There are typically two distinct functions performed at Layer 1. These usually include some form of transmission convergence (TC) sublayer and a physical medium dependent (PMD) sublayer. They sound quite technical but all they really mean is that the bits have to be placed in a particular order as per protocol between the endpoints and that the bits must be transmitted.
The process of placing the bits in a formalized, predetermined order is known as framing. The process of actually transmitting the bits is known as line coding. Both are somewhat generic terms and significantly factor into the overall functionality of Layer 1. When all is said and done, the job of the TC and PMD sublayers is to line up bits and kick them out the interface. If the bits are lined up incorrectly, they will arrive incorrectly and not be understood at the far end. If they are sent incorrectly, they will be seen as errors and dropped.
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