VDSL/VDSL2
Very-high-bit-rate Digital Subscriber Line for faster speeds over shorter distances.
The Next Leap in Copper Broadband
VDSL (Very High-speed Digital Subscriber Line) represents a significant evolution in the xDSL family, designed specifically to deliver ultra-high speeds over the short copper wires that constitute the final connection to a subscriber's home or office. It pushes the physical limits of copper lines far beyond what was achievable with earlier ADSL technologies, serving as a critical bridge technology for delivering fiber-like speeds before full fiber-to-the-home deployment.
The core principle of VDSL is to use a much wider frequency spectrum than ADSL, which allows for packing significantly more data into the signal, but at the cost of a much shorter effective range.
The VDSL Spectrum: Expanding the Highway
The primary method VDSL uses to achieve its impressive speeds is by aggressively expanding the range of frequencies used on the copper pair. While ADSL2+ maxed out at 2.2 MHz, VDSL and its successor, VDSL2, pushed this boundary much further.
Key VDSL Generations:
- VDSL1 (ITU-T G.993.1): The first generation of VDSL technology, utilizing frequencies up to 12 MHz. It allowed for symmetric speeds (e.g., 26 Mbps) or asymmetric speeds (e.g., 52 Mbps downstream / 16 Mbps upstream) over very short distances.
- VDSL2 (ITU-T G.993.2): The dominant standard, VDSL2 significantly extends the frequency spectrum, defined by various profiles. Common profiles include:
- Profile 17a: Uses up to 17.664 MHz.
- Profile 30a: Uses up to 30 MHz.
- Profile 35b (VDSL2-Vplus): A later enhancement using frequencies up to 35.328 MHz.
The Catch and the Solution: Distance and FTTx Deployment
The aggressive use of high frequencies in VDSL comes with a fundamental trade-off: severe distance limitations. High-frequency signals suffer from much greater (signal loss) over copper wires than lower frequencies. Consequently, VDSL is only effective on very short local loops, typically under 1.5 kilometers.
Typical VDSL2 Performance vs. Distance:
- Up to 300 meters: Speeds can reach or exceed 100/100 Mbps (symmetric).
- At 1 kilometer: Performance might drop to around 40 Mbps downstream and 10 Mbps upstream.
- Beyond 1.5 kilometers: Speeds become comparable to, or even worse than, ADSL2+.
The Solution: FTTN/FTTC Architecture
Because VDSL cannot operate effectively from a distant central office, it is almost exclusively deployed in an architecture. This involves bringing a high-capacity fiber optic line to a street cabinet near the subscribers. Inside the cabinet, a DSLAM converts the optical signal to VDSL signals, which then travel over the short, existing copper lines for the "last leg" of the journey to the user's home. This strategy maximizes VDSL performance by ensuring the copper segment is short.
Overcoming Interference: The Magic of Vectoring (G.vector)
On short copper lines, the main performance killer for VDSL2 is not attenuation but from other VDSL2 lines in the same cable bundle. This is where VDSL2 Vectoring (standardized as G.vector) comes into play.
How Vectoring Works:
Vectoring is an advanced noise-cancellation technique. The DSLAM in the street cabinet processes signals from all active VDSL2 lines in a cable bundle simultaneously. By knowing the signal it is sending on Line A, the DSLAM can predict the exact interference (crosstalk) that Line A will induce on Line B. It then generates a corresponding "anti-noise" signal and injects it into Line B's transmission, effectively canceling out the crosstalk from Line A. This process is repeated for all lines against all other lines in real-time, functioning as a sophisticated, coordinated noise-cancellation system.
This technique can dramatically improve the signal-to-noise ratio, allowing VDSL2 lines to achieve data rates significantly closer to their theoretical maximum for a given line length, greatly enhancing both speed and stability.