1. Field of the Invention
The present invention relates generally to digital communications, and more particularly, to switching to a lower data rate for power savings.
2. Description of Related Art
A communication system permits communication between two or more network devices. Communication between network devices can be conventionally achieved using a communication line (or link), formed by twisted pairs of wires (or cables), and transceivers, one transceiver positioned at each end of a twisted pair. For example, the IEEE 802.3 (10GBASE-T) standard targets data transmission rates with a total throughput of 10 Gbps over (4) pairs of twisted wires for distances of up to 100 m. The data transmission is generally performed in a simultaneous bidirectional fashion, thus each pair of wires simultaneously carries bidirectional data, each direction running effectively at 2.5 Gbps.
A common problem associated with a communication system using multiple twisted pairs of wires and multiple transceivers is noise in the form of interference signals. For example, due to the bidirectional nature of data transmission along a twisted pair (or channel), echo cancellation is typically performed which subtracts a transmitted signal from a received signal. Furthermore, since channel insertion loss is quite significant, the signal strength at the end of a communication line is typically very weak, and any noise and/or interference can significantly affect communication system bit error rate (BER). Thus, much effort is usually carried out to cancel any deterministic source of noise in a communication system.
Such deterministic noise sources include, for example, first reflection of a transmitted signal off of discontinuities in the communication line (so called echo), second intersymbol interference due to signal distortion in the communication line, and near-end crosstalk (NEXT) from channels adjacent to a given channel within a communication line, and differential signal wander caused by the AC coupled link and non-DC balanced data stream. Far-end crosstalk (FEXT) is another deterministic source of noise. Due to the high complexity of cancellation circuitry and the fact that far-end crosstalk is orders of magnitude weaker than other deterministic sources of noise, far-end crosstalk may not be typically cancelled. However, in the 10GBASE-T standard the very low system signal-to-noise ratio (SNR) requires FEXT cancellation as well. In the 10GBASE-T standard jitter and other alienated sources of interference are treated as random noise that are accounted for in the signal-to-noise (SNR) budget of the link.
After the cancellation of the major sources of deterministic noise, there is generally still not enough signal-to-noise ratio left to achieve a target bit error rate of 10E-12 for the link. Therefore, a low density parity check (LDPC) decoder typically follows the recovered data to provide coding gain and increase the effective signal-to-noise ratio. The overhead of the LDPC decoding is approximately 1/7th of total data throughput.