Networks, Physical Layer

[2.2] Physical layer (OSI 1) transports information physically over distance.
[2.3] Provides bit stream to data link layer. [2.4] Provides
- Digital-analogue conversion.
- Low-level buffering.
- Error detection but no error correction.
  - [2.5] Hence reliability important.
- Multiplexing
  - over time (time divison multiplexing), or
  - over frequencies (wave divison multiplexing).
[2.5] Reliability \(\leftrightarrow\) bandwidth tradeoff.
[2.6] Signalling is time-synchronized with some encoding.
- [2.6.1] Non-return to zero (NRZ) encoding: 1 = high, 0 = low.
  - [2.6.2] Clock drift problematic.
- [2.6.4] Manchester encoding: 1 = low→high, 0 = high→low (or opp.).
  - Sync clocks using transitions.
  - Read bits away from transmissions.
- [2.6.5] Differential Manchester: 1 = no-change, 0 = change (or opp.), in addition to change at middle of window.
  - Sync clocks using middle transitions.
  - No dependence on absolute values, only changes.
  - Early/late bit-read points.