Power Levels in DSL Services

DSL power levels are much higher than those levels used in voice, fax, or analog data services. This is simply because of the much greater attenuation of signals at DSL frequencies that are caused by the electrical characteristics of telephone lines.

Because the attenuation of a phone line is much greater at ADSL frequencies than at voice frequencies, the phone line is much more attenuative to DSL than to voice, fax, or analog modems. To recover a usable signal at the end of 18 kft of cable, ADSL is usually transmitted at +15 to +20 dBm. This is an exceptionally high power to be transmitted on a telephone line, but it is necessary to attain DSL rates at maximum reach.

The standard unit of measure for telephone system power levels is dBm, which is dB relative to 1 milliwatt of power dissipated across 600 Ohms. 0 dBm is often called milliwatt. 3 dBm refers to a level 3 dB higher than milliwatt, and -3 dBm refers to a level 3 dB less than milliwatt.

Broad spectrum power levels are usually measured in dBm per Hertz (dBm/Hz) rather than dBm. This is often referred to as power spectral density (PSD). Most ADSL DMT is transmitted at -39 to -41 dBm/Hz, and this falls in the range of +15 to +20 dBm.

The following is a simple formula to estimate dBm/Hz when dBm is specified:

(60 is derived from 10 x log 1e6.)

Example:

For an ADSL transmit power level of 20 dBm: P = 20 - 60 = -40 dBm/Hz.

To estimate dBm when dBm/Hz is specified:

For an ADSL power of -41 dBm/Hz: P = -41 + 60 = 19 dBm

NOTE This shortcut formula is for the full ADSL spectrum only, and is an approximation for demonstration purposes. This shortcut factor of 60 must not be used for G.Lite, SDSL, G.SHDSL, and so on.

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