Discrete multitone technology
The visual shows the trick behind the standard discrete multitone technology (DMT) line code used for ADSL (and G.lite). Instead of 256 V.34 modems all running in the voice passband of 0–4 kHz requiring 256 separate local loops—DMT chipsets establish 256 subcarriers on the same local loop pair. So DSL modems are truly modems, in the sense that digital information is sent as analog waveforms.
In DMT, the subcarriers are usually called bins. The 256 bins allow all 256 V.34 modems to share one local loop, mainly by running bin 1 at 0–4 kHz, bin 2 at 4–8 kHz, bin 3 at 8–12 kHz, and so on up to a little more than 1 MHz for bin 256. Each bin essentially has a separate V.34 modem using analog quadrature amplitude modulation (QAM) line coding that can train as fast as 33.6 kbps, but much lower if necessary, depending on the line conditions in each bin. This gives DMT an inherently rate adaptive digital subscriber line (RADSL) nature. Once DMT became the standard ADSL/G.lite line code, all standard ADSL modems effectively became RADSL modems automatically.
All 256 modems sit on a single ADSL modem board sharing a single chipset. Most of the complexity of DMT is figuring out what speed each bin is running at and then properly inverse multiplexing the single buffer at each end onto and off of the single local loop. There was some resistance to DMT initially among vendors due to its complexities.
DMT chipsets require a lot of power and computing power as well. 256 modem chips also give off a lot of heat. The evolution of DMT has just begun, though, and new efficiencies are introduced every few months.
DMT for ADSL
When DMT is used for ADSL, a 1.1 MHz bandwidth on a local loop is divided into 256 bins of 4.3125 kHz each. Some guard band is needed between each subcarrier, which is why this is not exactly 4 kHz. Technically, the total bandwidth used is 1.104 MHz (256 x 4.3125 kHz).
DMT bins are numbered 1–256 (not 0–255) and bin 1 starts at 0–4.3125 kHz. The first 5 bins are not used, since analog voice support is always part of ADSL. Starting the data at bin 6 (25.874 kHz) gives the voice a wide berth and prevents harmonics and crosstalk to some extent.
Upstream ADSL traffic uses bins 6 through 38 (at about 200 kHz). The unequal split of bins gives ADSL its distinctive asymmetrical character. Originally, downstream traffic used bins 39–256, but there was a problem with this simple frequency division multiplexing (FDM) scheme. Attenuation (i.e., signal loss) is always more serious the higher the frequency. Only more power (technically, the signal-to-noise ratio) can improve performance. That was the reason the upstream bins were placed below the downstream bins: less attenuation translated into lower powered consumer devices. The DSLAM could employ higher signal strengths to reach the home. However, early DMT trials showed that many of the higher bins, from the 240s on, were just about useless. So downstream traffic can now overlap with the upstream bins, meaning bins 7–256 can be used for downstream traffic as well. Some bins are reserved for pilot tones that help with the inverse multiplexing process. These are bins 16 (about 69 kHz) and 64 (about 276 kHz) for upstream and downstream, respectively.
Each bin will train independently at up to 33.6 kbps. So the bins can all differ in terms of “bits per bin,” and usually do. Some bins are totally turned off, especially those closer to 1 MHz. This wide range of possible speeds is referred to as ADSL granularity. Most vendors will limit granularity to about 32 kbps.
Sometimes line conditions will change during a connection. Since ADSL is always on, this can be a problem unless the bins can retrain on the fly. Early DMT chipsets did indeed fail the whole link and retrain everything, taking anywhere from 16 to 30 seconds to do so.