VDSL over short copper loops
VDSL (Very-high-speed Digital Subscriber Line) is an xDSL access technology for high data rates over short twisted-pair subscriber loops. ITU-T G.993.1 specifies the first VDSL generation, while ITU-T G.993.2 specifies VDSL2. Both use existing copper plant, so the electrical length and condition of the loop remain part of the service design.
VDSL uses a wider spectrum than ADSL. More usable subcarriers can carry data, while high-frequency and crosstalk reduce the available reach. The selected profile, band plan, cable and noise environment therefore determine the resulting rate.
VDSL1 examples included about 26 Mbit/s symmetric or 52 Mbit/s downstream and 16 Mbit/s upstream on very short loops. These figures describe physical-layer examples rather than the rate of every service.
Band plans and VDSL2 profiles
A VDSL profile selects usable bands, transmit masks and framing parameters. It sets a capability boundary rather than a guaranteed access rate. ADSL2+ uses spectrum up to about 2.2 MHz, whereas VDSL2 extends the upper band edge substantially.
Selected profiles
- VDSL1 (ITU-T G.993.1): Its band plans reach 12 MHz. The recommendation supports asymmetric and symmetric transmission; the rate depends on the selected band plan, loop and impairment model.
- VDSL2 (ITU-T G.993.2): The 2019 edition defines profiles with different upper band edges. The commonly deployed examples are:
- Profile 17a: upper band edge 17.664 MHz and a 100 Mbit/s minimum bidirectional net data rate capability.
- Profile 30a: upper band edge 30 MHz and a 200 Mbit/s minimum bidirectional net data rate capability.
- Profile 35b: Annex Q extends the band plan to 35.328 MHz; its annex specifies a 400 Mbit/s bidirectional net data rate capability under the stated conditions.
Reach and FTTx deployment
Higher frequencies increase attenuation and make the line more sensitive to crosstalk, bridge taps and changes of cable gauge. A universal VDSL distance limit is therefore not defined. The profile, copper diameter, loop topology, binder occupancy and vectoring status must be considered together.
Deployment examples show the rate falling as the loop gets longer. Up to 300 m, a profile can reach about 100/100 Mbit/s; at 1 km, a result around 40 Mbit/s downstream and 10 Mbit/s upstream is an example; beyond 1.5 km, performance can approach ADSL2+. Each figure must be related to the cable, profile and impairment environment.
Reference lengths in test specifications
- VDSL2 profile 17a: ETSI TS 102 533 uses a 750 m, 0.5 mm reference loop for its energy-consumption test.
- VDSL2 profile 30a: The same test specification uses a 300 m reference loop.
- These are test conditions, not service-rate promises. They illustrate why profiles with wider bands are normally placed closer to the subscriber.
FTTN and FTTC architecture
shortens the copper section. A cabinet DSLAM terminates the fiber uplink and provides VDSL line ports for the remaining copper loop.
Vectoring and self-FEXT cancellation
On short loops, far-end crosstalk from other VDSL2 pairs can become a dominant impairment. ITU-T G.993.5 defines G.vector for coordinated cancellation with VDSL2 transceivers. Its scope covers downstream and upstream self-FEXT inside a vectored group, used together with G.993.2.
How the coordination works
A vectoring control entity estimates the coupling between active lines. The transmitters use that estimate to precode their signals, so the coupled components arrive at the receivers with reduced interference. Coordination must cover the lines in the vectored group; an uncoordinated disturber still contributes noise.
Vectoring can raise the usable signal-to-noise margin and stabilize rates when the line set is coordinated. Alien crosstalk, changing line states, attenuation and the selected PSD mask continue to limit performance.