DSL Modem TrainUp and Framing
The start sequence conducted between two modems is referred to as training or train-up.
During train-up, DSL modems start with no data interleaving and no line coding. (Interleaving and line coding must be negotiated between the two modems.) The signal-to-noise level is calculated across the DSL spectrum, and a DSL line rate is established based on three essential factors:
• Provisioning—DSL modems will not establish a faster DSL rate than what is specified (provisioned) in the DSLAM. The DSL trained rate is usually specified in terms of available user bandwidth or data rate rather than the actual DSL spectrum used. That is, the reported DSL rate is the actual user-available bandwidth. Most systems subtract the DSL overhead before reporting the trained rate, and the actual line rate is slightly higher than the reported trained rate.
• Inappropriate DMT options—An unnecessarily high noise-margin setting results in an unnecessarily low DSL trained rate. The recommended noise margin settings for DMT are 6 dB downstream and 6 dB upstream. (The recommendation for CAP systems is traditionally 3 dB downstream and 6 dB upstream.)
• Line conditions—This includes several factors, but primarily two: attenuation and noise levels. More than one noise source can contribute to the total noise spectrum on the line, thus producing multiple noise frequencies with related noise (power) levels. Some noise sources are more detrimental to DSL performance than others, again depending on the spectrum and power level of the noise induced into the DSL circuit.
Continue reading here: Power Levels in DSL Services
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