Physical Layer - Transmission Media
Guided Transmission Media
In addition to the following you can also have transfer through physical media. This can give the best transfer rate (especially for large amounts of data).
- Twisted Pair
- You can twist the cables more to give better cross talk.
- Coaxial Cable - A copper core surrounded by a mesh sheath.
- Fibre Optics - Light travels through a fibre via total internal reflection.
Unguided Transmission Media
We can use various bandwidths of the electromagnetic spectrum to transmit data. Different frequency bands have different properties:
- Radio Transmission
- In VLF, LF and MF bands radio waves follow the curvature of the earth.
- The HF band can bounce off the ionosphere.
Communication Satellites
Satellites in different height orbits have different communication properties:
| Height | Altitude (KM) | Orbit Type | Latency (ms) | Satellites Needed |
|---|---|---|---|---|
| Geostationary | ~35000 | GEO | 270 | 3 |
| Medium Earth Orbit | 5000-15000 | MEO | 35-85 | 10 |
| Low Earth Orbit | ~0 | LEO | 1-7 | 50 |
Satellites operate on the following bands:
| Band | Down-link (GHz) | Up-link (GHz) | Bandwidth (MHz) | Problems |
|---|---|---|---|---|
| L | 1.5 | 1.6 | 15 | Low bandwidth, Crowded |
| S | 1.9 | 2.2 | 70 | Low bandwidth, Crowded |
| C | 4 | 6 | 500 | Terrestrial Interference |
| Ku | 11 | 14 | 500 | Rain |
| Ka | 20 | 30 | 3500 | Rain, Equipment cost |