The data link layer uses the service provided by the physical layer. Although there are many different implementations of the physical layer from a technological perspective, they all provide a service that enables the data link layer to send and receive bits between directly connected devices.

Most data link layer technologies impose limitations on the size of the frames:

  1. Some technologies only impose a maximum frame size.
  2. Others enforce both minimum and maximum frame sizes.
  3. Finally, some technologies only support a single frame size. In this case, the data link layer will usually include an adaptation sub-layer to allow the network layer to send and receive variable-length packets. This adaptation layer may include fragmentation and reassembly mechanisms.

Limitations Imposed By The Physical Layer

The physical layer service facilitates the sending and receiving of bits, but it’s usually far from perfect:

  • The physical layer may change the value of a bit being transmitted due to any reason, e.g., electromagnetic interferences.
  • The Physical layer may deliver more bits to the receiver than the bits sent by the sender.
  • The Physical layer may deliver fewer bits to the receiver than the bits sent by the sender.

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