The PSTN and supporting local access networks were designed with guidelines that limited transmissions to a 3,400 Hz analog voice channel. For example, telephones, dial modems, fax modems and private line modems limited their transmission over the local access phone lines to be in a frequency spectrum that exists between 0 Hz and 3,400 Hz. The highest achievable information rate using that 3,400 Hz frequency spectrum is less than 56 Kbps.

So, how does DSL technology achieve information rates in the millions of bits per second over those same copper loops?

The answer is simple – eliminate the 3,400 Hz boundary. DSL, much like traditional T1 or E1, uses a much broader range of frequencies than the voice channel. Such an implementation requires transmission of information over a wide range of frequencies – from one end of the copper wire loop to another complementary device, which receives the wide frequency signal at the far end of the copper loop.

Now, recognizing that we can choose to eliminate the 3,400 Hz frequency boundary and dramatically increase the information rates supported on copper wires, you may be asking, "Why don’t we just ignore POTS transmission guidelines and use the higher frequencies?" The answer can get far more complex than we want to cover here, so we will consider the three dominant issues associated with this question:

• Attenuation: The dissipation of the power of a transmitted signal as it travels over the copper wire line. In-home wiring also contributes to attenuation.

• Bridged taps: These are unterminated extensions of the loop that cause additional loop loss, with loss peaks surrounding the frequency of the quarter wavelength of the extension length.

• Crosstalk: The interference between two wires in the same bundle, caused by the electrical energy carried by each.

Attenuation and Resulting Distance Limitations

One might compare the transmission of an electric signal to driving a car. The faster you go, the more energy you burn over a given distance and the sooner you have to refuel. With electrical signals transmitted over a copper wire line, the use of higher frequencies to support higher-speed services also results in shorter loop reach. This is because high-frequency signals transmitted over metallic loops attenuate energy faster than the lower-frequency signals.

One way to minimize attenuation is to use lower-resistance wire. Thick wires have less resistance than thin wires, which in turn means less signal attenuation and thus, the signal can travel a longer distance. Of course, thicker-gauge wire means more copper, which translates into higher per-foot plant costs. Therefore, telephone companies have designed their cable plant using the thinnest gauge wire that could support the required services.

In the US, wire thickness is represented by the denominator composed of the fraction of an inch in wire size, assuming a numerator of 1. Therefore, a wire that is 1/24 inch in diameter is referred to as 24 American Wire Gauge (AWG). Wire gauges of 24, and more often 26, are present in most North American cable plants. The design rules used by nearly all telephone companies provided for a change in wire gauge, with a thinner gauge used near the entrance of a central office to minimize physical space requirements and changing to thicker gauges over long loops to maximize loop reach.

In most markets outside North America, wire gauges are referred to by their diameter in millimeters. For example, 0.4 mm, which is comparable to 26 gauge, and 0.5 mm, which is comparable to 24 gauge, are the most common; although in many developing countries, heavy gauges of 0.6 mm to 0.9 mm can be found in newly urbanized areas. This variation in wire gauge adds to the challenge of determining a particular DSL system’s performance over a particular loop.

Advanced Modulation Techniques minimize Attenuation

In the early 1980s, equipment vendors were working aggressively to develop Basic Rate ISDN, which would provide up to two 64 Kbps B-channels, plus a 16 Kbps D-channel used for signaling and packet data. The information payload, plus other overheads associated with implementation, resulted in 160 Kbps in total transmitted information. A key requirement of ISDN was that it had to reach customers over the existing non-loaded copper wire loops, equating to 18,000 feet. However, an AMI implementation of Basic Rate ISDN would require use of the lower 160,000 Hz, which resulted in too much signal attenuation and would fall short of the required 18,000 feet loop reach of 26 gauge wire.

By 1988, advancements in signal processing and line coding doubled the effectiveness of the legacy AMI code by sending two bits of information with each cycle of an analog waveform or baud. The line code was called 2 Binary, 1 Quaternary (2B1Q). A 2B1Q implementation of Basic Rate ISDN used frequencies ranging from 0 to approximately 80,000 Hz, which has less attenuation and results in the desired 18,000-foot loop reach.

HDSL enters the Scene

In the early 1990s, some vendors encouraged the use of 2B1Q at higher speeds as an alternate way to provisioning T1 and E1 services, without repeaters. The technique consisted of splitting the 1,544,000 bps service into two pairs (four wires), which each ran at 784,000 bps. By splitting the service across two lines and increasing the bits per baud, the per-line speed and resulting need for frequency spectrum could be reduced to allow longer loop reach. This technique was referred to as High-bit-rate Digital Subscriber Line or HDSL. The result was that an HDSL-based DS-1 service could be implemented over Carrier Serving Area (CSA) specified loops of up to 12,000 feet long (assuming 24 gauge; or 9,000 feet with 26 gauge wire), with no repeaters.

The early 2B1Q-based E1 HDSL initiatives split the 2.048 Mbps service across three wire pairs (a total of six wires) in an effort to achieve the targeted loop reach. As the technology matured and performance improved, E1 HDSL implementations migrated to a two-pair (four-wire) implementation, each operating at 1.168 Mbps, which was similar to the T1 implementation.

In parallel with the 2B1Q initiative, Paradyne began development of a similar HDSL transceiver using a line code called Carrierless Amplitude and Phase (CAP) modulation. Like 2B1Q, CAP was an advanced line-coding technique allowing multiple bits of information to be represented by a single frequency cycle or baud. However, CAP could be designed to transmit multiple bits ranging from two to nine bits per baud. This enabled CAP-based transceivers to transmit the same amount of information using a lower range of the frequency spectrum than 2B1Q, equating to less signal attenuation and greater loop reach. As a result of 2B1Q’s proven market acceptance with ISDN and CAP’s performance benefits, both line codes were endorsed with technical reports by both the American National Standards Institute (ANSI) and European Telecommunications Standardization Institute (ETSI) standards committees for HDSL.

There are some instances where vendors have developed HDSL products using line codes other than 2B1Q or CAP.

However, these examples are isolated and alternative line codes are not recognized by the standards organizations.

The higher-frequency signals associated with the AMI implementation get weak sooner than the HDSL transmissions. As a result, the CAP and similarly 2B1Q HDSL systems have substantially longer loop reach than AMI or HDB-3 based T1 or E1 systems, respectively.

Bridged Taps

Bridged taps are unterminated extensions for the loop that cause additional loop loss, with loss peaks surrounding the frequency of the quarter wavelength of the extension length. Since wavelength and frequency have an inverse relationship, short bridged taps have the greatest impact on wideband services, while long bridged taps have a greater impact on narrowband services. Most loops contain at least one bridged tap, and the effect of multiple taps is cumulative. Premises wiring contain additional bridged taps. The additional loss created is greatest on short bridged taps; consequently, technologies that operate at lower frequencies are less impacted.