How it Works
ADSL and ADSL2 make use of a 1,104 kHz band, divided into 256 channels (also called tones or bins) numbered zero through 255, each with a 4.3125 kHz bandwidth. Channels six through 31 are used for upstream (upload) while channels 33 through 255 are used for downstream (download). Channel one is used for voice (i.e., conversation using a regular phone set connected to the ADSL phone line). Channel zero is not used. Channels two through five are not used in order to create a separation between the voice and the data channels. Channel 32 is not used, in order to create a separation between the upstream and downstream bands. See Figure 1. Channels 16 and 64 are not used as well; this way, the upstream and downstream bands have 25 and 222 usable channels, respectively.
ADSL2+, on the other hand, uses a 2,208 kHz band, doubling the number of available channels. In the regular ADSL2+ standard, all additional channels are used for downstream, as you can see in Figure 2. In the ADSL2+ Annex M, however, the upstream band uses more channels, six through 56, and the downstream band uses channels 60 through 511. There are now three unused channels separating the upstream and downstream bands, as you can see in Figure 3. Similarly to ADSL and ADLS2, there is one channel in each band that is reserved and cannot be used.
This division of the available band into channels is known by several names, such as Frequency-Division Multiplexing (FDM), Orthogonal Frequency-Division Multiplexing (OFDM), and Discrete Multi-Tone (DMT).
When a connection is established, modems test the signal-to-noise ratio of each channel, in order to determine the maximum transfer rate possible. Each 3 dB of signal-to-noise ratio allows the transmission of one data bit. For example, a channel with a signal-to-noise ratio of 24 dB can transfer up to eight data bits per signaling element. The configuration of how many bits a channel can carry is called bits/bin.
On the standard ADSL, each channel must have a minimum signal-to-noise ratio of 6 dB, i.e., be able to carry two data bits. On ADSL2 and ADLS2+, the minimum is 3 dB, meaning one data bit. Channels with a signal-to-noise ratio below 6 dB (ADSL) or 3 dB (ADLS2 and ADSL2+) cannot be used and are marked as unusable. As you can see, one of the main differences between ADSL2 and ADSL is the ability, on ADSL2, to use channels that would be marked unusable on ADSL.
This signal-to-noise ratio test is done in order to check whether each channel can be really used and the transfer rate that is available at each channel. This will depend on the conditions of the telephone wiring from the installation point to the telephone company, the length of the wires, and the interference generated by AM radio and other sources of interference.
Each channel can transmit up to 15 bits, which equals to a maximum transfer rate of 56 kbps. In other words, each channel is equivalent to a traditional analog phone line, as long as the channel presents a signal-to-noise ratio of at least 45 dB. If the channel has a low signal-to-noise ratio, less bits can be carried per signaling element, lowering the speed of the channel, as explained.
ADSL and ADLS2 connections have a maximum theoretical transfer rate of 12,432 kbps (roughly 12 Mbps; 222 usable channels x 56 kbps per channel) for downstream, but limited to 8 Mbps on the regular ADSL connection. For upstream, ADSL and ADLS2 have a maximum theoretical transfer rate of 1.4 Mbps (25 usable channels x 56 kbps per channel).
ADSL2+ offers more channels, so its maximum theoretical transfer rate for downstream is of 26,768 kbps (roughly 26 Mbps, but limited to 24 Mbps; 478 usable channels x 56 kbps per channel). Its upstream transfer rate is the same as the regular ADSL connection, since it uses the same number of channels.
ADSL2+ Annex M offers a maximum theoretical transfer rate of 25,256 kbps for downstream (roughly 25 Mbps, but limited to 24 Mbps; 451 usable channels x 56 kbps per channel) and 2,800 kbps for upstream (2.8 Mbps; 50 usable channels x 56 kbps per channel).
Note: The exact number of guard (i.e., reserved) channels varies according to the phone company, so the maximum theoretical speeds vary according to the actual number of available channels.
However, the effective transfer rates can be far lower than the numbers presented. First, obviously you can hire a service with a lower speed. And, second, we saw that when a connection is established, modems test the signal-to-noise ratio of all channels to see how many bits can be transferred and, therefore, the maximum speed supported by each channel. In the calculations presented above, we assumed that all channels available could be used at their maximum transfer rates, which may not happen.
Transfer rates achieved by ADSL connections can vary drastically from one installation point to another, as the transfer rate depends, mostly, on the quality and length of the wires connecting the installation point to the phone company.
Another feature available on ADSL2+ connections not mentioned before is the ability to bond together more than one phone line. If two ADSL phone lines are bonded, we have a maximum theoretical download speed of 48 Mbps. In order for this feature to be used, the ADSL equipment at your home and at the phone company must support it.