Asymmetric Digital Subscriber Line

ADSL Capabilities

Many of the capabilities being considered with the DSL family are the services for converging voice, data, multimedia, video, and Internet streaming protocols services. The carriers see their future in the rollout of products and services to the general consuming public so they can access the Internet. Remember that the speeds and distances shown in this table are theoretical. If the copper has been damaged or impaired in any way, the speed and distances will change accordingly (downward). In reality the actual distances and speeds will most likely be less than those shown in this table.

Unshielded twisted-pair copper cable used for POTS has been given a stay of execution. The high cost of fiber to the home or even to the curb has given the copper pair infrastructure a life extension of several years. Innovations in the modem laboratories now allow the transmission of several megabits per second over regular copper pairs, which opens the door for video on demand and wideband Internet connections.

Asymmetric, describes the difference between broadband downstream transmission (1 to 9 Mb/s) and narrowband upstream transmission (64 to 800 kb/s). This technique eliminates the main bit-rate-limiting problem for copper pairs known as near end crosstalk (NEXT) where the signal transmitted along a cable on one pair interferes with incoming signals on adjacent or other pairs. In doing so, it creates a far end crosstalk (FEXT) problem where signals interfere with each other as they proceed along different pairs, so that by the time they reach the far end, the crosstalk is received and can cause severe distortion. The maximum bit rate for FEXT-limited transmission is still in the megabits-per-second region for 0.5-mm (24 gauge wire. The wire attenuation, which increases with frequency as the skin effect resistance becomes dominant, is still a limiting factor for distance. In the United States, about 50 percent of lines extend to 12,000 ft (3.66 km) and 80 percent to 18,000 ft (5.49 km), with splices at about 500 ft (152 m) on average. In Europe, 5.5 km would reach more than 90 percent of customers.

An important requirement is that ADSL shall coexist on the same line as existing telephony. Telephony uses tens of volts for ringing and other pulses, whereas ADSL signals at 25 kHz and above are less than 1 mV. Separation filters must have very high stopband attenuation to minimize the echo and sidetone interference of ADSL on telephony. Notice how the use of asymmetric echo cancellation allows the upstream and downstream spectra to overlap and make better use of the lower loss region for some of the downstream transmission.

The major technology required for ADSL is highly bandwidth efficient modulation. Three contenders were identified as candidates: QAM, carrierless amplitude/phase modulation (CAP), and discrete multitone modulation (DMT). CAP is very similar to QAM in its spectral shape, the difference being that the two half-rate bit streams are each passed through a digital transversal filter that has the same amplitude response but an orthogonal (differing by Tr/2) phase response instead of each stream being mixed with a sine and cosine carrier. Bandwidth efficiencies in the region of 8 b/s/Hz are used for several megabits-per-second downstream performance. This is equivalent to 256-QAM or higher.

DMT is a type of multicarrier modulation that divides time into symbol periods, each carrying a specific number of bits. Serial-to-parallel converters are used to split up the bit stream for modulation on several separate carriers. Groups of bits are assigned to different carriers and the bits for each carrier set the carrier's amplitude and phase for the duration of the symbol period.           

This is equivalent to several QAM systems operating in parallel, and the level of QAM on each carrier depends on the number of bits assigned to the carrier. This multicarrier modulation requires the carriers to be orthogonal, and fast Fourier transform (FFT) techniques are used to achieve DMT.

The number of bits that can be sent on each carrier varies as the noise or interference at that frequency changes. This allows adaptive bit rate variation to optimize performance. The symbol rate might vary from, say, 2 or 3 b/Hz to as much as 15 b/Hz depending on the frequency. DMT has received the vote of confidence from manufacturers and standards bodies alike. It has an inherent immunity to impulse noise, and larger bandwidth capability and higher transmission rates, particularly for the shorter loops. DMT has some disadvantages. The FFT processing causes transmission delays that might violate ISDN specifications (<1.25 ms). Echo cancellation is not trivial for DMT. In addition, FEC, which is used to enhance performance, is more complex for DMT than for CAP or QAM.

Considering that ADSL is a relatively new technology, its performance is already very impressive. Using 0.5-mm cable, 2 Mb/s over 5.75 km, 4 Mb/s over 4 km, and 6 Mb/s over 3.6 km for downstream transmission ranges are known to be successful. These figures are based on a pessimistic noise source including crosstalk and radio frequency interference (RFI) and include a 6-dB safety margin. The range performance values can be increased by additional signal processing, again at the expense of transmission delay. For example, a performance enhancement of 5 to 6 dB can be achieved by the combined use of (l) trellis coding, (2) echo cancellation, and (3) optimized transmit power distribution.