of switching technology.