Page 141 - Computer_Science_F5

Computer Science Logical addressing, such as IP addressing, To transmit this frame, it must be
is used to route each segment to the converted into a digital signal. Since a
correct network. The Network layer
frame is a logical group of 1s and 0s, the
protocol adds a control header to the
Physical layer encodes these bits into a
segment from the Transport layer, turning
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it into an IP packet or datagram. While
the same local network. The receiving
the Transport and Network layers work digital signal interpretable by devices on
together to reassemble a data stream on devices then synchronise with the digital
the receiving host, they do not handle signal and decode the embedded 1s and
placing their PDUs on a local network 0s. At this point, devices reconstruct the
segment for delivery to a router or host. frames, perform a Cyclic Redundancy
This task falls to the Data Link layer, Check (CRC), and compare the result
which receives packets from the Network with the frame check sequence (FCS)
layer and transmits them onto the network field. If they match, the packet is extracted
medium, whether cable or wireless. The from the frame, and the remainder of the
Data Link layer encapsulates each packet frame is discarded in a process called
within a frame, whose header includes
the hardware addresses of the source and de-encapsulation. The packet is then
destination hosts. If the destination is on forwarded to the Network layer, where its
a different network, the frame is sent to address is verified. If the address matches,
a router for further routing. Once on the the segment is extracted from the packet,
destination network, a new frame carries and the rest is discarded. Figure 3.6 shows
the packet to the target host. PDU and Layer addressing.

Figure 3.6: PDU and Layer addressing

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Computer Science Form 5.indd 132 23/07/2024 12:33