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Attenuation results from the signal strength falling off with distance. It depends on medium, the received signal strength:must be enough to be detected and must be sufficiently higher than noise to be received without error.
Attenuation distortion arises when attenuation is different for different frequencies and is afunction of frequency.
Delay distortion, which only occus in guided media, is due to the propagation velocity varying with frequency and some components of one bit position may spill over into
another bit position; causing inter-symbol interference. This is a major limitation to maximum bit rate.
The line attenuation figure describes the distance between the end-user and the exchange of the ADSL provider. It is expressed in dBs and can be found (usually) as part of the ADSL statistics of the router. The lower this number is, the better:
20dB and below is outstandingThe speed of the ADSL line is directly affected by attenuation.
20dB - 30dB is excellent
30dB - 40dB is very good
40dB - 50db is OK
50dB and above is bad
The decibel (dB) is a logarithmic unit that specifies the ratio of two powers. It was devised by the Bell Telephone Company to simplify calculations of gain and loss in cascaded transmission systems. The original unit was the Bel, and defined as:
Number of Bels = logarithm (base 10) of the ratioOrdinary twisted-pair telephone lines were never designed for the high frequencies used in ADSL transmission and a lot of the ADSL signal is lost between the exchange and the user.OutputPower InputPower.
Prior to 6 September 2004, the limiting attenuation figure for a UK ADSL installation was 60 dB (i.e. a gain of -60 dB). This is equal to 6 Bels, and therefore the logarithm of the power ratio is 6, the signal at the end of the line is just 10-6 (one-millionth) of the signal transmitted from the exchange. And as from 6 September 2004 the limits were relaxed further, with 75 dB being the level of attenuation at which a usable ADSL service is considered extremely unlikely, a power ratio of about 32 million.
An extension of the dB is the dBm, which is defined as dB referenced to 1 mW (one milliwatt). This is a convenient way to specify a wide range of power levels in fairly small numbers. So the ratio of the actual power of a device to 1 mW is converted to dB, and then the power is specified as that number of dBm.
Most modem/routers report the upstream and downstream power levels in dBm, although in some cases this information is only available by using the telnet interface. The upstream (or local) power is the power transmitted by the modem/router, and the downstream (or remote) power is the power transmitted by the DSLAM or MSAN in the exchange.
The router will probably display statistics for the upstream and the downstream traffic. These statistics will be different, as upstream/downstream speed is different.
The following theoretical relationship exists between the magnitude frequency response and the attenuation function:
| HK(f) | = 10The index K makes it clear, that the considered Linear time-invariant (LTI) system is a cable (German : Kabel). For the first calculation rule, the damping function aK(f) must be used in dB (decibel). Since in the second calculation rule, the damping function αK, Np(f) must be used in Np (Neper).-αK(f) 20= e-αK, Np(f).
The following conversions apply: 1 dB=0.05 * ln(10) Np = 0.1151 Np or 1 Np=20 * lg(e) dB = 8.6859 dB
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