DSL (G.SHDSL). The table lists their characteristics:

ADSL is designed to deliver more bandwidth downstream than upstream and supports data and voice simultaneously over existing copper lines. ADSL is oriented toward residential subscribers, where usually more bandwidth is required in the downstream for applications such as downloading music and movies, playing online games, surfing the Internet, or receiving e-mail with large attachments. The downstream rate ranges from 256 kbps to 8 Mbps, while upstream speed can reach 1 Mbps. ITU-T Recommendation G992.1 and ANSI Standard T1.413-1998 specify full rate ADSL.
RADSL is ADSL service with a data transmission rate that can be adapted to the local loop line conditions. Specifically, if noise or signal loss affects downstream speeds, upstream speeds will adjust downwards to as low as 64 kbps to provide better downstream performance. This allows it to function over longer distances.
VDSL can provide symmetrical or asymmetrical services. The downstream bandwidth ranges from 13 Mbps to 52 Mbps. Like ADSL, VDSL also supports data and voice over a single copper line. Cisco Long Reach Ethernet (LRE) solution is based on Ethernet over VDSL.
IDSL transmits data digitally (rather than via analog) on a twisted-pair copper telephone line across existing ISDN lines. IDSL delivers up to 144 kbps of symmetrical bandwidth derived from two bearer channels (B-channels at 64 kbps each) plus the signaling channel (D-channel at 16 kbps). This configuration makes IDSL essentially a leased-line ISDN BRI in which there is no D channel. IDSL does not support voice; it can only carry data but has an advantage over ISDN in that it is always on.
SDSL delivers 768 kbps both downstream and upstream over a single copper twisted pair. SDSL technology is proprietary and non-standardized, and can only carry data. The symmetrical nature of SDSL makes it ideal for commercial use when the end user must send large amounts of data by applications such as e-mail messaging to customers with large attachments, uploading data to corporate servers, or updating web pages.
HDSL delivers 1.544 Mbps or 2.048 Mbps of symmetrical bandwidth over two copper twisted pairs. Service providers have been using HDSL as a substitute for T1 and E1. HDSL only carries data.
G.SHDSL offers symmetrical data rates from 192 kbps to 2.3 Mbps. The International Telecommunication Union (ITU) developed and standardized G.SHDSL to address the worldwide SDSL market.

Content 2.4 Describing DSL Technology 2.4.4 Factors Affecting DSL Performance All DSL types are limited in distance and speed. Speed is inversely proportional to distance. That is, a longer distance in the local loop means a lower maximum speed that a particular DSL connection supports. Various impairments in the local loop that attenuate or distort the signal also affect the maximum speed of certain DSL connections:

Signal attenuation: Attenuation means signal loss over distance and is determined by the distance between a subscriber and the CO. The longer the distance between the two, the more attenuation occurs and therefore the lower the speed.
Bridge tap: A bridge tap is an extra telephone wire with an un-terminated cable end connected to the local loop. Such an un-terminated tap can cause noise and reflections and can radiate power that reduces signal strength and, consequently speed. DSL providers should remove bridge taps before installing a DSL connection.
Load coil: Provisioning of loading coils was a standard procedure used to improve plain old telephone service (POTS) voice quality on longer local loops. This procedure is called “conditioning” the loop. A loading coil is a wrap of wire placed at specific intervals along the local loop that extends the local loop distance. This wire creates a low-frequency band pass filter and cuts off, or blocks, the DSL frequencies. For the DSL to operate, load coils must be removed from the loop.
Wire gauge: Wire gauge is the thickness of the wire that is used in the local loop. For higher speeds, thicker wire is used.
Impedance mismatch: Noise or an echo in the local loop that is caused by changes in wire gauge, wire splices, or corrosion is called impedance mismatch.
Crosstalk: Crosstalk is the interference between two wires in a bundle. Electrical energy causes crosstalk.
AM radio interference: AM radio frequencies can interfere with a DSL signal and reduce speed. The interference is particularly a problem with in-house wiring when untwisted or poorly twisted wiring exists.
Fiber-optic cable: ADSL signals cannot pass through the conversion from analog to digital to analog that occurs if a portion of the telephone circuit traverses fiber-optic cable in transit.

Content 2.4 Describing DSL Technology 2.4.5 DSL Distance Limitations The table in Figure summarizes the maximum data rate and operational reach for each xDSL technology from the central office (CO), assuming that there are no defects or impairments in the copper wiring. The maximum data rate describes the maximum achievable downstream and upstream bandwidth with the shortest operational distance (distance between the subscriber and the CO). The maximum operational reach is the maximum achievable distance with the lowest operational data rate. The relationship between bandwidth and distance is inversely related. ADSL reaches greater distances than other DSL types, but the achievable speed of ADSL transmissions degrades as the distance increases. The maximum distance is limited to approximately 18,000 feet (5.5 km) from the CO. ADSL2 and ADSL2+ are enhancements to basic ADSL and provide a downstream bandwidth of up to 24 Mbps and an upstream bandwidth of up to 1.5 Mbps. VDSL offers the highest operational speed but has the shortest achievable distance. For VDSL to support the maximum speed of 52 Mbps, the subscriber has to be very close to the CO—within 1000 feet (300 meters). The maximum operational distance is 4500 feet (1.4 km). The maximum operating distance of IDSL is limited to 18,000 feet (5.5 km). An IDSL line can be configured for a speed of 64 kbps, 128 kbps, or 144 kbps. The line coding mechanism you use for IDSL is two binary, one quaternary (2B1Q). This line coding mechanism allows transparent operation through an ISDN interface. The use of a single twisted pair wiring limits the operating range of SDSL to about 22,000 feet (6.7 km) from the CO. The operating range of HDSL is limited to approximately 12,000 feet (3.7 km) from the CO. The maximum operational distance supported by G.SHDSL is about 28,000 feet (8.5 km) from the CO. G.SHDSL offers greater reach than other deployed DSL technologies do.

Content 2.5 Deploying ADSL 2.5.1 ADSL Figure shows how ADSL exists on the same twisted-pair telephone line as the POTS. Three information channels usually exist over the same wiring (depending on the variety of ADSL): a POTS channel for analog voice, a lower-speed ADSL upstream data channel, and a high-speed ADSL downstream data channel. A user can use the phone line and the ADSL connection simultaneously without adverse effects on either service. ADSL has asymmetric data rates, with higher data rates toward the user (downstream) and lower data rates toward the carrier (upstream). The distance between the end user and the CO provides the guideline for line speeds. Downstream, ADSL supports speeds up to slightly more than 8 Mbps. For upstream, the rate is approximately 1 Mbps. The maximum distance for the maximum upstream rate is 18,000 feet (5.5 km) over a one-wire pair without repeaters on an optimized loop. The maximum downstream speed can be achieved at distances up to 12,000 feet (3.7 km) using standard 0.6 mm (24-gauge) wire on an optimal loop. Standardized in 2004, newer ADSL variants offer improvements over regular ADSL:

ADSL2 (ITU G.992.3/4) offers higher downstream rates of up