From a Computer Science viewpoint, the physical transmission of information through a wire is often considered as a black box that allows to transmit bits. This black box is often referred to as the physical layer service and is represented by using the DATA.request and DATA.indication primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits. This service abstracts the technological details that are involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the physical layer service is imperfect and has the following characteristics :