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Data Encapsulation

Each PDU is attached to the data by encapsulating it at each layer of the

OSI model, and each has a specific name depending on the information pro-
vided in each header. This PDU information is only read by the peer layer on
the receiving device. After it's read, it's stripped off, and the data is then
handed to the next layer up.

Figure 1.19 shows the PDUs and how they attach control information to

each layer. This figure demonstrates how the upper-layer user data is con-
verted for transmission on the network. The data stream is then handed
down to the Transport layer, which sets up a virtual circuit to the receiving
device by sending over a synch packet. The data stream is then broken up
into smaller pieces, and a Transport layer header (a PDU) is created and
attached to the header of the data field; now the piece of data is called a seg-
ment. Each segment is sequenced so the data stream can be put back together
on the receiving side exactly as it was transmitted.

F I G U R E 1 . 1 9

Data encapsulation

Each segment is then handed to the Network layer for network addressing

and routing through the internetwork. Logical addressing (for example, IP)
is used to get each segment to the correct network. The Network layer pro-
tocol adds a control header to the segment handed down from the Transport
layer, and what we have now is called a packet or datagram. Remember that
the Transport and Network layers work together to rebuild a data stream on
a receiving host, but it's not part of their work to place their PDUs on a local

Application

Presentation

Session

Transport

Network

Data Link

Physical

Segment

PDU

Packet

Frame

Bits

Upper layer data

TCP header

Data

IP header

Data

LLC header

Data

MAC header

0101110101001000010

Upper layer data

FCS

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