Amplitude control in a variable load environment

A method performed by data processing apparatus, the method comprising:

• selecting a first fundamental frequency for symbols that are transmitted to endpoints in a communications system, the first fundamental frequency being selected so that a baseline harmonic of the first fundamental frequency is within a downstream channel over which data are transferred from a substation to the endpoints, the baseline harmonic being at least a second harmonic of the first fundamental frequency;
• receiving status data from the endpoints, the status data for each endpoint specifying a number of bit errors that have been detected by the endpoint;
• determining that the status data from at least a threshold number of the endpoints specify a number of bit errors that exceeds a threshold number of bit errors;
• and
• adjusting the first fundamental frequency to a second fundamental frequency that is higher than the first fundamental frequency, the second fundamental frequency being a frequency at which a lower harmonic than the baseline harmonic is within the downstream channel.

A method performed by data processing apparatus, the method comprising:

• transmitting symbols from a substation to endpoints in a communications network, the symbols being transmitted at a first frequency and amplified at an amplification factor, the first frequency being a harmonic of a fundamental frequency for the symbols, the harmonic being within a downstream channel over which the substation communicates with the endpoints;
• receiving transmission quality data specifying a measure of transmission quality for the symbols;
• determining that at least a threshold number of endpoints in the communications network are experiencing a same type of transmission error;
• adjusting the fundamental frequency based on the type of transmission error, the fundamental frequency being adjusted so that a different harmonic of the adjusted fundamental frequency is within the downstream channel;
• and
• transmitting the symbols over the downstream channel, the symbols being generated at the fundamental frequency and amplified at the amplification factor.

A method performed by data processing apparatus, the method comprising:

• generating a first symbol at a first fundamental frequency;
• filtering the first symbol with a filter having a pass-band that includes a harmonic frequency of the first fundamental frequency;
• amplifying the filtered first symbol at an amplification factor;
• determining that at least a threshold number of communications errors are occurring at endpoints to which the first filtered symbols are being transmitted;
• generating a second symbol at a second fundamental frequency that is different than the first fundamental frequency, the second fundamental frequency having a harmonic that is within the pass-band;
• filtering the second symbol with the filter;
• and
• amplifying the second filtered symbol at the amplification factor.

A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising:

• selecting a first fundamental frequency for symbols that are transmitted to endpoints in a communications system, the first fundamental frequency being selected so that a baseline harmonic of the first fundamental frequency is within a downstream channel over which data are transferred from a substation to the endpoints, the baseline harmonic being at least a second harmonic of the first fundamental frequency;
• receiving status data from the endpoints, the status data for each endpoint specifying a number of bit errors that have been detected by the endpoint;
• determining that the status data from at least a threshold number of the endpoints specify a number of bit errors that exceeds a threshold number of bit errors;
• and
• adjusting the first fundamental frequency to a second fundamental frequency that is higher than the first fundamental frequency, the second fundamental frequency being a frequency at which a lower harmonic than the baseline harmonic is within the downstream channel.