ADSL and DSL Technologies: A Comprehensive Overview

ADSL and DSL Technologies

ADSL (Asymmetric Digital Subscriber Line)

Advantages

• User: High-speed access, permanent connection, fast service provisioning.
• Telephone Company: Dual function of existing cable, no central office occupation, no risk of switched network collapse, service installed only on requested lines.

Disadvantages

• Service unavailability on certain lines (poor condition, distance from the central office).
• Potential for poor quality home cabling.
• Router cost.
• Requires a phone line.
• Cost in Spain.
• Theoretical limit for acceptable service at 5.5 km line length.

ADSL2+

Extends ADSL downstream capability with bit folding, achieving speeds of up to 24 Mbps downstream and 3.5 Mbps upstream. Doubles bandwidth from 1.1 MHz to 2.2 MHz.

ATM over ADSL

Allows multiple permanent virtual channels for different services with varying speeds and quality of service parameters.

Bandwidth Division

• Phone Band: 0 to 25 KHz (3.1 KHz used by ADSL to avoid interference).
• Upstream Data Band: 25 to 200 KHz.
• Downstream Data Band: 250 KHz to 1 Mbps.

ADSL Evolution in Spain

• 1999: Terra introduced 128 Kbps/256 Kbps.
• 2002: Doubled to 128 Kbps/512 Kbps.
• 2004: Increased to 300 Kbps/1024 Kbps. Current speeds range from 1 Mbps to 20 Mbps.

HDSL (High-bit-rate Digital Subscriber Line)

Developed in the late 1980s, offering speeds up to 2312 Kbps. With 24-pair cables, it can cover distances up to 3.6 km between the user and PBX.

RADSL (Rate-Adaptive Digital Subscriber Line)

Modem adjusts bandwidth dynamically, creating a wider downstream frequency band. More tolerant of noise and signal loss. Upstream speeds range from 128 Kbps to 1 Mbps, downstream up to 7 Mbps, and supports line lengths up to 5.4 km.

SDSL (Symmetric Digital Subscriber Line)

Offers symmetrical speeds up to 2048 Kbps. Can be provided asymmetrically (e.g., 52 Mbps downstream and 12 Mbps upstream) or symmetrically (e.g., 26 Mbps downstream and 26 Mbps upstream).

VDSL (Very-high-bit-rate Digital Subscriber Line)

Offers speeds up to 55 Mbps with a maximum distance of 1.5 km.