Cable Modem Technology
- The upstream channel is more tricky
- Typically, in a two-way activated cable network, the upstream (also known as the reverse path) is transmitted between 5 and 42 MHz
- This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling
- Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing
- Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment
- The drawback is that QPSK is "slower" than QAM.