Cable Modems
Downstream Characteristics
Downstream is the term used for the signal received by the subscriber. The electrical characteristics are outlined in the below table:
|
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) |
The raw data-rate (which includes also error correction) depends on the modulation and bandwidth as shown below:
|
|
64-QAM(6 bits per symbol) |
256-QAM(8 bits per symbol) |
|
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 (6.9Msym/s*6bits/sym= 41.4 Mbits/s) bandwidth and 5.2 Msym/s is used for 6 MHz bandwidth in the above calculations.
In addition, raw bit-rate is somewhat higher than the effective data-rate due to error-correction and other overhead.
Since the physical medium is shared and 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). Each cable modem filters out the data it needs from the stream of data.
Cable Modems
Downstream Characteristics