The OSI model consists of seven layers as shown in Fig. 7-1.
The physical layer is responsible for the transmission of a bit stream between two or more machines.
You could think of the physical layer as a pipe through which individual bits flow. The basic objective of the
physical layers is to make sure that when a ???1??? is sent, the other side receives a ???1.??? The physical layer defines
a medium through which messages can be sent.
The services provided by the physical layer are not that different from the services provided by the post
office. When you send a letter using standard first-class mail, the post office promises that it will attempt to
deliver the letter, but it provides no guarantees. In most cases the letter will arrive at the intended destination,
but there are times when the letter may be lost. In those cases where you require reliable mail delivery, you
might send a letter using a higher-level service like registered mail.
The data link layer uses the physical layer to provide reliable point-to-point delivery within a network.
While the underlying physical layer may not be entirely error free, the data link layer transforms the connection
into a facility that appears free of errors.
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