8. What is Downstream?
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.
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