Bits are the Layer PDU. Frames become electrical, optical, or radio signals.
Encoding schemes:
- NRZ: = high voltage, = low
- Manchester: transitions represent bits
- 4B/5B: maps bits to -bit patterns
Physical media:
- Copper: electrical, interference-prone
- Fiber: light, interference-immune
- Wireless: radio, subject to attenuation
Bit errors happen. Layer checksums detect them. Layer (TCP) can request retransmission.