It does this by means of error checking and retry mechanisms built into
the protocol.
Note that the term point-to-point means that the machines involved in the communication are directly
connected. As long as the two computers are directly connected, one to the other, with no intervening computers,
the data link layer provides a service similar to that when you send a letter via registered mail.
128 NETWORKING [CHAP. 7
Figure 7-1 ISO OSI reference model.
In addition to reliable transmission, the data link layer, unlike the physical layer, provides structure to
the messages that are being transmitted. The physical layer is concerned only with the delivery of the
individual bits that make up the messages, and does not care about the structure of those messages. The data
link layer, on the other hand, often divides messages into frames and adds additional information to the frames
to provide synchronization and flow control. By synchronization we mean that the sender and receiver
must agree on the start and end of each frame, and by flow control we mean that the receiver must have
a mechanism for pacing the sender, so that the sender does not flood the receiver with data at a pace faster
than the receiver can accept it.
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