Performance

On the outside, ADSL looks simple and transparent, but on the inside there is a miracle of modern technology…

In ADSL, downstream data rates can be between 1.5 Mbps and 8 Mbps, while upstream data rates are between 16 Kbps and 832 Kbps. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are also available.

Known Problems & Solutions

The factors that can influence the downstream data rates are the length of the line, the gauge of the line, presence of bridged taps and crosstalk from other wires in the same time that cause noise. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Also, some copper loops use different gauge wires at different points and this can cause reflections in the signal, effectively attenuating some frequencies. Ignoring bridged taps, ADSL will perform as follows:

Data Rate	Wire gauge	Wire size	Distance
1.5 or 2 Mbps	24 AWG	0.5 mm	5.5 km
1.5 or 2 Mbps	26 AWG	0.4 mm	4.6 km
6.1 Mbps	24 AWG	0.5 mm	3.7 km
1.5 or 2 Mbps	26 AWG	0.4 mm	2.7

Table 1. ADSL performance over different loop conditions

 Some solutions to the problems that influence the data rate in ADSL:

1. Placing an optical network unit closer to the neighborhoods which are located too far from the central office to obtain useful ADSL connection speeds. Data is transmitted over fiber to the optical network unit, which then distributes the signals to ADSL modems for connection to the residences.
2. Most ADSL providers select a series of speeds that are smaller than the maximum so that more customers can obtain higher speeds, because the speed varies with the length and the gauge of the line.
3. ADSL modems incorporate forward error correction that dramatically reduces errors caused by impulse noise. Error correction on a symbol-by-symbol basis also reduces errors caused by continuous noise coupled into a line.
   

Next: Framing