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The lowest layer in Figure 58-2 is the data-link layer, which consists of the device driver and the hardware interface (network card) to the underlying physical medium (e.g., a telephone line, a coaxial cable, or a fiber-optic cable). The data-link layer is concerned with transferring data across a physical link in a network.

To transfer data, the data-link layer encapsulates datagrams from the network layer into units called frames. In addition to the data to be transmitted, each frame includes a header containing, for example, the destination address and frame size. The data-link layer transmits the frames across the physical link and handles acknowledgements from the receiver. (Not all data-link layers use acknowledgements.) This layer may perform error detection, retransmission, and flow control. Some data-link layers also split large network packets into multiple frames and reassemble them at the receiver.


  

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