Very High Data Rate DSL
Very High Data Rate DSL (VDSL) pushes to the limit what can be transmitted over 24-gauge copper pairs. VDSL's intent is to provide the fastest bit rate regardless of distance, and thereby provide a competitive service to cable by supporting entertainment video. VDSL is enabled due to recent advances in DSP technology, which allow the implementation of sophisticated channel modulation and adaptive equalization schemes. VDSL will modulate single-pair wiring from 300 kHz up to (possibly) 30 MHz over distances from 300 meters to 1000 meters. VDSL starts at 300 kHz to protect the POTS/ISDN service. (Yes, it needs a splitter.) This yields the following service options:
| Upstream | Downstream | Distance |
|---|---|---|
| 12.96 Mbps | 12.96 Mbps | 1000 meters |
| 2 Mbps | 25.92 Mbps | 1000 meters |
| 25.84 Mbps | 25.92 Mbps | 300 meters |
| 2 Mbps | 51.84 Mbps or STS-1 | 300 meters |
At a distance of 1000 meters from the ONU, VDSL can support a single channel of HDTV, T1, or E1 data and analog or digital telephony simultaneously.
Some operators, particularly in Europe, believe that it is desirable to have both asymmetric and symmetric systems. Symmetric service, which has higher upstream speeds, would tend to reduce distance but would be more appealing to business-oriented users who would have servers at the client site. This requires agreement on a common spectrum allocation plan and is a prime issue for VDSL. Discussions are in process in ETSI, ADSL Forum, and the ITU (G.vdsl, G.993), with standards to be determined.
The key to providing the fastest bit rate possible is to have the shortest possible length of copper wire and therefore the longest run of fiber possible. To this end, fiber is terminated in an ONU very near the subscriber. Current thought is that the distance from fiber termination to ONU is about 200 meters, with an additional 100 meters on the customer premises, for a total distance of 300 meters from the fiber termination to the terminal equipment.
The ONU can be placed in many locations. If the subscriber resides within 200 meters of a CO or a remote terminal, the ONU can be in the CO or remote terminal, and VDSL service can be provided readily. For subscribers farther out, it will be necessary to locate the ONU in a street pedestal or controlled vault. For residents of a multiple-dwelling unit, where VDSL may have the greatest acceptance, the ONU can be inside the development. But with any of these options, the potential penetration rate is low compared with that of longer-reach services. It is unlikely that VDSL will ever reach a majority of homes in the United States, although it may achieve higher penetration in Asia and Europe because of the higher population densities.
Comparison with ADSL
Because standardization of VDSL is not complete as of this writing, we are limited to comparing the intent of VDSL with ADSL. Certain advantages and characteristics are compared as follows.