DSL: ADSL, VDSL, G.fast

DSL: Broadband Over Copper

DSL exploits the fact that telephone lines carry much more bandwidth than voice calls use. By using frequencies above 25 kHz, DSL can transmit data while simultaneously carrying voice on the same wire. The challenge: distance matters. Signal degrades over copper, limiting reach and speed.

DMT Modulation: How DSL Works

DSL uses DMT (Discrete Multi-Tone) modulation — essentially OFDM (Orthogonal Frequency-Division Multiplexing). The available frequency band is divided into independent sub-carriers, each modulated with QAM:

• ADSL1: 256 bins at 4.3125 kHz spacing spanning 25 kHz–1.104 MHz. Bins 7–31 carry upstream data (25.875–138.375 kHz), bins 32–255 carry downstream (138.375–1104 kHz). Each bin can carry 0–15 bits per symbol using QAM (up to 32768-QAM on the best bins).
• ADSL2+: 512 bins extending to 2208 kHz, doubling the downstream passband and achieving 24 Mbps. Uses the same 4.3125 kHz bin spacing but doubles the frequency range.
• VDSL2: Up to 4096 bins with profiles 8a/12a/17a/30a defining maximum frequencies of 8.5 MHz, 12 MHz, 17.664 MHz, and 30 MHz respectively.

The DSL modem probes each bin during training, measuring the signal-to-noise ratio (SNR)and allocating bits accordingly. Bins with high SNR carry more bits (up to 15 in ADSL); noisy bins carry fewer or zero bits. This adaptive allocation — called bit loading — maximizes throughput for each unique copper pair. SRA (Seamless Rate Adaptation)in ADSL2+ allows the modem to change bit allocation on-the-fly without dropping the connection.

The Splitter

A DSL splitter (also called a microfilter) separates voice and data frequencies at the demarcation point. It contains:

• Low-pass filter (0–4 kHz): Passes voice to the telephone
• High-pass filter (25 kHz+): Passes DSL data to the modem
• Isolation: >65 dB attenuation between voice and data ports, preventing DSL signals from interfering with phone calls and vice versa

Without a splitter, DSL signals would be audible as high-pitched noise on the phone line, and phone pickups could disrupt the data connection. Some ADSL implementations use in-line microfilters on each phone jack instead of a central splitter.

ADSL: Asymmetric Speeds

Asymmetric DSL (ADSL) was designed for typical home users who download more than they upload:

• ADSL1: 8 Mbps down, 1.3 Mbps up (max), 256 DMT bins, up to 5.5 km reach
• ADSL2: 12 Mbps down, 1.3 Mbps up, improved framing and power management
• ADSL2+: 24 Mbps down, 3.5 Mbps up, 512 bins extending to 2.2 MHz
• Reach: Up to 5 km from DSLAM, degrades rapidly beyond 3 km — at 5 km, speeds drop to 1–2 Mbps

The DSLAM: Central Office Equipment

The DSLAM (DSL Access Multiplexer) is the provider-side equipment that aggregates hundreds or thousands of DSL connections. Located in the central office or a remote cabinet, a DSLAM:

• Ports: Ranges from 192 to 2048 ports per chassis
• Function: Demodulates DMT signals from each subscriber, converts to ATM or Ethernet cells, and aggregates onto high-speed uplinks (GE/10GE)
• Line cards: Each card handles 16–64 DSL lines with dedicated DSP processors per port
• Distance: The DSLAM-to-subscriber distance (loop length) is the primary determinant of DSL speed

VDSL2: Very-high-bit-rate DSL

VDSL2 was designed for fiber-to-the-cabinet (FTTC) deployments where copper runs are short:

• VDSL1: 52 Mbps down, 16 Mbps up
• VDSL2: Up to 300 Mbps symmetric (bonded pairs), profiles 8a/12a/17a/30a
• Reach: Only 300–1500 meters from cabinet — speed degrades sharply with distance
• Vectoring (G.993.5): Uses far-end crosstalk (FEXT) cancellation to boost speeds 50–100%, similar to noise-canceling headphones but for copper pairs

G.fast: Copper's Last Hurrah

G.fast (ITU G.9701) uses frequencies up to 106 MHz (or 212 MHz with the 212 MHz profile) to achieve:

• G.fast: 500 Mbps–1 Gbps over short copper loops (<100m)
• GigaDSL: 2–5 Gbps using 212 MHz profile with 4096-QAM
• Target: Fiber to the curb (FTTC) with 100m copper drop — fiber brings data to the neighborhood node, G.fast bridges the last 50–100 meters
• Limitations: Extremely distance-sensitive — performance drops to <100 Mbps beyond 200m

DSL Technologies Explained

• ADSL: Asymmetric — download prioritized, 256–512 DMT bins
• SDSL: Symmetric — equal upload/download, single pair, business-oriented
• HDSL: High-bit-rate — 2B1Q encoding, T1/E1 alternatives over existing copper
• VDSL: Very-high-bit-rate — short loops only, profiles 8a/12a/17a/30a
• VDSL2: Extended VDSL with bonding and vectoring
• G.fast: Ultra-short loop — copper's final performance push