Cable TV Modem Theory

Cable Modem Theory

by: Rolf V. Østergaard

What Is Downstream - Page 8

Downstream_Chart.gif (5768 bytes)

Downstream is the term used for the signal received by the Cable Modem. The electrical characteristics are outlined in the below table. Notice that most CATV networks in Europe allows 8 MHz bandwidth TV channels, whereas the US CATV networks allows only 6 MHz. Again Europe runs a little faster...

Frequency	42-850 MHz in USA and 65-850 MHz in Europe
Bandwidth	6 MHz in USA and 8 MHz in Europe
Modulation	64-QAM with 6 bits per symbol (normal) 256-QAM with 8 bits per symbol (faster, but more sensitive to noise)

The raw data-rate depends on the modulation and bandwidth as shown below:

	64-QAM	256-QAM
6 MHz	31.2 Mbit/s	41.6 Mbit/s
8 MHz	41.4 Mbit/s	55.2 Mbit/s

Note: A symbol rate of 6.9 Msym/s is used for 8 MHz bandwidth and 5.2 Msym/s is used for 6 MHz bandwidth in the above calculations. Raw bit-rate is somewhat higher than the effective data-rate due to error-correction, framing and other overhead.

Since the downstream data are received by all Cable Modems, the total bandwidth is shared between all active Cable Modems on the system. This is similar to an Ethernet, only the wasted bandwidth on an Ethernet is much higher. Each Cable Modem filters out the data it needs from the stream of data