to 12 Mbps for spans of less than 8000 feet (2.5 km).
  • ADSL2+ (ITU G.992.5) provides up to 24 Mbps for spans of less than 5000 feet (1.5 km).
    • Figure lists key ADSL equipment and characteristics:
    Content 2.5 Deploying ADSL 2.5.2 ADSL and POTS Coexistence The major benefit of ADSL is the ability to provide data services with voice services. When the service provider puts analog voice and ADSL on the same wire, the provider splits the POTS channel from the ADSL modem using filters or splitters. This setup guarantees uninterrupted regular phone service even if ADSL fails. When filters or splitters are in place, the user can use the phone line and the ADSL connection simultaneously without adverse effects on either service. Figure shows how ADSL and voice traffic travel back and forth between the CO and the customer premises. ADSL offloads the data (modem) traffic from the voice switch and keeps analog POTS separate from data. Separating voice and data traffic provides fail-safe emergency-call services for POTS operation. The data channel is established between the CPE modem and the CO DSLAM. The voice channel is established between the telephone and the voice switch at the CO premises. ADSL signals distort voice transmission and are split or filtered at the customer premises. There are two ways to separate ADSL from voice at the customer premises: using a microfilter or using a splitter. A microfilter filters the ADSL signal from the voice signal. A microfilter is a passive low-pass filter with two ends. One end connects to the telephone, and the other end connects to the telephone wall jack. This solution eliminates the need for a technician to visit the premises and allows the user to use any jack in the house for voice or ADSL service. POTS splitters separate the DSL traffic from the POTS traffic. The POTS splitter is a passive device. In the event of a power failure, the voice traffic still travels to the voice switch in the CO. Splitters are located at the CO and, in some deployments, at the customer premises. At the CO, the POTS splitter separates the voice traffic that is coming back from the customer and going to the voice switch in the CO and the data traffic that goes to the DSLAM in the CO. Figure uses a splitter at the customer premises. The local loop terminates on the customer premises at the demarcation point in the network interface device (NID). This point is usually where the phone line enters the customer premises. At this point, a splitter is attached to the phone line. The splitter forks the phone line; one branch provides the original house telephone wiring for telephones, and the other branch connects to the ADSL modem. The splitter acts as a low-pass filter, allowing only the 0 to 4 kHz frequencies to pass to or from the telephone. Installing the POTS splitter at the NID usually means that a technician must go to the customer site. Because of this additional labor, most home installations today use microfilters. Also, since the splitter separates the ADSL and voice signals at the NID, there is usually only one ADSL outlet available in the house.
    Content 2.5 Deploying ADSL 2.5.3 ADSL Channel Separation ADSL uses two types of modulation techniques: a single-carrier CAP, which is proprietary, and multicarrier standardized DMT. Figure describes CAP modulation. CAP modulation is easy to implement and was used in many early ADSL installations. CAP-based DSL makes use of three separate channels on the wire by dividing the signals into three distinct bands: The three channels are widely separated to minimize the possibility of interference between them on one line or between the signals on different lines. A single-carrier notation means that only one frequency band carries either an upstream or a downstream channel. CAP is similar to Quadrature Amplitude Modulation (QAM) in how it manipulates the carrier wave to convey data. CAP produces a signal that filters the carrier frequency (suppresses the carrier). This results in less power required in transmission. A single frequency carrier, centered in the middle of the frequency range, is modulated (via CAP) and the resulting output signal with a suppressed carrier has sufficient detail to decode on the receiving end. Due to the use of the entire bandwidth and multibit encoding, the data throughput is quite high. As an example, 256 CAP at 1088 kbaud symbol rate results in 8 Mbps. Figure describes DMT modulation. DMT modulation is standardized with ANSI and ITU—ITU 992.1 (G.dmt), ITU 992.2 (G.lite), and ANSI T1.413 Issue 2. DMT is the prevailing modulation technique used in modern ADSL deployments. As with CAP-based DSL, DMT divides the signals on the wire into separate channels. The main difference is that DMT does not use only two wide channels for upstream and downstream data traffic. DMT divides the frequency band into 256 separate 4 kHz-wide channels. Channels 6 to 38 are duplex and used for both upstream and downstream data traffic. Downstream data traffic uses channels 39 and onwards. Carrier frequencies centered in each channel are each modulated using a complex form of QAM. To compensate for noise, the system constantly monitors each channel. When channel quality decreases, the system adjusts the number of bits per channel. If the quality is too impaired, the signal shifts to another channel. DMT modulation constantly searches for the best channels to use for transmission and reception. The system shifts signals among different channels as required. Implementing DMT modulation is more complex than implementing CAP modulation because DMT modulation uses a large number of channels. However, DMT modulation offers more flexibility when traversing lines of differing quality. G.lite is a less complex version of the DMT standard. G.lite, sometimes called half-rate DMT, uses only half the subchannels (128 channels). Having fewer channels determines a lower maximum downstream speed of 1.5 Mbps and a maximum upstream speed of 640 kbps.
    Content 2.5 Deploying ADSL 2.5.4 Data over ADSL Figure illustrates the