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For instance, ADSL is affected by several physical factors that accompany signaling through copper pair wiring. The most important is line attenuation. Additionally, the amount of electro-magnetic interference that copper is s ubjected to--especially from within the phone company itself--can cause reliability and throughput problems.

Line-Coding Techniques Three types of line coding can be implemented within ADSL modems: Discrete Multitone (DMT), Carrierless Amplitude/Phase modulation (CAP) and a less-commonly implemented Quadrature Amplitude Modulation (QAM). These modulation techniques are used to establish and maintain a signal between two ADSL modems. Among the jobs they perform are negotiation for rate adaption, channel identification and error correction between modems.

Of the three, DMT is considered superior, offering better performance, more flexible bandwidth options and an easier implementation. For this reason, the American National Standards Institute (ANSI) has adopted it as the standard for ADSL line coding.

However, some don't agree that DMT is better than CAP, so we decided to include both in our tests. Although the modem units used in our tests are early releases, all performed at exceptional levels. Overall, we found that the DMT-based ADSL modems were robust in signaling and were able to perform over longer distances (up to 18,000 feet).

Note that line-coding techniques are only of concern from one modem to the next (from your site into y our carrier's central office, for instance). If these devices are used in packet-switching networks, such as the Internet, you don't have to worry about conflicts between your sites.

We used 24-gauge copper pair wiring and found that the line signal attenuated at about 2 decibels (dB) to 3 dB per 1,000 feet. The specification of ADSL calls accounts for distances of up to 12,000 feet (roughly -20 dB), but good ADSL modems can perform reliably for much greater distances.

In fact, we found that most of the modems achieved distances of up to 15,000 feet (-26 dB). The DMT-based ADSL modems in our tests were able to operate up to our maximum cable distance of 18,000 feet at a rate of 791 Kbps downstream and 582 Kbps upstream (with a measured line attenu ation of 31 dB).

The CAP-based modems operated at a full speed of up to 4 Mbps downstream and 422 Kbps upstream until 12,000 feet was reached for one of the modems, and 15,000 feet for the other modem, which reached lower speeds of up to 2.2 Mbps.

We also performed other tests that are common to real-life copper deployment by phone companies, such as introducing bridge taps into the line. A bridge tap is an open-circuited branched phone line split off the main line for another extension line. This line mostly goes unused and doesn't add crosstalk to the main, but it greatly affects the attenuation of the main. We were surprised that some of the modems in our test could handle up to a 5,000-foot bridge tap at 12,000-foot main, and as much as a 1,000-foot tap at 15,000-foot main before the reliability of the signal started to degrade beyond an acceptable level.

Line Signal Interference Near-end crosstalk (NEXT) and far-end cro sstalk (FEXT) are forms of electromagnetic interference that cause the ADSL waveforms to distort and corrupt, thus affecting the decoding of the signal. This type of interference can be introduced at either end of the connection when the ADSL copper wire is adjacent to another line carrying a s ignal, such as a T1 line or another ADSL connection.

The electromagnetic field of wires bleeds into others, causing signals to couple in one direction or another, causing bit errors. For the modems in our tests, this bleed proved to be of minimum consequence as we streamed data through an ADSL line adjacent to a busy T1 line. This was done without compromising our ADSL signaling or T1 line. In a phone company's central office, the effects would most likely be much greater as there would most likely be multiple T1 lines and ADSL lines bundled together. The phone company would have to take care in wiring the centrally located ADSL circuits.