The speed and stability of DSL and Cable on any given line in the access network is determined by the signal to noise ratio at the end of that line, which in turn depends on:
- The length, quality and dimensions of the cable.
- The amount of crosstalk (directly related to "cable fill", the proportion of pairs in the cables carrying DSL).
- Interference from sources inside the business premises (including wiring).
- Noise picked up from the environment e.g. radio frequency interference.
- Any faults that might be present.
Geographic coverage achieved at a given headline speed also depends on:
- The topology of the access network.
- The statistical distribution of cable lengths
These factors are based on physical science and are identical for anyone operating ADSL on an access network. The speed delivered to the end user also can depend on retail and wholesale product definitions, the makes of DSLAM and modem (as standard specifications can be implemented by different designs) and other decisions on how the network operates the chosen DSL technology. In addition, the end user throughput depends on operational decisions on backhaul and contention ratios. The line data available to end users from modems is highly variable in extent and quality and is not a reliable guide to ADSL performance.
